Cinnamon extract prevents the insulin resistance induced by a high-fructose diet.

The aim of this study was to determine whether cinnamon extract (CE) would improve the glucose utilization in normal male Wistar rats fed a high-fructose diet (HFD) for three weeks with or without CE added to the drinking water (300 mg/kg/day). In vivo glucose utilization was measured by the euglycemic clamp technique. Further analyses on the possible changes in insulin signaling occurring in skeletal muscle were performed afterwards by Western blotting. At 3 mU/kg/min insulin infusions, the decreased glucose infusion rate (GIR) in HFD-fed rats (60 % of controls, p < 0.01) was improved by CE administration to the same level of controls (normal chow diet) and the improving effect of CE on the GIR of HFD-fed rats was blocked by approximately 50 % by N-monometyl-L-arginine. The same tendency was found during the 30 mU/kg/min insulin infusions. There were no differences in skeletal muscle insulin receptor (IR)-beta, IR substrate (IRS)-1, or phosphatidylinositol (PI) 3-kinase protein content in any groups. However, the muscular insulin-stimulated IR-beta and IRS-1 tyrosine phosphorylation levels and IRS-1 associated with PI 3-kinase in HFD-fed rats were only 70 +/- 9 %, 76 +/- 5 %, and 72 +/- 6 % of controls (p < 0.05), respectively, and these decreases were significantly improved by CE treatment. These results suggest that early CE administration to HFD-fed rats would prevent the development of insulin resistance at least in part by enhancing insulin signaling and possibly via the NO pathway in skeletal muscle.     

Effects of keishi-ka-jutsubu-to (traditional herbal medicine: Gui-zhi-jia-shu-fu-tang) on in vivo insulin action in streptozotocin-induced diabetic rats

This study investigated the effects of the traditional herbal medicine, Keishi-ka-jutsubu-to (KJT) on insulin action in vivo and insulin signaling in skeletal muscle in STZ-induced diabetes. Rats were divided into single and 7-days oral administration groups. Euglycemic clamp (insulin infusion rates: 3 and 30 mU/kg/min) was used in awaked rats and the insulin signaling in skeletal muscle was evaluated. At low-dose insulin infusion, the decreased metabolic clearance rates of glucose (MCR) in diabetic rats were improved by a single and 7-days administration of KJT (800 mg/kg BW, p.o.; acute effect: 6.7 +/- 0.6 vs. 12.3 +/- 1.2, and 7-days effect: 6.3 +/- 0.5 vs. 13.9 +/- 1.0 ml/kg/min, P<0.001, respectively). During high-dose insulin infusion, the MCR was increased in 7-days KJT treated diabetes compared with saline diabetes, but, these changes were not observed after a single KJT treatment. About 90% of the increasing effect in MCR induced by the 7-days KJT treatment was blocked by L-NMMA. However, no further additive effects were seen in KJT + SNP treatment. IRbeta protein increase and decreased IRS-1 protein expression in diabetes were significantly improved by KJT treatment. KJT had no effect on the GLUT4 protein content. The increased tyrosine phosphorylation level of IRbeta, IRS-1, and IRS-1 associated with PI 3-kinase were significantly inhibited in KJT treated diabetes. The present study suggests that the improvement of impaired insulin action in STZ-diabetes by administration of KJT may be due, at least in part, to enhanced insulin signaling, which may be involved with production of nitric oxide (NO).     

Evidence against a role for insulin-signaling proteins PI 3-kinase and Akt in insulin resistance in human skeletal muscle induced by short-term GH infusion

Prolonged growth hormone (GH) excess is known to be associated with insulin resistance, but the underlying mechanisms remain unknown. The aim of this study was to assess the impact of GH on insulin-stimulated glucose metabolism and insulin signaling in human skeletal muscle. In a cross-over design, eight healthy male subjects (age 26.0 +/- 0.8 yr and body mass index 24.1 +/- 0.5 kg/m2) were infused for 360 min with either GH (Norditropin, 45 ng.kg(-1).min(-1)) or saline. During the final 180 min of the infusion, a hyperinsulinemic euglycemic clamp was performed (insulin infusion rate: 1.2 mU.kg(-1).min(-1)). Muscle biopsies from vastus lateralis were taken before GH/saline administration and after 60 min of hyperinsulinemia. GLUT4 content and insulin signaling, as assessed by insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase and Akt activity were determined. GH levels increased to a mean (+/-SE) level of 20.0 +/- 2.3 vs. 0.5 +/- 0.2 microg/l after saline infusion (P < 0.01). During GH infusion, the glucose infusion rate during hyperinsulinemia was reduced by 38% (P < 0.01). In both conditions, free fatty acids were markedly suppressed during hyperinsulinemia. Despite skeletal muscle insulin resistance, insulin still induced a similar approximately 3-fold rise in IRS-1-associated PI 3-kinase activity (269 +/- 105 and 311 +/- 71% compared with baseline, GH vs. saline). GH infusion did not change Akt protein expression, and insulin caused an approximately 13-fold increase in Akt activity (1,309 +/- 327 and 1,287 +/- 173%) after both GH and saline infusion. No difference in total GLUT4 content was noted (114.7 +/- 7.4 and 107.6 +/- 16.7 arbitrary units, GH vs. saline, compared with baseline). In conclusion, insulin resistance in skeletal muscle induced by short-term GH administration is not associated with detectable changes in the upstream insulin-signaling cascade or reduction in total GLUT4. Yet unknown mechanisms in insulin signaling downstream of Akt may be responsible.     

The high-fat-fed lean Zucker rat: a spontaneous isocaloric model of fat-induced insulin resistance associated with muscle GSK-3 overactivity

High-fat feeding (HFF) is a well-accepted model for nutritionally-induced insulin resistance. The purpose of this investigation was to assess the metabolic responses of female lean Zucker rats provided regular chow (4% fat) or a high-fat chow (50% fat) for 15 wk. HFF rats spontaneously adjusted food intake so that daily caloric intake matched that of chow-fed (CF) controls. HFF animals consumed more (P < 0.05) calories from fat (31.9 +/- 1.2 vs. 2.4 +/- 0.2 kcal/day) and had significantly greater final body weights (280 +/- 10 vs. 250 +/- 5 g) and total visceral fat (24 +/- 3 vs. 10 +/- 1 g). Fasting plasma insulin was 2.3-fold elevated in HFF rats. Glucose tolerance (58%) and whole body insulin sensitivity (75%) were markedly impaired in HFF animals. In HFF plantaris muscle, in vivo insulin receptor beta-subunit (IR-beta) and insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation and phosphorylation of Akt Ser473 and glycogen synthase kinase-3beta (GSK-3beta) Ser9, relative to circulating insulin levels, were decreased by 40-59%. In vitro insulin-stimulated glucose transport in HFF soleus was decreased by 54%, as were IRS-1 tyrosine phosphorylation (26%) and phosphorylation of Akt Ser473 (38%) and GSK-3beta Ser9 (25%), the latter indicative of GSK-3 overactivity. GSK-3 inhibition in HFF soleus using CT98014 increased insulin-stimulated glucose transport (28%), IRS-1 tyrosine phosphorylation (28%) and phosphorylation of Akt Ser473 (38%) and GSK-3beta Ser9 (48%). In summary, the female lean Zucker rat fed a high-fat diet represents an isocaloric model of nutritionally-induced insulin resistance associated with moderate visceral fat gain, hyperinsulinemia, and impairments of skeletal muscle insulin-signaling functionality, including GSK-3beta overactivity.     

Immobilization depresses insulin signaling in skeletal muscle

Prolonged immobilization depresses insulin-induced glucose transport in skeletal muscle and leads to a catabolic state in the affected areas, with resultant muscle wasting. To elucidate the altered intracellular mechanisms involved in the insulin resistance, we examined insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit (IR-beta) and insulin receptor substrate (IRS)-1 and activation of its further downstream molecule, phosphatidylinositol 3-kinase (PI 3-K), after unilateral hindlimb immobilization in the rat. The contralateral hindlimb served as control. After 7 days of immobilization of the rat, insulin was injected into the portal vein, and tibialis anterior muscles on both sides were extracted. Immobilization reduced insulin-stimulated tyrosine phosphorylation of IR-beta and IRS-1. Insulin-stimulated binding of IRS-1 to p85, the regulatory subunit of PI 3-K, and IRS-1-associated PI 3-K activity were also decreased in the immobilized hindlimb. Although IR-beta and p85 protein levels were unchanged, IRS-1 protein expression was downregulated by immobilization. Thus prolonged immobilization may cause depression of insulin-stimulated glucose transport in skeletal muscle by altering insulin action at multiple points, including the tyrosine phosphorylation, protein expression, and activation of essential components of insulin signaling pathways.     

Does impaired mitochondrial function affect insulin signaling and action in cultured human skeletal muscle cells?

Insulin-resistant type 2 diabetic patients have been reported to have impaired skeletal muscle mitochondrial respiratory function. A key question is whether decreased mitochondrial respiration contributes directly to the decreased insulin action. To address this, a model of impaired cellular respiratory function was established by incubating human skeletal muscle cell cultures with the mitochondrial inhibitor sodium azide and examining the effects on insulin action. Incubation of human skeletal muscle cells with 50 and 75 microM azide resulted in 48 +/- 3% and 56 +/- 1% decreases, respectively, in respiration compared with untreated cells mimicking the level of impairment seen in type 2 diabetes. Under conditions of decreased respiratory chain function, insulin-independent (basal) glucose uptake was significantly increased. Basal glucose uptake was 325 +/- 39 pmol/min/mg (mean +/- SE) in untreated cells. This increased to 669 +/- 69 and 823 +/- 83 pmol/min/mg in cells treated with 50 and 75 microM azide, respectively (vs. untreated, both P < 0.0001). Azide treatment was also accompanied by an increase in basal glycogen synthesis and phosphorylation of AMP-activated protein kinase. However, there was no decrease in glucose uptake following insulin exposure, and insulin-stimulated phosphorylation of Akt was normal under these conditions. GLUT1 mRNA expression remained unchanged, whereas GLUT4 mRNA expression increased following azide treatment. In conclusion, under conditions of impaired mitochondrial respiration there was no evidence of impaired insulin signaling or glucose uptake following insulin exposure in this model system.     

诺沙坦改善非胰岛素依赖型糖尿病大鼠胰岛素敏感性的作用机制

本文研究血管紧张素Ⅱ受体拮抗剂诺沙坦对非胰岛素依赖型糖尿病(non-insulin-dependent diabetes mellitus,NIDDM)大鼠胰岛素敏感性的改善作用,并探讨其作用机制。从饮水中给予正常或高脂喂养加小剂量链脲佐菌素(STZ)诱发的NIDDM大鼠诺沙坦(4 mg/kg),连续6周。分离骨骼肌,用免疫印迹法检测诺沙坦对胰岛素受体底物1(insulin receptor substrate 1,IRS-1)、蛋白激酶B(protein kinase B,PKB)和葡萄糖转运因子4(glucose transporter 4,GLUT4)的表达,以及IRS-1的磷酸化、IRS-1与磷脂酰肌醇3激酶(phosphatidylinositol(PI)3-kinase)的结合。口服葡萄糖耐量试验表明,口服诺沙坦可改善糖尿病大鼠胰岛素敏感性。在骨骼肌组织,NIDDM和正常大鼠的IRS-1、PKB和GLUT4蛋白表达无差异,且不受诺沙坦处理的影响。NIDDM大鼠胰岛素刺激后的骨骼肌IRS-1酪氨酸磷酸化水平、PI 3-kinase结合IRS-1的活性和PKB活性较对照组显著降低(P<0.01),且不能被诺沙坦改善。诺沙坦显著增加NIDDM大鼠肌细胞质膜(plasma membrane,PM)和T管(T-tubules,TT)胰岛素诱导的GLUT4的 含量(P<0.05)。与该结果一致的是,诺沙坦处理的NIDDM大鼠血糖水平较未处理NIDDM大鼠下降(P<0.05)。结果表明,诺沙坦可改善胰岛素抵抗状态,主要是通过非PI 3-kinase依赖的     

Dysregulation of muscle lipid metabolism in rats selectively bred for low aerobic running capacity

As substrate for evaluation of metabolic diseases, we developed novel rat models that contrast for endurance exercise capacity. Through two-way artificial selection, we created rodent phenotypes of intrinsically low-capacity runners (LCR) and high-capacity runners (HCR) that also differed markedly for cardiovascular and metabolic disease risk factors. Here, we determined skeletal muscle proteins with putative roles in lipid and carbohydrate metabolism to better understand the mechanisms underlying differences in whole body substrate handling between phenotypes. Animals (generation 16) differed for endurance running capacity by 295%. LCR animals had higher resting plasma glucose (6.58 +/- 0.45 vs. 6.09 +/- 0.45 mmol/l), insulin (0.48 +/- 0.03 vs. 0.32 +/- 0.02 ng/ml), nonesterified fatty acid (0.57 +/- 0.14 v 0.35 +/- 0.05 mM), and triglyceride (TG; 0.47 +/- 0.11 vs. 0.25 +/- 0.08 mmol/l) concentrations (all P < 0.05). Muscle TG (72.3 +/- 14.7 vs. 38.9 +/- 6.2 mmol/kg dry muscle wt; P < 0.05) and diacylglycerol (96 +/- 28 vs. 42 +/- 8 pmol/mg dry muscle wt; P < 0.05) contents were elevated in LCR vs. HCR rats. Accompanying the greater lipid accretion in LCR was increased fatty acid translocase/CD36 content (1,014 +/- 80 vs. 781 +/- 70 arbitrary units; P < 0.05) and reduced TG lipase activity (0.158 +/- 0.0125 vs. 0.274 +/- 0.018 mmol.min(-1).kg dry muscle wt(-1); P < 0.05). Muscle glycogen, GLUT4 protein, and basal phosphorylation states of AMP-activated protein kinase-alpha1, AMP-activated protein kinase-alpha2, and acetyl-CoA carboxylase were similar in LCR and HCR. In conclusion, rats with low intrinsic aerobic capacity demonstrate abnormalities in lipid-handling capacity. These disruptions may, in part, be responsible for the increased risk of metabolic disorders observed in this phenotype.     

Chronic selective glycogen synthase kinase-3 inhibition enhances glucose disposal and muscle insulin action in prediabetic obese Zucker rats

Increasing evidence supports a negative role of glycogen synthase kinase-3 (GSK-3) in regulation of skeletal muscle glucose transport. We assessed the effects of chronic treatment of insulin-resistant, prediabetic obese Zucker (fa/fa) rats with a highly selective GSK-3 inhibitor (CT118637) on glucose tolerance, whole body insulin sensitivity, plasma lipids, skeletal muscle insulin signaling, and in vitro skeletal muscle glucose transport activity. Obese Zucker rats were treated with either vehicle or CT118637 (30 mg/kg body wt) twice per day for 10 days. Fasting plasma insulin and free fatty acid levels were reduced by 14 and 23% (P < 0.05), respectively, in GSK-3 inhibitor-treated animals compared with vehicle-treated controls. The glucose response during an oral glucose tolerance test was reduced by 18% (P < 0.05), and whole body insulin sensitivity was increased by 28% (P < 0.05). In vivo insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (50%) and IRS-1-associated phosphatidylinositol-3' kinase (79%) relative to fasting plasma insulin levels were significantly elevated (P < 0.05) in plantaris muscles of GSK-3 inhibitor-treated animals. Whereas basal glucose transport in isolated soleus and epitrochlearis muscles was unaffected by chronic GSK-3 treatments, insulin stimulation of glucose transport above basal was significantly enhanced (32-60%, P < 0.05). In summary, chronic treatment of insulin-resistant, prediabetic obese Zucker rats with a specific GSK-3 inhibitor enhances oral glucose tolerance and whole body insulin sensitivity and is associated with an amelioration of dyslipidemia and an improvement in IRS-1-dependent insulin signaling in skeletal muscle. These results provide further evidence that selective targeting of GSK-3 in muscle may be an effective intervention for the treatment of obesity-associated insulin resistance.     

Aging is associated with myocardial insulin resistance and mitochondrial dysfunction

Aging is associated with insulin resistance, often attributable to obesity and inactivity. Recent evidence suggests that skeletal muscle insulin resistance in aging is associated with mitochondrial alterations. Whether this is true of the senescent myocardium is unknown. Twelve young (Y, 4 years old) and 12 old (O, 11 years old) dogs, matched for body mass, were instrumented with left-ventricular pressure gauges, aortic and coronary sinus catheters, and flow probes on left circumflex artery. Before surgery, all dogs participated in a 6-wk exercise program. Dogs underwent measurements of hemodynamics and plasma substrates before and during a 2-h hyperinsulinemic-euglycemic clamp to measure whole body and myocardial glucose and nonesterified fatty acid uptake. Following the protocol, myocardial and skeletal samples were obtained to measure components of the insulin-signaling cascade and mitochondrial structure. There was no difference in plasma glucose (Y, 90 +/- 4 mg/dl; O, 87 +/- 4 mg/dl), but old dogs had higher (P < 0.02) nonesterified fatty acids (Y, 384 +/- 48 micromol/l; O, 952 +/- 97 micromol/l) and plasma insulin (Y, 39 +/- 11 pmol/l; O, 108 +/- 18 pmol/l). Old dogs had impaired total body glucose disposition (Y, 11.5 +/- 1 mg x kg(-1) x min(-1); O, 8.0 +/- 0.5 mg x kg(-1) x min(-1); P < 0.05) and insulin-stimulated myocardial glucose uptake (Y, 3.5 +/- 0.3 mg x min(-1) x g(-1); O, 1.8 +/- 0.3 mg x min(-1) x g(-1); P < 0.05). The impaired insulin action was associated with altered insulin signaling and glucose transporter (GLUT4) translocation. There were myocardial mitochondrial structural changes observed in association with decreased expression of uncoupling protein-3. Aging is associated with both whole body and myocardial insulin resistance, independent of obesity and inactivity, but involving altered mitochondrial structure and impaired cellular insulin action.     

Type 2 diabetic patients may have a mild form of an injury response: a clinical research center study

Patients with type 2 diabetes (DM) demonstrate inadequate insulin release, elevated gluconeogenesis, and diminished nonoxidative glucose disposal. Similar metabolic changes occur during systemic injury caused by infection, trauma, or cancer. Described here are metabolic changes occurring in 16 DM and 11 lung cancer patients (CA) and 13 normal volunteers (NV). After a 10-h overnight fast, all subjects had fasting hormone and substrate concentrations determined, along with rates of glucose production, leucine appearance (LA), and leucine oxidation (LO). Fasting insulin (data not shown) and C-peptide concentrations were elevated in DM and CA compared with weight-matched NV (0.72 +/- 0.09 and 0.64 +/- 0.08 vs. 0.51 +/- 0.03 mg/l, P < 0.05). C-reactive protein concentration was elevated in CA compared with DM and NV (23.3 +/- 6.0 vs. 4.2 +/- 1.4 and 2.1 +/- 0.5 mg/l, P < 0.01). All counterregulatory hormones were normal except for serum cortisol (11.4 +/- 1.0 and 12.1 +/- 1.0 vs. 8.9 +/- 0.7 microg/dl, DM and CA vs. NL, respectively, P < 0.05). Glucose production was increased in DM and CA compared with NV (4.22 +/- 0.6 and 3.53 +/- 0.3 vs. 2.76 +/- 0.2 mg x kg lean body wt(-1) x min(-1), P < 0.01). LO and LA were increased in DM and CA compared with NV (LO: 27.3 +/- 1.5 and 19.7 +/- 1.5 vs. 12.5 +/- 1.1 mmol x kg lean body wt(-1) x min(-1), P < 0.05; LA: 91.9 +/- 6.6 and 90.7 +/- 7.0 vs. 79.1 +/- 6.0 mmol. kg lean body wt(-1) x min(-1), P < 0.01). DM share similar metabolic derangements with CA. The increase in LA may be secondary to an increased glucose production where amino acids are mobilized to provide the liver with adequate substrate to make glucose. The increase in glucose production may also be part of the injury response, or it may represent a form of insulin resistance that exists in both the DM and (non-DM) CA patients.     

Reduced glucose uptake precedes insulin signaling defects in adipocytes from heterozygous GLUT4 knockout mice.

Decreased GLUT4 expression, impaired insulin receptor (IR), IRS-1, and pp60/IRS-3 tyrosine phosphorylation are characteristics of adipocytes from insulin-resistant animal models and obese NIDDM humans. However, the sequence of events leading to the development of insulin signaling defects and the significance of decreased GLUT4 expression in causing adipocyte insulin resistance are unknown. The present study used male heterozygous GLUT4 knockout mice (GLUT4(+/-)) as a novel model of diabetes to study the development of insulin signaling defects in adipocytes with the progression of whole body insulin resistance and diabetes. Male GLUT4(+/-) mice with normal fed glycemia and insulinemia (N/N), normal fed glycemia and hyperinsulinemia (N/H), and fed hyperglycemia with hyperinsulinemia (H/H) exist at all ages. The expression of GLUT4 protein and the maximal insulin-stimulated glucose transport was 50% decreased in adipocytes from all three groups. Insulin signaling was normal in N/N adipose cells. From 35 to 70% reductions in insulin-stimulated tyrosine phosphorylation of IR, IRS-1, and pp60/IRS-3 were noted with no changes in the cellular content of IR, IRS-1, and p85 in N/H adipocytes. Insulin-stimulated protein tyrosine phosphorylation was further decreased to 12-23% in H/H adipose cells accompanied by 42% decreased IR and 80% increased p85 expression. Insulin-stimulated, IRS-1-associated PI3 kinase activity was decreased by 20% in N/H and 68% reduced in H/H GLUT4(+/-) adipocytes. However, total insulin-stimulated PI3 kinase activity was normal in H/H GLUT4(+/-) adipocytes. Taken together, these results strongly suggest that hyperinsulinemia triggers a reduction of IR tyrosine kinase activity that is further exacerbated by the appearance of hyperglycemia. However, the insulin signaling cascade has sufficient plasticity to accommodate significant changes in specific components without further reducing glucose uptake. Furthermore, the data indicate that the cellular content of GLUT4 is the rate-limiting factor in mediating maximal insulin-stimulated glucose uptake in GLUT4(+/-) adipocytes.     

Diets enriched in sucrose or fat increase gluconeogenesis and G-6-Pase but not basal glucose production in rats

High-fat (HFD) and high-sucrose diets (HSD) reduce insulin suppression of glucose production in vivo, increase the capacity for gluconeogenesis in vitro, and increase glucose-6-phosphatase (G-6-Pase) activity in whole cell homogenates. The present study examined the effects of HSD and HFD on in vivo gluconeogenesis, the catalytic and glucose-6-phosphate translocase subunits of G-6-Pase, glucokinase (GK) translocation, and glucose cycling. Rats were fed a high-starch control diet (STD; 68% cornstarch), HSD (68% sucrose), or HFD (45% fat) for 7-13 days. The ratio of 3H in C6:C2 of glucose after 3H2O injection into 6- to 8-h-fasted rats was significantly increased in HSD (0.68 +/- 0.07) and HFD (0.71 +/- 0.08) vs. STD (0.40 +/- 0.10). G-6-Pase activity was significantly higher in HSD and HFD vs. STD in both intact and disrupted liver microsomes. HSD and HFD significantly increased the amount of the p36 catalytic subunit protein, whereas the p46 glucose-6-phosphate translocase protein was increased in HSD only. Despite increased nonglycerol gluconeogenesis and increased G-6-Pase, basal glucose and insulin levels as well as glucose production were not significantly different among groups. Hepatocyte cell suspensions were used to ascertain whether diet-induced adaptations in glucose phosphorylation and GK might serve to compensate for upregulation of G-6-Pase. Tracer-estimated glucose phosphorylation and glucose cycling (glucose <--> glucose 6-phosphate) were significantly higher in cells isolated from HSD only. After incubation with either 5 or 20 mM glucose and no insulin, GK activity (nmol. mg protein(-1). min(-1)) in digitonin-treated eluates (translocated GK) was significantly higher in HSD (32 +/- 4 and 146 +/- 6) vs. HFD (4 +/- 1 and 83 +/- 10) and STD (9 +/- 2 and 87 +/- 9). Thus short-term, chronic exposure to HSD and HFD increase in vivo gluconeogenesis and the G-6-Pase catalytic subunit. Exposure to HSD diet also leads to adaptations in glucose phosphorylation and GK translocation.     

Insulin action in skeletal muscle from patients with NIDDM

Insulin resistance in peripheral tissues is a common feature of non insulin-dependent diabetes mellitus (NIDDM). The decrease in insulin-mediated peripheral glucose uptake in NIDDM patients can be localized to defects in insulin action on glucose transport in skeletal muscle. Following short term in vitro exposure to both submaximal and maximal concentrations of insulin, 3-O-methylglucose transport rates are 40-50% lower in isolated skeletal muscle strips from NIDDM patients when compared to muscle strips from nondiabetic subjects. In addition, we have shown that physiological levels of insulin induce a 1.6-2.0 fold increase in GLUT4 content in skeletal muscle plasma membranes from control subjects, whereas no significant increase was noted in NIDDM skeletal muscle. Impaired insulin-stimulated GLUT4 translocation and glucose transport in NIDDM skeletal muscle is associated with reduced insulin-stimulated IRS-1 tyrosine phosphorylation and PI3-kinase activity. The reduced IRS-1 phosphorylation cannot be attributed to decreased protein expression, since the IRS-1 protein content is similar between NIDDM subjects and controls. Altered glycemia may contribute to decreased insulin-mediated glucose transport in skeletal muscle from NIDDM patients. We have shown that insulin-stimulated glucose transport is normalized in vitro in the presence of euglycemia, but not in the presence of hyperglycemia. Thus, the circulating level of glucose may independently regulate insulin stimulated glucose transport in skeletal muscle from NIDDM patients via a down regulation of the insulin signaling cascade.     

Endurance exercise training increases insulin responsiveness in isolated adipocytes through IRS/PI3-kinase/Akt pathway.

Endurance exercise training promotes important metabolic adaptations, and the adipose tissue is particularly affected. The aim of this study was to investigate how endurance exercise training modulates some aspects of insulin action in isolated adipocytes and in intact adipose tissue. Male Wistar rats were submitted to daily treadmill running (1 h/day) for 7 wk. Sedentary age-matched rats were used as controls. Final body weight, body weight gain, and epididymal fat pad weight did not show any statistical differences between groups. Adipocytes from trained rats were smaller than those from sedentary rats (205 +/- 16.8 vs. 286 +/- 26.4 pl; P < 0.05). Trained rats showed decreased plasma glucose (4.9 +/- 0.13 vs. 5.3 +/- 0.07 mM; P < 0.05) and insulin levels (0.24 +/- 0.012 vs. 0.41 +/- 0.049 mM; P < 0.05) and increased insulin-stimulated glucose uptake (23.1 +/- 3.1 vs. 12.1 +/- 2.9 pmol/cm(2); P < 0.05) compared with sedentary rats. The number of insulin receptors and the insulin-induced tyrosine phosphorylation of insulin receptor-beta subunit did not change between groups. Insulin-induced tyrosine phosphorylation insulin receptor substrates (IRS)-1 and -2 increased significantly (1.57- and 2.38-fold, respectively) in trained rats. Insulin-induced IRS-1/phosphatidylinositol 3 (PI3)-kinase (but not IRS-2/PI3-kinase) association and serine Akt phosphorylation also increased (2.06- and 3.15-fold, respectively) after training. The protein content of insulin receptor-beta subunit, IRS-1 and -2, did not differ between groups. Taken together, these data support the hypothesis that the increased adipocyte responsiveness to insulin observed after endurance exercise training is modulated by IRS/PI3-kinase/Akt pathway.     

Regular exercise enhances insulin activation of IRS-1-associated PI3-kinase in human skeletal muscle.

Insulin action in skeletal muscle is enhanced by regular exercise. Whether insulin signaling in human skeletal muscle is affected by habitual exercise is not well understood. Phosphatidylinositol 3-kinase (PI3-kinase) activation is an important step in the insulin-signaling pathway and appears to regulate glucose metabolism via GLUT-4 translocation in skeletal muscle. To examine the effects of regular exercise on PI3-kinase activation, 2-h hyperinsulinemic (40 mU. m(-2). min(-1))-euglycemic (5.0 mM) clamps were performed on eight healthy exercise-trained [24 +/- 1 yr, 71.8 +/- 2.0 kg, maximal O(2) uptake (VO(2 max)) of 56.1 +/- 2.5 ml. kg(-1). min(-1)] and eight healthy sedentary men and women (24 +/- 1 yr, 64.7 +/- 4.4 kg, VO(2 max) of 44.4 +/- 2.7 ml. kg(-1). min(-1)). A [6, 6-(2)H]glucose tracer was used to measure hepatic glucose output. A muscle biopsy was obtained from the vastus lateralis muscle at basal and at 2 h of hyperinsulinemia to measure insulin receptor substrate-1(IRS-1)-associated PI3-kinase activation. Insulin concentrations during hyperinsulinemia were similar for both groups (293 +/- 22 and 311 +/- 22 pM for trained and sedentary, respectively). Insulin-mediated glucose disposal rates (GDR) were greater (P < 0.05) in the exercise-trained compared with the sedentary control group (9.22 +/- 0.95 vs. 6.36 +/- 0.57 mg. kg fat-free mass(-1). min(-1)). Insulin-stimulated PI3-kinase activation was also greater (P < 0.004) in the trained compared with the sedentary group (3.8 +/- 0.5- vs. 1.8 +/- 0.2-fold increase from basal). Endurance capacity (VO(2 max)) was positively correlated with PI3-kinase activation (r = 0.53, P < 0.04). There was no correlation between PI3-kinase and muscle morphology. However, increases in GDR were positively related to PI3-kinase activation (r = 0.60, P < 0.02). We conclude that regular exercise leads to greater insulin-stimulated IRS-1-associated PI3-kinase activation in human skeletal muscle, thus facilitating enhanced insulin-mediated glucose uptake.     

Effects of hormone therapy on insulin signaling proteins in skeletal muscle of cynomolgus monkeys

We have previously shown that hormone therapy (HT) with medroxyprogesterone acetate (MPA) alone or in combination with conjugated equine estrogens (CEE) impairs insulin sensitivity. In the current study, we sought to determine if the effect of MPA on whole body insulin sensitivity is associated with alterations in insulin signaling proteins in skeletal muscle. Ovariectomized cynomolgus monkeys were treated for 2 years with either no hormones (n = 10), CEE (0.625 mg/day human equivalent, n = 11) or CEE + MPA (2.5 mg/day human equivalent, n = 12). At the end of the study, biopsies of rectus femoris muscle were flash frozen in the basal and insulin-stimulated (10 min post-intravenous insulin injection) state. Immunoblotting revealed that CEE + MPA monkeys had significantly less glucose transporter 4 (GLUT4) expression (ANOVA P = 0.001), but there was no significant treatment effect on expression of insulin receptor, insulin receptor substrate (IRS)-1, IRS-2, or the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). There was a tendency for decreased insulin receptor tyrosine phosphorylation with CEE + MPA treatment (ANOVA P = 0.14). These deficiencies in skeletal muscle insulin signaling likely contribute to the unfavorable changes in whole body insulin sensitivity associated with CEE + MPA treatment.     

Myricetin, a naturally occurring flavonol, ameliorates insulin resistance induced by a high-fructose diet in rats

The present study was conducted to explore the effects of myricetin on insulin resistance in rats fed for 6 weeks with a diet containing 60% fructose. Repeated intravenous (i.v.) injection of myricetin (1 mg/kg per injection, 3 times daily) for 14 days was found to significantly decrease the high glucose and triglyceride levels in plasma of fructose chow-fed rats. Also, the higher degree of insulin resistance in fructose chow-fed rats as measured by homeostasis model assessment of basal insulin resistance was significantly decreased by myricetin treatment. Myricetin increased the whole-body insulin sensitivity in fructose chow-fed rats, as evidenced by the marked elevation of composite whole-body insulin sensitivity index during the oral glucose tolerance test. Myricetin was found to reverse the defect in expression of insulin receptor substrate-1 (IRS-1) and the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase) in soleus muscle of fructose chow-fed rats under the basal state, despite the protein expression of insulin receptor (IR). Increased basal phosphorylation of IR and IRS-1 as well as Akt was observed in parallel. The reduced level of insulin action on phosphorylation of IR, IRS-1 and Akt in soleus muscle of fructose chow-fed rats was reversed by myricetin treatment. Furthermore, myricetin treatment improved the defective insulin action on the translocation of glucose transporter subtype 4 (GLUT 4) in insulin-resistant soleus muscle. These findings indicate that myricetin improves insulin sensitivity through the enhancement of insulin action on IRS-1-associated PI 3-kinase and GLUT 4 activity in soleus muscles of animals exhibiting insulin resistance.     

Voluntary exercise training enhances glucose transport but not insulin signaling capacity in muscle of hypertensive TG(mREN2)27 rats.

Male heterozygous TG(mREN2)27 rats (TGR) overexpress a murine renin transgene, display marked hypertension, and have insulin resistance of skeletal muscle glucose transport and insulin signaling. We have shown previously that voluntary exercise training by TGR improves insulin-mediated skeletal muscle glucose transport (Kinnick TR, Youngblood EB, O'Keefe MP, Saengsirisuwan V, Teachey MK, and Henriksen EJ. J Appl Physiol 93: 805-812, 2002). The present study evaluated whether this training-induced enhancement of muscle glucose transport is associated with upregulation of critical insulin signaling elements, including insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3-kinase, Akt, and glycogen synthase kinase-3. TGR remained sedentary or ran spontaneously in activity wheels for 6 wk, averaging 7.1 +/- 0.8 km/day by the end of week 3 and 4.3 +/- 0.5 km/day over the final week of training. Exercise training reduced total abdominal fat by 20% (P < 0.05) in TGR runners (2.64 +/- 0.01% of body weight) compared with sedentary TGR controls (3.28 +/- 0.01%). Insulin-stimulated (2 mU/ml) glucose transport activity in soleus muscle was 36% greater in TGR runners compared with sedentary TGR controls. However, the protein expression and functionality of tyrosine phosphorylation of insulin receptor and IRS-1, IRS-1 associated with the p85 regulatory subunit of phosphatidylinositol 3-kinase, and Ser473 phosphorylation of Akt were not altered by exercise training. Only insulin-stimulated glycogen synthase kinase-3beta Ser9 phosphorylation was increased (22%) by exercise training. These results indicate that voluntary exercise training in TGR can enhance insulin-mediated glucose transport in skeletal muscle, as well as reduce total abdominal fat mass. However, this adaptive response in muscle occurs independently of modifications in the proximal elements of the insulin signaling cascade.     

Absence of insulin receptor substrate-1 expression does not alter GLUT1 or GLUT4 abundance or contraction-stimulated glucose uptake by mouse skeletal muscle.

The purpose of this study was to determine the influence of insulin receptor substrate-1 (IRS-1) expression on GLUT1 and GLUT4 glucose transporter protein abundance, contraction-stimulated glucose uptake, and contraction-induced glycogen depletion by skeletal muscle. Mice (6 months old) from three genotypes were studied: wild-type (IRS-1(+/+)), heterozygous (IRS-1(+/-)) for the null allele, and IRS-1 knockouts (IRS-1(-/-)) lacking a functional IRS-1 gene. In situ muscle contraction was induced (electrical stimulation of sciatic nerve) in one hindlimb using contralateral muscles as controls. Soleus and extensor digitorum longus were dissected and 2-deoxyglucose uptake was measured in vitro. 2-Deoxyglucose uptake was higher in basal muscles (no contractions) from IRS-1(-/-) vs. both other genotypes. Contraction-stimulated 2-deoxyglucose uptake and glycogen depletion did not differ among genotypes. Muscle IRS-1 protein was undetectable for IRS-1(-/-) mice, and values were approximately 40 % lower in IRS-1(+/-) than in IRS-1(+/+) mice. No difference was found in IRS-1(+/+) compared to IRS-1(-/-) groups regarding muscle abundance of GLUT1 and GLUT4. Substantial reduction or elimination of IRS-1 did not alter the hallmark effects of contractions on muscle carbohydrate metabolism--activation of glucose uptake and glycogen depletion.