A low dose of dietary resveratrol partially mimics caloric restriction and retards aging parameters in mice

Resveratrol in high doses has been shown to extend lifespan in some studies in invertebrates and to prevent early mortality in mice fed a high-fat diet. We fed mice from middle age (14-months) to old age (30-months) either a control diet, a low dose of resveratrol (4.9 mg kg(-1) day(-1)), or a calorie restricted (CR) diet and examined genome-wide transcriptional profiles. We report a striking transcriptional overlap of CR and resveratrol in heart, skeletal muscle and brain. Both dietary interventions inhibit gene expression profiles associated with cardiac and skeletal muscle aging, and prevent age-related cardiac dysfunction. Dietary resveratrol also mimics the effects of CR in insulin mediated glucose uptake in muscle. Gene expression profiling suggests that both CR and resveratrol may retard some aspects of aging through alterations in chromatin structure and transcription. Resveratrol, at doses that can be readily achieved in humans, fulfills the definition of a dietary compound that mimics some aspects of CR.

Stimulation of glucose transport in skeletal muscle by hypoxia

Hypoxia caused a progressive cytochalasin B-inhibitable increase in the rate of 3-O-methylglucose transport in rat epitrochlearis muscles to a level approximately six-fold above basal. Muscle ATP concentration was well maintained during hypoxia, and increased glucose transport activity was still present after 15 min of reoxygenation despite repletion of phosphocreatine. However, the increase in glucose transport activity completely reversed during a 180-min-long recovery in oxygenated medium. In perfused rat hindlimb muscles, hypoxia caused an increase in glucose transporters in the plasma membrane, suggesting that glucose transporter translocation plays a role in the stimulation of glucose transport by hypoxia. The maximal effects of hypoxia and insulin on glucose transport activity were additive, whereas the effects of exercise and hypoxia were not, providing evidence suggesting that hypoxia and exercise stimulate glucose transport by the same mechanism. Caffeine, at a concentration too low to cause muscle contraction or an increase in glucose transport by itself, markedly potentiated the effect of a submaximal hypoxic stimulus on sugar transport. Dantrolene significantly inhibited the hypoxia-induced increase in 3-O-methylglucose transport. These effects of caffeine and dantrolene suggest that Ca2+ plays a role in the stimulation of glucose transport by hypoxia.

Increased phosphorylation of Akt substrate of 160 kDa (AS160) in rat skeletal muscle in response to insulin or contractile activity

In 3T3-L1 adipocytes, insulin-stimulated GLUT4 translocation requires phosphorylation of the protein designated Akt substrate of 160 kDa (AS160). Both insulin and contractions activate Akt in skeletal muscle. Therefore, we assessed the effects in skeletal muscle of each stimulus on phosphorylation of proteins, including AS160, on the Akt phosphomotif. Isolated rat epitrochlearis muscles were incubated with insulin (for time course and dose response), stimulated to contract, or incubated with 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) and used to assess the following: serine-phosphorylation of Akt (P-Akt), immunoreactivity with an antibody recognizing the Akt phosphomotif (alpha-phospho-[Ser/Thr] Akt substrate [PAS]), and PAS immunoreactivity of samples immunoprecipitated with anti-AS160. P-Akt peaked at 5 min of insulin, and PAS immunoreactivity subsequently peaked for proteins of 250 kDa (10 min) and 160 kDa (15 min). P-Akt, PAS-160, and PAS-250 increased significantly with 0.6 nmol/l insulin. Contractile activity led to increased P-Akt and PAS immunoreactivity of proteins of 160 and 250 kDa. The 160-kDa protein was confirmed to be AS160 based on elevated PAS immunoreactivity in AS160 immunoprecipitates. Wortmannin inhibited insulin (120 nmol/l) and contraction effects on AS160 phosphorylation. Incubation with AICAR caused increased phosphorylation of AMP-activated protein kinase and AS160 but not Akt. Our working hypothesis is that phosphorylation of these putative Akt substrates is important for some of the insulin and contraction bioeffects.

Skeletal muscle atrophy in old rats: differential changes in the three fiber types

This study was undertaken to reevaluate the effects of ageing on skeletal muscle mass and on mitochondrial and glycolytic enzyme levels in the different types of skeletal muscle in rats. It was found that some muscles atrophy with ageing, while others do not, in male rats. Atrophy appears to occur in weight-bearing muscles, and is most marked in those with a high proportion of type IIb fibers. The muscles that did not atrophy are non-weight-bearing, and include the epitrochlearis (predominantly type IIb fibers) and the adductor longus (predominantly type I fibers). The average cross-sectional area of muscle fibers in the plantaris muscles of 28-30-month-old rats was approximately 30% smaller than that of 9-10-month-old animals, providing evidence that the approximately 30% lower weight of the plantaris in the old group was entirely due to fiber atrophy. The proportion of type IIa fibers was decreased and the proportion of type I fibers was increased in the plantaris of the old rats. The respiratory capacity of the soleus muscle (predominantly type I fibers), and the glycolytic capacity of the superficial, white (type IIb) and deep, red (predominantly type IIa) portions of the vastus lateralis, were reduced in the old rats. Our results provide evidence that ageing has differential effects on the three types of skeletal muscle fiber, and on weight-bearing and non-weight-bearing muscles, in the rat.

AMPK directly activates mTORC2 to promote cell survival during acute energetic stress

AMP-activated protein kinase (AMPK) senses energetic stress and, in turn, promotes catabolic and suppresses anabolic metabolism coordinately to restore energy balance. We found that a diverse array of AMPK activators increased mTOR complex 2 (mTORC2) signaling in an AMPK-dependent manner in cultured cells. Activation of AMPK with the type 2 diabetes drug metformin (GlucoPhage) also increased mTORC2 signaling in liver in vivo and in primary hepatocytes in an AMPK-dependent manner. AMPK-mediated activation of mTORC2 did not result from AMPK-mediated suppression of mTORC1 and thus reduced negative feedback on PI3K flux. Rather, AMPK associated with and directly phosphorylated mTORC2 (mTOR in complex with rictor). As determined by two-stage in vitro kinase assay, phosphorylation of mTORC2 by recombinant AMPK was sufficient to increase mTORC2 catalytic activity toward Akt. Hence, AMPK phosphorylated mTORC2 components directly to increase mTORC2 activity and downstream signaling. Functionally, inactivation of AMPK, mTORC2, and Akt increased apoptosis during acute energetic stress. By showing that AMPK activates mTORC2 to increase cell survival, these data provide a potential mechanism for how AMPK paradoxically promotes tumorigenesis in certain contexts despite its tumor-suppressive function through inhibition of growth-promoting mTORC1. Collectively, these data unveil mTORC2 as a target of AMPK and the AMPK-mTORC2 axis as a promoter of cell survival during energetic stress.

Prolonged increase in insulin-stimulated glucose transport in muscle after exercise

Exercise can induce short-term increases in the sensitivity and responsiveness of skeletal muscle glucose transport to insulin. The purpose of this study was to determine the effect of carbohydrate deprivation on the persistence of increased insulin sensitivity and responsiveness after a bout of exercise. Three hours after a bout of exercise, epitrochlearis muscles from carbohydrate-deprived (fat fed) rats showed a 25% greater increase in 3-O-methylglucose (3-MG) transport in response to a maximal insulin stimulus compared with muscles of nonexercised rats; this increase in insulin responsiveness had reversed 18 h postexercise. Muscles of rats fed carbohydrate showed no increase in insulin responsiveness 3 h after exercise. The effect of 60 microU/ml of insulin on 3-MG transport was approximately twofold greater in muscles studied 3 h after exercise than in nonexercised controls regardless of dietary carbohydrate intake. This increase in insulin sensitivity was lost within 18 h in carbohydrate-fed rats but persisted for at least 48 h in carbohydrate-deprived rats. Muscle glycogen increased approximately 41 mumol/g in the rats fed carbohydrate for 18 h, and only approximately 14.5 mumol/g in the rats fed fat for 48 h, after exercise. The persistent increase in insulin sensitivity after exercise in carbohydrate-deprived rats was unrelated to caloric intake, as muscles of fasted and fat-fed rats behaved similarly.

Role of Akt substrate of 160 kDa in insulin-stimulated and contraction-stimulated glucose transport

Insulin and exercise, the most important physiological stimuli to increase glucose transport in skeletal muscle, trigger a redistribution of GLUT4 glucose transporter proteins from the cell interior to the cell surface, thereby increasing glucose transport capacity. The most distal insulin signaling protein that has been linked to GLUT4 translocation, Akt substrate of 160 kDa (AS160), becomes phosphorylated in insulin-stimulated 3T3-L1 adipocytes; this is important for insulin-stimulated GLUT4 translocation and glucose transport. Insulin also induces a rapid and dose-dependent increase in AS160 phosphorylation in skeletal muscle. Available data from skeletal muscle support the concepts developed in adipocytes with regard to the role AS160 plays in the regulation of insulin-stimulated glucose transport. In vivo exercise, in vitro contractions, or in situ contractions can also stimulate AS160 phosphorylation. AMP-activated protein kinase (AMPK) is likely important for phosphorylating AS160 in response to exercise/contractile activity, whereas Akt2 appears to be important for insulin-stimulated AS160 phosphorylation in muscle. Evidence of a role for AS160 in exercise/contraction-stimulated glucose uptake is currently inconclusive. The distinct signaling pathways that are stimulated by insulin and exercise/contraction converge at AS160. Although AS160 phosphorylation is apparently important for insulin-stimulated GLUT4 translocation and glucose transport, it is uncertain whether elevated AS160 phosphorylation plays a similar role with exercise/contraction.

Prolonged incubation in PUGNAc results in increased protein O-Linked glycosylation and insulin resistance in rat skeletal muscle

Increased flux through the hexosamine biosynthetic pathway and increased O-linked glycosylation (N-acetylglucosamine [O-GlcNAc]) of proteins have been implicated in insulin resistance. Previous research in 3T3-L1 adipocytes indicated that insulin-stimulated glucose uptake and phosphorylation of Akt were reduced after incubation with O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc; 100 micromol/l), an inhibitor of the O-GlcNAcase that catalyzes removal of O-GlcNAc from proteins. Therefore, in this study, we tested the effects of PUGNAc on skeletal muscle. Incubation of rat epitrochlearis muscles for 19 h with 100 micromol/l PUGNAc resulted in a marked increase in O-GlcNAcylation of multiple proteins. Incubation with PUGNAc reduced glucose transport with a physiologic insulin concentration without affecting glucose transport without insulin or with supraphysiologic insulin. PUGNAc did not significantly alter insulin-stimulated phosphorylation of Akt (serine and threonine) or its substrates glycogen synthase kinase (GSK)3 alpha and GSK3 beta. Insulin stimulated a dose-dependent (12.0 > 0.6 > 0 nmol/l) increase in the phosphorylation of a 160-kDa protein detected using an antibody against an Akt substrate phosphomotif. PUGNAc treatment did not alter phosphorylation of this protein. These results indicate that PUGNAc is an effective inhibitor of O-GlcNAcase in skeletal muscle and suggest that O-GlcNAc modification of proteins can induce insulin resistance in skeletal muscle independent of attenuated phosphorylation of Akt, GSK 3 alpha, GSK3 beta, and a 160-kDa protein with an Akt phosphomotif.

Roles of TBC1D1 and TBC1D4 in insulin- and exercise-stimulated glucose transport of skeletal muscle

This review focuses on two paralogue Rab GTPase activating proteins known as TBC1D1 Tre-2/BUB2/cdc 1 domain family (TBC1D) 1 and TBC1D4 (also called Akt Substrate of 160 kDa, AS160) and their roles in controlling skeletal muscle glucose transport in response to the independent and combined effects of insulin and exercise. Convincing evidence implicates Akt2-dependent TBC1D4 phosphorylation on T642 as a key part of the mechanism for insulin-stimulated glucose uptake by skeletal muscle. TBC1D1 phosphorylation on several insulin-responsive sites (including T596, a site corresponding to T642 in TBC1D4) does not appear to be essential for in vivo insulin-stimulated glucose uptake by skeletal muscle. In vivo exercise or ex vivo contraction of muscle result in greater TBC1D1 phosphorylation on S237 that is likely to be secondary to increased AMP-activated protein kinase activity and potentially important for contraction-stimulated glucose uptake. Several studies that evaluated both normal and insulin-resistant skeletal muscle stimulated with a physiological insulin concentration after a single exercise session found that greater post-exercise insulin-stimulated glucose uptake was accompanied by greater TBC1D4 phosphorylation on several sites. In contrast, enhanced post-exercise insulin sensitivity was not accompanied by greater insulin-stimulated TBC1D1 phosphorylation. The mechanism for greater TBC1D4 phosphorylation in insulin-stimulated muscles after acute exercise is uncertain, and a causal link between enhanced TBC1D4 phosphorylation and increased post-exercise insulin sensitivity has yet to be established. In summary, TBC1D1 and TBC1D4 have important, but distinct roles in regulating muscle glucose transport in response to insulin and exercise.

Recruitment of GLUT-4 glucose transporters by insulin in diabetic rat skeletal muscle

The cause of reduced insulin-stimulated glucose transport in skeletal muscle of diabetic rats was investigated. Basal and insulin-stimulated glucose uptake into hindquarter muscles of 7-day diabetic rats were 70% and 50% lower, respectively, than in nondiabetic controls. Subcellular fractionation of hindquarter muscles yielded total crude membranes, plasma membranes and intracellular membranes. The number of GLUT-4 glucose transporters was lower in crude membranes, plasma membranes and intracellular membranes, relative to non-diabetic rat muscles. These results were paralleled by reductions in D-glucose-protectable binding of cytochalasin B. Insulin caused a redistribution of GLUT-4 transporters from intracellular membranes to plasma membranes, in both control and diabetic rat muscles. This redistribution was also recorded using binding of cytochalasin B. The insulin-dependent decrement in glucose transporters in intracellular membranes was similar for both animal groups, but the gain and final amount of transporters in the plasma membrane were 50% lower in the diabetic group. The results suggest that insulin signalling and recruitment of GLUT-4 glucose transporters occur in diabetic rat muscle, and that the diminished insulin response may be due to fewer glucose transporters operating in the muscle plasma membrane.

Exercise-induced increase in glucose transporters in plasma membranes of rat skeletal muscle

A previously developed technique for the isolation of plasma and intracellular membrane fractions from rat skeletal muscle was used to investigate transporter migration after insulin treatment or a bout of exercise (45 min of treadmill). Glucose-inhibitable cytochalasin-B binding was used to estimate the number of glucose transporters. Insulin and exercise caused increases in glucose uptake into the hindlimb muscles of 5- and 3-fold, respectively. Each stimulus also caused a 2-fold increase in the number of glucose transporters in plasma membranes prepared from hindlimb muscles. The insulin-induced increase in plasma membrane transporters was accompanied by a concomitant decrease in transporters from the intracellular pool. In contrast to insulin, there was no concomitant decrease in the number of cytochalasin-B-binding sites in the intracellular membrane fraction from exercised muscles. The ability of both insulin and exercise to increase the number of transporters in the plasma membrane is in accordance with recruitment of transporters as one cause of increased transport activity. However, the inability of exercise to decrease the number of transporters in the insulin-sensitive intracellular pool suggests the existence of either a second recruitable transporter pool or masked glucose transporters in the plasma membrane that are unmasked by the muscle contractile activity.

Increased AS160 phosphorylation, but not TBC1D1 phosphorylation, with increased postexercise insulin sensitivity in rat skeletal muscle

A single exercise bout can increase insulin-independent glucose transport immediately postexercise and insulin-dependent glucose transport (GT) for several hours postexercise. Akt substrate of 160 kDa (AS160) and TBC1D1 are paralog Rab GTPase-activating proteins that have been proposed to contribute to these exercise effects. Previous research demonstrated greater AS160 and Akt threonine phosphorylation in rat skeletal muscle at 3-4 h postexercise concomitant with enhanced insulin-stimulated GT. To further probe whether these signaling events or TBC1D1 phosphorylation were important for the enhanced postexercise insulin-stimulated GT, male Wistar rats were studied using four experimental protocols (2-h swim exercise, differing with regard to timing of muscle sampling and whether food was provided postexercise) that were known to vary in their influence of insulin-independent and insulin-dependent GT postexercise. The results indicated that, in isolated rat epitrochlearis muscle, 1) elevated phosphorylation of AS160 (measured using anti-phospho-Akt substrate, PAS-AS160, and phosphospecific anti-Thr(642)-AS160, pThr(642)-AS160) consistently tracked with elevated insulin-stimulated GT; 2) PAS-TBC1D1 was not different from sedentary values at 3 or 27 h postexercise, when insulin sensitivity was increased; 3) insulin-stimulated Akt activity was not increased postexercise in muscles with increased insulin sensitivity; 4) PAS-TBC1D1 was increased immediately postexercise, when insulin-independent GT was elevated, and reversed at 3 and 27 h postexercise, when insulin-independent GT was also reversed; and 5) there was no significant effect of exercise or insulin on total abundance of AS160, TBC1D1, Akt, or GLUT4 protein with any of the protocols. The results are consistent with increased AS160 phosphorylation (PAS-AS160 or pThr(642)-AS160) but not increased PAS-TBC1D1 or Akt activity, which is important for increased postexercise insulin-stimulated GT in rat skeletal muscle. They also support the idea that increased TBC1D1 phosphorylation may play a role in the insulin-independent increase in GT postexercise.

Akt2 is essential for the full effect of calorie restriction on insulin-stimulated glucose uptake in skeletal muscle

Brief calorie restriction (CR; 20 days of 60% of ad libitum [AL] intake) improves insulin-stimulated glucose transport, concomitant with enhanced phosphorylation of Akt2. The purpose of this study was to determine whether Akt2 is essential for the calorie restriction-induced enhancement in skeletal muscle insulin sensitivity. We measured insulin-stimulated 2-deoxyglucose (2DG) uptake in isolated extensor digitorum longus (EDL) and soleus muscles from male and female wild-type (WT) and Akt2-null (knockout [KO]) mice after ad libitum or calorie-restricted (20 days at 60% of AL) feeding. In WT mice, calorie restriction significantly enhanced insulin-stimulated 2DG uptake in both muscles regardless of sex. However, in KO mice, calorie restriction did not enhance insulin-stimulated 2DG in male or female EDL or in female soleus. Only in male KO soleus did calorie restriction significantly increase insulin-stimulated 2DG through an Akt2-independent mechanism, although 2DG uptake of the KO-CR group was reduced compared with the WT-CR soleus group. Akt2 serine phosphorylation was enhanced approximately two- to threefold in insulin-stimulated WT-CR versus WT-AL muscles. Calorie restriction induced an approximately 1.5- to 2-fold elevation in Akt1 phosphorylation of insulin-treated muscles, regardless of genotype, but this increase was insufficient to replace Akt2 for insulin-stimulated 2DG in Akt2-deficient muscles. These results indicate that Akt2 is essential for the full effect of brief calorie restriction on insulin-stimulated glucose uptake in skeletal muscle with physiologic insulin.

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise

Enhanced skeletal muscle and whole body insulin sensitivity can persist for up to 24-48 h after one exercise session. This review focuses on potential mechanisms for greater postexercise and insulin-stimulated glucose uptake (ISGU) by muscle in individuals with normal or reduced insulin sensitivity. A model is proposed for the processes underlying this improvement; i.e., triggers initiate events that activate subsequent memory elements, which store information that is relayed to mediators, which translate memory into action by controlling an end effector that directly executes increased insulin-stimulated glucose transport. Several candidates are potential triggers or memory elements, but none have been conclusively verified. Regarding potential mediators in both normal and insulin-resistant individuals, elevated postexercise ISGU with a physiological insulin dose coincides with greater Akt substrate of 160 kDa (AS160) phosphorylation without improved proximal insulin signaling at steps from insulin receptor binding to Akt activity. Causality remains to be established between greater AS160 phosphorylation and improved ISGU. The end effector for normal individuals is increased GLUT4 translocation, but this remains untested for insulin-resistant individuals postexercise. Following exercise, insulin-resistant individuals can attain ISGU values similar to nonexercising healthy controls, but after a comparable exercise protocol performed by both groups, ISGU for the insulin-resistant group has been consistently reported to be below postexercise values for the healthy group. Further research is required to fully understand the mechanisms underlying the improved postexercise ISGU in individuals with normal or subnormal insulin sensitivity and to explain the disparity between these groups after similar exercise.

Prior exercise increases phosphorylation of Akt substrate of 160 kDa (AS160) in rat skeletal muscle

The main purpose of this study was to determine whether the increased glucose transport (GT) found immediately postexercise (IPEX) or 4 h postexercise (4hPEX) is accompanied by increased phosphorylation of Akt substrate of 160 kDa (AS160, a protein regulator of GLUT4 translocation). Paired epitrochlearis muscles were dissected from rats (sedentary or IPEX, 2-h swim) and used to measure protein phosphorylation and insulin-independent GT. IPEX values exceeded sedentary values for GT and phosphorylations of AS160, AMP-activated protein kinase (pAMPK) and acetyl-CoA carboxylase (pACC) but not for AS160 abundance or phosphorylation of Akt serine (pSerAkt), Akt threonine (pThrAkt), or glycogen synthase kinase-3 (pGSK3). AS160 phosphorylation was significantly correlated with GT (R=0.801, P<0.01) and pAMPK (R=0.655, P<0.05). Muscles from other rats were studied 4hPEX along with sedentary controls. One muscle per rat was incubated without insulin, and the contralateral muscle was incubated with insulin. 4hPEX values exceeded sedentary values for insulin-stimulated GT. The elevated pAMPK and pACC found IPEX had reversed by 4hPEX. Insulin caused a significant increase in pSerAkt, pThrAkt, pGSK3, and AS160 phosphorylation with or without exercise. Exercise significantly increased AS160 phosphorylation, regardless of insulin, with unchanged AS160 abundance. Among the signaling proteins studied, insulin-stimulated GT was significantly correlated only with insulin-stimulated pThrAkt (R=0.720, P<0.0005). The results are consistent with a role for increased AS160 phosphorylation in the increased insulin-independent GT IPEX, and the exercise effects on AS160 phosphorylation and/or pThrAkt at 4hPEX are potentially relevant to the increased insulin-stimulated glucose transport at this time.

Postexercise improvement in insulin-stimulated glucose uptake occurs concomitant with greater AS160 phosphorylation in muscle from normal and insulin-resistant rats

Earlier research on rats with normal insulin sensitivity demonstrated that acute exercise increased insulin-stimulated glucose uptake (GU) concomitant with greater phosphorylation of Akt substrate of 160 kDa (pAS160). Because mechanisms for exercise effects on GU in insulin-resistant muscle are unknown, our primary objective was to assess insulin-stimulated GU, proximal insulin signaling (insulin receptor [IR] tyrosine phosphorylation, IR substrate 1-phosphatidylinositol-3-kinase, and Akt phosphorylation and activity), and pAS160 in muscles from acutely exercised (one session) and sedentary rats fed either chow (low-fat diet [LFD]; normal insulin sensitivity) or a high-fat diet (HFD; for 2 weeks, insulin-resistant). At 3 h postexercise (3hPEX), isolated epitrochlearis muscles were used for insulin-stimulated GU and insulin signaling measurements. Although exercise did not enhance proximal signaling in either group, insulin-stimulated GU at 3hPEX exceeded respective sedentary control subjects (Sedentary) in both diet groups. Furthermore, insulin-stimulated GU for LFD-3hPEX was greater than HFD-3hPEX values. For HFD-3hPEX muscles, pAS160 exceeded HFD-Sedentary, but in muscle from LFD-3hPEX rats, pAS160 was greater still than HFD-3hPEX values. These results implicated pAS160 as a potential determinant of the exercise-induced elevation in insulin-stimulated GU for each diet group and also revealed pAS160 as a possible mediator of greater postexercise GU of insulin-stimulated muscles from the insulin-sensitive versus insulin-resistant group.

Muscle glucose transport: interactions of in vitro contractions, insulin, and exercise

Exercise increases permeability of muscle to glucose. Normally, the effects of exercise and a maximal insulin stimulus on glucose transport are additive. However, the combined effect on rat epitrochlearis muscle permeability to 3-O-methylglucose (3-MG) of a maximal insulin stimulus followed by in vitro contractile activity of 1.24 +/- 0.06 mumol.10 min-1.ml intracellular water-1 was no greater than that of either stimulus alone. We found that this absence of an additive effect was caused by prolonged exposure to an unphysiologically high insulin concentration (20,000 microU/ml for 60 min), which, in addition to stimulating glucose transport, appears to prevent further increases in permeability to glucose. When the treatments were reversed and muscles were first stimulated to contract and then incubated with 20,000 microU/ml insulin, 3-MG uptake (mumol.10 min-1.ml intracellular water-1) increased from a control value of 0.26 +/- 0.03 to 1.80 +/- 0.15, compared with 1.04 +/- 0.06 for contractile activity alone, 1.21 +/- 0.08 for insulin, and 1.88 +/- 0.11 for exercise (swimming) plus insulin. Swimming plus in vitro contractile activity did not have a greater effect than contractile activity alone. Our results provide evidence that 1) the effect of exercise on muscle permeability to glucose is mediated solely by a process associated with contractile activity, and 2) it is advisable to avoid the use of unphysiologically high insulin concentrations in studies designed to elucidate in vivo actions of insulin.

Increased submaximal insulin-stimulated glucose uptake in mouse skeletal muscle after treadmill exercise

The primary purpose of this study was to determine the effect of prior exercise on insulin-stimulated glucose uptake with physiological insulin in isolated muscles of mice. Male C57BL/6 mice completed a 60-min treadmill exercise protocol or were sedentary. Paired epitrochlearis, soleus, and extensor digitorum longus (EDL) muscles were incubated with [3H]-2-deoxyglucose without or with insulin (60 microU/ml) to measure glucose uptake. Insulin-stimulated glucose uptake for paired muscles was calculated by subtracting glucose uptake without insulin from glucose uptake with insulin. Muscles from other mice were assessed for glycogen and AMPK Thr172 phosphorylation. Exercised vs. sedentary mice had decreased glycogen in epitrochlearis (48%, P < 0.001), soleus (51%, P < 0.001), and EDL (41%, P < 0.01) and increased AMPK Thr172 phosphorylation (P < 0.05) in epitrochlearis (1.7-fold), soleus (2.0-fold), and EDL (1.4-fold). Insulin-independent glucose uptake was increased 30 min postexercise vs. sedentary in the epitrochlearis (1.2-fold, P < 0.001), soleus (1.4-fold, P < 0.05), and EDL (1.3-fold, P < 0.01). Insulin-stimulated glucose uptake was increased (P < 0.05) approximately 85 min after exercise in the epitrochlearis (sedentary: 0.266 +/- 0.045 micromol x g(-1) x 15 min(-1); exercised: 0.414 +/- 0.051) and soleus (sedentary: 0.102 +/- 0.049; exercised: 0.347 +/- 0.098) but not in the EDL. Akt Ser473 and Akt Thr308 phosphorylation for insulin-stimulated muscles did not differ in exercised vs. sedentary. These results demonstrate enhanced submaximal insulin-stimulated glucose uptake in the epitrochlearis and soleus of mice 85 min postexercise and suggest that it will be feasible to probe the mechanism of enhanced postexercise insulin sensitivity by using genetically modified mice.

Exercise modulates the insulin-induced translocation of glucose transporters in rat skeletal muscle

Insulin and acute exercise (45 min of treadmill run) increased glucose uptake into perfused rat hindlimbs 5-fold and 3.2-fold, respectively. Following exercise, insulin treatment resulted in a further increase in glucose uptake. The subcellular distribution of the muscle glucose transporters GLUT-1 and GLUT-4 was determined in plasma membranes and intracellular membranes. Neither exercise nor exercise----insulin treatment altered the distribution of GLUT-1 transporters in these membrane fractions. In contrast, exercise, insulin and exercise----insulin treatment caused comparable increases in GLUT-4 transporters in the plasma membrane. The results suggest that exercise might limit insulin-induced GLUT-4 recruitment and that following exercise, insulin may alter the intrinsic activity of plasma membrane glucose transporters.

Genetic selection of mice for high voluntary wheel running: effect on skeletal muscle glucose uptake

Effects of genetic selection for high wheel-running activity (17th generation) and access to running wheels on skeletal muscle glucose uptake were studied in mice with the following treatments for 8 wk: 1) access to unlocked wheels; 2) same as 1, but wheels locked 48 h before glucose uptake measurement; or 3) wheels always locked. Selected mice ran more than random-bred (nonselected) mice (8-wk mean +/- SE = 8,243 +/- 711 vs. 3,719 +/- 233 revolutions/day). Body weight was 5-13% lower for selected vs. nonselected groups. Fat pad/body weight was ~40% lower for selected vs. nonselected and unlocked vs. locked groups. Insulin-stimulated glucose uptake and fat pad/body weight were inversely correlated for isolated soleus (r = -0.333; P < 0.005) but not extensor digitorum longus (EDL) or epitrochlearis muscles. Insulin-stimulated glucose uptake was higher in EDL (P < 0.02) for selected vs. nonselected mice. Glucose uptake did not differ by wheel group, and amount of running did not correlate with glucose uptake for any muscle. Wheel running by mice did not enhance subsequent glucose uptake by isolated muscles.

Endurance training effects on striatal D2 dopamine receptor binding and striatal dopamine metabolite levels

We have previously shown that endurance training is associated with higher binding of [3H]spiperone to striatal D2 dopamine (DA) receptors of presenescent (21 months old) rats. In the present study we investigated the effects of 6 months of endurance training of young adults on the relationship between steady-state levels of DA and its metabolites in striatum and the affinity and density of striatal D2 DA receptors. The extent of training was confirmed by evaluating the maximal oxygen consumption (VO2 max) in the subjects. D2 DA binding was significantly increased at each of 3 [3H]spiperone concentrations in the young runners. A 'synaptic coupling ratio' calculated as the specific DA binding/DOPAC concentration was significantly increased in runners for the 0.1 and 0.4 nM radioligand concentrations. Across experimental groups levels of DA were highly and positively correlated with specific DA binding at the 0.1, 0.2 and 0.4 nM [3H]spiperone concentrations. Together, these results suggest that exercise can alter the number of DA binding sites and the metabolism of DA in young adult animals.

Inhibition of contraction-stimulated AMP-activated protein kinase inhibits contraction-stimulated increases in PAS-TBC1D1 and glucose transport without altering PAS-AS160 in rat skeletal muscle

OBJECTIVE

Phosphorylation of two members of the TBC1 domain family of proteins, Akt substrate of 160 kDa (AS160, also known as TBC1D4) and TBC1D1, has been implicated in the regulation of glucose transport in skeletal muscle. Insulin-stimulated phosphorylation (measured using the phospho-Akt substrate [PAS] antibody) of AS160 and TBC1D1 appears to occur in an Akt-dependent manner, but the kinases responsible for contraction-stimulated PAS-AS160 and PAS-TBC1D1 remain unclear. AMP-activated protein kinase (AMPK) and Akt, both activated by contraction, can each phosphorylate AS160 and TBC1D1 in cell-free assays.

RESEARCH DESIGN AND METHODS

To evaluate the roles of AMPK and Akt on insulin- or contraction-stimulated PAS-AS160, PAS-TBC1D1, and glucose transport, rat epitrochlearis was incubated with and without compound C (inhibitor of AMPK) or Wortmannin (inhibitor of phosphatidylinositol [PI] 3-kinase, which is upstream of Akt) before and during insulin stimulation or contraction.

RESULTS

Insulin-stimulated glucose transport and phosphorylation of both AS160 and TBC1D1 were completely inhibited by Wortmannin. Wortmannin eliminated contraction stimulation of phospho-Ser(21/9)glycogen synthase kinase 3alpha/beta (pGSK3; Akt substrate) and PAS-AS160 but did not significantly alter pAMPK, phospho-Ser79acetyl CoA carboxylase (pACC; AMPK substrate), PAS-TBC1D1, or glucose transport in contraction-stimulated muscle. Compound C completely inhibited contraction-stimulated pACC and PAS-TBC1D1 and partially blocked glucose transport, but it did not significantly alter pAkt, pGSK3, or PAS-AS160.

CONCLUSIONS

These data suggest that 1) insulin stimulates glucose transport and phosphorylation of AS160 and TBC1D1 in a PI 3-kinase/Akt-dependent manner, 2) contraction stimulates PAS-AS160 (but not PAS-TBC1D1 or glucose transport) in a PI 3-kinase/Akt-dependent manner, and 3) contraction stimulates PAS-TBC1D1 and glucose transport (but not PAS-AS160) in an AMPK-dependent manner.

Calorie restriction increases insulin-stimulated glucose transport in skeletal muscle from IRS-1 knockout mice

Calorie restriction (CR), even for brief periods (4-20 days), results in increased whole-body insulin sensitivity, in large part due to enhanced insulin-stimulated glucose transport by skeletal muscle. Evidence suggests that the cellular alterations leading to this effect are postreceptor steps in insulin signaling. To determine whether insulin receptor substrate (IRS)-1 is essential for the insulin-sensitizing effect of CR, we measured in vitro 2-deoxyglucose (2DG) uptake in the presence and absence of insulin by skeletal muscle isolated from wild-type (WT) mice and transgenic mice lacking IRS-1 (knockout [KO]) after either ad libitum (AL) feeding or 20 days of CR (60% of ad libitum intake). Three muscles (soleus, extensor digitorum longus [EDL], and epitrochlearis) from male and female mice (4.5-6 months old) were studied. In each muscle, insulin-stimulated 2DG uptake was not different between genotypes. For EDL and epitrochlearis, insulin-stimulated 2DG uptake was greater in CR compared to AL groups, regardless of sex. Soleus insulin-stimulated 2DG uptake was greater in CR compared with AL in males but not females. The diet effect on 2DG uptake was not different for WT and KO animals. Genotype also did not alter the CR-induced decrease in plasma constituents (glucose, insulin, and leptin) or body composition (body weight, fat pad/body weight ratio). Consistent with previous studies in rats, IRS-1 protein expression in muscle was reduced in WT-CR compared with WT-AL mice, and muscle IRS-2 abundance was unchanged by diet. Skeletal muscle IRS-2 protein expression was significantly lower in WT compared with KO mice. These data demonstrate that IRS-1 is not essential for the CR-induced increase in insulin-stimulated glucose transport in skeletal muscle, and the absence of IRS-1 does not modify any of the characteristic adaptations of CR that were evaluated.

Interaction between BTBR and C57BL/6J genomes produces an insulin resistance syndrome in (BTBR x C57BL/6J) F1 mice

Insulin resistance is a common syndrome that often precedes the development of noninsulin-dependent diabetes mellitus (NIDDM). Both diet and genetic factors are associated with insulin resistance. BTBR and C57BL/6J (B6) mice have normal insulin responsiveness and normal fasting plasma insulin levels. However, a cross between these two strains yielded male offspring with severe insulin resistance. Surprisingly, on a basal diet (6.5% fat), the insulin resistance was not associated with fasting hyperinsulinemia. However, a 15% fat diet produced significant hyperinsulinemia in the male mice (twofold at 10 weeks; P < .05). At 10 weeks of age, visceral fat contributed approximately 4.3% of the total body weight in the males versus 1.8% in females. In the males, levels of plasma triacylglycerol and total cholesterol increased 40% and 30%, respectively, compared to females. Plasma free fatty acid concentrations were unchanged. Oral glucose tolerance tests revealed significant levels of hyperglycemia and hyperinsulinemia 15 to 90 minutes after oral glucose administration in the male mice. This was particularly dramatic in males on a 15% fat diet. Glucose transport was examined in skeletal muscles in (BTBR x B6)F1 mice. In the nonhyperinsulinemic animals (females), insulin stimulated 2-deoxyglucose transport 3.5-fold in the soleus and 2.8-fold in the extensor digitorum longus muscles. By contrast, glucose transport was not stimulated in the hyperinsulinemic male mice. Hypoxia stimulates glucose transport through an insulin-independent mechanism. This is known to involve the translocation of GLUT4 from an intracellular pool to the plasma membrane. In the insulin-resistant male mice, hypoxia induced glucose transport as effectively as it did in the insulin-responsive mice. Thus, defective glucose transport in the (BTBR x B6)F1 mice is specific for insulin-stimulated glucose transport. This is similar to what has been observed in muscles taken from obese NIDDM patients. These animals represent an excellent genetic model for studying insulin resistance and investigating the transition from insulin resistance in the absence of hyperinsulinemia to insulin resistance with hyperinsulinemia.

Growth hormone reduces glucose transport but not GLUT-1 or GLUT-4 in adult and old rats

The primary purpose of this study was to investigate the influence of administration of recombinant-derived human growth hormone (rhGH) to adult male rats of several ages (9, 20, and 31 mo) on skeletal muscle glucose transport. Rats were injected with rhGH (0.7 mg/kg) or vehicle twice daily for 10 days. The rhGH treatment led to a doubling of circulating insulin-like growth factor I levels at each age. Skeletal muscle glucose transport activity was evaluated in isolated epitrochlearis muscle with use of 3-O-methylglucose at three insulin concentrations (0, 100, and 20,000 microU/ml). The results indicate that, after 10 days of rhGH administration, 1) an approximately 20-30% reduction in basal glucose transport activity was evident in muscles from every age group, 2) the ability of a submaximally effective insulin concentration (100 microU/ml) to increase glucose transport activity above basal values was not significantly reduced in any age group, 3) maximal insulin-stimulated glucose transport activity (with 20,000 microU/ml) was significantly reduced (approximately 40%) by rhGH treatment only in the oldest rats, and 4) the alterations in glucose transport activity occurred despite no change in skeletal muscle GLUT-1 or GLUT-4 protein levels.