Sex-Specific Skeletal Muscle Gene Expression Responses to Exercise Reveal Novel Direct Mediators of Insulin Sensitivity Change.

BACKGROUND: Understanding the causal pathways, systems, and mechanisms through which exercise impacts human health is complex. This study explores molecular signaling related to whole-body insulin sensitivity (Si) by examining changes in skeletal muscle gene expression. The analysis considers differences by biological sex, exercise amount, and exercise intensity to identify potential molecular targets for developing pharmacologic agents that replicate the health benefits of exercise. METHODS: The study involved 53 participants from the STRRIDE I and II trials who completed eight months of aerobic training. Skeletal muscle gene expression was measured using Affymetrix and Illumina technologies, while pre- and post-training Si was assessed via an intravenous glucose tolerance test. A novel gene discovery protocol, integrating three literature-derived and data-driven modeling strategies, was employed to identify causal pathways and direct causal factors based on differentially expressed transcripts associated with exercise intensity and amount. RESULTS: In women, the transcription factor targets identified were primarily influenced by exercise amount and were generally inhibitory. In contrast, in men, these targets were driven by exercise intensity and were generally activating. Transcription factors such as ATF1, CEBPA, BACH2, and STAT1 were commonly activating in both sexes. Specific transcriptional targets related to exercise-induced Si improvements included TACR3 and TMC7 for intensity-driven effects, and GRIN3B and EIF3B for amount-driven effects. Two key signaling pathways mediating aerobic exercise-induced Si improvements were identified: one centered on estrogen signaling and the other on phorbol ester (PKC) signaling, both converging on the epidermal growth factor receptor (EGFR) and other relevant targets. CONCLUSIONS: The signaling pathways mediating Si improvements from aerobic exercise differed by sex and were further distinguished by exercise intensity and amount. Transcriptional adaptations in skeletal muscle related to Si improvements appear to be causally linked to estrogen and PKC signaling, with EGFR and other identified targets emerging as potential skeletal muscle-specific drug targets to mimic the beneficial effects of exercise on Si.

Mitochondrial lipid oxidation is impaired in cultured myotubes from obese humans.

OBJECTIVE: The skeletal muscle of obese humans is characterized by an inability to appropriately respond to alterations in substrate availability. The purpose of this study was to determine if this metabolic inflexibility with obesity is retained in mitochondria of human skeletal muscle cells raised in culture (HSkMC) and to identify potential mechanisms involved. DESIGN: Mitochondrial respiration was measured in permeabilized myotubes cultured from lean and obese individuals before and after a 24-h lipid incubation. RESULTS: Mitochondrial respiration (state 3) in the presence of lipid substrate (palmitoyl carnitine) increased by almost twofold after lipid incubation in HSkMC from lean, but not obese subjects, indicative of metabolic inflexibility with obesity. The 24-h lipid incubation increased mitochondrial DNA (mtDNA) copy number in HSkMC from lean subjects by +16% (P<0.05); conversely, mtDNA copy number decreased in myotubes cultured from obese individuals (-13%, P=0.06). When respiration data were normalized to mtDNA copy number and other indices of mitochondrial content (COX-IV protein content and CS activity), the significant treatment effects of lipid incubation persisted in the lean subjects, suggesting concomitant alterations in mitochondrial function; no similar adjustment was evident in HSkMC from obese individuals. CONCLUSION: These data indicate that the skeletal muscle of obese individuals inherently lacks metabolic flexibility in response to lipid exposure, which consists of an inability to increase mitochondrial respiration in the presence of lipid substrate and perhaps by an inability to induce mitochondrial proliferation.

A high-fat diet elicits differential responses in genes coordinating oxidative metabolism in skeletal muscle of lean and obese individuals.

CONTEXT: In lean individuals, increasing dietary lipid can elicit an increase in whole body lipid oxidation; however, with obesity the capacity to respond to changes in substrate availability appears to be compromised. OBJECTIVE: To determine whether the responses of genes regulating lipid oxidation in skeletal muscle differed between lean and insulin resistant obese humans upon exposure to a high-fat diet (HFD). DESIGN AND SETTING: A 5-d prospective study conducted in the research unit of an academic center. PARTICIPANTS: Healthy, lean (n = 12; body mass index = 22.1 ± 0.6 kg/m(2)), and obese (n=10; body mass index = 39.6 ± 1.7 kg/m(2)) males and females, between ages 18 and 30. INTERVENTION: Participants were studied before and after a 5-d HFD (65% fat). MAIN OUTCOME MEASURES: Skeletal muscle biopsies (vastus lateralis) were obtained in the fasted and fed states before and after the HFD and mRNA content for genes involved with lipid oxidation determined. Skeletal muscle acylcarnitine content was determined in the fed states before and after the HFD. RESULTS: Peroxisome proliferator activated receptor (PPAR) α mRNA content increased in lean, but not obese, subjects after a single high-fat meal. From Pre- to Post-HFD, mRNA content exhibited a body size × HFD interaction, where the lean individuals increased while the obese individuals decreased mRNA content for pyruvate dehydrogenase kinase 4, uncoupling protein 3, PPARα, and PPARγ coactivator-1α (P ≤ 0.05). In the obese subjects medium-chain acylcarnitine species tended to accumulate, whereas no change or a reduction was evident in the lean individuals. CONCLUSIONS: These findings indicate a differential response to a lipid stimulus in the skeletal muscle of lean and insulin resistant obese humans.

Muscle insulin receptor concentrations in obese patients post bariatric surgery: relationship to hyperinsulinemia.

OBJECTIVE: Obesity results in insulin resistance. Bariatric surgery for obese individuals induces weight loss, improves insulin sensitivity, and lowers insulin levels. We investigated the mechanisms of this improvement. DESIGN: Insulin receptor (IR) content, IR signaling, and adiponectin levels were measured in nine morbidly obese subjects before and after bariatric surgery. SUBJECTS: Seven female and two male, average age 44+/-2y, BMI >40 kg/m(2) and/or at least 100 lbs over ideal body weight, undergoing elective bariatric surgery. MEASUREMENTS: Before surgery BMI, fasting plasma glucose, adiponectin, and insulin levels were measured. A fasting muscle biopsy was obtained from the vastus lateralis for IR concentration and autophosphorylation activity measurements. These procedures were repeated 1 y after surgery. RESULTS: At 1 y after surgery, the subjects had lost an average of 48.3+/-5.6 kg (P<0.001), insulin sensitivity had significantly increased as determined by the minimal model (SI 0.72+/-0.18 vs 3.86+/-1.43, P<0.05), and IR content had increased two-fold in muscle (2.1+/-0.4 vs 4.3+/-0.7 ng/mg protein, P<0.01). The increase in IR content was related to fasting insulin levels. In the subjects with the lowest IR function, there was also an increase in IR function. Plasma adiponectin increased by 40% following weight loss (7.4+/-1.6 pre vs 10.3+/-1.3 mg/ml post, P<0.05). There was no significant change in muscle content of the IR inhibitor, PC-1. CONCLUSION: Increased IR content, most likely regulated by insulin levels, may be one contributor to the increased insulin sensitivity that occurs when morbidly obese patients undergo bariatric surgery.

Studies of a targeted risk reduction intervention through defined exercise (STRRIDE).

PURPOSE: The Studies of a Targeted Risk Reduction Intervention through Defined Exercise (STRRIDE) trial is a randomized controlled clinical trial designed to study the effects of exercise training regimens differing in dose (kcal.wk-1) and/or intensity (relative to peak VO2) on established cardiovascular risk factors and to investigate the peripheral biologic mechanisms through which chronic physical activity alters carbohydrate and lipid metabolism to result in improvements in these parameters of cardiovascular risk in humans. METHODS: We will recruit 384 subjects and randomly assign them to one of three exercise training regimens or to a sedentary control group. The recruiting goal is to attain a subject population that is 50% female and 30% ethnic minority. The overall strategy is to use graded exercise training regimens in moderately overweight subjects with impairments in insulin action and mild to moderate lipid abnormalities to investigate whether there are dose or intensity effects and whether adaptations in skeletal muscle (fiber type, metabolic capacity, and/or capillary surface area) account for improvements in insulin action and parameters of lipoprotein metabolism. We will study these variables before and after exercise training, and over the course of a 2-wk detraining period. The study sample size is chosen to power the study to examine differences in responses between subjects of different gender and ethnicity to exercise training with respect to the least sensitive parameter-skeletal muscle capillary density. RESULTS: The driving hypothesis is that improvements in cardiovascular risk parameters derived from habitual exercise are primarily mediated through adaptations occurring in skeletal muscle. CONCLUSION: Identification that amount and intensity of exercise matter for achieving general and specific health benefits and a better understanding of the peripheral mechanisms mediating the responses in carbohydrate and lipid metabolism to chronic physical activity will lead to better informed recommendations for those undertaking an exercise program to improve cardiovascular risk.

Plasma cholesteryl ester transfer protein activity is not linked to insulin sensitivity.

Plasma cholesteryl ester transfer protein (CETP) activity has been reported to decline during a hyperinsulinemic-euglycemic clamp. It has been suggested that this suppressive effect of acute hyperinsulinemia is linked to whole body insulin sensitivity, and that the insulin resistance that accompanies obesity leads to high plasma CETP activity found in obese subjects. In the present study, we used 2 experimental approaches to examine the putative link between CETP and insulin action. First, we examined if the clamp-induced suppression of plasma CETP activity is linked to whole body insulin sensitivity. Plasma CETP activity was measured at the beginning and end of a 2-hour hyperinsulinemic-euglycemic clamp in 18 nondiabetic individuals before and after an exercise training regimen that improved insulin sensitivity without weight loss. CETP activity decreased in response to the clamp procedure in 16 of 18 subjects, and on average, by 9% (P <.001). While training decreased plasma CETP activity (10%, P <.05), the improvement in insulin sensitivity had no statistical effect on the clamp-induced suppression of plasma CETP activity (training*clamp, P =.26). Second, we examined if insulin resistance is associated with an elevation in fasting plasma CETP activity when the influence of adiposity and diabetes were negated. Plasma CETP activity was measured in 41 women (12 insulin-sensitive lean; 8 insulin-resistant lean; 10 insulin-sensitive obese; 11 insulin-resistant obese). The level of insulin sensitivity had no significant effect on fasting plasma CETP activity, but CETP levels were 25% higher in obese subjects (P <.01). Thus, neither experimental approach provided evidence that plasma CETP levels are linked to insulin and insulin sensitivity. These data suggest that the elevated CETP activity found in obese patients is less associated with hyperinsulinemia and the accompanying insulin resistance, but rather is more related to some other metabolic complication of obesity.

Relationship between fat-to-fat-free mass ratio and decrements in leg strength after downhill running.

The purpose of this study was to determine whether greater body fat mass (FM) relative to lean mass would result in more severe muscle damage and greater decrements in leg strength after downhill running. The relationship between the FM-to-fat-free mass ratio (FM/FFM) and the strength decline resulting from downhill running (-11% grade) was investigated in 24 male runners [age 23.4 +/- 0.7 (SE) yr]. The runners were divided into two groups on the basis of FM/FFM: low fat (FM/FFM = 0.100 +/- 0.008, body mass = 68.4 +/- 1.3 kg) and normal fat (FM/FFM = 0.233 +/- 0.020, body mass = 76.5 +/- 3.3 kg, P < 0.05). Leg strength was reduced less in the low-fat (-0.7 +/- 1.3%) than in the normal-fat individuals (-10.3 +/- 1.5%) 48 h after, compared with before, downhill running (P < 0.01). Multiple linear regression analysis revealed that the decline in strength could be predicted best by FM/FFM (r2 = 0.44, P < 0.05) and FM-to-thigh lean tissue cross-sectional area ratio (r2 = 0.53, P < 0.05), with no additional variables enhancing the prediction equation. There were no differences in muscle glycogen, creatine phosphate, ATP, or total creatine 48 h after, compared with before, downhill running; however, the change in muscle glycogen after downhill running was associated with a higher FM/FFM (r = -0.56, P < 0.05). These data suggest that FM/FFM is a major determinant of losses in muscle strength after downhill running.

Diabetic groups as defined by ADA and NDDG criteria have a similar aerobic capacity, blood pressure and body composition. American Diabetes Association and National Diabetes Data Group.

AIMS/HYPOTHESIS: In 1997 the American Diabetes Association (ADA) published new categories for diabetes based on fasting plasma glucose that classified diabetes as a plasma glucose of 7.0 mmol/l, or more, rather than one of 7.8 mmol/l or more, as published previously by the National Diabetes Data Group (NDDG) in 1979. We compared the cardiovascular disease risk factors of subjects classified as having Type II (non-insulin-dependent) diabetes mellitus under the NDDG and ADA criteria. METHODS: We examined a database of approximately 3,700 men (40.4 +/- 11.5 years old) and distributed them into four categories: normal fasting plasma glucose (NFG) of less than 6.1 mmol/l, impaired (IFG) 6.1 to 7.0 mmol/l, ADA diabetic 7.0 to 7.8 mmol/l and NDDG diabetic of 7.8 mmol/l or more. RESULTS: Fasting glucose was 5.2 +/- 0.5, 6.4 +/- 0.2, 7.3 +/- 0.2 and 11.2 +/- 2.9 mmol/l for the subjects of the NFG, IFG, ADA and NDDG groups, respectively. Estimated treadmill VO2max was 41.4 +/- 8.0, 36.0 +/- 7.8, 32.2 +/- 7.6, 30.6 +/- 7.0 ml x kg(-1) x min(-1) in the NFG, IFG, ADA, and NDDG groups, respectively (NFG and IFG > ADA and NDDG: p < 0.05). The ADA and NDDG groups were also similar for resting and exercise blood pressure and body composition. Triglycerides and total: HDL cholesterol ratios were higher and LDL cholesterol concentration was lower, in the NDDG group than in all other groups (p < 0.05). Total and LDL cholesterol in the ADA and NDDG groups were similar. CONCLUSION/INTERPRETATION: The similarities in the aerobic capacities, blood pressure and body composition of the ADA and NDDG groups indicate that the decision to lower the cut-off from 7.8 mmol/l to 7.0 mmol/l blood glucose for the clinical classification of diabetes was appropriate. The ADA and NDDG groups, however, might not have identical risks for cardiovascular disease because of differences between total:HDL cholesterol ratios, circulating HDL cholesterol and triglyceride concentrations.

Substrate utilization during exercise in formerly morbidly obese women.

The purpose of this study was to compare substrate utilization during fasting and submaximal exercise in morbidly obese women after weight loss (WL) with that in weight-matched controls (C). WL were studied in the weight-stable condition approximately 24 mo after gastric bypass surgery. Energy intake (self-reported) and expenditure ((2)H(2)(18)O) were also compared. The respiratory exchange ratio during exercise at the same absolute (15 W) workload was significantly (P < or = 0.05) elevated in WL vs. C (0.90 +/- 0.02 vs. 0.83 +/- 0.03); this was reflected as lower fat utilization in WL (29.7 +/- 4.8 vs. 53.2 +/- 9.7% of energy from fat). Respiratory exchange ratio during exercise at the same relative (65% of maximal O(2) uptake) intensity was also significantly (P < 0.05) elevated in WL (0.96 +/- 0.01 vs. 0.89 +/- 0.02), and fat use was concomitantly depressed (12.4 +/- 3.0 vs. 34.3 +/- 9.9% of energy from fat). Resting substrate utilization, daily energy expenditure, and self-reported relative macronutrient intake did not differ between groups. These data suggest that lipid oxidation is depressed during physical activity in WL. This defect may, at least in part, contribute to a propensity for the development of morbid obesity.

Enhanced muscle insulin receptor autophosphorylation with short-term aerobic exercise training.

Exercise training improves insulin action in skeletal muscle, but the mechanisms of this effect are not completely understood. In particular, the role of the insulin receptor (IR) is unclear. We examined the IR and an enzyme indicative of oxidative capacity in muscle in relation to improved insulin action in 20 previously sedentary individuals before and after a 7-day program of moderate-intensity cycle ergometry. After training, insulin sensitivity increased 33% (6.20 +/- 0.91 vs. 8.22 +/- 1.12 min. microU(-1). ml(-1) mean +/- SE, pre- vs. posttraining, respectively, P < 0.05). The mitochondrial marker enzyme cytochrome c oxidase (COX) increased in vastus lateralis biopsies by 21% (P < 0.05). After training, IR autophosphorylation, determined by ELISA, was significantly increased by approximately 40% at insulin concentrations from 1 to 100 nM (P < 0.05). The training-induced improvements in IR autophosphorylation were significantly correlated with changes in muscle COX content (r = 0.65, P < 0.05). These studies indicate that, in this model of increased physical activity, improvements in IR function are an early adaptation to exercise in humans, are correlated with increases in muscle oxidative capacity, and likely contribute to the beneficial effects of exercise training on insulin action.

Effect of intense training on plasma leptin in male and female swimmers.

PURPOSE: The purpose of this study was to determine whether fasting plasma leptin concentration was altered with an increase in training volume in competitive male and female athletes. METHODS: Intercollegiate male (N = 9) and female (N = 12) swimmers were examined during the preseason and at two times during the mid-season (mid-season 1 and mid-season 2) when training volume was relatively high (33,000 m.wk(-1)). Body composition (hydrostatic weighing), energy intake and expenditure, and fasting plasma leptin concentration were measured. RESULTS: In the women, there was a significant (P < 0.05) decline in fat mass (2 kg) with the increase in training volume, which was not accompanied by a reduction in fasting leptin (12.8 +/- 1.5 vs 11.0 +/- 1.2 vs 11.0 +/- 1.5 ng.mL(-1) for preseason, mid-season 1, and mid-season 2, respectively). In the men, there were no significant changes in body composition, body mass, or fasting leptin (4.4 +/- 0.8 vs 4.3 +/- 0.8 vs 4.6 +/- 0.8 ng.mL(-1), respectively). CONCLUSION: These findings suggest 1) plasma leptin is not sensitive to an increase in training volume and 2) leptin may not be indicative of changes in fat mass with an increase in training volume in female athletes. These data suggest that leptin may not be useful in monitoring relative training stress in athletes.

Lipid oxidation is reduced in obese human skeletal muscle.

The purpose of this study was to discern cellular mechanisms that contribute to the suppression of lipid oxidation in the skeletal muscle of obese individuals. Muscle was obtained from obese [body mass index (BMI), 38.3 +/- 3.1 kg/m(2)] and lean (BMI, 23.8 +/- 0.9 kg/m(2)) women, and fatty acid oxidation was studied by measuring (14)CO(2) production from (14)C-labeled fatty acids. Palmitate oxidation, which is at least partially dependent on carnitine palmitoyltransferase-1 (CPT-1) activity, was depressed (P < 0.05) by approximately 50% with obesity (6.8 +/- 2.2 vs. 13.7 +/- 1.4 nmole CO(2).g(-1).h(-1)). The CPT-1-independent event of palmitoyl carnitine oxidation was also depressed (P < 0.01) by approximately 45%. There were significant negative relationships (P < 0.05) for adiposity with palmitate (r = -0.76) and palmitoyl carnitine (r = -0.82) oxidation. Muscle CPT-1 and citrate synthase activity, an index of mitochondrial content, were also significantly (P < 0.05) reduced ( approximately 35%) with obesity. CPT-1 (r = -0.48) and citrate synthase (r = -0.65) activities were significantly (P < 0.05) related to adiposity. These data suggest that lesions at CPT-1 and post-CPT-1 events, such as mitochondrial content, contribute to the reduced reliance on fat oxidation evident in human skeletal muscle with obesity.

Association between muscle fiber composition and blood pressure levels during exercise in men.

Normotensive individuals with a magnified blood pressure (BP) level during exercise have an increased risk for developing hypertension. The purpose of this study was to determine if skeletal muscle fiber type is related to the BP level during exercise. Peak BP was determined in 35 normotensive, middle-aged (mean +/- SE, 46.0 +/- 1.8 years) men during maximal treadmill exercise. Fiber distribution (I, IIa, IIb) was measured in muscle samples (percutaneous needle biopsy) from the vastus lateralis and lateral gastrocnemius. The systolic BP during exercise was significantly (P < .05) related to the percentage of type IIb fibers in both the vastus lateralis (r = 0.37) and gastrocnemius (r = 0.38). Mean arterial pressure BP was also related to the percentage of type IIb fibers in the gastrocnemius (r = 0.39, P < .05), with a similar trend evident in the vastus lateralis (r = 0.31, P = 0.08). The percentage of type IIb muscle fibers in both muscle groups was associated with (P < .05) body fat (vastus lateralis, r = 0.44; gastrocnemius, r = 0.43). There were no relationships between the relative percentage of type I or IIa fibers with any BP parameters. Maximal oxygen consumption was negatively related to BP, but only when expressed relative to body weight (mL x kg(-1) x min(-1)). These data suggest that muscle morphology is related to the blood pressure level during exercise and provides insight into factors that may predispose individuals toward the development of hypertension and cardiovascular disease.

Effect of short-term exercise training on leptin and insulin action.

The purpose of the study was to determine the effect of short-term exercise training (7 consecutive days for 60 min/d at 75% maximal oxygen consumption [VO2 max]), which did not change body mass on fasting plasma leptin concentration and insulin action. Young, lean subjects (n = 16; age, 21.9 +/- 0.6 years; body fat, 17.5% +/- 1.5%) and older subjects with relatively more adipose tissue (n = 14; age, 58.6 +/- 1.4 years; body fat, 28.3% +/- 1.3%) were studied (mean +/- SE). Fasting plasma leptin was significantly (P < .05) related to adiposity (fat mass, r = .58; % body fat, r = .76) in this population. Body mass did not change (P < .05) in any of the groups with training (71.8 +/- 2.5 v 71.9 +/- 2.5 kg). The insulin sensitivity index (SI determined from an intravenous glucose tolerance test (IVGTT) improved significantly (P < .05) in both the young group (4.8 +/- 0.6 v6.9 +/- 0.8 x 10(-4)/ min (microU/mL) and the older group (3.2 +/- 0.6 v 5.9 +/- 1.0 x 10(-4)/min (microU/mL)). Fasting leptin did not change with training in either group (10.4 +/- 1.6 v 9.2 +/- 1.0 ng/mL). These findings suggest that exercise does not independently affect the fasting plasma leptin concentration and the improvement in insulin action with exercise is not associated with an alteration in fasting leptin in healthy sedentary lean and relatively lean subjects.

Impact of insulin resistance on lipoprotein subpopulation distribution in lean and morbidly obese nondiabetic women.

The purpose of this study was to examine the effects of insulin resistance on the lipoprotein subpopulation distribution of very-low-density, low-density, and high-density lipoproteins (VLDL, LDL, and HDL) in lean and morbidly obese nondiabetic women. Lean women (body mass index [BMI], 20 to 27 kg/m2) stratified by BMI were divided into insulin-sensitive (SL, n = 12) and insulin-resistant (RL, n = 8) groups according to Bergman's minimal model, SI. A group of obese women (BMI, 30 to 53 kg/m2), also stratified by BMI, were divided into insulin-sensitive (SO, n = 10) and insulin-resistant (RO, n = 11) groups in a similar fashion. Resistant groups were similar to sensitive groups (SL v RL and SO vRO) in age, weight, percent body fat, and waist circumference, ie, total and regional adiposity. VLDL, LDL, and HDL subpopulation distributions were determined in fasting plasma samples by nuclear magnetic resonance (NMR) spectroscopy. The average particle sizes of all 3 classes of lipoproteins were similar for the SL and RL groups. In contrast, RO subjects had larger VLDL, smaller LDL, and smaller HDL, than SO subjects (P < .05). Lower concentrations of large LDL and large HDL were found in RO compared with SO subjects (P < .05). In obese women, but not in lean women, VLDL size was associated with plasma insulin (r = .60, P < .005), while LDL size and HDL size were negatively correlated with plasma insulin (r = -.39, P < .05 and r = -.38, P < .05) and positively correlated with SI (r = .54, P < .01 and r = .42, P < .05). These results suggest that in obese women, insulin resistance may be involved in the formation of lipoprotein subpopulation distributions that are associated with vascular disease.

Effect of short-term exercise training on insulin-stimulated PI 3-kinase activity in human skeletal muscle.

The purpose of this study was to determine if the improvement in insulin sensitivity with exercise training is associated with enhanced phosphatidylinositol 3-kinase (PI 3-kinase) activity. Nine sedentary men were studied before and after 7 days of exercise training (1 h/day, approximately 75% maximal oxygen consumption). Insulin sensitivity was determined with a euglycemic-hyperinsulinemic glucose clamp in the sedentary state and 15-17 h after the final exercise bout. PI 3-kinase activity was determined from samples (vastus lateralis) obtained in the fasted condition and after 60 min of submaximal insulin stimulation during the clamp. After exercise, glucose infusion rate increased (P < 0. 05) significantly (means +/- SE, 7.8 +/- 0.5 vs. 9.8 +/- 0.8 mg. kg(-1). min(-1)), indicating improved insulin sensitivity. Insulin-stimulated (insulin stimulated/fasting) phosphotyrosine immunoprecipitable PI 3-kinase activity also increased significantly (P < 0.05) with exercise (3.1 +/- 0.8-fold) compared with the sedentary condition (1.3 +/- 0.1-fold). There was no change in fasting PI 3-kinase activity. These data suggest that an enhancement of insulin signal transduction in skeletal muscle may contribute to the improvement in insulin action with exercise.

Impact of hyperinsulinemia on myosin heavy chain gene regulation.

The purpose of this study was to determine whether hyperinsulinemia alters myosin heavy chain (MHC) gene expression in human skeletal muscle. A biopsy from the vastus lateralis was obtained in young, lean [age 24.6 +/- 1.0 (SE) yr, body fat 11.9 +/- 1.9%, body mass index 26.1 +/- 1.1 kg/m2; n = 10] men before and after 3 h of hyperinsulinemia (hyperinsulinemic-euglycemic clamp). Muscle was analyzed for mRNA of type I, IIa, and IIx MHC isoforms. Hyperinsulinemia (mean of 1,065.7 +/- 9.8 pmol/l during minutes 20 to 180) did not change (P > 0.05) the mRNA concentration of either the type I MHC or type IIA MHC isoforms. In contrast, type IIX MHC mRNA increased (P < 0.05) with hyperinsulinemia compared with the fasted condition. These data indicate that hyperinsulinemia rapidly increases type IIx MHC mRNA in human skeletal muscle.