Muscle GLUT4 Protein Research Articles

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.

Skeletal Muscle and Liver Glucose Transporter Protein Expression in Obese Offspring of the Gestational Diabetic Rat † 467

Insulin resistance in diabetes mellitus has been associated with aberrant expression of glucose transporter proteins. We examined the effect of gestational diabetes on skeletal muscle Glut-4 and liver Glut-2 protein expression in the macrosomic offspring of the hyperglycemic rat. Five time-dated pregnant Sprague-Dawley rats were made mildly hyperglycemic (plasma glucose 100-300mg/dl) by intraperitoneal injection of 37 mg/kg streptozotocin on day five of gestation. Two rats were injected with vehicle alone to serve as controls. Macrosomia was selectively defined as birth weight >2SD of mean control pup birth weight (Control pups' mean birth weight: 6.29g±0.49, Macrosomia pups' mean birth weight: 7.40g±0.16). Animals were sacrificed at 10-12 weeks of age. Skeletal muscle and liver samples were quickly removed and frozen in liquid nitrogen. 100 mg of muscle or liver homogenate was solubulized in Laemmli buffer. Protein content was determined by Bio-Rad assay with bovine serum albumin as the standard. Samples were separated by SDS-PAGE and transferred to nitrocellulose. Membranes were incubated with primary antibody (rabbit anti-rat Glut-4 for muscle and rabbit anti-rat Glut-2 for liver) followed by anti-rabbit Ig, horseradish peroxidase linked secondary antibody. Western blot analysis was performed using Amersham ECL reagent. Densitometric quantitation data (OD units) are as follows: Control muscle Glut-4 9.18±0.14, Macrosomic muscle Glut-4 3.51±0.07 p<0.01; Control liver Glut-2 2.38 ±0.54, Macrosomic liver Glut-2 3.49 ±0.19 p <0.05. In the macrosomic offspring of the diabetic dam, muscle GLUT-4 protein expression was decreased by 62% and liver Glut-2 protein expression was increased by 46% compared to controls. Decreased Glut-4 protein expression in muscle peripherally, in contrast to increased Glut-2 protein expression in liver centrally, may partially account for the presence of insulin resistance known to occur in the offspring of the gestational diabetic.

Training in swimming reduces blood pressure and increases muscle glucose transport activity as well as GLUT4 contents in stroke-prone spontaneously hypertensive rats.

Exercise improves muscle insulin sensitivity and GLUT4 contents. We investigated the beneficial effects of swimming training on insulin sensitivity and genetic hypertension using stroke-prone hypertensive rats (SHRSP). We studied the relationship between genetic hypertension and insulin resistance in SHRSP and Wistar Kyoto rats (WKY) as a control. The systolic blood pressure of SHRSP was significantly reduced by 4-week swimming training (208.4 +/- 6.8 mmHg vs. 187.2 +/- 4.1 mmHg, p < 0.05). The swimming training also resulted in an approximately 20% increase in the insulin-stimulated glucose transport activity (p < 0.05) of soleus muscle strips and an approximately 3-fold increase in the plasma membrane GLUT4 protein expression (p < 0.01) in SHRSP. However, basal and insulin-stimulated glucose transport activity and GLUT4 contents were not significantly different between WKY and SHRSP. There was no difference in insulin resistance in skeletal muscle of SHRSP as compared with WKY. Our results indicated swimming training exercise improved not only hypertension but also muscle insulin sensitivity and GLUT4 protein expression in SHRSP.

Training effects on muscle glucose transport during exercise.

Muscle glucose uptake is increased during exercise compared to rest. In general, muscle glucose uptake increases with increasing exercise intensity and duration. Whereas the arterio-venous concentration difference only increases 2-4-fold during exercise compared with rest the increase in muscle perfusion in 10-20 times and therefore quantitatively very important. During exercise the surface membrane glucose transport capacity increases in skeletal muscle primarily due to an increase in surface membrane GLUT4 protein content. Endurance training decreases muscle glucose uptake during exercise at a given absolute submaximal work-load despite a large increase in muscle GLUT4 protein content. We have shown that this decrease in glucose uptake at least in part is due to a blunted exercise-induced increase in sarcolemmal glucose transport capacity secondary to a blunted increase in sarcolemmal GLUT4 transporter number. Thus, endurance training leads to a marked reduction of the fraction of muscle GLUT4 that is translocated during a given submaximal exercise bout. Whether this is true also during exercise at higher intensities remains to be seen.

Sarcolemmal glucose transport and GLUT-4 translocation during exercise are diminished by endurance training.

Glucose utilization during exercise of a given submaximal power output is decreased after endurance training. The aim of the present study was to elucidate the mechanisms behind this phenomenon by utilizing the sarcolemmal giant vesicle technique. Eight healthy young untrained men endurance trained one thigh for 3 wk. They then exercised both thighs simultaneously at the same work load (77% of peak O2 uptake of the untrained thigh) for 40 min. Training increased muscle GLUT-4 protein by 70% (P < 0.05). Glucose uptake during exercise was 38% lower (P < 0.05) in the trained (T) thigh than in the untrained (UT) thigh because of both a lower (P < 0.05) glucose extraction and blood flow in T. During exercise, sarcolemmal GLUT-4 protein content and glucose transport capacity increased significantly less in T than in UT muscle, and muscle glucose concentration was lower in T compared with UT (P < 0.05) at the end of exercise. It is concluded that, despite a large increase in muscle GLUT-4 with endurance training, exercise of a given submaximal power output increases muscle glucose uptake less in T than in UT muscle. It is suggested that the mechanism behind this phenomenon is blunted exercise-induced translocation of GLUT-4 to the sarcolemma, resulting in a blunted increase in sarcolemmal glucose transport in T muscle.

Changes in insulin-stimulated glucose transport and GLUT-4 protein in rat skeletal muscle after training.

After running training, which increased GLUT-4 protein content in rat skeletal muscle by <40% compared with control rats, the training effect on insulin-stimulated maximal glucose transport (insulin responsiveness) in skeletal muscle was short lived (24 h). A recent study reported that GLUT-4 protein content in rat epitrochlearis muscle increased dramatically ( approximately 2-fold) after swimming training (J.-M. Ren, C. F. Semenkovich, E. A. Gulve, J. Gao, and J. O. Holloszy. J. Biol. Chem. 269, 14396-14401, 1994). Because GLUT-4 protein content is known to be closely related to skeletal muscle insulin responsiveness, we thought it possible that the training effect on insulin responsiveness may remain for >24 h after swimming training if GLUT-4 protein content decreases gradually from the relatively high level and still remains higher than control level for >24 h after swimming training. Therefore, we examined this possibility. Male Sprague-Dawley rats swam 2 h a day for 5 days with a weight equal to 2% of body mass. Approximately 18, 42, and 90 h after cessation of training, GLUT-4 protein concentration and 2-[1,2-3H]deoxy-D-glucose transport in the presence of a maximally stimulating concentration of insulin (2 mU/ml) were examined by using incubated epitrochlearis muscle preparation. Swimming training increased GLUT-4 protein concentration and insulin responsiveness by 87 and 85%, respectively, relative to age-matched controls when examined 18 h after training. Forty-two hours after training, GLUT-4 protein concentration and insulin responsiveness were still higher by 52 and 51%, respectively, in muscle from trained rats compared with control. GLUT-4 protein concentration and insulin responsiveness in trained muscle returned to sedentary control level within 90 h after training. We conclude that 1) the change in insulin responsiveness during detraining is directly related to muscle GLUT-4 protein content, and 2) consequently, the greater the increase in GLUT-4 protein content that is induced by training, the longer an effect on insulin responsiveness persists after the training.

Lipoic acid reduces glycemia and increases muscle GLUT4 content in streptozotocin-diabetic rats

Alpha lipoic acid (lipoate [LA]), a cofactor of α-ketodehydrogenase, exhibits unique antioxidant properties. Recent studies suggest a direct effect of LA on glucose metabolism in both human and experimental diabetes. This study examines the possbility that LA positively affects glucose homeostasis in streptozotocin (STZ)-induced diabetic rats by altering skeletal muscle glucose utilization. Blood glucose concentration in STZ-diabetic rats following 10 days of intraperitoneal (IP) injection of LA 30 mg/kg was reduced compared with that in vehicle-treated diabetic rats [495 ± 131 v 641 ± 125 mg/dL in fed state, P = .003, and 189 ± 48 v 341 ± 36 mg/dL after 12-hour fast, P = .001). No effect of LA on plasma insulin was observed. gastroenemius muscle crude membrane GLUT4 protein was elevated both in control and in diabetic rats treated with LA by 1.5- and 2.8-fold, respectively, without significant changes in GLUT4 mRNA levels. Gastroenemius lactic acid was increased in diabetic rats (19.9 ± 5.5 v 10.4 ± 2.8 μmol/g muscle, P < .05 v nondiabetic rats), and was normal in LA-treated diabetic rats (9.1 ± 5.0 μmol/g muscle). Insulin-stimulated 2-deoxyglucose (2 DG) uptake into isolated soleus muscle was reduced in treatment prevented this reduction, resulting in insulin-stimulated glucose uptake comparable to that of nondiabetic animals. These results suggest that daily LA treatment may reduce blood glucose concentrations in STZ-diabetic rats by enhancing muscle GLUT4 protein content and by increasing muscle glucose utilization.

Effects of feeding acarbose on muscle glucose transport and GLUT4 protein in lean and obese diabetic ( [formula omitted]) rats

The Zucker diabetic fatty rat ( ZDF Gmi -fa ), a model for noninsulin-dependent diabetes mellitus (NIDDM), was used to evaluate the action of dietary acarbose (20 week, 40 mg/100 g diet) in improving hyperglycemia. Muscle glucose uptake and glucose transport were measured during hindlimb perfusion. GLUT4 protein was also measured in the perfused muscle. Insulin resistance was not observed in skeletal muscle from diabetic animals compared with lean controls as concluded from equivalent hemicorpus glucose uptake and muscle glucose transport rates. Three fold increases in both GLUT4 protein and glucose transport were observed in acarbose-fed lean rats compared with lean controls. Acarbose feeding significantly decreased hyperglycemia in diabetic rats. This improvement seems not to be the result of an increase in insulin-stimulated glucose transport in muscle as insulin sensitivity and responsiveness were unchanged.

Plasma membrane content of insulin-regulated glucose transporter in skeletal muscle of the male Otsuka Long-Evans Tokushima Fatty rat, a model of non-insulin-dependent diabetes mellitus

The male Otsuka Long-Evans Tokushima Fatty (OLETF) rat shows insulin resistance in skeletal muscle and visceral obesity. To obtain information on the mechanism of the insulin resistance in the diabetic rats, we examined the content of insulin-regulated glucose transporter (GLUT4) in skeletal muscles. The results indicate that the total content of the transporter is significantly decreased ( P<0.05) in muscles of the diabetic rats. Plasma membrane content of the GLUT4 protein in muscles of the diabetic rats was increased in the basal state as compared to control rats. Hyperinsulinemic clamps increased GLUT4 levels in the plasma membrane of control rats but failed to do so in the diabetic rats. The distribution of GLUT4 in OLETF rat is reminiscent of the characteristics of human non-insulin-dependent diabetes mellitus.

THE EFFECTS OF IMMOBILIZATION AND SUBSEQUENT EXERCISE ON HUMAN MUSCLE GLUT4 mRNA LEVELS 1311

Muscle glucose transporter GLUT4 plays an important role in maintaining glucose homeostasis. Previous studies have shown that exercise increases muscle GLUT4 gene expression and protein level in animal models. The purpose of this study was to investigate the human skeletal muscle GLUT4 mRNA levels before and after immobilization and subsequent exercise. Four human subjects were immobilized with a full cast on the left leg for 3 weeks followed by 12 weeks of resistance exercise training. Total RNA was isolated from biopsy samples of the vastus lateralis muscle of the immobilized leg. Ribonuclease protection assay was used to measure the mRNA levels of GLUT4 transporter. Immobilization decreased GLUT4 mRNA levels to 60% of control(p<0.05). Subsequent resistance exercise training on the immobilized leg induced a 2.0-fold increase in GLUT4 mRNA levels vs immobilization(p<0.05). The results of this study suggest that human skeletal muscle glucose transporter GLUT4 can be regulated at the pre-translational level by immobilization and exercise.

EFFECT OF CHRONIC ELECTRICAL STIMULATION AND ??-GPA DIET ON MUSCLE GLUT4 PROTEIN CONCENTRATION 1291

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 (N) or a 2% β-guanidinoproprionic acid (β-GPA) diet for 8 wk and then the gastrocnemius of one hindlimb was subjected to 0, 14 or 28 d of chronic (24 h/d) low-frequency electrical stimulation (10 Hz). The β-GPA diet, in the absence of electrical stimulation, significantly reduced ATP, creatine phosphate, creatine and inorganic phosphate and elevated GLUT4 protein concentration by 60% (P<0.05) without altering adenylate cyclase activity or cAMP concentration. Following 14 d of electrical stimulation, GLUT4 protein concentration was elevated above non-stimulated muscle in both groups but was significantly more elevated (P<0.05) in the β-GPA group(99.3±5.5% heart standard, β-GPA vs. 65.3±8.4% heart standard, N). 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 d of electrical stimulation, GLUT4 protein concentration and cAMP stabilized and was not different between treatments. However, the high energy phosphates were significantly higher in the N rats as opposed to theβ-GPA rats. These findings therefore suggest that a reduction in the high energy phosphates initiates the contraction-induced increase in muscle GLUT4 protein concentration, but that a rise in cAMP may protentiate this effect.

Effect of in vivo injection of cholera and pertussis toxin on glucose transport in rat skeletal muscle.

Cholera toxin (CTX) and pertussis toxin (PTX) were examined for their ability to inhibit glucose transport in perfused skeletal muscle. Twenty-five hours after an intravenous injection of CTX, basal transport was decreased approximately 30%, and insulin- and contraction-stimulated transport was reduced at least 86 and 49%, respectively, in both the soleus and red and white gastrocnemius muscles. In contrast, PTX treatment was much less efficient. Impairment of glucose transport appeared to develop 10-15 h after CTX administration, which coincided with development of hyperglycemia despite hyperinsulinimia, increased plasma free fatty acid levels, increased adenosine 3',5'-cyclic monophosphate (cAMP) concentrations in muscle, but no difference in plasma catecholamines. Twenty-five hours after CTX treatment, GLUT-4 protein in both soleus and red gastrocnemius muscles was decreased, whereas no change in GLUT-1 protein content was found. In contrast, GLUT-4 mRNA was unchanged, but transcripts for GLUT-1 were increased > or = 150% in all three muscles from CTX-treated rats. The findings suggest that CTX via increased cAMP impairs basal as well as insulin- and contraction-stimulated muscle glucose transport, at least in part from a decrease in intramuscular GLUT-4 protein.

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.

Effect of reduced training and training cessation on insulin action and muscle GLUT-4.

This study examined the impact of a 50% reduction in training frequency or training cessation on insulin action and muscle GLUT-4 protein concentration. Middle-aged individuals were tested before and after 12 wk of exercise training (4 days/wk, 40-45 min/day). Subjects then either maintained training (n = 9), reduced training frequency by 50% (n = 11), or stopped exercising (n = 10) for the ensuing 2 wk. GLUT-4 protein concentration and insulin action (insulin sensitivity index, as determined by the minimal model) increased (P < or = 0.05) by an average of 1.6- and 1.9-fold, respectively, with the 12 wk of training. Insulin action and GLUT-4 did not increase further with the additional 2 wk of training in the maintained training group. Similarly, insulin sensitivity index and GLUT-4 concentration remained at trained levels when training frequency was reduced by 50% for 2 wk. GLUT-4 concentration and insulin action, however, were not different from sedentary values after 14 days of training cessation. These findings indicate that a 14-day 50% reduction in exercise frequency maintains the improvements in GLUT-4 protein concentration and insulin action gained with endurance training in moderately trained middle-aged adults; in contrast, these adaptations are largely lost with training cessation.

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.

Muscle contractile activity modulates GLUT4 protein content in the absence of insulin.

We examined whether muscle contractile activity directly modulates GLUT4 protein content in rat skeletal muscle without the participation of insulin action or via amplified insulin action. To attain this purpose, the effects of increased, by training, or eliminated, by denervation, muscle contractile activity on muscle GLUT4 protein concentration were investigated in severely insulin-deficient diabetic rats. For the first set of experiments, insulin-deficient diabetic rats (induced by injection of 80 mg/kg B.W. streptozotocin) were trained for three weeks by treadmill running (90 min/day, 19 m/min, 10%, 6 days/week). GLUT4 protein concentration in soleus muscle was increased by 48% (p < 0.01) as compared with diabetic sedentary animals. For the second set of experiments, rats were injected with streptozotocin (100 mg/kg). The muscles innervated by the sciatic nerve of one leg were denervated four days after injection of streptozotocin. Three days after denervation, soleus muscles in both legs were excised. Insulin deficiency decreased GLUT4 protein concentration in innervated soleus muscle. In insulin-deficient diabetic rats, denervation also decreased soleus GLUT4 protein concentration by 50% (p < 0.01) as compared with the contralateral innervated muscle. Furthermore, the effects of insulin-deficiency and denervation on GLUT4 protein concentration were additive. These results provide evidence that muscle contractile activity directly modulates skeletal muscle GLUT4 protein concentration independent of insulin action.

Effect of 7-10 days of cycle ergometer exercise on skeletal muscle GLUT-4 protein content

Previous studies in animals and humans have shown that endurance exercise-training protocols of several weeks to many months in duration induce adaptive increases in skeletal muscle GLUT-4 protein concentration. It is generally assumed that the increase in GLUT-4 concentration is a long-term adaptation to training. The present study examined whether 7-10 days of cycle ergometer exercise could induce increases in skeletal muscle GLUT-4 levels. Eight healthy subjects (4 men, 4 women) aged 31 +/- 2 (SE) yr exercised 2 h daily at 65-70% of peak O2 uptake (VO2peak) for either 7 (n = 3) or 10 (n = 5) consecutive days. Muscle biopsies (vastus lateralis) were obtained before initiation of the exercise program and 36-48 h after the final bout of exercise. Glucose transporter protein was quantitated by Western blotting using antiserum specific for GLUT-4. VO2peak was increased by 10% (from 3.0 +/- 0.2 to 3.3 +/- 0.2 l/min; P < 0.01) in response to the training. Body weight did not change (74.3 +/- 4.6 before vs. 75.0 +/- 4.2 kg after) as a result of training. Muscle GLUT-4 immunoreactivity was increased 98% (from 584 +/- 50 to 1,154 +/- 40 counts per minute 125I/25 micrograms protein; P < 0.001) in response to training. Increase in VO2peak and GLUT-4 protein were similar for 7 and 10 days of training. These results suggest that, given an adequate training stimulus, adaptations in skeletal muscle GLUT-4 protein occur very rapidly. Furthermore, the increase in GLUT-4 after 7-10 days of exercise is as large as that reported in studies employing long-term training protocols.

Eccentric muscle damage transiently decreases rat skeletal muscle GLUT-4 protein.

The effects of concentric and muscle-damaging eccentric contractions on muscle glucose transporter GLUT-4 content were studied in rat muscles. Rats were anesthetized, the calf muscles on one side were stimulated electrically for concentric or eccentric contractions, and bilateral calf muscles were obtained in the postexercise period. Inflammatory and phagocytic cells accumulated in the eccentric white and red gastrocnemius muscles, whereas there were only discrete changes in the eccentric soleus. Glycogen was depleted to the same extent in the white and red gastrocnemius muscles after both types of stimulation, and it remained decreased > 2 days in eccentric muscles. The total GLUT-4 protein content was decreased in the eccentric white and red gastrocnemius muscles 1 and 2 days after the eccentric stimulation, whereas the maximal activity of glycogen synthase was unaffected at these time points. In conclusion, our one-legged stimulation model caused eccentric muscle damage in the white and red gastrocnemius, whereas only minor damage was observed in the soleus muscle. In damaged muscle, muscle glycogen and GLUT-4 protein content were decreased for > 2 days. These findings may suggest (but do not prove) that decreased muscle GLUT-4 protein is involved in the delayed glycogen resynthesis after eccentric exercise.

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.