Exercise endurance and fuel utilization: a reevaluation of the effects of fasting

The effect of fasting on energy utilization during running or swimming was studied in adult male Wistar rats. Compared with fed rats, fasted animals displayed a decreased contribution of carbohydrates in energy supply, with decreased liver and muscle glycogen contents and decreased rate of glycogen breakdown. This was compensated by an enhanced rate of beta-oxidation. In addition, fasting induced an exaggerated sympathoadrenal response during exercise, reflected by a greater epinephrine plasma level and a higher norepinephrine turnover rate in both liver and soleus. Nevertheless, endurance capacity was similar in fasted and fed animals. These results contrast with the impairment of endurance observed in fasting humans but also with the improvement of endurance in rats previously reported by Dohm et al. (J. Appl. Physiol. 55: 830-833, 1983). These data suggest that the metabolic responses to exercise subsequent to food deprivation depend not only on the considered species but also, in the same species (rat), on the age of the animals and the duration of the fast. These factors probably determine the hormonal secretion and substrate utilization during prolonged exercise in fasting conditions.     

Influence of prolonged endurance cycling and recovery diet on intramuscular triglyceride content in trained males

Intramuscular triglycerides (IMTG) are assumed to form an important substrate source during prolonged endurance exercise in trained males. This study investigated the effects of endurance exercise and recovery diet on IMTG content in vastus lateralis muscle. Nine male cyclists were provided with a standardized diet for 3 days, after which they performed a 3-h exercise trial at a 55% maximum workload. Before and immediately after exercise and after 24 and 48 h of recovery, magnetic resonance spectroscopy (MRS) was performed to quantitate IMTG content. Muscle biopsies were taken after 48 h of recovery to determine IMTG content by using quantitative fluorescence microscopy. The entire procedure was performed two times; in one trial, a normal diet containing 39% energy (En%) as fat was provided (NF) and in the other a typical carbohydrate-rich athlete's diet (LF: 24 En% fat) was provided. During exercise, IMTG content decreased by 21.4 +/- 3.1%. During recovery, IMTG content increased significantly in the NF trial only, reaching preexercise levels within 48 h. In accord with MRS, fluorescence microscopy showed significantly higher IMTG content in the NF compared with the LF trial, with differences restricted to the type I muscle fibers (2.1 +/- 0.2 vs. 1.4 +/- 0.2% area lipid staining, respectively). In conclusion, IMTG content in the vastus lateralis muscle declines significantly during prolonged endurance exercise in male cyclists. When a normal diet is used, IMTG contents are subsequently repleted within 48 h of postexercise recovery. In contrast, IMTG repletion is impaired substantially when a typical, carbohydrate-rich athlete's diet is used. Data obtained by quantitative fluorescence microscopy correspond well with MRS results, implying that both are valid methods to quantify IMTG content.     

Intramyocellular lipid content is increased after exercise in nonexercising human skeletal muscle.

Intramyocellular lipid (IMCL) content has been reported to decrease after prolonged submaximal exercise in active muscle and, therefore, seems to form an important local substrate source. Because exercise leads to a substantial increase in plasma free fatty acid (FFA) availability with a concomitant increase in FFA uptake by muscle tissue, we aimed to investigate potential differences in the net changes in IMCL content between contracting and noncontracting skeletal muscle after prolonged endurance exercise. IMCL content was quantified by magnetic resonance spectroscopy in eight trained cyclists before and after a 3-h cycling protocol (55% maximal energy output) in the exercising vastus lateralis and the nonexercising biceps brachii muscle. Blood samples were taken before and after exercise to determine plasma FFA, glycerol, and triglyceride concentrations, and substrate oxidation was measured with indirect calorimetry. Prolonged endurance exercise resulted in a 20.4 +/- 2.8% (P < 0.001) decrease in IMCL content in the vastus lateralis muscle. In contrast, we observed a substantial (37.9 +/- 9.7%; P < 0.01) increase in IMCL content in the less active biceps brachii muscle. Plasma FFA and glycerol concentrations were substantially increased after exercise (from 85 +/- 6 to 1450 +/- 55 and 57 +/- 11 to 474 +/- 54 microM, respectively; P < 0.001), whereas plasma triglyceride concentrations were decreased (from 1498 +/- 39 to 703 +/- 7 microM; P < 0.001). IMCL is an important substrate source during prolonged moderate-intensity exercise and is substantially decreased in the active vastus lateralis muscle. However, prolonged endurance exercise with its concomitant increase in plasma FFA concentration results in a net increase in IMCL content in less active muscle.     

Antioxidative effect of CLA diet and endurance training in liver and skeletal muscles of rat

The purpose of this study is to investigate the effects of conjugated linoleic acid (CLA) supplementation and endurance exercise on the oxidative/anti-oxidative status in rat liver and skeletal muscles. Sprague-Dawley male rats were randomly divided into HS (high-fat diet sedentary group, n = 8), CS group (CLA supplemented sedentary group, n = 8), and CE group (CLA supplemented exercise group, n = 8). For CLA supplementation, 1.0% CLA was substituted for dietary fat. For endurance exercise, the rats swam for 60 min a day, 5 days a week for 8 weeks. The MDA content, Cu, Zn-SOD and Mn-SOD expression in the soleus muscle (SOM) of the CE group improved significantly compared to the HS (p < 0.01) and CS groups (p < 0.05). Moreover, Mn-SOD expression in both the SOM and extensor digitorum longus muscle (EDL) of the CS were enhanced significantly compared to the HS (p < 0.05). From these results, it was suggested that CLA supplementation under the endurance exercise condition may improve the oxidative status by decreasing the MDA content via potential scavenging of Cu,Zn-SOD, and Mn-SOD protein in red muscle, respectively. Therefore, our study demonstrated long-term endurance exercise with CLA supplementation plays a crucial role for maintenance of antioxidative properties in the skeletal muscle of rat.     

Epinephrine, glucose, and lactate infusion in exercising adrenodemedullated rats

The purpose of this study was to determine the metabolic function of the marked increase in plasma epinephrine which occurs in fasted rats during treadmill exercise. Fasted adrenodemedullated (ADM) and sham-operated (SHAM) rats were run on a rodent treadmill (21 m/min, 15% grade) for 30 min or until exhaustion. ADM rats were infused with saline, epinephrine, glucose, or lactate during the exercise bouts. ADM saline-infused rats showed markedly reduced endurance, hypoglycemia, elevated plasma insulin, reduced blood lactate, and reduced muscle glycogenolysis compared with exercising SHAM's. Epinephrine infusion corrected all deficiencies. Glucose infusion restored endurance run times and blood glucose to normal without correcting the deficiencies in blood lactate and muscle glycogenolysis. Infusion of lactate partially corrected the hypoglycemia at 30 min of exercise, but endurance was not restored to normal and rats were hypoglycemic at exhaustion. We conclude that in the fasted exercising rat, actions of epinephrine in addition to provision of gluconeogenic substrate are essential for preventing hypoglycemia and allowing the rat to run for long periods of time.     

Exercise hyperthermia as a factor limiting physical performance: temperature effect on muscle metabolism

The muscle contents of high-energy phosphates and their derivatives [ATP, ADP, AMP, creatine phosphate (CrP), and creatine], glycogen, some glycolytic intermediates, pyruvate, and lactate were compared in 11 dogs performing prolonged heavy exercise until exhaustion (at ambient temperature 20.0 +/- 1.0 degrees C) without and with trunk cooling using ice packs. Without cooling, dogs were able to run for 57 +/- 8 min, and their rectal (Tre) and muscle (Tm) temperatures increased to 41.8 +/- 0.2 and 43.0 +/- 0.2 degrees C, respectively. Compared with noncooling, duration of exercise with cooling was longer by approximately 45% while Tre and Tm at the time corresponding to the end of exercise without cooling were lower by 1.1 +/- 0.2 and 1.2 +/- 0.2 degrees C, respectively. The muscle contents of high-energy phosphates (ATP + CrP) decreased less, the rate of glycogen depletion was lower, and the increases in the contents of AMP, pyruvate, and lactate as well as in the muscle-to-blood lactate ratio were smaller. The muscle content of lactate was positively correlated with Tm. The data indicate that with higher body temperature equilibrium between high-energy phosphate breakdown and resynthesis was shifted to the lower values of ATP and CrP and glycolysis was accelerated. The results suggest that hyperthermia developing during prolonged muscular work exerts an adverse effect on muscle metabolism that may be relevant to limitation of endurance.     

Acute endurance exercise increases plasma membrane fatty acid transport proteins in rat and human skeletal muscle

Fatty acid transport proteins are present on the plasma membrane and are involved in the uptake of long-chain fatty acids into skeletal muscle. The present study determined whether acute endurance exercise increased the plasma membrane content of fatty acid transport proteins in rat and human skeletal muscle and whether the increase was accompanied by an increase in long-chain fatty acid transport in rat skeletal muscle. Sixteen subjects cycled for 120 min at ～60 ± 2% Vo(2) peak. Two skeletal muscle biopsies were taken at rest and again following cycling. In a parallel study, eight Sprague-Dawley rats ran for 120 min at 20 m/min, whereas eight rats acted as nonrunning controls. Giant sarcolemmal vesicles were prepared, and protein content of FAT/CD36 and FABPpm was measured in human and rat vesicles and whole muscle homogenate. Palmitate uptake was measured in the rat vesicles. In human muscle, plasma membrane FAT/CD36 and FABPpm protein contents increased 75 and 20%, respectively, following 120 min of exercise. In rat muscle, plasma membrane FAT/CD36 and FABPpm increased 20 and 30%, respectively, and correlated with a 30% increase in palmitate transport following 120 min of running. These data suggest that the translocation of FAT/CD36 and FABPpm to the plasma membrane in rat skeletal muscle is related to the increase in fatty acid transport and oxidation that occurs with endurance running. This study is also the first to demonstrate that endurance cycling induces an increase in plasma membrane FAT/CD36 and FABPpm content in human skeletal muscle, which is predicted to increase fatty acid transport.     

Vitamin E deficiency and vitamin C supplements: exercise and mitochondrial oxidation

The effects of dietary antioxidant vitamins E and C on exercise endurance capacity and mitochondrial oxidation were investigated in rats. The endurance capacity of both vitamin E-deficient and vitamin C-supplemented, E-deficient rats was significantly (P less than 0.05) lower (38.1 and 33.6%, respectively) than control animals. Compared with the normal and vitamin E-deficient rats, there was a significant (P less than 0.05) increase in the concentration of vitamin C in blood and liver of the vitamin E-deficient, C-supplemented animals. Hence dietary vitamin C supplementation does not prevent the inhibition of exercise endurance capacity or increased hemolysis seen in vitamin E deficiency. The mitochondrial activities for the oxidation of palmitoyl carnitine and alpha-ketoglutarate were significantly (P less than 0.05) decreased by a single bout of exercise in brown adipose tissue but not in muscle, heart, or liver from vitamin C-supplemented, E-deficient groups of rats when compared with the activities in the tissue from the same group of rats killed at rest. Similar results were also seen in brown adipose tissue from vitamin E-deficient rats. The results suggest a tissue-specific role for vitamins E and C in substrate oxidation and show that the poor endurance capacity of vitamin E-deficient rats cannot be attributed to any changes in the mitochondrial activity in skeletal or cardiac muscles. It is also concluded that vitamin C supplementation, at least at the dose employed in the present study, cannot counteract the detrimental effects associated with vitamin E deficiency.     

Lipid peroxidation and scavenger enzymes during exercise: adaptive response to training

This study was designed to determine whether endurance training would influence the production of lipid peroxidation (LI-POX) by-products as indicated by malondialdehyde (MDA) at rest and after an acute exercise run. Additionally, the scavenger enzymes catalase (CAT) and superoxide dismutase (SOD) were examined to determine whether changes in LIPOX are associated with alterations in enzyme activity both at rest and after exercise. Male Sprague-Dawley rats (n = 32) were randomly assigned to either trained or sedentary groups and were killed either at rest or after 20 min of treadmill running. The training program increased oxidative capacity 64% in leg muscle. After exercise, the sedentary group demonstrated increased LIPOX levels in liver and white skeletal muscle, whereas the endurance-trained group did not show increases in LIPOX after exercise. CAT activity was higher in both red and white muscle after exercise in the trained animals. Total SOD activity was unaffected by either acute or chronic exercise. These data suggest that endurance training can result in a reduction in LIPOX levels as indicated by MDA during moderate-intensity exercise. It is possible that activation of the enzyme catalase and the increase in respiratory capacity were contributory factors responsible for regulating LIPOX after training during exercise.     

Enzymatic down regulation with exercise in rat skeletal muscle

Maximal activities of rat skeletal muscle mitochondrial citrate synthase (CS), malate dehydrogenase (MDH), and alanine aminotransferase (ALT), as well as several other mitochondrial enzymes involved in various metabolic functions were significantly suppressed after a single bout of acute or exhaustive treadmill running. This enzymatic "down regulation" was maintained 24 and 48 h post exhaustion, especially in the untrained rats. Neither muscle cytosolic nor hepatic enzymes exhibited down regulation after exercise. Proteolysis was increased with exercise as assessed by the clearance of [3H]leucine previously incorporated into the proteins of the rats. Decreased CS, MDH, and ALT activities correlated with a significant loss of mitochondrial total protein sulfhydryl (r = 0.67, 0.68, 0.59, respectively, P less than 0.001) in untrained rats and both CS and MDH could be partially restored by incubation with dithiothreitol. Endurance-tested untrained and trained rats had significantly higher glutathione peroxidase (GPX) activity in both muscle mitochondria and cytosol which correlated significantly with endurance time (r = 0.70 and 0.74, respectively). It is concluded that enzymatic down regulation is not caused by proteolysis alone; i.e., peroxides and oxygen free radicals produced in prolonged exercise may alter the intramitochondrial redox state by oxidizing free thiols that may be required at active sites of these enzymes. Training may enhance the ability of the muscle to resist the toxic oxygen species by increasing GPX activity.     

Similar qualitative and quantitative changes of mitochondrial respiration following strength and endurance training in normoxia and hypoxia in sedentary humans

Endurance and strength training are established as distinct exercise modalities, increasing either mitochondrial density or myofibrillar units. Recent research, however, suggests that mitochondrial biogenesis is stimulated by both training modalities. To test the training "specificity" hypothesis, mitochondrial respiration was studied in permeabilized muscle fibers from 25 sedentary adults after endurance (ET) or strength training (ST) in normoxia or hypoxia [fraction of inspired oxygen (Fi(O(2))) = 21% or 13.5%]. Biopsies were taken from the musculus vastus lateralis, and cycle-ergometric incremental maximum oxygen uptake (VO(2max)) exercise tests were performed under normoxia, before and after the 10-wk training program. The main finding was a significant increase (P < 0.05) of fatty acid oxidation capacity per muscle mass, after endurance and strength training under normoxia [2.6- and 2.4-fold for endurance training normoxia group (ET(N)) and strength training normoxia group (ST(N)); n = 8 and 3] and hypoxia [2.0-fold for the endurance training hypoxia group (ET(H)) and strength training hypoxia group (ST(H)); n = 7 and 7], and higher coupling control of oxidative phosphorylation. The enhanced lipid oxidative phosphorylation (OXPHOS) capacity was mainly (87%) due to qualitative mitochondrial changes increasing the relative capacity for fatty acid oxidation (P < 0.01). Mitochondrial tissue-density contributed to a smaller extent (13%), reflected by the gain in muscle mass-specific respiratory capacity with a physiological substrate cocktail (glutamate, malate, succinate, and octanoylcarnitine). No significant increase was observed in mitochondrial DNA (mtDNA) content. Physiological OXPHOS capacity increased significantly in ET(N) (P < 0.01), with the same trend in ET(H) and ST(H) (P < 0.1). The limitation of flux by the phosphorylation system was diminished after training. Importantly, key mitochondrial adaptations were similar after endurance and strength training, regardless of normoxic or hypoxic exercise. The transition from a sedentary to an active lifestyle induced muscular changes of mitochondrial quality representative of mitochondrial health.     

Endurance training stimulates growth and survival pathways and the redox balance in rat pancreatic islets

Endurance training has been shown to increase pancreatic β-cell function and mass. However, whether exercise modulates β-cell growth and survival pathways signaling is not completely understood. This study investigated the effects of exercise on growth and apoptotic markers levels in rat pancreatic islets. Male Wistar rats were randomly assigned to 8-wk endurance training or to a sedentary control group. After that, pancreatic islets were isolated; gene expression and the total content and phosphorylation of several proteins related to growth and apoptotic pathways as well as the main antioxidant enzymes were determined by real-time polymerase chain reaction and Western blot analysis, respectively. Reactive oxygen species (ROS) production was measured by fluorescence. Endurance training increased the time to reach fatigue by 50%. Endurance training resulted in increased protein phosphorylation content of AKT (75%), AKT substrate (AS160; 100%), mTOR (60%), p70s6k (90%), and ERK1/2 (50%), compared with islets from control group. Catalase protein content was 50% higher, whereas ROS production was 49 and 77% lower in islets from trained rats under basal and stimulating glucose conditions, respectively. Bcl-2 mRNA and protein levels increased by 46 and 100%, respectively. Bax and cleaved caspase-3 protein contents were reduced by 25 and 50% in islets from trained rats, respectively. In conclusion, these results demonstrate that endurance training favors the β-cell growth and survival by activating AKT and ERK1/2 pathways, enhancing antioxidant capacity, and reducing ROS production and apoptotic proteins content.     

Leucine supplementation does not affect protein turnover and impairs the beneficial effects of endurance training on glucose homeostasis in healthy mice

Endurance exercise training as well as leucine supplementation modulates glucose homeostasis and protein turnover in mammals. Here, we analyze whether leucine supplementation alters the effects of endurance exercise on these parameters in healthy mice. Mice were distributed into sedentary (C) and exercise (T) groups. The exercise group performed a 12-week swimming protocol. Half of the C and T mice, designated as the CL and TL groups, were supplemented with leucine (1.5 % dissolved in the drinking water) throughout the experiment. As well known, endurance exercise training reduced body weight and the retroperitoneal fat pad, increased soleus mass, increased VO_2max, decreased muscle proteolysis, and ameliorated peripheral insulin sensitivity. Leucine supplementation had no effect on any of these parameters and worsened glucose tolerance in both CL and TL mice. In the soleus muscle of the T group, AS-160^Thr-642 (AKT substrate of 160 kDa) and AMPK^Thr-172 (AMP-Activated Protein Kinase) phosphorylation was increased by exercise in both basal and insulin-stimulated conditions, but it was reduced in TL mice with insulin stimulation compared with the T group. Akt phosphorylation was not affected by exercise but was lower in the CL group compared with the other groups. Leucine supplementation increased mTOR phosphorylation at basal conditions, whereas exercise reduced it in the presence of insulin, despite no alterations in protein synthesis. In trained groups, the total FoxO3a protein content and the mRNA for the specific isoforms E2 and E3 ligases were reduced. In conclusion, leucine supplementation did not potentiate the effects of endurance training on protein turnover, and it also reduced its positive effects on glucose homeostasis.     

Effect of endurance exercise training on muscle glycogen supercompensation in rats

Nakatani, Akira, Dong-Ho Han, Polly A. Hansen, Lorraine A. Nolte, Helen H. Host, Robert C. Hickner, and John O. Holloszy. Effect of endurance exercise training on muscle glycogensupercompensation in rats. J. Appl.Physiol. 82(2): 711-715, 1997.The purpose of this study was to test the hypothesis that the rate and extent ofglycogen supercompensation in skeletal muscle are increased byendurance exercise training. Rats were trained by using a 5-wk-long swimming program in which the duration of swimming was gradually increased to 6 h/day over 3 wk and then maintained at 6 h/day for anadditional 2 wk. Glycogen repletion was measured in trained anduntrained rats after a glycogen-depleting bout of exercise. The ratswere given a rodent chow diet plus 5% sucrose in their drinking waterad libitum during the recovery period. There were remarkabledifferences in both the rates of glycogen accumulation and the glycogenconcentrations attained in the two groups. The concentration ofglycogen in epitrochlearis muscle averaged 13.1 ± 0.9 mg/g wet wtin the untrained group and 31.7 ± 2.7 mg/g in the trained group(P < 0.001) 24 h after the exercise.This difference could not be explained by a training effect on glycogensynthase. The training induced ~50% increases in muscle GLUT-4glucose transporter protein and in hexokinase activity inepitrochlearis muscles. We conclude that endurance exercise trainingresults in increases in both the rate and magnitude of muscle glycogensupercompensation in rats.

     

Effect of ginger extract ingestion on skeletal muscle glycogen contents and endurance exercise in male rats

[Purpose]Skeletal muscle glycogen is a determinant of endurance capacity for some athletes. Ginger is well known to possess nutritional effects, such as anti-diabetic effects. We hypothesized that ginger extract (GE) ingestion increases skeletal muscle glycogen by enhancing fat oxidation. Thus, we investigated the effect of GE ingestion on exercise capacity, skeletal muscle glycogen, and certain blood metabolites in exercised rats. [Methods]First, we evaluated the influence of GE ingestion on body weight and elevation of exercise performance in rats fed with different volumes of GE. Next, we measured the skeletal muscle glycogen content and free fatty acid (FFA) levels in GE-fed rats. Finally, we demonstrated that GE ingestion contributes to endurance capacity during intermittent exercise to exhaustion. [Results]We confirmed that GE ingestion increased exercise performance (p<0.05) and elevated the skeletal muscle glycogen content compared to the non-GE-fed (CE, control exercise) group before exercise (Soleus: p<0.01, Plantaris: p<0.01, Gastrocnemius: p<0.05). Blood FFA levels in the GE group were significantly higher than those in the CE group after exercise (p<0.05). Moreover, we demonstrated that exercise capacity was maintained in the CE group during intermittent exercise (p<0.05). [Conclusion]These findings indicate that GE ingestion increases skeletal muscle glycogen content and exercise performance through the upregulation of fat oxidation.     

Myogenin and oxidative enzyme gene expression levels are elevated in rat soleus muscles after endurance training.

The intent of this study was to determine whether endurance exercise training regulates increases in metabolic enzymes, which parallel modulations of myogenin and MyoD in skeletal muscle of rats. Adult Sprague-Dawley rats were endurance trained (TR) 5 days weekly for 8 wk on a motorized treadmill. They were killed 48 h after their last bout of exercise. Sedentary control (Con) rats were killed at the same time as TR animals. Myogenin, MyoD, citrate synthase (CS), cytochrome-c oxidase (COX) subunits II and VI, lactate dehydrogenase (LDH), and myosin light chain mRNA contents were determined in soleus muscles by using RT-PCR. Myogenin mRNA content was also estimated by using dot-blot hybridization. Protein expression levels of myogenin and MyoD were measured by Western blots. CS enzymatic activity was also measured. RT-PCR measurements showed that the mRNA contents of myogenin, CS, COX II, COX VI, and LDH were 25, 20, 17, 16, and 18% greater, respectively, in TR animals compared with Con animals (P < 0.05). The ratio of myogenin to MyoD mRNA content estimated by RT-PCR in TR animals was 28% higher than that in Con animals (P < 0.05). Myosin light chain expression was similar in Con and TR muscles. Results from dot-blot hybridization to a riboprobe further confirmed the increase in myogenin mRNA level in TR group. Western blot analysis indicated a 24% greater level of myogenin protein in TR animals compared with Con animals (P < 0.01). The soleus muscles from TR animals had a 25% greater CS enzymatic activity than the Con animals (P < 0.01). Moreover, myogenin mRNA and protein contents were positively correlated to CS activity and mRNA contents of CS, COX II, and COX VI (P < 0.05). These data are consistent with the hypothesis that myogenin is in the pathway for exercise-induced changes in mitochondrial enzymes.     

Changes in tissue protein levels as a result of endurance exercise

We previously reported that endurance training increases amino acid catabolism. In this study, the effects of an acute endurance exercise bout on tissue protein levels and urea excretion have been investigated. Exhaustive exercising of trained rats resulted in an increase in ammonia excretion but there was no significant change in urea excretion. Protein levels of muscle and liver were significantly decreased by an exhaustive bout of exercise. In muscle, both the soluble and myofibrillar protein fractions were decreased in exhausted rats. These results demonstrate that during exercise there is a net loss of protein in muscle and liver.     

IGF-I has no effect on postexercise suppression of the ubiquitin-proteasome system in rat skeletal muscle.

Both exercise and insulin-like growth factor I (IGF-I) are known to have major hypertrophic effects in skeletal muscle; however, the interactive effect of exogenous IGF-I and exercise on muscle protein turnover or the ubiquitin-proteasome pathway has not been reported. In the present study, we have examined the interaction between endurance exercise training and IGF-I treatment on muscle protein turnover and the ubiquitin-proteasome pathway in the postexercise period. Adult male rats (270-280 g) were randomized to receive 5 consecutive days of progressive treadmill exercise and/or IGF-I treatment (1 mg. kg body wt(-1). day(-1)). Twenty-four hours after the last bout of exercise, the rate of protein breakdown in incubated muscles was significantly reduced compared with that in unexercised rats. This was associated with a significant reduction in the chymotrypsin-like activity of the proteasome and the rate of ubiquitin-proteasome-dependent casein hydrolysis in muscle extracts from exercised compared with unexercised rats. In contrast, the muscle expression of the 20S proteasome subunit beta-1, ubiquitin, and the 14-kDa E2 ubiquitin-conjugating enzyme was not altered by exercise or IGF-I treatment 24 h postexercise. Exercise had no effect on the rates of total mixed muscle protein synthesis in incubated muscles 24 h postexercise. IGF-I treatment had no effect on muscle weights or the rates of protein turnover 24 h after endurance exercise. These results suggest that a suppression of the ubiquitin-proteasome proteolytic pathway after endurance exercise may contribute to the acute postexercise net protein gain.     

Effects of endurance exercise-training on single-fiber contractile properties of insulin-treated streptozotocin-induced diabetic rats.

The purpose of this study was to characterize the contractile properties of individual skinned muscle fibers from insulin-treated streptozotocin-induced diabetic rats after an endurance exercise training program. We hypothesized that single-fiber contractile function would decrease in the diabetic sedentary rats and that endurance exercise would preserve the function. In the study, 28 rats were assigned to either a nondiabetic sedentary, a nondiabetic exercise, a diabetic sedentary, or a diabetic exercise group. Rats in the diabetic groups received subcutaneous intermediate-lasting insulin daily. The exercise-trained rats ran on a treadmill at a moderate intensity for 60 min, five times per week. After 12 wk, the extensor digitorum longus and soleus muscles were dissected. Single-fiber diameter, Ca(2+)-activated peak force, specific tension, activation threshold, and pCa(50) as well as the myosin heavy chain isoform expression (MHC) were determined. We found that in MHC type II fibers from extensor digitorum longus muscle, diameters were significantly smaller from diabetic sedentary rats compared with nondiabetic sedentary rats (P < 0.001). Among the nondiabetic rats, fiber diameters were smaller with exercise (P = 0.038). The absolute force-generating capacity of single fibers was lower in muscles from diabetic rats. There was greater specific tension (force normalized to cross-sectional area) by fibers from the rats that followed an endurance exercise program compared with sedentary. From the results, we conclude that alterations in the properties of contractile proteins are not implicated in the decrease in strength associated with diabetes and that endurance-exercise training does not prevent or increase muscle weakness in diabetic rats.     

Habitual exercise enhances neuromuscular transmission efficacy of rat soleus muscle in situ.

Rat motor nerve terminals and the endplates they interact with exhibit changes to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neuromuscular transmission in rat soleus muscle were investigated using a novel in situ approach. Consistent with previous reports, miniature endplate potential frequency was enhanced by habitual exercise. Other passive properties, such as resting membrane potential, miniature endplate potential amplitude, and "giant" miniature endplate potential characteristics were unaltered by the training program. Full-size endplate potentials were obtained by blocking soleus muscle action potentials with mu-conotoxin GIIIb. Quantal content values were 91.5 and 119.9 for control and active groups, respectively (P < 0.01). We also measured the rate and extent of endplate potential amplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10--20%) in a specific population of cells from active rats (P < 0.05). The results establish the degree of activity-dependent plasticity as it pertains to neuromuscular transmission in a mammalian slow-twitch muscle.