| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota 55905
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
A pulse ([14C]palmitate)-chase
([3H]palmitate) approach was used to study intramuscular
triglyceride (imTG) fatty acid and plasma free fatty acid (FFA)
kinetics during exercise at ~45% peak O2 consumption in
12 adults. Vastus lateralis muscle was biopsied before and after 90 min
of bicycle exercise; 3H2O production, breath
14CO2 excretion and lipid oxidation (indirect
calorimetry) rates were measured during exercise. Results: during
exercise, 8.2 ± 1.2 and 8.4 ± 0.7 µmol · kg
1 · min1 of imTG
fatty acids and plasma FFA, respectively, were oxidized according to
isotopic measurements. The sum of these two values was not different
(P = 0.6) from lipid oxidation by indirect calorimetry (15.4 ± 1.6 µmol · kg1 · min1); the
isotopic and indirect calorimetry values were correlated (r = 0.79, P < 0.005). During
exercise, imTG turnover rate was 0.32 ± 0.07%/min (6.0 ± 2.0 µmol of imTG · kg wet
muscle1 · min1) and plasma FFA were
incorporated into imTG at a rate of 0.7 ± 0.1 µmol · kg
wet muscle1 · min1. The imTG pool
size did not change during exercise. This pulse-chase, dual tracer
appears to be a reasonable approach to measure oxidation and synthesis
kinetics of imTG.
[14C]palmitate; body composition; free fatty acids
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
INTRAMUSCULAR TRIGLYCERIDE (imTG) fatty acids are an important energy source during exercise (21, 30) and possibly at rest (4). Thus far, however, imTG oxidation in vivo has only been assessed using an indirect approach: plasma free fatty acid (FFA) oxidation (isotopic measures) is subtracted from total fatty acid oxidation (indirect calorimetry) (38). Uncertainties with respect to isotopic measures of FFA oxidation (14, 31, 32, 37) and the unreliability of indirect calorimetry at times of changing acid-base status (9) could limit the confidence in quantitative estimates of imTG oxidation using this approach. Another concern is that plasma FFA incorporated into imTG (4) during the preexercise tracer infusion interval might be oxidized during exercise, resulting in an overestimate of plasma FFA oxidation. This possibility does not appear to have been evaluated to date. Therefore, a method that can measure imTG oxidation more directly should advance our understanding of imTG metabolism under different nutritional and physical conditions. The dual-tracer, pulse-chase technique has been used to study intramuscular fatty acid metabolism in isolated muscles ex vivo (5, 6). We wished to determine whether this approach would also be applicable to in vivo conditions to study intramuscular fatty acid kinetics within the normal neural and hormonal milieu.
In the present study, we use this pulse-chase approach to measure imTG fatty acid oxidation in healthy men and women during moderate exercise. The method combines prelabeling imTG by a 14[C]-FFA tracer infusion before exercise with infusion of a [3H]-FFA tracer during the exercise. Muscle biopsies were performed before and after the exercise to determine turnover of the imTG pool. The turnover rate of imTG was determined by analyzing the decay of 14C-fatty acids from the imTG pool. We related the 14CO2 production to the imTG 14C-fatty acid specific activity (SA) to estimate the oxidation rate of imTG fatty acids. The [3H]-FFA tracer was infused during exercise to allowed measurement of plasma FFA oxidation and incorporation of plasma FFA into imTG during exercise. We wished to test the hypothesis that the combined isotopic measures of fatty acid oxidation rates (imTG plus circulating FFA) would equal total fatty acid oxidation as determined by indirect calorimetry. The results suggest that this pulse-chase, dual-tracer and two-point muscle biopsy technique is useful for investigation of skeletal muscle lipid metabolism in vivo in humans.
| |
METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
Subjects
Written, informed consent was obtained from 12 healthy, nonobese adults (6 men). The women were premenopausal and were studied in the follicular phase of their menstrual cycle. All participants consumed an isocaloric diet in the Mayo Clinic General Clinical Research Center (GCRC) for 7 days before this study. The diet provided 35% of calories as fat, 15% as protein, and 50% as carbohydrate. All subjects were moderately active physically and maintained their usual level of physical activity during the week before the study. Weight stability was achieved before the study. These volunteers are the same individuals participating in a recently published study of leg lipolysis during exercise (1). The additional [14C]palmitate tracer and muscle biopsies were included with the intention of acquiring pilot data with respect to the feasibility of using this approach to study imTG kinetics; the results were sufficiently encouraging to warrant dissemination.Protocol
Body and leg fat and fat-free mass (FFM) were measured within 3 days of the study using dual-energy X-ray absorptiometry (model DPX-IQ, Lunar Radiation, Madison, WI). Each subject was scanned for 20 min, and whole body and regional analysis was performed by using software version 4.1. Total body water was measured ~1 wk before the study using the 3H2O technique (18). Each participant's peak O2 consumption (
O2 peak) was measured with a
continuous bicycle exercise test. Exercise was initiated at 25 and 50 W
for women and men, respectively, and increased by 25 or 30 W,
respectively, every 2 min until exhaustion. Breath-by-breath
O2 consumption (O2) and
CO2 production (CO2) were
measured throughout the test by indirect calorimetry (29).
Figure 1 provides an overview of the
study protocol. The participants were admitted to the GCRC the evening
before the study, and an 18-gauge intravenous infusion catheter was
placed in a forearm and kept patent with a controlled infusion of
0.45% NaCl. At 0400 the next day, a continuous infusion of
[1-14C]palmitate was initiated at 0.3 µCi/min to
prelabel the imTG pool. Resting energy expenditure was measured by
indirect calorimetry (DeltaTrac Metabolic Cart, Sensor Medics, Yorba
Linda, CA) at 0700, after which a radial artery catheter was placed
under local anesthesia for blood sampling. Femoral artery and vein
catheters were also placed (1). At 0830, an infusion of
[9,10-3H]palmitate (0.6 µCi/min) was started. After 30 min for isotopic equilibration, a series of four blood samples was
obtained over a 30-min period for measurement of plasma palmitate and
FFA concentration and SA. The tracer infusions were then stopped, and
the participants were prepared for the first biopsy from vastus
lateralis muscle using local anesthesia and sterile techniques. The
muscle samples (200-300 mg) were washed of blood in ice-cold
normal saline solution, dissected of all adipose tissue under a
stereomicroscope, further rinsed of lipid droplets, and saved
immediately in liquid N2. We discontinued the
[1-14C]palmitate infusion at least 30 min before the
first biopsy to minimize the possibility that additional tracer would
be incorporated into imTG after the biopsy. Immediately after the
biopsy, the participants were transferred to the Integrative Physiology
Core of the GCRC for the exercise study. The
[9,10-3H]palmitate infusion was restarted as soon as the
participant arrived in the exercise study room. After ~30 min, the
participants began to exercise on a bicycle ergometer at 45% of
O2 peak continuously for 90 min. Blood
samples were collected at 15-min intervals during the exercise, and
heart rate was monitored throughout the study. After the completion of
the exercise, a second muscle biopsy was taken from the contralateral
leg as quickly as possible by the same procedures. The catheters were
removed after completion of the study, and the volunteers remained
under observation in the GCRC until the following morning.
|
During the exercise, systemic O2 and
CO2 were measured at 15-min intervals by
using the same breath-by-breath mass spectrometry system used for the
fitness test (29). Breath samples for measurement of
14CO2 SA (10) were collected
before and at 15-min intervals during exercise.
Materials
[1-14C]palmitate, [9,10-3H]palmitate and 3H2O were obtained from Amersham (Arlington Heights, IL). The FFA were prepared for intravenous infusion as a solution of 0.3% albumin in 0.9% NaCl.Assays and Measurements
Plasma FFA and triglycerides. Plasma total FFA and palmitate concentrations, as well as the isotopic purity of radioactive tracers, were measured by HPLC (19, 26). Plasma triglyceride concentrations were measured by a microfluorometric method (15), and plasma triglyceride SA was measured on an HPLC-purified (3) Folch extract of plasma.
Muscle lipids. The frozen muscle samples were pulverized into fine powder by using a stainless steel mortar and pestle on dry ice. The powder was extracted for total lipids by the method of Folch and colleagues (8), from which triglyceride was purified by HPLC (3). The majority of the purified muscle triglyceride was counted on a liquid scintillation counter for total 3H and 14C radioactivity [disintegrations/min (dpm)]. A small aliquot of the triglyceride was directly transmethylated by using 2.5% H2SO4 in methanol at 70°C for 2 h (20). The concentration and composition of imTG palmitate were determined by gas chromatography/combustion/isotope ratio mass spectrometry with heptadecanoic acid as an internal standard (12), and the 14C and 3H SA of imTG-palmitate (dpm/µmol) were then calculated.
3H2O production. Plasma water 3H2O concentrations were determined as previously described by using blood samples obtained before and during exercise (14). The slope of the increase in 3H2O in total body water was used to calculate the 3H2O production rate for each participant.
Urinary nitrogen was measured using an Analox GM7 fast enzymatic metabolite analyzer (Analox Instruments, Lunenburg, MA).Calculations
Rate of net imTG turnover during exercise.
imTG turnover rate was calculated by using the 14C
imTG-palmitate data. The 14C imTG-palmitate SA values
before and after exercise were transformed by natural logarithm. The
decrease in the transformed value from the beginning to the end of
exercise was used to calculate the fractional turnover rate (expressed
as % decrease per minute). The APPENDIX provides the
theory and process of this approach. The net turnover rate of imTG
(µmol imTG · kg wet
muscle1 · min1) was calculated from
the imTG-palmitate concentration data, corrected by its abundance in
the imTG fatty acid pool (28%).
Rate of imTG fatty acid oxidation during exercise.
During the initial stages of exercise, the
14CO2 expired may reflect a rapid washout of
14CO2 produced and accumulated in the body
bicarbonate pool during the prolonged [14C]palmitate
infusion. We reasoned that, after 45 min of exercise, 14CO2 excretion should occur almost exclusively
from the immediate oxidation of 14C-labeled imTG.
Therefore, imTG fatty acid oxidation rates were calculated for the last
45 min of exercise. The 14CO2 production rates
at 45, 60, 75, and 90 min of the exercise were determined by
multiplying the 14CO2 SA of breath by the
CO2 production rate measured by indirect calorimetry. The
14CO2 production was corrected for
CO2 fixation on the basis of a previously published
CO2 fixation-exercise intensity relationship derived by
Sidossis et al. (32). The corrected
14CO2 production rate was divided by the imTG
fatty acid SA estimated to be present at that time to calculate the
rate (µmol · kg1 · min1)
of imTG fatty acid oxidation during that interval. Because there were
only two muscle biopsies, one before (~0 min) and one after exercise
(~90 min), it was necessary to estimate the SA of imTG-palmitate at
45, 60, and 75 min. These values were calculated for each individual by
using the initial SA value of imTG-palmitate corrected for decay over
time by the determined fractional imTG turnover rate. In doing so, we
assumed that the decay in SA of imTG 14C-fatty acids was at
a constant rate during the course of exercise (see
APPENDIX).
Plasma FFA oxidation during exercise. The rate of whole body 3H2O production (dpm/min) was determined by linear regression analysis of the concentration of 3H2O in body water vs. time. The rate of 3H2O production was divided by the average plasma [3H]-FFA SA during the same period to calculate plasma FFA oxidation rates during that period. The plasma palmitate SA value was converted to FFA SA on the basis of plasma FFA composition.
The "indirect" oxidation of plasma fatty acids via imTG (esterification into imTG before being oxidized) was estimated as follows: the increment in imTG 3H-fatty acid SA and the fractional turnover rate of imTG were used to determine the rate of 3H2O production resulting from indirect oxidation of plasma 3H-fatty acids via imTG pool of leg skeletal muscle (22% and 27% of body weight for men and women, respectively, as determined by dual-energy X-ray absorptiometry). The rate of 3H2O production estimated from this indirect pathway was subtracted from whole body 3H2O production in the calculation of plasma fatty acid oxidation.Whole body fat oxidation and total energy expenditure during
exercise.
Because plasma lactate concentrations were stable during exercise
(1), we calculated fatty acid oxidation rates by using O2,
CO2, and urinary nitrogen excretion
rates with published formulas (9).
imTG synthesis during exercise. The increment in imTG 3H fatty acid SA from the beginning to the end of exercise was considered to represent the rate of net incorporation of plasma FFA during [3H]palmitate infusion. This rate was divided by the average plasma [3H]-FFA SA to derive the amount of FFA incorporated into imTG pool from plasma fatty acid pool. Palmitate value was converted to that for all fatty acids on the basis of plasma fatty acid composition (28%).
Statistics. All values are means ± SE except as indicated. For comparisons of imTG pool size before and after exercise, and between the rates of fatty acid oxidation measured isotopically with those measured by indirect calorimetry, a paired Student's t-test was employed. Total fatty acid oxidation (indirect calorimetry) was related to the sum of isotopically measured fatty acid oxidation (plasma FFA + imTG) for each individual using linear regression analysis.
| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
Subject Characteristics and FFA Kinetics
The characteristics of the subjects participating in this study have been previously published (1). The subjects were 31 ± 2 yr old, with a mean body mass index of 25.0 ± 1.2 kg/m2 and peakO2 of 52 ± 2 ml · kg FFM1 · min1.
Basal FFA flux was 406 ± 43 µmol/min (5.8 ± 0.7 µmol · kg1 · min1) and
increased (P < 0.005) to an average of 881 ± 64 µmol/min (12.3 ± 1.2 µmol · kg1 · min1) during
the last 45 min of exercise.
imTG Synthesis and Turnover Kinetics During Exercise
Figure 2 shows simultaneous turnover and synthesis of imTG in vastus lateralis muscle during exercise. The preexercise imTG 14C-fatty acid SA was 447 ± 69 dpm/µmol. After 90 min of exercise, the imTG 14C fatty acid SA decreased (P = 0.04) to 197 ± 54 dpm/µmol. The fractional turnover rate of imTG was 0.32 ± 0.07%/min, and the absolute turnover rate of imTG was 6.0 ± 2.0 µmol of imTG · kg wet muscle1 · min1. Plasma FFA were
actively incorporated into the imTG pool during exercise as indicated
by the increase in the imTG fatty acid 3H SA (from 52 ± 21 to 146 ± 44 dpm/µmol, P = 0.004). The
rate of incorporation of plasma fatty acids into the imTG pool was 0.7 ± 0.1 µmol · kg wet
muscle1 · min1. Despite the
apparent differences between uptake and disappearance of fatty acids
from imTG, we did not find differences in the content of imTG fatty
acids (palmitate) before and after exercise (2.0 ± 0.5 vs.
2.4 ± 0.8 mmol imTG/kg wet muscle before vs. after exercise, respectively, P = 0.7).
|
imTG Fatty Acid Oxidation During Exercise: Isotopic Approach
As expected, there was virtually no [14C]palmitate in arterial or femoral venous plasma (Fig. 3) at the beginning of or during exercise because its infusion had been discontinued for >60 min. Despite this, considerable 14CO2 was expired throughout exercise (Fig. 4). With the estimated imTG-fatty acid SA used as the precursor for imTG oxidation during the last 45 min of exercise, the average imTG fatty acid oxidation rate was 8.2 ± 1.2 µmol · kg1 · min1. This
represents 49 ± 11% of whole body lipid oxidation and 24 ± 4% of total energy expenditure during exercise.
|
|
Plasma Fatty Acid Oxidation During Exercise
Figure 5 depicts the cumulative production of 3H2O during exercise. With the generation of 3H2O used as a means of measuring FFA oxidation, 8.4 ± 0.7 µmol · kg1 · min1 of
plasma FFA were oxidized during exercise. The indirect oxidation of
plasma fatty acids that had traversed imTG fatty acid pool before being
oxidized was estimated to be negligible (0.12 ± 0.2 µmol · kg1 · min1, or
1.2 ± 0.3%).
|
Systemic Energy Expenditure and Fatty Acid Oxidation During Exercise
The total energy expenditure during the last 45 min of exercise was 29.9 ± 3.0 kJ/min. Systemic fatty acid oxidation (indirect calorimetry) during exercise was 15.4 ± 1.6 µmol · kg1 · min1.
Fatty Acid Oxidation Determined by Isotopic Vs. Indirect Calorimetry Approaches
Isotopically measured total fatty acid oxidation during exercise (imTG + plasma FFA) was strongly correlated with that determined by indirect calorimetry (Fig. 6; r = 0.79, P < 0.005). In addition, the sum of isotopically measured rates of fatty acid oxidation (16.3 ± 1.6 µmol · kg1 · min1)
during exercise was not different (P = 0.6) from fatty
acid oxidation rates measured with indirect calorimetry.
|
imTG fatty acid oxidation rates calculated by subtracting plasma FFA
oxidation from total fatty acid oxidation (indirect calorimetry) were
7.2 ± 1.1 µmol · kg body
wt1 · min1, compared with the
isotopically measured rate of 8.2 ± 1.2 µmol · kg1 · min1
(P = 0.4).
Plasma Triglyceride Concentration, SA, and Leg Balance
During exercise, the arterial plasma triglyceride concentrations averaged 284 ± 37 µmol/l. The net balance across the leg was
26 ± 18 µmol/min for both legs [P = not
significant (NS) vs. 0]. The arterial plasma triglyceride SA averaged
2,481 ± 315 dpm/µmol, indicating that substantial incorporation
of [14C]palmitate had occurred during the isotope
infusion interval. We were unable to detect leg uptake of
[14C]triglyceride during exercise; the leg
[14C]triglyceride balance was 49 ± 39 dpm/triglyceride per ml plasma (P = NS vs. 0).
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
imTGs are an important energy source (4, 21, 30) and, when present in excess, they are associated with greater risk of adverse metabolic consequences of obesity (7, 17, 23, 24, 27, 28, 34, 36). Both highly trained athletes (22) and insulin-resistant obese individuals (23, 27) have increased imTG content, however, suggesting that the kinetics of imTG may be more important than the amount of imTG. In this study, the pulse-chase, dual-isotope approach, previously used to study rat muscle ex vivo (6), was adapted to study the in vivo kinetics of imTG in humans. We chose to study exercise because imTG are thought to be an especially important fuel source during exercise (16). imTG were readily labeled with the exogenous FFA tracer (e.g., [14C]palmitate) in humans, similar to our findings in rats (12). Furthermore, the excretion of 14CO2 relative to the change in imTG 14C SA that occurred during exercise provided estimates of imTG oxidation that were consistent with the previously applied model using indirect calorimetry and [13C]palmitate (38). The rate of circulating FFA oxidation, as determined by the generation of 3H2O, was similar to that reported by Sidossis et al. (32) using [13C]FFA tracers. The sum of isotopically measured fatty acid oxidation (imTG plus plasma FFA) was comparable to total fatty acid oxidation measured with indirect calorimetry, suggesting that the isotopic measures are credible. Finally, plasma FFA were incorporated into imTG during exercise, which could account for the stable imTG pool size. We conclude that the pulse-chase, dual-isotope, muscle biopsy approach to studying imTG metabolism is a potentially valuable approach for studying imTG kinetics in vivo.
Exercise was chosen as a means of examining the dual-isotope, pulse-chase, muscle biopsy approach to studying imTG kinetics because high rates of fatty acid oxidation could be expected (16, 25). This allowed us to compare the traditional approach of calculating imTG oxidation (total fatty acid oxidation by indirect calorimetry-FFA oxidation by isotopic measures) with the new approach under high-flux conditions. Comparing these two methods of measuring imTG oxidation may not have been possible in resting volunteers because nonskeletal muscle fatty acid oxidation contributes a sufficiently large portion of total fatty acid oxidation that the results would be difficult to interpret.
We did not attempt to take more than two muscle biopsy samples (one just before and one just after exercise) during these studies, making it necessary to estimate the imTG 14C SA corresponding to the breath 14CO2 production rate at each time point during exercise. To do this, we employed a mathematical model using the two-point muscle biopsy data to determine the imTG turnover rate. This approach seems reasonable, considering that the pool size of imTG did not change appreciably during exercise. In addition, studies using a similar pulse-chase approach to examine isolated rat muscle (6) have indicated a linear or log-linear accumulation of exogenous fatty acids into imTG over time. In order for the imTG pool size to remain stable during exercise, the accumulation of imTG from plasma FFA would likely need to mirror imTG fatty acid oxidation. Finally, the good agreement between the combined fatty acid oxidation estimated isotopically (plasma FFA and imTG) and total fatty acid oxidation as determined by indirect calorimetry is reassuring that the model is reasonable. Although it would be a considerable experimental design challenge, a study that included more than two biopsies (at least one during exercise) would be a welcome test of the assumption that imTG decay is log-linear during exercise.
Another means of examining the internal consistency of our data
is to compare the leg plasma fatty acid uptake (measured using arterial-venous [3H]palmitate kinetics across legs)
during exercise (1) with the sum of oxidation and
esterification of plasma FFA. We reasoned that leg FFA uptake during
exercise should be similar to, although somewhat less than, FFA
oxidation + reesterification because the majority of FFA oxidation
during bicycle exercise should be within leg muscle (leg O2
uptake during exercise was ~65-70% of total O2
consumption; Ref. 1). Leg total FFA uptake during exercise in these volunteers (1) averaged 7.1 ± 0.4 µmol · kg1 · min1.
Using the incorporation of [3H]palmitate into imTG,
the average [3H]palmitate SA during exercise, the imTG
concentration, and leg muscle mass, we estimated that ~20% of leg
FFA uptake during exercise was esterified into imTG. As expected, the
sum of plasma FFA oxidation rates (8.4 ± 0.7 µmol · kg1 · min1) and
leg incorporation of FFA into imTG (0.12 ± 0.2 µmol · kg1 · min1) is
greater (by 1.5 µmol · kg1 · min1) than
leg FFA uptake rates measured isotopically. This is consistent with
some FFA oxidation also occurring in tissues other than leg skeletal
muscle during exercise (heart, liver, etc.). We are reassured that
these two independent measures of leg muscle FFA metabolism are
internally consistent.
Previous studies of isolated rat muscle using the pulse-chase, dual-isotope approach yielded results that were interpreted as providing evidence for rapidly turning over "subpools" of imTG (6); a 32% loss of 14C in imTG was observed without loss of imTG. If the [14C]FFA tracer in the present study was incorporated into small, rapidly turning over subpools (6), our isotopic estimates of imTG oxidation should have greatly exceeded the values derived by indirect calorimetry. Instead, the isotopic estimates were in good agreement with the indirect calorimetry-plasma FFA oxidation approach. Thus our results do not support the concept that distinct, intracellular subpools of rapidly turning over imTG are present in vivo in humans.
We did not observe significant decreases in imTG content in response to
exercise. This is perhaps not surprising given the reciprocal losses of
14C and simultaneous gains in 3H in imTG (Fig.
2). The lack of change in imTG content after exercise is consistent
with some (22, 35) but not other (2) studies. Carlson et al. (2) observed a 20% decline in imTG content
after an intensive exercise (65% maximal
O2) for 100 min. Higher exercise intensities produce the combination of greater imTG utilization and
lesser FFA availability (Ref. 21; to provide "replacement" imTG
fatty acids) and should make it easier to detect changes in imTG
content. Our results suggest that during moderate, short-term exercise,
imTG undergoes simultaneous hydrolysis and esterification such that the
pool size is relatively well maintained.
Another potential source of fatty acids for muscle is circulating
triglycerides, primarily those contained in very-low-density lipoprotein (VLDL) particles. Although we could not detect leg triglyceride uptake during exercise, the high rates of blood flow would
make it difficult to detect modest rates of VLDL-triglyceride uptake.
Consistent with the findings of Havel et al. (13), the plasma triglycerides of our volunteers were labeled with the
[14C]palmitate that was infused to label imTGs. In fact,
the SA of plasma triglyceride was ~10-fold greater than imTG, which
could have effects on calculated imTG oxidation rates if
VLDL-triglyceride fatty acids are used for fuel instead of being stored
in adipose tissue. VLDL-triglyceride production rates average ~0.35
µmol · kg1 · min1 in
healthy adults (33) and, if maintained at this rate during exercise, would deliver a substantial amount of [14C]
triglyceride fatty acids to the circulation in this study. Wolfe et al.
(39) have reported that ~40% of VLDL fatty acids are
oxidized in healthy dogs. If the same is true in humans, we estimated
that as much as 13% of 14CO2 production could
originate from this source in our subjects, causing us to overestimate
imTG oxidation. In addition, if VLDL-triglyceride fatty acids were
taken up and esterified by exercising muscle, this would cause us to
underestimate imTG turnover. The potentially confounding variable of
isotopically labeled VLDL-triglyceride that occurs with this
experimental design can be addressed using the approach of Sidossis et
al. (33). Measurement of VLDL-triglyceride fatty acid
kinetics and oxidation during exercise would help determine the
relative contribution of plasma FFA vs. imTG vs. VLDL-triglyceride to
lipid fuel metabolism. Although VLDL-triglyceride are not thought to
contribute to muscle fuel metabolism during exercise (11), confirmation of this with isotopic tracers would simplify the model
needed to understand imTG kinetics.
Consistent with the results of pulse-chase studies of fatty acid metabolism in isolated rat muscle (6), we found no evidence of FFA release from imTG during exercise, a condition that should result in high rates of imTG hydrolysis (16). There was no increase in the level of [14C]palmitate in plasma (either arterial or femoral venous) with exercise despite the massive increase in 14CO2 production resulting from the oxidation of intracellular imTG [14C]palmitate. We conclude that intramuscular triacylglycerol stores are used primarily, if not solely, as local oxidative fuel and are not exported. Given the relatively high plasma [14C]triglyceride SA, the failure to find significant 14C in plasma free palmitate also suggests that lipoprotein lipase-mediated clearance of VLDL-triglyceride fatty acids does not result in significant entrance of triglyceride fatty acids into the plasma FFA pool.
In summary, the present studies have examined the use of a dual-isotope, pulse-chase experimental design to evaluate imTG metabolism during exercise. By measuring the imTG 14C SA before and after exercise, and relating these changes to 14CO2 production, we more directly assessed imTG kinetics. imTG oxidation as determined using this isotopic approach agreed well with the more traditional means of determining imTG oxidation. Moreover, our results suggest that human imTG is a more homogeneous pool than ex vivo studies of rat muscle have indicated. The lack of change in imTG content during moderate exercise appears to relate at least partly to simultaneous esterification of plasma FFA within muscle. If VLDL-triglyceride fatty acids are not a significant source of lipid fuel in humans, this approach to the study of imTG metabolism may offer promise in the study of intracellular triglyceride fatty acid kinetics in vivo.
| |
APPENDIX |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
Determination of Turnover Rate of imTG Fatty Acids and Its Use for the Prediction of Fatty Acid Label in the Triglyceride Pool
During the turnover of imTG-[14C]palmitate, the labeled palmitate is replaced by unlabeled palmitate so that the imTG-palmitate pool size remains unchanged, whereas the absolute amount of the label continuously diminishes (Fig. 7). At a constant turnover rate k, the amount of [14C]palmitate remaining in the imTG pool at any point of time can be described as following
|
(1) |
![<IT>k</IT><IT>=</IT>[ln(Q)<SUB><IT>t</IT></SUB><IT>−</IT>ln(Q)<SUB>0</SUB>]&cjs0823; <IT>t</IT>](/content/vol89/issue5/fulltext/2057/img002.gif)
|
(2) |
|
For data presented in Fig. 7, ln(Q)5 is 3.6 and
ln(Q)0 is 4.6. The value of k is thus
calculated, by Eq. 2, to be (3.6 4.6)/5 = 0.2
(the negative sign indicates removal of [14C]palmitate
from imTG pool). The same k value can be obtained in the
same way with any two or more data points. The data treatment is
necessary for a two-point approach such as that used in present study,
in which only two muscle biopsies were available. On the other hand,
the two-point approach should provide a k value that is
identical to that obtained by a multiple-point approach.
The assumptions for this model are 1) a homogeneous pool of stable size and 2) a constant turnover rate k. These assumptions satisfy a single-pool model. Therefore, in using this model, it is assumed that imTG is a metabolically homogeneous pool turning over at a constant rate during the exercise.
For calculation of remaining amount of [14C]palmitate in imTG pool at any time point during the exercise, Eq. 1 is directly used with appropriate substitutions of the k for the determined turnover rate and time variable t.
| |
ACKNOWLEDGEMENTS |
|---|
This research was supported by National Institutes of Health Grants DK-40484, DK-45343, and RR-0585, Minnesota Obesity Center Grant DK-50456, and the Mayo Foundation.
| |
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: M. D. Jensen, Endocrine Research Unit, 5-164 W. Joseph, Mayo Clinic, Rochester, MN 55905 (E-mail: jensen.michael{at}mayo.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 28 January 2000; accepted in final form 27 June 2000.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
|
|---|
1.
Burguera, B,
Proctor DN,
Dietz W,
Guo ZK,
Joyner MJ,
and
Jensen MD.
Leg FFA kinetics during exercise in men and women.
Am J Physiol Endocrinol Metab
278:
E113-E117,
2000
2.
Carlson, LA,
Ekelund LG,
and
Froberg SO.
Concentration of triglycerides, phospholipids and glycogen in skeletal muscle and of free fatty acids and 
-hydroxybutyric acid in blood in man in response to exercise.
Eur J Clin Invest
1:
248-254,
1971[Web of Science][Medline].
3.
Christie, WW.
Rapid separation and quantification of lipid classes by high pressure liquid chromatography and mass detection.
J Lipid Res
26:
507-512,
1985[Abstract].
4.
Dagenais, GR,
Tancredi RG,
and
Zierler KL.
Free fatty acid oxidation by forearm muscle at rest, and evidence for an intramuscular lipid pool in the human forearm.
J Clin Invest
58:
421-431,
1976.
5.
Dyck, DJ,
and
Bonen A.
Muscle contraction increases palmitate esterification and oxidation and triacylglycerol oxidation.
Am J Physiol Endocrinol Metab
275:
E888-E896,
1998
6.
Dyck, DJ,
Peters SJ,
Glatz J,
Gorski J,
Keizer H,
Kiens B,
Liu S,
Richter EA,
Spriet LL,
van der Vusse GJ,
and
Bonen A.
Functional differences in lipid metabolism in resting skeletal muscle of various fiber types.
Am J Physiol Endocrinol Metab
272:
E340-E351,
1997
7.
Falholt, K,
Jensen I,
Lindkaer Jensen S,
Mortensen H,
Volund A,
Heding LG,
Noerskov Petersen P,
and
Falholt W.
Carbohydrate and lipid metabolism of skeletal muscle in type 2 diabetic patients.
Diabet Med
5:
27-31,
1988[Web of Science][Medline].
8.
Folch, J,
Lees M,
and
Sloane-Stanley GH.
A simple method for the isolation and purification of total lipids from animal tissues.
J Biol Chem
226:
497-509,
1957
9.
Frayn, KN.
Calculation of substrate oxidation rates in vivo from gaseous exchange.
J Appl Physiol
55:
628-634,
1983
10.
Fromm, H,
and
Hofman AF.
Breath test for arterial bile acid metabolism.
Lancet
2:
621-625,
1971[Web of Science][Medline].
11.
Griffiths, AJ,
Humphreys SM,
Clark ML,
and
Frayn KN.
Forearm substrate utilization during exercise after a meal containing both fat and carbohydrate.
Clin Sci
86:
169-175,
1994[Medline].
12.
Guo, ZK,
and
Jensen MD.
Intramuscular fatty acid metabolism evaluated with stable isotopic tracers.
J Appl Physiol
84:
1674-1679,
1998
13.
Havel, RJ,
Kane JP,
Balasse EO,
Segel N,
and
Basso LV.
Splanchnic metabolism of free fatty acids and production of triglycerides of very low density lipoproteins in normotriglyceridemic and hypertriglyceridemic humans.
J Clin Invest
49:
2017-2035,
1970.
14.
Heiling, VJ,
Miles JM,
and
Jensen MD.
How valid are isotopic measurements of fatty acid oxidation?
Am J Physiol Endocrinol Metab
261:
E572-E577,
1991
15.
Humphreys, SM,
Fisher RM,
and
Frayn KN.
Micromethod for measurement of sub-nanomole amounts of triacylglycerol.
Ann Clin Biochem
27:
597-598,
1990.
16.
Hurley, BF,
Nemeth PM,
Martin WH,
Hagberg JM,
Dalsky GP,
and
Holloszy JO.
Muscle triglyceride utilization during exercise: effect of training.
J Appl Physiol
60:
562-567,
1986
17.
Jacob, S,
Machann J,
Rett K,
Brechtel K,
Volk A,
Renn W,
Maerker E,
Matthaei S,
Schick F,
Claussen CD,
and
Haring HU.
Association of increased intramyocellular lipid content with insulin resistance in lean nondiabetic offspring of type 2 diabetic subjects.
Diabetes
48:
1113-1119,
1999[Abstract].
18.
Jensen, MD,
Braun JS,
Vetter RJ,
and
Marsh HM.
Measurement of body potassium with a whole-body counter: relationship between lean body mass and resting energy expenditure.
Mayo Clin Proc
63:
864-868,
1988[Web of Science][Medline].
19.
Jensen, MD,
Rogers PJ,
Ellman MG,
and
Miles JM.
Choice of infusion-sampling mode for tracer studies of free fatty acid metabolism.
Am J Physiol Endocrinol Metab
254:
E562-E565,
1988
20.
Jones, AL,
Pruitt NL,
Lloyd D,
and
Harwood JL.
Temperature-induced changes in the synthesis of unsaturated fatty acids by Acanthamoeba castellanii.
J Protozool
38:
532-536,
1991.
21.
Kanaley, JA,
Mottram CD,
Scanlon PD,
and
Jensen MD.
Fatty acid kinetic responses to running above or below lactate threshold.
J Appl Physiol
79:
439-447,
1995
22.
Kiens, B,
Essen-Gustavsson B,
Christensen NJ,
and
Saltin B.
Skeletal muscle substrate utilization during submaximal exercise in man: effect of endurance training.
J Physiol (Lond)
469:
459-478,
1993
23.
Koyama, K,
Chen G,
Lee Y,
and
Unger RH.
Tissue triglycerides, insulin resistance, and insulin production: implications for hyperinsulinemia of obesity.
Am J Physiol Endocrinol Metab
273:
E708-E713,
1997.
24.
Krssak, M,
Falk Petersen K,
Dresner A,
DiPietro L,
Vogel SM,
Rothman DL,
Roden M,
and
Shulman GI.
Intramyocellular lipid concentrations are correlated with insulin sensitivity in humans: a 1H NMR spectroscopy study.
Diabetologia
42:
113-116,
1999[Web of Science][Medline].
25.
Martin, WHI
Effect of endurance training on fatty acid metabolism during whole body exercise.
Med Sci Sports Exerc
29:
635-639,
1997[Medline].
26.
Miles, JM,
Ellman MG,
McClean KL,
and
Jensen MD.
Validation of a new method for determination of free fatty acid turnover.
Am J Physiol Endocrinol Metab
252:
E431-E438,
1987
27.
Pan, DA,
Lillioja S,
Kriketos AD,
Milner MR,
Baur LA,
Bogardus C,
Jenkins AB,
and
Storlien LH.
Skeletal muscle triglyceride levels are inversely related to insulin action.
Diabetes
46:
983-988,
1997[Abstract].
28.
Perseghin, G,
Scifo P,
De Cobelli F,
Pagliato E,
Battezzati A,
Arcelloni C,
Vanzulli A,
Testolin G,
Pozza G,
Del Maschio A,
and
Luzi L.
Intramyocellular triglyceride content is a determinant of in vivo insulin resistance in humans: a 1H-13C nuclear magnetic resonance spectroscopy assessment in offspring of type 2 diabetic parents.
Diabetes
48:
1600-1606,
1999[Abstract].
29.
Proctor, DN,
and
Beck KC.
Delay time adjustments to minimize errors in breath-by-breath measurement of O2 during exercise.
J Appl Physiol
81:
2495-2499,
1996
30.
Romijn, JA,
Coyle EF,
Sidossis LS,
Gastaldelli A,
Horowitz JF,
Endert E,
and
Wolfe RR.
Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration.
Am J Physiol Endocrinol Metab
265:
E380-E391,
1993
31.
Schrauwen, P,
Aggel-Leijssen DP,
Lichtenbelt MW,
Baak M,
and
Gijsen AP.
Validation of the [1,2-13C] acetate recovery factor for correction of [U-13C] palmitate oxidation rates in humans.
J Physiol (Lond)
513:
215-223,
1998
32.
Sidossis, LS,
Coggan AR,
Gastaldelli A,
and
Wolfe RR.
A new correlation factor for use in tracer estimations of plasma fatty acid oxidation.
Am J Physiol Endocrinol Metab
269:
E649-E656,
1995
33.
Sidossis, LS,
Mittendorfer B,
Walser E,
Chinkes D,
and
Wolfe RR.
Hyperglycemia-induced inhibition of splanchnic fatty acid oxidation increases hepatic triacylglycerol secretion.
Am J Physiol Endocrinol Metab
275:
E798-E805,
1998
34.
Standl, E,
Lotz N,
Dexel TH,
Janka HU,
and
Kolb HJ.
Muscle triglycerides in diabetic subjects.
Diabetologia
18:
463-469,
1980[Web of Science][Medline].
35.
Stankiewicz-Choroszucha, B,
and
Gorski J.
Effect of decreased availability of substrates on intramuscular triglyceride utilization during exercise.
Eur J Appl Physiol
40:
27-35,
1978.
36.
Storlien, LH,
Jenkins AB,
Chisholm DJ,
Pascoe WS,
Khouri S,
and
Kraegen EW.
Influence of dietary fat composition on development of insulin resistance in rats: relationship to muscle triglyceride and 
-3 fatty acids in muscle phospholipid.
Diabetes
40:
280-289,
1991[Abstract].
37.
Veerkamp, JH,
van Moerkerk TB,
Glatz JF,
Zuurveld JG,
Jacobs AE,
and
Wagenmakers AJ.
14CO2 production is no adequate measure of [14C] fatty acid oxidation.
Biochem Med Metab Biol
35:
248-259,
1986[Web of Science][Medline].
38.
Wendling, PS,
Peters SJ,
Heigenhauser GJF,
and
Spriet LL.
Variability of triacylglycerol content in human skeletal muscle biopsy samples.
J Appl Physiol
81:
1150-1155,
1996
39.
Wolfe, RR,
Shaw JH,
and
Durkot MJ.
Effect of sepsis on VLDL kinetics: responses in basal state and during glucose infusion.
Am J Physiol Endocrinol Metab
248:
E732-E740,
1985
This article has been cited by other articles:
|
|
 |
|
  M. D. Jensen Role of Body Fat Distribution and the Metabolic Complications of Obesity J. Clin. Endocrinol. Metab., November 1, 2008; 93(11_Supplement_1): s57 - s63. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. C. Devries, S. A. Lowther, A. W. Glover, M. J. Hamadeh, and M. A. Tarnopolsky IMCL area density, but not IMCL utilization, is higher in women during moderate-intensity endurance exercise, compared with men Am J Physiol Regulatory Integrative Comp Physiol, December 1, 2007; 293(6): R2336 - R2342. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. A. Wallis, A. L. Friedlander, K. A. Jacobs, M. A. Horning, J. A. Fattor, E. E. Wolfel, G. D. Lopaschuk, and G. A. Brooks Substantial working muscle glycerol turnover during two-legged cycle ergometry Am J Physiol Endocrinol Metab, October 1, 2007; 293(4): E950 - E957. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. P Corcoran, S. Lamon-Fava, and R. A Fielding Skeletal muscle lipid deposition and insulin resistance: effect of dietary fatty acids and exercise Am. J. Clinical Nutrition, March 1, 2007; 85(3): 662 - 677. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. B. Krag, L. C. Gormsen, Z. Guo, J. S. Christiansen, M. D. Jensen, S. Nielsen, and J. O. L. Jorgensen Growth hormone-induced insulin resistance is associated with increased intramyocellular triglyceride content but unaltered VLDL-triglyceride kinetics Am J Physiol Endocrinol Metab, March 1, 2007; 292(3): E920 - E927. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. L. Friedlander, K. A. Jacobs, J. A. Fattor, M. A. Horning, T. A. Hagobian, T. A. Bauer, E. E. Wolfel, and G. A. Brooks Contributions of working muscle to whole body lipid metabolism are altered by exercise intensity and training Am J Physiol Endocrinol Metab, January 1, 2007; 292(1): E107 - E116. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. C. Hannukainen, P. Nuutila, B. Ronald, J. Kaprio, U. M. Kujala, T. Janatuinen, O. J. Heinonen, J. Kapanen, T. Viljanen, M. Haaparanta, et al. Increased physical activity decreases hepatic free fatty acid uptake: a study in human monozygotic twins J. Physiol., January 1, 2007; 578(1): 347 - 358. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Z. Guo, L. Zhou, and M. D. Jensen Acute hyperinsulinemia inhibits intramyocellular triglyceride synthesis in high-fat-fed obese rats J. Lipid Res., December 1, 2006; 47(12): 2640 - 2646. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Koutsari and M. D. Jensen Thematic review series: Patient-Oriented Research. Free fatty acid metabolism in human obesity J. Lipid Res., August 1, 2006; 47(8): 1643 - 1650. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. J. Horton, E. K. Miller, and K. Bourret No effect of menstrual cycle phase on glycerol or palmitate kinetics during 90 min of moderate exercise J Appl Physiol, March 1, 2006; 100(3): 917 - 925. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Qvisth, E. Hagstrom-Toft, S. Enoksson, E. Moberg, P. Arner, and J. Bolinder Human Skeletal Muscle Lipolysis Is More Responsive to Epinephrine Than to Norepinephrine Stimulation in Vivo J. Clin. Endocrinol. Metab., February 1, 2006; 91(2): 665 - 670. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. Kiens Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance Physiol Rev, January 1, 2006; 86(1): 205 - 243. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. C. Devries, M. J. Hamadeh, T. E. Graham, and M. A. Tarnopolsky 17{beta}-Estradiol Supplementation Decreases Glucose Rate of Appearance and Disappearance with No Effect on Glycogen Utilization during Moderate Intensity Exercise in Men J. Clin. Endocrinol. Metab., November 1, 2005; 90(11): 6218 - 6225. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. A. Jacobs, G. A. Casazza, S.-H. Suh, M. A. Horning, and G. A. Brooks Fatty acid reesterification but not oxidation is increased by oral contraceptive use in women J Appl Physiol, May 1, 2005; 98(5): 1720 - 1731. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. J. C. van Loon Use of intramuscular triacylglycerol as a substrate source during exercise in humans J Appl Physiol, October 1, 2004; 97(4): 1170 - 1187. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. Watt, A. G. Holmes, G. R. Steinberg, J. L. Mesa, B. E. Kemp, and M. A. Febbraio Reduced plasma FFA availability increases net triacylglycerol degradation, but not GPAT or HSL activity, in human skeletal muscle Am J Physiol Endocrinol Metab, July 1, 2004; 287(1): E120 - E127. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Z. Guo, M. J. Watt, G. G. F. Heigenhauser, and L. L. Spriet Muscle fat utilization during exercise: controversial only methodologically J Appl Physiol, April 1, 2004; 96(4): 1569 - 1570. [Full Text] [PDF]
|
|
|
|
|
|
S. R. Stannard and N. A. Johnson Insulin resistance and elevated triglyceride in muscle: more important for survival than 'thrifty' genes? J. Physiol., February 1, 2004; 554(3): 595 - 607. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. J C van Loon, R. Koopman, J. H C H Stegen, A. J M Wagenmakers, H. A Keizer, and W. H M Saris Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state J. Physiol., December 1, 2003; 553(2): 611 - 625. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. J. C. van Loon, V. B. Schrauwen-Hinderling, R. Koopman, A. J. M. Wagenmakers, M. K. C. Hesselink, G. Schaart, M. E. Kooi, and W. H. M. Saris Influence of prolonged endurance cycling and recovery diet on intramuscular triglyceride content in trained males Am J Physiol Endocrinol Metab, October 1, 2003; 285(4): E804 - E811. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. E Kimber, G. J F Heigenhauser, L. L Spriet, and D. J Dyck Skeletal muscle fat and carbohydrate metabolism during recovery from glycogen-depleting exercise in humans J. Physiol., May 1, 2003; 548(3): 919 - 927. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Hagstrom-Toft, V. Qvisth, I. Nennesmo, M. Ryden, H. Bolinder, S. Enoksson, J. Bolinder, and P. Arner Marked Heterogeneity of Human Skeletal Muscle Lipolysis at Rest Diabetes, December 1, 2002; 51(12): 3376 - 3383. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. Watt, G. J. F. Heigenhauser, and L. L. Spriet Intramuscular triacylglycerol utilization in human skeletal muscle during exercise: is there a controversy? J Appl Physiol, October 1, 2002; 93(4): 1185 - 1195. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G van Hall, M Sacchetti, G Radegran, and B Saltin Human skeletal muscle fatty acid and glycerol metabolism during rest, exercise and recovery J. Physiol., September 15, 2002; 543(3): 1047 - 1058. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J Watt, G. J F Heigenhauser, D. J Dyck, and L. L Spriet Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man J. Physiol., June 15, 2002; 541(3): 969 - 978. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M Sacchetti, B Saltin, T Osada, and G van Hall Intramuscular fatty acid metabolism in contracting and non-contracting human skeletal muscle J. Physiol., April 1, 2002; 540(1): 387 - 395. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Roepstorff, C. H. Steffensen, M. Madsen, B. Stallknecht, I.-L. Kanstrup, E. A. Richter, and B. Kiens Gender differences in substrate utilization during submaximal exercise in endurance-trained subjects Am J Physiol Endocrinol Metab, February 1, 2002; 282(2): E435 - E447. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. D. Jensen, V. Chandramouli, W. C. Schumann, K. Ekberg, S. F. Previs, S. Gupta, and B. R. Landau Sources of blood glycerol during fasting Am J Physiol Endocrinol Metab, November 1, 2001; 281(5): E998 - E1004. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. D. Jensen, K. Ekberg, and B. R. Landau Lipid metabolism during fasting Am J Physiol Endocrinol Metab, October 1, 2001; 281(4): E789 - E793. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. H. Steffensen, C. Roepstorff, M. Madsen, and B. Kiens Myocellular triacylglycerol breakdown in females but not in males during exercise Am J Physiol Endocrinol Metab, March 1, 2002; 282(3): E634 - E642. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
