Effect of hindlimb unweighting on anaerobic metabolism in rat skeletal muscle.

Female Sprague-Dawley rats (250 g) were hindlimb suspended for 14 days, and the effects of hindlimb unweighting (HU) on skeletal muscle anaerobic metabolism were investigated and compared with nonsuspended controls (C). Soleus (SOL), plantaris (PL), and red and white portions of the gastrocnemius (RG, WG) were sampled from resting and stimulated limbs. Muscle atrophy after HU was 46% in SOL, 22% in PL, and 24% in the gastrocnemius compared with nonsuspended C animals. The muscles innervated by the sciatic nerve were stimulated to contract with an occluded circulation for 60 s with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1.0 Hz. Peak tension development by the gastrocnemius-PL-SOL muscle group was similar in HU and C animals (13.0 +/- 1.2, 12.2 +/- 0.8 N/g wet muscle). Occlusion of the circulation before stimulation created a predominantly anaerobic environment, and in situ glycogenolysis and glycolysis were estimated from accumulations of glycolytic intermediates. Total glycogenolysis and glycolysis were higher in the RG muscle of HU animals (74.6 +/- 3.3, 58.1 +/- 1.1) relative to C (57.1 +/- 4.6, 46.1 +/- 2.9 mumol glucosyl units/g dry muscle). Consequently, total anaerobic ATP production was also increased (HU, 251.3 +/- 1.1; C, 204.6 +/- 8.9 mumol ATP/g dry muscle). Total ATP production, glycogenolysis, and glycolysis were unaffected by HU in SOL, PL, and WG muscles. The enhanced glycolytic activity in RG after HU may be attributed to a shift in the metabolic profile from oxidative to glycolytic in the fast oxidative-glycolytic fiber population.     

Effect of work and recovery duration on skeletal muscle oxygenation and fuel use during sustained intermittent exercise

The purpose of this study was to compare rates of substrate oxidation in two protocols of intermittent exercise, with identical treadmill speed and total work duration, to reduce the effect of differences in factors such as muscle fibre type activation, hormonal responses, muscle glucose uptake and non-esterified fatty acid (NEFA) availability on the comparison of substrate utilisation. Subjects (n = 7) completed 40 min of intermittent intense running requiring a work:recovery ratio of either 6 s:9 s (short-interval exercise, SE) or 24 s:36 s (long-interval exercise, LE), on separate days. Another experiment compared O(2) availability in the vastus lateralis muscle across SE (10 min) and LE (10 min) exercise using near-infrared spectroscopy (RunMan, NIM. Philadelphia, USA). Overall (i.e. work and recovery) O(2) consumption (VO(2)) and energy expenditure were lower during LE (P < 0.01, P < 0.05, respectively). Overall exercise intensity, represented as a proportion of peak aerobic power (VO2(peak)), was [mean (SEM)] 64.9 (2.7)% VO2(peak) (LE) and 71.4 (2.4)% VO2(peak) (SE). Fat oxidation was three times lower (P < 0.01) and carbohydrate oxidation 1.3 times higher (P < 0. 01) during LE, despite the lower overall exercise intensity. Plasma lactate was constant and was higher throughout exercise in LE [mean (SEM) 5.33 (0.53) mM, LE; 3.28 (0.31) mM, SE; P < 0.001)]. Plasma pyruvate was higher and glycerol was lower in LE [215 (17) microM, 151 (13) microM, P < 0.05, pyruvate; 197 (19) microM, 246 (19) microM, P < 0.05, glycerol]. There was no difference between protocols for plasma NEFA concentration (n = 4) or plasma noradrenaline and adrenaline. Muscle oxygenation declined in both protocols (P < 0.001), but the nadir during LE was lower [52.04 (0. 60)%] compared to SE [61.85 (0.51)%; P < 0.001]. The decline in muscle oxygenation during work was correlated with mean lactate concentration (r = 0.68; P < 0.05; n = 12). Lower levels of fat oxidation occurred concurrent with accelerated carbohydrate metabolism, increases in lactate and pyruvate and reduced muscle O(2) availability. These changes were associated with proportionately longer work and recovery periods, despite identical treadmill speed and total work duration. The proposal that a metabolic regulatory factor within the muscle fibre retards fat oxidation under these conditions is supported by the current findings.     

Effect of induced metabolic alkalosis on human skeletal muscle metabolism during exercise

The purpose of the study was to examine the roles of active pyruvate dehydrogenase (PDH(a)), glycogen phosphorylase (Phos), and their regulators in lactate (Lac(-)) metabolism during incremental exercise after ingestion of 0.3 g/kg of either NaHCO(3) [metabolic alkalosis (ALK)] or CaCO(3) [control (CON)]. Subjects (n = 8) were studied at rest, rest postingestion, and during constant rate cycling at three stages (15 min each): 30, 60, 75% of maximal O(2) uptake (VO(2 max)). Radial artery and femoral venous blood samples, leg blood flow, and biopsies of the vastus lateralis were obtained during each power output. ALK resulted in significantly (P < 0.05) higher intramuscular Lac(-) concentration ([Lac(-)]; ALK 72.8 vs. CON 65.2 mmol/kg dry wt), arterial whole blood [Lac(-)] (ALK 8.7 vs. CON 7.0 mmol/l), and leg Lac(-) efflux (ALK 10.0 vs. CON 4.2 mmol/min) at 75% VO(2 max). The increased intramuscular [Lac(-)] resulted from increased pyruvate production due to stimulation of glycogenolysis at the level of Phos a and phosphofructokinase due to allosteric regulation mediated by increased free ADP (ADP(f)), free AMP (AMP(f)), and free P(i) concentrations. PDH(a) increased with ALK at 60% VO(2 max) but was similar to CON at 75% VO(2 max). The increased PDH(a) may have resulted from alterations in the acetyl-CoA, ADP(f), pyruvate, NADH, and H(+) concentrations leading to a lower relative activity of PDH kinase, whereas the similar values at 75% VO(2 max) may have reflected maximal activation. The results demonstrate that imposed metabolic alkalosis in skeletal muscle results in acceleration of glycogenolysis at the level of Phos relative to maximal PDH activation, resulting in a mismatch between the rates of pyruvate production and oxidation resulting in an increase in Lac(-) production.     

Early activation and redistribution of calpain activity in skeletal muscle during hindlimb unweighting and reweighting

The aims of this study were the following: (i) to determine whether activation of the Ca2+-activated protease, calpain, is an early event during hindlimb unweighting (HU) in skeletal muscle; and (ii) to assess whether calpain activity is greater during reweighting compared with HU alone. Rats were exposed to 12, 24, and 72 h, or 9 d of HU, followed by reweighting for 0, 12, or 24 h. Calpain activities were assayed for total, soluble, and particulate fractions. Total calpain activity was increased in the soleus at all HU time points, whereas activities were elevated in the gastrocnemius only after 9 d of HU. With reweighting, calpain activity remained elevated at all time points for both muscles. In general, reweighting the gastrocnemius increased its calpain activity more than during HU only, whereas reweighting the soleus did not produce additional increases in its calpain activity. The increases in calpain activity were associated with a proportional increase in activity of the particulate (membrane- and protein-associated) fraction. The results suggest that calpain activation is an early event during HU in the soleus, and that the increases in calpain activity in both muscles are associated with a redistribution of activity from cytosolic to particulate fractions.     

Autonomic control of skeletal muscle vasodilation during exercise

Buckwalter, John B., Patrick J. Mueller, and Philip S. Clifford. Autonomic control of skeletal muscle vasodilation duringexercise. J. Appl. Physiol. 83(6):2037-2042, 1997.Despite extensive investigation, the control ofblood flow during dynamic exercise is not fully understood. The purposeof this study was to determine whether -adrenergic or muscarinicreceptors are involved in the vasodilation in exercising skeletalmuscle. Six mongrel dogs were instrumented with ultrasonic flow probeson both external iliac arteries and with a catheter in a branch of onefemoral artery. The dogs exercised on a treadmill at 6 miles/h whiledrugs were injected intra-arterially into one hindlimb. Isoproterenol(0.2 µg) or acetylcholine (1 µg) elicited increases in iliac bloodflow of 89.8 ± 14.4 and 95.6 ± 17.4%, respectively, withoutaffecting systemic blood pressure or blood flow in the contralateraliliac artery. Intra-arterial propranolol (1 mg) or atropine (500 µg)had no effect on iliac blood flow, although they abolished theisoproterenol and acetylcholine-induced increases in iliac blood flow.These data indicate that exogenous activation of -adrenergic ormuscarinic receptors in the hindlimb vasculature increases blood flowto dynamically exercising muscle. More importantly, because neitherpropranolol nor atropine affected iliac blood flow, we conclude that-adrenergic and muscarinic receptors are not involved in the controlof blood flow to skeletal muscle during moderate steady-state dynamicexercise in dogs.

     

Effect of ambient temperature on human skeletal muscle metabolism during fatiguing submaximal exercise.

To examine the effect of ambient temperature on metabolism during fatiguing submaximal exercise, eight men cycled to exhaustion at a workload requiring 70% peak pulmonary oxygen uptake on three separate occasions, at least 1 wk apart. These trials were conducted in ambient temperatures of 3 degrees C (CT), 20 degrees C (NT), and 40 degrees C (HT). Although no differences in muscle or rectal temperature were observed before exercise, both muscle and rectal temperature were higher (P < 0.05) at fatigue in HT compared with CT and NT. Exercise time was longer in CT compared with NT, which, in turn, was longer compared with HT (85 +/- 8 vs. 60 +/- 11 vs. 30 +/- 3 min, respectively; P < 0.05). Plasma epinephrine concentration was not different at rest or at the point of fatigue when the three trials were compared, but concentrations of this hormone were higher (P < 0.05) when HT was compared with NT, which in turn was higher (P < 0.05) compared with CT after 20 min of exercise. Muscle glycogen concentration was not different at rest when the three trials were compared but was higher at fatigue in HT compared with NT and CT, which were not different (299 +/- 33 vs. 153 +/- 27 and 116 +/- 28 mmol/kg dry wt, respectively; P < 0.01). Intramuscular lactate concentration was not different at rest when the three trials were compared but was higher (P < 0.05) at fatigue in HT compared with CT. No differences in the concentration of the total intramuscular adenine nucleotide pool (ATP + ADP + AMP), phosphocreatine, or creatine were observed before or after exercise when the trials were compared. Although intramuscular IMP concentrations were not statistically different before or after exercise when the three trials were compared, there was an exercise-induced increase (P < 0.01) in IMP. These results demonstrate that fatigue during prolonged exercise in hot conditions is not related to carbohydrate availability. Furthermore, the increased endurance in CT compared with NT is probably due to a reduced glycogenolytic rate.     

Effect of body temperature during exercise on skeletal muscle cytochrome c oxidase content.

This study determined the role of body temperature during exercise on cytochrome-c oxidase (CytOx) activity, a marker of mitochondrial content, and mitochondrial heat shock protein 70 (mtHSP70), which is required for import of nuclear-coded preproteins. Male, 10-wk-old, Sprague-Dawley rats exercised identically for 9 wk in ambient temperatures of 23 degrees C (n = 10), 8 degrees C with wetted fur (n = 8), and 4 degrees C with wetted fur and fan (n = 7). These conditions maintained exercising core temperature (T(c)) at 40.4, 39.2, or 38.0 degrees C (resting temperature), respectively. During weeks 3-9, exercisers ran 5 days/wk up a 6% grade at 20 m/min for 60 min. Animals were housed at 23 degrees C. Gastrocnemius CytOx activity in T(c)=38.0 degrees C (83.5 +/- 5.5 microatoms O x min(-1) x g wet wt(-1)) was greater than all other groups (P < 0.05), exceeding sedentary (n = 7) by 73.2%. T(c) of 40.4 and 39.2 degrees C also were higher than sedentary by 22.4 and 37.4%, respectively (P < 0.05). Quantification of CytOx content verified that the increased activity was due to an increase in protein content. In extensor digitorum longus, a nonactive muscle, CytOx was not elevated in T(c) = 38.0 degrees C. mtHSP70 was significantly elevated in gastrocnemius of T(c) = 38.0 degrees C compared with sedentary (P < 0.05) but was not elevated in extensor digitorum longus (P > 0.05). The data indicate that decreasing exercise T(c) may enhance mitochondrial biogenesis and that mtHSP70 expression is not dependent on temperature.     

Effect of exercise on insulin action in human skeletal muscle

The effect of 1 h of dynamic one-legged exercise on insulin action in human muscle was studied in 6 healthy young men. Four hours after one-legged knee extensions, a three-step sequential euglycemic hyperinsulinemic clamp combined with arterial and bilateral femoral vein catheterization was performed. Increased insulin action on glucose uptake was found in the exercised compared with the rested thigh at mean plasma insulin concentrations of 23, 40, and 410 microU/ml. Furthermore, prior contractions directed glucose uptake toward glycogen synthesis and increased insulin effects on thigh O2 consumption and at some insulin concentrations on potassium exchange. In contrast, no change in insulin effects on limb exchange of free fatty acids, glycerol, alanine or tyrosine were found after exercise. Glycogen concentration in rested vastus lateralis muscle did not increase measurably during the clamp even though indirect estimates indicated net glycogen synthesis. In contrast, in exercised muscle estimated and biopsy-verified increases in muscle glycogen concentration agreed. Local contraction-induced increases in insulin sensitivity and responsiveness play an important role in postexercise recovery of human skeletal muscle.     

Effect of temperature on skeletal muscle energy turnover during dynamic knee-extensor exercise in humans.

The present study examined the effect of elevated temperature on muscle energy turnover during dynamic exercise. Nine male subjects performed 10 min of dynamic knee-extensor exercise at an intensity of 43 W (SD 10) and a frequency of 60 contractions per minute. Exercise was performed under normal (C) and elevated muscle temperature (HT) through passive heating. Thigh oxygen uptake (V(O2)) was determined from measurements of thigh blood flow and femoral arterial-venous differences for oxygen content. Anaerobic energy turnover was estimated from measurements of lactate release as well as muscle lactate accumulation and phosphocreatine utilization based on analysis of muscle biopsies obtained before and after each exercise. At the start of exercise, muscle temperature was 34.5 degrees C (SD 1.7) in C compared with 37.2 degrees C (SD 0.5) during HT (P < 0.05). Thigh V(O2) after 3 min was 0.52 l/min (SD 0.11) in C and 0.63 l/min (SD 0.13) in HT, and at the end of exercise it was 0.60 l/min (SD 0.14) and 0.61 l/min (SD 0.10) in C and HT, respectively (not significant). Total lactate release was the same between the two temperature conditions, as was muscle lactate accumulation and PCr utilization. Total ATP production (aerobic + anaerobic) was the same between each temperature condition [505.0 mmol/kg (SD 107.2) vs. 527.1 mmol/kg (SD 117.6); C and HT, respectively]. In conclusion, within the range of temperatures studied, passively increasing muscle temperature before exercise has no effect on muscle energy turnover during dynamic exercise.     

Physical performance and skeletal muscle characteristics after 2-week hind-limb unweighting

It is well-known that 2 weeks of hind-limb suspension or space flight induce the sufficient decrease of the physical performance and simultaneously changes of muscle contractile properties and fiber size. However, the data on enzyme activities changes at present are contradictory. Numerous authors have pointed to the increase, reduction of its activity as well as its stability after experiments of the similar design. In previous studies it was shown that beta-GPA (beta-guanidino-propionic acid) administration increased the oxidative enzyme activities in the skeletal muscles and improved their contractile properties in hind-limb suspended rats. The aim of our study is to clear out what determines changes of the physical performance after 2 weeks of hind-limb suspension and beta-GPA administration.     

Effect of sodium citrate on performance and metabolism of human skeletal muscle during supramaximal cycling exercise

The aim of this study was to examine whether the alkalosis-induced improvement in supramaximal performance could be explained by a less-altered muscle metabolic status. Eight subjects first performed exhausting exercise at 120% peak oxygen uptake after ingesting either a placebo (PLC) or sodium citrate (CIT) at a dose of 0.5 g · kg⁻¹ body mass to determine exhaustion time (t _exh). They then, performed exercise (Lim-EX) at the same relative intensity lasting PLCt _exh minus 20 s in both treatments. Samples were taken from vastus lateralis muscle at rest (90-min after the ingestion) and at the end of Lim-EX. Arterial blood samples were obtained at rest (immediately prior to and 90 min after ingesting the drug) and during the 20-min post-exercise recovery. The t _exh was significantly increased by CIT [PLC 258 (SD 29) s, CIT 297 (SD 45) s]. The CIT raised the rest [citrate] in blood [PLC 0.11 (SD 0.01) mmol · l⁻¹, CIT 0.34 (SD 0.07) mmol · l⁻¹] and in muscle [PLC 0.78 (SD 0.23) mmol · kg⁻¹ dry mass, CIT 1.00 (SD 0.21) mmol · kg⁻¹ dry mass]. Resting muscle pH and buffering capacity were unchanged by CIT. The same fall in muscle pH was observed during Lim-EX in the two conditions. This was associated with similar variations in both the cardio-respiratory response and muscle energy and metabolism status in spite of a better blood acid-base status after CIT. Thus, CIT would not seem to allow the alkalinization of the muscle cytosolic compartment. Though sodium citrate works in a similar way to NaHCO₃ on plasma alkalinization and exercise performance, the exact nature of the mechanisms involved in the delay of exhaustion could be different and remains to be elucidated. Accepted: 26 November 1996     

Effect of exercise and obesity on skeletal muscle amino acid uptake

The genetically obese Zucker rat has a reduced capacity to deposit dietary protein in skeletal muscle. To determine whether amino acid uptake by muscle of obese Zucker rats is impaired, soleus strip (SOL) and epitrochlearis (EPI) muscles from 10-wk-old lean and obese Zucker rats were studied in vitro by use of [14C]alpha-aminoisobutyric acid (AIB). Muscles from fasted rats were incubated under basal conditions at rest or after a 1-h treadmill run at 8% grade. To equate total work completed, lean and obese rats ran at 27 and 20 m/min, respectively. Muscles were pinned at resting length, preincubated for 30 min at 37 degrees C in Krebs-Ringer bicarbonate buffer containing 5 mM glucose under 95% O2-5% CO2, and then incubated up to 3 h in Krebs-Ringer bicarbonate with 0.5 mM AIB, [14C]AIB, and [3H]inulin as a marker of extracellular fluid. Basal AIB uptake in EPI and SOL from obese rats was significantly reduced by 40 and 30% (P less than 0.01), respectively, compared with lean rats. For both lean and obese rats, exercise increased (P less than 0.05) basal AIB uptake in EPI and SOL, but the relative increases were greater in the obese rats (EPI 54% and SOL 71% vs. EPI 32% and SOL 37%). These results demonstrate that genetically obese Zucker rats have reduced basal skeletal muscle amino acid uptake and suggest that physical inactivity may partially contribute to this defect.     

Effect of exercise intensity on skeletal muscle malonyl-CoA and acetyl-CoA carboxylase

Rasmussen, B. B., and W. W. Winder. Effectof exercise intensity on skeletal muscle malonyl-CoA and acetyl-CoAcarboxylase. J. Appl. Physiol. 83(4):1104-1109, 1997.Malonyl-CoA is synthesized by acetyl-CoAcarboxylase (ACC) and is an inhibitor of fatty acid oxidation. Exerciseinduces a decline in skeletal muscle malonyl-CoA, which is accompaniedby inactivation of ACC and increased activity of AMP-activated proteinkinase (AMPK). This study was designed to determine the effect ofexercise intensity on the enzyme kinetics of ACC, malonyl-CoA levels,and AMPK activity in skeletal muscle. Male Sprague-Dawley rats werekilled (pentobarbital sodium anesthesia) at rest or after 5 min ofexercise (10, 20, 30, or 40 m/min at 5% grade). The fast-twitch redand white regions of the quadriceps muscle were excised and frozen inliquid nitrogen. A progressive decrease in red quadriceps ACC maximalvelocity (from 28.6 ± 1.5 to 14.3 ± 0.7 nmol · g¹ · min¹,P < 0.05), an increase in activationconstant for citrate, and a decrease in malonyl-CoA (from 1.9 ± 0.2 to 0.9 ± 0.1 nmol/g, P < 0.05) were seen with theincrease in exercise intensity from rest to 40 m/min. AMPK activityincreased more than twofold. White quadriceps ACC activity decreasedonly during intense exercise. We conclude that the extent of ACCinactivation during short-term exercise is dependent on exerciseintensity.