Journal of Applied Physiology current issue

Intermittent hypoxia: keeping it real

Morgan, B. J. — 2009-06-30

HRT affects skeletal muscle contractile characteristics: a definitive answer?

Onambele-Pearson, G. L. — 2009-06-30

Newton's force as countermeasure for disuse atrophy

Rennie, M. J., Phillips, S. M., Richter, E. A. — 2009-06-30

The interactions of some commonly consumed drugs with mitochondrial adaptations to exercise

Robinson, M. M., Hamilton, K. L., Miller, B. F. — 2009-06-30

The importance of mitochondrial dysfunctions in the progression of chronic disease has been well established. Patients with chronic diseases are often prescribed a variety of medications, many of which have been shown to induce mitochondrial dysfunction. Exercise is a known stimulus for mitochondrial biogenesis and also recommended to patients as a lifestyle modification to supplement drug therapy. However, the potential interference of those drugs with mitochondrial adaptations to exercise has not been thoroughly investigated. This review provides a summary and discussion of known and potential interactions of commonly consumed drugs with exercise-induced mitochondrial adaptations.

A new model of chronic intermittent hypoxia in humans: effect on ventilation, sleep, and blood pressure

2009-06-30

Obstructive sleep apnea is characterized by repetitive nocturnal upper airway obstructions that are associated with sleep disruption and cyclic intermittent hypoxia (CIH) The cyclic oscillations in O₂ saturation are thought to contribute to cardiovascular and other morbidity, but animal and patient studies of the pathogenic link between CIH and these diseases have been complicated by species differences and by the effects of confounding factors such as obesity, hypertension, and impaired glucose metabolism. To minimize these limitations, we set up a model of nocturnal CIH in healthy humans. We delivered O₂ for 15 s every 2 min during sleep while subjects breathed 13% O₂ in a hypoxic tent to create 30 cycles/h of cyclic desaturation-reoxygenation [saturation of peripheral O₂ (Sp_O₂) range: 95–85%]. We exposed subjects overnight for 8–9 h/day for 2 wk (10 subjects) and 4 wk (8 subjects). CIH exposure induced respiratory disturbances (central apnea hypopnea index: 3.0 ± 1.9 to 31.1 ± 9.6 events/h of sleep at 2 wk). Exposure to CIH for 14 days induced an increase in slopes of hypoxic and hypercapnic ventilatory responses (1.5 ± 0.6 to 3.1 ± 1.2 l·min^–1·% drop in Sp_O₂ and 2.2 ± 1.0 to 3.3 ± 0.9 l·min^–1·mmHg CO₂^–1, respectively), consistent with hypoxic acclimatization. Waking normoxic arterial pressure increased significantly at 2 wk at systolic (114 ± 2 to 122 ± 2 mmHg) and for diastolic at 4 wk (71 ± 1.3 to 74 ± 1.7 mmHg). We propose this model as a new technique to study the cardiovascular and metabolic consequences of CIH in human volunteers.

Postmenopausal hormone replacement therapy modifies skeletal muscle composition and function: a study with monozygotic twin pairs

2009-06-30

We investigated whether long-term hormone replacement therapy (HRT) is associated with mobility and lower limb muscle performance and composition in postmenopausal women. Fifteen 54- to 62-yr-old monozygotic female twin pairs discordant for HRT were recruited from the Finnish Twin Cohort. Habitual (HWS) and maximal (MWS) walking speeds over 10 m, thigh muscle composition, lower body muscle power assessed as vertical jumping height, and maximal isometric hand grip and knee extension strengths were measured. Intrapair differences (IPD%) with 95% confidence intervals (CI) were calculated. The mean duration of HRT use was 6.9 ± 4.1 yr. MWS was on average 7% (0.9 to 13.1%, P = 0.019) and muscle power 16% (–0.8 to 32.8%, P = 0.023) greater in HRT users than in their cotwins. Thigh muscle cross-sectional area tended to be larger (IPD% = 6%, 95% CI: –0.07 to 12.1%, P = 0.065), relative muscle area greater (IPD% = 8%, CI: 0.8 to 15.0%, P = 0.047), and relative fat area smaller (IPD% = –5%, CI: –11.3 to 1.2%, P = 0.047) in HRT users than in their sisters. There were no significant differences in maximal isometric strengths or HWS between users and nonusers. Subgroup analyses revealed that estrogen-containing therapies (11 pairs) significantly decreased total body and thigh fat content, whereas tibolone (4 pairs) tended to increase muscle cross-sectional area. This study showed that long-term HRT was associated with better mobility, greater muscle power, and favorable body and muscle composition among 54- to 62-yr-old women. The results indicate that HRT is a potential agent in preventing muscle weakness and mobility limitation in older women.

Artificial gravity maintains skeletal muscle protein synthesis during 21 days of simulated microgravity

2009-06-30

We sought to determine the effects of longitudinal loading (artificial gravity) on skeletal muscle protein kinetics in 15 healthy young males after 21 days of 6° head-down tilt bed rest [experimental treatment (Exp) group: n = 8, 31 ± 1 yr; control (Con) group; n = 7, 28 ± 1 yr, means ± SE]. On days 1 and 21 of bed rest, postabsorptive venous blood samples and muscle biopsies (vastus lateralis and soleus) were obtained during a 1-h pulse bolus infusion protocol (0 min, l-[ring-¹³C₆]phenylalanine, 35 µmol/kg; 30 min, l-[ring-¹⁵N]phenylalanine, 35 µmol/kg). Outcome measures included mixed muscle fractional synthesis (FSR) and breakdown rates (FBR). The Exp group experienced 1 h of longitudinal loading (2.5G at the feet) via a short-radius centrifuge during each day of bed rest. Mixed muscle FSR in the Con group was reduced by 48.5% (day 1, 0.081 ± 0.000%/h vs. day 21, 0.042 ± 0.000%/h; P = 0.001) in vastus lateralis after 21 days of bed rest, whereas the Exp group maintained their rate of protein synthesis. A similar but nonsignificant change in FSR was noted for the soleus muscle (Exp, –7%; Con, –22%). No changes in muscle protein breakdown were observed. In conclusion, 1 h of daily exposure to artificial gravity maintained the rate of protein synthesis of the vastus lateralis and may represent an effective adjunct countermeasure to combat the loss of muscle mass and functional during extended spaceflight.

Artificial gravity as a countermeasure to microgravity: a pilot study examining the effects on knee extensor and plantar flexor muscle groups

Caiozzo, V. J., Haddad, F., Lee, S., Baker, M., Paloski, W., Baldwin, K. M. — 2009-06-30

The goal of this project was to examine the effects of artificial gravity (AG) on skeletal muscle strength and key anabolic/catabolic markers known to regulate muscle mass. Two groups of subjects were selected for study: 1) a 21 day-bed rest (BR) group (n = 7) and 2) an AG group (n = 8), which was subjected to 21 days of 6° head-down tilt bed rest plus daily 1-h exposures to AG (2.5 G at the feet). Centrifugation was produced using a short-arm centrifuge with the foot plate ~220 cm from the center of rotation. The torque-velocity relationships of the knee extensors and plantar flexors of the ankle were determined pre- and posttreatment. Muscle biopsy samples obtained from the vastus lateralis and soleus muscles were used for a series of gene expression analyses (mRNA abundance) of key factors implicated in the anabolic vs. catabolic state of the muscle. Post/pre torque-velocity determinations revealed greater decrements in knee extensor performance in the BR vs. AG group (P < 0.04). The plantar flexors of the AG subjects actually demonstrated a net gain in the torque-velocity relationship, whereas in the BR group, the responses declined (AG vs. BR, P < 0.001). Muscle fiber cross-sectional area decreased by ~20% in the BR group, whereas no losses were evident in the AG group. RT-PCR analyses of muscle biopsy specimens demonstrated that markers of growth and cytoskeletal integrity were higher in the AG group, whereas catabolic markers were elevated in the BR group. Importantly, these patterns were seen in both muscles. We conclude that paradigms of AG have the potential to maintain the functional, biochemical, and structural homeostasis of skeletal muscle in the face of chronic unloading.

Effects of artificial gravity during bed rest on bone metabolism in humans

2009-06-30

We report results from a study designed to explore the utility of artificial gravity (AG) as a countermeasure to bone loss induced by microgravity simulation. After baseline testing, 15 male subjects underwent 21 days of 6° head-down bed rest to simulate the deconditioning associated with spaceflight. Eight of the subjects underwent 1 h of centrifugation (AG; 1 G_z at the heart, 2.5 G_z at the feet) each day for 21 days, whereas seven of the subjects served as untreated controls (Con). Blood and urine were collected before, during, and after bed rest for bone marker determinations. Bone mineral density (BMD) and bone mineral content (BMC) were determined by dual-energy X-ray absorptiometry and peripheral quantitative computerized tomography before and after bed rest. Urinary excretion of bone resorption markers increased during bed rest, but the AG and Con groups did not differ significantly. The same was true for serum C-telopeptide. During bed rest, bone alkaline phosphatase (ALP) and total ALP tended to be lower in the AG group (P = 0.08, P = 0.09). Neither BMC nor BMD changed significantly from the pre-bed rest period in AG or Con groups, and the two groups were not significantly different. However, when AG and Con data were combined, there was a significant (P < 0.05) effect of time for whole body total BMC and total hip and trochanter BMD. These data failed to demonstrate efficacy of this AG prescription to prevent the changes in bone metabolism observed during 3 wk of bed rest.

Effects of 21 days of bed rest, with or without artificial gravity, on nutritional status of humans

2009-06-30

Spaceflight and bed rest models of microgravity have profound effects on physiological systems, including the cardiovascular, musculoskeletal, and immune systems. These effects can be exacerbated by suboptimal nutrient status, and therefore it is critical to monitor nutritional status when evaluating countermeasures to mitigate negative effects of spaceflight. As part of a larger study to investigate the usefulness of artificial gravity as a countermeasure for musculoskeletal and cardiovascular deficits during bed rest, we tested the hypothesis that artificial gravity would have an effect on some aspects of nutritional status. Dietary intake was recorded daily before, during, and after 21 days of bed rest with artificial gravity (n = 8) or bed rest alone (n = 7). We examined body composition, hematology, general blood chemistry, markers of oxidative damage, and blood levels of selected vitamins and minerals before, during, and after the bed rest period. Several indicators of vitamin status changed in response to diet changes: serum - and -tocopherol and urinary 4-pyridoxic acid decreased (P < 0.001) and plasma β-carotene increased (P < 0.001) in both groups during bed rest compared with before bed rest. A decrease in hematocrit (P < 0.001) after bed rest was accompanied by a decrease in transferrin (P < 0.001), but transferrin receptors were not changed. These data provide evidence that artificial gravity itself does not negatively affect nutritional status during bed rest. Likewise, artificial gravity has no protective effect on nutritional status during bed rest.

Active middle-aged men have lower fasting inflammatory markers but the postprandial inflammatory response is minimal and unaffected by physical activity status

Dixon, N. C., Hurst, T. L., Talbot, D. C. S., Tyrrell, R. M., Thompson, D. — 2009-06-30

Physical activity modifies some postprandial responses such as glycemic control, although it is unclear whether this translates into lower postprandial inflammation. Our objective in this study was to determine whether postprandial inflammatory markers are lower in active compared with sedentary middle-aged men. Thirteen active and twelve sedentary middle-aged men consumed a mixed meal on one occasion. Blood was taken via a cannula before and up to 8 h after the meal and with a single-use needle before and 8 h after the meal. Active men had lower fasted IL-6 (0.6 ± 0.2 vs. 1.2 ± 0.3 pg/ml; P = 0.004) and C-reactive protein (1.3 ± 0.3 vs. 2.9 ± 0.6 mg/l; P = 0.04) concentrations than sedentary men. Cannula blood IL-6 concentrations increased by 3.49 pg/ml in the 8 h following the meal (P < 0.001); however, this increase was minimal (0.36 pg/ml) in blood taken via a single-use needle from the contralateral arm (P = 0.013). The sedentary group had larger glucose (P = 0.034), insulin (P = 0.013), and triacylglycerol (P = 0.057) responses to the meal. These results provide further evidence that physical activity is associated with lower inflammatory marker concentrations in a fasted state and a lower postprandial metabolic response to a meal. However, this does not translate into lower postprandial inflammatory markers since the only evidence of postprandial inflammation (a large increase in serum IL-6) was actually due to the cannula used for blood sampling.

A simple and valid method to determine thermoregulatory sweating threshold and sensitivity

2009-06-30

Sweating threshold temperature and sweating sensitivity responses are measured to evaluate thermoregulatory control. However, analytic approaches vary, and no standardized methodology has been validated. This study validated a simple and standardized method, segmented linear regression (SReg), for determination of sweating threshold temperature and sensitivity. Archived data were extracted for analysis from studies in which local arm sweat rate (m_sw; ventilated dew-point temperature sensor) and esophageal temperature (T_es) were measured under a variety of conditions. The relationship m_sw/T_es from 16 experiments was analyzed by seven experienced raters (Rater), using a variety of empirical methods, and compared against SReg for the determination of sweating threshold temperature and sweating sensitivity values. Individual interrater differences (n = 324 comparisons) and differences between Rater and SReg (n = 110 comparisons) were evaluated within the context of biologically important limits of magnitude (LOM) via a modified Bland-Altman approach. The average Rater and SReg outputs for threshold temperature and sensitivity were compared (n = 16) using inferential statistics. Rater employed a very diverse set of criteria to determine the sweating threshold temperature and sweating sensitivity for the 16 data sets, but interrater differences were within the LOM for 95% (threshold) and 73% (sensitivity) of observations, respectively. Differences between mean Rater and SReg were within the LOM 90% (threshold) and 83% (sensitivity) of the time, respectively. Rater and SReg were not different by conventional t-test (P > 0.05). SReg provides a simple, valid, and standardized way to determine sweating threshold temperature and sweating sensitivity values for thermoregulatory studies.

NRF2 intron 3 A/G polymorphism is associated with endurance athletes' status

2009-06-30

The aim of this study was to determine the frequency distribution of nuclear respiratory factor 2 (NRF2) intron 3 A/G polymorphism (rs7181866) among 155 Israeli athletes (endurance athletes and sprinters) and 240 healthy controls. Results showed that there was a significantly higher proportion of the AG genotype, rather than the AA genotype, in the group of endurance athletes compared with the sprinters (P = 0.014) and controls (P = 0.0008). However, the sprinters’ genotype and allele frequencies were similar to those of the control group (P = 0.62 for genotype distribution percentage). These results were even more pronounced when we compared between the subgroups of 20 elite endurance athletes (those who had represented Israel in a world track-and-field championship or in the Olympic Games) and 54 national-level endurance athletes. In the group of elite endurance athletes the G allele was more frequent than in the national-level endurance athletes (P = 0.047). We conclude that 1) in Israeli athletes the NRF2 AG genotype is more frequent in elite endurance athletes than in sprinters, and 2) within the endurance group the NRF2 AG genotype and the G allele are more frequent in elite athletes, suggesting a positive association between the AG genotype, and possibly the G allele, and the likelihood of being an elite endurance athlete.

Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles

2009-06-30

Single motor unit electromyographic (EMG) activity of the knee extensors was investigated at different knee angles with subjects (n = 10) exerting the same absolute submaximal isometric torque at each angle. Measurements were made over a 20° range around the optimum angle for torque production (AngleTmax) and, where feasible, over a wider range (50°). Forty-six vastus lateralis (VL) motor units were recorded at 20.7 ± 17.9 %maximum voluntary contraction (%MVC) together with the rectified surface EMG (rsEMG) of the superficial VL muscle. Due to the lower maximal torque capacity at positions more flexed and extended than AngleTmax, single motor unit recruitment thresholds were expected to decrease and discharge rates were expected to increase at angles above and below AngleTmax. Unexpectedly, the recruitment threshold was higher (P < 0.05) at knee angles 10° more extended (43.7 ± 22.2 N·m) and not different (P > 0.05) at knee angles 10° more flexed (35.2 ± 17.9 N·m) compared with recruitment threshold at AngleTmax (41.8 ± 21.4 N·m). Also, unexpectedly the discharge rates were similar (P > 0.05) at the three angles: 11.6 ± 2.2, 11.6 ± 2.1, and 12.3 ± 2.1 Hz. Similar angle independent discharge rates were also found for 12 units (n = 5; 7.4 ± 5.4 %MVC) studied over the wider (50°) range, while recruitment threshold only decreased at more flexed angles. In conclusion, the similar recruitment threshold and discharge behavior of VL motor units during submaximal isometric torque production suggests that net motor unit activation did not change very much along the ascending limb of the knee-angle torque relationship. Several factors such as length-dependent twitch potentiation, which may contribute to this unexpected aspect of motor control, are discussed.

Training improves the response in glucose flux to exercise in postmenopausal women

2009-06-30

We examined the effects of endurance training on parameters of glucose flux during rest and exercise in postmenopausal women. Ten sedentary, but healthy women (55 ± 1 yr) completed 12 wk of endurance exercise training on a cycle ergometer [5 days/wk, 1 h/day, 65% peak oxygen consumption (Vo_2peak)]. Flux rates were determined by primed continuous infusion of [6,6-²H]glucose (D₂-glucose) during 90 min of rest and 60 min of cycle ergometer exercise during one pretraining exercise trial [65% Vo_2peak (PRE)] and two posttraining exercise trials [the power output that elicited 65% pretraining Vo_2peak (ABT) and 65% posttraining Vo_2peak (RLT)]. Training increased Vo_2peak by 16.3 ± 3.9% (P < 0.05). Epinephrine and glucagon were lower during ABT and lactate was lower during ABT and RLT (P < 0.05), but the apparent insulin response was unchanged. Whole body glucose rate of appearance decreased posttraining during exercise at a given power output (4.58 ± 0.39 mg·kg^–1·min^–1 during ABT compared with 5.21 ± 0.48 mg·kg^–1·min^–1 PRE, P < 0.05), but not at the same relative workload (5.85 ± 0.36 mg·kg^–1·min^–1). Training resulted in a 35% increase in glucose MCR during exercise at the same relative intensity (7.16 ± 0.42 ml·kg^–1·min^–1 during RLT compared with 5.28 ± 0.42 ml·kg^–1·min^–1 PRE, P < 0.05). Changes in parameters of glucose kinetics during exercise were accomplished without changes in dietary composition, body weight, or body composition. We conclude that despite changes in the hormonal milieu that occur at menopause, endurance training results in a similar magnitude in training-induced alterations of glucose flux as seen previously in younger women.

Neuromuscular recovery pattern after medial collateral ligament disruption in rats

Laurin, J., Dousset, E., Mesure, S., Decherchi, P. — 2009-06-30

The medial collateral ligament (MCL) is one of the most injured ligaments during sport activities. The resulting joint damage effects on neuromuscular system remain unclear. Thus this study was designed to assess the changes in neuromuscular properties of vastus medialis muscle after MCL transection. Complete rupture of MCL was performed on rats, and dynamic functional assessment during locomotion was achieved before and once a week from 1–5 wk postlesion. Twitch properties and metabo- and mechanosensitive afferent fiber responses to specific stimuli were measured 1, 3, and 5 wk after MCL transection. Results indicated that maximum knee angle measured during the stance phase of the gait cycle was decreased during 3 wk after MCL injury and then recovered. Minimum knee angle measured during the stance phase was decreased during 2 wk and showed compensatory effects at week 5. A stepwise decrease in maximum relaxation rate-to-amplitude ratio concomitant with a stepwise increase in half-relaxation time were observed following MCL injury. Variations in metabosensitive afferent response to chemical (KCl and lactic acid) injections were decreased at week 1 and recovered progressively from week 3 to week 5 postlesion. Recovery of the mechanosensitive afferent response to vibrations was not totally complete after 5 wk. Our data indicate that alteration of the sensory pathways from the vastus medialis muscle could be considered as a source of neuromuscular deficits following MCL transection. Our results should be helpful in clinical purpose to improve the knowledge of the influence exerted by ligament rupture on the motor system and permit development of rehabilitation protocols and exercises more appropriate for recovery of functional stability.

Forced expiratory flows and volumes in intubated and paralyzed infants and children: normative data up to 5 years of age

von Ungern-Sternberg, B. S., Trachsel, D., Erb, T. O., Hammer, J. — 2009-06-30

Reference equations that express indexes obtained from forced expiratory maneuvers in relation to height and/or other independent variables are lacking for infants and children with artificial airways. The present study was performed to establish normative data of forced expiration by forced deflation in healthy intubated and paralyzed infants and children and to develop prediction equations in relation to height and to ulna length to enable pulmonary assessments in children whose height is difficult to measure. Measurements of forced and passive expiratory maneuvers after inflation to +40 cmH₂O inspiratory pressure were prospectively obtained in 100 healthy anesthetized children from 0 to 5 yr of age. Linear regressions of log-transformed forced vital capacity (FVC) and maximum expiratory flow at 25% and 10% FVC (MEF₂₅ and MEF₁₀, respectively) obtained by forced deflation (–40 cmH₂O airway opening pressure) and of analogous indexes obtained by passive deflation were used to develop prediction equations from height or ulna length. FVC was significantly dependent on age and height or ulna length. Prediction equations for FVC using height or ulna length were as follows: ln(FVC in ml) = –5.6 + 2.8 x ln(height in cm) and ln(FVC in ml) = 0.46 + 2.5 x ln(ulna length in cm). Younger subjects had a significantly steeper slope for FVC vs. height than the older age group. Normal reference data for forced expiratory maneuvers in intubated infants and children up to 5 yr of age will enable improved assessment of pulmonary dysfunction in acutely or chronically ventilator-dependent children. Using ulna length instead of height should facilitate respiratory assessment in ventilated children with spinal or joint deformities.

Increases in corticospinal responsiveness during a sustained submaximal plantar flexion

Hoffman, B. W., Oya, T., Carroll, T. J., Cresswell, A. G. — 2009-06-30

Studying the responsiveness of specific central nervous system pathways to electrical or magnetic stimulation can provide important information regarding fatigue processes in the central nervous system. We investigated the changes in corticospinal responsiveness during a sustained submaximal contraction of the triceps surae. Comparisons were made between the size of motor-evoked potentials (MEPs) elicited by motor cortical stimulation and cervicomedullary motor-evoked potentials (CMEPs) elicited by magnetic stimulation of the descending tracts to determine the site of any change in corticospinal responsiveness. Participants maintained an isometric contraction of triceps surae at 30% of maximal voluntary contraction (MVC) for as long as possible on two occasions. Stimulation was applied to the motor cortex or the cervicomedullary junction at 1-min intervals during contraction until task failure. Peripheral nerve stimulation was also applied to evoke maximal M waves (M_max) and a superimposed twitch. Additionally, MEPs and CMEPs were evoked during brief contractions at 80%, 90%, and 100% of MVC as a nonfatigue control. During the sustained contractions, MEP amplitude increased significantly in soleus (113%) and medial gastrocnemius (108%) muscles and, at task failure, matched MEP amplitude in the prefatigue MVC (~20–25% M_max). In contrast, CMEP amplitude increased significantly in medial gastrocnemius (51%), but not in soleus (63%) muscle and, at task failure, was significantly smaller than during prefatigue MVC (5–6% M_max vs. 11–13% M_max). The data indicate that cortical processes contribute substantially to the increase in corticospinal responsiveness during sustained submaximal contraction of triceps surae.

Hindlimb unweighting induces changes in the p38MAPK contractile pathway of the rat abdominal aorta

Summers, S. M., Hayashi, Y., Nguyen, S. V., Nguyen, T. M., Purdy, R. E. — 2009-06-30

Hindlimb unweighting (HLU) of rats is a model used to mimic the cephalic fluid shift potentially involved in the orthostatic intolerance experienced by astronauts. Certain arteries in these rats exhibit a decreased contractile response to adrenergic agonists. It was shown previously that this may be caused by changes in thick filament regulation (Summers et al., Vascul Pharmacol 48: 208–214, 2008). In the present study, it was hypothesized that HLU also modifies thin filament regulation by effects on p38^MAPK and ERK. Abdominal aorta rings from 20-day HLU rats and untreated controls were subjected to phenylephrine and phorbol 12,13-dibutyrate (PDBU) concentration response curves in the presence and absence of two inhibitors: the p38^MAPK inhibitor SB-203580 and the MEK inhibitor U-0126. SB-203580 decreased control sensitivity to both agonists, but HLU sensitivity was not significantly affected. U-0126, which blocks enzymes immediately upstream of ERK, affected sensitivity to both agonists equally between control and HLU. Western blot analysis revealed no change in total levels of p38^MAPK and its downstream target heat shock protein 27 but did reveal a decrease in phosphorylated levels of both after stimulation with PDBU and phenylephrine after HLU treatment. Neither total ERK nor phosphorylated levels after stimulation were affected by HLU. Total levels of caldesmon, a molecule downstream of both pathways, were decreased, but phosphorylated levels after stimulation were decreased by roughly twice as much. The results of this study demonstrate that HLU downregulates p38^MAPK, but not ERK, signaling. In turn, this may decrease actin availability for contraction.

Effect of short-term high-intensity interval training vs. continuous training on O2 uptake kinetics, muscle deoxygenation, and exercise performance

McKay, B. R., Paterson, D. H., Kowalchuk, J. M. — 2009-06-30

The early time course of adaptation of pulmonary O₂ uptake (Vo₂_p) (reflecting muscle O₂ consumption) and muscle deoxygenation kinetics (reflecting the rate of O₂ extraction) were examined during high-intensity interval (HIT) and lower-intensity continuous endurance (END) training. Twelve male volunteers underwent eight sessions of either HIT (8–12 x 1-min intervals at 120% maximal O₂ uptake separated by 1 min of rest) or END (90–120 min at 65% maximal O₂ uptake). Subjects completed step transitions to a moderate-intensity work rate (~90% estimated lactate threshold) on five occasions throughout training, and ramp incremental and constant-load performance tests were conducted at pre-, mid-, and posttraining periods. Vo₂_p was measured breath-by-breath by mass spectrometry and volume turbine. Deoxygenation (change in deoxygenated hemoglobin concentration; [HHb]) of the vastus lateralis muscle was monitored by near-infrared spectroscopy. The fundamental phase II time constants for Vo₂_p (Vo₂) and deoxygenation kinetics {effective time constant, ' = (time delay + ), [HHb]} during moderate-intensity exercise were estimated using nonlinear least-squares regression techniques. The Vo₂ was reduced by ~20% (P < 0.05) after only two training sessions and by ~40% (P < 0.05) after eight training sessions (i.e., posttraining), with no differences between HIT and END. The '[HHb] (~20 s) did not change over the course of eight training sessions. These data suggest that faster activation of muscle O₂ utilization is an early adaptive response to both HIT and lower-intensity END training. That [HHb] kinetics (a measure of fractional O₂ extraction) did not change despite faster Vo₂_p kinetics suggests that faster kinetics of muscle O₂ utilization were accompanied by adaptations in local muscle (microvascular) blood flow and O₂ delivery, resulting in a similar "matching" of blood flow to O₂ utilization. Thus faster kinetics of Vo₂_p during the transition to moderate-intensity exercise occurs after only 2 days HIT and END training and without changes to muscle deoxygenation kinetics, suggesting concurrent adaptations to microvascular perfusion.

Nitric oxide and carbon monoxide lung transfer in patients with advanced liver cirrhosis

2009-06-30

In advanced cirrhosis, decreased lung transfer for carbon monoxide (TLCO) and increased alveolar-arterial oxygen tension difference (PA-aO₂ ≥15 mmHg while breathing ambient air) are frequently detected. Pulmonary membrane diffusion capacity for CO (DmCO) and pulmonary capillary blood volume (Vcap) can be derived from the simultaneous measurement of TLCO and lung transfer for nitric oxide (TLNO). Measurements of single-breath TLNO and TLCO were performed in 49 cirrhotic patients with advanced liver cirrhosis and in 35 healthy controls to derive Vcap, DmCO, and TLNO:TLCO ratio. Twenty-five patients had increased PA-aO₂, of whom 11 had hepatopulmonary syndrome (HPS). Compared with controls, non-HPS patients with normal PA-aO₂ had a significant ~10% decrease in TLCO, DmCO, and Vcap but similar TLNO:TLCO ratios. Compared with non-HPS patients with normal PA-aO₂, non-HPS patients with increased PA-aO₂ had lower Vcap and higher TLNO:TLCO ratio but similar DmCO. HPS patients had lower Vcap and higher TLNO:TLCO ratios than both subgroups of non-HPS patients. In cirrhotic patients, TLNO:TLCO ratios correlated positively, and TLCO (percentage of the predicted value) and Vcap (percentage of the predicted value) correlated negatively with PA-aO₂ (r² = 0.25, P = 0.0003, r² = 0.48, P < 0.0001 and r² = 0.57, P < 0.0001, respectively). We concluded that, in cirrhotic patients, lower TLCO and increased PA-aO₂ are associated with lower Vcap. In addition, high TLNO:TLCO ratios in patients with increased PA-aO₂ suggest a decreased thickness of the capillary blood layer in these patients.

Taurine supplementation increases skeletal muscle force production and protects muscle function during and after high-frequency in vitro stimulation

2009-06-30

Recent studies report that depletion and repletion of muscle taurine (Tau) to endogenous levels affects skeletal muscle contractility in vitro. In this study, muscle Tau content was raised above endogenous levels by supplementing male Sprague-Dawley rats with 2.5% (wt/vol) Tau in drinking water for 2 wk, after which extensor digitorum longus (EDL) muscles were examined for in vitro contractile properties, fatigue resistance, and recovery from fatigue after two different high-frequency stimulation bouts. Tau supplementation increased muscle Tau content by ~40% and isometric twitch force by 19%, shifted the force-frequency relationship upward and to the left, increased specific force by 4.2%, and increased muscle calsequestrin protein content by 49%. Force at the end of a 10-s (100 Hz) continuous tetanic stimulation was 6% greater than controls, while force at the end of the 3-min intermittent high-frequency stimulation bout was significantly higher than controls, with a 12% greater area under the force curve. For 1 h after the 10-s continuous stimulation, tetanic force in Tau-supplemented muscles remained relatively stable while control muscle force gradually deteriorated. After the 3-min intermittent bout, tetanic force continued to slowly recover over the next 1 h, while control muscle force again began to decline. Tau supplementation attenuated F₂-isoprostane production (a sensitive indicator of reactive oxygen species-induced lipid peroxidation) during the 3-min intermittent stimulation bout. Finally, Tau transporter protein expression was not altered by the Tau supplementation. Our results demonstrate that raising Tau content above endogenous levels increases twitch and subtetanic and specific force in rat fast-twitch skeletal muscle. Also, we demonstrate that raising Tau protects muscle function during high-frequency in vitro stimulation and the ensuing recovery period and helps reduce oxidative stress during prolonged stimulation.

Improved predictive models for plasma glucose estimation from multi-linear regression analysis of exhaled volatile organic compounds

2009-06-30

Exhaled volatile organic compounds (VOCs) represent ideal biomarkers of endogenous metabolism and could be used to noninvasively measure circulating variables, including plasma glucose. We previously demonstrated that hyperglycemia in different metabolic settings (glucose ingestion in pediatric Type 1 diabetes) is paralleled by changes in exhaled ethanol, acetone, and methyl nitrate. In this study we integrated these gas changes along with three additional VOCs (2 forms of xylene and ethylbenzene) into multi-linear regression models to predict plasma glucose profiles in 10 healthy young adults, during the 2 h following an intravenous glucose bolus (matched samples of blood, exhaled and room air were collected at 12 separate time points). The four-gas model with highest predictive accuracy estimated plasma glucose in each subject with a mean R value of 0.91 (range 0.70–0.98); increasing the number of VOCs in the model only marginally improved predictions (average R with best 5-gas model = 0.93; with 6-gas model = 0.95). While practical development of this methodology into clinically usable devices will require optimization of predictive algorithms on large-scale populations, our data prove the feasibility and potential accuracy of breath-based glucose testing.

Effect of a peripheral nerve block on torque produced by repetitive electrical stimulation

Lagerquist, O., Walsh, L. D., Blouin, J.-S., Collins, D. F., Gandevia, S. C. — 2009-06-30

Neuromuscular electrical stimulation (NMES) generates contractions by activation of motor axons (peripheral mechanism), but the afferent volley also contributes by recruiting spinal motoneurons synaptically (central mechanism), which recruits motoneurons according to Henneman's size principle. Thus, we hypothesized that contractions that develop due to a combination of peripheral and central mechanisms will fatigue less rapidly than when electrically evoked contractions are generated by the activation of motor axons alone. Plantar-flexion torque evoked by NMES over the triceps surae was compared in five able-bodied subjects before (Intact) and during (Blocked) a complete anesthetic block of the tibial and common peroneal nerves. In the Blocked condition, plantar-flexion torque could only develop from the direct activation of motor axons beneath the stimulating electrodes. NMES was delivered using three protocols: protocol A, constant 100 Hz for 30 s; protocol B, four 2-s bursts of 100 Hz alternating with 20-Hz stimulation; and protocol C, alternating 100 Hz bursts (1 s on, 1 s off) for 30 s. The percent change in evoked plantar flexion torque from the beginning to the end of the stimulation differed (P < 0.05) between Intact and Blocked conditions for all protocols (Intact: protocol A = +125%, B = +230%, C = +78%; Blocked: protocol A = –79%, B = –15%, C = –35%). These results corroborate previous evidence that NMES can evoke contractions via the recruitment of spinal motoneurons in addition to the direct recruitment of motor axons. We now show that NMES delivered for periods of up to 30 s generates plantar-flexion torque which decreases when only motor axons are recruited and increases when the central nervous system can contribute.

A brief bout of exercise alters gene expression and distinct gene pathways in peripheral blood mononuclear cells of early- and late-pubertal females

Radom-Aizik, S., Zaldivar, F., Leu, S.-Y., Cooper, D. M. — 2009-06-30

Recent studies show that brief exercise alters circulating neutrophil and peripheral blood mononuclear cell (PBMC) gene expression, ranging from cell growth to both pro-and anti-inflammatory processes. These initial observations were made solely in males, but whether PBMC gene expression is altered by exercise in females is not known. Ten early-pubertal girls (8–11 yr old) and 10 late-pubertal girls (15–17 yr old) performed ten 2-min bouts of cycle ergometry (~90% peak heart rate) interspersed with 1-min rest intervals. Blood was obtained at rest and after exercise, and microarrays were performed in each individual subject. RNA was hybridized to Affymetrix U133+2.0 Arrays. Exercise induced significant changes in PBMC gene expression in early (1,320 genes)- and late (877 genes)-pubertal girls. The expression of 622 genes changed similarly in both groups. Exercise influenced a variety of established gene pathways (EASE < 0.04) in both older (6 pathways) and younger girls (11 pathways). Five pathways were the same in both groups and were functionally related to inflammation, stress, and apoptosis, such as natural killer cell-mediated cytotoxicity, antigen processing and presentation, B cell receptor signaling, and apoptosis. In summary, brief exercise alters PBMC gene expression in early- and late-pubertal girls. The pattern of change involves diverse genetic pathways, consistent with a global danger-type response, perhaps readying PBMCs for a range of physiological functions from inflammation to tissue repair that would be useful following a bout of physical activity.

Role of calcitonin gene-related peptide (CGRP) in ovine burn and smoke inhalation injury

2009-06-30

Concomitant smoke inhalation trauma in burn patients is a serious medical problem. Previous investigations in our sheep model revealed that these injuries lead to significant airway hyperemia, enhanced pulmonary fluid extravasation, and severely impaired pulmonary function. However, the pathophysiological mechanisms are still not fully understood. The lung is innervated by sensory nerves containing peptides such as substance P and calcitonin gene-related peptide. Noxious stimuli in the airways can induce a neurogenic inflammatory response, which has previously been implicated in several airway diseases. Calcitonin gene-related peptide is known to be a potent vasodilator. We hypothesized that calcitonin gene-related peptide is also a mediator of the pulmonary reaction to toxic smoke and planned experiments to evaluate its role in this model. We tested the effects of pretreatment with a specific antagonist of the major receptor for calcitonin gene-related peptide (BIBN4096BS; 32 µg/kg, followed by continuous infusion of 6.4 µg·kg^–1·h^–1) until the animal was killed 48 h after injury in an established ovine model of burn (40% total body surface, third degree) and smoke inhalation (48 breaths, <40°C) injury. In treated animals (n = 7), the injury-related increases in tracheal blood flow and lung lymph flow were significantly attenuated compared with untreated controls (n = 5). Furthermore, the treatment significantly attenuated abnormalities in respiratory gas exchange. The data suggest that calcitonin gene-related peptide contributes to early airway hyperemia, transvascular fluid flux, and respiratory malfunction following ovine burn and smoke inhalation injury. Future studies will be needed to clarify the potential therapeutic benefit for patients with this injury.

Airway wall attenuation: a biomarker of airway disease in subjects with COPD

2009-06-30

The computed tomographic (CT) densities of imaged structures are a function of the CT scanning protocol, the structure size, and the structure density. For objects that are of a dimension similar to the scanner point spread function, CT will underestimate true structure density. Prior investigation suggests that this process, termed contrast reduction, could be used to estimate the strength of thin structures, such as cortical bone. In this investigation, we endeavored to exploit this process to provide a CT-based measure of airway disease that can assess changes in airway wall thickening and density that may be associated with the mural remodeling process in subjects with chronic obstructive pulmonary disease (COPD). An initial computer-based study using a range of simulated airway wall sizes and densities suggested that CT measures of airway wall attenuation could detect changes in both wall thickness and structure density. A second phantom-based study was performed using a series of polycarbonate tubes of known density. The results of this again demonstrated the process of contrast reduction and further validated the computer-based simulation. Finally, measures of airway wall attenuation, wall thickness, and wall area (WA) divided by total cross-sectional area, WA percent (WA%), were performed in a cohort of 224 subjects with COPD and correlated with spirometric measures of lung function. The results of this analysis demonstrated that wall attenuation is comparable to WA% in predicting lung function on univariate correlation and remain as a statistically significant correlate to the percent forced expiratory volume in 1 s predicted when adjusted for measures of both emphysema and WA%. These latter findings suggest that the quantitative assessment of airway wall attenuation may offer complementary information to WA% in characterizing airway disease in subjects with COPD.

Effects of chronic hypoxia on soluble guanylate cyclase activity in fetal and adult ovine cerebral arteries

Pearce, W. J., Williams, J. M., White, C. R., Lincoln, T. M. — 2009-06-30

A broad variety of evidence obtained largely in pulmonary vasculature suggests that chronic hypoxia modulates vasoreactivity to nitric oxide (NO). The present study explores the general hypothesis that chronic hypoxia also modulates cerebrovascular reactivity to NO, and does so by modulating the activity of soluble guanylate cyclase (sGC), the primary target for NO in vascular smooth muscle. Pregnant and nonpregnant ewes were maintained at either sea level or at 3,820 m for the final 110 days of gestation, at which time middle cerebral arteries from term fetal lambs and nonpregnant adults were harvested. In both fetal and adult arteries, NO-induced vasodilatation was attenuated by chronic hypoxia and completely inhibited by 10 µM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of sGC. sGC abundance (in ng sGC/mg protein) measured via Western immunoblots was ~10-fold greater in fetal (17.6 ± 1.6) than adult (1.7 ± 0.3) arteries but was not affected by chronic hypoxia. The specific activity of sGC (in pmol cGMP·µg sGC^–1·min^–1) was similar in fetal (255 ± 64) and adult (280 ± 75) arteries and was inhibited by chronic hypoxia in both fetal (120 ± 10) and adult (132 ± 26) arteries. Rates of cGMP degradation (in pmol cGMP·mg protein^–1·min^–1) were similar in fetal (159 ± 59) and adult (134 ± 36) arteries but were not significantly depressed by chronic hypoxia in either fetal (115 ± 25) or adult (108 ± 25) arteries. The cGMP analog 8-(p-chlorophenylthio)-cGMP was a more potent vasorelaxant in fetal (pD₂ = 4.7 ± 0.1) than adult (pD₂ = 4.3 ± 0.1) arteries, but its ability to promote vasodilatation was not affected by chronic hypoxia in either age group. Together, these results reveal that hypoxic inhibition of NO-induced vasodilatation is attributable largely to attenuation of the specific activity of sGC and does not involve significant changes in sGC abundance, cGMP-phosphodiesterase activity, or the vasorelaxant activity of protein kinase G.

Greater systemic lipolysis in women compared with men during moderate-dose infusion of epinephrine and/or norepinephrine

Horton, T. J., Dow, S., Armstrong, M., Donahoo, W. T. — 2009-06-30

Women have lower circulating catecholamine levels during metabolic perturbations, such as exercise or hypoglycemia, but similar rates of systemic lipolysis. This suggests women may be more sensitive to the lipolytic action of catecholamines, while maintaining similar glucoregulatory effects. The aim of the present study, therefore, was to determine whether women have higher rates of systemic lipolysis compared with men in response to matched peripheral infusion of catecholamines, but similar rates of glucose turnover. Healthy, nonobese women (n = 11) and men (n = 10) were recruited and studied on 3 separate days with the following infusions: epinephrine (Epi), norepinephrine (NE), or the two combined. Tracer infusions of glycerol and glucose were used to determine systemic lipolysis and glucose turnover, respectively. Following basal measurements of substrate kinetics, the catecholamine infusion commenced, and measures of substrate kinetics continued for 60 min. Catecholamine concentrations were similarly elevated in women and men during each infusion: Epi, 182–197 pg/ml and NE, 417–507 pg/ml. There was a significant sex difference in glycerol rate of appearance and rate of disappearance with the catecholamine infusions (P < 0.0001), mainly due to a significantly greater glycerol turnover during the first 30 min of each infusion: glycerol rate of appearance during Epi was only 268 ± 18 vs. 206 ± 21 µmol/min in women and men, respectively; during NE, only 173 ± 13 vs. 153 ± 17 µmol/min, and during Epi+NE, 303 ± 24 vs. 257 ± 21 µmol/min. No sex differences were observed in glucose kinetics under any condition. In conclusion, these data suggest that women are more sensitive to the lipolytic action of catecholamines, but have no difference in their glucoregulatory response. Thus the lower catcholamine levels observed in women vs. men during exercise and other metabolic perturbations may allow women to maintain a similar or greater level of lipid mobilization while minimizing changes in glucose turnover.

L-2-Oxothiazolidine-4-carboxylate reverses glutathione oxidation and delays fatigue of skeletal muscle in vitro

Ferreira, L. F., Gilliam, L. A. A., Reid, M. B. — 2009-06-30

Fatiguing exercise promotes oxidation of intracellular thiols, notably glutathione. Interventions that oppose or reverse thiol oxidation can inhibit fatigue. The reduced cysteine donor l-2-oxothiazolidine-4-carboxylate (OTC) supports glutathione synthesis and is approved for use in humans but has not been evaluated for effects on skeletal muscle. We tested the hypotheses that OTC would 1) increase reduced glutathione (GSH) levels and decrease oxidized glutathione, and 2) inhibit functional indexes of fatigue. Diaphragm fiber bundles from adult male ICR mice were incubated for 1 or 2 h at 37°C with buffer (control, C) or OTC (10 mM). N-acetylcysteine (NAC; 10 mM) was used as a positive control. We measured GSH metabolites and fatigue characteristics. We found that muscle GSH content was increased after 1-h incubation with OTC or NAC but was not altered after 2-h incubation. One-hour treatment with OTC or NAC slowed the decline in force with repetitive stimulation [mean (SD) fatigue index at 300 s: OTC = 34 ± 6% vs. C = 50 ± 8%, P < 0.05; NAC = 55 ± 4% vs. C = 65 ± 8%, P < 0.05] as did the 2-h OTC treatment (OTC = 38 ± 9% vs. C = 51 ± 9%, P < 0.05). These results demonstrate that OTC modulates the muscle GSH pool and opposes fatigue under the current experimental conditions.

Spinal NMDA receptor activation is necessary for de novo, but not the maintenance of, A2a receptor-mediated phrenic motor facilitation

Golder, F. J. — 2009-06-30

Adenosine 2a (A2a) receptor agonists elicit persistent increases in phrenic nerve activity by transactivating the neurotrophin receptor, TrkB, near phrenic motoneurons. Our working model proposes that A2a receptor-mediated TrkB receptor activation strengthens glutamatergic synapses onto phrenic motoneurons. Activation of glutamate N-methyl d-aspartate (NMDA) receptors has been implicated in other models of phrenic motor plasticity. Thus we hypothesized that NMDA receptor activation also would contribute to A2a receptor-mediated phrenic motor facilitation. Adult male Sprague-Dawley rats were anesthetized with urethane, mechanically ventilated, neuromuscularly paralyzed, and bilaterally vagotomized. The A2a receptor agonist CGS-21680 and the NMDA receptor-channel blocker MK-801 were administered intrathecally over the C4 spinal segment. Phrenic nerve activity was recorded before, during, and after drug administration. MK-801 (concentration range 0.1, 1.0, 10.0, and 100 µM) was administered 30 min before CGS-21680 (50 µM). MK-801 dose-dependently blocked A2a receptor-mediated phrenic motor facilitation. When administered at 60 min post-CGS-21680, MK-801 prevented further increases in phrenic nerve activity compared with the CGS-21680 alone (CGS-21680 alone at 120 min: 114 ± 19%; CGS-21680 and MK-801 at 60 min post-CGS-21680: 61 ± 11%, above baseline, P < 0.05) but did not return phrenic motor output to baseline values. Our data suggest that NMDA receptor activation is necessary for de novo A2a receptor-mediated phrenic motor facilitation and that the maintenance of preexisting phrenic motor facilitation does not involve NMDA receptor-dependent mechanisms.

Absence of caspase-3 protects against denervation-induced skeletal muscle atrophy

Plant, P. J., Bain, J. R., Correa, J. E., Woo, M., Batt, J. — 2009-06-30

The ubiquitin-proteasome system is a key proteolytic pathway activated during skeletal muscle atrophy. The proteasome, however, cannot degrade intact myofibrils or actinomyosin complexes. In rodent models of diabetes mellitus and uremia, caspase-3 is involved in actinomyosin cleavage, generating fragments that subsequently undergo ubiquitin-proteasome-mediated degradation. Here, we demonstrate that caspase-3 also mediates denervation-induced muscle atrophy. At 2 wk after tibial nerve transection, the denervated gastrocnemius of caspase-3-knockout mice weighed more and demonstrated larger fiber-type-specific cross-sectional area than the denervated gastrocnemius of wild-type mice. However, there was no difference between caspase-3-knockout and wild-type denervated muscles in the magnitude or pattern of actinomyosin degradation, as determined by Western blotting for actin and the 14-kDa actin fragment. Similarly, there was no difference between caspase-3-knockout and wild-type denervated muscles in the magnitude of increase in proteasome activity, total protein ubiquitination, or atrogin-1 and muscle-specific ring finger protein 1 transcript levels. In contrast, there was an increase in TdT-mediated dUTP nick end label-positive nuclei in the denervated muscle of wild-type compared with caspase-3-knockout mice. Apoptotic signaling upstream of caspase-3 remained intact, with equivalent mitochondrial Bax translocation and cytochrome c release and caspase-9 activation in the denervated gastrocnemius muscle of wild-type and caspase-3-knockout mice. In contrast, diminished poly(ADP-ribose) polymerase cleavage in the denervated muscle of caspase-3-knockout compared with wild-type mice revealed that apoptotic signaling downstream of caspase-3 was impaired, suggesting that the absence of caspase-3 protects against denervation-induced muscle atrophy by suppressing apoptosis as opposed to ubiquitin-proteasome-mediated protein degradation.

Motor unit control and force fluctuation during fatigue

Contessa, P., Adam, A., De Luca, C. J. — 2009-06-30

During isometric contractions, the fluctuation of the force output of muscles increases as the muscle fatigues, and the contraction is sustained to exhaustion. We analyzed motor unit firing data from the vastus lateralis muscle to investigate which motor unit control parameters were associated with the increased force fluctuation. Subjects performed a sequence of isometric constant-force contractions sustained at 20% maximal force, each spaced by a 6-s rest period. The contractions were performed until the mean value of the force output could not be maintained at the desired level. Intramuscular EMG signals were detected with a quadrifilar fine-wire sensor. The EMG signals were decomposed to identify all of the firings of several motor units by using an artificial intelligence-based set of algorithms. We were able to follow the behavior of the same motor units as the endurance time progressed. The force output of the muscle was filtered to remove contributions from the tracking task. The coefficient of variation of the force was found to increase with endurance time (P < 0.001, R² = 0.51). We calculated the coefficient of variation of the firing rates, the synchronization of pairs of motor unit firings, the cross-correlation value of the firing rates of pairs of motor units, the cross-correlation of the firing rates of motor units and the force, and the number of motor units recruited during the contractions. Of these parameters, only the cross-correlation of the firing rates (P < 0.01, R² = 0.10) and the number of recruited motor units (P = 0.042, R² = 0.22) increased significantly with endurance time for grouped subjects. A significant increase (P < 0.001, R² = 0.16) in the cross-correlation of the firing rates and force was also observed. It is suggested that the increase in the cross-correlation of the firing rates is likely due to a decrease in the sensitivity of the proprioceptive feedback from the spindles.

Early brain swelling in acute hypoxia

Dubowitz, D. J., Dyer, E. A. W., Theilmann, R. J., Buxton, R. B., Hopkins, S. R. — 2009-06-30

Acute mountain sickness (AMS) and high-altitude cerebral edema share common clinical characteristics, suggesting cerebral swelling may be an important factor in the pathophysiology of AMS. Hypoxia and hypocapnia associated with high altitude are known to exert strong effects on the control of the cerebral circulation, yet how these effects interact during acute hypoxia, and whether AMS-susceptible subjects may have a unique response, is still unclear. To test if self-identified AMS-susceptible individuals show altered brain swelling in response to acute hypoxia, we used quantitative arterial spin-labeling and volumetric MRI to measure cerebral blood flow and cerebrospinal fluid (CSF) volume changes during 40 min of acute hypoxia. We estimated changes in cerebral blood volume (CBV) (from changes in cerebral blood flow) and brain parenchyma swelling (from changes in CBV and CSF). Subjects with extensive high-altitude experience in two groups participated: self-identified AMS-susceptible (n = 6), who invariably experienced AMS at altitude, and self-identified AMS-resistant (n = 6), who almost never experienced symptoms. During 40-min hypoxia, intracranial CSF volume decreased significantly [–10.5 ml (SD 6.9), P < 0.001]. There were significant increases in CBV [+2.3 ml (SD 2.5), P < 0.005] and brain parenchyma volume [+8.2 ml (SD 6.4), P < 0.001]. However, there was no significant difference between self-identified AMS-susceptible and AMS-resistant groups for these acute-phase changes. In acute hypoxia, brain swelling occurs earlier than previously described, with significant shifts in intracranial CSF occurring as early as 40 min after exposure. These acute-phase changes are present in all individuals, irrespective of susceptibility to AMS.

Application of muscle biopotential measurement for sustained, noninvasive blood glucose survey

Vol, A., Gribova, O., Berman, S., Siman-Tov, Y., Efrati, S. — 2009-06-30

Biopotential, the electric potential generated by living tissues, is affected by changes in extracellular electrolyte and glucose concentrations. We aimed to apply correlation between blood glucose concentrations (BGC) and biopotential of peripheral muscles for noninvasive blood glucose measurement. The study included 58 Wistar rats. In part of them, diabetes was induced by streptozotocin injection. Group 1, comprising 19 normal and 5 diabetic rats, received glucose-challenging protocol (intraperitoneal injection of 1 g/ml glucose). Group 2, 24 normal and 6 diabetic rats, received insulin-challenging protocol (three 30 IU insulin injections with 15-min intervals). Four control rats, group 3, were injected with 2-ml saline. BGC were measured by a standard ACCU-CHEK-Sensor Meter and compared with those estimated by biopotential sensor, further designated as GlucoSat, placed around proximal parts of the tails of the anaesthetized animals. GlucoSat results were calculated using the following biopotential equation: BGC(t) = k1 * F1(t) + k2 * F2(t) * k3 * F3(t) + k4, based on an experimental model involving estimation of pH, muscle metabolism, and tissue conductance, where t is time, k1–k4 are coefficients, and F1–F4 are functions. Mean biopotential system measured BGC was 181.7 ± 4.3 mg/dl, not differing statistically from 187.9 ± 4.3 mg/dl estimated by ACCU-CHEK. Pearson's correlation coefficient (r²) was 0.961 (P < 0.00001), indicating strong, direct correlation between the results. Within the nondiabetic group, r² was 0.944 (P < 0.00001), while, within the diabetic group, r² was 0.974 (P < 0.00001). No significant, adverse skin reactions were concomitantly observed in any experimental group. Biopotential measurements may be used for continuous, noninvasive estimation of changes in BGC. Further studies are needed to evaluate the applicability of this method to humans.

Swim training suppresses tumor growth in mice

2009-06-30

The present study was designed to determine the effects of physical training on the development of cancer induced by the injection of Ehrlich tumor cells in mice. Male Swiss mice were subjected to a swim training protocol (5 days/wk for 6 wk, 1 h at 50% of maximal capacity-trained groups) or remained sedentary in their cages (sedentary groups). The inoculation of Ehrlich tumor cells was performed at the end of the fourth week, and animals were killed after 6 wk of training. Heart and solid tumor weights were recorded, and tumor volumes were calculated. Portions of the tumors were used for the evaluation of macrophages and neutrophil accumulation or fixed in neutral 10% buffered formalin for histological analysis. The tumor volume and weight were, respectively, ~270% and 280% greater in sedentary mice than in trained mice. Macrophage infiltration in the tumor tissue was significantly lower in trained mice (0.65 ± 0.16 vs. 1.78 ± 0.43 macrophages x 10³ in the sedentary group). Moreover, neutrophil accumulation in tumors was slightly reduced after exercise training, and the amount of tumor cells was reduced in trained mice. Exercise capacity was substantially increased in trained mice, as determined by a 440% increase in the exercise time at 50% of maximal capacity. In summary, swim training retarded the development of Ehrlich tumors in mice, accompanied by a reduction in macrophage infiltration and neutrophil accumulation. These findings provide conceptual support for clinical observations that controlled physical activities may be a therapeutically important approach to preventing cancer progression and may improve the outcome of cancer treatment.

The prone position results in smaller ventilation defects during bronchoconstriction in asthma

2009-06-30

The effect of body posture on regional ventilation during bronchoconstriction is unknown. In five subjects with asthma, we measured spirometry, low-frequency (0.15-Hz) lung elastance, and resistance and regional ventilation by intravenous ¹³NN-saline positron emission tomography before and after nebulized methacholine. The subjects were imaged prone on 1 day and supine on another, but on both days the methacholine was delivered while prone. From the residual ¹³NN after washout, ventilation defective areas were defined, and their location, volume, ventilation, and fractional gas content relative to the rest of the lung were calculated. Independent of posture, all subjects developed ventilation defective areas. Although ventilation within these areas was similarly reduced in both postures, their volume was smaller in prone than supine (25 vs. 41%, P < 0.05). The geometric center of the ventilation defective areas was gravitationally dependent relative to that of the lung in both postures. Mean lung fractional gas content was greater in the prone position before methacholine and did not increase as much as in the supine position after methacholine. In the prone position at baseline, areas that became ventilation defects had lower gas content than the rest of the lung. In both positions at baseline, there was a gradient of gas content in the vertical direction. In asthma, the size and location of ventilation defects is affected by body position and likely affected by small differences in lung expansion during bronchoconstriction.

Hyperinflation-induced cardiorespiratory failure in rats

Simpson, J. A., Brunt, K. R., Collier, C. P., Iscoe, S. — 2009-06-30

We previously showed that severe inspiratory resistive loads cause acute (<1 h) cardiorespiratory failure characterized by arterial hypotension, multifocal myocardial infarcts, and diaphragmatic fatigue. The mechanisms responsible for cardiovascular failure are unknown, but one factor may be the increased ventricular afterload caused by the large negative intrathoracic pressures generated when breathing against an inspiratory load. Because expiratory threshold loads increase intrathoracic pressure and decrease left ventricular afterload, we hypothesized that anesthetized rats forced to breathe against such a load would experience only diaphragmatic failure. Loading approximately doubled end-expiratory lung volume, halved respiratory frequency, and caused arterial hypoxemia and hypercapnia, respiratory acidosis, and increased inspiratory drive. Although hyperinflation immediately reduced the diaphragm's mechanical advantage, fatigue did not occur until near load termination. Mean arterial pressure progressively fell, becoming significant (cardiovascular failure) midway through loading despite tachycardia. Loading was terminated (endurance 125 ± 43 min; range 82–206 min) when mean arterial pressure dropped below 50 mmHg. Blood samples taken immediately after load termination revealed hypoglycemia, hyperkalemia, and cardiac troponin T, the last indicating myocardial injury that was, according to histology, mainly in the right ventricle. This damage probably reflects a combination of decreased O₂ delivery (decreased venous return and arterial hypoxemia) and greater afterload due to hyperinflation-induced increase in pulmonary vascular resistance. Thus, in rats breathing at an increased end-expiratory lung volume, cardiorespiratory, not just respiratory, failure still occurred. Right heart injury and dysfunction may contribute to the increased morbidity and mortality associated with acute exacerbations of obstructive airway disease.

AMPK activation is fiber type specific in human skeletal muscle: effects of exercise and short-term exercise training

Lee-Young, R. S., Canny, B. J., Myers, D. E., McConell, G. K. — 2009-06-30

AMP-activated protein kinase (AMPK) has been extensively studied in whole muscle biopsy samples of humans, yet the fiber type-specific expression and/or activation of AMPK is unknown. We examined basal and exercise AMPK- Thr¹⁷² phosphorylation and AMPK subunit expression (₁, ₂, and ₃) in type I, IIa, and IIx fibers of human skeletal muscle before and after 10 days of exercise training. Before training basal AMPK phosphorylation was greatest in type IIa fibers (P < 0.05 vs. type I and IIx), while an acute bout of exercise increased AMPK phosphorylation in all fibers (P < 0.05), with the greatest increase occurring in type IIx fibers. Exercise training significantly increased basal AMPK phosphorylation in all fibers, and the exercise-induced increases were uniformly suppressed compared with pretraining exercise. Expression of AMPK-₁ and -₂ was similar between fibers and was not altered by exercise training. However, AMPK-₃ was differentially expressed in skeletal muscle fibers (type IIx > type IIa > type I), irrespective of training status. Thus skeletal muscle AMPK phosphorylation and AMPK expression are fiber type specific in humans in the basal state, as well as during exercise. Our findings reveal fiber type-specific differences that have been masked in previous studies examining mixed muscle samples.

Dynamic arterial blood gas analysis in conscious, unrestrained C57BL/6J mice during exposure to intermittent hypoxia

Lee, E. J., Woodske, M. E., Zou, B., O'Donnell, C. P. — 2009-06-30

Rodent models of chronic intermittent hypoxia (IH) are commonly used to investigate the pathophysiological sequelae that result from hypoxic exposure in patients experiencing obstructive sleep apnea (OSA). Despite the widespread use of IH models, little attention has been paid to carefully defining the degree of oxyhemoglobin desaturation that occurs during each hypoxic period. Therefore, we developed a rapid blood sampling technique to determine the arterial blood gas changes that occur in conscious unrestrained mice during a single IH event and hypothesized that the arterial Po₂ (Pa_O₂) at the nadir level of the inspired oxygen profile causes oxyhemoglobin saturation to fall to between 80% and 90%. Mice were exposed to 120–180 cycles of IH at a rate of 60 cycles/h, and arterial blood samples were withdrawn (<3 s) at baseline and at 10-s time intervals over the course of a single IH cycle. The IH regimen caused a decline in the fraction of inspired oxygen from room air levels to a transient nadir of 6.0 ± 0.2% over the 30-s hypoxic period. The Pa_O₂ and arterial oxyhemoglobin saturation reached a nadir of 47 ± 2 mmHg and 85 ± 2% at 30 s, respectively. Arterial Pco₂ decreased to a nadir of 26 ± 2 mmHg at 30 s, associated with a rise in arterial pH to 7.46 ± 0.2. We conclude that the magnitude of oxyhemoglobin desaturation that is induced in our murine model of IH is consistent with the degree of hypoxic stress that occurs in moderate to severe clinical OSA.

Exhaled nitric oxide estimation by a simple and efficient noninvasive technique and its utility as a marker of airway inflammation in mice

2009-06-30

Allergic airway inflammation (AI) is commonly associated with enhanced exhaled nitric oxide (ENO) in both humans and mice. Since mouse models are being used to understand various mechanisms of asthma, a noninvasive, simple, and reproducible method to determine ENO in mice is required for serial nonterminal assessment that can be used independent of environmental situations in which the ambient air contains substantial amounts of NO as a contaminant. The aim of this study was to noninvasively measure ENO in individual mice and to test its utility as a marker of AI in different models of allergic AI. We modified the existing ENO measuring methods by incorporating flushing and washout steps that allowed simple but reliable measurements under highly variable ambient NO conditions (1–100 ppb). This method was used to serially follow ENO in acute and chronic models of allergic AI in mice. ENO was reproducibly measured by this modified method and was positively correlated to AI in both acute and chronic models of asthma but was not independently related to airway remodeling. Resolution of AI and other related parameters in dexamethasone-treated mice resulted in reduction of ENO, further confirming this association. Restriction of allergen challenge to pulmonary but not nasal airways was associated with a smaller increase in ENO compared with allergen challenge to both. Hence, ENO can now be reliably measured in mice independent of ambient NO levels and is a valid biomarker for AI. However, nasal and pulmonary airways are likely to be independent sources of ENO, and any results must be interpreted as such.

Adaptation of the NDIR technology to 13CO2 breath tests under increased inspiratory O2 concentrations

2009-06-30

Nondispersive infrared spectroscopy (NDIR) allows the continuous analysis of respiratory gases. Due to its high selectivity, simple and robust setup, and small footprint, it is also used to support ¹³CO₂ breath tests to assess bacterial growth in the stomach, gut, or liver function. CO₂ NDIR signals, however, are biased by oxygen in the gas matrix. This complicates NDIR-based breath tests, if the inspired oxygen concentration has to be adjusted to the subject's requirements, or hyperoxia-induced effects were studied. To avoid the oxygen-induced bias, a "dilution" approach was developed: expired gas is mixed with N₂ to lower the oxygen content down to the usual range of 15–20%. Accuracy and precision were tested using synthetic gas mixtures with increasing ¹³CO₂-to-¹²CO₂ ratios (¹³CO₂/¹²CO₂), either based on synthetic air with ~20% volume O₂ or on pure O₂. For samples with ¹³C values smaller than 300 (or ¹³CO₂/¹²CO₂ smaller than 0.003), the dilution does not significantly increase the bias in the ¹³CO₂/¹²CO₂ determination, and the within-run imprecision is smaller than 1 ¹³C. The practical use of this approach was validated in a pig study using a sepsis model reflecting a clinical situation that requires an increased oxygen concentration for respiration. The N₂ dilution eliminated the high bias in NDIR measurement, thus allowing the determination of the impact of oxygenation on glucose oxidation in patients ventilated with increased oxygen.

Introducing the Highlighted Topic series

De Troyer, A., Tobin, M. J. — 2009-06-30

Pulmonary characteristics in COPD and mechanisms of increased work of breathing

Loring, S. H., Garcia-Jacques, M., Malhotra, A. — 2009-06-30

Mechanical characteristics and gas exchange inefficiencies of the lungs contribute to increased work of ventilation in chronic obstructive pulmonary disease (COPD) at rest and exercise, and the energy cost of ventilation is increased in COPD at any external work level. Assuming typical ventilatory variables and respiratory characteristics, we estimated the relative contributions of inspiratory and expiratory resistance, dynamic elastance, intrinsic positive end-expiratory pressure, and gas exchange inefficiency to the work of breathing, finding that the last of these is likely to be of major importance. Dynamic hyperinflation can be seen as both an impediment to inspiratory muscle function and an essential component of adaptation to severe obstruction. Extrinsic restriction, in which the chest wall fails to achieve and maintain abnormally high lung volumes in COPD, can limit ventilatory function and contribute to disability.

Effect of acute inflation on the mechanics of the inspiratory muscles

De Troyer, A., Wilson, T. A. — 2009-06-30

When the lung is inflated acutely, the capacity of the diaphragm to generate pressure, in particular pleural pressure (Ppl), is impaired because the muscle during contraction is shorter and generates less force. At very high lung volumes, the pressure-generating capacity of the diaphragm may be further reduced by an increase in the muscle radius of curvature. Lung inflation similarly impairs the pressure-generating capacity of the inspiratory intercostal muscles, both the parasternal intercostals and the external intercostals. In contrast to the diaphragm, however, this adverse effect is largely related to the orientation and motion of the ribs, rather than the ability of the muscles to generate force. During combined activation of the two sets of muscles, the change in Ppl is larger than during isolated diaphragm activation, and this added load on the diaphragm reduces the shortening of the muscle and increases muscle force. In addition, activation of the diaphragm suppresses the cranial displacement of the passive diaphragm that occurs during isolated intercostal contraction and increases the respiratory effect of the intercostals. As a result, the change in Ppl generated during combined diaphragm-intercostal activation is greater than the sum of the pressures generated during separate muscle activation. Although this synergistic interaction becomes particularly prominent at high lung volumes, lung inflation, either bilateral or unilateral, places a substantial stress on the inspiratory muscle pump.

Respiratory muscle fiber remodeling in chronic hyperinflation: dysfunction or adaptation?

Clanton, T. L., Levine, S. — 2009-06-30

The diaphragm and other respiratory muscles undergo extensive remodeling in both animal models of emphysema and in human chronic obstructive pulmonary disease, but the nature of the remodeling is different in many respects. One common feature is a shift toward improved endurance characteristics and increased oxidative capacity. Furthermore, both animals and humans respond to chronic hyperinflation by diaphragm shortening. Although in rodent models this clearly arises by deletion of sarcomeres in series, the mechanism has not been proven conclusively in human chronic obstructive pulmonary disease. Unique characteristics of the adaptation in human diaphragms include shifts to more predominant slow, type I fibers, expressing slower myosin heavy chain isoforms, and type I and type II fiber atrophy. Although some laboratories report reductions in specific force, this may be accounted for by decreases in myosin heavy chain content as the muscles become more oxidative and more efficient. More recent findings have reported reductions in Ca²⁺ sensitivity and reduced myofibrillar elastic recoil. In contrast, in rodent models of disease, there is no consistent evidence for loss of specific force, no consistent shift in fiber populations, and atrophy is predominantly seen only in fast, type IIX fibers. This review challenges the hypothesis that the adaptations in human diaphragm represent a form of dysfunction, secondary to systemic disease, and suggest that most findings can as well be attributed to adaptive processes of a complex muscle responding to unique alterations in its working environment.

Arterial and pulmonary arterial hemodynamics and oxygen delivery/extraction in normal humans exposed to hyperbaric air and oxygen

Weaver, L. K., Howe, S., Snow, G. L., Deru, K. — 2009-06-30

Divers and hyperbaric chamber attendants breathe hyperbaric air routinely. Hyperbaric oxygen (HBO₂) is used therapeutically frequently. Although much is understood about the hemodynamic physiology and gas exchange effects during hyperbaric air and HBO₂ exposure, arterial and pulmonary arterial (PA) catheter data, including blood gas values during hyperbaric air and HBO₂ exposure of normal humans, have not been reported. We exposed 10 healthy volunteers instrumented with arterial and PA catheters to air at 0.85, 3.0, 2.5, 2.0, 1.3 (decompression stop), 1.12 (decompression stop), and 0.85 atm abs (our altitude) and then at identical pressures breathing O₂ followed by atmospheric pressure air while we measured arterial and PA pressures (PAP), cardiac output (Q), and blood gas measurements from both arterial and PA catheters. Although hemodynamic changes occurred during exposure to both hyperbaric air and HBO₂, we observed a greater magnitude of change under HBO₂ conditions: heart rate changes ranged from –9 to –19% (air to O₂), respiratory rate from –12 to –17%, Q from –7 to –18%, PAP from –18 to –19%, pulmonary vascular resistance from –38 to –48%, and right-to-left shunt fraction from –87 to –107%. Mixed venous CO₂ fell 8% from baseline during HBO₂ despite mixed venous O₂ tensions of several hundred Torr. The stroke volume, O₂ delivery, and O₂ consumption did not change across exposures. The arterial and mixed venous partial pressures of O₂ and contents were elevated, as predicted. O₂ extraction increased 37% during HBO₂.

Decreased muscle ACE activity enhances functional response to endurance training in rats, without change in muscle oxidative capacity or contractile phenotype

2009-06-30

In the present study, we tested the hypothesis that chronic ANG I-converting enzyme (ACE) inhibition could improve the training-induced improvement in endurance exercise performance and that this could be related to enhanced skeletal muscle metabolic efficiency. Female Wistar rats were assigned to four groups comprising animals either maintained sedentary or endurance trained (Sed and Tr, respectively), and treated or not for 10 wk with an ACE inhibitor, perindopril (2 mg·kg^–1·day^–1) (Per and Ct, respectively) (n = 8 each). Trained rats underwent an 8-wk treadmill training protocol that consisted of 2 h/day running at 30 m/min on a 8% decline. Before the start of and 1 wk before the end of experimental conditioning, the running time to exhaustion of rats was measured on a treadmill. The training program led to an increase in endurance time, higher in Tr-Per than in Tr-Ct group (125% in Tr-Ct vs. 183% in Tr-Per groups, P < 0.05). Oxidative capacity, measured in saponin-permeabilized fibers of slow soleus and fast plantaris muscles, increased with training, but less in Tr-Per than in Tr-Ct rats. The training-induced increase in citrate synthase activity also was less in soleus from Tr-Per than Tr-Ct rats. The training-induced increase in the percentage of the type IIa isoform of myosin heavy chain (MHC) (45%, P < 0.05) and type IIx MHC (25%, P < 0.05) associated with decreased type IIb MHC (34%, P < 0.05) was minimized by perindopril administration. These findings demonstrate that the enhancement in physical performance observed in perindopril-treated animals cannot be explained by changes in mitochondrial respiration and/or MHC distribution within muscles involved in running exercise.

Point:Counterpoint: The interpolated twitch does/does not provide a valid measure of the voluntary activation of muscle

Taylor, J. L. — 2009-06-30

Counterpoint: The interpolated twitch does not provide a valid measure of the voluntary activation of muscle

de Haan, A., Gerrits, K. H. L., de Ruiter, C. J. — 2009-06-30

Rebuttal from Taylor

2009-06-30

Rebuttal from de Haan, Gerrits, and de Ruiter

2009-06-30

Comments on Point:Counterpoint: The interpolated twitch does/does not provide a valid measure of the voluntary activation of muscle

2009-06-30

Last Word on Point:Counterpoint: The interpolated twitch does/does not provide a valid measure of the voluntary activation of muscle

Taylor, J. L. — 2009-06-30

Last Word on Point:Counterpoint: The interpolated twitch does/does not provide a valid measure of the voluntary activation of muscle

de Haan, A., Gerrits, K. H. L., de Ruiter, C. J. — 2009-06-30

Steady-state venous return: residue in a recent model analysis of the notion that it is driven by elastic recoil of the venous system

Brengelmann, G. L. — 2009-06-30

Reply to Brengelmann

Magder, S. — 2009-06-30

Muscle oxygenation by near-infrared-based tissue oximeters

Quaresima, V., Ferrari, M. — 2009-06-30

Reply to Quaresima and Ferrari

2009-06-30