Chromium chloride increases insulin-stimulated glucose uptake in the perfused rat hindlimb

AIM

To determine the effect of chromium chloride (CrCl3 ) on healthy skeletal muscle glucose uptake in the absence and presence of submaximal insulin using the rat hindlimb perfusion technique.

METHODS

Sprague-Dawley rats were randomly assigned to an experimental group: basal (Bas), chromium chloride (Cr), submaximal insulin (sIns) or chromium chloride plus submaximal insulin (Cr-sIns).

RESULTS

Insulin significantly increased [H(3)]-2 deoxyglucose (2-DG) uptake in the gastrocnemius muscles. Additionally, Cr-sIns displayed greater rates of 2-DG uptake than sIns (Cr-sIns 6.86 ± 0.74 μmol g h(-1) vs. sIns 4.83 ± 0.42 μmol g h(-1)). There was no difference between Cr and Bas treatment groups. It has been speculated that chromium works to increase glucose uptake by increasing insulin signalling. We found that Akt and AS160 phosphorylation was increased in the sINS treatment group, while chromium treatment had no additional effect on Akt or AS160 phosphorylation in the absence or presence of insulin. Cr-sIns significantly increased plasma membrane GLUT4 concentration above that of sIns (Cr-sIns 72.22 ± 12.7%, sIns 53.4 ± 6.1%), but in the absence of insulin, chromium had no effect.

CONCLUSION

Exposure of healthy skeletal muscle to chromium may increase skeletal muscle insulin-stimulated GLUT4 translocation and glucose uptake. However, these effects do not appear to result from enhanced insulin signalling proximal to AS160.

Caffeinated nitric oxide-releasing lozenge improves cycling time trial performance

Boosting nitric oxide production during exercise by various means has been found to improve exercise performance. We investigated the effects of a nitric oxide releasing lozenge with added caffeine (70 mg) on oxygen consumption during steady-state exercise and cycling time trial performance using a double-blinded randomized, crossover experimental design. 15 moderately trained cyclists (7 females and 8 males) were randomly assigned to ingest the caffeinated nitric oxide lozenge or placebo 5 min before exercise. Oxygen consumption and blood lactate were assessed at rest and at 50%, 65% and 75% maximal oxygen consumption. Exercise performance was assessed by time to complete a simulated 20.15 km cycling time-trial course. No significant treatment effects for oxygen consumption or blood lactate at rest or during steady-state exercise were observed. However, time-trial performance was improved by 2.1% (p<0.01) when participants consumed the nitric oxide lozenge (2,424±69 s) compared to placebo (2,476±78 s) and without a significant difference in rating of perceived exertion. These results suggest that acute supplementation with a caffeinated nitric oxide releasing lozenge may be a practical and effective means of improving aerobic exercise performance.

Functional electrical stimulation cycling improves body composition, metabolic and neural factors in persons with spinal cord injury

Persons with spinal cord injury (SCI) are at a heightened risk of developing type II diabetes and cardiovascular disease. The purpose of this investigation was to conduct an analysis of metabolic, body composition, and neurological factors before and after 10 weeks of functional electrical stimulation (FES) cycling in persons with SCI. Eighteen individuals with SCI received FES cycling 2-3 times per week for 10 weeks. Body composition was analyzed by dual X-ray absorptiometry. The American Spinal Injury Association (ASIA) neurological classification of SCI test battery was used to assess motor and sensory function. An oral glucose tolerance (OGTT) and insulin-response test was performed to assess blood glucose control. Additional metabolic variables including plasma cholesterol (total-C, HDL-C, LDL-C), triglyceride, and inflammatory markers (IL-6, TNF-alpha, and CRP) were also measured. Total FES cycling power and work done increased with training. Lean muscle mass also increased, whereas, bone and adipose mass did not change. The ASIA motor and sensory scores for the lower extremity significantly increased with training. Blood glucose and insulin levels were lower following the OGTT after 10 weeks of training. Triglyceride levels did not change following training. However, levels of IL-6, TNF-alpha, and CRP were all significantly reduced.

Dietary strategies to promote glycogen synthesis after exercise

Muscle glycogen is an essential fuel for prolonged intense exercise, and therefore it is important that the glycogen stores be copious for competition and strenuous training regimens. While early research focused on means of increasing the muscle glycogen stores in preparation for competition and its day-to-day replenishment, recent research has focused on the most effective means of promoting its replenishment during the early hours of recovery. It has been observed that muscle glycogen synthesis is twice as rapid if carbohydrate is consumed immediately after exercise as opposed to waiting several hours, and that a rapid rate of synthesis can be maintained if carbohydrate is consumed on a regular basis. For example, supplementing at 30-min intervals at a rate of 1.2 to 1.5 g CHO x kg(-1) body wt x h(-1) appears to maximize synthesis for a period of 4- to 5-h post exercise. If a lighter carbohydrate supplement is desired, however, glycogen synthesis can be enhanced with the addition of protein and certain amino acids. Furthermore, the combination of carbohydrate and protein has the added benefit of stimulating amino acid transport, protein synthesis and muscle tissue repair. Research suggests that aerobic performance following recovery is related to the degree of muscle glycogen replenishment.

Attenuation of insulin resistance by chronic beta2-adrenergic agonist treatment possible muscle specific contributions

A possible mechanism by which chronic clenbuterol treatment causes multiple physiological changes in skeletal muscle that leads to reduced insulin resistance in the obese Zucker rat (falfa) was investigated. Animals were gavaged with clenbuterol (CB) (0.8 mg x kg(-1) day(-1)), terbutaline (TB) (1.0 mg x kg(-1)day(-1)), or control (CT) vehicle for six weeks. Oral glucose tolerance and insulin responses were markedly improved in CB rats and impaired in TB rats. CB treatment caused a 24-34% gain in muscle mass in all muscle fiber types, and increases in 3-O-methyglucose transport (2-fold) and GLUT4 concentration (57%) in fast twitch glycolytic (FG) muscle. Oxidative capacity was reduced in both FG (47%) and fast twitch oxidative (FO) muscle (30%), but not in slow twitch oxidative (SO) muscle. Null model analysis for receptor occlusion demonstrated that most functional beta-adrenoceptors were lost in FO (82%) and FG (89%) fibers, but not in SO fibers. We propose that hypertrophy is the result of continuous direct activation of beta-adrenoceptors while loss in oxidative capacity may be the result of receptor down regulation. Improvements in insulin resistance may have been due, in part, to both increases in lean body mass and specific adaptations in FG muscle.

Effects of clenbuterol on insulin resistance in conscious obese Zucker rats

The present study was conducted to determine the effect of chronic administration of the long-acting beta(2)-adrenergic agonist clenbuterol on rats that are genetically prone to insulin resistance and impaired glucose tolerance. Obese Zucker rats (fa/fa) were given 1 mg/kg of clenbuterol by oral intubation daily for 5 wk. Controls received an equivalent volume of water according to the same schedule. At the end of the treatment, rats were catheterized for euglycemic-hyperinsulinemic (15 mU insulin. kg(-1). min(-1)) clamping. Clenbuterol did not change body weight compared with the control group but caused a redistribution of body weight: leg muscle weights increased, and abdominal fat weight decreased. The glucose infusion rate needed to maintain euglycemia and the rate of glucose disappearance were greater in the clenbuterol-treated rats. Furthermore, plasma insulin levels were decreased, and the rate of glucose uptake into hindlimb muscles and abdominal fat was increased in the clenbuterol-treated rats. This increased rate of glucose uptake was accompanied by a parallel increase in the rate of glycogen synthesis. The increase in muscle glucose uptake could not be ascribed to an increase in the glucose transport protein GLUT-4 in clenbuterol-treated rats. We conclude that chronic clenbuterol treatment reduces the insulin resistance of the obese Zucker rat by increasing insulin-stimulated muscle and adipose tissue glucose uptake. The improvements noted may be related to the repartitioning of body weight between tissues.

Exercise training increases ERK2 activity in skeletal muscle of obese Zucker rats

Acute exercise and training increase insulin action in skeletal muscle, but the mechanism responsible for this effect is unknown. Activation of the insulin receptor initiates signaling through both the phosphatidylinositol (PI) 3-kinase and the mitogen-activated protein kinase [MAPK, also referred to as extracellular signal-regulated kinases (ERK1/2)] pathways. Acute exercise has no effect on the PI3-kinase pathway signaling elements but does activate the MAPK pathway, which may play a role in the adaptation of muscle to exercise. It is unknown whether training produces a chronic effect on basal activity or insulin response of the MAPK pathway. The present study was undertaken to determine whether exercise training improves the activity of the MAPK pathway or its response to insulin in obese Zucker rats, a well-characterized model of insulin resistance. To accomplish this, obese Zucker rats were studied by using the hindlimb perfusion method with or without 7 wk of treadmill training. Activation of the MAPK pathway was determined in gastrocnemius muscles exposed in situ to insulin. Compared with lean Zucker rats, untrained obese Zucker rats had reduced basal and insulin-stimulated activities of ERK2 and its downstream target p90 ribosomal S6 kinase (RSK2). Seven weeks of training significantly increased basal and insulin-stimulated ERK2 and RSK2 activities, as well as insulin stimulation of MAPK kinase activity. This effect was maintained for at least 96 h in the case of ERK2. The training-induced increase in basal ERK2 activity was correlated with the increase in citrate synthase activity. Therefore, 7 wk of training increases basal and insulin-stimulated ERK2 activity. The increase in basal ERK2 activity may be related to the response of muscle to training.

Prevention and treatment of non-insulin-dependent diabetes mellitus

The benefits of exercise training in the prevention and treatment of insulin resistance, impaired glucose homeostasis, and NIDDM are strongly supported by current research. The actual mechanisms involved have not been completely identified but occur at the systemic, tissue, and cellular levels. The adaptations that are responsible for the prophylactic effects of exercise training, however, start to subside rapidly once training ceases and are completely lost within 1 to 2 weeks of detraining [4, 17, 37, 68, 161]. Thus, the benefits of exercise training must be renewed on a regular basis. In addition, many of the systemic and cellular adaptations that are responsible for an improved skeletal muscle insulin action occur in only those muscles involved in the training program [4, 28]. Therefore, exercise training programs that consist of various modes of exercise, and which require the use of a large muscle mass, such as swimming, power walking, and strength training, may be the most advantageous for the prevention and treatment of insulin resistance and associated diseases.

Effect of carbohydrate supplementation on postexercise GLUT-4 protein expression in skeletal muscle

The effect of carbohydrate supplementation on skeletal muscle glucose transporter GLUT-4 protein expression was studied in fast-twitch red and white gastrocnemius muscle of Sprague-Dawley rats before and after glycogen depletion by swimming. Exercise significantly reduced fast-twitch red muscle glycogen by 50%. During a 16-h exercise recovery period, muscle glycogen returned to control levels (25.0 +/- 1.4 micromol/g) in exercise-fasted rats (24.2 +/- 0. 3 micro). However, when carbohydrate supplementation was provided during and immediately postexercise by intubation, muscle glycogen increased 77% above control (44.4 +/- 2.1 micromol/g). Exercise-fasting resulted in an 80% increase in fast-twitch red muscle GLUT-4 mRNA but only a 43% increase in GLUT-4 protein concentration. Conversely, exercise plus carbohydrate supplementation elevated fast-twitch red muscle GLUT-4 protein concentration by 88% above control, whereas GLUT-4 mRNA was increased by only 40%. Neither a 16-h fast nor carbohydrate supplementation had an effect on fast-twitch red muscle GLUT-4 protein concentration or on GLUT-4 mRNA in sedentary rats, although carbohydrate supplementation increased muscle glycogen concentration by 40% (35.0 +/- 0.9 micromol/g). GLUT-4 protein in fast-twitch white muscle followed a pattern similar to fast-twitch red muscle. These results indicate that carbohydrate supplementation, provided with exercise, will enhance GLUT-4 protein expression by increasing translational efficiency. Conversely, postexercise fasting appears to upregulate GLUT-4 mRNA, possibly to amplify GLUT-4 protein expression on an increase in glucose availability. These regulatory mechanisms may help control muscle glucose uptake in accordance with glucose availability and protect against postexercise hypoglycemia.

The effect of a carbohydrate--arginine supplement on postexercise carbohydrate metabolism

The effect of a carbohydrate-arginine supplement on postexercise muscle glycogen storage was investigated. Twelve well-trained cyclists rode for 2 hr on two separate occasions to deplete their muscle glycogen stores. At 0, 1, 2, and 3 hr after each exercise bout, the subjects ingested either a carbohydrate (CHO) supplement (1 g carbohydrate/kg body weight) or a carbohydrate-arginine (CHO/AA) supplement (1 g carbohydrate/kg body mass and 0.08 g arginine-hydrochloride/kg body weight). No difference in rate of glycogen storage was found between the CHO/AA and CHO treatments, although significance was approached. There were also no differences in plasma glucose, insulin, or blood lactate responses between treatments. Postexercise carbohydrate oxidation during the CHO/AA treatment was significantly reduced compared to the CHO treatment. These results suggest that the addition of arginine to a CHO supplement reduces the rate of CHO oxidation postexercise and therefore may increase the availability of glucose for muscle glycogen storage during recovery.

Role of carbohydrate in physical activity

Carbohydrate is an essential fuel for prolonged, strenuous exercise, although the carbohydrate stores of the body are limited. Research studies have provided evidence that carbohydrate depletion is associated with fatigue, decrease in exercise intensity, and even exercise cessation. With the appropriate diet and exercise protocol, however, the carbohydrate stores of the body can be substantially increased and exercise performance improved by carbohydrate supplementation before and during exercise. In this article, the role of carbohydrate supplementation for increasing carbohydrate stores before exercise, maintaining blood glucose during exercise, and the rapid replenishment of the carbohydrate stores after exercise are discussed. Considered in the discussion are the types, amounts and forms of carbohydrate supplements that are most effective, and the most appropriate times for their ingestion.

Increased amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals

We have recently demonstrated that insulin activates farnesyltransferase (FTase) and thereby increases the amounts of cellular farnesylated p21Ras in 3T3-L1 fibroblasts, adipocytes and vascular smooth muscle cells. We postulated that hyperinsulinaemia might considerably increase the the cellular pool of farnesylated p21Ras available for activation by other growth factors. To examine the role of in vivo hyperinsulinaemia in regulating farnesylated p21Ras, we measured the amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals. Liver, aorta, and skeletal muscle of ob/ob mice, and mice made obese and hyperinsulinaemic by injection of gold-thioglucose contained greater amounts of farnesylated p21Ras than tissues of their lean normoinsulinaemic counterparts. Similarly, farnesylated p21Ras was increased (67 vs. 35 % in control animals, p<0.01) in the livers of hyperinsulinaemic Zucker rats (fa/fa). Reduction of hyperinsulinaemia by exercise training (2 h/day for 7-8 weeks) resulted in decreases in the amounts of farnesylated p21Ras in these animals. Increased farnesylated p21Ras in hyperinsulinaemic animals reflected increasing increments in the activity of FTase in ob/ob mice (2-fold increase) and fa/fa Zucker rats (3.5-fold increase), while the total amounts of Ras proteins remained unchanged. In contrast to insulin-resistant hyperinsulinaemic animals, denervated insulin-resistant rat soleus muscle (in the presence of normoinsulinaemia) showed normal amounts of farnesylated p21Ras. In summary, these data confirm increased amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals.

Regulation of GLUT4 protein expression and glycogen storage after prolonged exercise

The purpose of this study was to determine the time course of GLUT4 protein accumulation following an exercise-carbohydrate supplementation regimen, and to evaluate the effect of this regimen on GLUT4 mRNA regulation. Rats were exercised by swimming and intubated with 1 mL of a 50% glucose solution immediately post-exercise. Exercise significantly reduced muscle glycogen by 50%. By 1.5 h of recovery, muscle glycogen was normalized, but continued to increase above the control level during the next 16 h. A faster and larger repletion of glycogen occurred in the fast-twitch red compared with the fast-twitch white muscle during the 16 h of recovery. GLUT4 protein concentration in fast-twitch red muscle was significantly increased above control by 1.5 h of recovery, and progressively increased throughout the recovery period. Fast-twitch white muscle demonstrated a similar trend, but the increase in GLUT4 protein did not reach significance until 5 h of recovery. Fast-twitch red muscle GLUT4 mRNA was increased by 53% above control immediately post-exercise, but returned to the control level by 1.5 h of recovery. GLUT4 mRNA associated with polysomes, however, increased significantly during this time and remained elevated for a minimum of 5 h. The results suggest that the increased GLUT4 protein expression following a regimen of exercise-carbohydrate supplementation occurs sufficiently fast to contribute to the resynthesis of muscle glycogen, and is controlled by both pre-translational and translational mechanisms.

Attenuating the decline in ATP arrests the exercise training-induced increases in muscle GLUT4 protein and citrate synthase activity

Thirty-two female Sprague-Dawley rats were assigned to one of four groups: control (CON); exercise training (TR); exercise training + clenbuterol treatment (0.8 mg kg body wt(-1) d(-1)) (TR + CL) or exercise training + clenbuterol treatment + 2% beta-guanidinoproprionic acid diet (TR + CL + beta) to examine whether alterations in the high energy phosphate state of the muscle mediates exercise training-induced increases in skeletal muscle GLUT4 protein concentration and citrate synthase activity. Exercise training consisted of running the rats 5 d week(-1) for 8 weeks on a motor-driven treadmill (32 m min(-1), 15% grade). Gastrocnemius GLUT4 protein concentration and citrate synthase activity were significantly elevated in the TR animals, but these adaptations were attenuated in the TR + CL animals. Providing beta-GPA in combination with clenbuterol enabled training to elevate GLUT4 protein concentration and citrate synthase activity, with the increase in GLUT4 being greater than that observed for the TR animals. Skeletal muscle ATP levels were reduced in the TR + CL + beta animals while ATP levels in the TR + CL animals were significantly elevated compared with CON. An acute 40-min bout of electrical stimulation of the sciatic nerve was found to lower skeletal muscle ATP levels by approximately 50% and elevate cAMP levels in all groups. No difference in post-contraction cAMP levels were observed among groups. However, post-contraction ATP levels in the TR + CL animals were significantly greater than the other groups. Collectively, these findings suggest that exercise training-induced increases in skeletal muscle GLUT4 protein concentration and citrate synthase activity are initiated in response to a reduction in the skeletal muscle ATP concentration.

Amylin-mediated inhibition of insulin-stimulated glucose transport in skeletal muscle

We examined the effects of amylin on 3-O-methyl-D-glucose (3-O-MG) transport in perfused rat hindlimb muscle under hyperinsulinemic (350 microU/ml, 2,100 pmol/l) conditions. Amylin at 100 nmol/l concentration inhibited 3-O-MG transport relative to control in all three basic muscle fiber types. Transport decreased in slow-twitch oxidative (from 5.65 +/- 1.13 to 3.46 +/- 0.71 micromol . g-1 . h-1), fast-twitch oxidative (from 6.84 +/- 0.90 to 4.84 +/- 0.76 micromol . g-1 . h-1), and fast-twitch glycolytic (from 1.27 +/- 0.20 to 0.60 +/- 0.05 micromol . g-1 . h-1) muscle. Amylin inhibition of insulin-stimulated glucose transport in skeletal muscle was accompanied by a 433 +/- 72% increase in intracellular glucose 6-phosphate (G-6-P) despite the absence of extracellular glucose. The source of hexose units for the formation and maintenance of G-6-P was likely glycogen. Amylin increased glycogenolysis, increased lactate formation, and decreased glycogen synthase activity. Furthermore, the kinetics of glycogen synthase suggest that this enzyme may control intracellular G-6-P concentration. Despite the large increase in G-6-P, no detectable increase in uridine diphosphate-N-acetylhexosamines occurred, suggesting that the proposed glucosamine pathway may not be involved in transport inhibition. However, decreases in uridine diphosphate hexoses were detected. Therefore, uridine or hexosamine-based metabolites may be involved in amylin action.

Effect of chronic electrical stimulation and beta-GPA diet on GLUT4 protein concentration in rat skeletal muscle

The present study investigated whether alterations in the muscle high energy phosphate state initiates the contraction-induced increase in skeletal muscle GLUT4 protein concentration. Sprague-Dawley rats were provided either a normal or a 2% beta-guanidinoproprionic acid (beta-GPA) diet for 8 weeks and then the gastrocnemius of one hind limb was subjected to 0, 14 or 28 days of chronic (24 h day-1) low-frequency electrical stimulation (10 Hz). The beta-GPA diet, in the absence of electrical stimulation, significantly reduced ATP, creatine phosphate, creatine and inorganic phosphate and elevated GLUT4 protein concentration by 60% without altering adenylate cyclase activity or cAMP concentration. Following 14 days of electrical stimulation, GLUT4 protein concentration was elevated above non-stimulated muscle in both groups but was significantly more elevated in the beta-GPA group. Concurrent with this greater rise in GLUT4 protein concentration was a greater decline in the high energy phosphates and a greater rise in cAMP. After 28 days of electrical stimulation, GLUT4 protein concentration and cAMP stabilized and was not different between diet treatments. However, the high energy phosphates were significantly higher in the normal diet rats as opposed to the beta-GPA rats. These findings therefore suggest that a reduction in cellular energy supply initiates the contraction-induced increase in muscle GLUT4 protein concentration, but that a rise in cAMP may potentiate this effect.

Glycogen resynthesis after exercise: effect of carbohydrate intake

To maximize glycogen resynthesis after exercise, a carbohydrate supplement in excess of 1.0 g x kg(-1) body wt should be consumed immediately after competition or a training bout. Continuation of supplementation every two hours will maintain a rapid rate of storage up to six hours post exercise. Supplements composed of glucose or glucose polymers are the most effective for replenishment of muscle glycogen, whereas fructose is most beneficial for the replenishment of liver glycogen. The addition of protein to a carbohydrate supplement may also increase the rate of glycogen storage due to the ability of protein and carbohydrate to act synergistically on insulin secretion.

Effect of pyruvate and dihydroxyacetone on metabolism and aerobic endurance capacity

Pyruvate and dihydroxyacetone are three carbon compounds that when infused directly into the blood or taken orally produce strong metabolic effects. When chronically fed to animals as part of their diet, pyruvate plus dihydroxyacetone reduce the rate of weight gain and body fat content during growth. These alterations in growth pattern appear to be the result of an increased loss of calories as heat at the expense of storage of lipid. Pyruvate-dihydroxyacetone supplementation has also been found to improve the insulin sensitivity of insulin resistant rats and reduce plasma cholesterol levels induced by a high cholesterol diet as well as lower blood pressure and heart rate in obese individuals. When infused in rats during prolonged treadmill running, pyruvate reduced run time to exhaustion by approximately 67%. However, when provided as an oral supplement for several days, it has enhanced aerobic endurance capacity. The mechanism of action is unclear, but available data suggest that the increase in performance following pyruvate-dihydroxyacetone supplementation may be a result of an increased reliance on blood glucose, thus sparing muscle glycogen. In summary, chronic supplementation of pyruvate-dihydroxyacetone may be beneficial from a preventive medicine prospective as well as for certain athletic endeavors.

Regulation of GLUT4 protein and glycogen synthase during muscle glycogen synthesis after exercise

The pattern of muscle glycogen synthesis following its depletion by exercise is biphasic. Initially, there is a rapid, insulin independent increase in the muscle glycogen stores. This is then followed by a slower insulin dependent rate of synthesis. Contributing to the rapid phase of glycogen synthesis is an increase in muscle cell membrane permeability to glucose, which serves to increase the intracellular concentration of glucose-6-phosphate (G6P) and activate glycogen synthase. Stimulation of glucose transport by muscle contraction as well as insulin is largely mediated by translocation of the glucose transporter isoform GLUT4 from intracellular sites to the plasma membrane. Thus, the increase in membrane permeability to glucose following exercise most likely reflects an increase in GLUT4 protein associated with the plasma membrane. This insulin-like effect on muscle glucose transport induced by muscle contraction, however, reverses rapidly after exercise is stopped. As this direct effect on transport is lost, it is replaced by a marked increase in the sensitivity of muscle glucose transport and glycogen synthesis to insulin. Thus, the second phase of glycogen synthesis appears to be related to an increased muscle insulin sensitivity. Although the cellular modifications responsible for the increase in insulin sensitivity are unknown, it apparently helps maintain an increased number of GLUT4 transporters associated with the plasma membrane once the contraction-stimulated effect on translocation has reversed. It is also possible that an increase in GLUT4 protein expression plays a role during the insulin dependent phase.

Amylin influences insulin-stimulated glucose metabolism by two independent mechanisms

The effects of amylin on fiber type-specific muscle glucose metabolism under hyperglycemic (10 mmol/l) and hyperinsulinemic (2.1 nmol/l) conditions were investigated using a rat hindlimb perfusion system. Amylin concentration ranged from 1 to 100 nM. Efficacy for inhibition of glucose uptake traced with 2-deoxyglucose by amylin was demonstrated in all three fiber types. The incorporation of 2-deoxy-[3H]glucose tracer decreased from control values by 41% in fast oxidative (FO), 36% in fast glycolytic (FG), and 37% in slow oxidative (SO) muscle with 100 nM amylin. Amylin increased intracellular glucose 6-phosphate (G-6-P), and G-6-P was negatively correlated with 2-deoxyglucose uptake in both FO (r = -0.65; P < 0.01) and FG (r = -0.53; P < 0.01) muscle. Muscle glycogen concentration increased under control conditions and decreased in the presence of 100 nM amylin. Lactate arteriovenous efflux across the hindlimb increased significantly above control with 100 nM amylin (5.03 +/- 0.81 to 11.28 +/- 0.94 mumol.g-1.h-1). Adenosine 3',5'-cyclic monophosphate (cAMP) increased in FO and FG muscle with amylin. Salmon calcitonin-(8-32), an amylin antagonist, ameliorated the effect of amylin on all responses other than 2-deoxyglucose uptake and G-6-P concentration. These results suggest that amylin may work through at least two independent mechanisms, a cAMP-mediated effect on glycogen metabolism and a non-cAMP-mediated inhibition of glycolysis.

Role of exercise training in the prevention and treatment of insulin resistance and non-insulin-dependent diabetes mellitus

Recent epidemiological studies indicate that individuals who maintain a physically active lifestyle are much less likely to develop impaired glucose tolerance and non-insulin-dependent diabetes mellitus (NIDDM). Moreover, it was found that the protective effect of physical activity was strongest for individuals at highest risk of developing NIDDM. Reducing the risk of insulin resistance and NIDDM by regularly performed exercise is also supported by several aging studies. It has been found that older individuals who vigorously train on a regular basis exhibit a greater glucose tolerance and a lower insulin response to a glucose challenge than sedentary individuals of similar age and weight. While the evidence is substantial that aerobic exercise training can reduce the risk of impaired glucose tolerance and NIDDM, the evidence that exercise training is beneficial in the treatment of NIDDM is not particularly strong. Many of the early studies investigating the effects of exercise training on NIDDM could not demonstrate improvements in fasting plasma glucose and insulin levels, or glucose tolerance. The adequacy of the training programmes in many of these studies, however, is questionable. More recent studies using prolonged, vigorous exercise-training protocols have produced more favourable results. There are several important adaptations to exercise training that may be beneficial in the prevention and treatment of insulin resistance, impaired glucose tolerance and NIDDM. An increase in abdominal fat accumulation and loss of muscle mass are highly associated with the development of insulin resistance. Exercise training results in preferential loss of fat from the central regions of the body and should therefore contribute significantly in preventing or alleviating insulin resistance due to its development. Likewise, exercise training can prevent muscle atrophy and stimulate muscle development. Several months of weight training has been found to significantly lower the insulin response to a glucose challenge without affecting glucose tolerance, and to increase the rate of glucose clearance during a euglycaemic clamp. Muscle glucose uptake is equal to the product of the arteriovenous glucose difference and the rate of glucose delivery or muscle blood flow. While it has been known for many years that insulin will accelerate blood glucose extraction by insulin-sensitive peripheral tissues, recent evidence suggests that it can also acutely vasodilate skeletal muscle and increase muscle blood flow in a dose-dependent manner. A reduced ability of insulin to stimulate muscle blood flow is a characteristic of insulin-resistant obese individuals and individuals with NIDDM. Exercise training, however, has been found to help alleviate this problem, and substantially improve the control of insulin over blood glucose. Improvements in insulin resistance and glucose tolerance with exercise training are highly related to an increased skeletal muscle insulin action. This increased insulin action is associated with an increase in the insulin-regulatable glucose transporters, GLUT4, and enzymes responsible for the phosphorylation, storage and oxidation of glucose. Changes in muscle morphology may also be important following training. With exercise training there is an increase in the conversion of fast twitch glycolytic IIb fibres to fast twitch oxidative IIa fibres, as well as an increase in capillary density. IIa fibres have a greater capillary density and are more insulin-sensitive and -responsive than IIb fibres. Evidence has been provided that morphological changes in muscle, particularly the capillary density of the muscle, are associated with changes in fasting insulin levels and glucose tolerance. Furthermore, significant correlations between glucose clearance, muscle capillary density and fibre type have been found in humans during a euglycaemic clamp. Exercise training may also improve control over hepatic glucose production by increasin

Chromium and exercise training: effect on obese women

Chromium supplementation may affect various risk factors for coronary artery disease (CAD) and non-insulin-dependent diabetes mellitus (NIDDM), including body weight and composition, basal plasma hormone and substrate levels, and response to an oral glucose load. This study examined the effects of chromium supplementation (400 micrograms.d-1), with or without exercise training, on these risk factors in young, obese women. Chromium picolinate supplementation resulted in significant weight gain in this population, while exercise training combined with chromium nicotinate supplementation resulted in significant weight loss and lowered the insulin response to an oral glucose load. We conclude that high levels of chromium picolinate supplementation are contraindicated for weight loss in young, obese women. Moreover, our results suggest that exercise training combined with chromium nicotinate supplementation may be more beneficial than exercise training alone for modification of certain CAD and NIDDM risk factors.

Exercise training reverses insulin resistance in muscle by enhanced recruitment of GLUT-4 to the cell surface

The effects of exercise training on cell surface GLUT-4 in skeletal muscle of the obese (fa/fa) Zucker rat were investigated using the impermeant glucose transporter photoaffinity reagent 2-N-4-(1-azi-2,2,2-trifluoroethyl)-benzoyl-1,3-bis- (D-mannos-4-yloxy)-2-propylamine (ATB-BMPA). In the absence of insulin, 3-O-methyl-D-glucose transport activity was no different in either fast-twitch (epitrochlearis) or slow-twitch (soleus) muscles of trained and sedentary obese rats. Likewise, basal ATB-BMPA-labeled GLUT-4 was not altered in these muscles with training. In contrast, the trained group exhibited significantly greater insulin-stimulated (2 mU/ml) glucose transport activity in epitrochlearis muscles than the sedentary group (0.53 +/- 0.03 vs. 0.18 +/- 0.03 mumol.g-1 x 10 min-1 for trained and sedentary, respectively), which was paralleled by a significant enhancement of insulin-stimulated cell surface GLUT-4 (5.33 +/- 0.20 vs. 1.57 +/- 0.14 disintegrations.min-1.mg-1 for trained and sedentary, respectively). Exercise training, however, did not alter insulin-stimulated glucose transport activity or cell surface GLUT-4 in soleus muscles. Finally, exercise training did not alter the ability of muscle contraction to elevate glucose transport activity or cell surface GLUT-4 in either epitrochlearis or soleus muscles of the obese rat. These results indicate that training improves insulin-stimulated glucose transport in muscle of the obese Zucker rat by increasing GLUT-4 content and by altering the normal intracellular distribution of these transporters such that they are now capable of migrating to the cell surface in response to the insulin stimulus.

Role of glycogen concentration and epinephrine on glucose uptake in rat epitrochlearis muscle

The effects of diet-manipulated variations in muscle glycogen concentration and epinephrine on glucose uptake were studied in epitrochlearis muscles from Wistar rats. Both basal and insulin-stimulated glucose uptake [measured with a tracer amount of 2-[1,2-3H(N)]deoxy-D-glucose] inversely correlated with initial glycogen concentration (glycogen concentration vs. basal glucose uptake: Spearman's rho = -0.76, n = 84, P < 0.000001; glycogen concentration vs. insulin-stimulated glucose uptake: Spearman's rho = -0.67, n = 44, P < 0.00001). Two fasting-refeeding procedures were used that resulted in differences in muscle glycogen concentrations, although with similar treatment for the last 48 h before the experiment. In the rats with the lower glycogen concentration, basal as well as insulin-stimulated glucose uptake was elevated. The muscle glycogen concentration had no effect on epinephrine-stimulated glycogenolysis. Epinephrine, however, was found to reduce basal glucose uptake in all groups. These results suggest that 1) the glycogen concentration participates in the regulation of both basal and insulin-stimulated glucose uptake in skeletal muscle, 2) the magnitude of epinephrine-stimulated glycogen breakdown is independent of the glycogen concentration, and 3) epinephrine inhibits basal glucose uptake at all glycogen concentrations.

Contraction-induced intracellular signals and their relationship to muscle GLUT-4 concentration

This investigation used a model of increased skeletal muscle contractile activity to evaluate whether the adenylate cyclase-adenosine 3',5'-cyclic monophosphate (cAMP) pathway and/or the high-energy phosphate state of the muscle might be temporally related to the contraction-induced increase in skeletal muscle GLUT-4 protein concentration. Plantaris and gastrocnemius muscles of Sprague-Dawley rats were subjected to 3, 7, 14, or 28 days of chronic low-frequency electrical stimulation (10 Hz, 24 h/day). GLUT-4 protein concentration was slightly reduced after 3 days of electrical stimulation, similar to control values at 7 days and significantly elevated above control at 14 days (53%, P < 0.05) and 28 days (338%, P < 0.05) of stimulation. ATP, creatine phosphate, creatine, and P, were inversely related to GLUT-4 protein concentration. Adenylate cyclase activity increased with electrical stimulation and was significantly related to the increased GLUT-4 protein. cAMP was significantly increased at 14 days of stimulation and remained elevated through 28 days. These results demonstrate that both the adenylate cyclase-cAMP pathway and the high-energy phosphate state of the muscle are temporally related to elevations in skeletal muscle GLUT-4 protein concentration in response to chronic low-frequency electrical stimulation and, as such, suggest that both may comprise a component of the intracellular signal that regulates the contraction-induced increase in skeletal muscle GLUT-4 protein concentration.

Interaction of exercise training and clenbuterol on GLUT-4 protein in muscle of obese Zucker rats

Chronic administration of clenbuterol, a beta 2-adrenergic agonist, attenuates the exercise training-induced improvement in muscle insulin resistance of the obese Zucker rat. The present study was conducted to determine whether clenbuterol also attenuates the increase in muscle GLUT-4 protein that occurs with exercise training and whether the action of clenbuterol is related to its ability to downregulate the beta-adrenergic receptors. Female obese Zucker rats were randomly assigned to one of the following four groups: control (CON, n = 7), clenbuterol (CL, n = 8), exercise training (TR, n = 8), and clenbuterol with exercise training (CL+TR, n = 8). Rats assigned to the training groups were run on a rodent motor-driven treadmill for 6-7 wk. Rats receiving clenbuterol were intubated with 0.8 mg/kg body weight 30 min before running each day. Red quadriceps (RQ) and white quadriceps (WQ) GLUT-4 protein concentrations of TR rats were significantly greater than those of CON and CL+TR rats. The RQ GLUT-4 protein concentration of the CL+TR rats was significantly greater than that of CON rats, but this difference did not occur in the WQ. GLUT-4 protein concentrations were not different between the CON and CL rats. The patterns of RQ and WQ GLUT-4 mRNA were similar to those of their respective GLUT-4 proteins. Rats receiving daily injections of propranolol (30 mg/kg body wt), a beta-adrenergic receptor antagonist, demonstrated no increase in GLUT-4 protein in RQ or WQ after 6 wk of exercise training. These results indicate that 1) clenbuterol can attenuate the increase in muscle GLUT-4 protein associated with exercise training and 2) this effect is likely mediated by a downregulation of the beta-adrenergic receptors.

Glucose transport and cell surface GLUT-4 protein in skeletal muscle of the obese Zucker rat

The relationship between 3-O-methyl-D-glucose transport and 2-N-4-(1-azi-2,2,2-trifluoroethyl)-benzoyl-1, 3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA)-labeled cell surface GLUT-4 protein was assessed in fast-twitch (epitrochlearis) and slow-twitch (soleus) muscles of lean and obese (fa/fa) Zucker rats. In the absence of insulin, glucose transport as well as cell surface GLUT-4 protein was similar in both epitrochlearis and soleus muscles of lean and obese rats. In contrast, insulin-stimulated glucose transport rates were significantly higher for lean than obese rats in both soleus (0.74 +/- 0.05 vs. 0.40 +/- 0.02 mumol.g-1.10 min-1) and epitrochlearis (0.51 +/- 0.05 vs. 0.17 +/- 0.02 mumol.g-1.10 min-1) muscles. The ability of insulin to enhance glucose transport in fast- and slow-twitch muscles from both lean and obese rats corresponded directly with changes in cell surface GLUT-4 protein. Muscle contraction elicited similar increases in glucose transport in lean and obese rats, with the effect being more pronounced in fast-twitch (0.70 +/- 0.07 and 0.77 +/- 0.04 mumol.g-1.10 min-1 for obese and lean, respectively) than in slow-twitch muscle (0.36 +/- 0.03 and 0.40 +/- 0.02 mumol.g-1.10 min-1 for obese and lean, respectively). The contraction-induced changes in glucose transport directly corresponded with the observed changes in cell surface GLUT-4 protein. Thus the reduced glucose transport response to insulin in skeletal muscle of the obese Zucker rat appears to result directly from an inability to effectively enhance cell surface GLUT-4 protein.

Overexpression of hexokinase II in transgenic mice. Evidence that increased phosphorylation augments muscle glucose uptake

Hexokinase II (HKII) is the predominant isozyme expressed in peripheral insulin-responsive tissues. To explore the role of HKII in muscle glucose metabolism, two lines of transgenic mice were generated where overexpression was restricted to striated muscle; HKII protein levels and activity were increased by 3-8-fold. Oral glucose tolerance, intravenous insulin tolerance, and insulin and lactate levels were unaffected in transgenic mice. There was a trend toward increased levels of muscle glycogen; however, glucose-6-phosphate levels were increased by 43% in transgenic skeletal muscle following in vivo glucose and insulin administration. Using 2-[3H]deoxyglucose as a tracer, in vitro basal and insulin-stimulated glucose uptake were determined in extensor digitorum longus, soleus, and epitrochlearis muscles. Maximal insulin-stimulated glucose uptake was increased by 17% (extensor digitorum longus), 34% (soleus), and 90% (epitrochlearis) in transgenic muscles; basal and submaximal glucose uptake was also modestly increased in soleus and epitrochlearis. These data suggest that increased muscle HKII (corresponding to the upper end of the physiologic range) may not be sufficient to augment net in vivo glucose homeostasis. However, glucose phosphorylation can represent a rate-limiting step for skeletal muscle glucose utilization since muscle glucose-6-phosphate levels are increased during in vivo hyperinsulinemia and hyperglycemia; furthermore, basal and insulin-mediated muscle glucose uptake can be increased by a selective increase in HKII expression.

Transgenic mice with muscle-specific insulin resistance develop increased adiposity, impaired glucose tolerance, and dyslipidemia

Impaired skeletal muscle insulin receptor function is a feature of common forms of insulin resistance, including obesity and noninsulin-dependent diabetes mellitus. However, the extent to which this defect accounts for impaired muscle glucose disposal or altered in vivo glucose homeostasis remains to be established. We recently showed that transgenic mice that overexpress dominant-negative insulin receptors specifically in striated muscle have a severe defect in muscle insulin receptor-mediated signaling and modest hyperinsulinemia. Here we performed hindlimb perfusion studies to determine the impact of this defect on muscle glucose uptake and metabolism. Maximal rates of insulin-stimulated muscle 3-O-methylglucose transport were reduced by 32-40% in transgenic mice with proportional defects involving total hindlimb [14C]glucose uptake, lactate production, and muscle glycogen synthesis. To address the hypothesis that muscle insulin resistance could promote an increase in the accretion of body fat, carcass analysis was performed using two independent lines of transgenic mice. Although body weights were normal, transgenic mice had a 22-38% increase in body fat, with a reciprocal decrease (10-15%) in body protein. Mean gonadal fat pad weight was also increased in transgenic mice. Skeletal muscle histology and fiber type distribution were not affected. To determine whether muscle-specific insulin resistance was sufficient to cause impaired glucose tolerance, oral glucose tolerance tests were performed with 6-month-old transgenic and control mice. Fasting glucose levels were increased by 25%, and peak values were 22-40% higher in transgenic mice. Transgenic mice also had a 37% decrease in plasma lactate levels and modest increases in levels of plasma triglycerides and FFA (29% and 15%, respectively). We conclude that 1) severe defects in muscle insulin receptor function result in impaired insulin-stimulated glucose uptake and metabolism in this tissue; 2) muscle-specific insulin resistance can contribute to the development of obesity; and 3) a "pure" defect in insulin-mediated muscle glucose disposal is sufficient to result in impaired glucose tolerance and other features of the insulin resistance syndrome, including hyperinsulinemia and dyslipidemia.

Glucose transport and GLUT4 protein distribution in skeletal muscle of GLUT4 transgenic mice

The aim of the present investigation was to determine whether the subcellular distribution and insulin-stimulated translocation of the GLUT4 isoform of the glucose transporter are affected when GLUT4 is overexpressed in mouse skeletal muscle, and if the overexpression of GLUT4 alters maximal insulin-stimulated glucose transport and metabolism. Rates of glucose transport and metabolism were assessed by hind-limb perfusion in GLUT4 transgenic (TG) mice and non-transgenic (NTG) controls. Glucose-transport activity was determined under basal (no insulin), submaximal (0.2 m-unit/ml) and maximal (10 m-units/ml) insulin conditions using a perfusate containing 8 mM 3-O-methyl-D-glucose. Glucose metabolism was quantified by perfusing the hind limbs for 25 min with a perfusate containing 8 mM glucose and 10 m-units/ml insulin. Under basal conditions, there was no difference in muscle glucose transport between TG (1.10 +/- 0.10 mumol/h per g; mean +/- S.E.M.) and NTG (0.93 +/- 0.16 mumol/h per g) mice. However, TG mice displayed significantly greater glucose-transport activity during submaximal (4.42 +/- 0.49 compared with 2.69 +/- 0.33 mumol/h per g) and maximal (11.68 +/- 1.13 compared with 7.53 +/- 0.80 mumol/h per g) insulin stimulation. Nevertheless, overexpression of the GLUT4 protein did not alter maximal rates of glucose metabolism. Membrane purification revealed that, under basal conditions, plasma-membrane (approximately 12-fold) and intracellular-membrane (approximately 4-fold) GLUT4 protein concentrations were greater in TG than NTG mice. Submaximal insulin stimulation did not increase plasma-membrane GLUT4 protein concentration whereas maximal insulin stimulation increased this protein in both NTG (4.1-fold) and TG (2.6-fold) mice. These results suggest that the increase in insulin-stimulated glucose transport following overexpression of the GLUT4 protein is limited by factors other than the plasma-membrane GLUT4 protein concentration. Furthermore, GLUT4 overexpression is not coupled to glucose-metabolic capacity.

Fiber type-specific effects of clenbuterol and exercise training on insulin-resistant muscle

The interrelationships among glucose uptake, GLUT-4 protein, and citrate synthase activity in insulin-resistant skeletal muscle were investigated. Female obese (fa/fa) Zucker rats were randomly assigned to treadmill training, ingestion of the selective beta 2-adrenergic agonist clenbuterol, or sedentary control groups. After 7-8 wk of treatment, hindlimbs were perfused to determine maximal insulin-stimulated (10 mU/ml) 2-[3H]deoxy-D-glucose (2-DG) uptake. Exercise training significantly enhanced 2-DG uptake and GLUT-4 protein in red gastrocnemius and plantaris. Alternatively, 2-DG uptake was not altered in soleus after exercise training despite a 52% increase in GLUT-4 protein. The increases in GLUT-4 protein in red gastrocnemius, plantaris, and soleus of the trained rats were accompanied by increases in citrate synthase activity. In contrast to exercise training, clenbuterol administration decreased citrate synthase activity in red and white gastrocnemius, yet had no effect on GLUT-4 protein levels or maximal insulin-stimulated 2-DG uptake. Clenbuterol treatment did, however, increase citrate synthase activity and GLUT-4 protein in soleus. These findings indicate that total GLUT-4 protein largely determines the maximal rate of insulin-stimulated glucose uptake in fast-twitch muscle, whereas in slow-twitch muscle it does not. In addition, the results demonstrate that coordination of proteins governing glucose uptake and disposal may be disrupted in a fiber type-specific manner. Overall, the findings raise important questions as to whether regulation of proteins governing glucose uptake and disposal differs significantly among fiber types.

Ventilation parallels plasma potassium during incremental and continuous variable intensity exercise

The aim of the present investigation was to examine the relationship between plasma potassium (K+) and ventilation (VE) during incremental and prolonged continuous exercise which varied between low and moderate intensity. Seven well-trained male cyclists who had a mean maximal aerobic power (VO2max) of 69.4 +/- 2.9 ml/kg/min were recruited to participate as subjects. The graded incremental exercise bout was composed of 3 min stages set to elicit 40, 50, 60, 70, 80 and 90% VO2max. The continuous variable intensity exercise consisted of 30 min of cycling at 45% VO2max and then 6 x 16 min periods which consisted of cycling for 8 min at 75% VO2max and 8 min at 45% VO2max. During prolonged continuous exercise, VE and plasma K+ changed in a coordinated manner between the low and moderate intensity exercise bouts with the responses during the moderate intensity intervals being significantly greater (p < 0.05) than the responses during the low intensity intervals. During the incremental exercise test, a strong positive relationship between VE and plasma K+ concentration was found for each subject. However, a positive relationship and slope was also found when the VE/K+ ratio was correlated with exercise intensity (r = 0.90-0.99). This indicates that with increasing exercise intensity, the rise in VE becomes increasingly greater than the rise in plasma K+. These findings suggest that the plasma K+ concentration contributes to but may not be the sole determinant of ventilatory regulation during exercise.

Training adaptations of baboons to light and moderate treadmill exercise

We evaluated training adaptations by 18 baboons (Papio cynocephalus anubis) to low and moderate quadrupedal walking exercise on a motorized treadmill. Moderate training produced 47% increases in lactate threshold, 63% increases in muscle citrate synthetase activity, increases in percentage of Type IIc muscle fibers, and reduced plasma insulin concentrations. Low training produced only reduced plasma insulin concentrations. Only results indicate that the baboon response to exercise training was similar to that of Homo sapiens, and dependent on exercise intensity.

Glucose uptake and GLUT-4 protein distribution in skeletal muscle of the obese Zucker rat

The rates of muscle glucose uptake of lean and obese Zucker rats were assessed by hindlimb perfusion under basal conditions (no insulin), in the presence of a maximally stimulating concentration of insulin (10 mU/ml), and after muscle contraction elicited by electrical stimulation of the sciatic nerve. After perfusion, plasma and microsomal membranes were isolated from selected hindlimb muscles for determination of GLUT-4 protein distribution. Under basal conditions, rates of glucose uptake were similar for lean and obese rats despite plasma membranes from lean rats containing 82% more GLUT-4 protein than obese rats. Insulin stimulation resulted in significant increases in plasma membrane GLUT-4 protein concentration in lean but not obese rats. Glucose uptake of lean rats (35.3 +/- 4.7 mumol.h-1.g-1) in the presence of insulin was approximately fourfold greater than that of obese rats (8.8 +/- 1.3 mumol.h-1.g-1), but this difference in glucose uptake could not be completely accounted for by the difference in plasma membrane GLUT-4 protein concentration. Stimulation by contraction resulted in significant increases in plasma membrane GLUT-4 protein concentration in both lean and obese rats and similar rates of glucose uptake. These results suggest that the muscle insulin resistance of the obese Zucker rat is due to 1) a reduced plasma membrane GLUT-4 protein concentration, which results in part from an impairment in the insulin-stimulated GLUT-4 protein translocation process, and 2) a defect in the insulin-stimulated activation of this protein. However, contraction-stimulated glucose uptake, GLUT-4 protein translocation, and activation are normal in the obese Zucker rat.

Dietary supplements affect the anabolic hormones after weight-training exercise

To examine the effect of carbohydrate and/or protein supplements on the hormonal state of the body after weight-training exercise, nine experienced male weight lifters were given water (Control) or an isocaloric carbohydrate (CHO; 1.5 g/kg body wt), protein (PRO; 1.38 g/kg body wt), or carbohydrate-protein (CHO/PRO; 1.06 g carbohydrate/kg body wt and 0.41 g protein/kg) supplement immediately and 2 h after a standardized weight-training workout. Venous blood samples were drawn before and immediately after exercise and during 8 h of recovery. Exercise induced elevations in lactate, glucose, testosterone, and growth hormone. CHO and CHO/PRO stimulated higher insulin concentrations than PRO and Control. CHO/PRO led to an increase in growth hormone 6 h postexercise that was greater than PRO and Control. Supplements had no effect on insulin-like growth factor I but caused a significant decline in testosterone. The decline in testosterone, however, was not associated with a decline in luteinizing hormone, suggesting an increased clearance of testosterone after supplementation. The results suggest that nutritive supplements after weight-training exercise can produce a hormonal environment during recovery that may be favorable to muscle growth by stimulating insulin and growth hormone elevations.

The effects of muscle contraction and insulin on glucose-transporter translocation in rat skeletal muscle

The effect of electrically induced muscle contraction, insulin (10 m-units/ml) and electrically-induced muscle contraction in the presence of insulin on insulin-regulatable glucose-transporter (GLUT-4) protein distribution was studied in female Sprague-Dawley rats during hindlimb perfusion. Plasma-membrane cytochalasin B binding increased approximately 2-fold, whereas GLUT-4 protein concentration increased approximately 1.5-fold above control with contractions, insulin, or insulin + contraction. Microsomal-membrane cytochalasin B binding and GLUT-4 protein concentration decreased by approx. 30% with insulin or insulin + contraction, but did not significantly decrease with contraction alone. The rate of muscle glucose uptake was assessed by determining the rate of 2-deoxy[3H]glucose accumulation in the soleus, plantaris, and red and white portions of the gastrocnemius. Both contraction and insulin increased glucose uptake significantly and to the same degree in the muscles examined. Insulin + contraction increased glucose uptake above that of insulin or contraction alone, but this effect was only statistically significant in the soleus, plantaris and white gastrocnemius. The combined effects of insulin + contraction of glucose uptake were not fully additive in any of the muscles investigated. These results suggest that (1) insulin and muscle contraction are mobilizing two separate pools of GLUT-4 protein, and (2) the increase in skeletal-muscle glucose uptake due to insulin + contraction is not due to an increase in plasma-membrane GLUT-4 protein concentration above that observed for insulin or contraction alone.

Effects of pyruvate on the metabolism and insulin resistance of obese Zucker rats

Female obese Zucker rats aged 5 wk were randomly assigned to a control diet or one of two experimental diets. Experimental diets contained 6% of energy as pyruvate in the form of calcium-pyruvate (Ca-pyr) or 6% pyruvylglycine (pyr-gly). Diets were pair-fed according to the experimental group with the lowest food consumption. During the 3 wk of dietary treatment, Ca-pyr- and pyr-gly-fed rats gained significantly less weight, had a lower food-conversion efficiency, and maintained a higher resting oxygen consumption (mL.min-1 x kg-0.67) than control rats. Ca-pyr and pyr-gly also lowered the respiratory exchange ratio of the rats resulting in a 90% increase in their lipid oxidation and a 50% decrease in their carbohydrate oxidation. Glucose tolerance, assessed by an oral glucose load, was not different among treatments, but the insulin response of the pyr-gly-fed rats was significantly less than that of the control rats despite elevated plasma triglyceride concentrations in the pyr-gly-fed rats (control, 1.43 +/- 0.16 vs pyr-gly, 3.76 +/- 0.87 mmol/L). These results suggest that pyr-gly, like Ca-pyr, favorably alters the metabolism of obese Zucker rats. In addition, pyr-gly appeared to reduce the insulin resistance that develops spontaneously in obese rats.

Interaction of aerobic exercise training and clenbuterol: effects on insulin-resistant muscle

The effects of aerobic exercise training, chronic administration of the selective beta 2-adrenergic agonist clenbuterol, and the combination of these two treatments on muscle insulin resistance were compared in female obese (fa/fa) Zucker rats. Rats were randomly assigned to trained, clenbuterol, clenbuterol-trained, or control groups. Training consisted of treadmill running for 2 h/day at 18 m/min up an 8% grade. Clenbuterol was administered by intubation (0.4-0.8 mg.kg body wt-1 x day-1) approximately 30 min before the rats ran each day. After 8 wk of treatment, muscle insulin resistance was assessed via hindlimb perfusion in the presence of 8 mM glucose and a submaximal (500 microU/ml) insulin concentration. Training increased citrate synthase activity (mumol.g wet wt-1 x min-1) by 32-74% and insulin-stimulated glucose uptake by 45%. Clenbuterol ingestion induced a 17-29% increase in muscle mass but decreased citrate synthase activity by 34-42% and had no effect on muscle glucose uptake. Administration of clenbuterol to rats that exercise trained prevented the training-induced improvement in insulin-stimulated glucose uptake and attenuated the increases in citrate synthase activity. In addition, both clenbuterol-treated groups displayed a 42% decrease in beta-adrenergic receptor density. The results indicate that clenbuterol administration, possibly through beta-adrenergic receptor downregulation, attenuated a cellular reaction essential for the exercise training-induced increase in citrate synthase activity and improvement in skeletal muscle insulin resistance of the obese Zucker rat.

Carbohydrate supplementation spares muscle glycogen during variable-intensity exercise

Effects of carbohydrate (CHO) supplementation on muscle glycogen utilization and endurance were evaluated in seven well-trained male cyclists during continuous cycling exercise that varied between low [45% maximal O2 uptake (VO2 max)] and moderate intensity (75% VO2 max). During each exercise bout the subjects received either artificially flavored placebo (P), 10% liquid CHO supplement (L; 3 x 18 g CHO/h), or solid CHO supplement (S; 2 x 25 g CHO/h). Muscle biopsies were taken from vastus lateralis during P and L trials immediately before exercise and after first (124 min) and second set (190 min) of intervals. Subjects then rode to fatigue at 80% VO2 max. Plasma glucose and insulin responses during L treatment reached levels of 6.7 +/- 0.7 mM and 70.6 +/- 17.2 microU/ml, respectively, and were significantly greater than those of P treatment (4.4 +/- 0.1 mM and 17.7 +/- 1.6 microU/ml) throughout the exercise bout. Plasma glucose and insulin responses of S treatment were intermediate to those of L and P treatments. Times to fatigue for S (223.9 +/- 3.5 min) and L (233.4 +/- 7.5 min) treatments did not differ but were significantly greater than that of P treatment (202.4 +/- 9.8 min). After the first 190 min of exercise, muscle glycogen was significantly greater during L (79 +/- 3.5 mumol/g wet wt) than during P treatment (58.5 +/- 7.2 mumol/g wet wt). Furthermore, differences in muscle glycogen concentrations between L and P treatments after 190 min of exercise and in time to fatigue for these treatments were positively related (r = 0.76, P < 0.05). These results suggest that CHO supplementation can enhance prolonged continuous variable-intensity exercise by reducing dependency on muscle glycogen as a fuel source.

Insulin- and contraction-stimulated translocation of GTP-binding proteins and GLUT4 protein in skeletal muscle

Low molecular weight GTP-binding proteins and GLUT4 protein were isolated in purified plasma membrane and low density microsome fractions from rat skeletal muscle. GTP-binding proteins were detected via the ability of these proteins to bind [32P]GTP subsequent to Western blotting. GLUT4 protein was detected via the anti-GLUT4 antibody F349 subsequent to Western blotting. The possible involvement of GTP-binding proteins in the regulation of GLUT4 protein movement was investigated by examining the subcellular distribution of GTP-binding proteins and GLUT4 protein under basal conditions and following stimulation by insulin or muscle contraction. Insulin stimulation caused a 111 +/- 34.8% increase in the plasma membrane content of GTP-binding proteins which was paralleled by a 74 +/- 19.1% increase in the plasma membrane content of GLUT4 protein. The insulin-stimulated increase in plasma membrane GTP-binding proteins and GLUT4 protein occurred coincident with 27 +/- 4.6 and 33 +/- 7.4% decreases, respectively, in the low density microsome content of these proteins. In addition, muscle contraction significantly increased the plasma membrane content of GTP-binding proteins (63 +/- 18.1%) and GLUT4 protein (67 +/- 22.2%). However, with muscle contraction the concentrations of GTP-binding proteins and GLUT4 protein were not altered in low density microsome fractions. The similar patterns with which the GTP-binding proteins and GLUT4 protein responded to stimulation by insulin and muscle contraction suggests a possible, but yet unidentified functional relationship between these proteins.

Effects of exercise training on muscle GLUT-4 protein content and translocation in obese Zucker rats

The rates of muscle glucose uptake of trained (TR) and untrained (UT) obese Zucker rats were assessed by hindlimb perfusion under basal conditions (no insulin) in the presence of a maximally stimulating concentration of insulin (10 mU/ml) and after muscle contraction elicited by electrical stimulation of the sciatic nerve. Perfusate contained 28 mM glucose and 7.5 microCi/mmol of 2-deoxy-D-[3H]glucose. Muscle GLUT-4 concentration was determined by Western blot analysis and expressed as a percentage of a heart standard. The rates of insulin-stimulated glucose uptake were significantly higher in the plantaris, red gastrocnemius (RG), and white gastrocnemius (WG), but not the soleus or extensor digatorum longus (EDL) of TR compared with UT rats. After muscle contraction the rates of glucose uptake in the TR rats were significantly higher in the soleus, plantaris, and RG. TR rats had significantly higher GLUT-4 protein concentration and citrate synthase activity than the UT rats in the soleus, plantaris, RG, and WG. Basal plasma membrane GLUT-4 protein concentration of TR rats was 144% above UT rats (P < 0.01). Stimulation by insulin and contraction resulted in a significant increase in plasma membrane GLUT-4 protein concentration in UT rats only. However, plasma membrane GLUT-4 protein concentration in insulin- and contraction-stimulated TR rats remained 53% and 30% greater than that of UT rats, respectively (P < 0.05). Exercise training did not alter basal, insulin-, or contraction-stimulated GLUT-4 functional activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic electrical stimulation on GLUT-4 protein content in fast-twitch muscle

Insulin- and contraction-stimulated skeletal muscle glucose transport is governed largely by the GLUT-4 isoform of the glucose transporter. Recently, it has been demonstrated that denervated muscle has decreased GLUT-4 protein content, suggesting that regulation of GLUT-4 protein is related to neuromuscular activity. However, until now the effects of the opposite situation, enhanced neuromuscular activity, could only be speculated on from exercise training studies. In the present investigation the effect of chronic low-frequency electrical stimulation (10 Hz, 8 h/day) on GLUT-4 protein content and citrate synthase activity was determined in the predominantly fast-twitch plantaris. Chronic electrical stimulation enhanced GLUT-4 protein content and citrate synthase activity in the muscles stimulated for 10-20 days. Electrical stimulation lasting 30-40 days resulted in no further enhancement of GLUT-4 protein content while citrate synthase activity continued to increase. Further prolongation of electrical stimulation (60-90 days) resulted in a plateauing of citrate synthase activity. The results suggest that increased neuromuscular activity can act independently of systemic changes to increase total GLUT-4 protein content. They also suggest that both GLUT-4 protein content and citrate synthase activity are positively related to increased neuromuscular activity but that their rates of increase differ substantially.

Exercise training and clenbuterol reduce insulin resistance of obese Zucker rats

This study compared the effects of aerobic exercise training and chronic administration of the selective beta 2-adrenergic agonist clenbuterol on whole body and skeletal muscle insulin resistance in obese (fa/fa) Zucker rats. Obese rats were randomly assigned to training, clenbuterol, or sedentary control groups. Lean littermates served as a second control group. After 4-5 wk of treatment, an oral glucose tolerance test was performed, followed 1 wk later by hindlimb perfusion, during which time the rates of glucose uptake and 3-O-methyl-D-glucose (3-MG) transport were assessed in the presence of a submaximal (500 microU/ml) insulin concentration. Training resulted in a significant increase in citrate synthase and cytochrome oxidase activity in the recruited muscles. Clenbuterol induced a large increase in muscle mass but provoked a significant decrease in oxidative enzyme activity and beta-adrenergic receptor density. Both treatments increased glucose tolerance and reduced the postglucose insulin response, with the improvements being more pronounced in the clenbuterol group. However, only exercise training improved insulin-stimulated hindlimb muscle glucose uptake (11.37 +/- 0.65, 8.73 +/- 0.77, and 8.27 +/- 0.41 mumol.g-1.h-1 for trained, clenbuterol, and sedentary control groups, respectively) and 3-MG transport. These results suggest that aerobic exercise training attenuated the insulin-resistant condition in the obese Zucker rat by a mechanism other than or in addition to beta 2-adrenergic receptor activation.

Effects of exercise training on skeletal muscle glucose uptake and transport

Exercise training increases the concentration of GLUT-4 protein in skeletal muscle that is associated with an increase in maximal insulin-stimulated glucose transport. The purpose of this study was to determine whether exercise training results in a long-lasting increase in insulin-stimulated glucose transport in rat skeletal muscle. Glucose uptake and skeletal muscle 3-O-methyl-D-glucose (3-MG) transport were determined during hindlimb perfusion in the presence of a maximally stimulating concentration of insulin (10 mU/ml). Hindlimb glucose uptake was approximately 29% above sedentary (Sed) levels in rats examined within 24 h (24H) of their last exercise session. However, when rats were examined 48 h (48H) after their last exercise session, hindlimb glucose uptake was not different from Sed levels. Maximal 3-MG transport was enhanced, above Sed levels, in red (RG; 72% increase) and white (WG; 44% increase) gastrocnemius and plantaris (Plan; 67% increase) muscles, but not soleus (Sol), of 24H rats. GLUT-4 protein content was significantly elevated in those muscles that exhibited enhanced 3-MG transport in 24H rats. GLUT-4 protein content was also elevated in RG, WG, and Plan of 48H rats and was not different from 24H rats. Despite the elevated GLUT-4 protein content, 3-MG transport in 48H rats was only slightly, although statistically not significantly, higher than in Sed rats. These results provide evidence that exercise training does not result in a persistent increase in skeletal muscle glucose uptake or transport, despite an increase in GLUT-4 protein content.

Carbohydrate metabolism during exercise in hot and thermoneutral environments

This study compared the effects of moderately intense exercise in hot and thermoneutral environments on muscle glycogen and carbohydrate utilization. Well-trained, heat acclimatized cyclists (n = 7) rode at 73.6 +/- 1.1% maximal oxygen consumption for 60 min in a thermoneutral room (23.5 +/- 0.6 degrees C, 52.7 +/- 2.9 relative humidity) or an environmental chamber (33.7 +/- 0.1 degrees C, 49.1 +/- 1.8% relative humidity). During each exercise bout, the subjects received 125 ml of water every 15 min. Muscle biopsies from the vastus lateralis were obtained prior to and following each exercise bout. Exercise in the heat significantly elevated rectal temperature and heart rate above and reduced body weight and plasma volume below that produced by exercise in a thermoneutral environment. Plasma glucose and blood lactate concentrations were similar between treatments prior to exercise, but increased to a greater concentration (p < 0.05) when exercise was performed in the heat. No differences between treatments were found for blood glycerol or free fatty acid concentrations, carbohydrate oxidation or muscle glycogen utilization. These results suggest that moderately intense exercise in the heat, as opposed to a thermoneutral environment, does not increase the rate of muscle glycogenolysis or carbohydrate oxidation in well conditioned, heat acclimatized subjects.

Muscle glucose transport, GLUT-4 content, and degree of exercise training in obese Zucker rats

The effects of high (HI)- and low (LI)-intensity exercise training were examined on insulin-stimulated 3-O-methyl-D-glucose (3-MG) transport and concentration of insulin-regulatable glucose transporter protein (GLUT-4) in the red (fast-twitch oxidative) and white (fast-twitch glycolytic) quadriceps of the obese Zucker rat. Sedentary obese (SED) and lean (LN) Zucker rats were used as controls. 3-MG transport was determined during hindlimb perfusion in the presence of 8 mM 3-MG, 2 mM mannitol, 0.3 mM pyruvate, and 0.5 mU/ml insulin. HI and LI rats displayed greater rates of red quadriceps 3-MG transport and GLUT-4 concentrations than SED rats. No significant differences in rates of 3-MG transport or GLUT-4 concentrations were observed in the red quadriceps of HI and LI rats. There were no differences found in the rates of 3-MG transport in the white quadriceps of HI, LI, and SED rats although the difference between the HI and SED rats approached significance (P < 0.07). The GLUT-4 concentration and citrate synthase activity of HI rats were significantly greater than SED rats. The 3-MG transport rates of LN rats were twofold greater than SED rats regardless of fiber type, but a difference in GLUT-4 content between the LN and SED rats was observed only in the white quadriceps. GLUT-4 content of the obese rats was significantly correlated with citrate synthase activity (r = 0.93) and 3-MG transport (r = 0.82).(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction-activated glucose uptake is normal in insulin-resistant muscle of the obese Zucker rat

The rates of muscle glucose uptake of lean and obese Zucker rats were assessed via hindlimb perfusion under basal conditions (no insulin), in the presence of a maximal insulin concentration (10 mU/ml), and after electrically stimulated muscle contraction in the absence of insulin. The perfusate contained 28 mM glucose and 7.5 microCi/mmol of 2-deoxy-D-[3H-(G)]glucose. Glucose uptake rates in the soleus (slow-twitch oxidative fibers), red gastrocnemius (fast-twitch oxidative-glycolytic fibers), and white gastrocnemius (fast-twitch glycolytic fibers) under basal conditions and after electrically stimulated muscle contraction were not significantly different between the lean and obese rats. However, the rate of glucose uptake during insulin stimulation was significantly lower for obese than for lean rats in all three fiber types. Significant correlations were found for insulin-stimulated glucose uptake and glucose transporter protein isoform (GLUT-4) content of soleus, red gastrocnemius, and white gastrocnemius of lean (r = 0.79) and obese (r = 0.65) rats. In contrast, the relationships between contraction-stimulated glucose uptake and muscle GLUT-4 content of lean and obese rats were negligible because of inordinately low contraction-stimulated glucose uptakes by the solei. These results suggest that maximal skeletal muscle glucose uptake of obese Zucker rats is resistant to stimulation by insulin but not to contractile activity. In addition, the relationship between contraction-stimulated glucose uptake and GLUT-4 content appears to be fiber-type specific.