Insulin resistance is associated with reduced fasting and insulin-stimulated glycogen synthase phosphatase activity in human skeletal muscle.

Insulin-stimulated glycogen synthase activity in human skeletal muscle correlates with insulin-mediated glucose disposal rate (M) and is reduced in insulin-resistant subjects. We have previously reported reduced insulin-stimulated glycogen synthase activity associated with reduced fasting glycogen synthase phosphatase activity in skeletal muscle of insulin-resistant Pima Indians. In this study we investigated the time course for insulin stimulation of glycogen synthase and synthase phosphatase during a 2-h high-dose insulin infusion (600 mU/min per m2) in six insulin-sensitive caucasians (group S) and in five insulin-resistant Pima Indians (group R). Percutaneous muscle biopsies were obtained from the quadriceps femoris muscle after insulin infusion for 0, 10, 20, 40, and 120 min. In group S, insulin-stimulated glycogen synthase activity increased with time and was significantly higher than in group R. In group S, synthase phosphatase activity increased significantly by 25% at 10 min and then decreased gradually. No significant change in synthase phosphatase was seen in group R and activity was lower than group S at 0 to 20 min. These data suggest that a low basal synthase phosphatase activity and a defect in its response to insulin explain, at least in part, reduced insulin stimulation of skeletal muscle glycogen synthase associated with insulin resistance.     

Defective insulin response of phosphorylase phosphatase in insulin-resistant humans.

Insulin-stimulated glycogen synthase activity in human muscle is reduced in insulin-resistant subjects. Insulin regulation of human muscle glycogen synthase may require activation of a type-1 protein phosphatase (PP-1). We investigated the change of phosphorylase phosphatase and glycogen synthase activities in muscle biopsies obtained during a 2-h hyperinsulinemic euglycemic clamp in 12 insulin-sensitive (group S) and 8 insulin-resistant (group R) subjects. Fasting phosphorylase phosphatase activity was lower in group R than in group S, and did not increase significantly with insulin infusion in group R until 20 min. In group S, phosphorylase phosphatase was significantly stimulated by 10 min, remaining significantly higher than in group R at all time points. The insulin-mediated changes in phosphatase activities were not decreased by 3 nM okadaic acid but were completely inhibited by 1 microM okadaic acid, thereby verifying that insulin-stimulated phosphorylase phosphatase is accounted for by a PP-1. Subcellular fractionation demonstrated reduced fasting PP-1 activities in both the glycogen and cytosolic fractions of muscle obtained from subjects in group R compared to those in group S. These results suggest that insulin activation of PP-1 could contribute to the stimulation of glycogen synthase by this hormone in human muscle. Lower fasting PP-1 activity in cytosol and glycogen fractions plus lower insulin-stimulated PP-1 activity could explain, in part, reduced insulin-stimulated glycogen synthase in skeletal muscle of insulin-resistant subjects.     

Regulation of glycogen synthase and phosphorylase activities by glucose and insulin in human skeletal muscle.

We examined the insulin dose-response characteristics of human muscle glycogen synthase and phosphorylase activation. We also determined whether increasing the rate of glucose disposal by hyperglycemia at a fixed insulin concentration activates glycogen synthase. Physiological increments in plasma insulin but not glucose increased the fractional activity of glycogen synthase. The ED50: s for insulin stimulation of whole body and forearm glucose disposal were similar and unaffected by glycemia. Glycogen synthase activation was exponentially related to the insulin-mediated component of whole body and forearm glucose disposal at each glucose concentration. Neither insulin nor glucose changed glycogen phosphorylase activity. These results suggest that insulin but not the rate of glucose disposal per se regulates glycogen synthesis by a mechanism that involves dephosphorylation of glycogen synthase but not phosphorylase. This implies that the low glycogen synthase activities found in insulin-resistant states are a consequence of impaired insulin action rather than reduced glucose disposal.     

Activation of skeletal muscle casein kinase II by insulin is not diminished in subjects with insulin resistance.

Insulin resistance, which may precede the development of non-insulin-dependent diabetes mellitus in Pima Indians, appears to result from a postreceptor defect in signal transduction in skeletal muscle. To identify the putative postreceptor lesion responsible for insulin resistance in Pima Indians, we investigated the influence of insulin on the activity of casein kinase II (CKII) in skeletal muscle of seven insulin-sensitive, four insulin-resistant, nondiabetic, and five insulin-resistant diabetic Pima Indians during a 2 h hyperinsulinemic, euglycemic clamp. In sensitive subjects, CKII was transiently activated reaching a maximum over basal activity (42%) at 45 min before declining. CKII was also stimulated in resistant (19%) and diabetic (34%) subjects. Basal CKII activity in resistant subjects was 40% higher than in either sensitive or diabetic subjects, although the concentration of CKII protein, as determined by Western blotting, was equal among the three groups. Basal CKII activity was correlated with fasting plasma insulin concentrations, suggesting that the higher activity in resistant subjects resulted from insulin action. Extracts of muscle obtained from all three groups either before or after insulin administration were treated with immobilized alkaline phosphatase, which reduced and equalized CKII activity. These results suggest that insulin stimulates CKII activity in human skeletal muscle by a mechanism involving phosphorylation of either CKII or of an effector molecule, and support the idea that elevated basal activity in resistant subjects results from insulin action. It appears that the ability of insulin to activate CKII in skeletal muscle is not impaired in insulin-resistant Pima Indians, and that the biochemical lesion responsible for insulin resistance occurs either downstream from CKII or in a different pathway of insulin action.     

Impaired insulin-stimulated muscle glycogen synthase activation in vivo in man is related to low fasting glycogen synthase phosphatase activity.

Insulin-mediated glycogen synthase activity in skeletal muscle correlates with the rate of insulin-mediated glycogen deposition and is reduced in human subjects with insulin resistance. To assess the role of glycogen synthase phosphatase as a possible mediator of reduced glycogen synthase activity, we studied 30 Southwestern American Indians with a broad range of insulin action in vivo. Percutaneous biopsies of the vastus lateralis muscle were performed before and during a 440-min euglycemic clamp at plasma insulin concentrations of 89 +/- 5 and 1,470 +/- 49 microU/ml (mean +/- SEM); simultaneous glucose oxidation was determined by indirect calorimetry. After insulin stimulation, glycogen synthase activity was correlated with the total and nonoxidative glucose disposal at both low (r = 0.73, P less than 0.0001; r = 0.68, P less than 0.0001) and high (r = 0.75, P less than 0.0001; r = 0.74, P less than 0.0001) plasma insulin concentrations. Fasting muscle glycogen synthase phosphatase activity was correlated with both total and nonoxidative glucose disposal rates at the low (r = 0.48, P less than 0.005; r = 0.41, P less than 0.05) and high (r = 0.47, P less than 0.05; r = 0.43, P less than 0.05) plasma insulin concentrations. In addition, fasting glycogen synthase phosphatase activity was correlated with glycogen synthase activity after low- (r = 0.47, P less than 0.05) and high- (r = 0.50, P less than 0.01) dose insulin stimulations. These data suggest that the decreased insulin-stimulated glucose disposal and reduced glycogen synthase activation observed in insulin resistance could be secondary to a low fasting glycogen synthase phosphatase activity.     

Skeletal muscle glycogenolysis is more sensitive to insulin than is glucose transport/phosphorylation. Relation to the insulin-mediated inhibition of hepatic glucose production.

The effects of minimal increments in plasma insulin concentrations on hepatic glucose production and glucose uptake, skeletal muscle net glycogen synthesis and glycogenolysis, glycogen synthase and phosphorylase activity, glucose-6-phosphate and uridinediphosphoglucose (UDPG) concentrations were examined in 24-h and in 6-h fasted conscious rats. Insulin was infused for 120 min at rates of 1.5, 3, 6, 12, 24, and 108 pmol/kg per min in 24-h fasted rats and at rates of 3, 6, 9, 12, 36, and 108 pmol/kg per min in 6-h fasted rats while endogenous insulin release was inhibited by SRIF infusion and plasma glucose was maintained at the basal level. All rats received an infusion of [3-3H]glucose. The portion of the muscle glucose-6-phosphate (G6P) pool derived from net glycogenolysis was estimated from the ratio of specific activities of muscle UDPG and plasma glucose. Minimal increments in the circulating insulin levels, which did not stimulate glucose uptake, caused: (a) the increase in skeletal muscle glycogen synthase activity and the decrease in the rate of muscle glycogenolysis and in the G6P concentration; (b) the inhibition of hepatic glucose production. Net muscle glycogen synthesis was not stimulated despite submaximal activation of glycogen synthase, and its onset correlated with the rise in muscle G6P levels. Thus, insulin's inhibition of muscle glycogenolysis is the most sensitive insulin action on skeletal muscle and its dose-response characteristics resemble those for the inhibition of hepatic glucose production. These findings indicate that skeletal muscle glycogen synthase may play a major role in carbohydrate homeostasis even under postabsorptive (basal insulin) conditions and support the notion that insulin may exert some of its effects on the liver through an indirect or peripheral mechanism.     

Multiple defects in muscle glycogen synthase activity contribute to reduced glycogen synthesis in non-insulin dependent diabetes mellitus.

To define the mechanisms of impaired muscle glycogen synthase and reduced glycogen formation in non-insulin dependent diabetes mellitus (NIDDM), glycogen synthase activity was kinetically analyzed during the basal state and three glucose clamp studies (insulin approximately equal to 300, 700, and 33,400 pmol/liter) in eight matched nonobese NIDDM and eight control subjects. Muscle glycogen content was measured in the basal state and following clamps at insulin levels of 33,400 pmol/liter. NIDDM subjects had glucose uptake matched to controls in each clamp by raising serum glucose to 15-20 mmol/liter. The insulin concentration required to half-maximally activate glycogen synthase (ED50) was approximately fourfold greater for NIDDM than control subjects (1,004 +/- 264 vs. 257 +/- 110 pmol/liter, P less than 0.02) but the maximal insulin effect was similar. Total glycogen synthase activity was reduced approximately 38% and glycogen content was approximately 30% lower in NIDDM. A positive correlation was present between glycogen content and glycogen synthase activity (r = 0.51, P less than 0.01). In summary, defects in muscle glycogen synthase activity and reduced glycogen content are present in NIDDM. NIDDM subjects also have less total glycogen synthase activity consistent with reduced functional mass of the enzyme. These findings and the correlation between glycogen synthase activity and glycogen content support the theory that multiple defects in glycogen synthase activity combine to cause reduced glycogen formation in NIDDM.     

In vivo kinetics of insulin action on peripheral glucose disposal and hepatic glucose output in normal and obese subjects.

To determine whether abnormal kinetics of insulin's biologic actions contribute to the overall insulin resistance in obesity, we compared the rate of activation and deactivation of insulin's effects to stimulate glucose disposal rate (Rd) and inhibit hepatic glucose output (HGO) in 12 nonobese and 10 obese subjects using the euglycemic clamp technique at insulin infusion rates of 15, 40, 120, and 1,200 mU/M2 per min. In both groups, stimulation of Rd was faster the higher the insulin infusion rate and the time to reach half maximal stimulation (A50 value) in normals was 52 +/- 4, 44 +/- 2, 29 +/- 3, and 21 +/- 2 min at infusion rates of 15, 40, 120, and 1,200 mU/M2 per min, respectively. In the obese subjects, the rate of activation was slower (higher A50 values) with A50 values of 74 +/- 6, P less than 0.001 (compared to normal), 64 +/- 8 min, P less than 0.001, and 28 +/- 3 min, P less than 0.01, at the 40, 120, and 1,200 mU/M2 per min insulin infusions. Deactivation of the insulin effect to stimulate glucose disposal rate (Rd) was faster in the obese group compared with normal individuals after all comparable insulin infusions. In summary: for both groups, the higher the insulin infusion rate, the higher the steady state Rd value, the faster the rate of activation and the slower the subsequent rate of deactivation. In insulin-resistant obese subjects, the rate of activation of insulin action was slower and the rate of deactivation faster at comparable insulin infusion rates. The rate of suppression of HGO was comparable in normal and obese subjects, but the rate of recovery of HGO back to basal values was faster in the obese group. And in view of the phasic manner in which insulin is normally secreted following meals, steady state insulin action is not normally achieved. Therefore, the abnormal kinetics of insulin action in insulin-resistant obese individuals may represent functionally important manifestations of the insulin resistance in this condition.     

Postabsorptive respiratory quotient and insulin-stimulated glucose storage rate in nondiabetic pima indians are related To glycogen synthase fractional activity in cultured myoblasts.

A decreased ratio of fat to carbohydrate oxidation rate (an elevated respiratory quotient) predicts the development of obesity. Skeletal muscle accounts for a major fraction of total body lipid oxidation and is the principle site for reduced glucose storage in insulin-resistant subjects. The potentially important role that muscle has in promoting obesity or insulin resistance may be based on metabolic control intrinsic to skeletal muscle. Cultured skeletal muscle provides a system to examine the importance of inherent metabolic traits in muscle biopsies from obese and insulin-resistant subjects. Glycogen synthase fractional activity (GSFA) was measured in cultured myoblasts from 21 Pima Indians characterized in vivo using indirect calorimetry and a euglycemic hyperinsulinemic clamp. Basal GSFA in cultured muscle cells is inversely correlated with postabsorptive respiratory quotient of the muscle donors (r = -0.66, P = 0.001) and with in vivo high dose insulin-stimulated glucose storage rates (r = 0.47, P = 0.04). These results indicate that the postabsorptive respiratory quotients and insulin-mediated glucose storage rates in vivo share a common regulatory mechanism with GSFA in cultured myoblasts. Abnormal regulation of glycogen synthase phosphorylation state may be an intrinsic defect in skeletal muscle associated with obesity and insulin resistance.     

Effect of muscle glycogen depletion on in vivo insulin action in man.

In rats, muscle glycogen depletion has been associated with increased insulin action. Whether this also occurs in man has not been reported. After 4 d rest, 13 males (E Group) had a percutaneous muscle biopsy of the vastus lateralis muscle followed by a euglycemic clamp at plasma insulin congruent to 100 microU/ml and congruent to 1,900 microU/ml, with simultaneous indirect calorimetry. This was repeated 1 wk later, but after glycogen-depleting exercise the night before the euglycemic clamp. Seven subjects underwent the same protocol but were also re-fed 100 g carbohydrate (CHO) after the exercise (EF group). In both groups, the mean muscle glycogen content was approximately 40% lower (P less than 0.01) after exercise compared with the muscle glycogen content measured after rest. In the E group, the mean muscle glycogen synthase activity (percent independent of glucose-6-phosphate) increased threefold (P less than 0.001) after exercise, but increased only twofold in the EF group (P less than 0.02 between groups). In both groups, the mean basal and insulin-stimulated CHO oxidation rates were lower in the post-exercise, glycogen-depleted condition compared with the rested, glycogen-replete condition. The mean insulin-stimulated CHO storage rate increased significantly in the E group after exercise but not in the EF group. In the E group, the total insulin-stimulated CHO disposal rate (M) was 17 (P less than 0.04) and 10% (P less than 0.03) higher after exercise during the low and high dose insulin infusion, respectively. No significant changes in M were observed in the EF group. For all subjects, after rest and exercise, the M correlated with the CHO storage rates during the low (r = 0.80, P less than 0.001) and high dose (r = 0.77, P less than 0.001) insulin infusions. After exercise, the muscle glycogen synthase activity correlated with the CHO storage rate (r = 0.73, P less than 0.002; r = 0.75, P less than 0.002) during the low and high dose insulin infusions, respectively, and also with M (r = 0.64, P less than 0.008; r = 0.57; P less than 0.02).     

Changes in muscle glucose metabolism in type 1 diabetes

Patients with type 1 diabetes are characterized by an average 40% reduction in the insulin sensitivity. In newly diagnosed patients, insulin resistance is due to insulin deficiency and its metabolic consequences. After the beginning of insulin therapy, insulin sensitivity transiently improves, but deteriorates again after 6-9 months of insulin therapy. Insulin resistance is mainly due to a reduction in glucose uptake by muscle tissue. There are similar relative reductions in both oxidative and nonoxidative glucose disposal. When glucose disposal is determined under similar plasma glucose and insulin concentrations, glucose oxidation, the activity of pyruvate dehydrogenase and glycogen synthase are all reduced. If glucose disposal rate in diabetic patients is normalized by glucose mass action, both oxidative and nonoxidative glucose disposal and glycogen synthase activity become normal. As the normalization of glucose disposal occurs in the face of unchanged muscle glucose-6-phosphate concentrations, this suggest that reduced glucose disposal is secondary to reduced glucose transport in type 1 diabetes.     

The effect of insulin on adenosine 3':5'-monophosphate and guanosine 3':5'-monophosphate concentrations in human plasma.

1. The action of insulin on plasma cyclic nucleotide concentrations in normal human subjects has been studied after intravenous injection, alone and in combination with glucagon. 2. After injection of insulin alone there was an initial small, though not significant, decrease in plasma cyclic AMP at 15 min followed by an increase to more than twice the initial concentration at 30 min. The increase was absent when hypoglycaemia was lessened by infusion of glucose after insulin injection. 3. Injection of insulin caused no significant change in plasma cyclic GMP concentration, whether or not glucose was infused after the hormone. 4. Glucagon (3-300 nmol, 10-1000 mug), caused a dose-dependent increase in plasma cyclic AMP concentration. The rise in plasma cyclic AMP produced by 3 or 30 nmol of glucagon was not significantly modified by simultaneous injection of insulin (44 nmol; 6 units).     

Abnormal regulation of ribosomal protein S6 kinase by insulin in skeletal muscle of insulin-resistant humans.

Insulin resistance in Pima Indians appears to result from a post-receptor impairment of insulin signal transduction that affects only some responses to insulin. To identify the primary lesion responsible for insulin resistance, we investigated the influence of insulin on ribosomal protein S6 kinase activities in skeletal muscle of insulin-sensitive and insulin-resistant nondiabetic Pima Indians during a 2-h hyperinsulinemic, euglycemic clamp. In sensitive subjects, S6 kinase activity was transiently activated fivefold over basal activity by 45 min of insulin infusion. Although basal activities in the two groups were similar, the response to insulin was delayed and restricted to about threefold over basal in subjects resistant to insulin. Two major S6 kinase activities in extracts of human muscle were resolved by chromatography on Mono Q. Peak 1, which accounted for basal activity owes to an enzyme antigenically related to the 90-kD S6 kinase II, a member of the rsk gene family. The major insulin-stimulated S6 kinase eluted as peak 2 and is antigenically related to a 70-kD S6 kinase. Our results show that insulin resistance impairs signaling to the 70-kD S6 kinase.     

Correction of chronic hyperglycemia with vanadate, but not with phlorizin, normalizes in vivo glycogen repletion and in vitro glycogen synthase activity in diabetic skeletal muscle.

Vanadate has insulin-like activity in vitro and in vivo. To characterize the in vivo mechanism of action of vanadate, we examined meal tolerance, insulin-mediated glucose disposal, in vivo liver and muscle glycogen synthesis, and in vitro glycogen synthase activity in 90% partially pancreatectomized rats. Four groups were studied: group I, sham-operated controls; group II, diabetic rats; group III, diabetic rats treated with vanadate; and group IV, diabetic rats treated with phlorizin. Insulin sensitivity, assessed with the euglycemic hyperinsulinemic clamp technique in awake, unstressed rats, was reduced by approximately 28% in diabetic rats. Both vanadate and phlorizin treatment completely normalized meal tolerance and insulin-mediated glucose disposal. Muscle glycogen synthesis was reduced by approximately 80% in diabetic rats (P less than 0.01) and was completely restored to normal by vanadate, but not by phlorizin treatment. Glycogen synthase activity was reduced in skeletal muscle of diabetic rats (P less than 0.05) compared with controls and was increased to supranormal levels by vanadate treatment (P less than 0.01). Phlorizin therapy did not reverse the defect in muscle glycogen synthase. These results suggest that (a) the defect in muscle glycogen synthesis is the major determinant of insulin resistance in diabetic rats; (b) both vanadate and phlorizin treatment normalize meal tolerance and insulin sensitivity in diabetic rats; (c) vanadate treatment specifically reverses the defect in muscle glycogen synthesis in diabetic rats. This effect cannot be attributed to the correction of hyperglycemia because phlorizin therapy had no direct influence on the glycogenic pathway.     

Oral vanadyl sulfate improves hepatic and peripheral insulin sensitivity in patients with non-insulin-dependent diabetes mellitus.

We examined the in vivo metabolic effects of vanadyl sulfate (VS) in non-insulin-dependent diabetes mellitus (NIDDM). Six NIDDM subjects treated with diet and/or sulfonylureas were examined at the end of three consecutive periods: placebo for 2 wk, VS (100 mg/d) for 3 wk, and placebo for 2 wk. Euglycemic hyperinsulinemic (30 mU/m2.min) clamps and oral glucose tolerance tests were performed at the end of each study period. Glycemic control at baseline was poor (fasting plasma glucose 210 +/- 19 mg/dl; HbA1c 9.6 +/- 0.6%) and improved after treatment (181 +/- 14 mg/dl [P < 0.05], 8.8 +/- 0.6%, [P < 0.002]); fasting and post-glucose tolerance test plasma insulin concentrations were unchanged. After VS, the glucose infusion rate during the clamp was increased (by approximately 88%, from 1.80 to 3.38 mg/kg.min, P < 0.0001). This improvement was due to both enhanced insulin-mediated stimulation of glucose uptake (rate of glucose disposal [Rd], +0.89 mg/kg.min) and increased inhibition of HGP (-0.74 mg/kg.min) (P < 0.0001 for both). Increased insulin-stimulated glycogen synthesis (+0.74 mg/kg.min, P < 0.0003) accounted for > 80% of the increased Rd after VS, and the improvement in insulin sensitivity was maintained after the second placebo period. The Km of skeletal muscle glycogen synthase was lowered by approximately 30% after VS treatment (P < 0.05). These results indicate that 3 wk of treatment with VS improves hepatic and peripheral insulin sensitivity in insulin-resistant NIDDM humans. These effects were sustained for up to 2 wk after discontinuation of VS.     

Effect of the antilipolytic nicotinic acid analogue acipimox on whole-body and skeletal muscle glucose metabolism in patients with non-insulin-dependent diabetes mellitus.

Increased nonesterified fatty acid (NEFA) levels may be important in causing insulin resistance in skeletal muscles in patients with non-insulin-dependent diabetes mellitus (NIDDM). The acute effect of the antilipolytic nicotinic acid analogue Acipimox (2 X 250 mg) on basal and insulin-stimulated (3 h, 40 mU/m2 per min) glucose metabolism was therefore studied in 12 patients with NIDDM. Whole-body glucose metabolism was assessed using [3-3H]glucose and indirect calorimetry. Biopsies were taken from the vastus lateralis muscle during basal and insulin-stimulated steady-state periods. Acipimox reduced NEFA in the basal state and during insulin stimulation. Lipid oxidation was inhibited by Acipimox in all patients in the basal state (20 +/- 2 vs. 33 +/- 3 mg/m2 per min, P less than 0.01) and during insulin infusion (8 +/- 2 vs. 17 +/- 2 mg/m2 per min, P less than 0.01). Acipimox increased the insulin-stimulated glucose disposal rate (369 +/- 49 vs. 262 +/- 31 mg/m2 per min, P less than 0.01), whereas the glucose disposal rate was unaffected by Acipimox in the basal state. Acipimox increased glucose oxidation in the basal state (76 +/- 4 vs. 50 +/- 4 mg/m2 per min, P less than 0.01). During insulin infusion Acipimox increased both glucose oxidation (121 +/- 7 vs. 95 +/- 4 mg/m2 per min, P less than 0.01) and nonoxidative glucose disposal (248 +/- 47 vs. 167 +/- 29 mg/m2 per min, P less than 0.01). Acipimox enhanced basal and insulin-stimulated muscle fractional glycogen synthase activities (32 +/- 2 vs. 25 +/- 3%, P less than 0.05, and 50 +/- 5 vs. 41 +/- 4%, P less than 0.05). Activities of muscle pyruvate dehydrogenase and phosphofructokinase were unaffected by Acipimox. In conclusion, Acipimox acutely improved insulin action in patients with NIDDM by increasing both glucose oxidation and nonoxidative glucose disposal. This supports the hypothesis that elevated NEFA concentrations may be important for the insulin resistance in NIDDM. The mechanism responsible for the increased insulin-stimulated nonoxidative glucose disposal may be a stimulatory effect of Acipimox on glycogen synthase activity in skeletal muscles.     

Decreased insulin sensitivity and muscle enzyme activity in elderly subjects

Skeletal muscle glycogen deposition, and the activation of muscle glycogen synthase and pyruvate dehydrogenase during a hyerinsulinaemic euglycaemic clamp have been measured in six young and six elderly males matched for body mass index, physical activity and diet. Clamp glucose requirement (insulin, 0.1 U kg-1 h-1) was significantly lower in the older subjects (8.0 +/- 0.4 mg kg-1 min-1) than in younger subjects (10.5 +/- 0.6 mg kg-1 min-1, P less than 0.02). Although the older subjects had a 6.5% decrease in lean body mass, clamp glucose requirement expressed per unit of lean body mass was also significantly decreased in the older subjects (10.2 +/- 0.5 vs. 12.4 +/- 0.6 mg kg-1 min-1, P less than 0.05). The increase in muscle glycogen with the clamp was decreased by 33% in the older subjects (elderly: 13.1 +/- 1.3 mg g-1 protein, young: 19.6 +/- 2.2 mg g-1 protein; P less than 0.05), and was strongly correlated with clamp glucose requirement (r = 0.72, P less than 0.01). Glucose-6-phosphate independent glycogen synthase activity increased significantly between fasting and the end of the clamps in both groups (P less than 0.001), but was lower at the end of the clamp in the older subjects (P less than 0.05). Glycogen synthase activity at the end of the clamp correlated with both clamp glucose requirement (r = 0.83, P less than 0.01) and muscle glycogen deposition (r = 0.73, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Skeletal muscle glucose uptake, glycogen synthase activity and GLUT 4 content during hypoglycaemia in type 1 diabetic subjects

In healthy subjects, hypoglycaemia induces a profound 80% reduction in skeletal muscle glucose uptake and a similar suppression of glycogen synthase activity. The aim of this study was to examine the efficacy of this counterregulatory mechanism in type 1 diabetic subjects, who are especially prone to hypoglycaemic incidents. Nine type 1 diabetic male subjects were examined twice; during 120 min of hyperinsulinaemic (1.5 mU x kg(-1) x min(-1)) euglycaemia followed by (i) 240 min of graded hypoglycaemia (glucose nadir 2.8 mM) or (ii) 240 min of euglycaemia. At 345-360 min a muscle biopsy was taken and indirect calorimetry was performed at 210-240 and 320-340 min. The sensitivity of glycogen synthase to glucose-6-P was reduced by hypoglycaemia, as shown by an increase in A0.5 for glucose-6-P (at 0.07 mmol/L) from 0.21+/-0.02 to 0.28+/-0.03 mM (p=0.06). Likewise, the fractional velocity for glycogen synthase was reduced by 25%; i.e. from 20.8+/-2.0 to 15.5+/-1.4% (p<0.05). Total glucose disposal was decreased during hypoglycaemia (5.3+/-0.6 vs. 8.3+/-0.7 mg x kg(-1) x min(-1) (euglycaemia), n = 9; p<0.05), primarily due to a reduction of non-oxidative glucose disposal (2.7+/-0.3 vs. 5.1+/-0.6 mg x kg(-1) x min(-1) (euglycaemia), n=7; p<0.05). Forearm arteriovenous glucose differences were decreased by 50% in the hypoglycaemic situation (0.7+/-0.1 vs. 1.4+/-0.3 mmol/L (320-340 min)), and counterregulatory hormonal responses seemed less conspicuous than described in healthy subjects. We conclude that hypoglycaemia induces decrements of forearm glucose uptake and glycogen synthase activity in type 1 diabetic subjects. The study indicates a decreased magnitude of these responses, but this remains to be confirmed.     

Effects of troglitazone in young first-degree relatives of patients with type 2 diabetes

OBJECTIVE: Insulin resistance is a key characteristic of first-degree relatives of patients with type 2 diabetes. We therefore treated young, glucose-tolerant relatives with the insulin action enhancer troglitazone in order to determine the effects on insulin sensitivity, glucose metabolism, and glycogen synthase activity. RESEARCH DESIGN AND METHODS: Relatives were randomized in a double-blind manner and treated for 12 weeks with either 200 mg troglitazone or placebo. Before and after treatment, an oral glucose tolerance test (OGTT) and a euglycemic-hyperinsulinemic clamp (40 mU. m(-2). min(-1)) were performed, including 3-(3)H glucose infusion, glycolytic flux calculations, indirect calorimetry, and muscle biopsies. RESULTS: Twelve relatives received troglitazone and 12 placebo (aged 30.8 +/- 2.0 vs. 30.3 +/- 1.6 years, BMI 29.6 +/- 0.8 vs. 30.5 +/- 1.3 kg/m(2); means +/- SE). Area under the curve (AUC) for plasma glucose at the second OGTT was unchanged after troglitazone. In contrast, troglitazone reduced fasting (from 70.3 +/- 6.9 to 52.2 +/- 5.8 vs. 73.6 +/- 11.0 to 73.3 +/- 6.5 pmol/l, P < 0.02) and AUC plasma insulin (mean [CI] from 335.7 [230.9-488.1] to 277.4 [179.4-428.8] vs. 313.8 [218.2-451.2] to 353.9 [208.3-601.3] pmol/l, P < 0.05). Additionally, fasting plasma triglycerides were reduced by troglitazone (from 1.86 +/- 0.33 to 1.38 +/- 0.27 vs. 2.22 +/- 0.44 to 2.35 +/- 0.46 mmol/l, P < 0.01). Insulin-stimulated glucose disposal increased in the troglitazone group (from 208.3 +/- 23.7 to 263.5 +/- 30.4 vs. 197.1 +/- 20.0 to 200.8 +/- 20.8 mg. m(-2). min(-1), P < 0.02) mainly due to increased glucose storage (from 99.9 +/- 17.9 to 146.0 +/- 25.3 vs. 87.1 +/- 16.7 to 87.9 +/- 15.7 mg. m(-2). min(-1), P < 0.02), which took place without altering insulin-stimulated glycogen synthase activity. CONCLUSIONS: In glucose-tolerant first-degree relatives, treatment with troglitazone improved insulin sensitivity almost 50%, primarily due to increased glucose storage. It is suggested that the use of insulin action enhancers can be especially valuable in this group of subjects with a known high risk for developing type 2 diabetes.     

Intracellular glucose oxidation and glycogen synthase activity are reduced in non-insulin-dependent (type II) diabetes independent of impaired glucose uptake.

To examine whether reduced rates of oxidative (Gox) and non-oxidative (Nox) glucose metabolism in non-insulin-dependent diabetes mellitus (NIDDM) are due to reduced glucose uptake, intrinsic defects in intracellular glucose metabolism or increased fat oxidation (Fox), indirect calorimetry was performed at similar glucose uptake rates in eight nonobese NIDDM and eight comparable nondiabetic subjects. Three glucose clamp studies were performed: one in the nondiabetic and two in the NIDDM subjects. In the nondiabetic subjects, glucose uptake was increased to 7.62 +/- 0.62 mg/kg of fat-free mass (FFM) per min by increasing serum insulin to 309 pmol/liter at a glucose concentration of 5.1 mmol/liter. By raising the concentration of either serum glucose or insulin fourfold in the NIDDM subjects, glucose uptake was matched to nondiabetic subjects (8.62 +/- 0.49 and 8.59 +/- 0.51 mg/kg FFM per min, respectively, P = NS). Skeletal muscle glycogen synthase activity and plasma lactate levels were measured to characterize Nox. When glucose uptake was matched to nondiabetics by hyperglycemia or hyperinsulinemia, Gox was reduced by 26-28% in NIDDM (P less than 0.025) whereas Fox was similar. Nox was greater in NIDDM (P less than 0.01) and was accompanied by increases in circulating lactate levels. Glycogen synthase activity was reduced by 41% (P less than 0.025) when glucose uptake was matched by hyperglycemia. Glycogen synthase activity was normalized in NIDDM, however, when glucose uptake was matched by hyperinsulinemia. Therefore, a defect in Gox exists in nonobese NIDDM subjects which cannot be overcome by increasing glucose uptake or insulin. Since both glucose uptake and Fox were similar in the two subject groups these factors were not responsible for reduced Gox. Increased Nox in NIDDM is primarily into lactate. Reduced glycogen synthase activity in NIDDM is independent of glucose uptake but can be overcome by increasing the insulin concentration.