R. E. Bourey

Insulin Resistance in Aging Is Related to Abdominal Obesity

Studies have shown that insulin resistance increases with age, independent of changes in total adiposity. However, there is growing evidence that the development of insulin resistance may be more closely related to abdominal adiposity. To evaluate the independent effects of aging and regional and total adiposity on insulin resistance, we performed hyperinsulinemic euglycemic clamps on 17 young (21–33 yr) and 67 older (60–72 yr) men and women. We assessed FFM and total and regional adiposity by hydrodensitometry and anthropometry. Insulin-stimulated GORs at a plasma insulin concentration of ∼450 pM averaged 45.6 ± 3.3 μmol · kg FFM−1 · min−1 (mean ± SE) in the young subjects, 45.6 ± 10.0 μmol · kg FFM−1 · min−1 in 24 older subjects who were insulin sensitive, and 23.9 ± 11.7 μmol · kg FFM−1 · min−1 in 43 older subjects who were insulin resistant. Few significant differences were apparent in skin-fold and circumference measurements between young and insulin-sensitive older subjects, but measurements at most central body sites were significantly larger in the insulin-resistant older subjects. Waist girth accounted for >40% of the variance in insulin action, whereas age explained only 10–20% of the total variance and <2% of the variance when the effects of waist circumference were statistically controlled. These results suggest that insulin resistance is more closely associated with abdominal adiposity than with age.

Mechanism of enhanced insulin sensitivity in athletes. Increased blood flow, muscle glucose transport protein (GLUT-4) concentration, and glycogen synthase activity.

UNLABELLED We examined the mechanisms of enhanced insulin sensitivity in 9 male healthy athletes (age, 25 +/- 1 yr; maximal aerobic power [VO2max], 57.6 +/- 1.0 ml/kg per min) as compared with 10 sedentary control subjects (age, 28 +/- 2 yr; VO2max, 44.1 +/- 2.3 ml/kg per min). In the athletes, whole body glucose disposal (240-min insulin clamp) was 32% (P < 0.01) and nonoxidative glucose disposal (indirect calorimetry) was 62% higher (P < 0.01) than in the controls. Muscle glycogen content increased by 39% in the athletes (P < 0.05) but did not change in the controls during insulin clamp. VO2max correlated with whole body (r = 0.60, P < 0.01) and nonoxidative glucose disposal (r = 0.64, P < 0.001). In the athletes forearm blood flow was 64% greater (P < 0.05) than in the controls, whereas their muscle capillary density was normal. Basal blood flow was related to VO2max (r = 0.63, P < 0.05) and glucose disposal during insulin infusion (r = 0.65, P < 0.05). The forearm glucose uptake in the athletes was increased by 3.3-fold (P < 0.01) in the basal state and by 73% (P < 0.05) during insulin infusion. Muscle glucose transport protein (GLUT-4) concentration was 93% greater in the athletes than controls (P < 0.01) and it was related to VO2max (r = 0.61, P < 0.01) and to whole body glucose disposal (r = 0.60, P < 0.01). Muscle glycogen synthase activity was 33% greater in the athletes than in the controls (P < 0.05), and the basal glycogen synthase fractional activity was closely related to blood flow (r = 0.88, P < 0.001). IN CONCLUSION (a) athletes are characterized by enhanced muscle blood flow and glucose uptake. (b) The cellular mechanisms of glucose uptake are increased GLUT-4 protein content, glycogen synthase activity, and glucose storage as glycogen. (c) A close correlation between glycogen synthase fractional activity and blood flow suggests that they are causally related in promoting glucose disposal.

Effects of altered glucose homeostasis on glucose transporter expression in skeletal muscle of the rat.

Previous studies have suggested that alteration in the expression of the insulin-regulatable glucose transporter of muscle (GLUT-4 protein) may be an important determinant of insulin action. In the present studies, we have examined GLUT-4 mRNA and protein concentrations in muscle after variations in the metabolic status of the intact animal (i.e., 7 d streptozotocin-induced diabetes, 7 d insulin-induced hypoglycemia, and 3 d fasting). These changes in glucose homeostasis were associated with the following changes in GLUT-4 gene products: a decrease of approximately 30% in both mRNA and protein with diabetes; a 50% increase in mRNA and a 2.4-fold increase in protein with insulin injection; and normal mRNA in spite of a 2.7-fold increase in protein with fasting. Fasted diabetics exhibited an increase of 50% in GLUT-4 mRNA and a 2.4-fold increase in protein relative to fed diabetics. In diabetic and insulin-injected groups, the changes in GLUT-4 protein were similar to changes in mRNA, but in fasting, GLUT-4 protein increased without a concomitant change in mRNA. Overall there was no correlation between muscle concentrations of GLUT-4 protein and mRNA. Muscle GLUT-4 protein concentration tended to correlate with plasma glucose (r = -0.57, P less than 0.001), but not with plasma insulin. These results indicate that (a) chronic changes in glucose homeostasis are associated with changes in expression of GLUT-4 protein in muscle; (b) GLUT-4 protein increased in fasted soleus muscle without change in mRNA, thereby differing from fasted adipocytes in which both GLUT-4 products diminish; and (c) no simple relationship exists between total muscle GLUT-4 protein content and whole-body insulin sensitivity.

Glucose transporter gene expression in rat brain: Pretranslational changes associated with chronic insulin-induced hypoglycemia, fasting, and diabetes

Steady-state levels of the major glucose transporter gene (GLUT-1) of the brain were evaluated under three conditions that induced chronic changes in plasma glucose and insulin in adult rats: (i) repeated injection of insulin for 5 days, resulting in plasma glucose levels of 60-70 mg/dl for at least 3 days; (ii) fasting for 3 days; and (iii) moderate streptozotocin-induced diabetes of 1 week duration. Brain GLUT-1 mRNA was measured by dot blot hybridization with a HepG2/erythrocyte (GLUT1) [(32)P]cRNA probe, and GLUT-1 protein by immunoblot analysis with a polyclonal antibody (11493). Insulin injection resulted in hypoglycemia, increased GLUT-1 mRNA (143 +/- 15%, P < 0.05), and increased GLUT-1 protein (141 +/- 6%, P < 0.05). The increase in GLUT-1 mRNA was specific for brain, as no change was observed in liver or kidney. Fasting resulted in mild hypoglycemia, lower plasma insulin, increased GLUT-1 mRNA (131 +/- 17%, P < 0.05 vs control), and no change in GLUT-1 protein (125 +/- 9%, N.S.). Mild streptozotocin diabetes resulted in hyperglycemia, undetectable plasma insulin, decreased GLUT-1 mRNA (65 +/- 6%, P < 0.05 vs control), and no change in GLUT-1 protein (84 +/- 9%, N.S.). A negative correlation (r = -0.61, P < .0001) between GLUT-1 mRNA levels in brain and plasma glucose concentrations was observed among the three experimental groups and control animals, suggesting that the plasma glucose concentration may be at least one determinant of GLUT-1 levels in rat brain. The importance of these results is the finding that GLUT-1 gene expression in rat brain is regulated in vivo by the nutritional and endocrine status of the animal.

Level of skeletal muscle glucose transporter protein correlates with insulin-stimulated whole body glucose disposal in man

SummaryThe content of GLUT4 glucose transporter mRNA and protein were measured in samples of the vastus lateralis muscle of normal volunteers subjected to a 4-h hyperinsulinaemic, euglycaemic clamp. Plasma glucose concentration was clamped at 5.3±0.1 mmol/l, and serum insulin concentration was maintained at 740±5 pmol/l. Whole body glucose uptake averaged 38.3±2.2 μmol · kg−1 · min−1, 62% of this being due to disposal via non-oxidative pathways. A significant correlation existed between basal levels of GLUT4 protein and the rate of whole body glucose disposal (r=0.77, p<0.02) and non-oxidative glucose disposal (r=0.80, p<0.02). There was no correlation between GLUT4 protein content and oxidative glucose disposal (r=0.08, NS). These observations are consistent with an important role for skeletal muscle GLUT4 protein in whole body glucose disposal.

Athletes with IDDM exhibit impaired metabolic control and increased lipid utilization with no increase in insulin sensitivity.

Physical exercise is traditionally recommended to diabetic patients as part of their treatment. Although healthy athletes exhibit enhanced skeletal muscle insulin sensitivity, the metabolic effects of vigorous training in patients with insulin-dependent diabetes mellitus (IDDM) are not known. This study was designed to examine the effects of competitive sports on fuel homeostasis and insulin sensitivity in athletes with IDDM. We studied 11 athletes and 12 matched sedentary men with IDDM. In each subject, we measured glycemic control, insulin-stimulated glucose uptake in the whole body and forearm, rates of glucose and lipid oxidation, and muscle glycogen, glycogen synthase, and glucose transport protein (GLUT4) concentrations. The athletes had higher VO2max (52 ± 1 vs. 42 ± 1 ml.kg−1 · min−1, P < 0.001) and HbA1c levels (8.4 ± 0.4 vs. 7.2 ± 0.2%, P < 0.05) than sedentary patients, but took smaller insulin doses (41 ± 3 vs. 53 ± 3 U/day, P < 0.05). The insulin-stimulated rates of whole-body and forearm glucose uptake and glucose oxidation were similar in the two groups, whereas both energy expenditure and lipid oxidation were increased in the athletes. Lipid oxidation correlated inversely with glycogen synthase activity. The mean glucose arterialized venous blood-deep venous blood (A-V) difference during the insulin infusion (60-240 min) correlated with the whole-body glucose disposal throughout the insulin infusion (after 60 min, r > 0.73, P < 0.001 for all 30-min periods). This association is accounted for by the relationship between glucose A-V difference and nonoxidative glucose disposal. Muscle glycogen and GLUT4 protein contents were not different in the two groups. In conclusion, in athletes with IDDM: 1) competitive exercise performed at variable schedules and intensities leads to a decrease in required insulin dose, impairment of metabolic control, and increase in lipid utilization; 2) insulin sensitivity is not enhanced; and 3) glucose A-V difference, not blood flow, is the major determinant of body sensitivity to insulin. Thus, more intense glucose monitoring and education may be required for the maintenance of good control in patients with IDDM involved in competitive sports.

Effect of insulin on GLUT-4 mRNA and protein concentrations in skeletal muscle of patients with NIDDM and their first-degree relatives

SummaryWe examined whether insulin resistance, i. e. impaired insulin stimulated glucose uptake in NIDDM patients and their first-degree relatives is associated with alterations in the effect of insulin on the expression of the GLUT-4 gene in skeletal muscle in vivo. Levels of GLUT-4 mRNA and protein were measured in muscle biopsies taken before and after a euglycaemic insulin clamp from 14 NIDDM patients, 13 of their first-degree relatives and 17 control subjects. Insulin stimulated glucose uptake was decreased in the diabetic subjects (19.8±3.0 μmol · kg LBM−1 · min−1, both p<0.001) compared with control subjects (44.1±2.5 μmol · kg LBM−1 · min−1) and relatives (39.9±3.3 μmol · kg LBM−1 · min−1). Basal GLUT-4 mRNA levels were significantly higher in diabetic subjects and relatives compared to control subjects (99±8 and 108±9 pg/μg RNA vs 68±5 pg/μg RNA; both p<0.01). Insulin increased GLUT-4 mRNA levels in all control subjects (from 68±5 to 92±6 pg/ug RNA; p<0.0001), but not in the diabetic patients (from 99±8 to 90±8 pg/μg RNA, NS), or their relatives (from 94±9 to 101±11 pg/μg RNA, NS). In the relatives, individual basal GLUT-4 mRNA concentrations varied between 55 and 137 pg/μg RNA. Insulin-resistant (n=6, mean glucose uptake rate=30.6±3.4 μmol · kg LBM−1 · min−1) but not insulin-sensitive relatives (n=7, mean glucose uptake rate=47.4±3.2 μmol · kg LBM−1 · min−1) had higher basal GLUT-4 mRNA concentrations compared to control subjects (108±9 vs 68±5 pg/ug RNA, p<0.01). GLUT-4 protein content in muscle did not differ between the groups in the basal state and remained unchanged in all groups after insulin infusion. Neither insulin-stimulated GLUT-4 mRNA nor protein concentrations correlated with insulin-stimulated glucose uptake in any of the groups studied. We conclude, that impaired glucose uptake in NIDDM is not related to insulin-stimulated GLUT-4 mRNA or protein concentrations. Acute stimulation of GLUT-4 mRNA by insulin is altered in skeletal muscle of NIDDM patients and their first-degree relatives. This might be a consequence of chronic hyperinsulinaemia elevating basal GLUT-4 mRNA concentrations rather than the cause of insulin resistance.

Coordinate Reduction of Rat Pancreatic Islet Glucokinase and Proinsulin mRNA by Exercise Training

Exercise training results not only in enhanced insulin sensitivity but also in a reduction in insulin secretion. In this study, we examined the effects of exercise training on the expression of genes potentially related to insulin synthesis and glucose-stimulated insulin release by measuring pancreatic islet proinsulin, glucose-transporter (GLUT2), and glucokinase mRNAs. Female Wistar rats were subjected to 100 min of running at 25 m · min−1 up a 15% incline for 90 min/day for 6 days/wk for 3 wk. Pancreatic mRNA was evaluated by Northern- and dot-blot analysis with [32P]cRNA probes. We found no change in the pancreatic content of GLUT2 mRNA but found marked decreases in the content of proinsulin mRNA (78%, P < 0.005) and glucokinase mRNA (65%, P < 0.001). These results suggest that exercise modulates both islet glucose metabolism and insulin synthesis at the level of gene expression. Furthermore, there was a significant correlation between the decreases in glucokinase and proinsulin mRNA concentrations (r = 0.95, P < 0.001), suggesting that expression of these genes is regulated in parallel.

Differential expression of rat pancreatic islet Beta-cell glucose transporter (GLUT 2), proinsulin and islet amyloid polypeptide genes after prolonged fasting, insulin-induced hypoglycaemia and dexamethasone treatment

SummaryThe question posed by these studies was whether chronic adaptive changes in glucose-stimulated insulin secretion are accompanied by comparable changes in islet Betacell glucose transporter (GLUT 2) gene expression. Control, fasted (3-day), insulin-injected hypoglycaemic (5-day), and dexamethasone-treated (4-day) rats (n=5 for each condition), were studied. After fasting significant decrements in proinsulin mRNA/μg RNA (−32 %, p<0.05) and islet amyloid polypeptide mRNA/μg RNA (−44%, p<0.05) were observed, while there was no change in GLUT 2 mRNA/μg RNA (−13%, p>0.05). After insulin-induced hypoglycaemia, decrements in proinsulin mRNA/μg RNA (−49%, p<0.01) and islet amyloid polypeptide mRNA/μg RNA (−44 %, p<0.01) were also observed, with no change in islet GLUT 2 mRNA/μg RNA (−18 %, p>0.05). Dexamethasone treatment resulted in a marked stimulatory effect on proinsulin mRNA/μg RNA (+236%, p<0.001) and islet amyloid polypeptide mRNA/μg RNA (+221 %, p<0.01), while again there was no change in islet GLUT 2 mRNA/μg RNA (+0.3%, p>0.05). Quantitative immunoblot analysis with a GLUT 2 specific antibody revealed no change in islet GLUT 2 protein with fasting, but a small decrease (−39±11%) in islet GLUT2/μg protein after insulin-induced hypoglycaemia. These results do not support the hypothesis that chronic changes in glucose-stimulated insulin secretion are accompanied by changes in GLUT 2 expression. In contrast to the lack of correlation with GLUT 2, there was a striking correlation between proinsulin and islet amyloid polypeptide mRNAs for all experimental conditions (r=0.974, p<0.001). These results suggest common transcriptional or turnover regulatory mechanisms or both for proinsulin and islet amyloid polypeptide gene expression, which differ for GLUT 2 gene expression.