Enhanced stimulation of glucose uptake by insulin increases exercise-stimulated glucose uptake in skeletal muscle in humans: studies using [15O]O2, [15O]H2O, [18F]fluoro-deoxy-glucose, and positron emission tomography

In vitro studies have shown that insulin and exercise stimulate glucose uptake in part via distinct mechanisms. We determined whether a high rate of insulin-stimulated glucose uptake (good insulin sensitivity) is associated with an enhanced ability of exercise to increase glucose uptake in vivo in humans. In our study, 22 normal subjects performed one-legged isometric exercise for 105 min (45-150 min) under intravenously maintained euglycemic-hyperinsulinemic conditions (0-150 min). Rates of oxygen consumption, blood flow, and glucose uptake were quantitated simultaneously in skeletal muscle of both legs using [15O]O2, [15O]H2O, [18F]fluoro-deoxy-glucose, and positron emission tomography. The one-legged exercise, performed at an intensity of 11% of maximal isometric force, was designed to induce similar increases in oxygen consumption in both groups. In the entire group, exercise increased oxygen consumption from 2.3 +/- 0.3 ml x kg(-1) muscle x min(-1) (insulin) to 34.2 +/- 3. ml x kg(-1) muscle x min(-1) (insulin and exercise) (P < 0.001) and muscle glucose uptake from 60 +/- 6 pmol x kg(-1) muscle x min(-1) (insulin) to 220 +/- 22 micromol x kg(-1) muscle x min(-1) (insulin and exercise) (P < 0.001). The exercise-induced increase in glucose uptake was due to marked increases in blood flow (36 +/- 5 ml x kg(-1) muscle x min(-1) [insulin] vs. 262 +/- 20 ml x kg(-1) muscle x min(-1) [insulin and exercise], P < 0.001) rather than glucose extraction, which decreased from 2.0 +/- 0.2 mmol/l (insulin) to 1.0 +/- 0.1 mmol/1 (insulin and exercise) (P < 0.001). The subjects were classified according to their mean rate of whole-body insulin-stimulated glucose uptake into those with high (49 +/- 3 micromol x kg(-1) x min(-1)) and normal (27 +/- 2 micromol x kg(-1) x min(-1)) rates of insulin-stimulated glucose uptake. Both insulin-stimulated (2.4 +/- 1.1 vs. 2.3 +/- 1.2 ml x kg(-1) muscle x min(-1), normal vs. high insulin sensitivity) and exercise- and insulin-stimulated (33 +/- 6 vs. 34 +/- 4 ml x kg(-1) muscle x min(-1)) rates of oxygen consumption were comparable between the groups. Exercise increased glucose uptake more in the group with high insulin sensitivity (195 +/- 25 pmol x kg(-1) muscle x min(-1)) than in the group with normal insulin sensitivity (125 +/- 19 micromol x kg(-1) muscle x min(-1)) (P < 0.05). Muscle blood flow was closely correlated with the rate of oxygen consumption (r = 0.91, P < 0.0001), and insulin-stimulated (30 +/- 5 vs. 35 +/- 6 ml x kg(-1) muscle x min(-1)) and exercise-induced increments (222 +/- 31 vs. 228 +/- 23 ml x kg(-1) muscle x min(-1)) in muscle blood flow were similar between the groups. Glucose extraction remained higher in the group with high insulin sensitivity (1.2 +/- 0.2 mmol/l) than in the group with normal insulin sensitivity (0.7 +/- 0.1 mmol/l, P < 0.05). We conclude that whereas acute exercise per se increases glucose uptake via increasing glucose delivery, good insulin sensitivity modulates exercise-induced increases in glucose uptake by enhancing cellular glucose extraction.     

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.     

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.     

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.     

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.     

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.     

Reversal of nonalcoholic hepatic steatosis, hepatic insulin resistance, and hyperglycemia by moderate weight reduction in patients with type 2 diabetes

To examine the mechanism by which moderate weight reduction improves basal and insulin-stimulated rates of glucose metabolism in patients with type 2 diabetes, we used (1)H magnetic resonance spectroscopy to assess intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) content in conjunction with hyperinsulinemic-euglycemic clamps using [6,6-(2)H(2)]glucose to assess rates of glucose production and insulin-stimulated peripheral glucose uptake. Eight obese patients with type 2 diabetes were studied before and after weight stabilization on a moderately hypocaloric very-low-fat diet (3%). The diabetic patients were markedly insulin resistant in both liver and muscle compared with the lean control subjects. These changes were associated with marked increases in IHL (12.2 +/- 3.4 vs. 0.6 +/- 0.1%; P = 0.02) and IMCL (2.0 +/- 0.3 vs. 1.2 +/- 0.1%; P = 0.02) compared with the control subjects. A weight loss of only approximately 8 kg resulted in normalization of fasting plasma glucose concentrations (8.8 +/- 0.5 vs. 6.4 +/- 0.3 mmol/l; P < 0.0005), rates of basal glucose production (193 +/- 7 vs. 153 +/- 10 mg/min; P < 0.0005), and the percentage suppression of hepatic glucose production during the clamp (29 +/- 22 vs. 99 +/- 3%; P = 0.003). These improvements in basal and insulin-stimulated hepatic glucose metabolism were associated with an 81 +/- 4% reduction in IHL (P = 0.0009) but no significant change in insulin-stimulated peripheral glucose uptake or IMCL (2.0 +/- 0.3 vs. 1.9 +/- 0.3%; P = 0.21). In conclusion, these data support the hypothesis that moderate weight loss normalizes fasting hyperglycemia in patients with poorly controlled type 2 diabetes by mobilizing a relatively small pool of IHL, which reverses hepatic insulin resistance and normalizes rates of basal glucose production, independent of any changes in insulin-stimulated peripheral glucose metabolism.     

Increased fat mass compensates for insulin resistance in abdominal obesity and type 2 diabetes: a positron-emitting tomography study

To evaluate the relative impact of abdominal obesity and newly diagnosed type 2 diabetes on insulin action in skeletal muscle and fat tissue, we studied 61 men with (n = 31) or without (n = 30) diabetes, subgrouped into abdominally obese or nonobese according to the waist circumference. Adipose tissue depots were quantified by magnetic resonance imaging, and regional glucose uptake was measured using 2-[(18)F]fluoro-2-deoxyglucose/positron emission tomography during euglycemic hyperinsulinemia. Across groups, glucose uptake per unit tissue weight was higher in visceral (20.5 +/- 1.4 micromol . min(-1) . kg(-1)) than in abdominal (9.8 +/- 0.9 micromol min(-1) . kg(-1), P < 0.001) or femoral (12.3 +/- 0.6 micromol . min(-1) . kg(-1), P < 0.001) subcutaneous tissue and approximately 40% lower than in skeletal muscle (33.1 +/- 2.5 micromol . min(-1) . kg(-1), P < 0.0001). Abdominal obesity was associated with a marked reduction in glucose uptake per unit tissue weight in all fat depots and in skeletal muscle (P < 0.001 for all regions). Recent type 2 diabetes per se had little additional effect. In both intra-abdominal adipose (r = -0.73, P < 0.0001) and skeletal muscle (r = -0.53, P < 0.0001) tissue, glucose uptake was reciprocally related to intra-abdominal fat mass in a curvilinear fashion. When regional glucose uptake was multiplied by tissue mass, total glucose uptake per fat depot was similar irrespective of abdominal obesity or type 2 diabetes, and its contribution to whole-body glucose uptake increased by approximately 40% in obese nondiabetic and nonobese diabetic men and was doubled in obese diabetic subjects. We conclude that 1) in abdominal obesity, insulin-stimulated glucose uptake rate is markedly reduced in skeletal muscle and in all fat depots; 2) in target tissues, this reduction is reciprocally (and nonlinearly) related to the amount of intra-abdominal fat; 3) mild, recent diabetes adds little insulin resistance to that caused by abdominal obesity; and 4) despite fat insulin resistance, an expanded fat mass (especially subcutaneous) provides a sink for glucose, resulting in a compensatory attenuation of insulin resistance at the whole-body level in men.     

Endothelin limits insulin action in obese/insulin-resistant humans

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.     

Weight loss-induced plasticity of glucose transport and phosphorylation in the insulin resistance of obesity and type 2 diabetes

We tested the hypothesis that weight loss alleviates insulin resistance in skeletal muscle within the proximal steps of glucose metabolism, namely substrate delivery, glucose transport, and glucose phosphorylation. In obese subjects with and without type 2 diabetes, in vivo skeletal muscle assessments were obtained with dynamic positron emission tomography (PET) imaging performed during euglycemic clamps at moderate hyperinsulinemia (40 mU x min(-1) x m(-2)), using [(15)O]H(2)O and [(18)F]fluoro-deoxyglucose ([(18)F]FDG) to quantify tissue perfusion and glucose metabolism. Dynamic [(18)F]FDG PET data were analyzed using both a novel muscle-specific compartmental model and a compartmental model originally developed for the brain and often used for [(18)F]FDG muscle image quantification. Weight loss in obese subjects with (n = 9) and without (n = 9) type 2 diabetes over a 4-month intervention was substantial (14 +/- 2 kg, P < 0.05). Muscle insulin resistance, assessed by insulin-stimulated [(18)F]FDG uptake, decreased threefold in diabetic subjects and twofold in nondiabetic subjects (P < 0.001). Kinetic parameters for [(18)F]FDG transport and phosphorylation improved substantially in both groups, whereas tissue blood flow did not change. In particular, clinically significant weight loss fully corrected insulin resistance in type 2 diabetes at the step of glucose phosphorylation and largely, but incompletely, corrected insulin resistance at the glucose transport step.     

Obesity blunts insulin-mediated microvascular recruitment in human forearm muscle

We have previously shown that skeletal muscle capillaries are rapidly recruited by physiological doses of insulin in both humans and animals. This facilitates glucose and insulin delivery to muscle, thus augmenting glucose uptake. In obese rats, both insulin-mediated microvascular recruitment and glucose uptake are diminished; however, this action of insulin has not been studied in obese humans. Here we used contrast ultrasound to measure microvascular blood volume (MBV) (an index of microvascular recruitment) in the forearm flexor muscles of lean and obese adults before and after a 120-min euglycemic-hyperinsulinemic (1 mU . min(-1) . kg(-1)) clamp. We also measured brachial artery flow, fasting lipid profile, and anthropomorphic variables. Fasting plasma glucose (5.4 +/- 0.1 vs. 5.1 +/- 0.1 mmol/l, P = 0.05), insulin (79 +/- 11 vs. 38 +/- 6 pmol/l, P = 0.003), and percent body fat (44 +/- 2 vs. 25 +/- 2%, P = 0.001) were higher in the obese than the lean adults. After 2 h of insulin infusion, whole-body glucose infusion rate was significantly lower in the obese versus lean group (19.3 +/- 3.2 and 37.4 +/- 2.6 mumol . min(-1) . kg(-1) respectively, P < 0.001). Compared with baseline, insulin increased MBV in the lean (18.7 +/- 3.3 to 25.0 +/- 4.1, P = 0.019) but not in the obese group (20.4 +/- 3.6 to 18.8 +/- 3.8, NS). Insulin increased brachial artery diameter and flow in the lean but not in the obese group. We observed a significant, negative correlation between DeltaMBV and BMI (R = -0.482, P = 0.027) in response to insulin. In conclusion, obesity eliminated the insulin-stimulated muscle microvascular recruitment and increased brachial artery blood flow seen in lean individuals.     

Insulin-regulated mitochondrial gene expression is associated with glucose flux in human skeletal muscle.

To identify abnormally expressed genes contributing to muscle insulin resistance in type 2 diabetes, we screened the mRNA populations from normal and diabetic human skeletal muscle using cDNA differential display and isolated abnormally expressed cDNA clones of mitochondrial-encoded NADH dehydrogenase 1 (ND1), cytochrome oxidase 1, tRNA(leu), and displacement loop. We then measured mRNA expression of these mitochondrial genes using a relative quantitative polymerase chain reaction method in biopsies taken before and after an insulin clamp in 12 monozygotic twin pairs discordant for type 2 diabetes and 12 matched control subjects and in muscle biopsies taken after an insulin clamp from 13 subjects with type 2 diabetes, 15 subjects with impaired glucose tolerance, and 14 subjects with normal glucose tolerance. Insulin infusion increased mRNA expression of ND1 from 1.02 +/- 0.04 to 2.55 +/- 0.30 relative units (P < 0.001) and of cytochrome oxidase 1 from 0.80 +/- 0.01 to 1.24 +/- 0.10 relative units (P < 0.001). The ND1 response to insulin correlated with glucose uptake (r = 0.46, P = 0.002). Although the rate of insulin-mediated glucose uptake was decreased in the diabetic versus the nondiabetic twins (5.2 +/- 0.7 vs. 8.5 +/- 0.8 mg x kg(-1) fat-free mass x min(-1), P < 0.01), insulin-stimulated ND1 expression was not significantly different between them (2.4 +/- 0.5 vs. 2.7 +/- 0.5 relative units). Neither was there any significant intrapair correlation of ND1 expression between the monozygotic twins (r = -0.15, NS). We conclude that insulin upregulates mitochondrial-encoded gene expression in skeletal muscle. Given the positive correlation between ND1 expression and glucose uptake and the lack of intrapair correlation between monozygotic twins, mitochondrial gene expression may represent an adaptation to intracellular glucose flux rather than an inherited trait.     

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.     

Nitric oxide synthase inhibition reduces glucose uptake during exercise in individuals with type 2 diabetes more than in control subjects

Nitric oxide (NO) synthase inhibition reduces leg glucose uptake during cycling without reducing leg blood flow (LBF) in young, healthy individuals. This study sought to determine the role of NO in glucose uptake during exercise in individuals with type 2 diabetes. Nine men with type 2 diabetes and nine control subjects matched for age, sex, peak pulmonary oxygen uptake (VO(2) peak), and weight completed two 25-min bouts of cycling exercise at 60 +/- 2% VO(2) peak, separated by 90 min. N(G)-monomethyl-L-arginine (L-NMMA) (total dose 6 mg/kg) or placebo was administered into the femoral artery for the final 15 min of exercise in a counterbalanced, blinded, crossover design. LBF was measured by thermodilution in the femoral vein, and leg glucose uptake was calculated as the product of LBF and femoral arteriovenous glucose difference. During exercise with placebo, glucose uptake was not different between control subjects and individuals with diabetes; however, LBF was lower and arterial plasma glucose and insulin levels were higher in individuals with diabetes. L-NMMA had no effect on LBF or arterial plasma glucose and insulin concentrations during exercise in both groups. L-NMMA significantly reduced leg glucose uptake in both groups, with a significantly greater reduction (P = 0.04) in the diabetic group (75 +/- 13%, 5 min after L-NMMA) compared with the control group (34 +/- 14%, 5 min after L-NMMA). These data suggest a greater reliance on NO for glucose uptake during exercise in individuals with type 2 diabetes compared with control subjects.     

Independent association of type 2 diabetes and coronary artery disease with myocardial insulin resistance

Clustering of classical cardiovascular risk factors is insufficient to account for the excess coronary artery disease (CAD) of patients with diabetes, and chronic hyperglycemia and insulin resistance (IR) are obvious culprits. Whole-body and skeletal muscle IR is characteristic of patients with type 2 diabetes, but whether it extends to the normally contracting cardiac muscle is controversial. We investigated whether type 2 diabetes is associated with myocardial IR independent of CAD in a case-control series (n = 55) of male nondiabetic and diabetic (type 2 and type 1) patients with or without angiographically documented CAD. Baseline blood flow ((15)O-water) and insulin-stimulated glucose uptake ((18)F-fluoro-deoxyglucose) during euglycemic (5.6 mmol/l), physiological hyperinsulinemia (40 mU x min(-1) x m(-2) insulin clamp) were measured by positron emission tomography in skeletal muscle and normally contracting myocardium. Skeletal muscle glucose uptake was reduced in association with both CAD and type 2 diabetes. In regions with normal baseline perfusion, insulin-mediated myocardial glucose uptake was reduced in non-CAD type 2 diabetic (0.36 +/- 0.14 micro mol x min(-1). g(-1)) and nondiabetic CAD patients (0.44 +/- 0.15 micro mol x min(- 1) x g(-1)) in comparison with healthy control subjects (0.61 +/- 0.08) or with non-CAD type 1 diabetic patients (0.80 +/- 0.13; P < 0.001 for both CAD and diabetes). Neither basal skeletal muscle nor basal myocardial blood flow differed across groups; both skeletal muscle and myocardial IR were directly related to whole-body IR. We conclude that type 2 diabetes is specifically associated with myocardial IR that is independent of and nonadditive with angiographic CAD and proportional to skeletal muscle and whole-body IR.     

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.     

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.     

Pioglitazone reduces hepatic fat content and augments splanchnic glucose uptake in patients with type 2 diabetes

The effect of pioglitazone on splanchnic glucose uptake (SGU), endogenous glucose production (EGP), and hepatic fat content was studied in 14 type 2 diabetic patients (age 50 +/- 2 years, BMI 29.4 +/- 1.1 kg/m(2), HbA(1c) 7.8 +/- 0.4%). Hepatic fat content (magnetic resonance spectroscopy) and SGU (oral glucose load- insulin clamp technique) were quantitated before and after pioglitazone (45 mg/day) therapy for 16 weeks. Subjects received a 7-h euglycemic insulin (100 mU. m(-2). min(-1)) clamp, and a 75-g oral glucose load was ingested 3 h after starting the insulin clamp. Following glucose ingestion, the steady-state glucose infusion rate during the insulin clamp was decreased appropriately to maintain euglycemia. SGU was calculated by subtracting the integrated decrease in glucose infusion rate during the 4 h after glucose ingestion from the ingested glucose load. 3-[(3)H]glucose was infused during the initial 3 h of the insulin clamp to determine rates of EGP and glucose disappearance (R(d)). Pioglitazone reduced fasting plasma glucose (10.0 +/- 0.7 to 7.5 +/- 0.6 mmol/l, P < 0.001) and HbA(1c) (7.8 +/- 0.4 to 6.7 +/- 0.3%, P < 0.001) despite increased body weight (83 +/- 3 to 86 +/- 3 kg, P < 0.001). During the 3-h insulin clamp period before glucose ingestion, pioglitazone improved R(d) (6.9 +/- 0.5 vs. 5.2 +/- 0.5 mg. kg(-1). min(- 1), P < 0.001) and insulin-mediated suppression of EGP (0.21 +/- 0.04 to 0.06 +/- 0.02 mg. kg(-1). min(-1), P < 0.01). Following pioglitazone treatment, hepatic fat content decreased from 19.6 +/- 3.6 to 10.4 +/- 2.1%, (P < 0.005), and SGU increased from 33.0 +/- 2.8 to 46.2 +/- 5.1% (P < 0.005). Pioglitazone treatment in type 2 diabetes 1) decreases hepatic fat content and improves insulin-mediated suppression of EGP and 2) augments splanchnic and peripheral tissue glucose uptake. Improved splanchnic/peripheral glucose uptake and enhanced suppression of EGP contribute to the improvement in glycemic control in patients with type 2 diabetes.     

Skeletal muscle insulin resistance in normoglycemic subjects with a strong family history of type 2 diabetes is associated with decreased insulin-stimulated insulin receptor substrate-1 tyrosine phosphorylation

Normoglycemic subjects with a strong family history of type 2 diabetes are insulin resistant, but the mechanism of insulin resistance in skeletal muscle of such individuals is unknown. The present study was undertaken to determine whether abnormalities in insulin-signaling events are present in normoglycemic, nonobese subjects with a strong family history of type 2 diabetes. Hyperinsulinemic-euglycemic clamps with percutaneous muscle biopsies were performed in eight normoglycemic relatives of type 2 diabetic patients (FH(+)) and eight control subjects who had no family history of diabetes (FH(-)), with each group matched for age, sex, body composition, and ethnicity. The FH(+) group had decreased insulin-stimulated glucose disposal (6.64 +/- 0.52 vs. 8.45 +/- 0.54 mg. kg(-1) fat-free mass. min(-1); P < 0.05 vs. FH(-)). In skeletal muscle, the FH(+) and FH(-) groups had equivalent insulin stimulation of insulin receptor tyrosine phosphorylation. In contrast, the FH(+) group had decreased insulin stimulation of insulin receptor substrate (IRS)-1 tyrosine phosphorylation (0.522 +/- 0.077 vs. 1.328 +/- 0.115 density units; P < 0.01) and association of PI 3-kinase activity with IRS-1 (0.299 +/- 0.053 vs. 0.466 +/- 0.098 activity units; P < 0.05). PI 3-kinase activity was correlated with the glucose disposal rate (r = 0.567, P = 0.02). In five subjects with sufficient biopsy material for further study, phosphorylation of Akt was 0.266 +/- 0.061 vs. 0.404 +/- 0.078 density units (P < 0.10) and glycogen synthase activity was 0.31 +/- 0.06 vs. 0.50 +/- 0.12 ng. min(-1). mg(-1) (P < 0.10) for FH(+) and FH(-) subjects, respectively. Therefore, despite normal insulin receptor phosphorylation, postreceptor signaling was reduced and was correlated with glucose disposal in muscle of individuals with a strong genetic background for type 2 diabetes.     

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.