Increased metabolic rate and insulin sensitivity in male mice lacking the carcino-embryonic antigen-related cell adhesion molecule 2.

AIMS/HYPOTHESIS: The carcino-embryonic antigen-related cell adhesion molecule (CEACAM)2 is produced in many feeding control centres in the brain, but not in peripheral insulin-targeted tissues. Global Ceacam2 null mutation causes insulin resistance and obesity resulting from hyperphagia and hypometabolism in female Ceacam2 homozygous null mutant mice (Cc2 [also known as Ceacam2](-/-)) mice. Because male mice are not obese, the current study examined their metabolic phenotype. METHODS: The phenotype of male Cc2(-/-) mice was characterised by body fat composition, indirect calorimetry, hyperinsulinaemic-euglycaemic clamp analysis and direct recording of sympathetic nerve activity. RESULTS: Despite hyperphagia, total fat mass was reduced, owing to the hypermetabolic state in male Cc2(-/-) mice. In contrast to females, male mice also exhibited insulin sensitivity with elevated ß-oxidation in skeletal muscle, which is likely to offset the effects of increased food intake. Males and females had increased brown adipogenesis. However, only males had increased activation of sympathetic tone regulation of adipose tissue and increased spontaneous activity. The mechanisms underlying sexual dimorphism in energy balance with the loss of Ceacam2 remain unknown. CONCLUSIONS/INTERPRETATION: These studies identified a novel role for CEACAM2 in the regulation of metabolic rate and insulin sensitivity via effects on brown adipogenesis, sympathetic nervous outflow to brown adipose tissue, spontaneous activity and energy expenditure in skeletal muscle.

Postmarathon paradox: insulin resistance in the face of glycogen depletion.

Acute physical exercise enhances insulin sensitivity in healthy subjects. We examined the effect of a 42-km marathon run on insulin sensitivity and lipid oxidation in 19 male runners. In the morning after the marathon run, basal serum free fatty acid concentration was 2.2-fold higher, muscle glycogen content 37% lower (P < 0.01), glycogen synthase fractional activity 56% greater (P < 0.01), and glucose oxidation reduced by 43% (P < 0.01), whereas lipid oxidation was increased by 55% (P < 0.02) compared with the control study. During euglycemic-hyperinsulinemic clamp, whole body glucose disposal was decreased by 12% (P < 0.01) because of a 36% lower glucose oxidation rate (P < 0.05), whereas the rate of lipid oxidation was 10-fold greater (P < 0.02) than in the control study. After the marathon, muscle glycogen content correlated positively with lipid oxidation (r = 0.60, P < 0.05) and maximal aerobic power (Vo2peak; r = 0.61, P < 0.05). Vo2peak correlated positively with basal lipid oxidation (r = 0.57, P < 0.05). In conclusion, 1) after the marathon run, probably because of increased lipid oxidation, the insulin-stimulated glucose disposal is decreased despite muscle glycogen depletion and the activation of glycogen synthase; 2) the contribution of lipid oxidation in energy expenditure is increased in proportion to physical fitness; 3) these adaptations of fuel homeostasis may contribute to the maintenance of physical performance after prolonged exercise.

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of exercise training on alpha-adrenergic mediated pressor responses and baroreflex function in older subjects.

Endurance exercise training increases maximal O2 uptake (VO2max) in older subjects, and training also improves cardiac function in older men. Although the effects of training on beta-adrenergic responses have been investigated, little information is available regarding the effects of alpha-adrenergic responses and baroreflex function in older men and women. The purpose of the study was to determine whether endurance exercise training can affect alpha-adrenergic responses and baroreflex function in the elderly. We studied 13 men and women, 63 +/- 4 yrs old (mean +/- SE). VO2max was determined during treadmill exercise. Baroreflex function was determined from the change in heart rate (HR) relative to the change in systolic blood pressure (delta HR/delta SBP) during infusion of phenylephrine. VO2max was increased by 23% (1.9 +/- 0.16 vs 2.34 +/- 0.20 l/min; p < .01) in response to training. Maximal heart rate did not change, but HR during submaximal exercise at the same absolute exercise intensity was 17% lower after training. Resting heart rate was slower in the trained state. During alpha-adrenergic stimulation induced by graded doses of phenylephrine infusion, heart rate was lower after training because of training-induced bradycardia at rest. However, the elevation in systolic blood pressure (delta SBP) and mean blood pressure (delta MBP) from basal levels in response to a given dose of phenylephrine were significantly larger (delta SBP:18 +/- 3 vs 26 +/- 3 mmHg, p < .01; and delta MBP 10 +/- 2 vs 15 +/- 3 mmHg, p < .01) after than before training.(ABSTRACT TRUNCATED AT 250 WORDS)

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

We 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 mumol.kg LBM-1.min-1, both p < 0.001) compared with control subjects (44.1 +/- 2.5 mumol.kg LBM-1.min-1) and relatives (39.9 +/- 3.3 mumol.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/micrograms RNA vs 68 +/- 5 pg/micrograms RNA; both p < 0.01). Insulin increased GLUT-4 mRNA levels in all control subjects (from 68 +/- 5 to 92 +/- 6 pg/micrograms RNA; p < 0.0001), but not in the diabetic patients (from 99 +/- 8 to 90 +/- 8 pg/micrograms RNA, NS), or their relatives (from 94 +/- 9 to 101 +/- 11 pg/micrograms RNA, NS). In the relatives, individual basal GLUT-4 mRNA concentrations varied between 55 and 137 pg/micrograms RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Relationship between glucose tolerance and glucose-stimulated insulin response in 65-year-olds.

BACKGROUND: Decreased insulin secretion may contribute to the deterioration of glucose tolerance associated with aging. METHODS: We studied the insulin response to a 3-hour hyperglycemic clamp (10 mM) of 19 young (24 +/- 1 y) subjects with normal glucose tolerance and 60 older (65 +/- 1 y) subjects with various levels of glucose tolerance. RESULTS: The noninsulin dependent diabetic (NIDDM) group had a diminished first phase immunoreactive (IR)-insulin response compared to young and nondiabetic older groups (p < .05). The older groups had a lower rate of change in IR insulin concentration during the third hour of hyperglycemia compared to the young group (p < .05). This was not, however, a universal finding, because a decreased third hour response was not seen in a subgroup of older subjects whose glucose tolerance was similar to that of the young group. Another subgroup of older subjects with a decrease in glucose tolerance mild enough to be considered normal by the National Diabetes Group Criteria tended to have both an increase in the early insulin response and a decrease in the third hour response. More severe decreases in glucose tolerance were associated with blunting of the early response. CONCLUSION: Aberrations in early and late phase glucose-stimulated insulin responses appear to be present in older subjects with even mildly decreased glucose tolerance. Some individuals, however, show no evidence of deterioration of glucose tolerance or insulin response to glucose with aging, at least up to age 70 years.

Endurance exercise training reduces glucose-stimulated insulin levels in 60- to 70-year-old men and women.

BACKGROUND: Aging is frequently associated with development of insulin resistance and deterioration of glucose tolerance. Plasma glucose and insulin concentrations tend to be higher than in young people, even in those older individuals whose glucose tolerance is within the normal range. A sedentary life style could play a role in the development of insulin resistance and hyperinsulinemia with advancing age. METHODS: We evaluated the effect of 9 mo of vigorous endurance exercise training (approximately 80% of maximal heart rate) on the glucose-stimulated insulin response and glucose disposal rate, using the hyperglycemic clamp procedure, in 12 people aged 65 +/- 1 yr (mean +/- SE) with normal glucose tolerance. The post-training hyperglycemic clamps were performed approximately 16 h after a usual exercise session. RESULTS: VO2max increased approximately 23% in response to the exercise program. The plasma insulin concentration (I) during hyperglycemia (180 mg.dL-1) was significantly lower (mean 36 +/- 6 microU.mL-1 before vs 26 +/- 5 microU.mL-1 after; p < .05) after the exercise program. Insulin action was improved by the exercise, as the glucose disposal rate (M) during hyperglycemia was unchanged despite the blunted insulin response, resulting in an increase in the M/I ratio from 24 +/- 5 to 30 +/- 5 (p < .05), a value similar to the M/I ratio of 33 +/- 4 found in normally active young subjects. CONCLUSION: These results provide evidence that regular exercise is effective in reducing hyperinsulinemia and improving insulin action in 65-yr-olds to levels typical of young people.

Exercise reduces muscle glucose transport protein (GLUT-4) mRNA in type 1 diabetic patients.

We examined the effect of acute exercise on muscle glucose transporter (GLUT-4) protein and mRNA concentrations in nine male type 1 diabetic patients (age 31 +/- 3 yr, body mass index 23.6 +/- 0.7 kg/m2, insulin dose 44 +/- 4 U/day, glycosylated hemoglobin 7.8 +/- 0.4%) and in nine healthy control subjects (34 +/- 1 yr, 25.3 +/- 0.8 kg/m2). Three hours of cycle ergometer exercise was performed after an overnight fast. A needle biopsy (100-150 mg) was taken from the quadriceps femoris 40 min before and immediately after the end of exercise. During exercise, plasma glucose, insulin, cortisol, and growth hormone concentrations were higher in the diabetic patients than in the control subjects. In the basal state, GLUT-4 protein and mRNA concentrations were similar in the two groups. During exercise, GLUT-4 mRNA concentration decreased by 30-45% in the diabetic patients but remained unchanged in the control subjects. GLUT-4 protein content remained unchanged in both groups. These data suggest an abnormal GLUT-4 mRNA production or degradation or both in type 1 diabetic patients during physical exercise.

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 GDRs at a plasma insulin concentration of approximately 450 pM averaged 45.6 +/- 3.3 mumol.kg FFM-1 x min-1 (mean +/- SE) in the young subjects, 45.6 +/- 10.0 mumol.kg FFM-1 x min-1 in 24 older subjects who were insulin sensitive, and 23.9 +/- 11.7 mumol.kg FFM-1 x 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.

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.

The question posed by these studies was whether chronic adaptive changes in glucose-stimulated insulin secretion are accompanied by comparable changes in islet Beta-cell 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/microgram RNA (-32%, p < 0.05) and islet amyloid polypeptide mRNA/microgram RNA (-44%, p < 0.05) were observed, while there was no change in GLUT 2 mRNA/microgram RNA (-13%, p > 0.05). After insulin-induced hypoglycaemia, decrements in proinsulin mRNA/microgram RNA (-49%, p < 0.01) and islet amyloid polypeptide mRNA/microgram RNA (-44%, p < 0.01) were also observed, with no change in islet GLUT 2 mRNA/microgram RNA (-18%, p > 0.05). Dexamethasone treatment resulted in a marked stimulatory effect on proinsulin mRNA/microgram RNA (+236%, p < 0.001) and islet amyloid polypeptide mRNA/microgram RNA (+221%, p < 0.01), while again there was no change in islet GLUT 2 mRNA/microgram 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 GLUT 2/microgram 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).(ABSTRACT TRUNCATED AT 250 WORDS)

Defect in insulin action on expression of the muscle/adipose tissue glucose transporter gene in skeletal muscle of type 1 diabetic patients.

Recently several members of the glucose transporter family have been identified by molecular cloning techniques. We determined the effect of a 4-h insulin infusion on the expression of the muscle/adipose tissue (GLUT-4) glucose transporter mRNA and protein in 14 insulin-treated type 1 diabetic patients and 15 matched nondiabetic subjects. GLUT-4 mRNA and protein concentrations were determined in muscle biopsies taken before and at the end of the insulin infusion during maintenance of normoglycemia. In response to insulin, muscle GLUT-4 mRNA increased in the nondiabetic subjects from 24 +/- 3 to 36 +/- 4 pg/microgram RNA (P less than 0.001) but remained unchanged in the insulin-resistant diabetic patients (24 +/- 2 vs. 26 +/- 2 pg/microgram RNA, before vs. after insulin). The glucose transporter protein concentrations were similar in the basal state and decreased by 21 +/- 7% (P less than 0.02) in the normal subjects but remained unchanged in the diabetic patients. The increase of the GLUT-4 mRNA and the decrease in the GLUT-4 protein correlated with the rate of glucose uptake [correlation coefficient (r) = -0.55, P less than 0.01, and r = -0.44, P less than 0.05, respectively]. We conclude that the insulin response of both the GLUT-4 glucose transporter mRNA and protein are absent in skeletal muscle of insulin-resistant type 1 diabetic patients. Thus, impaired insulin regulation of glucose transporter gene expression can be one of the underlying mechanisms of insulin resistance in type 1 diabetes.

A variant insulin promoter in non-insulin-dependent diabetes mellitus.

To test the hypothesis that alterations in regulatory regions of the insulin gene occur in a subset of patients with non-insulin-dependent diabetes mellitus (NIDDM), the promoter region was studied by polymerase chain reaction (PCR) amplification directly from genomic DNA, followed by high-resolution polyacrylamide gel electrophoresis under nondenaturing conditions. By using this method a previously identified HincII polymorphism (GTTGAC to GTTGAG at position-56) in American Blacks was readily detected, indicating that single base changes could be observed. In the course of screening the insulin promoter from 40 American Black subjects with NIDDM, an apparent larger allele was found in two individuals. Both patients were shown to have in addition to a normal allele, a larger allele containing an 8-bp repeat, TGGTCTAA from positions -322 to -315 of the insulin promoter. To facilitate rapid screening for the 8-bp repeat, a high-resolution agarose gel electrophoretic analysis was adopted. DNA from American Black NIDDM subjects (n = 100) and nondiabetic subjects (n = 100) was PCR amplified and analyzed. The 8-bp repeat was present in five NIDDM subjects, and one nondiabetic subject. DNA from Mauritius Creoles, also of African ancestry, was analyzed, and the 8-bp repeat was present in 3 of 41 NIDDM subjects, and 0 of 41 nondiabetic subjects. Analysis of glucose metabolism in three presumed normal sibs of an NIDDM patient with an 8-bp repeat revealed that one sib had overt diabetes, and two sibs were glucose intolerant, but there was no consistent segregation of the insulin promoter variant with the diabetes phenotype. The variant promoter was not present in 35 Caucasian NIDDM patients or in 40 Pima Indians. To test the biological consequences of the 8-bp repeat sequence in the insulin promoter, a normal and variant promoter were subcloned into a luciferase plasmid, and reporter gene activity assessed by transient transfection into mouse insulinoma (beta TC1) and hamster insulinoma (HIT) cells. The promoter activity of the variant allele was found to be reduced to 37.9 +/- 10.3% of the activity of the normal promoter in HIT cells (P less than 0.01, n = 4), and 49.1 +/- 6.4% in beta TC1 cells (P less than 0.01, n = 6). These data thus suggest that a naturally occurring variant of the insulin promoter may contribute to the diabetes phenotype in 5-6% of Black NIDDM patients.

Insulin resistance in type 2 (non-insulin-dependent) diabetic patients and their relatives is not associated with a defect in the expression of the insulin-responsive glucose transporter (GLUT-4) gene in human skeletal muscle.

To study whether insulin resistance in Type 2 (non-insulin-dependent) diabetes mellitus is due to a defect in the expression of the insulin-responsive glucose transporter gene (GLUT-4) in human skeletal muscle, we measured the level of GLUT-4 mRNA and (in some of the subjects) its protein in muscle biopsies taken from 14 insulin-resistant patients with Type 2 diabetes, 10 first-degree relatives of the diabetic patients and 12 insulin-sensitive control subjects. Insulin sensitivity was measured with a + 45 mU.m2(-1).min-1 euglycaemic insulin clamp in combination with indirect calorimetry and infusion of [3-3H]glucose. GLUT-4 mRNA was measured using a human GLUT-4 cDNA probe and GLUT-4 protein with a polyclonal antibody specific for the 15 amino acid carboxy-terminal peptide. Both Type 2 diabetic patients and their relatives showed impaired stimulation of total-body glucose disposal by insulin compared with control subjects (29.5 +/- 2.1 and 34.0 +/- 4.8 vs 57.9 +/- 3.1 mumol.kg lean body mass-1.min-1; p less than 0.01). This impairment in glucose disposal was primarily accounted for by a reduction in insulin-stimulated storage of glucose as glycogen (13.0 +/- 2.4 and 15.6 +/- 3.9 vs 36.9 +/- 2.2 mumol.kg lean body mass-1.min-1; p less than 0.01). The levels of GLUT-4 mRNA expressed both per microgram of total RNA and per microgram DNA, were higher in the diabetic patients compared with the control subjects (116 +/- 25 vs 53 +/- 10 pg/microgram RNA and 177 +/- 35 vs 112 +/- 29 pg/microgram DNA; p less than 0.05, p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of gender, age, and fitness level on response of VO2max to training in 60-71 yr olds.

The adaptive response of maximal aerobic power (VO2max) to endurance exercise training was compared in 53 men and 57 women, aged 60-71 yr. The subjects were healthy and had been sedentary for at least 2 yr. Pretraining VO2max was measured during graded treadmill walking on two occasions. These values were reproducible (24.4 +/- 4.7 vs. 24.4 +/- 4.6 (SD) ml.min-l.kg-1; r = 0.96). Subjects trained primarily by walking and running for 9-12 mo, averaging 3.9 +/- 0.6 days/wk and 45 +/- 5 min/day at 80 +/- 5% of maximal heart rate (HRmax). Average improvement in VO2max (ml.min-1.kg-1) was 24 +/- 12% (range 0-58%). Relative improvement was not significantly different in men and women (26 +/- 12 vs. 23 +/- 12%, ml.min-1.kg-1; 21 +/- 10 vs 19 +/- 10%, l/min). When subjects were divided into three groups by age (60-62, 63-66, 67-71 yr), there were no significant differences among the groups in the relative increase in VO2max (21% vs. 19% vs. 18%, 1/min). Correlation analysis also yielded a nonsignificant relationship between improvement and age (r = -0.13). To examine the effect of initial fitness level on the adaptive response to exercise, pretraining VO2max was correlated with the absolute improvement in VO2max. This relationship was not significant in either men (r = 0.04) or women (r = -0.23). In conclusion, in healthy people aged 60-71 yr, VO2max adapts to endurance exercise training to the same relative extent as in young people, and this adaptation is independent of gender, age, and initial level of fitness.

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

The 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 mumol.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 less than 0.02) and non-oxidative glucose disposal (r = 0.80, p less than 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.

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 less than 0.005) and glucokinase mRNA (65%, P less than 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 less than 0.001), suggesting that expression of these genes is regulated in parallel.

Effects of treadmill exercise to exhaustion on the insulin response to hyperglycemia in untrained men.

The effects of a single bout of exercise to exhaustion on pancreatic insulin secretion were determined in seven untrained men by use of a 3-h hyperglycemic clamp with plasma glucose maintained at 180 mg/100 ml. Clamps were performed either 12 h after an intermittent treadmill run at approximately 77% maximum O2 consumption or without prior exercise. Arterialized blood samples for glucose, insulin, and C-peptide determination were obtained from a heated hand vein. The peak insulin response during the early phase (0-10 min) of the postexercise clamp was higher (81 +/- 8 vs. 59 +/- 9 microU/ml; P less than 0.05) than in the nonexercise clamp. Incremental areas under the insulin (376 +/- 33 vs. 245 +/- 51 microU.ml-1.min) and C-peptide (17 +/- 2 vs. 12 +/- 1 ng.ml-1.min) curves were also greater (P less than 0.05) during the early phase of the postexercise clamp. No differences were observed in either insulin concentrations or whole body glucose disposal during the late phase (15-180 min). Area under the C-peptide curve was greater during the late phase of the postexercise clamp (650 +/- 53 vs. 536 +/- 76 ng.ml-1.min, P less than 0.05). The exercise bout induced muscle soreness and caused an elevation in plasma creatine kinase activity (142 +/- 32 vs. 305 +/- 31 IU/l; P less than 0.05) before the postexercise clamp. We conclude that in untrained men a bout of running to exhaustion increased pancreatic beta-cell insulin secretion during the early phase of the hyperglycemic clamp. Increased insulin secretion during the late phase of the clamp appeared to be compensated by increased insulin clearance.

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.

Effect of exercise on glucose disposal: response to a maximal insulin stimulus.

We used the euglycemic clamp to assess the effects of exercise on maximally insulin-stimulated glucose disposal. In 11 young men, a 60-min bout of exercise had no significant effect on the rate of glucose disposal during a euglycemic clamp performed approximately 30 min postexercise in which plasma insulin was raised to approximately 2,500 microU/ml (a maximal insulin stimulus). The maximal rate of glucose disposal attained during the clamp averaged 15.7 +/- 1.0 mg.kg lean body mass-1.min-1 after exercise vs. a control value of 15.4 mg.kg lean body mass-1.min-1. In a second experiment, eight men performed supine cycle exercise during the 3rd h of a 4-h euglycemic clamp with a plasma insulin concentration of approximately 2,500 microU/ml. Exercise during the hyperinsulinemic clamp resulted in a 70% increase in glucose disposal rate. There was no measurable increase in glucose 6-phosphate in the quadriceps muscle during the insulin infusion at rest. We conclude that prior exercise does not enhance maximally insulin-stimulated glucose disposal in young healthy men. Our results are compatible with the interpretations that glucose availability rather than glucose metabolism limits the rate of glucose disposal in response to a maximal insulin stimulus in resting subjects and that the increase in glucose uptake in response to superimposed exercise is primarily due to an increase in glucose availability.