Hormonal regulation of the rate of the glycogen/glucose-1-phosphate cycle in skeletal muscle

The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise

Initially it was believed that phosphorylase was responsible for both glycogen breakdown and synthesis in the living cell. The discovery of glycogen synthase and McArdle's disease (lack of phosphorylase activity), together with the high P_i/glucose 1-P ratio in skeletal muscle, demonstrated that glycogen synthesis could not be attributed to reversal of the phosphorylase reaction. Rather, glycogen synthesis was attributable solely to the activity of glycogen synthase, subsequent to the transport of glucose into the cell. However, the well-established observation that phosphorylase was inactivated (i.e., dephosphorylated) during the initial recovery period after prior exercise, when the rate of glycogen accumulation is highest and independent of insulin, suggested that phosphorylase could play an active role in glycogen accumulation. But the quantitative contribution of phosphorylase inactivation was not established until recently, when studying isolated murine muscle preparations during recovery from repeated contractions at temperatures ranging from 25 to 35 °C. Thus, in both slow-twitch, oxidative and fast-twitch, glycolytic muscles, inactivation of phosphorylase accounted for 45%-75% of glycogen accumulation during the initial hours of recovery following repeated contractions. Such data indicate that phosphorylase inactivation may be the most important mechanism for glycogen accumulation under defined conditions. These results support the initial belief that phosphorylase plays a quantitative role in glycogen formation in the living cell. However, the mechanism is not via activation of phosphorylase, but rather via inactivation of the enzyme.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Heating after intense repeated contractions inhibits glycogen accumulation in mouse EDL muscle: role of phosphorylase in postexercise glycogen metabolism

The effects of heating on glycogen synthesis (incorporation of [¹⁴C]glucose into glycogen) and accumulation after intense repeated contractions were investigated. Isolated mouse extensor digitorum longus muscle (type II) was stimulated electrically to perform intense tetanic contractions at 25°C. After 120 min recovery at 25°C, glycogen accumulated to almost 80% of basal, whereas after recovery at 35°C, glycogen remained low (~25% of basal). Glycogen synthesis averaged 0.97 ± 0.07 µmol·30 min^-1·g wet wt^-1 during recovery at 25°C and 1.48 ± 0.08 during recovery at 35°C ( P < 0.001). There were no differences in phosphorylase and glycogen synthase total activities nor in phosphorylase fractional activity, whereas glycogen synthase fractional activity was increased by ~50% after recovery at 35°C vs. 25°C. Inorganic phosphate (P_i, substrate for phosphorylase) was markedly increased (~300% of basal) following contraction but returned to control levels after 120 min recovery at 25°C. In contrast, P_i remained elevated after recovery at 35°C (>2-fold higher than recovery at 25°C). Estimates of glycogen breakdown indicated that phosphorylase activity (either via inhibition at 25°C or activation at 35°C) was responsible for ~60% of glycogen accumulation during recovery at 25°C and ~45% during recovery at 35°C. These data demonstrate that despite the enhancing effect of heating on glycogen synthesis during recovery from intense contractions, glycogen accumulation is inhibited owing to P_i-mediated activation of phosphorylase. Thus phosphorylase can play a quantitatively important role in glycogen biogenesis during recovery from repeated contractions in isolated type II muscle.

Increased temperature accelerates glycogen synthesis and delays fatigue in isolated mouse muscle during repeated contractions

AIM

Elevated glycogen content in muscle delays fatigue during exercise. We examined if increasing muscle temperature during recovery from exercise affects glycogen synthesis and muscle performance during a subsequent bout of exercise.

METHODS

Isolated mouse extensor digitorum longus muscles were stimulated electrically to perform repeated tetanic contractions until force decreased to 40% of initial at 25°C. Thereafter, muscles recovered for 120 minutes at 25°C (control), 120 minutes at 35°C or 60 minutes at 35°C followed by 60 minutes at 25°C. After recovery, muscles were again stimulated to fatigue at 25°C.

RESULTS

In the control group, the number of contractions in the second run was slightly less than during the first run (92 ± 5%). Following recovery for 120 minutes at 35°C, the number of contractions was similar to the first run (98 ± 6%). Allowing recovery for 120 minutes at 35°C in the presence of the antioxidant N-acetylcysteine also did not alter the number of contractions in the second run (98 ± 3%). However, recovery for 60 minutes at 35°C followed by 60 minutes at 25°C resulted in an increase in the number of contractions during the second run (110 ± 2%, P < .001). Incorporation of [¹⁴ C]glucose into glycogen (glycogen synthesis) during recovery was 1.7-fold higher at 35°C vs 25°C (1.44 ± 0.08 μmol (30 min)^-1  (g wet muscle)^-1 vs 0.84 ± 0.04; P < .001).

CONCLUSION

These data demonstrate that, under the conditions studied, elevating muscle temperature for 60 minutes following a bout of repeated contractions delays muscle fatigue during a subsequent bout of repeated contractions and this is associated with enhanced glycogen synthesis in isolated muscle.

Effects of insulin and transgenic overexpression of UDP-glucose pyrophosphorylase on UDP-glucose and glycogen accumulation in skeletal muscle fibers

UDP-glucose (UDP-Glc) and glycogen levels in skeletal muscle fibers of defined fiber type were measured using microanalytical methods. Infusing rats with insulin increased glycogen in both Type I and Type II fibers. Insulin was without effect on UDP-Glc in Type I fibers but decreased UDP-Glc by 35-40% in Type IIA/D and Type IIB fibers. The reduction in UDP-Glc suggested that UDP-Glc pyrophosphorylase (PPL) activity might limit glycogen synthesis in response to insulin. To explore this possibility, we generated mice overexpressing a UDP-Glc PPL transgene in skeletal muscle. The transgene increased both UDP-Glc PPL activity and levels of UDP-Glc in skeletal muscles by approximately 3-fold. However, overexpression of UDP-Glc PPL was without effect on either the levels of skeletal muscle glycogen or glucose tolerance in vivo. The transgene was also without effect on either control or insulin-stimulated rates of (14)C-glucose incorporation into glycogen in muscles incubated in vitro. The results indicate that UDP-Glc PPL activity is not limiting for glycogen synthesis.

Insulin-independent glycogen supercompensation in isolated mouse skeletal muscle: role of phosphorylase inactivation

Glycogen supercompensation (increase in muscle glycogen content above basal) is an established phenomenon induced by unknown mechanisms. It consists of both insulin-dependent and -independent components. Here, we investigate insulin-independent glycogen supercompensation in isolated, intact extensor digitorum longus muscles from mice. Muscles were stimulated electrically, incubated in vitro with 5.5 mM glucose for up to 16 h and then analysed for glycogen, glucose uptake and enzyme activities. Basal glycogen was 84+/-6 micro mol glucosyl units/g dry muscle and was depleted by 80% after 10 min contraction. Glycogen increased after contraction, reaching a peak value of 113+/-9 micro mol glucosyl units/g dry muscle ( P<0.05 vs. basal) by 6 h, and returned to basal values by 16 h (84+/-8). Maximal activities of glycogen synthase, phosphorylase and alpha-glucosidase were not significantly altered by contraction or during the 6-h recovery period. Glycogen synthase fractional activity (0.17/7.2 mM glucose-6-P; inversely related to phosphorylation state of the enzyme) was increased about twofold early after contraction but then decreased and was slightly lower than baseline during the period of supercompensation (4-6 h). Phosphorylase fractional activity (+/-adenosine monophosphate; directly related to phosphorylation state of the enzyme) decreased to 60% of basal after contraction and decreased further during the initial 4 h of recovery to 40% of basal ( P<0.01 vs. basal). After 4 h recovery, glucose uptake was slightly (50%) higher in the stimulated than in the non-stimulated muscle ( P<0.01). Thus, insulin-independent glycogen supercompensation involves inactivation of phosphorylase and hence an inhibition of glycogen breakdown.

Insulin increases near-membrane but not global Ca2+ in isolated skeletal muscle

It has long been debated whether changes in Ca2+ are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca2+ concentration and the near-membrane free Ca2+ concentration ([Ca2+]mem) in intact, single skeletal muscle fibers from mice by using fluorescent Ca2+ indicators. Insulin has no effect on the global free myoplasmic Ca2+ concentration. However, insulin increases [Ca2+]mem by approximately 70% and the half-maximal increase in [Ca2+]mem occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca2+]mem by insulin persists when sarcoplasmic reticulum Ca2+ release is inhibited but is lost by perfusing the fiber with a low Ca2+ medium or by addition of L-type Ca2+ channel inhibitors. Thus, insulin appears to stimulate Ca2+ entry into muscle cells via L-type Ca2+ channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca2+]mem. These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca2+]mem in skeletal muscle may underlie important actions of the hormone.

Mechanisms of whole-body glycogen deposition after oral glucose in normal subjects. Influence of the nutritional status

It is known that prior fasting enhances whole-body glycogen retention after glucose ingestion. To identify the involved mechanisms, 33 normal volunteers underwent a total fast, varying between 14 h and 4 days, and ingested thereafter 75 g glucose labeled with [14C]glucose. Measurements of oral glucose oxidation (expired 14CO2, corrected for incomplete recovery) and total carbohydrate (CHO) oxidation (indirect calorimetry) were performed over the following 5 h. These data allowed us to calculate oral glucose storage (uptake oxidation), glycogen oxidation (CHO oxidation - oral glucose oxidation), and net CHO balance (oral glucose uptake - CHO oxidation). As compared with an overnight fast, prolonged fasting (4 days) inhibited the uptake (64.8 vs. 70.3 g/5 h; P < 0.01) and the oxidation (10.9 vs. 20.0 g/5 h; P < 0.001) of oral glucose and stimulated slightly its conversion to glycogen (53.9 vs. 50.3 g/5 h; P < 0.05). The latter effect played only a minor role in the marked increase in net CHO balance (52.3 vs. 25.2 g/5 h; P < 0.001), which was almost entirely related to a decrease in glycogen oxidation (1.6 vs. 25.1 g/5 h; P < 0.001). Considering the whole series of data, including intermediate durations of fast, it was observed that the modifications in postprandial CHO metabolism, induced by fasting, correlated strongly with basal CHO oxidation, suggesting that the degree of initial glycogen depletion is a major determinant of glycogen oxidation and net CHO storage. Thus, prior fasting stimulates postprandial glycogen retention, mainly through an inhibition of the glycogen turnover that exists in overnight-fasted subjects, during the absorptive period.

Re-feeding after starvation involves a temporal shift in the control site of glycogen synthesis in rat muscle

The starved-to-fed transition is accompanied by rapid glycogen deposition in skeletal muscles. On the basis of recent findings [Bräu, Ferreira, Nikolovski, Raja, Palmer and Fournier (1997) Biochem. J. 322, 303-308] that during recovery from exercise there is a shift from a glucose 6-phosphate/phosphorylation-based control of glycogen synthesis to a phosphorylation-based control alone, this paper seeks to establish whether a similar shift occurs in muscle during re-feeding after starvation in the rat. Chow re-feeding after 48 h of starvation resulted in glycogen deposition in all muscles examined (white, red and mixed quadriceps, soleus and diaphragm) to levels higher than those in the fed state. Although the early phase of re-feeding was associated with increases in glucose 6-phosphate levels in all muscles, there was no accompanying increase in the fractional velocity of glycogen synthase except in the white quadriceps muscle. This finding, together with the observation that the fractional velocity of glycogen synthase in most muscles was already high in the starved state, suggests that in the initial phase of glycogen deposition the phosphorylation state of the enzyme may be adequate to support net glycogen synthesis. In the later phase of re-feeding, the progressive decrease in the fractional velocity of glycogen synthase in association with a decrease in the rate of glycogen deposition suggests that glycogen synthesis is controlled primarily by changes in the phosphorylation state of glycogen synthase. In conclusion, this study suggests that there is a temporal shift in the site of control of glycogen synthesis as glycogen deposition progresses during re-feeding after starvation.

Regulation of glycogen synthase and phosphorylase during recovery from high-intensity exercise in the rat

The aim of this study was to determine the role of the phosphorylation state of glycogen synthase and glycogen phosphorylase in the regulation of muscle glycogen repletion in fasted animals recovering from high-intensity exercise. Groups of rats were swum to exhaustion and allowed to recover for up to 120 min without access to food. Swimming to exhaustion caused substantial glycogen breakdown and lactate accumulation in the red, white and mixed gastrocnemius muscles, whereas the glycogen content in the soleus muscle remained stable. During the first 40 min of recovery, significant repletion of glycogen occurred in all muscles examined except the soleus muscle. At the onset of recovery, the activity ratios and fractional velocities of glycogen synthase in the red, white and mixed gastrocnemius muscles were higher than basal, but returned to pre-exercise levels within 20 min after exercise. In contrast, after exercise the activity ratios of glycogen phosphorylase in the same muscles were lower than basal, and increased to pre-exercise levels within 20 min. This pattern of changes in glycogen synthase and phosphorylase activities, never reported before, suggests that the integrated regulation of the phosphorylation state of both glycogen synthase and phosphorylase might be involved in the control of glycogen deposition after high-intensity exercise.

Inhibition of muscle glycogen synthase activity and non-oxidative glucose disposal during hypoglycaemia in normal man

The purpose of the present study was to evaluate the role of muscle glycogen synthase activity in the reduction of glucose uptake during hypoglycaemia. Six healthy young men were examined twice; during 120 min of hyperinsulinaemic (1.5 mU.kg-1. min-1) euglycaemia followed by: 1)240 min of graded hypoglycaemia (plasma glucose nadir 2.8 mmol/l) or 2) 240 min of euglycaemia. At 350-360 min a muscle biopsy was taken and indirect calorimetry was performed at 210-240 and 330-350 min. Hypoglycaemia was associated with markedly increased levels of adrenaline, growth hormone and glucagon and also with less hyperinsulinaemia. During hypoglycaemia the fractional velocity for glycogen synthase was markedly reduced; from 29.8 +/- 2.3 to 6.4 +/- 0.9%, p < 0.05. Total glucose disposal was decreased during hypoglycaemia (5.58 +/- 0.55 vs 11.01 +/- 0.75 mg.kg-1. min-1 (euglycaemia); p < 0.05); this was primarily due to a reduction of non-oxidative glucose disposal (2.43 +/- 0.41 vs 7.15 +/- 0.7 mg.kg-1 .min-1 (euglycaemia); p < 0.05), whereas oxidative glucose disposal was only suppressed to a minor degree. In conclusion hypoglycaemia virtually abolishes the effect of insulin on muscle glycogen synthase activity. This is in keeping with the finding of a marked reduction of non-oxidative glucose metabolism.

Dose-dependent elevation of cyclic AMP, activation of glycogen phosphorylase, and release of lactate by amylin in rat skeletal muscle

We report here our investigation of the role of cyclic AMP (cAMP) in amylin signal transduction in isolated strips of soleus muscle. Rat amylin, at 100 nM, increased cAMP levels, from 0.431 +/- 0.047 to a peak of 1.24 +/- 0.01 pmol cAMP/mg wet wt. after 5 min, in the absence of added phosphodiesterase inhibitor. The EC50 of the response was 0.48 nM (+/- 0.12 log units) in the absence of insulin and 0.3 nM (+/- 0.18 log units) in the presence of 7.1 nM insulin. The response seen with a maximally effective concentration of amylin (10 nM) was similar to that seen with a maximally effective concentration of epinephrine (1 microM) under the same conditions. Consistent with the observed rise in cAMP there was an increase in glycogen phosphorylase a (EC50 2.2 nM +/- 0.25 log units), decreased glycogen content (EC50 0.9 nM +/- 0.22 log units) and enhanced production of lactate (EC50 1.5 nM +/- 0.33 log units). These data support the concept that amylin promotes glycogenolysis in skeletal muscle and enhances production of lactate through glycolysis as a result of activation of Gs coupled receptors, stimulation of adenylate cyclase, elevation of cAMP levels and activation of glycogen phosphorylase.

Glycogen turnover in the isolated working rat heart

The isolated working rat heart was adapted for simultaneous determination of glycogen synthesis and degradation using a dual isotope technique. After prelabeling of glycogen with [U-14C]glucose, glycogenolysis was determined continuously from the washout of 14CO2 plus [14C]lactate. Glycogen synthesis was determined during the same period from incorporation of [5-3H]glucose. In the absence of added hormones, hearts were predominantly glycogenolytic (1.5 mumol/min/g, dry weight), and there was simultaneous synthesis (11% of the rate of glycogenolysis). The percentage of glucose taken up by the heart that could traverse the glycogen pool as a consequence of glycogen turnover was minor (5%). Insulin (10 milliunits/ml) predictably stimulated glycogen synthesis (3.6-fold) and nearly abolished glycogenolysis. Addition of glucagon (1 microgram/ml) increased contractile performance and initially stimulated glycogenolysis (3.8-fold) until glycogen was largely depleted. Net tritium incorporation was unaffected by glucagon. Both hormones stimulated glycolytic flux from exogenous glucose (3H2O from [5-3H]glucose) as well as total glycolytic flux (3H2O plus glycogenolysis). The initial stimulation in total glycolytic flux with glucagon was largely from glycogen, explaining the lag in stimulation from exogenous glucose. The relationship between the specific radioactivity and amount of glycogen remaining after different degrees of glycogenolysis suggests that the preference of glycogenolysis for newly synthesized glycogen is only partial.

On the mechanism of inactivation of muscle glycogen phosphorylase by insulin

Glucose 6-phosphate, an allosteric inhibitor of skeletal muscle phosphorylase b, inhibits at physiological concentrations and conditions the phosphorylation and activation of the enzyme by phosphorylase b kinase. AMP inhibits the dephosphorylation of phosphorylase a, but is without effect on the phosphorylation of phosphorylase b. Glucose 6-phosphate has no effect on the activity of phosphorylase a and does not affect its dephosphorylation by phosphatases 1 or 2A. The inhibition of the phosphorylation of phosphorylase b by glucose 6-phosphate may explain the reported decreased phosphorylation of phosphorylase in muscle following insulin treatment, which elevates intracellular levels of glucose 6-phosphate.

Beta-adrenoceptor-agonist and insulin actions on glucose metabolism in rat skeletal muscle in different thyroid states

1. The actions of the beta-adrenoceptor agonist isoprenaline on glucose and glycogen metabolism, in the presence of various concentrations of insulin, were investigated in isolated soleus muscle preparations taken from eu-, hyper- and hypothyroid rats. 2. Hyperthyroidism, induced by 3,3',5-tri-iodo-D-thyronine (T3) administration for 5 days, increased the rate of lactate formation and suppressed the rate of glycogen synthesis in soleus muscle in response to isoprenaline, even in the presence of physiological or supraphysiological insulin concentrations. 3. Hypothyroidism, induced by administration of 6-n-propyl-2-thiouracil for 4 weeks, decreased the rate of isoprenaline-stimulated lactate formation at all insulin concentrations, but significantly decreased the responsiveness of lactate formation only at low insulin concentrations. In the presence of 100 or 10,000 mu-units of insulin/ml, the ability of isoprenaline to suppress the rate of glycogen synthesis was markedly impaired (inhibition at 100 mu-units of insulin/ml and 1 micro-M-isoprenaline: eu- 72.6 +/- 2.9%; hypo-41.0 +/- 2.1%; P less than 0.001). 4. Hyperthyroidism had no effect on the number or affinity of beta-adrenoceptors, defined by 125I-pindolol binding, or beta-adrenoceptor- or forskolin-stimulated adenylate cyclase activity in membrane preparations of gastrocnemius muscle, whereas hypothyroidism increased the beta-adrenoceptor density and decreased the beta-adrenoceptor-stimulated adenylate cyclase activity, without affecting the receptor affinity or forskolin-stimulated adenylate cyclase activity. 5. It is concluded that there is a complex interplay between insulin, catecholamines and thyroid hormones to regulate skeletal-muscle glucose metabolism. The changes observed in muscles in hypothyroidism may be explained, at least in part, by changes in beta-adrenoceptor-G-protein-adenylate cyclase coupling affecting the generation of cyclic AMP and the regulation of some of the key enzymes of glycogen metabolism; in contrast, the changes observed in muscles in hyperthyroidism do not appear to result from alterations at the level of the receptor-mediated second-messenger generation.

Quantitation of glycogen/glucose-1-P cycling in liver

A method is introduced for quantitating cycling between hepatic glycogen and glucose-1-P in humans. It depends on the administration of trace [2-3H,6-14C]galactose, a glucose load, and acetaminophen. The ratio of 3H to 14C in the glucuronide of the acetaminophen excreted in urine to that in the administered galactose provides the measure of the fraction of glycogen synthesized that is synthesized from glucose-1-P formed from glycogen. The quantity of glucose-1-P formed from glycogen that is not reconverted to glycogen is not measured. It is assumed that the glucuronide samples the UDP-glucose pool in liver from which glycogen is formed, the last glucosyl units formed from UDP-glucose in glycogen synthesis are the first broken down, and the equilibration of [2-3H]glucose-1-P with fructose-6-P is rapid relative to its conversion to UDP-glucose. During a 5-hour period, while three normal subjects and three non-insulin-dependent diabetics, who had fasted overnight, were infused with 4 mg/kg/min of glucose, the rate of glycogen breakdown, as measured using the method, was only a small percentage of the rate of glycogen synthesis.

Epinephrine inhibits insulin-mediated glycogenesis but enhances glycolysis in human skeletal muscle

The effect of epinephrine (E) infusion on insulin-mediated glucose metabolism in humans has been studied. Eight glucose-tolerant men were studied on two separate occasions: 1) during 120 min of euglycemic hyperinsulinemia (UH, approximately 5 mM; 40 mU.m-2.min-1); and 2) during UH while E was infused (UHE, 0.05 microgram.kg-1.min-1). Biopsies were taken from the quadriceps femoris muscle before and after each clamp. Glucose disposal, correcting for endogenous glucose production, was 36 +/- 3 and 18 +/- 2 (SE) mumol.kg fat-free mass (FFM)-1.min-1 during the last 40 min of UH and UHE, respectively (P less than 0.001). Nonoxidative glucose disposal (presumably glycogenesis) averaged 23.0 +/- 3.0 and 4.0 +/- 1.1 (P less than 0.001), whereas carbohydrate oxidation (which is proportional to glycolysis) averaged 13.1 +/- 1.4 and 15.3 +/- 1.1 mumol.kg FFM-1.min-1 (P less than 0.05) during UH and UHE, respectively. UHE resulted in significantly higher contents of UDP-glucose, hexose monophosphates, postphosphofructokinase intermediates, and glucose 1,6-bisphosphate (G-1,6-P2) in muscle (P less than 0.05-0.001), but there were no significant differences in high-energy phosphates or fructose 2,6-bisphosphate (F-2,6-P2) between treatments. Fractional activities of phosphorylase increased (P less than 0.01), and glycogen synthase decreased (P less than 0.001) during UHE. It is concluded that E inhibits insulin-mediated glycogenesis because of an inactivation of glycogen synthase and an activation of glycogenolysis. E also appears to inhibit insulin-mediated glucose utilization, at least partly, because of an increase in G-6-phosphate (which inhibits hexokinase) and enhances glycolysis by G-1,6-P2-, fructose 6-phosphate-, and F-1,6-P2-mediated activation of PFK.

Hyperthyroid myopathy and the response to treatment

Hyperthyroidism is accompanied by significant dysfunction of both proximal and distal skeletal muscles. The purpose of this study was to quantitate the degree of muscle weakness in newly diagnosed patients with Graves' disease and to assess the response to treatment. Ten patients were prospectively studied with objective measures of strength and endurance of proximal and distal muscles while hyperthyroid (stage I), after 2 weeks of propranolol (stage II), and about 6 months later when euthyroid (stage III). Propranolol treatment for 2 weeks resulted in a subjective decrease in weakness, which was accompanied by a statistically significant improvement in grip strength (P less than 0.01), shoulder strength (P less than 0.02), and grip endurance (P less than 0.01) but not shoulder endurance. Muscle function further improved and attained control levels when the patients were chemically and clinically euthyroid. In contrast, a control group subjected to the same muscle testing protocol before and after 1 week of propranolol treatment showed no improvement in grip, shoulder strength, or shoulder endurance but had decreased grip endurance (P less than 0.01) and increased subjective weakness. These results confirm that muscle weakness commonly is associated with hyperthyroidism and can be quantitatively profound. In contrast to the effects of beta-blockade in normal controls, propranolol partially improves muscle weakness in thyrotoxic patients. We conclude that thyroid hormone and catecholamines in concert mediate the muscle dysfunction of hyperthyroidism.

The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle

The ability of insulin to promote the phosphorylation of some proteins and the dephosphorylation of others is paradoxical. An insulin-stimulated protein kinase is shown to activate the type-1 protein phosphatase that controls glycogen metabolism, by phosphorylating its regulatory subunit at a specific serine. Furthermore, the phosphorylation of this residue is stimulated by insulin in vivo. Increased and decreased phosphorylation of proteins by insulin can therefore be explained through the same basic underlying mechanism.

Glucose utilization by skeletal muscles in vivo in experimental hyperthyroidism in the rat

In the fed state, hyperthyroidism increased glucose utilization indices (GUIs) of skeletal muscles containing a lower proportion of oxidative fibres. Glycogen concentrations were unchanged, but active pyruvate dehydrogenase (PDHa) activities were decreased. Hyperthyroidism attenuated the effects of 48 h of starvation to decrease muscle GUI. Glycogen concentrations and PDHa activities after 48 h of starvation were low and similar in euthyroid and hyperthyroid rats. The increase in glucose uptake and phosphorylation relative to oxidation and storage in skeletal muscle induced by hyperthyroidism may contribute to increased glucose re-cycling in the fed hyperthyroid state and to glucose turnover in the starved hyperthyroid state.

Simultaneous synthesis and degradation of rat liver glycogen. An in vivo nuclear magnetic resonance spectroscopic study

Using 13C nuclear magnetic resonance spectroscopic methods we examined in vivo the synthesis of liver glycogen during the infusion of D-[1-13C]glucose and the turnover of labeled glycogen during subsequent infusion of D-[1-13C]glucose. In fasted rats the processes of glycogen synthesis and degradation were observed to occur simultaneously with the rate of synthesis much greater than degradation leading to net glycogen synthesis. In fed rats, incorporation of infused D-[1-13C]glucose occurred briskly; however, over 2 h there was no net glycogen accumulated. Degradation of labeled glycogen was greater in the fed versus the fasted rats (P less than 0.001), and the lack of net glycogen synthesis in fed rats was due to degradation and synthesis occurring at similar rates throughout the infusion period. There was no indication that suppression of phosphorylase a or subsequent activation of glycogen synthase was involved in modulation of the flux of tracer into liver glycogen. We conclude that in both fed and fasted rats, glycogen synthase and phosphorylase are active simultaneously and the levels of liver glycogen reached during refeeding are determined by the balance between ongoing synthetic and degradative processes.

Regulation of plasma lactate concentration in resting human subjects

We evaluated the relative contributions of glucose, insulin, and the rate of glucose disposal to the regulation of the plasma lactate concentration. Rates of glucose disposal were measured in 88 separate studies in whole body and across the forearm at varying plasma insulin (9, 50, 160, and 1,800 microU/mL) concentrations, and at each insulin concentration at four different glucose concentrations (90, 160, 250, and 400 mg/dL) in healthy male subjects. The rate of glucose disposal was positively correlated with the plasma lactate concentration (r = .83, n = 88, P less than .0001). When the plasma lactate concentration was adjusted for the rate of glucose disposal, plasma glucose or insulin concentrations did not contribute significantly to the residual variation in plasma lactate. When plasma lactate concentrations were compared at matched rates of glucose disposal, the lactate levels were similar regardless of whether glucose disposal was induced by hyperglycemia or hyperinsulinemia. At the lowest glucose and insulin concentrations, forearm tissues released lactate, but at all other glucose and insulin concentrations, no significant net lactate flux was observed. After subtraction of the rate of forearm glucose disposal from whole-body glucose disposal, the plasma lactate concentration correlated with the remaining, extramuscular, rate of glucose disposal (r = .60, P less than .0001). These data suggest that in resting normal subjects the plasma lactate concentration may be determined by the rate of glucose disposal in extramuscular tissues, rather than the ambient glucose or insulin concentration.

Fuel selection and carbon flux during the starved-to-fed transition

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Calcitonin gene-related peptide-1 (CGRP-1) is a potent regulator of glycogen metabolism in rat skeletal muscle

We investigated the effects of CGRP on glucose metabolism in intact rat skeletal muscle preparations that are largely composed of either type I (soleus) or II fibres (e.g. extensor digitorum longus (EDL) or epitrochlearis muscles). CGRP-1 inhibited insulin-stimulated glycogen synthesis in both soleus and EDL muscle preparations. Rat CGRP-1 was a potent stimulator of glycogenolysis only in muscles composed of type II fibres, which depend on high rates of glycogenolysis to produce high power outputs. These results may provide the basis for understanding how CGRP regulate glycogenolysis in type II fibres in vivo.

Effects of hypothyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin in the soleus muscle of the rat

1. The effects of hypothyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin were investigated in the isolated, incubated soleus muscle of the rat. 2. Hypothyroidism, which was induced by administration of propylthiouracil to the rats, decreased fasting plasma levels of free fatty acids and increased plasma levels of glucose but did not significantly change plasma levels of insulin. 3. The sensitivity of the rates of glycogen synthesis to insulin was increased at physiological, but decreased at supraphysiological, concentrations of insulin. 4. The rates of glycolysis in the hypothyroid muscles were decreased at all insulin concentrations studied and the EC50 for insulin was increased more than 8-fold; the latter indicates decreased sensitivity of this process to insulin. However, at physiological concentrations of insulin, the rates of glucose phosphorylation in the soleus muscles of hypothyroid rats were not different from controls. This suggests that hypothyroidism affects glucose metabolism in muscle not by affecting glucose transport but by decreasing the rate of glucose 6-phosphate conversion to lactate and increasing the rate of conversion of glucose 6-phosphate to glycogen. 5. The rates of glucose oxidation were decreased in the hypothyroid muscles at all insulin concentrations.

Effects of hyperthyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin in the soleus muscle of the rat

1. The effects of hyperthyroidism on the sensitivity and responsiveness of glycolysis and glycogen synthesis to insulin were investigated in the isolated incubated soleus muscle of the rat. 2. Hyperthyroidism, which was induced by administration of tri-iodothyronine (T3) to rats for 2, 5 or 10 days, increased fasting plasma concentrations of glucose, insulin and free fatty acids. 3. Administration of T3 for 2 or 5 days increased the rates of glycolysis at all insulin concentrations studied: this was due to increased rates of both glucose phosphorylation and glycogen breakdown, but there was no effect of T3 on the sensitivity of glycolysis to insulin. However, administration of T3 for 10 days increased the sensitivity of the rate of glycolysis to insulin. 4. The concentration of adenosine in the gastrocnemius muscles of the rats was not different from controls after 5 days, but it was markedly decreased after 10 days of T3 administration. If these changes are indicative of changes in the soleus muscle, the increased sensitivity of glycolysis to insulin found after 10 days' T3 administration could be due to the decrease in the concentration of adenosine. 5. Administration of T3 decreased the sensitivity of glycogen synthesis to insulin and the glycogen content of the soleus muscles. This may explain the decreased rates of non-oxidative glucose disposal found in spontaneous and experimental hyperthyroidism in man. 6. The rates of glucose oxidation did not change after 2 days, but they were increased after 5 and 10 days of T3 administration.

Time course of the response of carbohydrate metabolism to unloading of the soleus

The time course of the response of carbohydrate metabolism to unloading was studied in the soleus muscle of rats subjected to tail-cast suspension. In the fresh soleus, just 12 hours of unloading led to higher concentrations of glycogen and lower activity ratios of both glycogen synthase and glycogen phosphorylase. These changes were still evident on day 3. This initial accumulation of glycogen was likely due to its decreased degradation in response to muscle disuse. Thereafter, the increased glycogen concentration apparently diminished the activity ratio of glycogen synthase, leading to a subsequent fall in the total glycogen content after day 1. After 24 hours of unloading, when no significant atrophy was detectable, there was no differential response to insulin for in vitro glucose metabolism. As reported for day 6 (reference 6), on day 3 the soleus atrophied significantly and displayed a greater sensitivity to insulin for most of these parameters compared to the weight-bearing control muscle. However, insulin sensitivity for glycogen synthesis was unchanged. These results showed that the increased sensitivity to insulin of the unloaded soleus is associated with the degree of muscle atrophy, likely due to an increased insulin binding capacity relative to muscle mass. This study also showed that insulin regulation of glucose uptake and of glycogen synthesis is affected differentially in the unloaded soleus muscle.