Resistance to exercise-induced increase in glucose uptake during hyperinsulinemia in insulin-resistant skeletal muscle of patients with type 1 diabetes

Insulin and exercise have been shown to activate glucose transport at least in part via different signaling pathways. However, it is unknown whether insulin resistance is associated with a defect in the ability of an acute bout of exercise to enhance muscle glucose uptake in vivo. We compared the abilities of insulin and isometric exercise to stimulate muscle blood flow and glucose uptake in 12 men with type 1 diabetes (age 24 +/- 1 years, BMI 23.0 +/- 0.4 kg/m(2)) and in 11 age- and weight-matched nondiabetic men (age 25 +/- 1 years, BMI 22.3 +/- 0.6 kg/m(2)) during euglycemic hyperinsulinemia (1 mU. kg(-1). min(-1) insulin infusion for 150 min). One-legged exercise was performed at an intensity of 10% of maximal isometric force for 105 min (range 45-150). Rates of muscle blood flow, oxygen consumption, and glucose uptake were quantitated simultaneously in both legs using [(15)O]water, [(15)O]oxygen, [(18)F]-2-fluoro-2-deoxy-D-glucose, and positron emission tomography. Resting rates of oxygen consumption were similar during hyperinsulinemia between the groups (2.4 +/- 0.3 vs. 2.0 +/- 0.5 ml. kg(-1) muscle. min(-1); normal subjects versus patients with type 1 diabetes, NS), and exercise increased oxygen consumption similarly in both groups (25.3 +/- 4.3 vs. 20.1 +/- 3.0 ml. kg(-1) muscle. min(-1), respectively, NS). Rates of insulin-stimulated muscle blood flow and the increments in muscle blood flow induced by exercise were also similar in normal subjects (129 +/- 14 ml. kg(-1). min(-1)) and in patients with type 1 diabetes (115 +/- 12 ml. kg(-1). min(-1)). The patients with type 1 diabetes exhibited resistance to both insulin stimulation of glucose uptake (34 +/- 6 vs. 76 +/- 9 micromol. kg(-1) muscle. min(-1), P < 0.001) and also to the exercise-induced increment in glucose uptake (82 +/- 15 vs. 162 +/- 29 micromol. kg(-1) muscle. min(-1), P < 0.05). We conclude that the ability of exercise to increase insulin-stimulated glucose uptake in vivo is blunted in patients with insulin-resistant type 1 diabetes compared with normal subjects. This could be caused by either separate or common defects in exercise- and insulin-stimulated pathways.     

Caffeine-induced impairment of insulin action but not insulin signaling in human skeletal muscle is reduced by exercise

We investigated the effects of caffeine ingestion on skeletal muscle glucose uptake, glycogen synthase (GS) activity, and insulin signaling intermediates during a 100-min euglycemic-hyperinsulinemic (100 microU/ml) clamp. On two occasions, seven men performed 1-h one-legged knee extensor exercise at 3 h before the clamp. Caffeine (5 mg/kg) or placebo was administered in a randomized, double-blind fashion 1 h before the clamp. During the clamp, whole-body glucose disposal was reduced (P < 0.05) in caffeine (37.5 +/- 3.1 micromol x min(-1) x kg(-1)) vs. placebo (54.1 +/- 2.9 micromol x min(-1) x kg(-1)). In accordance, the total area under the curve over 100 min (AUC(0--100 min)) for insulin-stimulated glucose uptake in caffeine was reduced (P < 0.05) by approximately 50% in rested and exercised muscle. Caffeine also reduced (P < 0.05) GS activity before and during insulin infusion in both legs. Exercise increased insulin sensitivity of leg glucose uptake in both caffeine and placebo. Insulin increased insulin receptor tyrosine kinase (IRTK), insulin receptor substrate 1-associated phosphatidylinositol (PI) 3-kinase activities, and Ser(473) phosphorylation of protein kinase B (PKB)/Akt significantly but similarly in rested and exercised legs. Furthermore, insulin significantly decreased glycogen synthase kinase-3alpha (GSK-3alpha) activity equally in both legs. Caffeine did not alter insulin signaling in either leg. Plasma epinephrine and muscle cAMP concentrations were increased in caffeine. We conclude that 1) caffeine impairs insulin-stimulated glucose uptake and GS activity in rested and exercised human skeletal muscle; 2) caffeine-induced impairment of insulin-stimulated muscle glucose uptake and downregulation of GS activity are not accompanied by alterations in IRTK, PI 3-kinase, PKB/Akt, or GSK-3alpha but may be associated with increases in epinephrine and intramuscular cAMP concentrations; and 3) exercise reduces the detrimental effects of caffeine on insulin action in muscle.     

Interaction of insulin and prior exercise in control of hepatic metabolism of a glucose load

To determine if prior exercise enhances insulin-stimulated extraction of glucose by the liver, chronically catheterized dogs were submitted to 150 min of treadmill exercise or rest. After exercise or rest, dogs received portal glucose (18 micro mol x kg(-1) x min(-1)), peripheral somatostatin, and basal portal glucagon infusions from t = 0 to 150 min. A peripheral glucose infusion was used to clamp arterial blood glucose at 8.3 mmol/l. Insulin was infused into the portal vein to create either basal levels or mild hyperinsulinemia. Prior exercise did not increase whole-body glucose disposal in the presence of basal insulin (25.5 +/- 1.5 vs. 20.3 +/- 1.7 micro mol x kg(-1) x min(-1)), but resulted in a marked enhancement in the presence of elevated insulin (97.2 +/- 15.1 vs. 64.4 +/- 7.4 micro mol x kg(-1) x min(-1)). Prior exercise also increased net hepatic glucose uptake in the presence of both basal insulin (7.5 +/- 1.2 vs. 2.9 +/- 2.4 micro mol x kg(-1) x min(-1)) and elevated insulin (22.0 +/- 3.5 vs. 11.5 +/- 1.8 micro mol x kg(-1) x min(-1)). Likewise, net hepatic glucose fractional extraction was increased by prior exercise with both basal insulin (0.04 +/- 0.01 vs. 0.01 +/- 0.01 micro mol x kg(-1) x min(-1)) and elevated insulin (0.10 +/- 0.01 vs. 0.05 +/- 0.01). Hepatic glycogen synthesis was increased by elevated insulin, but was not enhanced by prior exercise. Although the increase in glucose extraction after exercise could be ascribed to increased insulin action, the increase in hepatic glycogen synthesis was independent of it.     

Effects of free fatty acids and glycerol on splanchnic glucose metabolism and insulin extraction in nondiabetic humans.

The present study sought to determine whether elevated plasma free fatty acids (FFAs) alter the ability of insulin and glucose to regulate splanchnic as well as muscle glucose metabolism. To do so, FFAs were increased in 10 subjects to approximately 1 mmol/l by an 8-h Intralipid/heparin (IL/Hep) infusion, whereas they fell to levels near the detection limit of the assay (<0.05 mmol/l) in 13 other subjects who were infused with glycerol alone at rates sufficient to either match (n = 5, low glycerol) or double (n = 8, high glycerol) the plasma glycerol concentrations observed during the IL/Hep infusion. Glucose was clamped at approximately 8.3 mmol/l, and insulin was increased to approximately 300 pmol/l to stimulate both muscle and hepatic glucose uptake. Insulin secretion was inhibited with somatostatin. Leg and splanchnic glucose metabolism were assessed using a combined catheter and tracer dilution approach. Leg glucose uptake (21.7 +/- 3.5 vs. 48.3 +/- 9.3 and 57.8 +/- 11.7 micromol x kg(-1) leg x min(-1)) was lower (P < 0.001) during IL/Hep than the low- or high-glycerol infusions, confirming that elevated FFAs caused insulin resistance in muscle. IL/Hep did not alter splanchnic glucose uptake or the contribution of the extracellular direct pathway to UDP-glucose flux. On the other hand, total UDP-glucose flux (13.2 +/- 1.7 and 12.5 +/- 1.0 vs. 8.1 +/- 0.5 micromol x kg(-1) x min(-1)) and flux via the indirect intracellular pathway (8.4 +/- 1.2 and 8.1 +/- 0.6 vs. 4.8 +/- 0.05 micromol x kg(-1) x min(-1)) were greater (P < 0.05) during both the IL/Hep and high-glycerol infusions than the low-glycerol infusion. In contrast, only IL/Hep increased (P < 0.05) splanchnic glucose production, indicating that elevated FFAs impaired the ability of the liver to autoregulate. Splanchnic insulin extraction, directly measured using the arterial and hepatic vein catheters, did not differ (67 +/- 3 vs. 71 +/- 5 vs. 69 +/- 1%) during IL/Hep and high- and low-glycerol infusions. We conclude that elevated FFAs exert multiple effects on glucose metabolism. They inhibit insulin- and glucose-induced stimulation of muscle glucose uptake and suppression of splanchnic glucose production. They increase the contribution of the indirect pathway to glycogen synthesis and impair hepatic autoregulation. On the other hand, they do not alter either splanchnic glucose uptake or splanchnic insulin extraction in nondiabetic humans.     

Differential effects of rosiglitazone and metformin on adipose tissue distribution and glucose uptake in type 2 diabetic subjects

We evaluated the effects of rosiglitazone (4 mg b.i.d.) and metformin (1 g b.i.d.) monotherapy for 26 weeks on adipose tissue insulin-stimulated glucose uptake in patients (n = 41) with type 2 diabetes. Before and after the treatment, glucose uptake was measured using 2-[(18)F]fluoro-2-deoxyglucose and positron emission tomography and adipose tissue masses were quantified using magnetic resonance imaging. Rosiglitazone improved insulin-stimulated whole-body glucose uptake by 44% (P < 0.01 vs. placebo). Mean body weight was unchanged in the rosiglitazone group, while it decreased by 2.0 kg in the metformin group (P < 0.05 vs. placebo). In visceral adipose tissue, glucose uptake increased by 29% (from 17.8 +/- 2.0 to 23.0 +/- 2.6 micro mol x kg(-1) x min(-1), P < 0.05 vs. placebo) in the rosiglitazone group but to a lesser extent (17%) in the metformin group (from 16.2 +/- 1.5 to 18.9 +/- 1.7 micro mol x kg(-1) x min(-1), P < 0.05 vs. baseline). Because the visceral adipose tissue mass simultaneously decreased with both treatments (P < 0.05), no change was observed in total visceral glucose uptake per depot. Rosiglitazone significantly enhanced glucose uptake in the femoral subcutaneous area, either when expressed per tissue mass (from 10.8 +/- 1.2 to 17.1 +/- 1.7 micro mol x kg(-1) x min(-1), P < 0.01 vs. placebo) or per whole-fat depot (P < 0.05 vs. placebo). In conclusion, metformin treatment resulted in improvement of glycemic control without enhancement of peripheral insulin sensitivity. The improved insulin sensitivity of the nonabdominal subcutaneous adipose tissue during treatment with rosiglitazone partly explains the enhanced whole-body insulin sensitivity and underlies the central role of adipose tissue for action of peroxisome proliferator-activated receptor gamma agonist in vivo.     

Mechanism of troglitazone action in type 2 diabetes

To examine the metabolic pathways by which troglitazone improves insulin responsiveness in patients with type 2 diabetes, the rate of muscle glycogen synthesis was measured by 13C-nuclear magnetic resonance (NMR) spectroscopy. The rate-controlling steps of insulin-stimulated muscle glucose metabolism were assessed using 31P-NMR spectroscopic measurement of intramuscular glucose-6-phosphate (G-6-P) combined with a novel 13C-NMR method to assess intracellular glucose concentrations. Seven healthy nonsmoking subjects with type 2 diabetes were studied before and after completion of 3 months of troglitazone (400 mg/day) therapy. After troglitazone treatment, rates of insulin-stimulated whole-body glucose uptake increased by 58+/-11%, from 629+/-82 to 987+/-156 micromol x m(-2) x min(-1) (P = 0.008), which was associated with an approximately 3-fold increase in rates of insulin-stimulated glucose oxidation (from 119+/-41 to 424+/-70 micromol x m(-2) x min(-1); P = 0.018) and muscle glycogen synthesis (26+/-17 vs. 83+/-35 micromol x l(-1) muscle x min(-1); P = 0.025). After treatment, muscle G-6-P concentrations increased by 0.083+/-0.019 mmol/l (P = 0.008 vs. pretreatment) during the hyperglycemic-hyperinsulinemic clamp, compared with no significant changes in intramuscular G-6-P concentrations in the pretreatment study, reflecting an improvement in glucose transport and/or hexokinase activity. The concentrations of intracellular free glucose did not differ between the pre- and posttreatment studies and remained >50-fold lower in concentration (<0.1 mmol/l) than what would be expected if hexokinase activity was rate-controlling. These results indicate that troglitazone improves insulin responsiveness in skeletal muscle of patients with type 2 diabetes by facilitating glucose transport activity, which thereby leads to increased rates of muscle glycogen synthesis and glucose oxidation.     

Direct and indirect effects of insulin on glucose uptake and storage by the liver

Studies were conducted in conscious 42-h-fasted dogs to determine how much of insulin's effect on hepatic glucose uptake arises from its direct hepatic action versus its indirect (extrahepatic) action. Each experiment consisted of equilibration, basal, and experimental periods. During the latter, somatostatin, basal intraportal glucagon, portal glucose (21.3 micromol x kg(-1) x min(-1)), and peripheral glucose (to double the hepatic glucose load) were infused. During the experimental period, insulin was infused intraportally at a basal rate (BI, n = 6), at a fourfold basal rate (PoI, n = 6), or via a peripheral vein to create a selective increase in the arterial insulin level similar to that in PoI (PeI, n = 6). Arterial and hepatic sinusoidal insulin levels (in picomoles per liter) during the experimental period were 31 +/- 5 and 113 +/- 15 in BI, 97 +/- 11 and 394 +/- 66 in PoI, and 111 +/- 13 and 96 +/- 9 in PeI, respectively. Net hepatic glucose uptake (NHGU) averaged 7.0 +/- 1.1 micromol x kg(-1) x min(-1), 15.7 +/- 2.7 micromol x kg(-1) x min(-1) (P < 0.05 vs. BI), and 12.0 +/- 2.4 micromol x kg(-1) x min(-1) (P < 0.05 vs. BI) in BI, PoI, and PeI, respectively. Net hepatic carbon retention was 4.4 +/- 1.2 micromol glucose equivalents. kg(-1) x min(-1), 12.3 +/- 2.5 micromol glucose equivalents x kg(-1) x min(-1) (P < 0.05 vs. BI, P < 0.05 vs. PeI), and 7.1 +/- 1.0 micromol glucose equivalents x kg(-1) x min(-1) (P < 0.05 vs. BI) in BI, PoI, and PeI, respectively. Both direct and indirect insulin actions increase NHGU, but the rise in hepatic sinusoidal insulin appears critical for efficient storage of glucose as hepatic glycogen.     

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog.

Intraportal infusion of small amounts of fructose markedly augmented net hepatic glucose uptake (NHGU) during hyperglycemic hyperinsulinemia in conscious dogs. In this study, we examined whether the inclusion of catalytic amounts of fructose with a glucose load reduces postprandial hyperglycemia and the pancreatic beta-cell response to a glucose load in conscious 42-h-fasted dogs. Each study consisted of an equilibration (-140 to -40 min), control (-40 to 0 min), and test period (0-240 min). During the latter period, glucose (44.4 micromol x kg(-1) x min(-1)) was continuously given intraduodenally with (2.22 micromol x kg(-1) x min(-1)) or without fructose. The glucose appearance rate in portal vein blood was not significantly different with or without the inclusion of fructose (41.3 +/- 2.7 vs. 37.3 +/- 8.3 micromol x kg(-1) x min(-1), respectively). In response to glucose infusion without the inclusion of fructose, the net hepatic glucose balance switched from output to uptake (from 10 +/- 2 to 11 +/- 4 micromol x kg(-1) x min(-1)) by 30 min and averaged 17 +/- 6 micromol x kg(-1) x min(-1). The fractional extraction of glucose by the liver during the infusion period was 7 +/- 2%. Net glycogen deposition was 2.44 mmol glucose equivalent/kg body wt; 49% of deposited glycogen was synthesized via the direct pathway. Net hepatic lactate production was 1.4 mmol/kg body wt. Arterial blood glucose rose from 4.1 +/- 0.2 to 7.3 +/- 0.4 mmol/l, and arterial plasma insulin rose from 42 +/- 6 to 258 +/- 66 pmol/l at 30 min, after which they decreased to 7.0 +/- 0.5 mmol/l and 198 +/- 66 pmol/l, respectively. Arterial plasma glucagon decreased from 54 +/- 7 to 32 +/- 3 ng/l. In response to intraduodenal glucose infusion in the presence of fructose, net hepatic glucose balance switched from 9 +/- 1 micromol x kg(-1) x min(-1) output to 12 +/- 3 and 28 +/- 5 micromol x kg(-1) x min(-1) uptake by 15 and 30 min, respectively. The average NHGU (28 +/- 5 micromol x kg(-1) x min(-1)) and fractional extraction during infusion period (12 +/- 2%), net glycogen deposition (3.68 mmol glucose equivalent/kg body wt), net hepatic lactate production (3.27 mmol/kg), and glycogen synthesis via the direct pathway (68%) were significantly higher (P < 0.05) compared to that in the absence of fructose. The increases in arterial blood glucose (from 4.4 +/- 0.1 to 6.4 +/- 0.2 mmol/l at 30 min) and arterial plasma insulin (from 48 +/- 6 to 126 +/- 30 pmol/l at 30 min) were significantly smaller (P < 0.05). In summary, the inclusion of small amounts of fructose with a glucose load augmented NHGU, increased hepatic glycogen synthesis via the direct pathway, and augmented hepatic glycolysis. As a result, postprandial hyperglycemia and insulin release by the pancreatic beta-cell were reduced. In conclusion, catalytic amounts of fructose have the ability to improve glucose tolerance.     

Independent regulation of in vivo insulin action on glucose versus K(+) uptake by dietary fat and K(+) content

Insulin stimulates both glucose and K(+) uptake, and high-fat feeding is known to decrease insulin-stimulated glucose uptake. The purpose of this study was to examine whether insulin's actions on glucose and K(+) uptake are similarly decreased by a high-fat diet. Wistar rats were fed a standard control (12.2% fat; n = 6) or high-fat (66.5% fat; n = 13) diet for 15 days. Because K(+) content was 1% in the control and 0.5% in the high-fat diet and because the rats ate less of the high-fat diet, we also compared the high-fat diet with 0.5% K(+) (HFD; n = 7) to a high-fat diet supplemented with 1.5% K(+) (HFD+K; n = 6). K(+) intake was matched between the control and HFD+K groups (246 +/- 8 vs. 224 +/- 2 mg/day), but was lower in the HFD group (78 +/- 10 mg/day; P < 0.05). Insulin-stimulated glucose and K(+) uptake were determined by hyperinsulinemic (5 mU.kg(-1).min(-1)) glucose and K(+) clamps. The HFD depressed both insulin-stimulated glucose uptake compared to the control (133 +/- 5 vs. 166 +/- 7 micromol.kg(-1).min(-1); P < 0.05) and K(+) uptake (5.5 +/- 0.9 vs. 8.9 +/- 1.0 micromol.kg(-1).min(-1); P < 0.05) compared to the control. However, insulin-stimulated K(+) uptake was unchanged in the HFD+K versus in the control group (10.0 +/- 0.6 vs. 8.9 +/- 1.0 micromol.kg(-1).min(-1); P > 0.05), whereas insulin-stimulated glucose uptake in the HFD+K group was decreased to a rate (137 +/- 9 micromol.kg(-1).min(-1)), similar to that of the HFD group. We concluded that the decrease in insulin-stimulated K(+) uptake during high-fat feeding was a result of decreased K(+) intake, and that insulin's actions on glucose uptake and K(+) uptake are independently regulated by dietary fat and K(+) content, respectively.     

Glucose-mediated glucose disposal in insulin-resistant normoglycemic relatives of type 2 diabetic patients

With the aim of investigating glucose-mediated glucose disposal (glucose effectiveness [GE]) in 15 (3 female and 12 male subjects) insulin-resistant normoglycemic relatives of patients with type 2 diabetes (DM2), and 15 age-, sex-, and BMI-matched control subjects without a family history of DM2, we performed 2 studies: 1) a 5-h euglycemic near-normoinsulinemic pancreatic clamp with somatostatin (360 microg/h), insulin (0.25 mU x kg(-1) x min(-1)), glucagon (0.5 ng x kg(-1) x min(-1)), growth hormone (6 ng x kg(-1) x min(-1)), and tritiated glucose infusion and indirect calorimetry; and 2) on a separate day, an identical 5-h clamp but at hyperglycemia (approximately 12 mmol/l) over the last 2 h. Fasting plasma insulin (PI) concentrations were elevated in the relatives compared with control subjects (49 +/- 6 vs. 32 +/- 5 pmol/l, P < 0.04), whereas plasma glucose (PG) was not (5.6 +/- 0.1 vs. 5.5 +/-0.1 mmol/l). At the end (i.e., 4.5-5.0 h) of the euglycemic clamp (PG, 6.1 +/- 0.4 vs. 5.6 +/- 0.1 mmol/l; PI, 78 +/- 5 vs. 73 +/-6 pmol/l), peripheral glucose uptake (Rd(euglycemia)) was decreased in the relatives (2.93 +/- 0.08 vs. 3.70 +/-0.23 mg x min(-1) x kg(-1) fat free mass [FFM], P < 0.005), due to a decreased nonoxidative glucose disposal (0.83 +/-0.21 vs. 1.62 +/- 0.19 mg x min(-1) x kg(-1) FFM, P < 0.01), but hepatic glucose production (HGP) was increased (1.97 +/-0.19 vs. 1.50 +/- 0.13 mg x min(-1) x kg(-1) FFM, P < 0.05). At the matched end of the hyperglycemic clamp (PG, 12.7 +/-0.2 vs. 12.6 +/- 0.2 mmol/l; PI, 87 +/- 5 vs. 78 +/- 7 pmol/l), peripheral glucose disposal (Rd(hyperglycemia)) (5.52 +/- 0.22 vs. 5.92 +/- 0.29 mg x min(-1) x kg(-1) FFM, NS), nonoxidative glucose disposal (2.93 +/- 0.18 vs. 2.78 +/- 0.25 mg x min(-1) x kg(-1) FFM, NS), and HGP(hyperglycemia) (1.20 +/- 0.09 vs. 1.37 +/-0.23 mg x min(-1) x kg(-1) FFM, NS) were all identical. When the effectiveness of glucose itself on glucose uptake and production [(Rd(hyperglycemia) - Rd(euglycemia))/deltaPG and (HGP(euglycemia)- HGP(hyperglycemia))/deltaPG] was calculated, the relatives had a 22% increase in peripheral uptake (0.022 +/- 0.002 vs. 0.018 +/- 0.002 mg x min(-1) x kg(-1) FFM per mg/dl), due to a significantly increased nonoxidative glucose metabolism and enhanced suppression of HGP (0.0076 +/- 0.0021 vs. 0.0011 +/- 0.0022 mg x min(-1) x kg(-1) FFM per mg/dl, P < 0.05). In conclusion, in insulin-resistant relatives of DM2 patients, whole-body glucose-mediated glucose disposal is increased by GE enhancement of the muscle nonoxidative glucose pathway and by GE enhancement of the suppression of HGP. These mechanisms may represent a compensatory mechanism to the ongoing insulin resistance of these relatives.     

In normal men, free fatty acids reduce peripheral but not splanchnic glucose uptake

Raising plasma free fatty acid (FFA) levels reduces muscle glucose uptake, but the effect of FFAs on splanchnic glucose uptake, total glucose output, and glucose cycling may also be critical to producing lipid-induced glucose intolerance. In eight normal volunteers, we measured glucose turnover and cycling rates ([2H7]glucose infusion) during a moderately hyperglycemic (7.7 mmol/l) hyperinsulinemic clamp, before and after ingestion of a labeled (dideuterated) oral glucose load (700 mg/kg). Each test was performed twice, with either a lipid or a saline infusion; four subjects also had a third test with a glycerol infusion. As shown by similar rates of exogenous glucose appearance, the lipid infusion did not reduce first-pass splanchnic glucose uptake (saline 1.48+/-0.18, lipid 1.69+/-0.17, and glycerol 1.88+/-0.17 mmol/kg per 180 min; NS), but it reduced peripheral glucose uptake by 40% (P < 0.01 vs. both saline and glycerol infusions). Before oral ingestion of glucose, total glucose output was similarly increased by the lipid and glycerol infusions. Total glucose output was significantly increased by FFAs after oral ingestion of glucose (saline 3.68+/-1.15, glycerol 3.68+/-1.70, and lipid 7.92+/-0.88 micromol x kg(-1) x min(-1); P < 0.01 vs. saline and P < 0.05 vs. glycerol). The glucose cycling rate was approximately 2.7 micromol x kg(-1) x min(-1) with the three infusions and tended to decrease all along the lipid infusion, which argues against a stimulation of glucose-6-phosphatase by FFAs. It is concluded that in situations of moderate hyperinsulinemia-hyperglycemia, FFAs reduce peripheral but not splanchnic glucose uptake. Total glucose output is increased by FFAs, by a mechanism that does not seem to involve stimulation of glucose-6-phosphatase.     

Normalization of plasma glucose concentration by insulin therapy improves insulin-stimulated glycogen synthesis in type 2 diabetes.

Considerable evidence suggests that skeletal muscle insulin resistance is an inherent feature of type 2 diabetes and contributes to the pathogenesis of the disease. In patients with poorly controlled diabetes, hyperglycemia is thought to produce additional insulin resistance in muscle. The magnitude and nature of hyperglycemia-induced insulin resistance is not known. The purpose of the present study was to determine the biochemical mechanisms responsible for increased insulin-stimulated glucose disposal after the achievement of tight glycemic control with a mixed-split regimen. We performed hyperinsulinemic-euglycemic clamps with indirect calorimetry and vastus lateralis muscle biopsies in eight type 2 diabetic patients who had poor glycemic control (HbA(1c) 10.1%) and again after 3 months of intensive insulin therapy designed to produce near-normoglycemia (HbA(1c) 6.6%). Improved glycemic control increased insulin-stimulated glucose disposal (5.16 +/- 0.32 vs. 3.69 +/- 0.33 mg x kg(-1) x min(-1); P < 0.01); nonoxidative glucose disposal, which primarily reflects glycogen synthesis (2.11 +/- 0.26 vs. 0.90 +/- 0.16 mg x kg(-1) x min(-1); P < 0.01); and glycogen synthase fractional velocity (0.094 +/- 0.017 vs. 0.045 +/- 0.007; P < 0.05). There was no improvement in insulin-stimulated glucose oxidation (3.05 +/- 0.25 vs. 2.79 +/- 0.20 mg x kg(-1) x min(-1)), hexokinase II mRNA expression (increase over basal values), or hexokinase II enzymatic activity (0.51 +/- 0.16 vs. 0.42 +/- 0.18 pmol x min(-1) x microg(-1) protein). All of the increase in insulin-stimulated glucose disposal could be accounted for by increased glycogen synthesis, which is likely attributable to increased activation of glycogen synthase by insulin.     

Intramyocellular lipid is associated with resistance to in vivo insulin actions on glucose uptake, antilipolysis, and early insulin signaling pathways in human skeletal muscle

To examine whether and how intramyocellular lipid (IMCL) content contributes to interindividual variation in insulin action, we studied 20 healthy men with no family history of type 2 diabetes. IMCL was measured as the resonance of intramyocellular CH(2) protons in lipids/resonance of CH(3) protons of total creatine (IMCL/Cr(T)), using proton magnetic resonance spectroscopy in vastus lateralis muscle. Whole-body insulin sensitivity was measured using a 120-min euglycemic-hyperinsulinemic (insulin infusion rate 40 mU/m(2). min) clamp. Muscle biopsies of the vastus lateralis muscle were taken before and 30 min after initiation of the insulin infusion to assess insulin signaling. The subjects were divided into groups with high IMCL (HiIMCL; 9.5 +/- 0.9 IMCL/Cr(T), n = 10) and low IMCL (LoIMCL; 3.0 +/- 0.5 IMCL/Cr(T), n = 10), the cut point being median IMCL (6.1 IMCL/Cr(T)). The groups were comparable with respect to age (43 +/- 3 vs. 40 +/- 3 years, NS, HiIMCL versus LoIMCL), BMI (26 +/- 1 vs. 26 +/- 1 kg/m(2), NS), and maximal oxygen consumption (33 +/- 2 vs. 36 +/- 3 ml. kg(-1). min(-1), NS). Whole-body insulin-stimulated glucose uptake was lower in the HiIMCL group (3.0 +/- 0.4 mg. kg(-1). min(-1)) than the LoIMCL group (5.1 +/- 0.5 mg. kg(-1). min(-1), P < 0.05). Serum free fatty acid concentrations were comparable basally, but during hyperinsulinemia, they were 35% higher in the HiIMCL group than the LoIMCL group (P < 0.01). Study of insulin signaling indicated that insulin-induced tyrosine phosphorylation of the insulin receptor (IR) was blunted in HiIMCL compared with LoIMCL (57 vs. 142% above basal, P < 0.05), while protein expression of the IR was unaltered. IR substrate-1-associated phosphatidylinositol (PI) 3-kinase activation by insulin was also lower in the HiIMCL group than in the LoIMCL group (49 +/- 23 vs. 84 +/- 27% above basal, P < 0.05 between HiIMCL and LoIMCL). In conclusion, IMCL accumulation is associated with whole-body insulin resistance and with defective insulin signaling in skeletal muscle independent of body weight and physical fitness.     

Exercise-induced changes in insulin and glucagon are not required for enhanced hepatic glucose uptake after exercise but influence the fate of glucose within the liver

To test whether pancreatic hormonal changes that occur during exercise are necessary for the postexercise enhancement of insulin-stimulated net hepatic glucose uptake, chronically catheterized dogs were exercised on a treadmill or rested for 150 min. At the onset of exercise, somatostatin was infused into a peripheral vein, and insulin and glucagon were infused in the portal vein to maintain basal levels (EX-Basal) or simulate the response to exercise (EX-Sim). Glucose was infused as needed to maintain euglycemia during exercise. After exercise or rest, somatostatin infusion was continued in exercised dogs and initiated in dogs that had remained sedentary. In addition, basal glucagon, glucose, and insulin were infused in the portal vein for 150 min to create a hyperinsulinemic-hyperglycemic clamp in EX-Basal, EX-Sim, and sedentary dogs. Steady-state measurements were made during the final 50 min of the clamp. During exercise, net hepatic glucose output (mg x kg(-1) x min(-1)) rose in EX-Sim (7.6 +/- 2.8) but not EX-Basal (1.9 +/- 0.3) dogs. During the hyperinsulinemic-hyperglycemic clamp that followed either exercise or rest, net hepatic glucose uptake (mg x kg(-1) x min(-1)) was elevated in both EX-Basal (4.0 +/- 0.7) and EX-Sim (4.6 +/- 0.5) dogs compared with sedentary dogs (2.0 +/- 0.3). Despite this elevation in net hepatic glucose uptake after exercise, glucose incorporation into hepatic glycogen, determined using [3-3H]glucose, was not different in EX-Basal and sedentary dogs, but was 50 +/- 30% greater in EX-Sim dogs. Exercise-induced changes in insulin and glucagon, and consequent glycogen depletion, are not required for the increase in net hepatic glucose uptake after exercise but result in a greater fraction of the glucose consumed by the liver being directed to glycogen.     

Inhibition of lipolysis stimulates peripheral glucose uptake but has no effect on endogenous glucose production in HIV lipodystrophy

HIV-infected patients with lipodystrophy (HIV lipodystrophy) are insulin resistant and have elevated plasma free fatty acid (FFA) concentrations. We aimed to explore the mechanisms underlying FFA-induced insulin resistance in patients with HIV lipodystrophy. Using a randomized, placebo-controlled, cross-over design, we studied the effects of an overnight acipimox-induced suppression of FFAs on glucose and FFA metabolism by using stable isotope-labeled tracer techniques during basal conditions and a two-stage euglycemic-hyperinsulinemic clamp (20 and 50 mU insulin/m(2) per min, respectively) in nine patients with nondiabetic HIV lipodystrophy. All patients received antiretroviral therapy. Biopsies from the vastus lateralis muscle were obtained during each stage of the clamp. Acipimox treatment reduced basal FFA rate of appearance by 68.9% (95% CI 52.6-79.5) and decreased plasma FFA concentration by 51.6% (42.0-58.9) (both, P < 0.0001). Endogenous glucose production was not influenced by acipimox. During the clamp, the increase in glucose uptake was significantly greater after acipimox treatment compared with placebo (acipimox: 26.85 micromol x kg(-1) x min(-1) [18.09-39.86] vs. placebo: 20.30 micromol x kg(-1) x min(-1) [13.67-30.13]; P < 0.01). Insulin increased phosphorylation of Akt Thr(308) and glycogen synthase kinase-3beta Ser(9), decreased phosphorylation of glycogen synthase (GS) site 3a + b, and increased GS activity (percent I-form) in skeletal muscle (P < 0.01). Acipimox decreased phosphorylation of GS (site 3a + b) (P < 0.02) and increased GS activity (P < 0.01) in muscle. The present study provides direct evidence that suppression of lipolysis in patients with HIV lipodystrophy improves insulin-stimulated peripheral glucose uptake. The increased glucose uptake may in part be explained by increased dephosphorylation of GS (site 3a + b), resulting in increased GS activity.     

Decreased insulin responsiveness of glucose uptake in cultured human skeletal muscle cells from insulin-resistant nondiabetic relatives of type 2 diabetic families

To investigate the contribution of inherited biochemical defects to the peripheral insulin resistance of type 2 diabetes, we studied cultured skeletal muscle from 10 insulin-resistant nondiabetic first-degree relatives of type 2 diabetic families and 6 control subjects. Insulin stimulation of glucose uptake and glycogen synthesis was maximal in myoblasts. Insulin-stimulated glucose uptake (fold-stimulation over basal uptake) was decreased in relative compared with control myoblasts at 0.001 micromol/l (0.93 +/- 0.05 [mean +/- SE] vs. 1.15 +/- 0.06, P < 0.05) and 0.1 micromol/l (1.38 +/- 0.10 vs. 1.69 +/- 0.08, P = 0.025) insulin. Insulin responsiveness was markedly impaired in 5 of the relative myoblast cultures, and in 4 of these, there was an associated increase in basal glucose uptake (76.7 +/- 7.0 vs. 47.4 +/- 5.5 pmol x min(-1) x mg(-1) protein, relative vs. control; P < 0.02). Expression of insulin receptor substrate 1, phosphatidylinositol 3-kinase, protein kinase B, and glycogen synthase was normal in the relative cultures with impaired insulin responsiveness. Glycogen synthesis was also normal in the relative cultures. We conclude that the persistence of impaired insulin responsiveness in some of the relative cultures supports the role of inherited factors in the insulin resistance of type 2 diabetes and that the association with increased basal glucose uptake suggests that the 2 abnormalities may be linked.     

Exercise-stimulated glucose turnover in the rat is impaired by glucosamine infusion

The infusion of glucosamine causes insulin resistance, presumably by entering the hexosamine biosynthetic pathway; it has been proposed that this pathway plays a role in hyperglycemia-induced insulin resistance. This study was undertaken to determine if glucosamine infusion could influence exercise-stimulated glucose uptake. Male SD rats were infused with glucosamine at 0.1 mg x kg(-1) x min(-1) (low-GlcN group), 6.5 mg x kg(-1) x min(-1) (high-GlcN group), or saline (control group) for 6.5 h and exercised on a treadmill for 30 min (17 m/min) at the end of the infusion period. Glucosamine infusion caused a modest increase in basal glycemia in both experimental groups, with no change in tracer-determined basal glucose turnover. During exercise, glucose turnover increased approximately 2.2-fold from 46 +/- 2 to 101 +/- 5 pmol x kg(-1) x min(-1) in the control group. Glucose turnover increased to a lesser extent in the glucosamine groups and was limited to 88% of control in the low-GlcN group (47 +/- 2 to 90 +/- 3 pmol x kg(-1) x min(-1); P < 0.01) and 72% of control in the high-GlcN group (43 +/- 1 to 73 +/- 3 pmol kg(-1) 1 min(-1); P < 0.01). Similarly, the metabolic clearance rate (MCR) in the control group increased 72% from 6.1 +/- 0.2 to 10.5 +/- 0.7 ml kg(-1) x min(-1) in response to exercise. However, the increase in MCR was only 83% of control in the low-GlcN group (5.2 +/- 0.5 to 8.7 +/- 0.5 ml x kg(-1) x min(-1); P < 0.01) and 59% of control in the high-GlcN group (4.5 +/- 0.2 to 6.2 +/- 0.3 ml x kg(-1) x min(-1); P < 0.01). Neither glucosamine infusion nor exercise significantly affected plasma insulin or free fatty acid (FFA) concentrations. In conclusion, the infusion of glucosamine, which is known to cause insulin resistance, also impaired exercise-induced glucose uptake. This inhibition was independent of hyperglycemia and FFA levels.     

Impaired free fatty acid uptake in skeletal muscle but not in myocardium in patients with impaired glucose tolerance: studies with PET and 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid.

Free fatty acids (FFAs) are an important substrate for myocardial and skeletal muscle metabolism, and increased availability and oxidation of FFA are suggested to be associated with insulin resistance. This study was undertaken to assess whether myocardial or muscle uptake of FFA is altered in patients with impaired glucose tolerance (IGT). Eight healthy men (control group; age 48+/-1 years, BMI 25+/-1 kg/m2, mean +/- SE) and eight men with IGT (glucose-intolerant group; age 49+/-1 years, BMI 29+/-1 kg/m2) were studied in the fasting state. Myocardial oxygen consumption and blood flow and myocardial and femoral muscle FFA uptake rates were measured with positron emission tomography (PET) and [15O]O2, [15O]H2O, [15O]CO, and 14(R, S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA), a fatty acid tracer trapped into the cell after undergoing initial steps of beta-oxidation. Serum glucose and insulin concentrations were higher in the glucose-intolerant group during the PET study, but FFA concentrations were comparable between the groups. No differences between the groups were observed in the myocardial blood flow, oxygen consumption, fractional FTHA uptake rates, or FFA uptake indices (5.6+/-0.4 vs. 5.2+/-0.4 pmol x 100 g(-1) x min(-1), glucose-intolerant versus control, NS). In the femoral muscle, fractional FTHA uptake (0.0062+/-0.0003 vs. 0.0072+/-0.0003 min(-1), P = 0.044) and FFA uptake indices (0.30+/-0.02 vs. 0.43+/-0.04 min(-1), P = 0.020) were significantly lower in the glucose-intolerant group than in the control group. In conclusion, when studied at the fasting state and normal serum FFA concentrations, subjects with IGT have similar myocardial but lowered femoral muscle FFA uptake. This finding argues against the hypothesis that an increased oxidation of serum FFA, via the competition of glucose and FFA as fuel sources, is the primary cause for impaired peripheral glucose utilization and insulin resistance commonly observed in IGT.     

Effect of a sustained reduction in plasma free fatty acid concentration on intramuscular long-chain fatty Acyl-CoAs and insulin action in type 2 diabetic patients

To investigate the effect of a sustained (7-day) decrease in plasma free fatty acid (FFA) concentrations on insulin action and intramyocellular long-chain fatty acyl-CoAs (LCFA-CoAs), we studied the effect of acipimox, a potent inhibitor of lipolysis, in seven type 2 diabetic patients (age 53 +/- 3 years, BMI 30.2 +/- 2.0 kg/m2, fasting plasma glucose 8.5 +/- 0.8 mmol/l, HbA 1c 7.5 +/- 0.4%). Subjects received an oral glucose tolerance test (OGTT) and 120-min euglycemic insulin (80 mU/m2 per min) clamp with 3-[3H]glucose/vastus lateralis muscle biopsies to quantitate rates of insulin-mediated whole-body glucose disposal (Rd) and intramyocellular LCFA-CoAs before and after acipimox (250 mg every 6 h for 7 days). Acipimox significantly reduced fasting plasma FFAs (from 563 +/- 74 to 230 +/- 33 micromol/l; P < 0.01) and mean plasma FFAs during the OGTT (from 409 +/- 44 to 184 +/- 22 micromol/l; P < 0.01). After acipimox, decreases were seen in fasting plasma insulin (from 78 +/- 18 to 42 +/- 6 pmol/l; P < 0.05), fasting plasma glucose (from 8.5 +/- 0.8 to 7.0 +/- 0.5 mmol/l; P < 0.02), and mean plasma glucose during the OGTT (from 14.5 +/- 0.8 to 13.0 +/- 0.8 mmol/l; P < 0.05). After acipimox, insulin-stimulated Rd increased from 3.3 +/- 0.4 to 4.4 +/- 0.4 mg x kg(-1) x min(-1) (P < 0.03), whereas suppression of endogenous glucose production (EGP) was similar and virtually complete during both insulin clamp studies (0.16 +/- 0.10 vs. 0.14 +/- 0.10 mg x kg(-1) x min(-1); P > 0.05). Basal EGP did not change after acipimox (1.9 +/- 0.2 vs. 1.9 +/- 0.2 mg x kg(-1) x min(-1)). Total muscle LCFA-CoA content decreased after acipimox treatment (from 7.26 +/- 0.58 to 5.64 +/- 0.79 nmol/g; P < 0.05). Decreases were also seen in muscle palmityl CoA (16:0; from 1.06 +/- 0.10 to 0.75 +/- 0.11 nmol/g; P < 0.05), palmitoleate CoA (16:1; from 0.48 +/- 0.05 to 0.33 +/- 0.05 nmol/g; P = 0.07), oleate CoA (18:1; from 2.60 +/- 0.11 to 1.95 +/- 0.31 nmol/g; P < 0.05), linoleate CoA (18:2; from 1.81 +/- 0.26 to 1.38 +/- 0.18 nmol/g; P = 0.13), and linolenate CoA (18:3; from 0.27 +/- 0.03 to 0.19 +/- 0.02 nmol/g; P < 0.03) levels after acipimox treatment. Muscle stearate CoA (18:0) did not decrease after acipimox treatment. The increase in R(d) correlated strongly with the decrease in muscle palmityl CoA (r = 0.75, P < 0.05), oleate CoA (r = 0.76, P < 0.05), and total muscle LCFA-CoA (r = 0.74, P < 0.05) levels. Plasma adiponectin did not change significantly after acipimox treatment (7.9 +/- 1.8 vs. 7.5 +/- 1.5 microg/ml). These data demonstrate that the reduction in intramuscular LCFA-CoA content is closely associated with enhanced insulin sensitivity in muscle after a chronic reduction in plasma FFA concentrations in type 2 diabetic patients despite the lack of an effect on plasma adiponectin concentration.