Skeletal Muscle Glycogen Synthase Activity Research Articles

Insulin Resistance in Skeletal Muscle of Chronic Stroke.

A stroke can lead to reduced mobility affecting skeletal muscle mass and fatty infiltration which could lead to systemic insulin resistance, but this has not been examined and the mechanisms are currently unknown. The objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in paretic (P) and nonparetic (NP) skeletal muscle in chronic stroke, and to compare to nonstroke controls. Participants were mild to moderately disabled adults with chronic stroke (n = 30, 60 ± 8 years) and sedentary controls (n = 35, 62 ± 8 years). Insulin sensitivity (M) and bilateral GS activity were determined after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Stroke subjects had lower aerobic capacity than controls, but M was not significantly different. Insulin-stimulated activities of GS (independent, total, fractional), as well as absolute differences (insulin minus basal) and the percent change (insulin minus basal, relative to basal) in GS activities, were all significantly lower in P versus NP muscle. Basal GS fractional activity was 3-fold higher, and the increase in GS fractional activity during the clamp was 2-fold higher in control versus P and NP muscle. Visceral fat and intermuscular fat were associated with lower M. The effect of in vivo insulin to increase GS fractional activity was associated with M in control and P muscle. A reduction in insulin action on GS in paretic muscle likely contributes to skeletal muscle-specific insulin resistance in chronic stroke.

Progresses of the Past Decade on Factors Contributing to Skeletal Muscle Glycogen Synthase Regulation

Glycogen is the primary storage form of glucose, which is metabolized actively in the liver and skeletal muscle of mammals. The skeletal muscle glycogen is used intracellularly to provide starting substrate for glycolysis, whereas liver glycogen is used mainly for the maintenance of blood glucose homeostasis. The conversion of glucose to glycogen is an ordered process, in which the enzyme glycogen synthase plays an essential role. The key events in the synthesis of glycogen have been elucidated in both the liver and muscle. The process of understanding the regulatory mechanism of glycogen synthase activity has led to the discoveries of many important players in cellular metabolism. Research in identifying factors modulating muscle glycogen synthesis has been of popular interest within the past decade. The goal of this review is to summarize potential factors affecting skeletal muscle glycogen synthase expression level and activity in literatures published for the past decade.

Dysregulation of muscle glycogen synthase in recovery from exercise in type 2 diabetes.

Insulin and exercise stimulate skeletal muscle glycogen synthase (GS) activity by dephosphorylation and changes in kinetic properties. The aim of this study was to investigate the effects of insulin, exercise and post-exercise insulin stimulation on GS phosphorylation, activity and substrate affinity in obesity and type 2 diabetes. Obese men with type 2 diabetes (n = 13) and weight-matched controls (n = 14) underwent euglycaemic-hyperinsulinaemic clamps in the rested state and 3 h after 60 min of cycling (70% maximal pulmonary oxygen uptake [VO2max]). Biopsies from vastus lateralis muscle were obtained before and after clamps, and before and immediately after exercise. Insulin-stimulated glucose uptake was lower in diabetic patients vs obese controls with or without prior exercise. Post exercise, glucose partitioning shifted away from oxidation and towards storage in both groups. Insulin and, more potently, exercise increased GS activity (fractional velocity [FV]) and substrate affinity in both groups. Both stimuli caused dephosphorylation of GS at sites 3a + 3b, with exercise additionally decreasing phosphorylation at sites 2 + 2a. In both groups, changes in GS activity, substrate affinity and dephosphorylation at sites 3a + 3b by exercise were sustained 3 h post exercise and further enhanced by insulin. Post exercise, reduced GS activity and substrate affinity as well as increased phosphorylation at sites 2 + 2a were found in diabetic patients vs obese controls. Exercise-induced activation of muscle GS in obesity and type 2 diabetes involves dephosphorylation of GS at sites 3a + 3b and 2 + 2a and enhanced substrate affinity, which is likely to facilitate glucose partitioning towards storage. Lower GS activity and increased phosphorylation at sites 2 + 2a in type 2 diabetes in the recovery period imply an impaired response to exercise.

Aerobic exercise plus weight loss improves insulin sensitivity and increases skeletal muscle glycogen synthase activity in older men.

The purpose of this study was to determine the effects of 6-month aerobic exercise training + weight loss (AEX + WL) on basal and insulin activation of glycogen synthase, basal citrate synthase activity, and Akt and AS160 phosphorylation in older, overweight/obese insulin-resistant men (n = 14; 63 ± 2 years; body mass index, 32 ± kg/m(2)). Muscle samples of the vastus lateralis were collected before and during a 3-hour 80 mU/m(2)/min hyperinsulinemic-euglycemic clamp. AEX + WL increased VO2max by 11% (p < .05) and decreased body weight (-9%, p < .001). AEX + WL increased basal citrate synthase activity by 46% (p < .01) and insulin activation of independent (2.9-fold) and fractional (2.3-fold) activities (both p < .001) of glycogen synthase. AEX + WL had no effect on phosphorylation of Akt or AS160. Glucose utilization (M) improved 25% (p < .01), and the change tended to be related to the increase in insulin activation of glycogen synthase fractional activity (r = .50, p = .08) following AEX + WL. In summary, AEX + WL has a robust effect on insulin activation of skeletal muscle glycogen synthase activity that likely contributes to improved glucose utilization in older insulin-resistant men.

Exercise with calorie restriction improves insulin sensitivity and glycogen synthase activity in obese postmenopausal women with impaired glucose tolerance

Our objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in normal (NGT) vs. impaired glucose-tolerant (IGT) obese postmenopausal women and to determine whether an increase in insulin activation of GS is associated with an improvement in insulin sensitivity (M) following calorie restriction (CR) and/or aerobic exercise plus calorie restriction (AEX + CR) in women with NGT and IGT. We did a longitudinal, clinical intervention study of CR compared with AEX + CR. Overweight and obese women, 49-76 yr old, completed 6 mo of CR (n = 46) or AEX + CR (n = 50) with Vo(2 max), body composition, and glucose tolerance testing. Hyperinsulinemic euglycemic (80 mU·m(-2)·min(-1)) clamps (n = 73) and skeletal muscle biopsies (before and during clamp) (n = 58) were performed before and after the interventions (n = 50). After 120 min of hyperinsulinemia during the clamp, GS fractional activity and insulin's effect to increase GS fractional activity (insulin - basal) were significantly lower in IGT vs. NGT (P < 0.01) at baseline. GS total activity increased during the clamp in NGT (P < 0.05), but not IGT, at baseline. CR and AEX + CR resulted in a significant 8% weight loss with reductions in total fat mass, visceral fat, subcutaneous fat, and intramuscular fat. Overall, M increased (P < 0.01), and the change in M (postintervention - preintervention) was associated with the change in insulin-stimulated GS fractional activity (partial r = 0.44, P < 0.005). In IGT, the change (postintervention - preintervention) in insulin-stimulated GS total activity was greater following AEX + CR than CR alone (P < 0.05). In IGT, insulin-stimulated GS-independent (P < 0.005) and fractional activity (P = 0.06) increased following AEX + CR. We conclude that the greatest benefits at the whole body and cellular level (insulin activation of GS) in older women at highest risk for diabetes are derived from a lifestyle intervention that includes exercise and diet.

Time course of insulin sensitivity and skeletal muscle glycogen synthase activity after a single bout of exercise in horses

The time course of insulin sensitivity, skeletal muscle glycogen and GLUT4 content, and glycogen synthase (GS) activity after a single bout of intense exercise was examined in eight horses. On separate days, a euglycemic-hyperinsulinemic clamp (EHC) was undertaken at 0.5, 4, or 24 h after exercise or after 48 h of rest [control (Con)]. There was no increase in mean glucose infusion rate (GIR) with exercise (0.5-, 4-, and 24-h trials), and GIR was significantly decreased at 0.5 h postexercise (GIR: 8.6 +/- 2.7, 6.7 +/- 2.0, 9.0 +/- 2.0, and 10.6 +/- 2.2 mg.kg(-1).min(-1) for Con and at 0.5, 4, and 24 h, respectively). Before each EHC, muscle glycogen content (mmol glucosyl units/kg dry muscle) was higher (P < 0.05) for Con (565 +/- 102) than for other treatments (317 +/- 84, 362 +/- 79, and 382 +/- 74 for 0.5, 4, and 24 h, respectively) and muscle GLUT4 content was unchanged. Pre-EHC active-to-total GS activity ratio was higher (P < 0.05) at 0.5, 4, and 24 h after exercise than in Con. Post-EHC active GS and GS activity ratio were higher (P < 0.05) in Con and at 24 h. There was a significant inverse correlation (r = -0.43, P = 0.02) between glycogen content and GS activity ratio but no relationship between GS activity and GIR. The lack of increase in insulin sensitivity, determined by EHC, after exercise that resulted in a significant reduction in muscle glycogen content is consistent with the slow rate of muscle glycogen resynthesis observed in equine studies.

Reduced plasma adiponectin concentrations may contribute to impaired insulin activation of glycogen synthase in skeletal muscle of patients with type 2 diabetes

Circulating levels of adiponectin are negatively associated with multiple indices of insulin resistance, and the concentration is reduced in humans with insulin resistance and type 2 diabetes. However, the mechanisms by which adiponectin improves insulin sensitivity remain unclear. Combining euglycaemic-hyperinsulinaemic clamp studies with indirect calorimetry and skeletal muscle biopsies, we examined the relationship between plasma adiponectin and parameters of whole-body glucose and lipid metabolism, and muscle glycogen synthase (GS) activity in 51 Caucasians (ten lean, 21 obese and 20 with type 2 diabetes). Plasma adiponectin was significantly reduced in type 2 diabetic compared with obese and lean subjects. In lean and obese subjects, insulin significantly reduced plasma adiponectin, but this response was blunted in patients with type 2 diabetes. Plasma adiponectin was positively associated with insulin-stimulated glucose disposal (r = 0.48), glucose oxidation (r = 0.54), respiratory quotient (r = 0.58) and non-oxidative glucose metabolism (r = 0.38), and negatively associated with lipid oxidation during insulin stimulation (r = -0.60) after adjustment for body fat (all p < 0.01). Most notably, we found a positive association between plasma adiponectin and insulin stimulation of GS activity in skeletal muscle (r = 0.44, p < 0.01). Our results indicate that plasma adiponectin may enhance insulin sensitivity by improving the capacity to switch from lipid to glucose oxidation and to store glucose as glycogen in response to insulin, and that low adiponectin may contribute to impaired insulin activation of GS in skeletal muscle of patients with type 2 diabetes.

In Vivo Insulin Regulation of Skeletal Muscle Glycogen Synthase in Calorie-Restricted and in Ad Libitum–Fed Rhesus Monkeys

Chronic calorie restriction in primates has been shown to have profound and unexpected effects on basal and on in vivo insulin action on skeletal muscle glycogen synthase (GS) activity. The decreased ability of insulin to activate skeletal muscle GS is a hallmark of insulin resistance and type 2 diabetes. The mechanism and role of in vivo insulin regulation of skeletal muscle GS are not fully understood. Two pathways for the activation of GS by insulin have been described by Larner and others: 1) insulin activates glucose transport that results in an increase in glucose-6-phosphate (G6P), thereby activating protein phosphatase-1, which in turn dephosphorylates and activates GS, therefore, pushing substrate into glycogen; and 2) insulin activates GS (perhaps by forming low-molecular-weight mediators which may activate protein phosphatase-1 and 2C) and activated GS subsequently pulls intermediates (e.g., G6P and uridine 5'-diphosphoglucose) into glycogen. To determine whether in vivo insulin regulates glycogen synthesis primarily via a push or pull mechanism and how this mechanism might be affected by long-term calorie restriction, skeletal muscle samples were obtained before and during a euglycemic hyperinsulinemic clamp from 41 rhesus monkeys. The monkeys varied widely in their degree of insulin sensitivity and age and included chronically calorie-restricted (CR) monkeys and ad libitum-fed monkeys. The ad libitum-fed monkeys included spontaneously type 2 diabetic, prediabetic and clinically normal animals. The apparent affinity of GS for the allosteric activator G6P (G6P Ka of GS) was measured and compared with G6P content in the muscle samples. Basal G6P Ka of GS was lower in the CR monkeys compared with the 3 ad libitum-fed groups (P: < or = 0.05). Only the normal ad libitum-fed monkeys had a decrease in the G6P Ka of GS with insulin (P: < 0.005). The insulin effect (insulin-stimulated minus basal) on the G6P Ka of GS was strongly positively related to the insulin effect on G6P content (r = 0.80, P: < 0.0001) across the entire group of monkeys. This finding supports the hypothesis that activation/dephosphorylation of GS by insulin is related to a decrease in G6P content and that paradoxical inactivation/phosphorylation of GS by insulin is related to an increase in G6P content (as demonstrated in 4 of 6 CR monkeys). Therefore, during a euglycemic hyperinsulinemic clamp, insulin regulates skeletal muscle glycogen synthesis primarily via a pull mechanism in both CR and in ad libitum-fed rhesus monkeys.

The intracellular mechanism of insulin resistance in the hamster pancreatic ductal adenocarcinoma model.

Diabetes associated with pancreatic cancer is characterized by profound peripheral insulin resistance. The intracellular mechanism of insulin resistance was investigated in skeletal muscles from N-nitrosobis(2-oxopropyl)amine (BOP)-treated hamsters. Effects of high-fat diet and exercise also were studied. BOP (20 mg/kg body weight) was administrated weekly for 2 weeks. Hyperinsulinemia was found in BOP-treated hamsters at 20 weeks after BOP treatment, suggesting the peripheral insulin resistance is an early feature in pancreatic cancer. Hamsters were killed at 42 weeks, and soleus muscles were taken for the analysis. Skeletal muscle insulin-receptor binding and insulin receptor tyrosine kinase activities were similar between the control and BOP-treated hamsters. However, maximal muscle glycogen synthase activity was significantly reduced in BOP-treated hamsters compared with the control group. Muscle glycogen phosphorylase activity was increased in the BOP-treated group fed with high-fat diet as well as in BOP-treated groups with exercise. These findings indicate that insulin resistance in the hamster pancreatic cancer model is caused by a postreceptor defect, which led to significant decrease of muscle glycogen synthase activity. Whereas a high-fat diet causes more severe insulin resistance in BOP-treated hamsters, high-fat diet and exercising had no significant effects on skeletal muscle insulin-receptor function and glycogen synthase activity. Furthermore, both high-fat diet and exercise enhanced glycogen phosphorylase activity in BOP-treated hamsters.

Defects of insulin action and skeletal muscle glucose metabolism in growth hormone-deficient adults persist after 24 months of recombinant human growth hormone therapy.

We have previously reported that GH-deficient (GHD) adults are severely insulin resistant. In the present study, we determined the effects of 6 months (n = 7) and 24 months (long-term; n = 11) of recombinant human GH (rhGH) therapy (approximately 0.22 IU/kg.week) on body composition and fasting biochemical (including lipid) parameters, and baseline and insulin-stimulated: 1) rates of hepatic glucose production, total glucose disposal (Rd), total glycolysis (GF) and glucose storage (GS); and 2) skeletal muscle glucose processing [using the euglycemic-hyperinsulinemic (approximately 60 mU/L) clamp technique with tritiated glucose infusion coupled with skeletal muscle biopsies]. To allow baseline comparison, these measurements were also obtained from 10 control subjects matched to the pretreated GHD adults for age, sex, and body mass index. Long-term rhGH therapy in GHD adults induced significant improvements in fat mass, abdominal fat mass and fat free mass, and reductions in fasting cholesterol and low-density lipoprotein-cholesterol levels (P < 0.05-0.01 vs. pretreatment values). However, there was a significant increase in fasting insulin (13.1 +/- 0.9 vs. 8.6 +/- 1.1 mU/L; P < 0.01) and connecting peptide (0.56 +/- 0.05 vs. 0.41 +/- 0.06 nmol/L; P < 0.05). Although rates of baseline hepatic glucose production, GF, and GS were unchanged, the insulin-stimulated increment (delta) in Rd, GF, and GS remained markedly attenuated in the long-term rhGH-treated GHD adults [pretreatment: delta Rd 16.6 +/- 3.4, delta GF 3.0 +/- 1.2, delta GS 13.6 +/- 3.0 vs. 24 months of rhGH: delta Rd 17.2 +/- 3.3, delta GF 3.1 +/- 0.9, delta GS 14.1 +/- 2.5 vs. controls: delta Rd 42.6 +/- 4.3, delta GF 9.2 +/- 1.9, delta GS 35.9 +/- 4.5 mumol/kg fat free mass.min; P < 0.05-0.01 vs. controls]. Additionally, there was a sustained reduction in the insulin-stimulated skeletal muscle glycogen synthase fractional velocity (pretreatment: 0.29 +/- 0.03 vs. 24 months of rhGH: 0.24 +/- 0.03 vs. controls: 0.48 +/- 0.04; both P < 0.05 vs. controls), which was accompanied by a sustained 44% decrease in baseline glycogen content and a 70% increase in baseline im glucose concentrations in the presence of low-to-normal glucose 6-phosphate levels and persisting euglycemia. Stepwise regression analysis revealed that body weight and fasting free fatty acid and high-density lipoprotein (HDL)-cholesterol accounted for 82% of the variance in the insulin sensitivity index in long-term rhGH-treated adults, and that the 24-month fasting insulin-like growth factor 1 was a negative predictor of the change in insulin sensitivity (r = -0.82; P < 0.01). In conclusion, despite improvements in body composition and lipid profiles, the severe defects of in vivo insulin sensitivity and skeletal muscle intracellular glucose phosphorylation and glycogen synthase activity, which are associated with modestly elevated insulin-like growth factor 1 levels, normal free fatty acid levels, and the development of hyperinsulinemia, persist with long-term rhGH therapy.

Intracellular Glucose and Fat Metabolism in Identical Twins Discordant for Non-insulin-dependent Diabetes Mellitus (NIDDM): Acquired Versus Genetic Metabolic Defects?

Intracellular glucose and lipid metabolism was studied in 12 identical twin pairs discordant for non-insulin-dependent (Type 2) diabetes mellitus (NIDDM) and 13 control subjects without family history of diabetes during low (baseline) and high plasma insulin concentrations, using the hyperinsulinaemic clamp technique combined with indirect calorimetry, tritiated water glycolytic flux rates and biopsy skeletal muscle glycogen synthase activity determinations. Baseline and insulin stimulated rates of lipid oxidation were elevated--and glucose oxidation decreased--in the NIDDM twins compared with the non-diabetic co-twins and controls (all p < 0.05). Baseline and insulin stimulated rates of glucose and lipid oxidation were similar in non-diabetic twins and controls. Exogenous glycolytic flux was decreased in NIDDM twins compared with both their non-diabetic co-twins and controls during clamp insulin measurements (p < 0.02), but similar in all study groups during baseline measurements. Insulin stimulated glucose disposal, exogenous glucose storage (glucose disposal-exogenous glycolytic flux) and skeletal muscle glycogen synthase activity were all significantly decreased in NIDDM twins compared with both their non-diabetic co-twins and controls. Furthermore, glucose disposal and glucose storage were decreased in the non-diabetic twins (n = 12) compared with controls (p < 0.05 both). However, insulin stimulated fractional skeletal muscle glycogen synthase activity was not significantly decreased in non-diabetic twins compared with controls. (1) the glucose fatty acid cycle plays a major role in the secondary--but not the primary--abnormalities of glucose metabolism in NIDDM; (2) insulin resistance in non-diabetic identical co-twins of NIDDM patients is restricted exclusively to the pathway of exogenous glucose storage; (3) however, the decreased glucose storage is not explained solely by an impairment of insulin stimulated skeletal muscle glycogen synthase activity; and finally (4) the impairment of skeletal muscle glycogen synthase activity in NIDDM has an apparent non-genetic component and can be escaped (or postponed) in individuals (twins) with a 100% genetic predisposition to NIDDM.

Glucose processing during the intravenous glucose tolerance test

The impact of the dynamic changes in plasma glucose and insulin levels observed during a frequently sampled intravenous (IV) glucose tolerance test (FSIGT) on whole-body glucose processing and muscle glycogen metabolism is not known. Paired randomized FSIGTs were performed in eight healthy subjects (age, 31 years; range, 28 to 35; BMI, 25.4 kg/m 2; range, 22.3 to 32.1), one with muscle biopsy samples and one without. The mean time average (0- to 40- and 0- to 120-minute) insulin levels during the test were 26.6 and 11.4 mU/I, respectively. Glucose oxidation increased following the IV gluclose bolus (basal 1.34 ± 0.21 v mean value at 0 to 120 minutes 2.09 ± 0.22 mg/kg fat-free mass [FFM]/min, P < .02). In contrast, fractional glucose-6-phosphate [G-6-P]) ( 0.1 10 mmol/L) skeletal muscle glycogen synthase activity in muscle biopsies obtained before and following the IV glucose bolus (−30, 30, 60, and 120 minutes, respectively) were unchanged (38.1% ± 3.3%, 38.3% ± 2.9%, 38.1% ± 2.3%, 35.4% ± 2.3%, NS). Skeletal muscle glycogen concentration decreased slightly (449 ± 54, 439 ± 55, and 383 ± 29, and 438 ± 48 mmol/kg dry weight, P = .05), indicating no net storage of glucose into glycogen during the FSIGT. G-6-P decreased (0.77 ± 0.08, 0.64 ± 0.07, 0.66 ± 0.07, and 0.54 ± 0.04 mmol/kg dry weight, P < .05). Levels of the insulin-regulatable glucose transporter, GLUT-4, were unchanged. Insulin sensitivity (Si), glucose effectiveness, and insulin secretion parameters (Ø1 and Ø2) were not affected by the muscle biopsy procedure. In conclusion, the FSIGT is associated predominantly with increased whole-body glucose oxidation with no apparent activation of muscle glucose storage as glycogen. Thus, the Si measured by the FSIGT, although similar in magnitude to the clamp-derived parameter, represents primarily glucose oxidation, in contrast to the euglycemic clamp, which involves glucose oxidation and storage.

Long-term effects of fluoxetine on glycemic control in obese patients with non-insulin-dependent diabetes mellitus or glucose intolerance: Influence on muscle glycogen synthase and insulin receptor kinase activity

Fluoxetine (F) is a specific serotonin-reuptake inhibitor that has been shown to promote weight loss and improve glycemic control in obese diabetic patients. To study its long-term metabolic effect, 40 obese patients with non-insulin-dependent diabetes mellitus (NIDDM) or impaired glucose tolerance (IGT) were included in a 12-month, randomized, placebo-controlled study. Patients were assigned to receive either 60 mg F or placebo (P) daily in conjunction with a 5.0-MJ/d diet (> 50% carbohydrate). Both groups showed a significant weight loss, with a nadir after 6 months without group differences (mean ± SD: F, 10.1 ± 10.0 kg; P, 9.4 ± 11.5 kg). Fifteen patients from the F group and 14 from the P group completed the 12-month study without weight loss differences. Glycemic regulation improved along with the weight loss, but with a larger decline in plasma C-peptide and fasting glucose levels in the F group ( P < .05). Total skeletal muscle glycogen synthase (GS) activity increased by 31% in the F group ( P < .01) and by 17% in the P group (nonsignificant) after 6 months of treatment, but was still less than the activity in normal-weight controls (aged 28.0 ± 6.3 years; body mass index, 23.5 ± 2.2). After adjustment for fasting glucose, insulin, weight loss, and diabetic state, a positive effect of F remained on the total GS activity, which accounted for 27% of the variation ( P < .05). The waist to hip ratio was reduced in P subjects as compared with F subjects ( P < .05). Fat-free mass (FFM) tended to be more reduced in the F group as compared with P subjects (4.9 v 1.9 kg), although the difference did not reach statistical significance. In conclusion, F seems to improve insulin sensitivity beyond the effect mediated through weight loss by a possible effect on GS activity in skeletal muscle tissue.

Multiple defects of both hepatic and peripheral intracellular glucose processing contribute to the hyperglycaemia of NIDDM

Non-insulin-dependent diabetic (NIDDM) patients were studied during a modified euglycaemic state when fasting hyperglycaemia was normalized by a prior (−210 to −150 min) — and later withdrawn (−150–0 min) — intravenous insulin infusion. Glucose metabolism was assessed in NIDDM patients (n=10) and matched control subjects (n=10) using tritiated glucose turnover rates, indirect calorimetry and skeletal muscle glycogen synthase activity determinations. Total and non-oxidative exogenous glycolytic flux rates were measured using appearance rates of tritiated water. A+180 min euglycaemic hyperinsulinaemic (40 mU·m−2·min−1) clamp was performed to determine the insulin responsiveness of the various metabolic pathways. Plasma glucose concentration increased spontaneously during baseline measurements in the NIDDM patients (−120 to 0 min: 4.8±0.3 to 7.0±0.3 mmol/l; p<0.01), and was primarily due to an elevated rate of hepatic glucose production (3.16±0.13 vs 2.51±0.16 mg·kg FFM−1·min−1; p<0.01). In the NIDDM subjects baseline glucose oxidation was decreased (0.92±0.17 vs 1.33±0.14 mg·kg FFM−1·min−1; p<0.01) in the presence of a normal rate of total exogenous glycolytic flux and skeletal muscle glycogen synthase activity. The simultaneous finding of an increased lipid oxidation rate (1.95±0.13 vs 1.61±0.07 mg·kg FFM−1·min−1; p=0.05) and increased plasma lactate concentrations (0.86±0.05 vs 0.66±0.03 mmol/l; p=0.01) are consistent with a role for both the glucose-fatty acid cycle and the Cori cycle in the maintenance and development of fasting hyperglycaemia in NIDDM during decompensation. Insulin resistance was demonstrated during the hyperinsulinaemic clamp in the NIDDM patients with a decrease in the major peripheral pathways of intracellular glucose metabolism (oxidation, storage and muscle glycogen synthase activity), but not in the pathway of non-oxidative glycolytic flux which was not completely suppressed during insulin infusion in the NIDDM patients (0.55±0.15 mg·kg FFM−1·min−1; p<0.05 vs 0; control subjects: 0.17±0.29; NS vs 0). Thus, these data also indicate that the defect(s) of peripheral (skeletal muscle) glucose processing in NIDDM goes beyond the site of glucose transport across the cell membrane.

Role of basal insulin in maintenance of intracellular glucose metabolic pathways in non—insulin-dependent diabetes mellitus

Impairments of both basal and insulin-stimulated oxidative (G ox) and nonoxidative (N ox) glucose metabolism are documented to exist in non—insulin-dependent diabetes mellitus (NIDDM). Although these defects have been well characterized during insulin stimulation, little is known about the effects of basal insulin or its deficiency on intracellular glucose metabolism in NIDDM. To determine the physiological significance of basal insulin in the maintenance of glucose metabolism in NIDDM, we studied nine subjects with NIDDM in the basal and insulin-deficient state produced by 3 hours of somatostatin (SRIF) infusion (0.08 pmol/kg/min). Glucose turnover rates were quantified by [3- 3H]glucose turnover, and substrate oxidation was assessed by a combination of indirect calorimetry and urinary nitrogen measurements. Skeletal muscle glycogen synthase (GS) and pyruvate dehydrogenase (PDH) activities were also measured in the basal state and during SRIF infusion. Basal glucose levels were maintained during SRIF infusion by exogenous glucose infusion (12.5 ± 0.9 mmol/L in the basal state v 12.8 ± 0.8 during SRIF infusion, P = NS). During the last hour of SRIF infusion, plasma C-peptide levels declined by 88% from 0.73 ± 0.11 to 0.09 ± 0.02 nmol/L ( P < .001), and serum insulin concentrations were undetectable (<14 pmol/L). During insulinopenic conditions, rates of glucose uptake (GU) were decreased by 12% from basal level of 2.26 ± 0.13 to 1.99 ± 0.12 mg/kg/min ( P < .05), and were entirely accounted for by reduced rates of G ox (1.01 ± 0.10 to 0.65 ± 0.14 mg/kg/min, P < .01). Corresponding measurements of active PDH (PDHa) activity in skeletal muscle were similarly decreased from 0.50 ± 0.14 to 0.31 ± 0.09 nmol/min/mg protein ( P < .05) during insulinopenia, whereas rates of fat oxidation (F ox) were increased from 0.87 ± 0.05 to 1.05 ± 0.07 mg/kg/min ( P < .01). The reduction in energy expenditure (EE) from lower G ox was completely offset by increased energy from enhanced F ox and resulted in unchanged total EE (1.42 ± 0.10 v 1.41 ± 0.11 kcal/min, P = NS). Although skeletal muscle GS activity was significantly decreased (2.6% ± 0.7% to 1.8% ± 0.8%, P < .05), calculated rates of N ox remained unchanged (1.24 ± 0.17 to 1.34 ± 0.21 mg/kg/min, P = NS) and were significantly correlated with non—insulin-mediated GU (NIMGU) during insulinopenia ( r = .76, P < .05). In conclusion, approximately 35% of basal G ox in NIDDM was dependent on basal insulin action. Basal N ox was independent of basal insulin and was determined by the rate of NIMGU. Basal insulin regulates intracellular glucose oxidation by multiple mechanisms that include effects on glucose uptake, free fatty acid (FFA) substrate availability, and PDH enzyme activity.

Relationship of skeletal muscle glucose 6-phosphate to glucose disposal rate and glycogen synthase activity in insulin-resistant and non-insulin-dependent diabetic rhesus monkeys.

Reduced insulin action on skeletal muscle glycogen synthase activity and reduced whole-body insulin-mediated glucose disposal rates in insulin-resistant subjects may be associated with an alteration in muscle glucose transport (or phosphorylation) or with a defect distal to glucose 6-phosphate. To examine this issue we determined the glucose 6-phosphate concentration and glycogen synthase activity in muscle samples obtained under basal and euglycaemic hyperinsulinaemic clamp conditions in 27 rhesus monkeys (Macaca mulatta). They ranged from metabolically normal (n = 11) to insulin-resistant (n = 8) to overtly diabetic (non-insulin-dependent) (n = 8). The glucose 6-phosphate measured under insulin-stimulated conditions was inversely correlated to insulin-stimulated glycogen synthase independent activity (r = -0.54, p < 0.005), the change in glycogen synthase independent activity (insulin-stimulated minus basal) (r = -0.58, p < 0.002) and to whole-body insulin-mediated glucose disposal rate (r = -0.60, p < 0.002). The insulin-resistant and diabetic monkeys had significantly higher insulin-stimulated glucose 6-phosphate concentrations (0.57 +/- 0.11 and 0.62 +/- 0.11 nmol/mg dry weight, respectively) compared to the normal monkeys (0.29 +/- 0.05 nmol/mg dry weight) (p's < 0.05). We conclude that under euglycaemic/hyperinsulinaemic conditions, a defect distal to glucose 6-phosphate is a major contributor to reduced whole-body insulin-mediated glucose disposal rates and to reduced insulin action on glycogen synthase in insulin-resistant and diabetic monkeys.

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.

The impact of metformin therapy on hepatic glucose production and skeletal muscle glycogen synthase activity in overweight type II diabetic patients

The effect of metformin therapy on glucose metabolism was examined in eight overweight newly presenting untreated type II diabetic patients (five males, three females). Patients were treated for 12 weeks with either metformin (850 mg × 3) or matching placebo using a double-bling crossover study design; patients were studied at presentation and at the end of each treatment period. Insulin action was assessed by measuring activation of skeletal muscle glycogen synthase (GS) before and during a 4-hour hyperinsulinemic euglycemic clamp (100 mU · kg −1 · h −1). Metformin therapy was associated with a significant decrease in fasting blood glucose (6.8 ± 0.6 v 8.3 ± 0.9 mmol · L −1, P < .01) and glycosylated hemoglobin ([HbA 1] 7.7% ± 0.4% v 8.5% ± 0.5%, P < .01) levels. Fasting hepatic glucose production (HGP) was also significantly decreased following metformin therapy (1.98 ± 0.13 v 2.41 ± 0.20 mg · kg −1 · min −1, P < .02), whereas fasting insulin and C-peptide concentrations remained unaltered. The decrease in basal HGP correlated closely with the decrease in fasting blood glucose concentration ( r = .92, P < .001). Insulin-stimulated glucose uptake was assessed using the hyperinsulinemic euglycemic clamp technique and was increased post-metformin (3.8 ± 0.6 v 3.1 ± 0.7 mg · kg −1 · min −1, P < .05). This was primarily the result of increased nonoxidative glucose metabolism (1.1 ± 0.6 v 0.4 ± 0.6 mg · kg −1 · min −1, P < .05); oxidative glucose metabolism did not change. Metformin had no measurable effect on insulin activation of skeletal muscle GS, the rate-limiting enzyme controlling muscle glucose storage. Thus metformin is an effective drug for use in type II diabetes mellitus, and its hypoglycemic action is likely to result from increased insulin sensitivity at the liver and periphery.

Intracellular glucose metabolism after long term metabolic control with glyburide: improved glucose oxidation with unchanged glycogen synthase activity.

To determine whether improved metabolic control with long term glyburide treatment alters intracellular glucose metabolism independent of effects on glucose uptake (GU), we studied eight obese patients with noninsulin-dependent diabetes mellitus before and 7 months after glyburide therapy. Indirect calorimetry and skeletal muscle biopsies were performed in the basal state and during 300 pmol/m2.min insulin infusions, with glucose turnover rates determined by [3-3H]glucose turnover. During the glucose clamps, rates of GU were matched before and after treatment using equivalent hyperinsulinemia and variable levels of hyperglycemia. After glyburide treatment, rates of GU were decreased in the basal state [4.16 +/- 0.57 vs. 3.29 +/- 0.37 mg/kg fat free mass (FFM)/min; P < 0.05], but similar during glucose clamps (11.53 +/- 1.42 vs. 11.93 +/- 1.32 mg/kg FFM.min; P = NS) according to study design. In both the basal state and during glucose clamps after glyburide therapy, rates of glucose oxidative metabolism (Gox) increased by 68-78% [1.21 +/- 0.16 vs. 2.03 +/- 0.31 mg/kg FFM.min (P < 0.05) and 3.13 +/- 0.51 vs. 5.58 +/- 0.55 mg/kg FFM.min (P < 0.05), respectively], and rates of nonoxidative glucose metabolism decreased [2.96 +/- 0.68 vs. 1.25 +/- 0.21 mg/kg FFM.min (P < 0.05) and 8.40 +/- 1.50 to 6.30 +/- 1.40 mg/kg FFM.min (P < 0.01), respectively]. Circulating plasma FFA levels and rates of fat oxidation (Fox) remained unchanged in both the basal state and during clamp studies. Skeletal muscle glycogen synthase (GS) activity, expressed as fractional velocity, was unchanged by glyburide therapy (2.2 +/- 0.8 vs. 2.7 +/- 0.3% in the basal state and 7.3 +/- 1.8 vs. 6.1 +/- 0.9% during clamps; both P = NS). In summary, at both matched (during clamp studies) and unmatched (during basal studies) rates of GU, improved metabolic control with glyburide therapy resulted in marked improvement of Gox independent of the effects on GU. The improvement in Gox was not associated with changes in Fox, circulating FFA, or muscle GS activity. These data indicate that long term metabolic control achieved by glyburide therapy markedly improves Gox, but not skeletal muscle GS activity, in noninsulin-dependent diabetes mellitus independent of GU and Fox.

Abdominal obesity is associated with insulin resistance and reduced glycogen synthase activity in skeletal muscle

Insulin resistance is commonly associated with obesity. The present study was performed to investigate the relative importance of total fat mass versus localization of adipose tissue in insulin-stimulated glucose disposal (R d) and skeletal muscle glycogen synthase (GS) activity in obese individuals. Twenty obese women with an average body mass index (BMI) of 37.8 ± 1.3 kg/m 2 and a waist to hip ratio (WHR) ranging from 0.78 to 1.02 were examined during basal conditions and following hyperinsulinemia (hyperinsulinemic euglycemic clamp). To accurately determine body composition, the following three methods were used: anthropometric measurements, dual-energy x-ray absorptiometry scanning (DEXA-scan), and bioelectric impedance measurements. In addition, indirect calorimetry and muscle biopsy were performed. Insulin-stimulated glucose R d was negatively correlated with WHR ( R = −.52, P < .025) whereas there were no correlations with BMI or percent fat ( R = .16, NS and R = .16, NS, respectively). Furthermore, a negative correlation between WHR and insulin stimulation of GS activity in skeletal muscle was found ( R = −.62, P < .005). In contrast, BMI and percent fat were not correlated with the insulin effect on GS activity in skeletal muscle ( R = .34, NS and R = −.35, NS, respectively). The concentration of nonesterified fatty acids (NEFA) during hyperinsulineamia was strongly correlated with WHR and abdominal localization of adipose tissue (determined by DEXA-scan; R = .60, P < .005 and R = .60, P < .007, respectively). Moreover, lipid oxidation during hyperinsulinemia was positively correlated with WHR ( R = .58, P < .007), whereas there was no correlation with BMI or percent fat ( R = .31, NS and R = .29, NS, respectively). Finally, NEFA concentrations during hyperinsulinemia were negatively correlated with insulin-stimulated glucose R d ( R = .73, P < .0009). We conclude that in a group of massively obese women localization of adipose tissue in the abdominal region (assessed by various techniques) was accompanied by impaired insulin-stimulated glucose R d, decreased capacity of insulin to stimulate GS activity in skeletal muscle, and higher circulating NEFA concentrations during the hyperinsulinemic period. In contrast, no correlations between BMI, percent fat, or other measurements of total amount of adipose tissue and these variables were found. Finally, the sophisticated measurements (DEXA-scan and bioelectric measurements) were not superior to BMI and WHR in predicting insulin resistance in this group of obese women.