No abstract

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 ⅐ kg ؊1 fat-free mass ⅐ 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 ⅐ min ؊1 ⅐ 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. Diabetes 50: 2572-2578, 2001

It is well known that amino acids, one of the three major classes of nutrients, are involved in the metabolism of carbohydrates.1,2) Among various amino acids, L-leucine (leucine) and L-isoleucine (isoleucine) show potent activity for controlling blood glucose. In normal or cirrhotic rats, acute administration of leucine or isoleucine improves the glucose tolerance curve after an oral glucose challenge, 3,4) while chronic leucine supplementation reduces obesity and improves glucose metabolism in mice on a high-fat diet (HFD).5) The glucose-lowering activity of isoleucine beyond normoglycemia seems to be relatively mild and becomes saturated as its blood level increases.6) If the glucose-lowering activity of isoleucine in diabetic animals was as strong as in normal animals, this amino acid or another agent with the same target molecule could be a promising candidate as an anti-diabetic agent. Accordingly, we determined the acute effects of isoleucine in glucose-intolerant and diabetic mice. We also investigated the chronic effect of isoleucine supplementation on glucose metabolism in mice fed a highfat/high-sucrose diet. MATERIALS AND METHODS AnimalsThe study protocol was designed to comply with the relevant institutional guidelines and was approved by the Animal Care Committee of Ajinomoto Co., Inc. Male or female C57BL/6J mice (10-16 weeks) and male BKS.Cg-mϩ/ϩLepr db /J(ϩLeprdb/ϩLeprdb) mice (db/db mice, 7-8 weeks) were purchased from Charles River Japan (Yokohama, Japan). The animals were maintained in an airconditioned room (24Ϯ1°C) with a 12 : 12-h light-dark cycle and were given free access to regular chow (CRF-1; Oriental Yeast Co., Tokyo, Japan). In the HFD study, female C57BL/6J mice were fed control chow (D12450B; 4% w/w fat, Research Diet, New Brunswick, NJ, U.S.A.) or an HFD (D12492; 35% w/w fat, Research Diet) for 8 weeks before undergoing a glucose tolerance test. In the high-fat/high-sucrose (HFHS) diet study, female C57BL/6J mice also had free access to drinking water containing 20% sucrose without or with 1 or 2% of isoleucine for 6 weeks. Mice were deprived of food and had access to isoleucine-and sucrose-free water 18 h before undergoing a glucose tolerance test. Since it was reported that female C57BL/6J mice on a HFD develop impaired glucose tolerance after a relatively short period, 7) we used female mice for our glucose-intolerance model.Oral Glucose Tolerance Test Mice fasted for 18 h were given isoleucine orally, which was immediately followed by bolus administration of glucose (1 or 2 g/kg as indicated). Distilled water or 0.5% methylcellulose was given to mice in the control group. To determine the blood glucose and plasma insulin levels, approximately 25 ml of blood was taken from the tail vein at each time indicated. Blood glucose levels were measured by the glucose oxidase method using a Fuji Dri-Chem 5500 autoanalyzer (Fuji Medical Systems, Tokyo, Japan). Plasma was separated by centrifugation and the plasma insulin level was measured by an ELISA kit (Morinaga, Tokyo, Japan). The p...

Insulin and exercise independently increase glucose metabolism in muscle. Moreover, exercise training or a prior bout of exercise increases insulin-stimulated glucose uptake in resting skeletal muscle. The present study was undertaken to compare how physiological hyperinsulinemia and moderate intensity aerobic exercise affect the tyrosine phosphorylation state and activity of insulin signaling molecules in healthy, physically inactive volunteers. Subjects had biopsies of the vastus lateralis muscle before and immediately after 30 min of either hyperinsulinemia (euglycemic insulin clamp) or moderate-intensity exercise on a cycle ergometer (approximately 60% of VO2max). Insulin receptor and IRS-1 tyrosine phosphorylation, association of the p85 regulatory subunit of PI 3-kinase with IRS-1, IRS-1 associated PI 3-kinase activity, and glycogen synthase activity were determined in muscle biopsy specimens taken from healthy subjects before and after insulin or exercise. Physiological hyperinsulinemia increased the rate of glucose disposal from 11.4 +/- 1.5 to 25.6 +/- 6.7 micromol x kg(-1) x min(-1) (P < 0.01), insulin receptor and IRS-1 tyrosine phosphorylation (173 +/- 19% and 159 +/- 35% of basal values, respectively, P < 0.05), association of the p85 regulatory subunit of PI 3-kinase with IRS-1 (159 +/- 10%, P < 0.05), and glycogen synthase fractional velocity (136 +/- 11%, P < 0.01). Exercise also increased glucose disposal, from 10.4 +/- 0.5 to 15.6 +/- 1.7 micromol x kg(-1) x min(-1) (P < 0.01) and glycogen synthase fractional velocity (253 +/- 35% of basal, P < 0.01). The exercise-induced increase in glycogen synthase was greater than that due to insulin (P < 0.05). In contrast to insulin, exercise decreased tyrosine phosphorylation of the insulin receptor to 72 +/- 10% of basal values (P < 0.05 vs basal and P < 0.05 vs insulin) and had no effect on IRS-1 tyrosine phosphorylation, or association of p85 with IRS-1. The exercise-induced decreased insulin receptor tyrosine phosphorylation could explain the well-known effect of exercise to enhance the sensitivity of muscle to insulin.