Journal of Applied Physiology recent issues

NO and CO have got to GO for enhanced chemoreceptor sympathoexcitation in heart failure

Mifflin, S. W. — 2008-07-18

Not so fast: intrinsic heart rate vs. {beta}-adrenergic responsiveness in the aging human heart

Joyner, M. J. — 2008-07-18

Dermatologic therapy with cardiotonic digitalis?

Naz, R. K. — 2008-07-18

High rates of muscle glycogen resynthesis after exhaustive exercise when carbohydrate is coingested with caffeine

2008-07-18

We determined the effect of coingestion of caffeine (Caff) with carbohydrate (CHO) on rates of muscle glycogen resynthesis during recovery from exhaustive exercise in seven trained subjects who completed two experimental trials in a randomized, double-blind crossover design. The evening before an experiment subjects performed intermittent exhaustive cycling and then consumed a low-CHO meal. The next morning subjects rode until volitional fatigue. On completion of this ride subjects consumed either CHO [4 g/kg body mass (BM)] or the same amount of CHO + Caff (8 mg/kg BM) during 4 h of passive recovery. Muscle biopsies and blood samples were taken at regular intervals throughout recovery. Muscle glycogen levels were similar at exhaustion [~75 mmol/kg dry wt (dw)] and increased by a similar amount (~80%) after 1 h of recovery (133 ± 37.8 vs. 149 ± 48 mmol/kg dw for CHO and Caff, respectively). After 4 h of recovery Caff resulted in higher glycogen accumulation (313 ± 69 vs. 234 ± 50 mmol/kg dw, P < 0.001). Accordingly, the overall rate of resynthesis for the 4-h recovery period was 66% higher in Caff compared with CHO (57.7 ± 18.5 vs. 38.0 ± 7.7 mmol·kg dw^–1·h^–1, P < 0.05). After 1 h of recovery plasma Caff levels had increased to 31 ± 11 µM (P < 0.001) and at the end of the recovery reached 77 ± 11 µM (P < 0.001) with Caff. Phosphorylation of CaMK^Thr286 was similar after exercise and after 1 h of recovery, but after 4 h CaMK^Thr286 phosphorylation was higher in Caff than CHO (P < 0.05). Phosphorylation of AMP-activated protein kinase (AMPK)^Thr172 and Akt^Ser473 was similar for both treatments at all time points. We provide the first evidence that in trained subjects coingestion of large amounts of Caff (8 mg/kg BM) with CHO has an additive effect on rates of postexercise muscle glycogen accumulation compared with consumption of CHO alone.

Downregulation of carbon monoxide as well as nitric oxide contributes to peripheral chemoreflex hypersensitivity in heart failure rabbits

Ding, Y., Li, Y.-L., Schultz, H. D. — 2008-07-18

Peripheral chemoreflex sensitivity is potentiated in clinical and experimental chronic heart failure (CHF). Downregulation of nitric oxide (NO) synthase (NOS) in the carotid body (CB) is involved in this effect. However, it remains poorly understood whether carbon monoxide (CO) also contributes to the altered peripheral chemoreflex sensitivity in CHF. This work highlights the effect of NO and CO on renal sympathetic nerve activity (RSNA) in response to graded hypoxia in conscious rabbits. Renal sympathetic nerve responses to graded hypoxia were enhanced in CHF rabbits compared with sham rabbits. The NO donor S-nitroso-N-acetylpenicillamine (SNAP, 1.2 µg·kg^–1·min^–1) and the CO-releasing molecule tricarbonyldichlororuthenium (II) dimer {[Ru(CO)₃Cl₂]₂, 3.0 µg·kg^–1·min^–1} each attenuated hypoxia-induced RSNA increases in CHF rabbits (P < 0.05), but the degree of attenuation of RSNA induced by SNAP or [Ru(CO)₃Cl₂]₂ was smaller than that induced by SNAP + [Ru(CO)₃Cl₂]₂. Conversely, treatment with the NOS inhibitor N-nitro-l-arginine (30 mg/kg) + the heme oxygenase (HO) inhibitor Cr (III) mesoporphyrin IX chloride (0.5 mg/kg) augmented the renal sympathetic nerve response to hypoxia in sham rabbits to a greater extent than treatment with either inhibitor alone and was without effect in CHF rabbits. In addition, using immunostaining and Western blot analyses, we found that expression of neuronal NOS, endothelial NOS, and HO-2 protein (expressed as the ratio of NOS or HO-2 expression to β-tubulin protein expression) was lower in CBs from CHF (0.19 ± 0.04, 0.17 ± 0.06, and 0.15 ± 0.02, respectively) than sham (0.63 ± 0.04, 0.56 ± 0.06, and 0.27 ± 0.03, respectively) rabbits (P < 0.05). These results suggest that a deficiency of NO and CO in the CBs augments peripheral chemoreflex sensitivity to hypoxia in CHF.

Decreased maximal heart rate with aging is related to reduced {beta}-adrenergic responsiveness but is largely explained by a reduction in intrinsic heart rate

Christou, D. D., Seals, D. R. — 2008-07-18

A decrease in maximal exercise heart rate (HR_max) is a key contributor to reductions in aerobic exercise capacity with aging. However, the mechanisms involved are incompletely understood. We sought to gain insight into the respective roles of intrinsic heart rate (HR_int) and chronotropic β-adrenergic responsiveness in the reductions in HR_max with aging in healthy adults. HR_max (Balke treadmill protocol to exhaustion), HR_int (HR during acute ganglionic blockade with intravenous trimethaphan), and chronotropic β-adrenergic responsiveness (increase in HR with incremental intravenous infusion of isoproterenol during ganglionic blockade) were determined in 15 older (65 ± 5 yr) and 15 young (25 ± 4 yr) healthy men. In the older men, HR_max was lower (162 ± 9 vs. 191 ± 11 beats/min, P < 0.0001) and was associated with a lower HR_int (58 ± 7 vs. 83 ± 9 beats/min, P < 0.0001) and chronotropic β-adrenergic responsiveness (0.094 ± 0.036 vs. 0.154 ± 0.045 HR/[isoproterenol]: P < 0.0001). Both HR_int (r = 0.87, P < 0.0001) and chronotropic β-adrenergic responsiveness (r = 0.61, P < 0.0001) were positively related to HR_max. Accounting for the effects of HR_int and chronotropic β-adrenergic responsiveness reduced the age-related difference in HR_max by 83%, rendering it statistically nonsignificant (P = 0.2). Maximal oxygen consumption was lower in the older men (34.9 ± 8.1 vs. 48.6 ± 6.7 ml·kg^–1·min^–1, P < 0.0001) and was positively related to HR_max (r = 0.62, P < 0.0001), HR_int (r = 0.51, P = 0.002), and chronotropic β-adrenergic responsiveness (r = 0.47, P = 0.005). Our findings indicate that, together, reductions in HR_int and chronotropic responsiveness to β-adrenergic stimulation largely explain decreases in HR_max with aging, with the reduction in HR_int playing by far the greatest role.

Effects of cardiotonic steroids on dermal collagen synthesis and wound healing

2008-07-18

We previously reported that cardiotonic steroids stimulate collagen synthesis by cardiac fibroblasts in a process that involves signaling through the Na-K-ATPase pathway (Elkareh et al. Hypertension 49: 215–224, 2007). In this study, we examined the effect of cardiotonic steroids on dermal fibroblasts collagen synthesis and on wound healing. Increased collagen expression by human dermal fibroblasts was noted in response to the cardiotonic steroid marinobufagenin in a dose- and time-dependent fashion. An eightfold increase in collagen synthesis was noted when cells were exposed to 10 nM marinobufagenin for 24 h (P < 0.01). Similar increases in proline incorporation were seen following treatment with digoxin, ouabain, and marinobufagenin (10 nM x 24 h, all results P < 0.01 vs. control). The coadministration of the Src inhibitor PP2 or N-acetylcysteine completely prevented collagen stimulation by marinobufagenin. Next, we examined the effect of digoxin, ouabain, and marinobufagenin on the rate of wound closure in an in vitro model where human dermal fibroblasts cultures were wounded with a pipette tip and monitored by digital microscopy. Finally, we administered digoxin in an in vivo wound healing model. Olive oil was chosen as the digoxin carrier because of a favorable partition coefficient observed for labeled digoxin with saline. This application significantly accelerated in vivo wound healing in rats wounded with an 8-mm biopsy cut. Increased collagen accumulation was noted 9 days after wounding (both P < 0.01). The data suggest that cardiotonic steroids induce increases in collagen synthesis by dermal fibroblasts, as could potentially be exploited to accelerate wound healing.

The effectiveness of hand cooling at reducing exercise-induced hyperthermia and improving distance-race performance in wheelchair and able-bodied athletes

Goosey-Tolfrey, V., Swainson, M., Boyd, C., Atkinson, G., Tolfrey, K. — 2008-07-18

The purpose of this study was to examine the effectiveness of reducing core temperature in postexercise hyperthermic subjects and to assess if hand cooling (HC) improves subsequent timed distance performance. Following a detailed measurement check on the use of insulated auditory canal temperature (T_ac), eight wheelchair (WA) athletes and seven male able-bodied (AB) athletes performed two testing sessions, comprising a 60-min exercise protocol and 10-min recovery period, followed by a performance trial (1 km and 3 km for WA and AB, respectively) at 30.8°C (SD 0.2) and 60.6% (SD 0.2) relative humidity. In a counterbalanced order, HC and a no-cooling condition was administered during the 10-min recovery period before the performance trial. Nonsignificant condition x time interactions for both WA (F_15,75 = 1.5, P = 0.14) and AB (F_15,90 = 1.2, P = 0.32) confirmed that the exercise-induced changes () in T_ac were similar before each intervention. However, the exercise-induced increase was evidently greater in AB compared with WA (2.0 vs. 1.3°C change, respectively). HC produced T_ac of –0.4°C (SD 0.4) and –1.2°C (SD 0.2) in comparison (WA and AB, respectively), and simple-effects analyses suggested that the reductions in T_ac were noteworthy after 4 min of HC. HC had an impact on improving AB performances by –4.0 s (SD 11.5) (P < 0.05) and WA by –20.5 s (SD 24.2) (P > 0.05). In conclusion, extraction of heat through the hands was effective in lowering T_ac in both groups and improving 3-km performance in the AB athletes and trends toward positive gains for the 1-km performance times of the WA group.

Effect of sleep stage on breathing in children with central hypoventilation

2008-07-18

The early literature suggests that hypoventilation in infants with congenital central hypoventilation syndrome (CHS) is less severe during rapid eye movement (REM) than during non-REM (NREM) sleep. However, this supposition has not been rigorously tested, and subjects older than infancy have not been studied. Given the differences in anatomy, physiology, and REM sleep distribution between infants and older children, and the reduced number of limb movements during REM sleep, we hypothesized that older subjects with CHS would have more severe hypoventilation during REM than NREM sleep. Nine subjects with CHS, aged (mean ± SD) 13 ± 7 yr, were studied. Spontaneous ventilation was evaluated by briefly disconnecting the ventilator under controlled circumstances. Arousal was common, occurring in 46% of REM vs. 38% of NREM trials [not significant (NS)]. Central apnea occurred during 31% of REM and 54% of NREM trials (NS). Although minute ventilation declined precipitously during both REM and NREM trials, hypoventilation was less severe during REM (drop in minute ventilation of 65 ± 23%) than NREM (drop of 87 ± 16%, P = 0.036). Despite large changes in gas exchange during trials, there was no significant change in heart rate during either REM or NREM sleep. We conclude that older patients with CHS frequently have arousal and central apnea, in addition to hypoventilation, when breathing spontaneously during sleep. The hypoventilation in CHS is more severe during NREM than REM sleep. We speculate that this may be due to increased excitatory inputs to the respiratory system during REM sleep.

Effect of an inhaled glucocorticoid on endothelial function in healthy smokers

2008-07-18

Cigarette smoking is associated with attenuated endothelium-dependent vasodilation (endothelial dysfunction) in the systemic circulation, including the airway circulation. We wished to determine whether an inhaled corticosteroid could restore endothelial function in the airway of lung-healthy current smokers, ex-smokers, and nonsmokers. We measured baseline airway blood flow (Qaw) and Qaw reactivity to inhaled albuterol as an index of endothelium-dependent vasodilation and to sublingual nitroglycerin as an index of endothelium-independent vasodilation in lung-healthy current smokers, ex-smokers, and nonsmokers. Current smokers were then treated with inhaled fluticasone for 3 wk, and all measurements were repeated after fluticasone treatment and after a subsequent 3-wk fluticasone washout period. Baseline mean Qaw and endothelium-independent Qaw reactivity were similar in the three groups. Mean endothelium-dependent Qaw reactivity was 49.5% in nonsmokers, 42.7% in ex-smokers, and 10.8% in current smokers (P < 0.05 vs. nonsmokers). In current smokers, mean baseline Qaw was unchanged after fluticasone treatment, but endothelium-dependent Qaw reactivity significantly increased to 34.9%. Qaw reactivity was again blunted after fluticasone washout. Endothelial dysfunction, as assessed by vascular reactivity, can be corrected with an inhaled corticosteroid in the airway of lung-healthy current smokers. This proof of concept can serve as the basis for future clinical investigations on the effect of glucocorticoids on endothelial function in smokers.

Effects of dietary protein content on IGF-I, testosterone, and body composition during 8 days of severe energy deficit and arduous physical activity

2008-07-18

Energy restriction coupled with high energy expenditure from arduous work is associated with an altered insulin-like growth factor-I (IGF-I) system and androgens that are coincident with losses of fat-free mass. The aim of this study was to determine the effects of two levels of dietary protein content and its effects on IGF-I, androgens, and losses of fat-free mass accompanying energy deficit. We hypothesized that higher dietary protein content would attenuate the decline of anabolic hormones and, thus, prevent losses of fat-free mass. Thirty-four men [24 (SD 0.3) yr, 180.1 (SD 1.1) cm, and 83.0 (SD 1.4) kg] participated in an 8-day military exercise characterized by high energy expenditure (16.5 MJ/day), low energy intake (6.5 MJ/day), and sleep deprivation (4 h/24 h) and were randomly divided into two dietary groups: 0.9 and 0.5 g/kg dietary protein intake. IGF-I system analytes, androgens, and body composition were assessed before and on days 4 and 8 of the intervention. Total, free, and nonternary IGF-I and testosterone declined 50%, 64%, 55%, and 45%, respectively, with similar reductions in both groups. There was, however, a diet x time interaction on day 8 for total IGF-I and sex hormone-binding globulin. Decreases in body mass (3.2 kg), fat-free mass (1.2 kg), fat mass (2.0 kg), and percent body fat (1.5%) were similar in both groups (P = 0.01). Dietary protein content of 0.5 and 0.9 g/kg minimally attenuated the decline of IGF-I, the androgenic system, and fat-free mass during 8 days of negative energy balance associated with high energy expenditure and low energy intake.

Greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans

Sauder, C. L., Leonard, T. O., Ray, C. A. — 2008-07-18

Otolith organs have been shown to activate the sympathetic nervous system in the prone position by head-down rotation (HDR) in humans. To date, otolithic stimulation by HDR has not been comprehensively studied in the upright posture. The purpose of the present study was to determine whether otolithic stimulation increases muscle sympathetic nerve activity (MSNA) in the upright posture. It was hypothesized that stimulation of the otolith organs would increase MSNA in the upright posture, despite increased baseline sympathetic activation due to unloading of the baroreceptors. MSNA, arterial blood pressure, heart rate, and degree of head rotation were measured during HDR in 18 volunteers (23 ± 1 yr) in different postures. Study 1 (n = 11) examined HDR in the prone and sitting positions and study 2 (n = 7) examined HDR in the prone and 60° head-up tilt positions. Baseline MSNA was 8 ± 4, 15 ± 4, and 33 ± 2 bursts/min for prone, sitting, and head-up tilt, respectively. HDR significantly increased MSNA in the prone (4 ± 1 and 105 ± 37% for burst frequency and total activity, respectively), sitting (5 ± 1 and 43 ± 12%), and head-up tilt (7 ± 1 and 110 ± 41%; P < 0.05). Sensitivity of the vestibulosympathetic reflex (%MSNA/HDR; degree of head rotation) was significantly greater in the sitting and head-up tilt than prone position (prone = 74 ± 22; sitting = 109 ± 30; head-up tilt = 276 ± 103; P < 0.05). These data indicate that stimulation of the otolith organs can mediate increases in MSNA in the upright posture and suggest a greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans.

Contralateral muscle activity and fatigue in the human first dorsal interosseous muscle

Post, M., Bayrak, S., Kernell, D., Zijdewind, I. — 2008-07-18

During effortful unilateral contractions, muscle activation is not limited to the target muscles but activity is also observed in contralateral muscles. The amount of this associated activity is depressed in a fatigued muscle, even after correction for fatigue-related changes in maximal force. In the present experiments, we aimed to compare fatigue-related changes in associated activity vs. parameters that are used as markers for changes in central nervous system (CNS) excitability. Subjects performed brief maximal voluntary contractions (MVCs) with the index finger in abduction direction before and after fatiguing protocols. We followed changes in MVCs, associated activity, motor-evoked potentials (MEP; transcranial magnetic stimulation), maximal compound muscle potentials (M waves), and superimposed twitches (double pulse) for 20 min after the fatiguing protocols. During the fatiguing protocols, associated activity increased in contralateral muscles, whereas afterwards the associated force was reduced in the fatigued muscle. This force reduction was significantly larger than the decline in MVC. However, associated activity (force and electromyography) remained depressed for only 5–10 min, whereas the MVCs stayed depressed for over 20 min. These decreases were accompanied by a reduction in MEP, MVC electromyography activity, and voluntary activation in the fatigued muscle. According to these latter markers, the decrease in CNS motor excitability lasted much longer than the depression in associated activity. Differential effects of fatigue on (associated) submaximal vs. maximal contractions might contribute to these differences in postfatigue behavior. However, we cannot exclude differences in processes that are specific to either voluntary or to associated contractions.

Focal CO2 dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO2 dialysis in the retrotrapezoid nucleus

Dias, M. B., Li, A., Nattie, E. — 2008-07-18

Simultaneous inhibition of the retrotrapezoid nucleus (RTN) and raphe obscurus (ROb) decreased the systemic CO₂ response by 51%, an effect greater than inhibition of RTN (–24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigated this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO₂ in two probes located in or adjacent to the RTN and ROb in conscious adult male rats. Ventilation was measured in a whole body plethysmograph at 30°C. There were four groups (n = 5): 1) probes correctly placed in both RTN and ROb (RTN-ROb); 2) one probe correctly placed in RTN and one incorrectly placed in areas adjacent to ROb (RTN-peri-ROb); 3) one probe correctly placed in ROb and one probe incorrectly placed in areas adjacent to RTN (peri-RTN-ROb); and 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased ventilation (Ve) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased Ve significantly by up to 15% compared with baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased Ve by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.

Quantitative analysis of serum sodium concentration after prolonged running in the heat

Baker, L. B., Lang, J. A., Kenney, W. L. — 2008-07-18

This study compared measured serum [Na⁺] (S_[Na+]; brackets denote concentration) with that predicted by the Nguyen-Kurtz equation after manipulating ingested [Na⁺] and changes in body mass (BM) during prolonged running in the heat. Athletes (4 men, 4 women; 22–36 yr) ran for 2 h, followed by a run to exhaustion and 1-h recovery. During exercise and recovery, subjects drank a 6% carbohydrate solution without Na⁺ (Na⁺0), 6% carbohydrate solution with 18 mmol/l Na⁺ (Na⁺18), or 6% carbohydrate solution with 30 mmol/l Na⁺ (Na⁺30) to maintain BM (0%BM), increase BM by 2%, or decrease BM by 2% or 4% in 12 separate trials. Net fluid, Na⁺, and K⁺ balance were measured to calculate the Nguyen-Kurtz predicted S_[Na+] for each trial. For all beverages, predicted and measured S_[Na+] were not significantly different during the 0%, –2%, and –4%BM trials (–0.2 ± 0.2 mmol/l) but were significantly different during the +2%BM trials (–2.6 ± 0.5 mmol/l). Overall, Na⁺ consumption attenuated the decline in S_[Na+] (–2.0 ± 0.5, –0.9 ± 0.5, –0.5 ± 0.5 mmol/l from pre- to postexperiment of the 0%BM trials for Na⁺30, Na⁺18, and Na⁺0, respectively) but the differences among beverages were not statistically significant. Beverage [Na⁺] did not affect performance; however, time to exhaustion was significantly shorter during the –4% (8 ± 3 min) and –2% (14 ± 3 min) vs. 0% (22 ± 5 min) and +2% (26 ± 6 min) BM trials. In conclusion, when athletes maintain or lose BM, changes in S_[Na+] can be accurately predicted by changes in the mass balance of fluid, Na⁺, and K⁺ during prolonged running in the heat.

Upper airway pressure-flow relationships and pharyngeal constrictor EMG activity during prolonged expiration in awake goats

O'Halloran, K. D., Bisgard, G. E. — 2008-07-18

We undertook the present investigation to establish whether narrowing/closure of the upper airway occurs during spontaneous and provoked respiratory rhythm disturbances and whether pharyngeal constrictor muscle recruitment occurs coincident with upper airway occlusion during prolonged expiratory periods. Upper airway pressure-flow relationships and middle pharyngeal constrictor (mPC) EMG activities were recorded in 11 adult female goats during spontaneous and provoked prolongations in expiratory time (Te). A total of 213 spontaneous prolongations of expiration were recorded. Additionally, 169 prolonged expiratory events preceded by an augmented breath were included in the analyses. In separate trials on different days, Te was prolonged by systemic administration of dopamine, by raising the inspired fraction of O₂ from 0.10 to 1.00 during poikilocapnic conditions or by systemic administration of clonidine. Continuous tonic activation of the mPC EMG was observed during each prolonged Te period regardless of the duration or initiating cause. However, significant increases in subglottic tracheal pressure, with expiratory airflow braking indicative of upper airway narrowing or closure, was only observed during spontaneous events without a preceding augmented breath and during clonidine-induced events. Tonic mPC activation proved an unreliable indicator of airway occlusion. Furthermore, mPC muscle activation alone is not sufficient to induce pharyngeal occlusion during prolonged expiration. Our data suggest that airway closure is not a common occurrence during provoked respiratory disturbances in awake goats. We propose that airway closure, when present during prolonged Te, is more likely dependent on activation of laryngeal adductor muscles with glottic braking independent of pharyngeal narrowing.

Fractal scaling properties of heart rate dynamics following resistance exercise training

2008-07-18

With aging and disease, there is a breakdown of the natural fractal-like organization of heart rate (HR). Fractal-like correlation properties of HR can be assessed with detrended fluctuation analysis (DFA). A short-time scaling exponent (_s) value of 1 is associated with healthy HR dynamics, whereas values that deviate away from 1, in either direction, indicate fractal collapse. The purpose of this study was to examine the effect of resistance exercise training (RT) on fractal correlation properties of HR dynamics. Resting ECG was collected at baseline, following a 4-wk time control period and 6 wk of RT (3 days per wk) in 34 men (23 ± 1 years of age). Fractal properties of HR were assessed with DFA. There was no change in _s following either the time control period or RT (1.01 ± 0.06 to 0.98 ± 0.06 to 0.93 ± 0.04, P > 0.05). Given the potential bidirectional nature of fractal collapse, subjects were retrospectively separated into two groups (higher _s and lower _s) on the basis of the initial _s by using cluster analysis. An interaction was detected for _s following RT (P < 0.05). There was no change in _s in either group following the time control, but _s increased following RT in the lower _s group (n = 18; 0.73 ± 0.04 to 0.69 ± 0.04 to 0.88 ± 0.04) and _s decreased following RT in the higher _s group (n = 16; 1.20 ± 0.04 to 1.24 ± 0.04 to 0.98 ± 0.04). In conclusion, RT improves fractal properties of HR dynamics.

Effect of prior chronic contractile activity on mitochondrial function and apoptotic protein expression in denervated muscle

O'Leary, M. F. N., Hood, D. A. — 2008-07-18

Skeletal muscle is highly adaptable in response to increases and decreases in contractile activity. The purpose of this study was to determine whether the preconditioning of skeletal muscle has a protective effect against subsequent denervation-induced apoptotic protein expression. To investigate this, we chronically stimulated the tibialis anterior and extensor digitorum longus muscles for 7 days (10 Hz, 3 h/day) before 7 days of denervation. Denervation reduced total cytochrome-c oxidase activity by 39%, which was likely a consequence of a decrease in subsarcolemmal (SS) mitochondria. This decrease in the SS subfraction was prevented by prior chronic stimulation and, as a result, maintained total mitochondrial content at control levels. The expression of Bax was elevated 2.2-fold by denervation, and prior chronic stimulation did not attenuate this increase. This produced a increase in the Bax-to-Bcl-2 ratio, indicating greater muscle apoptotic susceptibility. Denervation also decreased state 3 respiration in SS and intermyofibrillar mitochondria and elevated state 4 reactive oxygen species production within both mitochondrial subfractions. These changes were not prevented by prior chronic stimulation. Furthermore, the antioxidant protein MnSOD was also reduced by denervation, whereas Beclin-1 was markedly elevated. This suggests that autophagic cell death could also play a significant part in denervation-induced muscle atrophy. Thus, despite prior chronic stimulation, denervation increases the apoptotic susceptibility of skeletal muscle by altering the Bax-to-Bcl-2 ratio, by increasing reactive oxygen species production, and by reducing the expression of MnSOD. Whether a more extensive stimulation paradigm would be more effective in attenuating apoptosis before muscle disuse remains to be determined.

Endogenous and exogenous female sex hormones and renal electrolyte handling: effects of an acute sodium load on plasma volume at rest

Sims, S. T., Rehrer, N. J., Bell, M. L., Cotter, J. D. — 2008-07-18

This study was conducted to investigate effects of an acute sodium load on resting plasma volume (PV) and renal mechanisms across the menstrual cycle of endurance-trained women with natural (NAT) or oral contraceptive pill (OCP) controlled cycles. Twelve women were assigned to one of two groups, according to their usage status: 1) OCP [n = 6, 29 yr (SD 6), 59.4 kg (SD 3.2)], or 2) NAT [n = 6, 24 yr (SD 5), 61.3 kg (SD 3.6)]. The sodium load was administered as a concentrated sodium chloride/citrate beverage (164 mmol Na⁺/l, 253 mosmol/kgH₂O, 10 ml/kg body mass) during the last high-hormone week of the OCP cycle (OCP_high) or late luteal phase of the NAT cycle (NAT_high) and during the low-hormone sugar pill week of OCP (OCP_low) or early follicular phase of the NAT cycle (NAT_low). The beverage (~628 ml) was ingested in seven portions across 60 min. Over the next 4 h, PV expanded more in the low-hormone phase for both groups (time-averaged change): OCP_low 6.1% (SD 1.1) and NAT_low 5.4% (SD 1.2) vs. OCP_high 3.9% (SD 0.9) and NAT_high 3.5% (SD 0.8) (P = 0.02). The arginine vasopressin increased less in the low-hormone phase [1.63 (SD 0.2) and 1.30 pg/ml (SD 0.2) vs. 1.82 (SD 0.3) and 1.57 pg/ml (SD 0.5), P = 0.0001], as did plasma aldosterone concentration (~64% lower, P = 0.0001). Thus PV increased more and renal hormone sensitivity was decreased in the low-hormone menstrual phase following sodium/fluid ingestion, irrespective of OCP usage.

Sensitizing effects of chronic exposure and acute inhalation of ovalbumin aerosol on pulmonary C fibers in rats

Zhang, G., Lin, R.-L., Wiggers, M. E., Lee, L.-Y. — 2008-07-18

The effect of ovalbumin (Ova) sensitization on pulmonary C-fiber sensitivity was investigated. Brown-Norway rats were sensitized by intraperitoneal injection of Ova followed by aerosolized Ova three times per week for 3 wk. Control rats received the vehicle. At the end of the third week, single-unit fiber activities (FA) of pulmonary C fibers were recorded in anesthetized, artificially ventilated rats. Our results showed the following: 1) Ova sensitization induced airway inflammation (infiltration of eosinophils and neutrophils) and airway hyperresponsiveness in rats; 2) baseline FA in sensitized rats was significantly higher than that in control ones; 3) similarly, the pulmonary C-fiber response to right atrial injection of capsaicin was markedly higher in sensitized rats, which were significantly amplified after the acute Ova inhalation challenge; and 4) similar patterns, but smaller magnitudes of the differences in C-fiber responses to adenosine and lung inflation, were also found between sensitized and control rats. In conclusion, Ova sensitization elevated the baseline FA and excitability of pulmonary C fibers, and the hypersensitivity was further potentiated after the acute Ova inhalation challenge in sensitized rats. Chronic allergic inflammatory reactions in the airway probably contributed to the sensitizing effect on these lung afferents.

Central nervous adaptations following 1 wk of wrist and hand immobilization

Lundbye-Jensen, J., Nielsen, J. B. — 2008-07-18

Plastic neural changes have been documented in relation to different types of physical activity, but little is known about central nervous system plasticity accompanying reduced physical activity and immobilization. In the present study we investigated whether plastic neural changes occur in relation to 1 wk of immobilization of the nondominant wrist and hand and a corresponding period of recovery in 10 able-bodied volunteers. After immobilization, maximal voluntary contraction torque decreased and the variability of submaximal static contractions increased significantly without evidence of changes in muscle contractile properties. Hoffmann (H)-reflex amplitudes and the ratios of H-slope to M-slope increased significantly in flexor carpi radialis and abductor pollicis brevis at rest and during contraction without changes in corticospinal excitability, estimated from motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation. Corticomuscular coherence measures were derived from EEG and EMG obtained during static contractions. After immobilization, corticomuscular coherence in the 15- to 35-Hz range associated with maximum negative cumulant values at lags corresponding to MEP latencies decreased. One week after cast removal, all measurements returned to preimmobilization levels. The increased H-reflex amplitudes without changes in MEPs may suggest that presynaptic inhibition or postactivation depression of Ia afferents is reduced following immobilization. Reduced corticomuscular coherence may be caused by changes in afferent input at spinal and cortical levels or by changes in the descending drive from motor cortex. Further studies are needed to elucidate the mechanisms underlying the observed increased spinal excitability and reduced coupling between motor cortex and spinal motoneuronal activity following immobilization.

Influence of hypercapnic vasodilation on cerebrovascular autoregulation and pial arteriolar bed resistance in piglets

Narayanan, N., Leffler, C. W., Daley, M. L. — 2008-07-18

Changes in both pial arteriolar resistance (PAR) and simulated arterial-arteriolar bed resistance (SimR) of a physiologically based biomechanical model of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure and intracranial pressure, are used to test the hypothesis that hypercapnia disrupts autoregulatory reactivity. To evaluate pressure reactivity, vasopressin-induced acute hypertension was administered to normocapnic and hypercapnic (N = 12) piglets equipped with closed cranial windows. Pial arteriolar diameters were used to compute arteriolar resistance. Percent change of PAR (%PAR) and percent change of SimR (%SimR) in response to vasopressin-induced acute hypertension were computed and compared. Hypercapnia decreased cerebrovascular resistance. Indicative of active autoregulatory reactivity, vasopressin-induced hypertensive challenge resulted in an increase of both %PAR and %SimR for all normocapnic piglets. The hypercapnic piglets formed two statistically distinct populations. One-half of the hypercapnic piglets demonstrated a measured decrease of both %PAR and %SimR to pressure challenge, indicative of being pressure passive, whereas the other one-half demonstrated an increase in these percentages, indicative of active autoregulation. No other differences in measured variables were detectable between regulating and pressure-passive piglets. Changes in resistance calculated from using the model mirrored those calculated from arteriolar diameter measurements. In conclusion, vasodilation induced by hypercapnia has the potential to disrupt autoregulatory reactivity. Our physiologically based biomechanical model of cerebrovascular pressure transmission accurately estimates the changes in arteriolar resistance during conditions of active and passive cerebrovascular reactivity.

Influence of phosphagen concentration on phosphocreatine breakdown kinetics. Data from human gastrocnemius muscle

Francescato, M. P., Cettolo, V., di Prampero, P. E. — 2008-07-18

At the onset of a square-wave exercise of moderate intensity, in the absence of any detectable lactate production, the hydrolysis of phosphocreatine (PCr) fills the gap between energy requirement and energy yield by oxidative pathways, thus representing a readily available source of energy for the muscle. We verified experimentally the relationships between high-energy phosphates and/or their changes and the time constant of PCr concentration ([PCr]) kinetics in humans (_PCr). High-energy phosphate concentration (by ³¹P-NMR spectroscopy) in the calf muscles were measured during three repetitions of the rest-to-work transition of moderate aerobic square-wave exercise on nine healthy volunteers, while resting [PCr] was estimated from the appropriate spectroscopy data. PCr concentration decreased significantly (22 ± 6%) from rest to steady-state exercise, without differences among the three repetitions. Absolute resting [PCr] and _PCr were consistent with literature values, amounting to 27.5 ± 2.2 mM and 23.9 ± 2.9 s, respectively. No significant relationships were detected between individual _PCr and mechanical power, fraction or absolute amount of PCr hydrolyzed, or change in ADP concentration. On the contrary, individual _PCr (s) was linearly related to absolute resting [PCr] (mM), the relationship being described by: _PCr = 0.656 + 0.841·[PCr] (n = 9, R = 0.708, P < 0.05). These data support the view that in humans PCr concentration sets the time course of the oxidative metabolism in skeletal muscle at the start of exercise, being one of the main controllers of oxidative phosphorylation.

Intramuscular {beta}2-agonist administration enhances early regeneration and functional repair in rat skeletal muscle after myotoxic injury

2008-07-18

Systemic administration of β₂-adrenoceptor agonists (β₂-agonists) can improve skeletal muscle regeneration after injury. However, therapeutic application of β₂-agonists for muscle injury has been limited by detrimental cardiovascular side effects. Intramuscular administration may obviate some of these side effects. To test this hypothesis, the right extensor digitorum longus (EDL) muscle from rats was injected with bupivacaine hydrochloride to cause complete muscle fiber degeneration. Five days after injury, half of the injured muscles received an intramuscular injection of formoterol (100 µg). Muscle function was assessed at 7, 10, and 14 days after injury. A single intramuscular injection of formoterol increased muscle mass and force-producing capacity at day 7 by 17 and 91%, respectively, but this effect was transient because these values were not different from control levels at day 10. A second intramuscular injection of formoterol at day 7 prolonged the increase in muscle mass and force-producing capacity. Importantly, single or multiple intramuscular injections of formoterol did not elicit cardiac hypertrophy. To characterize any potential cardiovascular effects of intramuscular formoterol administration, we instrumented a separate group of rats with indwelling radio telemeters. Following an intramuscular injection of formoterol, heart rate increased by 18%, whereas systolic and diastolic blood pressure decreased by 31 and 44%, respectively. These results indicate that intramuscular injection can enhance functional muscle recovery after injury without causing cardiac hypertrophy. Therefore, if the transient cardiovascular effects associated with intramuscular formoterol administration can be minimized, this form of treatment may have significant therapeutic potential for muscle-wasting conditions.

Type 5 adenylyl cyclase plays a major role in stabilizing heart rate in response to microgravity induced by parabolic flight

2008-07-18

It is well known that autonomic nervous activity is altered under microgravity, leading to disturbed regulation of cardiac function, such as heart rate. Autonomic regulation of the heart is mostly determined by β-adrenergic receptors/cAMP signal, which is produced by adenylyl cyclase, in cardiac myocytes. To examine a hypothesis that a major cardiac isoform, type 5 adenylyl cyclase (AC5), plays an important role in regulating heart rate during parabolic flights, we used transgenic mouse models with either disrupted (AC5KO) or overexpressed AC5 in the heart (AC5TG) and analyzed heart rate variability. Heart rate had a tendency to decrease gradually in later phases within one parabola in each genotype group, but the magnitude of decrease was smaller in AC5KO than that in the other groups. The inverse of heart rate, i.e., the R-R interval, was much more variable in AC5KO and less variable in AC5TG than that in wild-type controls. The standard deviation of normal R-R intervals, a marker of total autonomic variability, was significantly greater in microgravity phase in each genotype group, but the magnitude of increase was much greater in AC5KO than that in the other groups, suggesting that heart rate regulation became unstable in the absence of AC5. In all, AC5 plays a major role in stabilizing heat rate under microgravity.

Resistance training induces qualitative changes in muscle morphology, muscle architecture, and muscle function in elderly postoperative patients

2008-07-18

Although the negative effects of bed rest on muscle strength and muscle mass are well established, it still remains a challenge to identify effective methods to restore physical capacity of elderly patients recovering from hospitalization. The present study compared different training regimes with respect to muscle strength, muscle fiber size, muscle architecture, and stair walking power in elderly postoperative patients. Thirty-six patients (60–86 yr) scheduled for unilateral hip replacement surgery due to hip osteoarthritis were randomized to either 1) resistance training (RT: 3/wk x 12 wk), 2) electrical stimulation (ES: 1 h/day x 12 wk), or 3) standard rehabilitation (SR: 1 h/day x 12 wk). All measurements were performed at baseline, at 5 wk and 12 wk postsurgery. After 12 wk of resistance training, maximal dynamic muscle strength increased by 30% at 60°/s (P < 0.05) and by 29% at 180°/s (P < 0.05); muscle fiber area increased for type I (+17%, P < 0.05), type IIa (+37%, P < 0.05), and type IIx muscle fibers (+51%, P < 0.05); and muscle fiber pennation angle increased by 22% and muscle thickness increased by 15% (P < 0.05). Furthermore, stair walking power increased by 35% (P < 0.05) and was related to the increase in type II fiber area (r = 0.729, P < 0.05). In contrast, there was no increase in any measurement outcomes with electrical stimulation and standard rehabilitation. The present study is the first to demonstrate the effectiveness of resistance training to induce beneficial qualitative changes in muscle fiber morphology and muscle architecture in elderly postoperative patients. In contrast, rehabilitation regimes based on functional exercises and neuromuscular electrical stimulation had no effect. The present data emphasize the importance of resistance training in future rehabilitation programs for elderly individuals.

Adenosine induces a cholinergic tracheal reflex contraction in guinea pigs in vivo via an adenosine A1 receptor-dependent mechanism

Reynolds, S. M., Docherty, R., Robbins, J., Spina, D., Page, C. P. — 2008-07-18

Adenosine induces dyspnea, cough, and airways obstruction in asthma, a phenomenon that also occurs in various sensitized animal models in which a neuronal involvement has been implicated. Although adenosine has been suggested to activate cholinergic nerves, the precise mechanism has not been established. In the present study, the adenosine A₁ receptor agonist N⁶-cyclopentyladenosine (CPA) induced a cholinergic reflex, causing tracheal smooth muscle contraction that was significantly inhibited by the adenosine A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 100 µg/kg) (P < 0.05) in anesthetized animals. Furthermore, the adenosine A₂ agonist 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) induced a small reflex, whereas the A₃ selective agonist N⁶-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) was without effect. The tracheal reflex induced by CPA was also inhibited by recurrent nerve ligation or muscarinic receptor blockade (P < 0.001), indicating that a cholinergic neuronal mechanism of action accounted for this response. The cholinergic reflex in response to aerosolized CPA was significantly greater in passively sensitized compared with naive guinea pigs (P < 0.01). Chronic capsaicin treatment, which inhibited sensory nerve function, failed to inhibit CPA-induced reflex tracheal contractions in passively sensitized guinea pigs, although the local anesthetic lidocaine inhibited CPA-induced tracheal contractions. The effects of CPA on the reflex response was not dependent on the release of histamine from tissue mast cells or endogenous prostaglandins as shown by the lack of effect of the histamine H₁ receptor antagonist pyrilamine (1 mg/kg) or the cyclooxygenase inhibitor meclofenamic acid (3 mg/kg), respectively. In conclusion, activation of pulmonary adenosine A₁ receptors can stimulate cholinergic reflexes, and these reflexes are increased in allergic guinea pigs.

Upper airway neuromuscular compensation during sleep is defective in obstructive sleep apnea

2008-07-18

Obstructive sleep apnea is the result of repeated episodes of upper airway obstruction during sleep. Recent evidence indicates that alterations in upper airway anatomy and disturbances in neuromuscular control both play a role in the pathogenesis of obstructive sleep apnea. We hypothesized that subjects without sleep apnea are more capable of mounting vigorous neuromuscular responses to upper airway obstruction than subjects with sleep apnea. To address this hypothesis we lowered nasal pressure to induce upper airway obstruction to the verge of periodic obstructive hypopneas (cycling threshold). Ten patients with obstructive sleep apnea and nine weight-, age-, and sex-matched controls were studied during sleep. Responses in genioglossal electromyography (EMG_GG) activity (tonic, peak phasic, and phasic EMG_GG), maximal inspiratory airflow (V_Imax), and pharyngeal transmural pressure (P_TM) were assessed during similar degrees of sustained conditions of upper airway obstruction and compared with those obtained at a similar nasal pressure under transient conditions. Control compared with sleep apnea subjects demonstrated greater EMG_GG, V_Imax, and P_TM responses at comparable levels of mechanical and ventilatory stimuli at the cycling threshold, during sustained compared with transient periods of upper airway obstruction. Furthermore, the increases in EMG_GG activity in control compared with sleep apnea subjects were observed in the tonic but not the phasic component of the EMG response. We conclude that sustained periods of upper airway obstruction induce greater increases in tonic EMG_GG, V_Imax, and P_TM in control subjects. Our findings suggest that neuromuscular responses protect individuals without sleep apnea from developing upper airway obstruction during sleep.

Acute norepinephrine reuptake inhibition decreases performance in normal and high ambient temperature

2008-07-18

Combined inhibition of dopamine (DA)/norepinephrine (NE) reuptake improves exercise performance and increases core temperature in the heat. A recent study demonstrated that this effect may primarily be related to increased DA activity. NE reuptake inhibition (NERI), however, has received little attention in humans, certainly in the heat, where central fatigue appears to be a main factor influencing performance. Therefore the present study examines the effect of NERI (reboxetine) on exercise capacity, thermoregulation, and hormonal response in normal and high temperature. Nine healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (Pla; 2 x 8 mg) or reboxetine (Rebox; 2 x 8 mg). Subjects exercised in temperate (18°C) or warm (30°C) conditions and cycled for 60 min at 55% W_max immediately followed by a time trial (TT; Pla18/Rebox18; Pla30/Rebox30) to measure exercise performance. Acute NERI decreased power output and consequently exercise performance in temperate (P = 0.018) and warm (P = 0.007) conditions. Resting heart rate was significantly elevated by NERI (18°C: P = 0.02; 30°C: P = 0.018). In Rebox18, heart rate was significantly higher than in the Pla18, while in the heat no effect of the drug treatment was reported during exercise. In Rebox30, all hormone concentrations increased during exercise, except for growth hormone (GH), which was significantly lower during exercise. In Rebox18, prolactin (PRL) concentrations were significantly elevated; GH was significantly higher at rest, but significantly lower during exercise. In conclusion, manipulation of the NE system decreases performance and modifies hormone concentrations, thereby indicating a central NE effect of the drug. These findings confirm results from previous studies that predominantly increased DA activity is important in improving performance.

Mechanisms of orthostatic intolerance following very prolonged exercise

2008-07-18

Nine men completed a 24-h exercise trial, with physiological testing sessions before (T1, ~0630), during (T2, ~1640; T3, ~0045; T4, ~0630), and 48-h afterwards (T5, ~0650). Participants cycled and ran/trekked continuously between test sessions. A 24-h sedentary control trial was undertaken in crossover order. Within testing sessions, participants lay supine and then stood for 6 min, while heart rate variability (spectral analysis of ECG), middle cerebral artery perfusion velocity (MCAv), mean arterial pressure (MAP; Finometer), and end-tidal Pco₂ (Pet_CO₂) were measured, and venous blood was sampled for cardiac troponin I. During the exercise trial: 1) two, six, and four participants were orthostatically intolerant at T2, T3, and T4, respectively; 2) changes in heart rate variability were only observed at T2; 3) supine MAP (baseline = 81 ± 6 mmHg) was lower (P < 0.05) by 14% at T3 and 8% at T4, whereas standing MAP (75 ± 7 mmHg) was lower by 16% at T2, 37% at T3, and 15% at T4; 4) Pet_CO₂ was reduced (P < 0.05) at all times while supine (–3–4 Torr) and standing (–4–5 Torr) during exercise trial; 5) standing MCAv was reduced (P < 0.05) by 23% at T3 and 30% at T4 during the exercise trial; 6) changes in MCAv with standing always correlated (P < 0.01) with changes in Pet_CO₂ (r = 0.78–0.93), but only with changes in MAP at T1, T2, and T3 (P < 0.05; r = 0.62–0.84); and 7) only two individuals showed minor elevations in cardiac troponin I. Recovery was complete within 48 h. During prolonged exercise, postural-induced hypotension and hypocapnia exacerbate cerebral hypoperfusion and facilitate syncope.

Effects of contraction intensity on muscle fascicle and stretch reflex behavior in the human triceps surae

2008-07-18

The aims of this study were to examine changes in the distribution of a stretch to the muscle fascicles with changes in contraction intensity in the human triceps surae and to relate fascicle stretch responses to short-latency stretch reflex behavior. Thirteen healthy subjects were seated in an ankle ergometer, and dorsiflexion stretches (8°; 250°/s) were applied to the triceps surae at different moment levels (0–100% of maximal voluntary contraction). Surface EMG was recorded in the medial gastrocnemius, soleus, and tibialis anterior muscles, and ultrasound was used to measure medial gastrocnemius and soleus fascicle lengths. At low forces, reflex amplitudes increased despite a lack of change or even a decrease in fascicle stretch velocities. At high forces, lower fascicle stretch velocities coincided with smaller stretch reflexes. The results revealed a decline in fascicle stretch velocity of over 50% between passive conditions and maximal force levels in the major muscles of the triceps surae. This is likely to be an important factor related to the decline in stretch reflex amplitudes at high forces. Because short-latency stretch reflexes contribute to force production and stiffness regulation of human muscle fibers, a reduction in afferent feedback from muscle spindles could decrease the efficacy of human movements involving the triceps surae, particularly where high force production is required.

The involvement of norepinephrine, neuropeptide Y, and nitric oxide in the cutaneous vasodilator response to local heating in humans

Hodges, G. J., Kosiba, W. A., Zhao, K., Johnson, J. M. — 2008-07-18

Presynaptic blockade of cutaneous vasoconstrictor nerves (VCN) abolishes the axon reflex (AR) during slow local heating (SLH) and reduces the vasodilator response. In a two-part study, forearm sites were instrumented with microdialysis fibers, local heaters, and laser-Doppler flow probes. Sites were locally heated from 33 to 40°C over 70 min. In part 1, we tested whether this effect of VCN acted via nitric oxide synthase (NOS). In five subjects, treatments were as follows: 1) untreated; 2) bretylium, preventing neurotransmitter release; 3) N^G-nitro-l-arginine methyl ester (l-NAME) to inhibit NOS; and 4) combined bretylium + l-NAME. At treated sites, the AR was absent, and there was an attenuation of the ultimate vasodilation (P < 0.05), which was not different among those sites (P > 0.05). In part 2, we tested whether norepinephrine and/or neuropeptide Y is involved in the cutaneous vasodilator response to SLH. In seven subjects, treatments were as follows: 1) untreated; 2) propranolol and yohimbine to antagonize - and β-receptors; 3) BIBP-3226 to antagonize Y₁ receptors; and 4) combined propranolol + yohimbine + BIBP-3226. Treatment with propranolol + yohimbine or BIBP-3226 significantly increased the temperature at which AR occurred (n = 4) or abolished it (n = 3). The combination treatment consistently eliminated it. Importantly, ultimate vasodilation with SLH at the treated sites was significantly (P < 0.05) less than at the control. These data suggest that norepinephrine and neuropeptide Y are important in the initiation of the AR and for achieving a complete vasodilator response. Since VCN and NOS blockade in combination do not have an inhibition greater than either alone, these data suggest that VCN promote heat-induced vasodilation via a nitric oxide-dependent mechanism.

Resistance training and timed essential amino acids protect against the loss of muscle mass and strength during 28 days of bed rest and energy deficit

2008-07-18

Spaceflight and bed rest (BR) result in losses of muscle mass and strength. Resistance training (RT) and amino acid (AA) supplementation are potential countermeasures to minimize these losses. However, it is unknown if timing of supplementation with exercise can optimize benefits, particularly with energy deficit. We examined the effect of these countermeasures on body composition, strength, and insulin levels in 31 men (ages 31–55 yr) during BR (28 days) followed by active recovery (14 days). Subjects were randomly assigned to essential AA supplementation (AA group, n = 7); RT with AA given 3 h after training (RT group, n = 12); or RT with AA given 5 min before training (AART group, n = 12). Energy intake was reduced by 8 ± 6%. Midthigh muscle area declined with BR for the AA > RT > AART groups: –11%, –3%, –4% (P = 0.05). Similarly, greatest losses in lower body muscle strength were seen in the AA group (–22%). These were attenuated in the exercising groups [RT (–8%) and AART (–6%; P < 0.05)]. Fat mass and midthigh intramuscular fat increased after BR in the AA group (+3% and +14%, respectively), and decreased in the RT (–5% and –4%) and AART groups (–1 and –5%; P = 0.05). Muscle mass and strength returned toward baseline after recovery, but the AA group showed the lowest regains. Combined resistance training with AA supplementation pre- or postexercise attenuated the losses in muscle mass and strength by approximately two-thirds compared with AA supplement alone during BR and energy deficit. These data support the efficacy of combined AA and RT as a countermeasure against muscle wasting due to low gravity.

Effects of leptin deficiency on postnatal lung development in mice

2008-07-18

Leptin modulates energy metabolism and lung development. We hypothesize that the effects of leptin on postnatal lung development are volume dependent from 2 to 10 wk of age and are independent of hypometabolism associated with leptin deficiency. To test the hypotheses, effects of leptin deficiency on lung maturation were characterized in age groups of C57BL/6J mice with varying Lep^ob genotypes. Quasi-static pressure-volume curves and respiratory impedance measurements were performed to profile differences in respiratory system mechanics. Morphometric analysis was conducted to estimate alveolar size and number. Oxygen consumption was measured to assess metabolic rate. Lung volume at 40-cmH₂O airway pressure (V₄₀) increased with age in each genotypic group, and V₄₀ was significantly (P < 0.05) lower in leptin-deficient (ob/ob) mice beginning at 2 wk. Differences were amplified through 7 wk of age relative to wild-type (+/+) mice. Morphometric analysis showed that alveolar surface area was lower in ob/ob compared with +/+ and heterozygote (ob/+) mice beginning at 2 wk. Unlike the other genotypic groups, alveolar size did not increase with age in ob/ob mice. In another experiment, ob/ob at 4 wk received leptin replacement (5 µg·g^–1·day^–1) for 8 days, and expression levels of the Col1a1, Col3a1, Col6a3, Mmp2, Tieg1, and Stat1 genes were significantly increased concomitantly with elevated V₄₀. Leptin-induced increases in V₄₀ corresponded with enlarged alveolar size and surface area. Gene expression suggested a remodeling event of lung parenchyma after exogenous leptin replacement. These data support the hypothesis that leptin is critical to postnatal lung remodeling, particularly related to increased V₄₀ and enlarged alveolar surface area.

Acute moderate-intensity exercise in middle-aged men has neither an anti- nor proinflammatory effect

Markovitch, D., Tyrrell, R. M., Thompson, D. — 2008-07-18

Strenuous exercise induces an initial pro- and subsequent anti-inflammatory response, and it has been suggested that this may be one of the ways that regular exercise reduces chronic inflammation and therefore the risk of cardiovascular disease. However, public health recommendations emphasize moderate-intensity physical activity, and it is important to understand whether moderate-intensity exercise has a similar anti-inflammatory effect. Twelve sedentary male volunteers (age 54 ± 4 yr) completed two main trials, moderate-intensity exercise and rest (30 min at 50% maximal oxygen uptake vs. sitting, respectively). There were no significant changes in circulating neutrophils, lymphocytes, monocytes, or serum interleukin-6, interleukin-10, and C-reactive protein concentration over the 7 days following exercise. Similarly, lymphocyte adhesion to cultured endothelial cells and heme oxygenase-1 (HO-1) expression in lymphocytes and monocytes were not affected by walking at any time point. These results suggest that the long-term anti-inflammatory and antiatherogenic effects of regular moderate-intensity physical activity must be explained by something other than a profound net anti-inflammatory response to each exercise bout since a single bout of walking did not lead to a change in various markers of inflammation or lymphocyte adherence to cultured endothelial cells.

Regulation of middle cerebral artery blood velocity during dynamic exercise in humans: influence of aging

Fisher, J. P., Ogoh, S., Young, C. N., Raven, P. B., Fadel, P. J. — 2008-07-18

Although cerebral autoregulation (CA) appears well maintained during mild to moderate intensity dynamic exercise in young subjects, it is presently unclear how aging influences the regulation of cerebral blood flow during physical activity. Therefore, to address this question, middle cerebral artery blood velocity (MCAV), mean arterial pressure (MAP), and the partial pressure of arterial carbon dioxide (Pa_CO₂) were assessed at rest and during steady-state cycling at 30% and 50% heart rate reserve (HRR) in 9 young (24 ± 3 yr; mean ± SD) and 10 older middle-aged (57 ± 7 yr) subjects. Transfer function analysis between changes in MAP and mean MCAV (MCAV_mean) in the low-frequency (LF) range were used to assess dynamic CA. No age-group differences were found in Pa_CO₂ at rest or during cycling. Exercise-induced increases in MAP were greater in older subjects, while changes in MCAV_mean were similar between groups. The cerebral vascular conductance index (MCAV_mean/MAP) was not different at rest (young 0.66 ± 0.04 cm·s^–1·mmHg^–1 vs. older 0.67 ± 0.03 cm·s^–1·mmHg^–1; mean ± SE) or during 30% HRR cycling between groups but was reduced in older subjects during 50% HRR cycling (young 0.67 ± 0.03 cm·s^–1·mmHg^–1 vs. older 0.56 ± 0.02 cm·s^–1·mmHg^–1; P < 0.05). LF transfer function gain and phase between MAP and MCAV_mean was not different between groups at rest (LF gain: young 0.95 ± 0.05 cm·s^–1·mmHg^–1 vs. older 0.88 ± 0.06 cm·s^–1·mmHg^–1; P > 0.05) or during exercise (LF gain: young 0.80 ± 0.05 cm·s^–1·mmHg^–1 vs. older 0.72 ± 0.07 cm·s^–1·mmHg^–1 at 50% HRR; P > 0.05). We conclude that despite greater increases in MAP, the regulation of MCAV_mean is well maintained during dynamic exercise in healthy older middle-aged subjects.

Protein-containing nutrient supplementation following strength training enhances the effect on muscle mass, strength, and bone formation in postmenopausal women

2008-07-18

We evaluated the response of various muscle and bone adaptation parameters with 24 wk of strength training in healthy, early postmenopausal women when a nutrient supplement (protein, carbohydrate, calcium, and vitamin D) or a placebo supplement (a minimum of energy) was ingested immediately following each training session. At inclusion, each woman was randomly and double-blindedly assigned to a nutrient group or a placebo (control) group. Muscle hypertrophy was evaluated from biopsies, MRI, and dual-energy X-ray absorptiometry (DEXA) scans, and muscle strength was determined in a dynamometer. Bone mineral density (BMD) was measured using DEXA scans, and bone turnover was determined from serum osteocalcin and collagen type I cross-linked carboxyl terminal peptide. The nutrient group improved concentric and isokinetic (60°/s) muscle strength from 6 to 24 wk by 9 ± 3% (P < 0.01), whereas controls showed no change (1 ± 2%, P > 0.05). Only the nutrient group improved lean body mass (P < 0.05) over the 24 wk. BMD responded similarly at the lumbar spine but changed differently in the two groups at the femoral neck (P < 0.05) [control: 0.943 ± 0.028 to 0.930 ± 0.024 g/mm³ (–1.0 ± 1.4%); nutrient group: 0.953 ± 0.051 to 0.978 ± 0.043 g/mm³ (3.8 ± 3.4%)] when adjusted for age, body mass index, and BMD at inclusion. Bone formation displayed an interaction (P < 0.05), mainly caused by increased osteocalcin at 24 wk in the nutrient group. In conclusion, we report that nutrient supplementation results in superior improvements in muscle mass, muscle strength, femoral neck BMD, and bone formation during 24 wk of strength training. The observed differences following such a short intervention emphasize the significance of postexercise nutrient supply on musculoskeletal maintenance.

Are the dynamic response characteristics of brachial artery flow-mediated dilation sensitive to the magnitude of increase in shear stimulus?

Pyke, K. E., Hartnett, J. A., Tschakovsky, M. E. — 2008-07-18

The purpose of this study was to determine the dynamic characteristics of brachial artery dilation in response to step increases in shear stress [flow-mediated dilation (FMD)]. Brachial artery diameter (BAD) and mean blood velocity (MBV) (Doppler ultrasound) were obtained in 15 healthy subjects. Step increases in MBV at two shear stimulus magnitudes were investigated: large (L; maximal MBV attainable), and small (S; MBV at 50% of the large step). Increase in shear rate (estimate of shear stress: MBV/BAD) was 76.8 ± 15.6 s^–1 for L and 41.4 ± 8.7 s^–1 for S. The peak %FMD was 14.5 ± 3.8% for L and 5.7 ± 2.1% for S (P < 0.001). Both the L (all subjects) and the S step trials (12 of 15 subjects) elicited a biphasic diameter response with a fast initial phase (phase I) followed by a slower final phase. Relative contribution of phase I to total FMD when two phases occurred was not sensitive to shear rate magnitude (r² = 0.003, slope P = 0.775). Parameters quantifying the dynamics of the FMD response [time delay (TD), time constant ()] were also not sensitive to shear rate magnitude for both phases (phase I: TD r² = 0.03, slope P = 0.376, r² = 0.04, slope P = 0.261; final phase: TD r² = 0.07, slope P = 0.169, r² = 0.07, slope P = 0.996). These data support the existence of two distinct mechanisms, or sets of mechanisms, in the human conduit artery FMD response that are proportionally sensitive to shear stimulus magnitude and whose dynamic response is not sensitive to shear stimulus magnitude.

High sensitivity of the sheep pulmonary vein antrum to acetylcholine stimulation

2008-07-18

Isolation of the pulmonary vein antrum can terminate atrial fibrillation, but the rationale has not been elucidated. In the present study, we show that sheep atrial effective refractory period (ERP) was heterogeneously shortened by acetylcholine administration. After perfusion with 15 µM acetylcholine, the shortest ERP occurred in the pulmonary vein antrum, which was recorded with the standard intracellular microelectrode technique (the ERP results in the pulmonary vein antrum, left atrial posterior wall, roof, free wall and appendage, and right atrial free wall were 52.0 ± 1.6, 75.1 ± 2.0, 77.2 ± 1.7, 85.6 ± 1.7, 64.3 ± 2.1, and 90.5 ± 1.3 ms, respectively; P < 0.05). Immunofluorescent staining revealed that muscarinic type 2 receptors (M₂R) were also distributed heterogeneously in the atrial myocardium, with the highest density in the antrum (the relative fluorescent intensity results of the M₂R in the pulmonary vein antrum, left atrial posterior wall, roof, free wall and appendage, and right atrial free wall were 62.64 ± 2.56, 53.12 ± 2.76, 51.83 ± 2.45, 47.90 ± 2.33, 55.27 ± 2.08, and 45.53 ± 2.02, respectively; P < 0.05), which was in accordance with the heterogeneity of ERP distribution. Thus the pulmonary vein antrum is a unique electrophysiological region with high sensitivity to acetylcholine, and its intensive response to acetylcholine is most likely associated with the dense M₂R distribution of this region. Such an acetylcholine-induced ERP heterogeneity is possibly a substrate for atrial fibrillation and hence one of the potential electrophysiological bases for the isolation therapy.

Comparative MRI analysis of T2 changes associated with single and repeated bouts of downhill running leading to eccentric-induced muscle damage

Marqueste, T., Giannesini, B., Fur, Y. L., Cozzone, P. J., Bendahan, D. — 2008-07-18

Although the exact mechanisms are still unclear, it is commonly acknowledged that acute eccentric exercise alters muscle performance, whereas the repetition of successive bouts leads to the disappearance of the deleterious signs. To clarify this issue, we measured blood creatine kinase and lactate dehydrogenase activities and proton transverse relaxation time (T2) in various leg muscles 72 h after single and repeated bouts of exhausting downhill running sessions (–15°, 1.5 km/h) with either 4 or 7 days elapsed between bouts. After a single exercise bout, T2 and enzyme activities initially increased and recovered rapidly. When exercise bouts were repeated over a short time period (4 days), initial changes did not recover and endurance time throughout additional exercise sessions was significantly reduced. On the contrary, with a longer resting time between exercises (7 days), the endurance time of additional running sessions was significantly longer and muscle changes (T2 increase, muscle edema, and enzyme activity changes) slowly and completely reversed. Significant correlations were found between T2 changes and enzyme activities. T2 changes in the soleus and gastrocnemius muscle heads were differently affected by lengthening contractions, suggesting a muscle specificity and indicating that muscle alterations might be linked to different anatomical properties, such as fiber pennation angles, typology, and/or the exhausting nature of the downhill running sessions. We documented a "less muscle injury" effect due to the repetition of exercise bouts at a low frequency (i.e., 1 session per week) in accordance with the delayed muscle inflammation. This effect was not observed when the between-exercise resting time was shorter.

Technique for quantitative RT-PCR analysis directly from single muscle fibers

Wacker, M. J., Tehel, M. M., Gallagher, P. M. — 2008-07-18

The use of single-cell quantitative RT-PCR has greatly aided the study of gene expression in fields such as muscle physiology. For this study, we hypothesized that single muscle fibers from a biopsy can be placed directly into the reverse transcription buffer and that gene expression data can be obtained without having to first extract the RNA. To test this hypothesis, biopsies were taken from the vastus lateralis of five male subjects. Single muscle fibers were isolated and underwent RNA isolation (technique 1) or placed directly into reverse transcription buffer (technique 2). After cDNA conversion, individual fiber cDNA was pooled and quantitative PCR was performed using primer-probes for β₂-microglobulin, glyceraldehyde-3-phosphate dehydrogenase, insulin-like growth factor I receptor, and glucose transporter subtype 4. The no RNA extraction method provided similar quantitative PCR data as that of the RNA extraction method. A third technique was also tested in which we used one-quarter of an individual fiber's cDNA for PCR (not pooled) and the average coefficient of variation between fibers was <8% (cycle threshold value) for all genes studied. The no RNA extraction technique was tested on isolated muscle fibers using a gene known to increase after exercise (pyruvate dehydrogenase kinase 4). We observed a 13.9-fold change in expression after resistance exercise, which is consistent with what has been previously observed. These results demonstrate a successful method for gene expression analysis directly from single muscle fibers.

Assessing recruitment of lung diffusing capacity in exercising guinea pigs with a rebreathing technique

Yilmaz, C., Dane, D. M., Hsia, C. C. W. — 2008-07-18

Noninvasive techniques for assessing cardiopulmonary function in small animals are limited. We previously developed a rebreathing technique for measuring lung volume, pulmonary blood flow, diffusing capacity for carbon monoxide (Dl_CO) and its components, membrane diffusing capacity (Dm_CO) and pulmonary capillary blood volume (Vc), and septal volume, in conscious nonsedated guinea pigs at rest. Now we have extended this technique to study guinea pigs during voluntary treadmill exercise with a sealed respiratory mask attached to a body vest and a test gas mixture containing 0.5% SF₆ or Ne, 0.3% CO, and 0.8% C₂H₂ in 40% or 98% O₂. From rest to exercise, O₂ uptake increased from 12.7 to 25.5 ml·min^–1·kg^–1 while pulmonary blood flow increased from 123 to 239 ml/kg. The measured Dl_CO, Dm_CO, and Vc increased linearly with respect to pulmonary blood flow as expected from alveolar microvascular recruitment; body mass-specific relationships were consistent with those in healthy human subjects and dogs studied with a similar technique. The results show that 1) cardiopulmonary interactions from rest to exercise can be measured noninvasively in guinea pigs, 2) guinea pigs exhibit patterns of exercise response and alveolar microvascular recruitment similar to those of larger species, and 3) the rebreathing technique is widely applicable to human (~70 kg), dog (20–30 kg), and guinea pig (1–1.5 kg). In theory, this technique can be extended to even smaller animals provided that species-specific technical hurdles can be overcome.

Learning from children: the emergence of pediatric exercise science

Rowland, T., Saltin, B. — 2008-07-18

Arterial function in youth: window into cardiovascular risk

Fernhall, B., Agiovlasitis, S. — 2008-07-18

Noninvasive measures of arterial function, such as intima-media thickness (IMT), endothelial function, and arterial stiffness are associated with and are prognostic of cardiovascular events in adults. Postmortem evidence, however, has established that the atherosclerotic process starts in childhood. Furthermore, cardiovascular morbidities in childhood disrupt arterial health and may lead to adverse outcomes in adulthood. Thus it is important to examine the developmental changes in IMT, endothelial function, and arterial stiffness in healthy youth in contrast to the arterial health profile of youth with cardiovascular morbidities and to examine the effect of lifestyle interventions. In healthy youth, IMT may increase slightly, arterial stiffness increases, but there is no change in endothelial function from 5 to 20 years of age. In youth with cardiovascular risk factors there are larger increases in IMT and arterial stiffness, and reductions in endothelial function compared with healthy youth. The reduced arterial function in youth with cardiovascular risk factors may be related to the atherosclerotic process. Exercise and physical activity appear to exert a protective effect on arterial function, and exercise training can improve arterial function in children with cardiovascular risk factors. Furthermore, although diet alone can improve arterial function in children, the combination of exercise and diet appears to be more effective than either intervention alone. Future studies need to focus on the mechanism by which exercise and diet improve arterial function, the most effective types of diet and exercise, and if intervening in childhood leads to favorable outcomes in adulthood.