Effects of dexamethasone treatment on insulin-stimulated rates of glycolysis and glycogen synthesis in isolated incubated skeletal muscles of the rat.

1. Rats were treated with dexamethasone for 4 days before measurement of the rates of lactate formation [which is an index of hexose transport; see Challiss, Lozeman, Leighton & Newsholme (1986) Biochem. J. 233, 377-381] and glycogen synthesis in response to various concentrations of insulin in isolated incubated soleus and extensor digitorum longus muscle preparations. 2. The concentration of insulin required to stimulate these processes half-maximally in soleus and extensor digitorum longus muscles isolated from control rats was about 100 muunits/ml. 3. Dexamethasone increases the concentration of insulin required to stimulate glycolysis half-maximally in soleus and extensor digitorum longus preparations to 250 and 300 muunits/ml respectively. The respective insulin concentrations necessary to stimulate glycogen synthesis half-maximally were about 430 and 370 muunits/ml for soleus and extensor digitorum longus muscle preparations isolated from steroid-treated rats. 5. Dexamethasone treatment did not change the amount of insulin bound to soleus muscle.     

Studies on the conversion of pyruvate into fatty acids in white adipose tissue. Effects of insulin, alloxan-diabetes and starvation

The effect of insulin on the conversion of pyruvate into fatty acids in the presence and in the absence of glucose was studied in epididymal adipose tissue of the rat. 1. In adipose tissue from the normal rat, conversion of pyruvate into fatty acids is directly related to its concentration, the maximal rates occurring with 40mm- and the half-maximal rates with approx. 4mm-pyruvate. Insulin treatment did not greatly influence the maximal rates, but the half-maximal rates were at much lower pyruvate concentrations. This effect of insulin could be seen with physiological concentrations of this hormone (50-100muunits/ml). 2. In adipose tissue from acute-alloxan-diabetic and 36h-starved rats the conversion of pyruvate into fatty acids was almost zero until its concentration exceeded 3mm and then increased markedly as the concentration of pyruvate was increased. The lag phase of this S-shaped curve was decreased but not eliminated when insulin was present. This could account for the very low rates of glucose conversion into fatty acids in these metabolic states. Maximum rates of fatty acid synthesis were similar in the presence and in the absence of insulin, but only when 30-40mm-pyruvate was employed. Re-feeding of the starved rats or insulin treatment of the diabetic rats in vivo for several days restored these patterns to normal.     

Effects of analogues of adenosine and methyl xanthines on insulin sensitivity in soleus muscle of the rat

The concentration of insulin that produces half-maximal stimulation of glycolysis by stripped soleus muscle preparations is markedly increased by the adenosine analogues, 2-chloroadenosine and N6-phenylisopropyladenosine, but is markedly decreased by the methyl xanthine analogue, 8-phenyltheophylline. 2-Chloroadenosine increases the concentration of insulin required to stimulate glycolysis half maximally, from about 100 to 2000 mu units/ml. 8-Phenyltheophylline decreases this concentration of insulin from about 100 to 10 mu units/ml, an effect which is similar to that produced either by addition of adenosine deaminase to the medium or to exercise-training of the donor animals for 4 weeks.     

Insulin sensitivity and responsiveness of epitrochlearis and soleus muscles from fed and starved rats. Recognition of differential changes in insulin sensitivities of protein synthesis and glucose incorporation into glycogen.

The insulin sensitivity of protein synthesis and glucose incorporation into glycogen by the soleus and epitrochlearis muscles from fed rats and 24 h-starved rats was determined in vitro during the first and second hours of incubation after isolation of the muscles. Rates of protein synthesis by both muscles from fed rats in the first hour of incubation were 2-fold higher than in the second hour and were not increased by insulin. Rates of protein synthesis during the first hour in the presence of 6000 microunits of insulin/ml were increased in soleus, but not in epitrochlearis, muscles from starved rats. Rates of protein synthesis in both muscles from fed and starved rats were increased significantly by insulin during the second hour. High concentrations of insulin caused a marked stimulation of the rates of glucose incorporation by both muscles from fed and starved rats in both the first and second hours of incubation. The insulin sensitivity of glucose incorporation during the second hour, defined as the concentration of insulin causing half-maximal stimulation, was increased 10-fold for both muscle types from starved rats (soleus, 65 microunits/ml; epitrochlearis, 45 microunits/ml) relative to muscles from fed rats (soleus, 600 microunits/ml; epitrochlearis, 500 microunits/m). The insulin sensitivity of protein synthesis in the second hour was greater for soleus muscles from starved rats (65 microunits/ml) than from fed rats (500 microunits/ml). In contrast, the insulin sensitivity of protein synthesis in epitrochlearis muscles from starved rats was significantly decreased (225 microunits/ml) compared with fed rats (25 microunits/ml Maximal rates achieved by high concentrations of insulin were not different from those in the same muscle from fed rats. It is suggested that protein synthesis, in distinction to glucose utilization, may be resistant to insulin stimulation during periods of acute starvation in muscles with fibre compositions similar to the epitrochlearis, but not in muscles with fibre compositions similar to the soleus. Partial reversal of the resistance observed in vitro for epitrochlearis muscles from starved rats may be due to the loss of factors which suppress the effect of insulin in vivo.     

Insulin resistance in soleus muscle from obese Zucker rats. Involvement of several defective sites

1. The effect of insulin upon glucose transport and metabolism in soleus muscles of genetically obese (fa/fa) and heterozygote lean Zucker rats was investigated at 5–6 weeks and 10–11 weeks of age. Weight-standardized strips of soleus muscles were used rather than the intact muscle in order to circumvent problems of diffusion of substrates. 2. In younger obese rats (5–6 weeks), plasma concentrations of immunoreactive insulin were twice those of controls, whereas their circulating triacylglycerol concentrations were normal. Insulin effects upon 2-deoxyglucose uptake and glucose metabolism by soleus muscles of these rats were characterized by both a decreased sensitivity and a decrease in the maximal response of this tissue to the hormone. 3. In older obese rats (10–11 weeks), circulating concentrations of insulin and triacylglycerols were both abnormally elevated. A decrease of 25–35% in insulin-binding capacity to muscles of obese rats was observed. The soleus muscles from the older obese animals also displayed decreased sensitivity and maximal response to insulin. However, at a low insulin concentration (0.1m-i.u./ml), 2-deoxyglucose uptake by muscles of older obese rats was stimulated, but such a concentration was ineffective in stimulating glucose incorporation into glycogen, and glucose metabolism by glycolysis. 4. Endogenous lipid utilization by muscle was calculated from the measurements of O₂ consumption, and glucose oxidation to CO₂. The rate of utilization of fatty acids was normal in muscles of younger obese animals, but increased in those of the older obese rats. Increased basal concentrations of citrate, glucose 6-phosphate and glycogen were found in muscles of older obese rats and may reflect intracellular inhibition of glucose metabolism as a result of increased lipid utilization. 5. Thus several abnormalities are responsible for insulin resistance of muscles from obese Zucker rats among which we have observed decreased insulin binding, decreased glucose transport and increased utilization of endogenous fatty acid which could inhibit glucose utilization.     

Sensitivity to insulin of glycolysis and glycogen synthesis of isolated soleus-muscle strips from sedentary, exercised and exercise-trained rats.

The half-maximal stimulation of the rates of glycolysis and glycogen synthesis in soleus-muscle strips from sedentary animals occurred at a concentration of insulin of about 100 microunits/ml. In soleus-muscle strips from exercise-trained rats (5 weeks of treadmill training), half-maximal stimulation of the rate of glycolysis occurred at about 10 microunits of insulin/ml, whereas that for glycogen synthesis occurred between 10 and 100 microunits of insulin/ml. The sensitivity of glycolysis to insulin after exercise training is similar to that of adipose tissue from sedentary animals. This finding suggests that, in sedentary animals, the effects of normal changes in insulin concentration may affect muscle primarily indirectly via the anti-lipolytic effect on adipose tissue, whereas after training insulin may effect the rate of glycolysis in muscle directly. A single period of exercise did not change the sensitivity of glycolysis in soleus muscle to insulin, nor probably that of glycogen synthesis. It is suggested that the improvement in insulin sensitivity of glycolysis in muscle caused by exercise-training could account, in part, for the well-established improvement in glucose tolerance and insulin sensitivity observed in man and rats after exercise-training.     

Effects of aging on the responsiveness and sensitivity of glucose metabolism to insulin in the incubated soleus muscle isolated from Sprague-Dawley and Wistar rats.

1. The effects of aging on the sensitivity and responsiveness of glucose transport, lactate formation and glycogen synthesis to insulin were studied in the incubated stripped soleus muscle isolated from aging Sprague-Dawley and Wistar rats. 2. As Sprague-Dawley rats aged from 5 to 13 weeks, there were marked increases in the concentrations of insulin that were required for half-maximal stimulation (i.e. EC50 value, which is a measure of sensitivity) of glucose transport, lactate formation and glycogen synthesis. 3. In marked contrast, there were no alterations in sensitivities of any of these processes to insulin in soleus muscle prepared from Wistar rats aged between 6 and 12 weeks. 4. However, in soleus muscles from 85-week-old Wistar rats the rates of glycogen synthesis in response to basal, sub-maximal and maximal concentrations of insulin were markedly decreased. The insulin EC50 value of glycogen synthesis was increased 4-fold, but was unchanged for lactate formation. 5. The insulin-stimulated rates of glucose transport in soleus muscles from 5- or 85-week-old Wistar rats were not significantly different.     

Effect of acetoacetate on glucose metabolism in the soleus and extensor digitorum longus muscles of the rat.

1. The effect of acetoacetate on glucose metabolism was compared in the soleus, a slow-twitch red muscle, and the extensor digitorum longus, a muscle composed of 50% fast-twitch red and 50% white fibres. 2. When incubated for 2h in a medium containing 5 mM-glucose and 0.1 unit of insulin/ml, rates of glucose uptake, lactate release and glucose oxidation in the soleus were 19.6, 18.6 and 1.47 micronmol/h per g respectively. Acetoacetate (1.7 mM) diminished all three rates by 25-50%; however, it increased glucose conversion into glycogen. In addition, it caused increases in tissue glucose, glucose 6-phosphate and fructose 6-phosphate, suggesting inhibition of phosphofructokinase. The concentrations of citrate, an inhibitor of phosphofructokinase, and of malate were also increased. 3. Rates of glucose uptake and lactate release in the extensor digitorum longus were 50-80% of those in the soleus. Acetoacetate caused moderate increases in tissue glucose 6-phosphate and possibly citrate, but it did not decrease glucose uptake or lactate release. 4. The rate of glycolysis in the soleus was approximately five times that previously observed in the perfused rat hindquarter, a muscle preparation in which acetoacetate inhibits glucose oxidation, but does not alter glucose uptake or glycolysis. A similar rate of glycolysis was observed when the soleus was incubated with a glucose-free medium. Under these conditions, tissue malate and the lactate/pyruvate ratio in the medium were decreased, and acetoacetate did not decrease lactate release or increase tissue citrate or glucose 6-phosphate. An intermediate rate of glycolysis, which was not decreased by acetoacetate, was observed when the soleus was incubated with glucose, but not insulin. 5. The data suggest that acetoacetate glucose inhibits uptake and glycolysis in red muscle under conditions that resemble mild to moderate exercise. They also suggest that the accumulation of citrate in these circumstances is linked to the rate of glycolysis, possibly through the generation of cytosolic NADH and malate formation.     

Beta-adrenergic effects on carbohydrate metabolism in the unweighted rat soleus muscle

The effects of insulin on carbohydrate metabolism in atrophied rat soleus muscle are increased after unweighting by tail-cast suspension. This work has been extended by testing the effect of unweighting on the response of carbohydrate metabolism to isoproterenol, a beta-adrenergic agonist. Isoproterenol promoted glycogen degradation more in the unweighted than in the weight-bearing soleus but showed no differences in the extensor digitorum longus, which is unresponsive to hindlimb unweighting. In soleus muscles depleted of glycogen, to avoid varied inhibitory effects of glycogen on glycogen synthesis, isoproterenol inhibited this process more in the unweighted muscle. Isoproterenol did not have a greater inhibitory effect on net uptake of 2-deoxy-D[1,2-3H]glucose by the unweighted muscle. Measurements of intracellular 2-deoxy-[3H]glucose 6-phosphate and 3-O-methyl-D-[1-3H]glucose, which cannot be phosphorylated, showed that isoproterenol inhibited glucose phosphorylation but not transport. This effect could be explained by an increase of glucose 6-phosphate, an inhibitor of hexokinase. At 100 microU insulin/ml but not at a lower amount (10 microU/ml), isoproterenol inhibited hexose phosphorylation more in the control than in the unweighted muscle. This result may be explained by greater insulin antagonism in the unweighted muscle owing to increased insulin sensitivity. However, insulin antagonism of isoproterenol stimulation of glycogenolysis or inhibition of glycogenesis was not altered by unweighting. Therefore, for some aspects of carbohydrate metabolism, the unweighted muscle has an increased response to beta-adrenergic activation, just as this muscle shows increased responses to insulin.     

The effect of glucose, insulin and adrenaline on glycerol metabolism in vitro in rat adipose tissue.

The uptake and utilization of [1-14C]glycerol was determined in pieces of rat epididymal fat-pads incubated in Krebs--Ringer bicarbonate buffer containing albumin. Insulin (200 muunits/ml), adrenaline (epinephrine; 0.5 mug/ml) and glucose (0, 5, 15 and 20 mM) were added to the medium. Changes in the specific radioactivity of the tracer during the incubation were taken into account in calculating the rate of glycerol utilization. Adrenaline decreased glycerol uptake, whereas insulin plus adrenaline increased it. The rate of incorporation of glycerol into glycerides was decreased by adrenaline and insulin, singly or together. Insulin increased the rate of formation of CO2 and fatty acids from glycerol. The formation of CO2 and fatty acids was further enhanced by insulin plus adrenaline. The decrease in glycerol uptake induced by adrenaline, the decrease in incorporation of glycerol into glycerides induced by insulin and insulin plus adrenaline and the synthesis of fatty acids were dependent on the presence of glucose in the medium. Thus insulin and adrenaline act on glycerol utilization in adipose tissue and some of their effects are mediated by action on glucose metabolism, but others are independent of this.     

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.     

Effects of adenosine deaminase on the sensitivity of glucose transport, glycolysis and glycogen synthesis to insulin in muscles of the rat

1. Soleus, extensor digitorum longus (EDL) or hemi-diaphragm muscles of the rat were incubated in the presence of insulin and rates of the processes of glycolysis and glycogen synthesis were measured. 2. The concentrations of insulin required to cause half-maximal stimulation of glycolysis in both soleus and EDL preparations were significantly decreased by the presence of adenosine deaminase in the medium. 3. Adenosine deaminase increased the sensitivity of the process of hexose transport to insulin (in an identical manner to the change in sensitivity of glycolysis) in the EDL preparation. 4. None of the adenosine mediated effects on insulin-stimulated rates of glycolysis were observed in the hemi-diaphragm preparation or on the rates of glycogen synthesis in any of the three muscle preparations. 5. Therefore, changes in the adenosine system in skeletal muscle influence insulin sensitivity regardless of fibre type composition of the muscle.     

The effects of amylin on carbohydrate metabolism in skeletal muscle in vitro and in vivo.

1. The effects of synthetic human amylin on basal and insulin-stimulated (100 and 1000 microunits/ml) rates of lactate formation, glucose oxidation and glycogen synthesis were measured in the isolated rat soleus muscle preparation incubated in the presence of various concentrations of glucose (5, 11 and 22 mM). 2. The rate of glucose utilization was increased by about 2-fold by increasing the glucose concentration from 5 to 22 mM. 3. Synthetic human amylin (10 nM) significantly inhibited (by 46-56%) glycogen synthesis, irrespective of the concentration of insulin or glucose present in the incubation medium. 4. Amylin (10 nM) did not affect insulin-stimulated rates of 2-deoxy[3H]glucose transport and phosphorylation. 5. Intraperitoneal administration of insulin (100 micrograms/kg) to rats in vivo stimulated the rate of [U-14C]glucose incorporation into glycogen in the diaphragm by about 80-fold. This rate was decreased (by 28%) by co-administration of amylin (66 micrograms/kg).     

The effects of peroxovanadate and peroxovanadyl on glucose metabolism in vivo and identification of signal transduction proteins involved in the mechanism of action in isolated soleus muscle

The insulin-like effects of peroxovanate (POV) and peroxovanadyl (PSV) on rates of lactate formation and glycogen synthesis were measured in isolated incubated soleus muscle preparations. In another experiment rats were made insulin deficient by streptozotocin injection and treated with POV and PSV (0.25 mM) administered in the drinking water and in the course of 7 days glycemia were determined. Also, signal transduction proteins ERK 1 and ERK 2 involved in the insulin signaling were measured in soleus muscle of diabetic rats treated with POV and PSV. Peroxides of vanadate and vanadyl significantly stimulated glucose utilization in soleus muscle preparations in vitro. The stimulation of glycogen synthesis and lactate formation by POV and PSV was similar to insulin stimuli. Rats treated with POV or PSV presented reduction of glycemia, food and fluid intake with amelioration of the diabetic state during the short period of treatment (7 days). POV and PSV modulated ERK1/2 phosphorilation and the insulin administration in these rats caused an addictive effect on phosphorilation state of these proteins.     

Substrate regulation of the glucose transport system in rat skeletal muscle. Characterization and kinetic analysis in isolated soleus muscle and skeletal muscle cells in culture

A self-regulatory mechanism of the glucose transport in rat skeletal muscle cells is described. In isolated rat soleus muscles and rat skeletal myocytes and myotubes in culture, pre-exposure to varying glucose concentrations modulated the rate of 2-deoxyglucose uptake. Maximal uptake was observed at glucose concentrations below 3 mM. Between 2.5 and 4.0 mM glucose it was reduced by 25-35%; further elevation of the glucose concentration resulted in a gradual decrease of the transport rate by approximately 2% for each millimolar glucose. The effect of glucose was time-dependent and fully reversible. Insulin rapidly increased the 2-deoxyglucose uptake in the soleus muscle; however, the insulin effect depended on the glucose concentration of the preincubation. Insulin was totally ineffective in muscles pre-exposed to 1.0-3.0 mM glucose, whereas its stimulatory action increased with increasing glucose concentrations above 4 mM. The effect of low glucose and insulin were not additive, and the maximal 2-deoxyglucose uptake rates induced by both conditions were of identical magnitude. It is postulated that glucose may "up- and down-regulate" its transport by affecting the number of active glucose transporters in the plasma membrane, and that insulin exerts its stimulatory effect only when the extracellular glucose reaches a threshold concentration.     

Hepatic glucose production is more sensitive to insulin-mediated inhibition than hepatic VLDL-triglyceride production

Insulin is an important inhibitor of both hepatic glucose output and hepatic VLDL-triglyceride (VLDL-TG) production. We investigated whether both processes are equally sensitive to insulin-mediated inhibition. To test this, we used euglycemic clamp studies with four increasing plasma concentrations of insulin in wild-type C57Bl/6 mice. By extrapolation, we estimated that half-maximal inhibition of hepatic glucose output and hepatic VLDL-TG production by insulin were obtained at plasma insulin levels of approximately 3.6 and approximately 6.8 ng/ml, respectively. In the same experiments, we measured that half-maximal decrease of plasma free fatty acid levels and half-maximal stimulation of peripheral glucose uptake were reached at plasma insulin levels of approximately 3.0 and approximately 6.0 ng/ml, respectively. We conclude that, compared with insulin sensitivity of hepatic glucose output, peripheral glucose uptake and hepatic VLDL-TG production are less sensitive to insulin.     

Nonacute effects of H-FABP deficiency on skeletal muscle glucose uptake in vitro

Heart-type fatty acid-binding protein (H-FABP) is required for high rates of skeletal muscle long-chain fatty acid (LCFA) oxidation and esterification. Here we assessed whether H-FABP affects soleus muscle glucose uptake when measured in vitro in the absence of LCFA. Wild-type and H-FABP null mice were fed a standard chow or high-fat diet before muscle isolation. With the chow, the mutation increased insulin-dependent deoxyglucose uptake by 141% (P < 0.01) at 0.02 mU/ml of insulin but did not cause a significant effect at 2 mU/ml of insulin; skeletal muscle triglyceride and long-chain acyl-CoA (LCA-CoA) levels remained normal. With the high-fat diet, the mutation increased insulin-dependent deoxyglucose uptake by 190% (P < 0.01) at 2 mU/ml of insulin, thus partially preventing insulin resistance, and it completely prevented the threefold (P < 0.001) diet-induced increase of muscle triglyceride levels; however, muscle LCA-CoA levels showed little or no reduction. With both diets, the mutation reduced the basal (insulin-independent) soleus muscle deoxyglucose uptake by 28% (P < 0.05). These results establish a close relation between FABP-dependent lipid pools and insulin sensitivity and indicate the existence of a nonacute, antagonistic, and H-FABP-dependent fatty acid regulation of basal and insulin-dependent muscle glucose uptake.     

Effect of the fatty acid oxidation inhibitor 2-tetradecylglycidic acid (TDGA) on glucose and fatty acid oxidation in isolated rat soleus muscle

1. The effect of 2-tetradecylglycidic acid (TDGA), a potent, specific inhibitor of long-chain fatty acid oxidation, on fatty acid and glucose oxidation by isolated rat soleus muscle was studied. 2. TDGA inhibited [1-14C]palmitate oxidation by soleus muscle in a concentration-dependent manner. 3. TDGA inhibited the activity of soleus muscle mitochondrial carnitine palmitoyltransferase A (CPT-A). 4. Added palmitate (0.5 mM) significantly inhibited D-[U-14C]glucose oxidation and, under conditions where TDGA inhibited palmitate oxidation, the oxidation of D-[U-14C]glucose by isolated soleus muscle was significantly stimulated. 5. TDGA stimulation of glucose oxidation was reversed by octanoate, a medium-chain fatty acid whose oxidation is not inhibited by TDGA. 6. When nondiabetic rats were treated with TDGA (10 mg/kg p.o./day x 3 days), fasting plasma glucose was significantly lowered and the ability of isolated contralateral soleus muscles to oxidize palmitate was inhibited while glucose oxidation was significantly stimulated.     

Regulation of glycogen synthase activity in the isolated mouse soleus muscle effect of insulin,epinephrine, glucose and anti-insulin receptor antibodies

The effects of insulin, epinephrine, glucose and anti-insulin receptor antibodies on enzymes involved in the regulation of glycogen synthesis were investigared in the isolated mouse soleus muscle. Insulin maximally increased the percentage of glycogen synthase active form after 15 min in the absence of glucose in the extracellular medium; half-maximal and maximal effects were obtained with 1.5 and 33 nM insulin, respectively. The basal percentage of glycogen phosphorylase active form was not altered by insulin. Antibodies to the insulin receptor had similar effects to those of insulin on both enzymes. The percentage of glycogen synthase active form was maximally decreased and that of phosphorylase maximally increased after a 2 min exposure to epinephrine in the absence of extracellular glucose. Glucose alone had no effect on muscle glycogen synthase. When muscles were incubated with insulin (33 nM) plus glucose (20 mM) for 5–10 min, the increase in the percentage of glycogen synthase active form was greater than with insulin alone. This enhancing effect of glucose on insulin activation of glycogen synthase disappeared after 20 min. The results suggest the existence of two mechanisms whereby insulin activates muscle glycogen synthase. The main effect is operative in the absence of extracellular glucose and occurs at insulin concentrations close to the physiological range. The other effect requires glucose and may result from the stimulation by insulin of glucose transport and/or metabolism.     

Regulation of glycogen synthase activity in the isolated mouse soleus muscle. Effect of insulin, epinephrine, glucose and anti-insulin receptor antibodies

The effects of insulin, epinephrine, glucose and anti-insulin receptor antibodies on enzymes involved in the regulation of glycogen synthesis were investigated in the isolated mouse soleus muscle. Insulin maximally increased the percentage of glycogen synthase active form after 15 min in the absence of glucose in the extracellular medium; half-maximal and maximal effects were obtained with 1.5 and 33 mM insulin, respectively. The basal percentage of glycogen phosphorylase active form was not altered by insulin. Antibodies to the insulin receptor had similar effects to those of insulin on both enzymes. The percentage of glycogen synthase active form was maximally decreased and that of phosphorylase maximally increased after a 2 min exposure to epinephrine in the absence of extracellular glucose. Glucose alone had no effect on muscle glycogen synthase. When muscles were incubated with insulin (33 nM) plus glucose (20 mM) for 5-10 min, the increase in the percentage of glycogen synthase active form was greater than with insulin alone. This enhancing effect of glucose on insulin activation of glycogen synthase disappeared after 20 min. The results suggest the existence of two mechanisms whereby insulin activates muscle glycogen synthase. The main effect is operative in the absence of extracellular glucose and occurs at insulin concentrations close to the physiological range. The other effect requires glucose and may result from the stimulation by insulin of glucose transport and/or metabolism.