Regulation of glycogen metabolism in rat respiratory muscles during exercise

The effect of prolonged exercise on the glycogen level in the respiratory muscles (diaphragm--D, external intercostal--IE and internal--II) has been studied in four groups of rats: 1-control, 2-fasted for 24 h, 3-treated with nicotinic acid and 4-treated with propranolol. There was a sharp reduction in glycogen level in each muscle after 30 min exercise in the control and fasted groups. Exercise till exhaustion further lowered the glycogen level in D in the control group and in IE and II in the fasted group. In the fasted group, the level of glycogen in each muscle, at rest, and after 30 min exercise, and in IE and II muscles after exercise till exhaustion was lower than in the control group. Nicotinic acid did not affect the glycogen level either at rest or during exercise as compared with the control group. Propranolol increased the glycogen level in the muscles at rest and during 30 min exercise. It partially prevented glycogen mobilization in D and IE and fully in II during exercise till exhaustion. In the control group, 24 and 48 h after exercise till exhaustion, the level of glycogen in each muscle exceeded the resting control value. It is concluded that exercise-induced glycogen metabolism in the respiratory muscles differs in some respects from that in the limb or heart muscles.     

Sympatho-endocrine and metabolic responses to exercise under post-ganglionic blockade in rats

The purpose of this study was to further document the role of locally released norepinephrine (NE) in the control of metabolic and endocrine responses to exercise in rats. Post-ganglionic blockade with bretylium (20 mg.kg-1, i.v.) reduced NE release from sympathetic nerve endings and triggered a compensatory increase in epinephrine (E) release from the adrenal medulla, as reflected by plasma NE and E concentrations at rest and exercise (E/NE ratio = 2.92 +/- 0.53 and 2.48 +/- 0.51 vs 0.62 +/- 0.15 and 1.48 +/- 0.18 in control rats; mean +/- SE). Following bretylium administration a reduction in running time to exhaustion (28 m.min-1, 8% slope: 33 +/- 2 min vs 74 +/- 10 min) was associated with 1) a faster decrease in blood glucose concentration (3.58 +/- 0.80 mM vs 8.09 +/- 0.38 mM in control rats exercised for 33 min); and 2) an increased glycogen store utilization in fast-twitch muscles (superficial vastus lateralis and gastrocnemius lateralis). Glycogen utilization was not modified in soleus muscle and in the liver. Taken together these results suggest that post-ganglionic blockade increased carbohydrate store and peripheral blood glucose utilization. This could reflect an impairment in fat mobilization and utilization which might be secondary to a reduction of NE release in the adipose tissue and/or in the endocrine pancreas.     

Fiber type specific glycogen utilization in rat diaphragm during treadmill exercise

To examine the significance of endogenous stores of glycogen in specific fiber types (I, IIa, IIb) of the costal region of the diaphragm, adult male Wistar rats performed continuous running (25 m/min, 8 degrees grade) exercise for either 30 min or until fatigue. At 30 min of exercise, glycogen loss, as measured microphotometrically using the periodic acid-Schiff technique averaged between 73 and 80% (P less than 0.05) in the different fiber types. When exercise was performed to exhaustion, representing an additional 94 min, no further reduction in glycogen was observed in any fiber type. Biochemical determinations of glycogen from the diaphragm confirmed the extensive reduction in glycogen concentration with exercise. Large reductions (P less than 0.05) in glycogen were also noted in the soleus, plantaris, and vastus lateralis red. Although significant depletion (P less than 0.05) occurred in the vastus lateralis white, it was not as pronounced as in these other muscles. Repletion to preexercise glycogen concentration was complete by 4 h of recovery in all muscles except the vastus lateralis white. It is concluded that endogenous glycogen is a significant substrate in all muscles sampled regardless of fiber composition. In the case of the costal region of the diaphragm, the increased work of breathing resulting from heavy exercise leads to the recruitment of all fiber types, and each fiber type depends on glycogen as a substrate at least early in the exercise.     

Effect of exhaustive ultra-endurance exercise in muscular glycogen and both Alpha1 and Alpha2 Ampk protein expression in trained rats

Glycogen is the main store of readily energy in skeletal muscle and plays a key role in muscle function, demonstrated by the inability to sustain prolonged high-intensity exercise upon depletion of these glycogen stores. With prolonged exercise, glycogen depletion occurs and 5′-AMP-activated protein kinase (AMPK), a potent regulator of muscle metabolism and gene expression, is activated promoting molecular signalling that increases glucose uptake by muscular skeletal cells. The aim of this study was primarily to determine the effect of ultra-endurance exercise on muscle glycogen reserves and secondly to verify the influence of this type of exercise on AMPK protein expression. Twenty-four male Wistar rats, 60 days old, were divided into four experimental groups: sedentary, sedentary exhausted (SE), endurance trained (T) and endurance trained exhausted (TE). The animals ran for 10 to 90 min/day, 5 days/week, for 12 weeks to attain trained status. Rats were killed immediately after the exhaustion protocol, which consisted of running on a treadmill (at approximately 60 % V _max until exhaustion). Optical density of periodic acid-Schiff was detected and glycogen depletion observed predominantly in type I muscle fibres of the TE group and in both type I and II muscle fibres in the SE group. Plasma glucose decreased only in the TE group. Hepatic glycogen was increased in T group and significantly depleted in TE group. AMPK protein expression was significantly elevated in TE and T groups. In conclusion, acute exhaustive ultra-endurance exercise promoted muscle glycogen depletion. It seems that total AMPK protein and gene expression is more influenced by status training.     

Muscle and liver glycogen, protein, and triglyceride in the rat. Effect of exercise and of the sympatho-adrenal system

We have previously found that during exercise net muscle glycogen breakdown is impaired in adrenodemedullated rats, as compared with controls. The present study was carried out to elucidate whether, in rats with deficiencies of the sympatho-adrenal system, diminished exercise-induced glycogenolysis in skeletal muscle was accompanied by increased breakdown of triglyceride and/or protein. Thus, the effect of exhausting swimming and of running on concentrations of glycogen, protein, and triglyceride in skeletal muscle and liver were studied in rats with and without deficiencies of the sympatho-adrenal system. In control rats, both swimming and running decreased the concentration of glycogen in fast-twitch red and slow-twitch red muscle whereas concentrations of protein and triglyceride did not decrease. In the liver, swimming depleted glycogen stores but protein and triglyceride concentrations did not decrease. In exercising rats, muscle glycogen breakdown was impaired by adrenodemedullation and restored by infusion of epinephrine. However, impaired glycogen breakdown during exercise was not accompanied by a significant net breakdown of protein or triglyceride. Surgical sympathectomy of the muscles did not influence muscle substrate concentrations. The results indicate that when glycogenolysis in exercising muscle is impeded by adrenodemedullation no compensatory increase in breakdown of triglyceride and protein in muscle or liver takes place. Thus, indirect evidence suggests that, in exercising adrenodemedullated rats, fatty acids from adipose tissue were burnt instead of muscle glycogen.     

Adaptations to a high-fat diet that increase exercise endurance in male rats

Eighty-seven male Sprague-Dawley rats (245-300 g) were randomly assigned to one of two experimental groups. The first group consumed a diet high in fat and low in carbohydrate (LCD), whereas the second group ate a normal diet (ND). After either 1 or 5 wk on the diets, rats from each group were killed either before or after an exhausting run on a rodent treadmill (35 m X min-1, 0% grade). The LCD animals ran significantly longer before exhaustion at both week 1 (44.9 +/- 5.1 vs. 41.6 +/- 4.2 min) and week 5 (47.1 +/- 3.6 vs. 35.5 +/- 3.1 min) (P less than 0.05). Adaptations to the LCD included lower muscle and liver glycogen content, decreased rate of glycogen breakdown during exercise, decreased lactate production, and elevated blood ketone levels. In addition to these substrate changes, the LCD caused increased enzyme activities of muscular 3-hydroxyacyl-CoA dehydrogenase (35-110%) and citrate synthase (15-20%). These data indicate that rats exposed to a high-fat diet are capable of prolonged intense exercise in spite of limited glycogen stores. This improved capacity for exercise appears to be partially the result of muscular adaptations to the diet, which apparently increase the ability to oxidize fat and concomitantly spare glycogen.     

Exercise-induced glycogen utilization by the respiratory muscles

Some controversy exists in the literature as to whether or not diaphragmatic glycogen is utilized during exercise. In this study male Sprague-Dawley rats were used to determine whether prolonged treadmill exercise would result in a significant reduction of glycogen concentration in the respiratory muscles. Untrained rats were run to exhaustion at a speed of 24 m/min, up a 10% grade. Run time averaged 48:30 min. After exercise a significant reduction in glycogen was observed in the diaphragm (43% of control), intercostals (43%), heart (39%), and plantaris (76%). In the diaphragm a significant reduction was shown in both types I and II fibers using the periodic acid-Schiff (PAS) stain for glycogen. These findings show that muscles with vastly different aerobic capacities utilize endogenous glycogen during moderately intense submaximal endurance exercise and that the costal diaphragm muscle is not an exception as has recently been suggested.     

Glycogen concentrations and endurance capacity of rats with chronic heart failure

The endurance capacities of rats with myocardial infarctions (MI) and of rats having undergone sham operations (SHAM) were tested during a submaximal exercise regimen that consisted of swimming to exhaustion. During this test, a decrement in the endurance capacity of the MI rat was demonstrated as the SHAM rat swam 25% longer than the MI rat (65 +/- 4 vs. 52 +/- 4 min). Glycogen concentrations were measured in the liver and the white gastrocnemius, plantaris, and soleus muscles of SHAM and MI rats that were randomly divided into four subgroups, which consisted of resting control, swim to exhaustion, swim to exhaustion + 24 h recovery, and swim to exhaustion + 24 h recovery + a second swim to exhaustion. The results demonstrated that the glycogen concentrations found in the liver, white gastrocnemius, plantaris, and soleus muscles of the SHAM and MI rats belonging to the resting control groups were similar. After swimming to exhaustion the glycogen concentrations in these tissues were significantly reduced compared with those found in the resting control groups of rats, and after 24 h of recovery the glycogen concentrations in these tissues were again similar to those found in the resting control groups of rats. Since the magnitude of the glycogen depletion in the liver and the white gastrocnemius, plantaris, and soleus muscles was similar in the SHAM and MI rats and because the SHAM rats consistently swam for longer periods of time in each of the experimental groups, it would be logical to assume that the rates of glycogen utilization for the various tissues may have been greater in the MI rat during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute glucocorticoid effects on glycogen utilization, O2 uptake, and endurance

This study was undertaken to determine the effects of increased substrate availability (glycogen + plasma fatty acids) by glucocorticoids on energy metabolism during exercise to exhaustion. Female rats received a single subcutaneous injection of cortisol acetate (CA) (100 mg.kg body wt-1) 21 h before treadmill running (30.8 m/min). At the start of exercise in the CA-treated rats, plasma fatty acids and liver glycogen were increased by 40%. Glycogen levels were also increased by CA treatment in slow-twitch soleus (61%), fast-twitch white vastus (38%), and fast-twitch red vastus lateralis (85%) muscles. Exercise time to exhaustion was increased by CA treatment (114 +/- 5 vs. 95 +/- 6 min, P less than 0.05). During the exercise, total glycogen depletion was greater in the CA-treated than in the control animals, whereas estimated relative rates of carbohydrate utilization (R = 0.90) were similar. However, while running the CA-treated group consumed 11% more O2 than the controls (P less than 0.05). These results show that a single injection of glucocorticoids is capable of improving endurance. Yet the increased O2 uptake during exercise may have minimized the impact of the initial increased availability of carbohydrates and fatty acids in prolonging exercise capacity. This decreased running economy by the CA-treated runners may be secondary to alterations in energy production or utilization.     

Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women.

This study examined the ability of well-trained eumenorrheic women to increase muscle glycogen content and endurance performance in response to a high-carbohydrate diet (HCD; approximately 78% carbohydrate) compared with a moderate-carbohydrate diet (MD; approximately 48% carbohydrate) when tested during the luteal phase of the menstrual cycle. Six women cycled to exhaustion at approximately 80% maximal oxygen uptake (VO(2 max)) after each of the randomly assigned diet and exercise-tapering regimens. A biopsy was taken from the vastus lateralis before and after exercise in each trial. Preexercise muscle glycogen content was high after the MD (625.2 +/- 50.1 mmol/kg dry muscle) and 13% greater after the HCD (709.0 +/- 44.8 mmol/kg dry muscle). Postexercise muscle glycogen was low after both trials (MD, 91.4 +/- 34.5; HCD, 80.3 +/- 19.5 mmol/kg dry muscle), and net glycogen utilization during exercise was greater after the HCD. The subjects also cycled longer at approximately 80% VO(2 max) after the HCD vs. MD (115:31 +/- 10:47 vs. 106:35 +/- 8:36 min:s, respectively). In conclusion, aerobically trained women increased muscle glycogen content in response to a high-dietary carbohydrate intake during the luteal phase of the menstrual cycle, but the magnitude was smaller than previously observed in men. The increase in muscle glycogen, and possibly liver glycogen, after the HCD was associated with increased cycling performance to volitional exhaustion at approximately 80% VO(2 max).     

Effects of acute hypobaric hypoxia and exhaustive exercise on AMP-activated protein kinase phosphorylation in rat skeletal muscle

The present study was aimed to explore the changes of phosphorylated AMP-activated protein kinase (pAMPK) level in skeletal muscle after exposure to acute hypobaric hypoxia and exhaustive exercise. Thirty-two male Sprague-Dawley (SD) rats were randomly divided into sea level and high altitude groups. The rats in high altitude group were submitted to simulated 5 000 m of high altitude in a hypobaric chamber for 24 h, and sea level group was maintained at normal conditions. All the rats were subjected to exhaustive swimming exercise. The exhaustion time was recorded. Before and after the exercise, blood lactate and glycogen content in skeletal muscle were determined; AMPK and pAMPK levels in skeletal muscle were detected by Western blot. The results showed that the exhaustion time was significantly decreased after exposure to high altitude. At the moment of exhaustion, high altitude group had lower blood lactate concentration and higher surplus glycogen content in gastrocnemius compared with sea level group. Exhaustive exercise significantly increased the pAMPK/AMPK ratio in rat skeletal muscles from both sea level and high altitude groups. However, high altitude group showed lower pAMPK/AMPK ratio after exhaustion compared to sea level group. These results suggest that, after exposure to acute hypobaric hypoxia, the decrement in exercise capacity may not be due to running out of glycogen, accumulation of lactate or disturbance in energy status in skeletal muscle.     

Postexercise Muscle Triacylglycerol and Glycogen Metabolism in Obese Insulin‐Resistant Zucker Rats

Objective: To determine the impact of insulin resistance and obesity on muscle triacylglycerol (IMTG) and glycogen metabolism during and after prolonged exercise. Research Methods and Procedures: Female lean (fa/?; N = 40, ZL) and obese insulin-resistant (fa/fa; N = 40, ZO) Zucker rats performed an acute bout of swimming exercise (8 times for 30 minutes) followed by 6 hours of carbohydrate supplementation (CHO) or fasting (FAST). IMTG and glycogen were measured in the extensor digitorum longus (EDL) and red vastus lateralis (RVL) muscles. Results: Despite resting IMTG content being 4-fold higher in ZO compared with ZL rats, IMTG levels were unchanged in either EDL or RVL muscles immediately after exercise. Resting glycogen concentration in EDL and RVL muscles was similar between genotypes, with exercise resulting in glycogen use in both muscles from ZL rats (∼85%, p < 0.05). However, in ZO rats, there was a much smaller decrease in postexercise glycogen content in both EDL and RVL muscles (∼30%). During postexercise recovery, there was a decrease in EDL muscle levels of IMTG in ZL rats supplemented with CHO after 30 and 360 minutes (p < 0.05). In contrast, IMTG content was increased above resting levels in RVL muscles of ZO rats fasted for 360 minutes. Six hours of CHO refeeding restored glycogen content to resting levels in both muscles in ZL rats. However, after 6 hours of FAST in ZO animals, RVL muscle glycogen content was still lower than resting levels (p < 0.05). At this time, IMTG levels were elevated above basal (p < 0.05). Discussion: In both healthy and insulin-resistant skeletal muscle, there was negligible net IMTG degradation after a single bout of prolonged exercise. However, during postexercise recovery, there was differential metabolism of IMTG between phenotypes.     

Prolonged hyperadrenalinemia impairs exercise ability in rats

This study was performed on rats with sustained (24 h) hyperadrenalinemia produced by sc implantation of retard adrenaline (A) tablets. Comparing with control, sham-operated animals, in A-treated rats duration of endurance exercise until exhaustion was shortened by approx. 40%. This was accompanied by: significant decreases of the pre-exercise muscle glycogen and creatine phosphate values in both "slow twitching" (soleus) and "fast twitching" (white portion of gastrocnemius) muscles, and of ATP content in soleus. Muscle lactate and pyruvate contents as well as blood glucose and FFA levels were elevated. After exercise muscle substrate and metabolite contents were similar in both groups in spite of the difference in exercise duration. It is concluded that prolonged hyperadrenalinemia diminishes the intramuscular energy substrate content, thereby reducing endurance capacity of rats.     

Effect of cocaine on exercise endurance and glycogen use in rats

To determine the effects of cocaine on exercise endurance, male rats were injected intraperitoneally with cocaine (20 mg/kg body wt) or saline and then run to exhaustion 20 min later at 22 m/min and 15% grade. Saline-injected animals ran 74.9 +/- 16.5 (SD) min, whereas cocaine-treated rats ran only 29 +/- 11.6 min. The drug had no effect on resting blood glucose or lactate levels, nor did it affect resting glycogen levels in liver or red and white vastus muscle. However, it did reduce resting soleus glycogen content by 30%. During exercise liver and soleus glycogen depletion occurred at the same rate in saline- and cocaine-treated animals. In contrast, the rate of glycogen depletion during exercise in red and white vastus was markedly increased in cocaine-treated rats with a corresponding elevation in blood lactate (12 vs. only 5 mM in saline group) at exhaustion. These data suggest that cocaine administration (20 mg/kg) before submaximal exercise dramatically alters glycogen metabolism during exercise, and this effect has a negative impact on exercise endurance.     

Effect of various doses of cocaine on endurance capacity in rats

To determine the effects of a variety of doses of cocaine on endurance capacity, rats were injected intraperitoneally with either 0.1, 0.5, 2.5, 12.5, or 20 mg/kg body wt 20 min before running to exhaustion at 26 m/min up a 10% grade. Animals given saline ran 116 +/- 9 (SE) min. At doses of 12.5 and 20 mg/kg, cocaine reduced endurance time significantly (34 and 74%, respectively). At rest the drug had no effect on liver or fast-twitch muscle glycogen but significantly reduced (20-40%) soleus glycogen at the two highest doses. However, at exhaustion, the quantity of glycogen depleted in the fast-twitch red and white vastus muscles was similar in all groups despite the reduced run times of the animals receiving a higher dose implying a greater rate of glycogenolysis due to cocaine. Blood lactate in the 20 mg/kg group (9.9 +/- 1.2 mM) at exhaustion was nearly twice that of the saline controls at exhaustion (5.1 +/- 0.6). Before exercise plasma norepinephrine (at doses of 2.5, 12.5 and 20 mg/kg) was higher than saline controls and remained higher (20 mg/kg groups) at exhaustion. We conclude that high doses of cocaine cause rapid muscle glycogen depletion and early fatigue. The mechanism by which cocaine causes these effects is not clear.     

Effect of estradiol on tissue glycogen metabolism in exercised oophorectomized rats

The effect of both physiological and pharmacological doses of estradiol on exercise performance and tissue glycogen utilization was determined in oophorectomized estradiol-replaced (ER) rats. Doses of beta-estradiol 3-benzoate (0.02, 0.04, 0.1, 0.2, 1, 2, 4, or 10 micrograms.0.1 ml of sunflower oil-1.100 g body wt-1) were injected 5 days/wk for 4 wk. Controls were sham injected (SI). After treatment, the animals were run to exhaustion on a motorized treadmill. ER animals receiving the 0.02-microgram dose ran significantly longer and completed more total work than the SI group. ER animals receiving doses of greater than or equal to 0.04 microgram ran longer and performed more work than the 0.02-microgram group. At exhaustion, myocardial glycogen content was significantly decreased in animals that were ER with less than or equal to 0.1 microgram, whereas those replaced with doses greater than 0.1 microgram utilized significantly less glycogen. With the 10-micrograms dose no significant decrease in heart glycogen content was observed at exhaustion. A submaximal 2-h run significantly reduced glycogen content in heart, red and white portions of the vastus lateralis, and the livers of SI animals. The latter effect was attenuated in skeletal muscle and liver, and there was no effect in the hearts of the ER animals receiving 2 micrograms. These data indicate that estradiol replacement in oophorectomized rats influenced myocardial glycogen utilization during exhaustive exercise and spared tissue glycogen during submaximal exercise. These glycogen sparing effects may have contributed to the significant improvements in exercise performance observed in this study.     

Glucose feedings and exercise in rats: glycogen use, hormone responses, and performance

This study compared the effects of glucose feeding and water on endurance performance, glycogen utilization, and endocrine responses to exhaustive running in rats. Forty-eight trained rats ran at approximately 70% peak O2 consumption (VO2) while receiving, via gavage, 1 ml of an 18% glucose solution or water every 30 min. Glucose- (GF) and water-fed rats (WF) were pair matched and killed at rest, at 25 or 50% of their previously determined run time to exhaustion, or at exhaustion. Run times to exhaustion were 4.6 +/- 1.0 and 3.0 +/- 0.9 h in GF and WF rats, respectively. In WF rats, plasma glucose declined continuously from a resting value of 7.4 +/- 0.5 to 1.8 +/- 0.5 mM at exhaustion and was lower than in GF rats at all exercise time points. In GF rats, glucose was maintained at 7.4 +/- 0.5 mM for 3 h before dropping to 3.9 +/- 0.6 mM at exhaustion. In both groups, liver and muscle glycogen decreased dramatically during the 1st h and changed only slightly thereafter. During the 3rd h, glycogen levels were maintained in GF rats but continued to decrease in WF rats (P less than 0.05). Insulin decreased during exercise and was not significantly different between groups. Glucagon, epinephrine, norepinephrine, and corticosterone increased to a greater extent in WF than in GF rats during the first 3 h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of insulin in the control of triacyglycerol (TG) in the rat skeletal muscles

The role of insulin in the control of triacyglycerol (TG) in different types of skeletal muscle has not been fully recognized so the aim of the present study was to fill this gap. The experiments were carried out on control rats, those fed with olive oil or fed with the oil and treated with insulin and on streptozotocin diabetic animals at rest and there after exercise till exhaustion. The level of TG was measured in the white and red layers of the vastus lateralis, the soleus and the diaphragm. It was found that acute feeding with olive oil had no effect on TG level in either muscle type examined. Insulin administered to rats fed with oil increased TG level in the red vastus. Streptozotocin diabetes caused an increase in TG level in muscles with high oxidative potential. Exercise lowered the level of TG only in the red vastus of the diabetic rats. It is concluded that insulin may increase muscle TG level. Accumulation of TG in muscles of rats with acute diabetes is likely to be a result of the high plasma free fatty acid concentration. Acute insulin deficiency did not affect the muscle TG response to exercise.     

Effect of streptozotocin-induced diabetes on glycogen resynthesis in fasted rats post-high-intensity exercise

It has recently been shown that food intake is not essential for the resynthesis of the stores of muscle glycogen in fasted animals recovering from high-intensity exercise. Because the effect of diabetes on this process has never been examined before, we undertook to explore this issue. To this end, groups of rats were treated with streptozotocin (60 mg/kg body mass ip) to induce mild diabetes. After 11 days, each animal was fasted for 24 h before swimming with a lead weight equivalent to 9% body mass attached to the tail. After exercise, the rate and the extent of glycogen repletion in muscles were not affected by diabetes, irrespective of muscle fiber composition. Consistent with these findings, the effect of exercise on the phosphorylation state of glycogen synthase in muscles was only minimally affected by diabetes. In contrast to its effects on nondiabetic animals, exercise in fasted diabetic rats was accompanied by a marked fall in hepatic glycogen levels, which, surprisingly, increased to preexercise levels during recovery despite the absence of food intake.     

Aging and acute exercise enhance free radical generation in rat skeletal muscle.

Reactive oxygen species (ROS) are implicated in the mechanism of biological aging and exercise-induced oxidative damage. The present study examined the effect of an acute bout of exercise on intracellular ROS production, lipid and protein peroxidation, and GSH status in the skeletal muscle of young adult (8 mo, n = 24) and old (24 mo, n = 24) female Fischer 344 rats. Young rats ran on a treadmill at 25 m/min and 5% grade until exhaustion (55.4 +/- 2.7 min), whereas old rats ran at 15 m/min and 5% grade until exhaustion (58.0 +/- 2.7 min). Rate of dichlorofluorescin (DCFH) oxidation, an indication of ROS and other intracellular oxidants production in the homogenate of deep vastus lateralis, was 77% (P < 0.01) higher in rested old vs. young rats. Exercise increased DCFH oxidation by 38% (P < 0.09) and 50% (P < 0.01) in the young and old rats, respectively. DCFH oxidation in isolated deep vastus lateralis mitochondria with site 1 substrates was elevated by 57% (P < 0.01) in old vs. young rats but was unaltered with exercise. Significantly higher DCFH oxidation rate was also found in aged-muscle mitochondria (P < 0.01), but not in homogenates, when ADP, NADPH, and Fe(3+) were included in the assay medium without substrates. Lipid peroxidation in muscle measured by malondialdehyde content showed no age effect, but was increased by 20% (P < 0.05) with exercise in both young and old rats. Muscle protein carbonyl formation was unaffected by either age or exercise. Mitochondrial GSH/ GSSG ratio was significantly higher in aged vs. young rats (P < 0.05), whereas exercise increased GSSG content and decreased GSH/GSSG in both age groups (P < 0.05). These data provided direct evidence that oxidant production in skeletal muscle is increased in old age and during prolonged exercise, with both mitochondrial respiratory chain and NADPH oxidase as potential sources. The alterations of muscle lipid peroxidation and mitochondrial GSH status were consistent with these conclusions.