Levodopa with carbidopa diminishes glycogen concentration, glycogen synthase activity, and insulin-stimulated glucose transport in rat skeletal muscle.

We hypothesized that levodopa with carbidopa, a common therapy for patients with Parkinson's disease, might contribute to the high prevalence of insulin resistance reported in patients with Parkinson's disease. We examined the effects of levodopa-carbidopa on glycogen concentration, glycogen synthase activity, and insulin-stimulated glucose transport in skeletal muscle, the predominant insulin-responsive tissue. In isolated muscle, levodopa-carbidopa completely prevented insulin-stimulated glycogen accumulation and glucose transport. The levodopa-carbidopa effects were blocked by propranolol, a beta-adrenergic antagonist. Levodopa-carbidopa also inhibited the insulin-stimulated increase in glycogen synthase activity, whereas propranolol attenuated this effect. Insulin-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was reduced by levodopa-carbidopa, although Akt phosphorylation was unaffected by levodopa-carbidopa. A single in vivo dose of levodopa-carbidopa increased skeletal muscle cAMP concentrations, diminished glycogen synthase activity, and reduced tyrosine phosphorylation of IRS-1. A separate set of rats was treated intragastrically twice daily for 4 wk with levodopa-carbidopa. After 4 wk of treatment, oral glucose tolerance was reduced in rats treated with drugs compared with control animals. Muscles from drug-treated rats contained at least 15% less glycogen and approximately 50% lower glycogen synthase activity compared with muscles from control rats. The data demonstrate beta-adrenergic-dependent inhibition of insulin action by levodopa-carbidopa and suggest that unrecognized insulin resistance may exist in chronically treated patients with Parkinson's disease.     

Metformin, but not exercise training, increases insulin responsiveness in skeletal muscle of Sprague-Dawley rats

We assessed the effects of combined metformin treatment and exercise training on body composition, on insulin concentration following glucose loading, on insulin-stimulated glucose transport in skeletal muscle, and on muscle glycogen content. Male Sprague-Dawley rats were treated for 35 days with or without metformin (320 mg/kg/day) and/or treadmill exercise training (20 min at 20 m/min, 5 days/wk). Because metformin reduces food intake, pair-fed controls were included. Metformin, training, and pair-feeding all decreased food intake, body weight, and insulin concentration following glucose loading. Metformin and training reduced intra-abdominal fat, but pair feeding did not. In isolated strips derived from soleus, epitrochlearis and extensor carpi ulnaris muscles, metformin increased insulin-stimulated transport of [3H]-2-deoxyglucose by 90%, 89% and 125%, respectively (P < 0.02) and training increased [3H]-2-deoxyglucose transport in the extensor carpi ulnaris muscle only (66%, P < 0.05). Pair-feeding did not alter [3H]-2-deoxyglucose transport. Training increased gastrocnemius muscle glycogen by 100% (P < 0.001). Metformin and pair-feeding did not alter muscle glycogen. We conclude that metformin reverses the maturation-induced impairment of insulin responsiveness in Sprague-Dawley rats by increasing insulin-stimulated glucose transport in skeletal muscle and that this effect is not secondary to reduced food intake. We also conclude that metformin and exercise training may increase insulin sensitivity by different mechanisms, with training causing increased glucose transport only in some muscles and also causing increased muscle glycogen storage.     

Hemodynamic and metabolic responses to exercise after adrenoceptor blockade in humans

The effects of acute alpha 1-adrenoceptor blockade with prazosin, beta 1-adrenoceptor blockade with atenolol, and nonselective beta-adrenoceptor blockade with propranolol were compared in a placebo-controlled crossover study of the hemodynamic and metabolic responses to acute exercise 2 h after prolonged prior exercise to induce skeletal muscle glycogen depletion, enhancing the dependence on hepatic glucose output and circulating free fatty acids (FFA). Plasma catecholamines were higher during exercise after, as opposed to before, glycogen depletion and were elevated further by all three drugs. Propranolol failed to produce a significant reduction in systolic blood pressure and elevated diastolic blood pressure. Atenolol reduced systolic blood pressure and did not change diastolic blood pressure. Both beta-blockers reduced FFA levels, but only propranolol lowered plasma glucose relative to placebo during exercise after glycogen depletion. In contrast, prazosin reduced systolic and diastolic blood pressures and resulted in elevated FFA and glucose levels. The results indicate important differences in the hemodynamic effects of beta 1-selective vs. nonselective beta-blockade during exercise after skeletal muscle glycogen depletion. Furthermore they confirm the importance of beta 2-mediated hepatic glucose production in maintaining plasma glucose levels during exercise. Acute alpha 1-blockade with prazosin induces reflex elevation of catecholamines, which in the absence of blockade of hepatic beta 2-receptors produces elevation of plasma glucose. The results suggest there is little role for alpha 1-mediated hepatic glucose production during exercise in humans.     

Effect of isoproterenol on the plasma C-peptide and insulin levels in humans

There are conflicting reports on the effect of stimulation of the beta-adrenergic receptors on insulin removal by the liver. It was, therefore, the aim of the present study to clarify that problem. Four experiments have been carried out on a group of 8 healthy female volunteers: (1) isoproterenol was infused intravenously, (2) glucose was infused intravenously, (3) isoproterenol was infused with glucose, and (4) infusion of isoproterenol and glucose was preceded by administration of propranolol (the beta-adrenergic blocking agent). The concentration of C-peptide and insulin was determined in plasma from the antecubital vein. It has been found that stimulation of the beta-adrenergic receptors with isoproterenol reduces insulin removal by the human liver. This effect of isoproterenol is prevented by blockade of the beta-adrenergic receptors with propranolol.     

Endothelial Na+-D-glucose cotransporter: no role in insulin-mediated glucose uptake.

A recent report indicates that the Na+-D-glucose cotransporter SGLT1 is present in capillaries of skeletal muscle and is required for insulin-mediated glucose uptake in myocytes. This result is based on the complete inhibition of insulin-mediated muscle glucose uptake by phlorizin, an inhibitor of SGLT1. Using the pump-perfused rat hind limb, we measured glucose uptake, lactate efflux, and radioactive 2-deoxyglucose uptake into individual muscles with saline (control), phlorizin, insulin, and insulin plus phlorizin, as well as with saline and insulin using normal and low Na+ perfusion buffer. Insulin-mediated glucose uptake was not inhibited after correction for phlorizin interference in the glucose assay. Lactate efflux and 2-deoxyglucose uptake by individual muscles were unaffected by phlorizin. Low Na+ buffer did not affect insulin-mediated glucose uptake, lactate efflux, or 2-deoxyglucose uptake. We conclude that endothelial SGLT1 exerts no barrier for glucose delivery to myocytes.     

Influence of alpha and beta adrenergic receptors blockade on the adrenolytic effect of muscular work

The changes in the response of adrenergic receptors alpha and beta in the blood vessels in the working muscles in a hindlimb in cats were studied after intra-arterial administration of noradrenaline, isoprenaline and during electric stimulation of the sympathetic trunk. The experiments were carried out during alpha-adrenergic receptors blockade with dihydroergotamine (0.3 mg/kg) beta-adrenergic receptors blockade with propranolol (1 mg/kg) and blockade of acetylcholine M receptors with atropine (0.5 mg/kg). The investigations were performed at rest, during exercise (electric stimulation of the sciatic nerve) and after the exercise. The following results deserve attention: 1) beta-adrenergic receptors blockade reduced significantly the alpha-adrenolytic effect of exercise restoring the ability of blood vessel to constriction in response to noradrenaline; 2) the vasodilator effect of isoprenaline evident in resting state and maintained to some extent during exercise was abolished completely by preceding alpha-adrenergic blockade. The changes in the reactivity of resistance vessels in working skeletal muscles to noradrenaline, with abolition of its vasoconstrictor effect, have been shown by Rein [7] and others authors [2, 5]. Similarly, it is well known that the resistance vessels contain two types of adrenergic receptors alpha and beta, and that the response of the vessels to stimulation of these receptors are different [1]. In view of the recently published observations of Jarhult and Lundvall suggesting that the beta-adrenergic receptors play an important physiological role [6] in the arterial part of the microcirculation [6] and in view of the hypothesis put forward by Kunos and Szentivanyj that alpha and beta receptors can be transformed depending on the intensity of tissue metabolism [8] it seemed worth while to study more systematically the changes of the reactivity of alpha and beta adrenergic receptors in the vascular bed of the skeletal muscles during and after muscle exercise.     

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.     

Modulation of muscle insulin resistance by selective inhibition of GSK-3 in Zucker diabetic fatty rats

A role for elevated glycogen synthase kinase-3 (GSK-3) activity in the multifactorial etiology of insulin resistance is now emerging. However, the utility of specific GSK-3 inhibition in modulating insulin resistance of skeletal muscle glucose transport is not yet fully understood. Therefore, we assessed the effects of novel, selective organic inhibitors of GSK-3 (CT-98014 and CT-98023) on glucose transport in insulin-resistant muscles of Zucker diabetic fatty (ZDF) rats. Incubation of type IIb epitrochlearis and type I soleus muscles from ZDF rats with CT-98014 increased glycogen synthase activity (49 and 50%, respectively, P < 0.05) but did not alter basal glucose transport (2-deoxyglucose uptake). In contrast, CT-98014 significantly increased the stimulatory effects of both submaximal and maximal insulin concentrations in epitrochlearis (37 and 24%) and soleus (43 and 26%), and these effects were associated with increased cell-surface GLUT4 protein. Lithium enhanced glycogen synthase activity and both basal and insulin-stimulated glucose transport in muscles from ZDF rats. Acute oral administration (2 x 30 mg/kg) of CT-98023 to ZDF rats caused elevations in GSK-3 inhibitor concentrations in plasma and muscle. The glucose and insulin responses during a subsequent oral glucose tolerance test were reduced by 26 and 34%, respectively, in the GSK-3 inhibitor-treated animals. Thirty minutes after the final GSK-3 inhibitor treatment, insulin-stimulated glucose transport was significantly enhanced in epitrochlearis (57%) and soleus (43%). Two hours after the final treatment, insulin-mediated glucose transport was still significantly elevated (26%) only in the soleus. These results indicate that specific inhibition of GSK-3 enhances insulin action on glucose transport in skeletal muscle of the insulin-resistant ZDF rat. This unique approach may hold promise as a pharmacological treatment against insulin resistance of skeletal muscle glucose disposal.     

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.     

Effect of beta-adrenergic blockade on lactate turnover in exercising dogs

Dogs with indwelling catheters in the jugular vein and in the carotid artery ran on the treadmill (slope: 15%, speed: 133 m/min). Lactate turnover and glucose turnover were measured using [U-14C]lactate and [3-3H]glucose as tracers, according to the primed constant-rate infusion method. In addition, the participation of plasma glucose in lactate production (Ra-L) was measured with [U-14C]glucose. Propranolol was given either (A) before exercise (250 micrograms/kg, iv) or (B) in form of a primed infusion administered to the dog running at a steady rate. Measurements of plasma propranolol concentration showed that in type A experiments plasma propranolol fell in 45 min below the lower limit of the complete beta-blockade. In the first 15 min of work Ra-L rose rapidly; then it fell below that of the control (exercise) values. During steady exercise, the elevated Ra-L was decreased by propranolol infusion close to resting values. beta-Blockade doubled the response of glucose production, utilization, and metabolic clearance rate to exercise. In exercising dogs approximately 40-50% of Ra-L arises from plasma glucose. This value was increased by the blockade to 85-90%. It is concluded that glycogenolysis in the working muscle has a dual control: 1) an intracellular control operating at the beginning of exercise, and 2) a hormonal control involving epinephrine and the beta-adrenergic receptors.     

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.     

Effect of epinephrine on glucose disposal during exercise in humans: role of muscle glycogen

This study examined the effect of epinephrine on glucose disposal during moderate exercise when glycogenolytic flux was limited by low preexercise skeletal muscle glycogen availability. Six male subjects cycled for 40 min at 59 +/- 1% peak pulmonary O2 uptake on two occasions, either without (CON) or with (EPI) epinephrine infusion starting after 20 min of exercise. On the day before each experimental trial, subjects completed fatiguing exercise and then maintained a low carbohydrate diet to lower muscle glycogen. Muscle samples were obtained after 20 and 40 min of exercise, and glucose kinetics were measured using [6,6-2H]glucose. Exercise increased plasma epinephrine above resting concentrations in both trials, and plasma epinephrine was higher (P < 0.05) during the final 20 min in EPI compared with CON. Muscle glycogen levels were low after 20 min of exercise (CON, 117 +/- 25; EPI, 122 +/- 20 mmol/kg dry matter), and net muscle glycogen breakdown and muscle glucose 6-phosphate levels during the subsequent 20 min of exercise were unaffected by epinephrine infusion. Plasma glucose increased with epinephrine infusion (i.e., 20-40 min), and this was due to a decrease in glucose disposal (R(d)) (40 min: CON, 33.8 +/- 3; EPI, 20.9 +/- 4.9 micromol. kg(-1). min(-1), P < 0.05), because the exercise-induced rise in glucose rate of appearance was similar in the trials. These results show that glucose R(d) during exercise is reduced by elevated plasma epinephrine, even when muscle glycogen availability and utilization are low. This suggests that the effect of epinephrine does not appear to be mediated by increased glucose 6-phosphate, secondary to enhanced muscle glycogenolysis, but may be linked to a direct effect of epinephrine on sarcolemmal glucose transport.     

Propranolol prevents epinephrine from limiting insulin-stimulated muscle glucose uptake during contraction.

Beta-blockade results in rapid glucose clearance and premature fatigue during exercise. To investigate the cause of this increased glucose clearance, we studied the acute effects of propranolol on insulin-stimulated muscle glucose uptake during contraction in the presence of epinephrine with an isolated rat muscle preparation. Glucose uptake increased in both fast- (epitrochlearis) and slow-twitch (soleus) muscle during insulin or contraction stimulation. In the presence of 24 nM epinephrine, glucose uptake during contraction was completely suppressed when insulin was present. This suppression of glucose uptake by epinephrine was accompanied by a decrease in insulin receptor substrate (IRS)-1-phosphatidylinositol 3 (PI3)-kinase activity. Propranolol had no direct effect on insulin-stimulated glucose uptake during contraction. However, epinephrine was ineffective in attenuating insulin-stimulated glucose uptake during contraction in the presence of propranolol. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake during contraction occurred in conjunction with its inability to completely suppress IRS-1-PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake during contraction is severely diminished in muscle exposed to propranolol. Thus the increase in glucose clearance and premature fatigue associated with beta-blockade could result from the inability of epinephrine to attenuate insulin-stimulated muscle glucose uptake.     

Tissue glucose utilization during epinephrine-induced hyperglycemia

The aim of this study was to investigate glucose utilization by individual tissues during epinephrine infusion. First, the applicability of the 2-deoxyglucose (2-DG) tracer technique during in vivo hyperglycemia was investigated in model systems in vitro. Epitrochlearis muscle and spleen cells were incubated with 1.25-20 mM glucose. The discrimination against 2-[14C]DG in glucose metabolic pathways, expressed by the lumped constant, remained unchanged over this wide range of glucose concentrations. It was concluded that in vivo hyperglycemia does not preclude the application of the 2-DG method. In a series of in vivo experiments, chronically catheterized conscious rats fasted for 24 h and were infused with epinephrine (0.2 microgram.kg-1.min-1), which produced a two-fold increase in plasma glucose concentration. 2-[14C]DG was injected 30 min after starting the epinephrine infusion and glucose utilization rates of individual tissues were calculated based on the concentration of phosphorylated 2-DG in samples excised at 70 min. The epinephrine infusion increased glucose utilization rates by 40-160% in hindlimb muscles, skin, ileum, liver, spleen, lung, epididymal fat, and kidney, although no change was found in the brain. Mass action of the increased plasma glucose is likely to play an important role in the enhanced rate of glucose utilization.     

Adrenergic regulation of glucose metabolism in rat heart. A calcium-dependent mechanism mediated by both alpha- and beta-adrenergic receptors

Epinephrine treatment of the perfused rat heart led to an increase in glucose uptake, detritiation of [5-3H] glucose, glycogenolysis, and the formation of lactate. The change in the rate of formation of 3H2O from [5-3H]glucose was slower to develop (commencing at approximately 30 s) than changes in cyclic AMP concentration, hexose-6-P concentration, and the phosphorylase a/(a + b) ratio which were maximal at 24 s. Epinephrine plus propranolol (alpha-adrenergic combination) treatment of the perfused heart also led to increases in glucose uptake, detritiation of [5-3H]glucose, and the formation of lactate, but these occurred without significant changes in cyclic AMP concentration, hexose-6-P concentration, or the phosphorylase a/(a + b) ratio. Half-maximal stimulation of glucose uptake occurred at 0.2 microM epinephrine, 1.5 microM methoxamine, and 1 microM isoproterenol. The increase in glucose uptake mediated by 1 microM epinephrine was blocked by 10 microM prazosin but unaffected by 10 microM propranolol. The increase in glucose uptake mediated by 10 microM epinephrine plus 10 microM propranolol was partly blocked by yohimbine and completely blocked by prazosin. A role for Ca2+ in the adrenergic regulation of glucose uptake was indicated by the sensitivity of the epinephrine dose curve to Ca2+ and the dependence of epinephrine on Ca2+. In addition the increases in glucose uptake mediated by 1 microM epinephrine, 1 microM epinephrine plus 10 microM propranolol, 1 microM isoproterenol, and by 10 mM CaCl2 were each blocked by the Ca2+ channel blocker nifedipine (1 microM). It is concluded that Ca2+-dependent alpha- and beta-adrenergic receptor mechanisms are present in rat heart for controlling glucose uptake. At submicromolar levels of epinephrine the predominant receptors utilized appear to be alpha 1.     

Insulin-like action of catecholamines and Ca2+ to stimulate glucose transport and GLUT4 translocation in perfused rat heart

The uptake of 2-deoxyglucose by perfused rat hearts was compared to the distribution of the insulin-regulatable glucose transporter (GLUT4) in membrane preparations from the same hearts. The hearts were treated with the alpha-adrenergic combination of epinephrine + propranolol, the beta-adrenergic agonist isoproterenol, high (8 mM) Ca2+ concentrations, insulin and the alpha adrenergic combination or insulin alone. Epinephrine (1 microM) + propranolol (10 microM), isoproterenol (10 microM), high Ca2+, insulin (1 microM) + epinephrine (1 microM) + propranolol (10 microM) and insulin (1 microM) each led to an increase in 2-deoxyglucose uptake and a shift in the recovery of the GLUT4 from a high-speed pellet membrane fraction (putatively intracellular) to a low-speed pellet membrane fraction (putatively sarcolemmal). There were significant correlations (r = -0.673, P less than 0.001) between the stimulation of 2-deoxyglucose uptake and the loss of GLUT4 from the intracellular membrane fraction, or the increase in the sarcolemmal fraction. The data provide evidence that the GLUT4 is translocated by agents that stimulate glucose transport in heart, and therefore this mechanism is not restricted to insulin.     

Insulin promotes glycogen synthesis in the absence of GSK3 phosphorylation in skeletal muscle

Insulin promotes dephosphorylation and activation of glycogen synthase (GS) by inactivating glycogen synthase kinase (GSK) 3 through phosphorylation. Insulin also promotes glucose uptake and glucose 6-phosphate (G-6-P) production, which allosterically activates GS. The relative importance of these two regulatory mechanisms in the activation of GS in vivo is unknown. The aim of this study was to investigate if dephosphorylation of GS mediated via GSK3 is required for normal glycogen synthesis in skeletal muscle with insulin. We employed GSK3 knockin mice in which wild-type GSK3 alpha and -beta genes are replaced with mutant forms (GSK3 alpha/beta S21A/S21A/S9A/S9A), which are nonresponsive to insulin. Although insulin failed to promote dephosphorylation and activation of GS in GSK3 alpha/beta S21A/S21A/S9A/S9A mice, glycogen content in different muscles from these mice was similar compared with wild-type mice. Basal and epinephrine-stimulated activity of muscle glycogen phosphorylase was comparable between wild-type and GSK3 knockin mice. Incubation of isolated soleus muscle in Krebs buffer containing 5.5 mM glucose in the presence or absence of insulin revealed that the levels of G-6-P, the rate of [14C]glucose incorporation into glycogen, and an increase in total glycogen content were similar between wild-type and GSK3 knockin mice. Injection of glucose containing 2-deoxy-[3H]glucose and [14C]glucose also resulted in similar rates of muscle glucose uptake and glycogen synthesis in vivo between wild-type and GSK3 knockin mice. These results suggest that insulin-mediated inhibition of GSK3 is not a rate-limiting step in muscle glycogen synthesis in mice. This suggests that allosteric regulation of GS by G-6-P may play a key role in insulin-stimulated muscle glycogen synthesis in vivo.     

TNF-alpha acutely inhibits vascular effects of physiological but not high insulin or contraction

TNF-alpha is elevated in many states of insulin resistance, and acutely administered TNF-alpha in vivo inhibits insulin-mediated hemodynamic effects and glucose uptake in muscle. In this study, we assess whether the inhibitory effects of TNF-alpha are affected by insulin dose or muscle contraction. Whole body glucose infusion rate (GIR), femoral blood flow (FBF), hindleg vascular resistance, hindleg glucose uptake (HGU), 2-deoxyglucose uptake into muscles of the lower leg (R'g) and hindleg metabolism of infused 1-methylxanthine (1-MX), a measure of capillary recruitment, were determined. Three groups were studied with and without infusion of TNF-alpha: euglycemic insulin-clamped, one-leg field-stimulated (2 Hz, 0.1 ms at 30 V), and saline-infused control anesthetized rats. Insulin infusions were 3, 10, or 30 mU x kg-1 x min-1 for 2 h x 1-MX metabolism was maximally increased by all three doses of insulin. GIR, HGU, and R'g were maximal at 10 mU and FBF was maximal at 30 mU of insulin. Contraction increased FBF, HGU, and 1-MX. TNF-alpha (0.5 microg x kg-1 x h-1) totally blocked the 3 and 10 mU insulin-mediated increases in FBF and 1-MX, and partly blocked GIR, HGU, and R'g. None of the increases due to twitch contraction was affected by TNF-alpha, and only the increase in FBF due to 30 mU of insulin was partly affected. We conclude that muscle capillary recruitment and glucose uptake due to high levels of insulin or muscle contraction under twitch stimuli at 2 Hz are resistant to TNF-alpha. These findings may have implications for ameliorating muscle insulin resistance resulting from increased plasma TNF-alpha and for the differing mechanisms by which contraction and insulin recruit capillary flow in muscle.     

A steroidal glycoside from Polygonatum odoratum (Mill.) Druce. improves insulin resistance but does not alter insulin secretion in 90% pancreatectomized rats

The aim of this study was to investigate the effects of three steroidal glycosides (SG-100, SG-280, and SG-460) obtained from Polygonatum odoratum (Mill.) Druce. on insulin secretion, insulin action, and relative glucose uptake in various tissues of 90% pancreatectomized male Sprague-Dawley rats. One of the compounds (30 mg/kg body weight daily) with a 40%-fat diet was orally administered to a group of such rats for 13 weeks. On the day after a hyperglycemic clamp, euglycemic hyperinsulinemic clamp with 1 microCi of [1-(14)C]2-deoxyglucose per 100 g body weight was used. Serum glucose levels were lowest in the rats receiving SG-100. Insulin secretion from pancreatic beta-cells did not change with SG administration. Whole-body glucose disposal rates increased with SG-100 administration by 39%. SG-100 increased the glycogen contents and glycogen synthase activity in the soleus muscle of pancreatectomized rats. Uptake of [1-(14)C]2-deoxyglucose into the soleus muscle was higher in such rats receiving SG-100 than in rats receiving other compounds. In conclusion, SG-100 has an antihyperglycemic effect by promoting peripheral insulin sensitivity without changing insulin secretion.     

Glucose infusion causes insulin resistance in skeletal muscle of rats without changes in Akt and AS160 phosphorylation

Hyperglycemia is a defining feature of Type 1 and 2 diabetes. Hyperglycemia also causes insulin resistance, and our group (Kraegen EW, Saha AK, Preston E, Wilks D, Hoy AJ, Cooney GJ, Ruderman NB. Am J Physiol Endocrinol Metab Endocrinol Metab 290: E471-E479, 2006) has recently demonstrated that hyperglycemia generated by glucose infusion results in insulin resistance after 5 h but not after 3 h. The aim of this study was to investigate possible mechanism(s) by which glucose infusion causes insulin resistance in skeletal muscle and in particular to examine whether this was associated with changes in insulin signaling. Hyperglycemia (~10 mM) was produced in cannulated male Wistar rats for up to 5 h. The glucose infusion rate required to maintain this hyperglycemia progressively lessened over 5 h (by 25%, P < 0.0001 at 5 h) without any alteration in plasma insulin levels consistent with the development of insulin resistance. Muscle glucose uptake in vivo (44%; P < 0.05) and glycogen synthesis rate (52%; P < 0.001) were reduced after 5 h compared with after 3 h of infusion. Despite these changes, there was no decrease in the phosphorylation state of multiple insulin signaling intermediates [insulin receptor, Akt, AS160 (Akt substrate of 160 kDa), glycogen synthase kinase-3beta] over the same time course. In isolated soleus strips taken from control or 1- or 5-h glucose-infused animals, insulin-stimulated 2-deoxyglucose transport was similar, but glycogen synthesis was significantly reduced in the 5-h muscle sample (68% vs. 1-h sample; P < 0.001). These results suggest that the reduced muscle glucose uptake in rats after 5 h of acute hyperglycemia is due more to the metabolic effects of excess glycogen storage than to a defect in insulin signaling or glucose transport.