Muscle glycogen and metabolic regulation

Muscle glycogen is an important fuel for contracting skeletal muscle during prolonged strenuous exercise, and glycogen depletion has been implicated in muscle fatigue. It is also apparent that glycogen availability can exert important effects on a range of metabolic and cellular processes. These processes include carbohydrate, fat and protein metabolism during exercise, post-exercise glycogen resynthesis, excitation–contraction coupling, insulin action and gene transcription. For example, low muscle glycogen is associated with reduced muscle glycogenolysis, increased glucose and NEFA uptake and protein degradation, accelerated glycogen resynthesis, impaired excitation–contraction coupling, enhanced insulin action and potentiation of the exercise-induced increases in transcription of metabolic genes. Future studies should identify the mechanisms underlying, and the functional importance of, the association between glycogen availability and these processes.

Effect of short-term training on GLUT-4 mRNA and protein expression in human skeletal muscle

Six untrained, male subjects (23 ± 1 years old, 84 ± 5 kg, V_O2peak= 3.7 ± 0.8 l min^–1) exercised for 60 min at 75 ± 1%V_O2peak on 7 consecutive days.  Muscle samples were obtained before the start of cycle exercise training and 24 h after the first and seventh exercise sessions and analysed for citrate synthase activity, glycogen and glucose transporter 4 (GLUT-4) mRNA and protein expression. Exercise training increased (P < 0.05) citrate synthase by ~20% and muscle glycogen concentration by ~40%. GLUT-4 mRNA levels 24 h after the first and seventh exercise sessions were similar to those  measured before the start of exercise training. In contrast, GLUT-4 protein expression was increased after 7 days of exercise training (12.4 ± 1.5 versus 3.4 ± 1.0 arbitray units (a.u.), P < 0.05) and although it tended to be higher 24 h after the first exercise session (6.0 ± 3.0 versus 3.4 ± 1.0 a.u.), this was not significantly different (P= 0.09). These results support the suggestion that the adaptive increase in skeletal muscle GLUT-4 protein expression with short-term exercise training arises from the repeated, transient increases in GLUT-gene transcription following each exercise bout leading to a gradual accumulation of GLUT-4 protein, despite GLUT-4 mRNA returning to basal levels between exercise stimuli.

Exercise does not alter subcellular localization, but increases phosphorylation of insulin-signaling proteins in human skeletal muscle

The subcellular localization of insulin signaling proteins is altered by various stimuli such as insulin, insulin-like growth factor I, and oxidative stress and is thought to be an important mechanism that can influence intracellular signal transduction and cellular function. This study examined the possibility that exercise may also alter the subcellular localization of insulin signaling proteins in human skeletal muscle. Nine untrained males performed 60 min of cycling exercise (~67% peak pulmonary O₂ uptake). Muscle biopsies were sampled at rest, immediately after exercise, and 3 h postexercise. Muscle was fractionated by centrifugation into the following crude fractions: cytosolic, nuclear, and a high-speed pellet containing membrane and cytoskeletal components. Fractions were analyzed for protein content of insulin receptor, insulin receptor substrate (IRS)-1 and -2, p85 subunit of phosphatidylinositol 3-kinase, Akt, and glycogen synthase kinase-3 (GSK-3). There was no significant change in the protein content of the insulin signaling proteins in any of the crude fractions after exercise or 3 h postexercise. Exercise had no significant effect on the phosphorylation of IRS-1 Tyr612 in any of the fractions. In contrast, exercise increased (P < 0.05) the phosphorylation of Akt Ser⁴⁷³ and GSK-3α/ß Ser^9/21 in the cytosolic fraction only. In conclusion, exercise can increase phosphorylation of downstream insulin signaling proteins specifically in the cytosolic fraction but does not result in changes in the subcellular localization of insulin signaling proteins in human skeletal muscle. Change in the subcellular protein localization is therefore an unlikely mechanism to influence signal transduction pathways and cellular function in skeletal muscle after exercise.

Acute exercise and GLUT4 expression in human skeletal muscle: influence of exercise intensity

To examine the influence of exercise intensity on the increases in vastus lateralis GLUT4 mRNA and protein after exercise, six untrained men exercised for 60 min at 39 ± 3% peak oxygen consumption (VO2 peak) (Lo) or 27 ± 2 min at 83 ± 2% VO2 peak (Hi) in counterbalanced order. Preexercise muscle glycogen levels were not different between trials (Lo: 408 ± 35 mmol/kg dry mass; Hi: 420 ± 43 mmol/kg dry mass); however, postexercise levels were lower (P < 0.05) in Hi (169 ± 18 mmol/kg dry mass) compared with Lo (262 ± 35 mmol/kg dry mass). Thus calculated muscle glycogen utilization was greater (P < 0.05) in Hi (251 ± 24 mmol/kg) than in Lo (146 ± 34). Exercise resulted in similar increases in GLUT4 gene expression in both trials. GLUT4 mRNA was increased immediately at the end of exercise (~2-fold; P < 0.05) and remained elevated after 3 h of postexercise recovery. When measured 3 h after exercise, total crude membrane GLUT4 protein levels were 106% higher in Lo (3.3 ± 0.7 vs. 1.6 ± 0.3 arbitrary units) and 61% higher in Hi (2.9 ± 0.5 vs. 1.8 ± 0.5 arbitrary units) relative to preexercise levels. A main effect for exercise was observed, with no significant differences between trials. In conclusion, exercise at ~40 and ~80% VO2 peak, with total work equal, increased GLUT4 mRNA and GLUT4 protein in human skeletal muscle to a similar extent, despite differences in exercise intensity and duration.

Interaction of exercise and diet on GLUT-4 protein and gene expression in type I and type II rat skeletal muscle

We determined the interaction of exercise and diet on glucose transporter (GLUT-4) protein and mRNA expression in type I (soleus) and type II [extensor digitorum longus (EDL)] skeletal muscle. Forty-eight Sprague Dawley rats were randomly assigned to one of two dietary conditions: high-fat (FAT, n =24) or high-carbohydrate (CHO, n =24). Animals in each dietary condition were allocated to one of two groups: control (NT, n =8) or a group that performed 8 weeks of treadmill running (4 sessions week^–1 of 1000 m @ 28 m min^–1 , RUN, n =16). Eight trained rats were killed after their final exercise bout for determination of GLUT-4 protein and mRNA expression: the remainder were killed 48 h after their last session for measurement of muscle glycogen and triacylglycerol concentration. GLUT-4 protein expression in NT rats was similar in both muscles after 8 weeks of either diet. However, there was a main effect of training such that GLUT-4 protein was increased in the soleus of rats fed with either diet (P < 0.05) and in the EDL in animals fed with CHO (P < 0.05). There was a significant diet–training interaction on GLUT-4 mRNA, such that expression was increased in both the soleus (100% ↑P < 0.05) and EDL (142% ↑P < 0.01) in CHO-fed animals. Trained rats fed with FAT decreased mRNA expression in the EDL (↓ 45%, P < 0.05) but not the soleus (↓ 14%, NS). We conclude that exercise training in CHO-fed rats increased both GLUT-4 protein and mRNA expression in type I and type II skeletal muscle. Despite lower GLUT-4 mRNA in muscles from fat-fed animals, exercise-induced increases in GLUT-4 protein were largely preserved, suggesting that control of GLUT-4 protein and gene expression are modified independently by exercise and diet.

ß- adrenergic stimulation of skeletal muscle HSL can be overridden by AMPK signaling

Hormone-sensitive lipase (HSL), an important regulatory enzyme for triacylglycerol hydrolysis within skeletal muscle, is controlled by β-adrenergic signaling as well as intrinsic factors related to contraction and energy turnover. In the current study, we tested the capacity of 5′AMP-activated protein kinase (AMPK) to suppress β-adrenergic stimulation of HSL activity. Eight male subjects completed 60 min of cycle exercise at 70% VO2 peak on two occasions: either with normal (CON) or low (LG) pre-exercise muscle glycogen content, which is known to enhance exercise-induced AMPK activity. Muscle samples were obtained before and immediately after exercise. Pre-exercise glycogen averaged 375 ± 35 and 163 ± 27 mmol·kg–1 dm for CON and LG, respectively. AMPK α-2 was not different between trials at rest and was increased (3.7-fold, P<0.05) by exercise during LG only. HSL activity did not differ between trials at rest and increased (0 min: 1.67 ± 0.13; 60 min: 2.60 ± 0.26 mmol·min–1·kg–1 dm) in CON. The exercise-induced increase in HSL activity was attenuated by AMPK α-2 activation in LG. The attenuated HSL activity during LG occurred despite higher plasma epinephrine levels (60 min: CON, 1.96 ± 0.29 vs LG, 4.25 ± 0.60 nM, P<0.05) compared with CON. Despite the attenuated HSL activity in LG, IMTG was decreased by exercise (0 min: 27.1 ± 2.0; 60 min: 22.5 ± 2.0 mmol.kg–1 dm, P<0.05), whereas no net reduction occurred in CON. To confirm the apparent effect of AMPK on HSL activity, we performed experiments in muscle cell culture. The epineprine-induced increase in HSL activity was totally attenuated (P<0.05) by AICAR administration in L6 myotubes. These data provide new evidence indicating that AMPK is a major regulator of skeletal muscle HSL activity that can override β-adrenergic stimulation. However, the increased IMTG degradation in LG suggests factors other than HSL activity are important for IMTG degradation.

Regulation of metabolic genes in human skeletal muscle by short-term exercise and diet manipulation

Changes in dietary macronutrient intake alter muscle and blood substrate availability and are important for regulating gene expression. However, few studies have examined the effects of diet manipulation on gene expression in human skeletal muscle. The aim of this study was to quantify the extent to which altering substrate availability impacts on subsequent mRNA abundance of a subset of carbohydrate (CHO)- and fat-related genes. Seven subjects consumed either a low- (LOW; 0.7 g/kg body mass CHO) or high- (HIGH; 10 g/kg body mass CHO) CHO diet for 48 h after performing an exhaustive exercise bout to deplete muscle glycogen stores. After intervention, resting muscle and blood samples were taken. Muscle was analyzed for the gene abundances of GLUT4, glycogenin, pyruvate dehydrogenase kinase-4 (PDK-4), fatty acid translocase (FAT/CD36), carnitine palmitoyltransferase I (CPT I), hormone-sensitive lipase (HSL), β-hydroxyacyl-CoA dehydrogenase (΄β-HAD), and uncoupling binding protein-3 (UCP3), and blood samples for glucose, insulin, and free fatty acid (FFA) concentrations. Glycogen-depleting exercise and HIGH-CHO resulted in a 300% increase in muscle glycogen content (P < 0.001) relative to the LOW-CHO condition. FFA concentrations were twofold higher after LOW- vs. HIGH-CHO (P < 0.05). The exercise-diet manipulation exerted a significant effect on transcription of all carbohydrate-related genes, with an increase in GLUT4 and glycogenin mRNA abundance and a reduction in PDK-4 transcription after HIGH-CHO (all P < 0.05). FAT/CD36 (P < 0.05) and UCP3 (P < 0.01) gene transcriptions were increased following LOW-CHO. We conclude that 1) there was a rapid capacity for a short-term exercise and diet intervention to exert coordinated changes in the mRNA transcription of metabolic related genes, and 2) genes involved in glucose regulation are increased following a high-carbohydrate diet.

Effect of AICAR treatment on gycogen metabolism in skeletal muscle

AMP-activated protein kinase (AMPK) is proposed to stimulate fat and carbohydrate catabolism to maintain cellular energy status. Recent studies demonstrate that pharmacologic activation of AMPK and mutations in the enzyme are associated with elevated muscle glycogen content in vivo. Our purpose was to determine the mechanism for increased muscle glycogen associated with AMPK activity in vivo. AMPK activity and glycogen metabolism were studied in red and white gastrocnemius muscles from rats treated with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) in vivo, and also in muscles incubated with AICAR in vitro. In vivo AICAR treatment reduced blood glucose and increased blood lactate compared with basal values. AICAR increased muscle α2 AMPK activity, glycogen, and glucose-6-phosphate concentrations. Glycogen synthase activity was increased in the red gastrocnemius but was decreased in the white gastrocnemius. Glycogen phosphorylase activity increased in both muscles, with an inhibition initially observed in the red gastrocnemius. In vitro incubation with AICAR activated α2 AMPK but had no effect on either glycogen synthase or glycogen phosphorylase. These results suggest that AICAR treatment does not promote glycogen accumulation in skeletal muscle in vivo by altering glycogen synthase and glycogen phosphorylase. Rather, the increased glycogen is due to the well-known effects of AICAR to increase glucose uptake.

Global and targeted gene expression and protein content in skeletal muscle of young men following short-term creatine monohydrate supplementation

Creatine monohydrate (CrM) supplementation has been shown to increase fat-free mass and muscle power output possibly via cell swelling. Little is known about the cellular response to CrM. We investigated the effect of short-term CrM supplementation on global and targeted mRNA expression and protein content in human skeletal muscle. In a randomized, placebo-controlled, crossover, double-blind design, 12 young, healthy, nonobese men were supplemented with either a placebo (PL) or CrM (loading phase, 20 g/day x 3 days; maintenance phase, 5 g/day x 7 days) for 10 days. Following a 28-day washout period, subjects were put on the alternate supplementation for 10 days. Muscle biopsies of the vastus lateralis were obtained and were assessed for mRNA expression (cDNA microarrays + real-time PCR) and protein content (Kinetworks KPKS 1.0 Protein Kinase screen). CrM supplementation significantly increased fat-free mass, total body water, and body weight of the participants (P < 0.05). Also, CrM supplementation significantly upregulated (1.3- to 5.0-fold) the mRNA content of genes and protein content of kinases involved in osmosensing and signal transduction, cytoskeleton remodeling, protein and glycogen synthesis regulation, satellite cell proliferation and differentiation, DNA replication and repair, RNA transcription control, and cell survival. We are the first to report this large-scale gene expression in the skeletal muscle with short-term CrM supplementation, a response that suggests changes in cellular osmolarity.

Uteroplacental insufficiency and reducing litter size alters skeletal muscle mitochondrial biogenesis in a sex-specific manner in the adult rat

Uteroplacental insufficiency has been shown to impair insulin action and glucose homeostasis in adult offspring and may act in part via altered mitochondrial biogenesis and lipid balance in skeletal muscle. Bilateral uterine vessel ligation to induce uteroplacental insufficiency in offspring (Restricted) or sham surgery was performed on day 18 of gestation in rats. To match the litter size of Restricted offspring, a separate cohort of sham litters had litter size reduced to five at birth (Reduced Litter), which also restricted postnatal growth. Remaining litters from sham mothers were unaltered (Control). Offspring were studied at 6 mo of age. In males, both Restricted and Reduced Litter offspring had reduced gastrocnemius PPAR γ coactivator-1α (PGC-1 α) mRNA and protein, and mitochondrial transcription factor A (mtTFA) and cytochrome oxidase (COX) III mRNA (P < 0.05), whereas only Restricted had reduced skeletal muscle COX IV mRNA and protein and glycogen (P < 0.05), despite unaltered glucose tolerance, homeostasis model assessment (HOMA) and intramuscular triglycerides. In females, only gastrocnemius mtTFA mRNA was lower in Reduced Litter offspring (P < 0.05). Furthermore, glucose tolerance was not altered in any female offspring, although HOMA and intramuscular triglycerides increased in Restricted offspring (P < 0.05). It is concluded that restriction of growth due to uteroplacental insufficiency alters skeletal muscle mitochondrial biogenesis and metabolic characteristics, such as glycogen and lipid levels, in a sex-specific manner in the adult rat in the absence of impaired glucose tolerance. Furthermore, an adverse postnatal environment induced by reducing litter size also restricts growth and alters skeletal muscle mitochondrial biogenesis and metabolic characteristics in the adult rat.

Carbohydrate ingestion does not alter skeletal muscle AMPK signaling during exercise in humans

There is evidence that increasing carbohydrate (CHO) availability during exercise by raising preexercise muscle glycogen levels attenuates the activation of AMPK{alpha}2 during exercise in humans. Similarly, increasing glucose levels decreases AMPK{alpha}2 activity in rat skeletal muscle in vitro. We examined the effect of CHO ingestion on skeletal muscle AMPK signaling during exercise in nine active male subjects who completed two 120-min bouts of cycling exercise at 65 ± 1% VO2 peak. In a randomized, counterbalanced order, subjects ingested either an 8% CHO solution or a placebo solution during exercise. Compared with the placebo trial, CHO ingestion significantly (P < 0.05) increased plasma glucose levels and tracer-determined glucose disappearance. Exercise-induced increases in muscle-calculated free AMP (17.7- vs. 11.8-fold), muscle lactate (3.3- vs. 1.8-fold), and plasma epinephrine were reduced by CHO ingestion. However, the exercise-induced increases in skeletal muscle AMPK{alpha}2 activity, AMPK{alpha}2 Thr172 phosphorylation and acetyl-CoA Ser222 phosphorylation, were essentially identical in the two trials. These findings indicate that AMPK activation in skeletal muscle during exercise in humans is not sensitive to changes in plasma glucose levels in the normal range. Furthermore, the rise in plasma epinephrine levels in response to exercise was greatly suppressed by CHO ingestion without altering AMPK signaling, raising the possibility that epinephrine does not directly control AMPK activity during muscle contraction under these conditions in vivo.

Effect of exercise intensity and hypoxia on skeletal muscle AMPK signaling and substrate metabolism in humans

We compared in human skeletal muscle the effect of absolute vs. relative exercise intensity on AMP-activated protein kinase (AMPK) signaling and substrate metabolism under normoxic and hypoxic conditions. Eight untrained males cycled for 30 min under hypoxic conditions (11.5% O2, 111 ± 12 W, 72 ± 3% hypoxia VO2 peak; 72% Hypoxia) or under normoxic conditions (20.9% O2) matched to the same absolute (111 ± 12 W, 51 ± 1% normoxia VO2 peak; 51% Normoxia) or relative (to VO2 peak) intensity (171 ± 18 W, 73 ± 1% normoxia VO2 peak; 73% Normoxia). Increases (P < 0.05) in AMPK activity, AMPK{alpha} Thr172 phosphorylation, ACCbeta Ser221 phosphorylation, free AMP content, and glucose clearance were more influenced by the absolute than by the relative exercise intensity, being greatest in 73% Normoxia with no difference between 51% Normoxia and 72% Hypoxia. In contrast to this, increases in muscle glycogen use, muscle lactate content, and plasma catecholamine concentration were more influenced by the relative than by the absolute exercise intensity, being similar in 72% Hypoxia and 73% Normoxia, with both trials higher than in 51% Normoxia. In conclusion, increases in muscle AMPK signaling, free AMP content, and glucose disposal during exercise are largely determined by the absolute exercise intensity, whereas increases in plasma catecholamine levels, muscle glycogen use, and muscle lactate levels are more closely associated with the relative exercise intensity.

Skeletal muscle fat metabolism during post-exercise recovery in humans

Recovery after prolonged or high-intensity exercise is characterised by a substantial increase in adipose tissue lipolysis, resulting in elevated rates of plasma-derived fat oxidation. Despite the large increase in circulating fatty acids (FAs) after exercise, only a small fraction of this is taken up by exercised muscle in the lower extremities. Indeed, the predominant fate of non-oxidised FAs derived from post-exercise lipolysis is reesteriflcation hi the liver. During recovery from endurance exercise, a number of changes also occur hi skeletal muscle that allow for a high metabolic priority towards glycogen resynthesis. Reducing muscle glycogen during exercise potentiates these effects, however the cellular and molecular mechanisms regulating substrate oxidation following exercise remain poorly defined. The broad arm of this thesis was to examine the regulation of fat metabolism during recovery from glycogen-lowering exercise hi the presence of altered fat and glucose availability. In study I, eight endurance-trained males completed a bout of exhaustive exercise followed by ingestion of carbohydrate (CHO)-rich meals (64-70% of energy from CHO) at 1, 4, and 7 h of recovery. Duplicate muscle biopsies were obtained at exhaustion and 3, 6 and 18 h of recovery. Despite the large intake of CHO during recovery (491 ± 28 g or 6.8 + 0.3 g • kg-1), respiratory exchange ratio values of 0.77 to 0.84 indicated a greater reliance on fat as an oxidative fuel. Intramuscular triacylglycerol (IMTG) content remained unchanged in the presence of elevated glucose and insulin levels during recovery , suggesting IMTG has a negligible role in contributing to the enhanced fat oxidation after exhaustive exercise. It appears that the partitioning of exogenous glucose towards glycogen resynthesis is of high metabolic priority during immediate post-exercise recovery, supported by the trend towards reduced pyruvate dehydrogenase (PDH) activity and increased fat oxidation. The effect of altering plasma FA availability during post-exercise recovery was examined in study II. Eight endurance-trained males performed three trials consisting of glycogen-lowering exercise, followed by infusion of either saline (CON), saline + nicotinic acid (NA) (LFA) or Intralipid and heparin (HFA). Muscle biopsies were obtained at the end of exercise (0 h) and at 3 and 6 h in recovery. Altering the availability of plasma FAs during recovery induced changes in whole-body fat oxidation that were unrelated to differences in skeletal muscle malonyl-CoA. Furthermore, fat oxidation and acetyl-CoA carboxylase (ACC) phosphorylation appear to be dissociated after exercise, suggesting mechanisms other than phosphorylation-mediated changes in ACC activity have an important role in regulating malonyl-CoA and fat metabolism in human skeletal muscle after exercise. Alternative mechanisms include citrate and long-chain fatty acyl-CoA mediated changes in ACC activity, or differences in malonyl-CoA decarboxylase (MCD) activity. Reducing plasma FA concentrations with NA attenuated the post-exercise increase in MCD and pyruvate dehydrogenase kinase 4 (PDK4) gene expression, suggesting that FAs and/or other factors induced by NA are involved hi the regulation of these genes. Despite marked changes hi plasma FA availability, no significant changes in IMTG concentration were detected, providing further evidence that plasma-derived FAs are the preferential fuel source contributing to the enhanced fat oxidation post-exercise during recovery. To further examine the effect of substrate availability after exercise, Study III investigated the regulation of fat metabolism during a 6 h recovery period with or without glucose infusion. Enhanced glucose availability significantly increased CHO oxidation compared with the fasted state, although no differences in whole-body fat oxidation were apparent. Consistent with the similar rates of fat metabolism, no difference hi AMPK or ACCβ phosphorylation were observed between trials. In addition, no significant treatment or time effects for IMTG concentration were detected during recovery. The large exercise-induced PDK4 gene expression was attenuated when plasma FAs were reduced during glucose infusion, supporting the hypothesis that PDK4 is responsive to sustained changes in lipid availability and/or changes in plasma insulin. Furthermore, the possibility exists that the suppression of PDK4 mRNA also reduced PDK activity and thus maintained PDH activity to account for the higher rates of CHO oxidation observed during glucose infusion compared with the control trial.

Regulation of skeletal muscle oxidative capacity and insulin signaling by the Mitochondrial Rhomboid Protease PARL

Type 2 diabetes mellitus (T2DM) and aging are characterized by insulin resistance and impaired mitochondrial energetics. In lower organisms, remodeling by the protease pcp1 (PARL ortholog) maintains the function and lifecycle of mitochondria. We examined whether variation in PARL protein content is associated with mitochondrial abnormalities and insulin resistance. PARL mRNA and mitochondrial mass were both reduced in elderly subjects and in subjects with T2DM. Muscle knockdown of PARL in mice resulted in malformed mitochondrial cristae, lower mitochondrial content, decreased PGC1α protein levels, and impaired insulin signaling. Suppression of PARL protein in healthy myotubes lowered mitochondrial mass and insulin-stimulated glycogen synthesis and increased reactive oxygen species production. We propose that lower PARL expression may contribute to the mitochondrial abnormalities seen in aging and T2DM.

Adrenaline and glycogenolysis in skeletal muscle during exercise : a study in adrenalectomised humans

1. The role of adrenaline in regulating muscle glycogenolysis and hormonesensitive lipase (HSL) activity during exercise was examined in six adrenalinedeficient bilaterally adrenalectomised, adrenocorticohormonalsubstituted humans (Adr) and in six healthy control individuals (Con).

2. Subjects cycled for 45 min at •70% maximal pulmonary Oμ uptake (ýO2,max) followed by 15 min at •86% ýO2,max either without (−Adr and Con) or with (+Adr) adrenaline infusion that elevated plasma adrenaline levels (45 min, 4·49 ± 0·69 nmol l¢; 60 min, 12·41 ± 1·80 nmol l¢). Muscle samples were obtained at 0, 45 and 60 min of exercise.

3. In −Adr and Con, muscle glycogen was similar at rest (−Adr, 409 ± 19 mmol (kg dry wt)¢; Con, 453 ± 24 mmol (kg dry wt)¢) and following exercise (−Adr, 237 ± 52 mmol (kg dry wt)¢; Con, 227 ± 50 mmol (kg dry wt)¢). Muscle lactate, glucose6phosphate and glucose were similar in −Adr and Con, whereas glycogen phosphorylase (aÏa + b ² 100 %) and HSL (% phosphorylated) activities increased during exercise in Con only. Adrenaline infusion increased activities of phosphorylase and HSL as well as blood lactate concentrations compared with those in −Adr, but did not enhance glycogen breakdown (+Adr, glycogen following exercise: 274 ± 55 mmol (kg dry wt)¢) in contracting muscle.

4. The present findings demonstrate that during exercise muscle glycogenolysis can occur in the absence of adrenaline, and that adrenaline does not enhance muscle glycogenolysis in exercising adrenalectomised subjects. Although adrenaline increases the glycogen phosphorylase activity it is not essential for glycogen breakdown in contracting muscle. Finally, a novel finding is that the activity of HSL in human muscle is increased in exercising man and this is due, at least partly, to stimulation by adrenaline.