Pro- and macroglycogenolysis during repeated exercise: roles of glycogen content and phosphorylase activation.

This study examined the relationship between preexercise muscle glycogen content and glycogen utilization in two physiological pools, pro- (PG) and macroglycogen (MG). Male subjects (n = 6) completed an exercise and dietary protocol before the experiment that resulted in one leg with high glycogen (HL) and one with low glycogen (LL). Preexercise PG levels were 312 +/- 29 and 208 +/- 31 glucosyl units/kg dry wt (dw) (P < or = 0.05) in the HL and LL, respectively, and the corresponding values for MG were 125 +/- 37 and 89 +/- 43 mmol glucosyl units/kg dw (P < or = 0.05). Subjects then performed two 90-s exercise bouts at 130% maximal oxygen uptake separated by a 10-min rest period. Biopsies were obtained at rest and after each exercise bout. Preexercise glycogen concentration was correlated to net glycogenolysis for both PG and MG for bout 1 and bouts 1 and 2 (r < or = 0.60). In bout 1, there was no difference in the rate of PG or MG catabolism between HL and LL despite a 26% increase (P < or = 0.05) in glycogen phosphorylase transformation (phos a %) in the HL. In the second bout, more PG was catabolized in the HL vs. LL (38 +/- 9 vs. 9 +/- 6 mmol glucosyl units. kg dw(-1). min(-1)) (P < or = 0.05) with no difference between legs in phos a %. phos a % was increased in HL vs. LL but does not necessarily increase glycogenolysis in either PG or MG. Despite both legs performing the same exercise and having identical metabolic demands, the HL catabolized 2.3 (P < or = 0.05) times more PG and 1.5 (P < or = 0.05) times more MG vs. LL in bouts 1 and 2, indicating that preexercise glycogen concentration is a regulator of glycogenolysis.     

Glycogenin activity and mRNA expression in response to volitional exhaustion in human skeletal muscle.

Glycogenolysis results in the selective catabolism of individual glycogen granules by glycogen phosphorylase. However, once the carbohydrate portion of the granule is metabolized, the fate of glycogenin, the protein primer of granule formation, is not known. To examine this, male subjects (n = 6) exercised to volitional exhaustion (Exh) on a cycle ergometer at 75% maximal O2 uptake. Muscle biopsies were obtained at rest, 30 min, and Exh (99 +/- 10 min). At rest, total glycogen concentration was 497 +/- 41 and declined to 378 +/- 51 mmol glucosyl units/kg dry wt following 30 min of exercise (P < 0.05). There were no significant changes in proglycogen, macroglycogen, glycogenin activity, or mRNA in this period (P > or = 0.05). Exh resulted in decreases in total glycogen, proglycogen, and macroglycogen as well as glycogenin activity (P < 0.05). These decrements were associated with a 1.9 +/- 0.4-fold increase in glycogenin mRNA over resting values (P < 0.05). Glycogenolysis in the initial exercise period (0-30 min) was not adequate to induce changes in glycogenin; however, later in exercise when concentration and granule number decreased further, decrements in glycogenin activity and increases in glycogenin mRNA were demonstrated. Results show that glycogenin becomes inactivated with glycogen catabolism and that this event coincides with an increase in glycogenin gene expression as exercise and glycogenolysis progress.     

Glucose formation in human skeletal muscle. Influence of glycogen content.

Eight men exercised at 66% of their maximal isometric force to fatigue after prior decrease in the glycogen store in one leg (low-glycogen, LG). The exercise was repeated with the contralateral leg (control) at the same relative intensity and for the same duration. Muscle (quadriceps femoris) glycogen content decreased in the LG leg from 199 +/- 17 (mean +/- S.E.M.) to 163 +/- 16 mmol of glucosyl units/kg dry wt. (P less than 0.05), and in the control leg from 311 +/- 23 to 270 +/- 18 mmol/kg (P less than 0.05). The decrease in glycogen corresponded to a similar accumulation of glycolytic intermediates. Muscle glucose increased in the LG leg during the contraction, from 1.8 +/- 0.1 to 4.3 +/- 0.6 mmol/kg dry wt. (P less than 0.01), whereas no significant increase occurred in the control leg (P greater than 0.05). It is concluded that during exercise glucose is formed from glycogen through the debranching enzyme when muscle glycogen is decreased to values below about 200 mmol/kg dry wt.     

No limiting role for glycogenin in determining maximal attainable glycogen levels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 micromol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles ( approximately 25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels ( approximately 170 micromol/g wet wt), approximately 30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.     

Pro- and macroglycogenolysis: relationship with exercise intensity and duration.

We examined the net catabolism of two pools of glycogen, proglycogen (PG) and macroglycogen (MG), in human skeletal muscle during exercise. Male subjects (n = 21) were assigned to one of three groups. Group 1 exercised 45 min at 70% maximal O(2) uptake (VO(2 max)) and had muscle biopsies at rest, 15 min, and 45 min. Group 2 exercised at 85% VO(2 max) to exhaustion (45.4 +/- 3.4 min) and had biopsies at rest, 10 min, and exhaustion. Group 3 performed three 3-min bouts of exercise at 100% VO(2 max) separated by 6 min of rest. Biopsies were taken at rest and after each bout. Group 1 had small MG and PG net glycogenolysis rates (ranging from 3.8 +/- 1.0 to 2.4 +/- 0.6 mmol glucosyl units. kg(-1). min(-1)) that did not change over time. In group 2, the MG glycogenolysis rate remained low and unchanged over time, whereas the PG rate was initially elevated (11.3 +/- 2.3 mmol glucosyl units. kg(-1). min(-1)) and declined (P < or = 0.05) with time. During the first 10 min, PG concentration ([PG]) declined (P < or = 0.05), whereas MG concentration ([MG]) did not. Similarly, in group 3, in both the first and the second bouts of exercise [PG] declined (P < or = 0.05) and [MG] did not, although by the end of the second exercise period the [MG] was lower (P < or = 0.05) than the rest level. The net catabolic rates for PG in the first two exercises were 22.6 +/- 6.8 and 21.8 +/- 8.2 mmol glucosyl units. kg(-1). min(-1), whereas the corresponding values for MG were 17.6 +/- 6.0 and 10.8 +/- 5.6. The MG pool appeared to be more resistant to mobilization, and, when activated, its catabolism was inhibited more rapidly than that of PG. This suggests that the metabolic regulation of the two pools must be different.     

Time course of insulin sensitivity and skeletal muscle glycogen synthase activity after a single bout of exercise in horses.

The time course of insulin sensitivity, skeletal muscle glycogen and GLUT4 content, and glycogen synthase (GS) activity after a single bout of intense exercise was examined in eight horses. On separate days, a euglycemic-hyperinsulinemic clamp (EHC) was undertaken at 0.5, 4, or 24 h after exercise or after 48 h of rest [control (Con)]. There was no increase in mean glucose infusion rate (GIR) with exercise (0.5-, 4-, and 24-h trials), and GIR was significantly decreased at 0.5 h postexercise (GIR: 8.6 +/- 2.7, 6.7 +/- 2.0, 9.0 +/- 2.0, and 10.6 +/- 2.2 mg.kg(-1).min(-1) for Con and at 0.5, 4, and 24 h, respectively). Before each EHC, muscle glycogen content (mmol glucosyl units/kg dry muscle) was higher (P < 0.05) for Con (565 +/- 102) than for other treatments (317 +/- 84, 362 +/- 79, and 382 +/- 74 for 0.5, 4, and 24 h, respectively) and muscle GLUT4 content was unchanged. Pre-EHC active-to-total GS activity ratio was higher (P < 0.05) at 0.5, 4, and 24 h after exercise than in Con. Post-EHC active GS and GS activity ratio were higher (P < 0.05) in Con and at 24 h. There was a significant inverse correlation (r = -0.43, P = 0.02) between glycogen content and GS activity ratio but no relationship between GS activity and GIR. The lack of increase in insulin sensitivity, determined by EHC, after exercise that resulted in a significant reduction in muscle glycogen content is consistent with the slow rate of muscle glycogen resynthesis observed in equine studies.     

Effects of endurance exercise training on muscle glycogen accumulation in humans.

The purpose of this investigation was to determine whether endurance exercise training increases the ability of human skeletal muscle to accumulate glycogen after exercise. Subjects (4 women and 2 men, 31 +/- 8 yr old) performed high-intensity stationary cycling 3 days/wk and continuous running 3 days/wk for 10 wk. Muscle glycogen concentration was measured after a glycogen-depleting exercise bout before and after endurance training. Muscle glycogen accumulation rate from 15 min to 6 h after exercise was twofold higher (P < 0.05) in the trained than in the untrained state: 10.5 +/- 0.2 and 4.5 +/- 1.3 mmol. kg wet wt(-1). h(-1), respectively. Muscle glycogen concentration was higher (P < 0.05) in the trained than in the untrained state at 15 min, 6 h, and 48 h after exercise. Muscle GLUT-4 content after exercise was twofold higher (P < 0.05) in the trained than in the untrained state (10.7 +/- 1.2 and 4.7 +/- 0.7 optical density units, respectively) and was correlated with muscle glycogen concentration 6 h after exercise (r = 0.64, P < 0.05). Total glycogen synthase activity and the percentage of glycogen synthase I were not significantly different before and after training at 15 min, 6 h, and 48 h after exercise. We conclude that endurance exercise training enhances the capacity of human skeletal muscle to accumulate glycogen after glycogen-depleting exercise.     

Time course of glycogen accumulation after eccentric exercise.

This study examined the time course of glycogen accumulation in skeletal muscle depleted by concentric work and subsequently subjected to eccentric exercise. Eight men exercised to exhaustion on a cycle ergometer [70% of maximal O2 consumption (VO2max)] and were placed on a carbohydrate-restricted diet. Approximately 12 h later they exercised one leg to subjective failure by repeated eccentric action of the knee extensors against a resistance equal to 120% of their one-repetition maximum concentric knee extension force (ECC leg). The contralateral leg was not exercised and served as a control (CON leg). During the 72-h recovery period, subjects consumed 7 g carbohydrate.kg body wt-1.day-1. Moderate soreness was experienced in the ECC leg 24-72 h after eccentric exercise. Muscle biopsies from the vastus lateralis of the ECC and CON legs revealed similar glycogen levels immediately after eccentric exercise (40.2 +/- 5.2 and 47.6 +/- 6.4 mmol/kg wet wt, respectively; P greater than 0.05). There was no difference in the glycogen content of ECC and CON legs after 6 h of recovery (77.7 +/- 7.9 and 85.1 +/- 4.9 mmol/kg wet wt, respectively; P greater than 0.05), but 18 h later, the ECC leg contained 15% less glycogen than the CON leg (90.2 +/- 8.2 vs. 105.8 +/- 8.9 mmol/kg wet wt; P less than 0.05). After 72 h of recovery, this difference had increased to 24% (115.8 +/- 8.0 vs. 153.0 +/- 12.2 mmol/kg wet wt; P less than 0.05). These data confirm that glycogen accumulation is impaired in eccentrically exercised muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Skeletal muscle energy metabolism during prolonged, fatiguing exercise.

A depletion of phosphocreatine (PCr), fall in the total adenine nucleotide pool (TAN = ATP + ADP + AMP), and increase in TAN degradation products inosine 5'-monophosphate (IMP) and hypoxanthine are observed at fatigue during prolonged exercise at 70% maximal O(2) uptake in untrained subjects [J. Baldwin, R. J. Snow, M. F. Carey, and M. A. Febbraio. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R295-R300, 1999]. The present study aimed to examine whether these metabolic changes are also prevalent when exercise is performed below the blood lactate threshold (LT). Six healthy, untrained humans exercised on a cycle ergometer to voluntary exhaustion at an intensity equivalent to 93 +/- 3% of LT ( approximately 65% peak O(2) uptake). Muscle biopsy samples were obtained at rest, at 10 min of exercise, approximately 40 min before fatigue (F-40 =143 +/- 13 min), and at fatigue (F = 186 +/- 31 min). Glycogen concentration progressively declined (P < 0.01) to very low levels at fatigue (28 +/- 6 mmol glucosyl U/kg dry wt). Despite this, PCr content was not different when F-40 was compared with F and was only reduced by 40% when F was compared with rest (52. 8 +/- 3.7 vs. 87.8 +/- 2.0 mmol/kg dry wt; P < 0.01). In addition, TAN concentration was not reduced, IMP did not increase significantly throughout exercise, and hypoxanthine was not detected in any muscle samples. A significant correlation (r = 0.95; P < 0. 05) was observed between exercise time and glycogen use, indicating that glycogen availability is a limiting factor during prolonged exercise below LT. However, because TAN was not reduced, PCr was not depleted, and no correlation was observed between glycogen content and IMP when glycogen stores were compromised, fatigue may be related to processes other than those involved in muscle high-energy phosphagen metabolism.     

Manganese sulfate-dependent glycosylation of endogenous glycoproteins in human skeletal muscle is catalyzed by a nonglucose 6-P-dependent glycogen synthase and not glycogenin

Glycogenin, a Mn2+-dependent, self-glucosylating protein, is considered to catalyze the initial glucosyl transfer steps in glycogen biogenesis. To study the physiologic significance of this enzyme, measurements of glycogenin mediated glucose transfer to endogenous trichloroacetic acid precipitable material (protein-bound glycogen, i.e., glycoproteins) in human skeletal muscle were attempted. Although glycogenin protein was detected in muscle extracts, activity was not, even after exercise that resulted in marked glycogen depletion. Instead, a MnSO4-dependent glucose transfer to glycoproteins, inhibited by glycogen and UDP-pyridoxal (which do not affect glycogenin), and unaffected by CDP (a potent inhibitor of glycogenin), was consistently detected. MnSO4-dependent activity increased in concert with glycogen synthase fractional activity after prolonged exercise, and the MnSO4-dependent enzyme stimulated glucosylation of glycoproteins with molecular masses lower than those glucosylated by glucose 6-P-dependent glycogen synthase. Addition of purified glucose 6-P-dependent glycogen synthase to the muscle extract did not affect MnSO4-dependent glucose transfer, whereas glycogen synthase antibody completely abolished MnSO4-dependent activity. It is concluded that: (1) MnSO4-dependent glucose transfer to glycoproteins is catalyzed by a nonglucose 6-P-dependent form of glycogen synthase; (2) MnSO4-dependent glycogen synthase has a greater affinity for low molecular mass glycoproteins and may thus play a more important role than glucose 6-P-dependent glycogen synthase in the initial stages of glycogen biogenesis; and (3) glycogenin is generally inactive in human muscle in vivo.     

Effects of exercise on GLUT-4 and glycogenin gene expression in human skeletal muscle.

To investigate the effect of exercise on GLUT-4, hexokinase, and glycogenin gene expression in human skeletal muscle, 10 untrained subjects (6 women and 4 men, 21.4 +/- 1.2 yr, 66.3 +/- 5.0 kg, peak oxygen consumption = 2.30 +/- 0.19 l/min) exercised for 60 min on a cycle ergometer at a power output requiring 73 +/- 4% peak oxygen consumption. Muscle samples were obtained by needle biopsy before, immediately after, and 3 h after exercise. Gene expression was quantified, relative to 29S ribosomal protein cDNA, by RT-PCR. GLUT-4 gene expression was increased immediately after exercise (1.7 +/- 0.4 vs. 0.9 +/- 0.3 arbitrary units; P < 0.05) and remained significantly higher than baseline 3 h after the end of exercise (2. 2 +/- 0.4 vs. 0.9 +/- 0.3 arbitrary units; P < 0.05). Hexokinase II gene expression was significantly higher than the resting value 3 h after the end of exercise (2.9 +/- 0.4 vs. 1.3 +/- 0.3 arbitrary units; P < 0.05). Exercise increased glycogenin mRNA more than twofold (2.8 +/- 0.6 vs. 1.2 +/- 0.2 arbitrary units; P < 0.05) 3 h after the end of exercise. For the first time, we report that a single bout of exercise is sufficient to cause upregulation of GLUT-4 and glycogenin gene expression in human skeletal muscle. Whether these increases, together with the associated increase in hexokinase II gene expression, lead to increased expression of these key proteins in skeletal muscle and contribute to the enhanced skeletal muscle glucose uptake, glycogen synthesis, and insulin action observed following exercise remains to be determined.     

Regulation of plasma long-chain fatty acid oxidation in relation to uptake in human skeletal muscle during exercise

In the present study, we investigated possible sites of regulation of long-chain fatty acid (LCFA) oxidation in contracting human skeletal muscle. Leg plasma LCFA kinetics were determined in eight healthy men during bicycling (60 min, 65% peak oxygen uptake) with either high (H-FOX) or low (L-FOX) leg fat oxidation (H-FOX: 1,098 +/- 140; L-FOX: 494 +/- 84 micromol FA/min, P < 0.001), which was achieved by manipulating preexercise muscle glycogen (H-FOX: 197 +/- 21; L-FOX: 504 +/- 25 mmol/kg dry wt, P < 0.001). Several blood metabolites and hormones were kept nearly similar between trials by allocating a preexercise meal and infusing glucose intravenously during exercise. During exercise, leg plasma LCFA fractional extraction was identical between trials (H-FOX: 17.8 +/- 1.6; L-FOX: 18.2 +/- 1.8%, not significant), suggesting similar LCFA transport capacity in muscle. On the contrary, leg plasma LCFA oxidation was 99% higher in H-FOX than in L-FOX (421 +/- 47 vs. 212 +/- 37 micromol/min, P < 0.001). Probably due to the slightly higher (P < 0.01) plasma LCFA concentration in H-FOX than in L-FOX, leg plasma LCFA uptake was nonsignificantly (P = 0.17) higher (25%) in H-FOX than in L-FOX, yet the fraction of plasma LCFA uptake oxidized was 61% higher (P < 0.05) in H-FOX than in L-FOX. Accordingly, the muscle content of several lipid-binding proteins did not differ significantly between trials, although fatty acid translocase/CD36 and caveolin-1 were elevated (P < 0.05) by the high-intensity exercise and dietary manipulation allocated on the day before the experimental trial. The present data suggest that, in contracting human skeletal muscle with different fat oxidation rates achieved by manipulating preexercise glycogen content, transsarcolemmal transport is not limiting plasma LCFA oxidation. Rather, the latter seems to be limited by intracellular regulatory mechanisms.     

Epinephrine-induced changes in muscle carbohydrate metabolism during exercise in male subjects

Epinephrine increases glycogenolysis in resting skeletal muscle, but less is known about the effects of epinephrine on exercising muscle. To study this, epinephrine was given intraarterially to one leg during two-legged cycle exercise in nine healthy males. The epinephrine-stimulated (EPI) and non-stimulated (C) legs were compared with regard to glycogen, glucose, glucose 6-phosphate (G6P), alpha-glycerophosphate (alpha-GP), and lactate contents in muscle biopsies taken before and after the 45-min submaximal exercise, as well as brachial arterial-femoral venous (a-fv) differences for epinephrine, norepinephrine, lactate, glucose, and O2 during exercise. During exercise the arterial plasma epinephrine concentration was 4.8 +/- 0.8 nmol/l and the femoral venous epinephrine concentrations were 10.3 +/- 2.1 and 3.9 +/- 0.6 nmol/l, respectively, in the EPI and C leg. During exercise the a-fv difference for lactate was greater (-0.41 +/- 0.14 vs. -0.21 +/- 0.14 mmol/l; P less than 0.001), and the a-fv difference for glucose was smaller (0.07 +/- 0.12 vs. 0.24 +/- 0.12 mmol/l; P less than 0.01) in the EPI than in the C leg, but the a-fv differences for O2 were similar. Muscle glycogen depletion (137 +/- 63 vs. 99 +/- 43 mmol/kg dry muscle; P less than 0.1) and the muscle concentrations of glucose (P less than 0.05), alpha-GP (P less than 0.1), G6P (P greater than 0.1), and lactate (P greater than 0.1) tended to be higher in the EPI than the C leg after exercise. These findings suggest that physiological concentrations of epinephrine may enhance muscle glycogenolysis during submaximal exercise in male subjects.     

Direct assessment of muscle glycogen storage after mixed meals in normal and type 2 diabetic subjects

To understand the day-to-day pathophysiology of impaired muscle glycogen storage in type 2 diabetes, glycogen concentrations were measured before and after the consumption of sequential mixed meals (breakfast: 190.5 g carbohydrate, 41.0 g fat, 28.8 g protein, 1253 kcal; lunch: 203.3 g carbohydrate, 48.1 g fat, 44.0 g protein, 1497.5 kcal) by use of natural abundance (13)C magnetic resonance spectroscopy. Subjects with diet-controlled type 2 diabetes (n = 9) and age- and body mass index-matched nondiabetic controls (n = 9) were studied. Mean fasting gastrocnemius glycogen concentration was significantly lower in the diabetic group (57.1 +/- 3.6 vs. 68.9 +/- 4.1 mmol/l; P < 0.05). After the first meal, mean glycogen concentration in the control group rose significantly from basal (97.1 +/- 7.0 mmol/l at 240 min; P = 0.005). After the second meal, the high level of muscle glycogen concentration in the control group was maintained, with a further rise to 108.0 +/- 11.6 mmol/l by 480 min. In the diabetic group, the postprandial rise was markedly lower than that of the control group (65.9 +/- 5.2 mmol/l at 240 min, P < 0.005, and 70.8 +/- 6.7 mmol/l at 480 min, P = 0.01) despite considerably greater serum insulin levels (752.0 +/- 109.0 vs. 372.3 +/- 78.2 pmol/l at 300 min, P = 0.013). This was associated with a significantly greater postprandial hyperglycemia (10.8 +/- 1.3 vs. 5.3 +/- 0.2 mmol/l at 240 min, P < 0.005). Basal muscle glycogen concentration correlated inversely with fasting blood glucose (r = -0.55, P < 0.02) and fasting serum insulin (r = -0.57, P < 0.02). The increment in muscle glycogen correlated with initial increment in serum insulin only in the control group (r = 0.87, P < 0.002). This study quantitates for the first time the subnormal basal muscle glycogen concentration and the inadequate glycogen storage after meals in type 2 diabetes.     

Impaired muscle glycogen resynthesis after eccentric exercise

Eight men performed 10 sets of 10 eccentric contractions of the knee extensor muscles with one leg [eccentrically exercised leg (EL)]. The weight used for this exercise was 120% of the maximal extension strength. After 30 min of rest the subjects performed two-legged cycling [concentrically exercised leg (CL)] at 74% of maximal O2 uptake for 1 h. In the 3 days after this exercise four subjects consumed diets containing 4.25 g CHO/kg body wt, and the remainder were fed 8.5 g CHO/kg. All subjects experienced severe muscle soreness and edema in the quadriceps muscles of the eccentrically exercised leg. Mean (+/- SE) resting serum creatine kinase increased from a preexercise level of 57 +/- 3 to 6,988 +/- 1,913 U/l on the 3rd day of recovery. The glycogen content (mmol/kg dry wt) in the vastus lateralis of CL muscles averaged 90, 395, and 592 mmol/kg dry wt at 0, 24, and 72 h of recovery. The EL muscle, on the other hand, averaged 168, 329, and 435 mmol/kg dry wt at these same intervals. Subjects receiving 8.5 g CHO/kg stored significantly more glycogen than those who were fed 4.3 g CHO/kg. In both groups, however, significantly less glycogen was stored in the EL than in the CL.     

Selected contribution: skeletal muscle capillarity and enzyme activity in rats selectively bred for running endurance.

To attempt to explain the difference in intrinsic (untrained) endurance running capacity in rats selectively bred over seven generations for either low (LCR) or high running capacity (HCR), the relationship among skeletal muscle capillarity, fiber composition, enzyme activity, and O(2) transport was studied. Ten females from each group [body wt: 228 g (HCR), 247 g (LCR); P = 0.03] were studied at 25 wk of age. Peak normoxic maximum O(2) consumption and muscle O(2) conductance were previously reported to be 12 and 33% higher, respectively, in HCR, despite similar ventilation, arterial O(2) saturation, and a cardiac output that was <10% greater in HCR compared with LCR. Total capillary and fiber number in the medial gastrocnemius were similar in HCR and LCR, but, because fiber area was 37% lower in HCR, the number of capillaries per unit area (or mass) of muscle was higher in HCR by 32% (P < 0.001). A positive correlation (r = 0.92) was seen between capillary density and muscle O(2) conductance. Skeletal muscle enzymes citrate synthase and beta-hydroxyacyl-CoA dehydrogenase were both approximately 40% higher (P < 0.001) in HCR (12.4 +/- 0.7 vs. 8.7 +/- 0.4 and 3.4 +/- 0.2 vs. 2.4 +/- 0.2 mmol. kg(-1). min(-1), respectively), whereas phosphofructokinase was significantly (P = 0.02) lower in HCR (27.8 +/- 1.2 vs. 35.2 +/- 2.5 mmol. kg(-1). min(-1)) and hexokinase was the same (0.65 +/- 0.04 vs. 0.65 +/- 0.03 mmol. kg(-1). min(-1)). Resting muscle ATP, phosphocreatine, and glycogen contents were not different between groups. Taken together, these data suggest that, in rats selectively bred for high-endurance exercise capacity, most of the adaptations for improved O(2) utilization occur peripherally in the skeletal muscles and not in differences at the level of the heart or lung.     

The discovery of glycogenin and the priming mechanism for glycogen biogenesis

The biogenesis of glycogen in skeletal muscle requires a priming mechanism that has recently been elucidated. The first step is catalysed by a protein tyrosine glucosyltransferase and involves the formation of a novel glycosidic linkage, namely the covalent attachment of glucose to a single tyrosine residue (Tyr194) on a priming protein, termed glycogenin. The next stage is the extension of the glucan chain from Tyr194 and involves the sequential addition of up to seven further glucosyl residues. This reaction is brought about autocatalytically by glycogenin itself, which is a Mn2+/Mg(2+)-dependent UDP-Glc-requiring glucosyltransferase. The glucan primer is elongated by glycogen synthase, but only when glycogenin and glycogen synthase are complexed together. Glycogen synthase dissociates from glycogenin during the synthesis of a glycogen molecule, enabling glycogen molecules to reach their maximum theoretical size. Each mature glycogen beta particle in muscle contains one molecule of glycogenin attached covalently, and an average one glycogen synthase catalytic subunit bound non-covalently. As evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general, the molecular details of mammalian glycogen biogenesis may serve as a useful model for other systems.     

Human muscle fatigue after glycogen depletion: a 31P magnetic resonance study.

To differentiate the effects of high energy phosphates, pH, and [H2PO4-] on skeletal muscle fatigue, intracellular acidosis during handgrip exercise was attenuated by prolonged submaximal exercise. Healthy human subjects (n = 6) performed 5-min bouts of maximal rhythmic handgrip (RHG) before (CONTROL) and after prolonged (60-min) handgrip exercise (ATTEN-EX) designed to attenuate lactic acidosis in active muscle by partially depleting muscle glycogen. Concentrations of free intracellular phosphocreatine ([PCr]), adenosine triphosphate ([ATP]), and orthophosphate ([P(i)]) and pH were measured by 31P nuclear magnetic resonance spectroscopy and used to calculate adenosine diphosphate [ADP], [H2PO4-], and [HPO4(2-)]. Handgrip force output was measured with a dynamometer, and fatigue was determined by loss of maximal contractile force. After ATTEN-EX, the normal exercise-induced muscle acidosis was reduced. At peak CONTROL RHG, pH fell to 6.3 +/- 0.1 (SE) and muscle fatigue was correlated with [PCr] (r = 0.83), [P(i)] (r = 0.82), and [H2PO4-] (r = 0.81); [ADP] was 22.0 +/- 5.7 mumol/kg. At peak RHG after ATTEN-EX, pH was 6.9 +/- 0.1 and [ADP] was 116.1 +/- 18.2 mumol/kg, although [PCr] and [P(i)] were not different from CONTROL RHG (P greater than 0.05). After ATTEN-EX, fatigue correlated most closely with [ADP] (r = 0.84). The data indicate that skeletal muscle fatigue 1) is multifactorial, 2) can occur without decreased pH or increased [H2PO4-], and 3) is correlated with [ADP] after exercise-induced glycogen depletion.     

Adaptations in skeletal muscle exercise metabolism to a sustained session of heavy intermittent exercise

The purpose of this study was to investigate the hypothesis that a single, extended session of heavy exercise would be effective in inducing adaptations in energy metabolism during exercise in the absence of increases in oxidative potential. Ten healthy males [maximal aerobic power (VO(2 peak)) = 43.4 +/- 2.2 (SE) ml x kg(-1) x min(-1)] participated in a 16-h training session involving cycling for 6 min each hour at approximately 90% of maximal oxygen consumption. Measurements of metabolic changes were made on tissue extracted from the vastus lateralis during a two-stage standardized submaximal cycle protocol before (Pre) and 36-48 h after (Post) the training session. At Pre, creatine phosphate (PCr) declined (P < 0.05) by 32% from 0 to 3 min and then remained stable until 20 min of exercise at 60% VO(2 peak) before declining (P < 0.05) by a further 35% during 20 min of exercise at 75% VO(2 peak). Muscle lactate (mmol/kg dry wt) progressively increased (P < 0.05) from 4.59 +/- 0.64 at 0 min to 17.8 +/- 2.7 and 30.9 +/- 5.3 at 3 and 40 min, respectively, whereas muscle glycogen (mmol glucosyl units/kg dry wt) declined (P < 0.05) from a rest value of 360 +/- 24 to 276 +/- 31 and 178 +/- 36 at similar time points. During exercise after the training session, PCr and glycogen were not as depressed (P < 0.05), and increases in muscle lactate were blunted (P < 0.05). All of these changes occurred in the absence of increases in oxidative potential as measured by the maximal activities of citrate synthase and malate dehydrogenase. These findings are consistent with other studies, namely, that muscle metabolic adaptations to regular exercise are an early adaptive event that occurs before increases in oxidative potential.