Mechanisms underlying impaired GLUT-4 translocation in glycogen-supercompensated muscles of exercised rats

Exercise training induces an increase in GLUT-4 in muscle. We previously found that feeding rats a high-carbohydrate diet after exercise, with muscle glycogen supercompensation, results in a decrease in insulin responsiveness so severe that it masks the effect of a training-induced twofold increase in GLUT-4 on insulin-stimulated muscle glucose transport. One purpose of this study was to determine whether insulin signaling is impaired. Maximally insulin-stimulated phosphatidylinositol (PI) 3-kinase activity was not significantly reduced, whereas protein kinase B (PKB) phosphorylation was approximately 50% lower (P < 0.01) in muscles of chow-fed, than in those of fasted, exercise-trained rats. Our second purpose was to determine whether contraction-stimulated glucose transport is also impaired. The stimulation of glucose transport and the increase in cell surface GLUT-4 induced by contractions were both decreased by approximately 65% in glycogen-supercompensated muscles of trained rats. The contraction-stimulated increase in AMP kinase activity, which has been implicated in the activation of glucose transport by contractions, was approximately 80% lower in the muscles of the fed compared with the fasted rats 18 h after exercise. These results show that both the insulin- and contraction-stimulated pathways for muscle glucose transport activation are impaired in glycogen-supercompensated muscles and provide insight regarding possible mechanisms.     

Rapid reversal of adaptive increases in muscle GLUT-4 and glucose transport capacity after training cessation

Previous studies have shown that when exercise isstopped there is a rapid reversal of the training-induced adaptiveincrease in muscle glucose transport capacity. Endurance exercisetraining brings about an increase in GLUT-4 in skeletal muscle. Theprimary purpose of this study was to determine whether the rapidreversal of the increase in maximally insulin-stimulated glucosetransport after cessation of training can be explained by a similarlyrapid decrease in GLUT-4. A second purpose was to evaluate thepossibility, suggested by previous studies, that the magnitude of theadaptive increase in muscle GLUT-4 decreases when exercise training is extended beyond a few days. We found that both GLUT-4 and maximally insulin-stimulated glucose transport were increased approximately twofold in epitrochlearis muscles of rats trained by swimming for 6 h/day for 5 days or 5 wk. GLUT-4 was 90% higher, citrate synthaseactivity was 23% higher, and hexokinase activity was 28% higher intriceps muscle of the 5-day trained animals compared with the controls.The increases in GLUT-4 protein and in insulin-stimulated glucosetransport were completely reversed within 40 h after the last exercisebout, after both 5 days and 5 wk of training. In contrast, theincreases in citrate synthase and hexokinase activities were unchanged40 h after 5 days of exercise. These results support the conclusionthat the rapid reversal of the increase in the insulin responsivenessof muscle glucose transport after cessation of training is explained bythe short half-life of the GLUT-4 protein.

     

Prevention of glycogen supercompensation prolongs the increase in muscle GLUT4 after exercise

Exercise induces an increase in GLUT4 in skeletal muscle with a proportional increase in glucose transport capacity. This adaptation results in enhanced glycogen accumulation, i.e., "supercompensation," in response to carbohydrate feeding after glycogen-depleting exercise. The increase in GLUT4 reverses within 40 h after exercise in carbohydrate-fed rats. The purpose of this study was to determine whether prevention of skeletal muscle glycogen supercompensation after exercise results in maintenance of the increases in GLUT4 and the capacity for glycogen supercompensation. Rats were exercised by means of three daily bouts of swimming. GLUT4 mRNA was increased approximately 3-fold and GLUT4 protein was increased approximately 2-fold 18 h in epitrochlearis muscle after exercise. These increases in GLUT4 mRNA and protein reversed completely within 42 h after exercise in rats fed a high-carbohydrate diet. In contrast, the increases in GLUT4 protein, insulin-stimulated glucose transport, and increased capacity for glycogen supercompensation persisted unchanged for 66 h in rats fed a carbohydrate-free diet that prevented glycogen supercompensation after exercise. GLUT4 mRNA was still elevated at 42 h but had returned to baseline by 66 h after exercise in rats fed the carbohydrate-free diet. Glycogen-depleted rats fed carbohydrate 66 h after exercise underwent muscle glycogen supercompensation with concomitant reversal of the increase in GLUT4. These findings provide evidence that prevention of glycogen supercompensation after exercise results in persistence of exercise-induced increases in GLUT4 protein and enhanced capacity for glycogen supercompensation.     

Short-term exercise enhances insulin-stimulated GLUT-4 translocation and glucose transport in adipose cells

This investigation examined the effectsof short-term exercise training on insulin-stimulated GLUT-4 glucosetransporter translocation and glucose transport activity in rat adiposecells. Male Wistar rats were randomly assigned to a sedentary (Sed) orswim training group (Sw, 4 days; final 3 days: 2 × 3 h/day). Adipose cell size decreased significantly but minimally(~20%), whereas total GLUT-4 increased by 30% in Sw vs. Sed rats.Basal3-O-methyl-D-[¹⁴C]glucosetransport was reduced by 62%, whereas maximally insulin-stimulated (MIS) glucose transport was increased by 36% in Sw vs. Sed rats. MIScell surface GLUT-4 photolabeling was 44% higher in the Sw vs. Sedanimals, similar to the increases observed in MIS glucose transportactivity and total GLUT-4. These results suggest that increases intotal GLUT-4 and GLUT-4 translocation to the cell surface contribute tothe increase in MIS glucose transport with short-term exercisetraining. In addition, the results suggest that the exercisetraining-induced adaptations in glucose transport occur more rapidlythan previously thought and with minimal changes in adipose cell size.

     

Effects of AICAR and exercise on insulin-stimulated glucose uptake, signaling, and GLUT-4 content in rat muscles.

Physical activity is known to increase insulin action in skeletal muscle, and data have indicated that 5'-AMP-activated protein kinase (AMPK) is involved in the molecular mechanisms behind this beneficial effect. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) can be used as a pharmacological tool to repetitively activate AMPK, and the objective of this study was to explore whether the increase in insulin-stimulated glucose uptake after either long-term exercise or chronic AICAR administration was followed by fiber-type-specific changes in insulin signaling and/or changes in GLUT-4 expression. Wistar rats were allocated into three groups: an exercise group trained on treadmill for 5 days, an AICAR group exposed to daily subcutaneous injections of AICAR, and a sedentary control group. AMPK activity, insulin-stimulated glucose transport, insulin signaling, and GLUT-4 expression were determined in muscles characterized by different fiber type compositions. Both exercised and AICAR-injected animals displayed a fiber-type-specific increase in glucose transport with the most marked increase in muscles with a high content of type IIb fibers. This increase was accompanied by a concomitant increase in GLUT-4 expression. Insulin signaling as assessed by phosphatidylinositol 3-kinase and PKB/Akt activity was enhanced only after AICAR administration and in a non-fiber-type-specific manner. In conclusion, chronic AICAR administration and long-term exercise both improve insulin-stimulated glucose transport in skeletal muscle in a fiber-type-specific way, and this is associated with an increase in GLUT-4 content.     

Effects of continuous low-carbohydrate diet after long-term exercise on GLUT-4 protein content in rat skeletal muscle.

Stimulation of AMPK and decreased glycogen levels in skeletal muscle have a deep involvement in enhanced insulin action and GLUT-4 protein content after exercise training. The present study examined the chronic effects of a continuous low-carbohydrate diet after long-term exercise on GLUT-4 protein content, glycogen content, AMPK, and insulin signaling in skeletal muscle. Rats were divided randomly into four groups: normal chow diet sedentary (N-Sed), low carbohydrate diet sedentary (L-Sed), normal chow diet exercise (N-Ex), and low carbohydrate diet exercise (L-Ex) groups. Rats in the exercise groups (N-Ex and L-Ex) were exercised by swimming for 6 hours/day in two 3-hour bouts separated by 45 minutes of rest. The 10-day exercise training resulted in a significant increase in the GLUT-4 protein content (p<0.01). Additionally, the GLUT-4 protein content in L-Ex rats was increased by 29% above that in N-Ex rats (p<0.01). Finally, the glycogen content in skeletal muscle of L-Ex rats was decreased compared with that of N-Ex rats. Taken together, we suggest that the maintenance of glycogen depletion after exercise by continuous low carbohydrate diet results in the increment of the GLUT-4 protein content in skeletal muscle.     

Increased GLUT-4 translocation mediates enhanced insulin sensitivity of muscle glucose transport after exercise

The purpose of this study was to determinewhether the increase in insulin sensitivity of skeletal muscle glucosetransport induced by a single bout of exercise is mediated by enhancedtranslocation of the GLUT-4 glucose transporter to the cell surface.The rate of3-O-[³H]methyl-D-glucosetransport stimulated by a submaximally effective concentration ofinsulin (30 µU/ml) was approximately twofold greater in the musclesstudied 3.5 h after exercise than in those of the sedentary controls(0.89 ± 0.10 vs. 0.43 ± 0.05 µmol · ml¹ · 10 min¹; means ± SE forn = 6/group). GLUT-4 translocation wasassessed by using theATB-[2-³H]BMPAexofacial photolabeling technique. Prior exercise resulted in greatercell surface GLUT-4 labeling in response to submaximal insulintreatment (5.36 ± 0.45 dpm × 10³/g in exercised vs. 3.00 ± 0.38 dpm × 10³/g insedentary group; n = 10/group) thatclosely mirrored the increase in glucose transport activity. The signalgenerated by the insulin receptor, as reflected in the extent ofinsulin receptor substrate-1 tyrosine phosphorylation, was unchangedafter the exercise. We conclude that the increase in muscle insulinsensitivity of glucose transport after exercise is due to translocationof more GLUT-4 to the cell surface and that this effect is not due topotentiation of insulin-stimulated tyrosine phosphorylation.

     

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.     

Effects of high-fat diet and exercise training on intracellular glucose metabolism in rats

We examined the effects of high-fat diet (HFD) and exercise training on insulin-stimulated whole body glucose fluxes and several key steps of glucose metabolism in skeletal muscle. Rats were maintained for 3 wk on either low-fat (LFD) or high-fat diet with or without exercise training (swimming for 3 h per day). After the 3-wk diet/exercise treatments, animals underwent hyperinsulinemic euglycemic clamp experiments for measurements of insulin-stimulated whole body glucose fluxes. In addition, muscle samples were taken at the end of the clamps for measurements of glucose 6-phosphate (G-6-P) and GLUT-4 protein contents, hexokinase, and glycogen synthase (GS) activities. Insulin-stimulated glucose uptake was decreased by HFD and increased by exercise training (P < 0.01 for both). The opposite effects of HFD and exercise training on insulin-stimulated glucose uptake were associated with similar increases in muscle G-6-P levels (P < 0.05 for both). However, the increase in G-6-P level was accompanied by decreased GS activity without changes in GLUT-4 protein content and hexokinase activities in the HFD group. In contrast, the increase in G-6-P level in the exercise-trained group was accompanied by increased GLUT-4 protein content and hexokinase II (cytosolic) and GS activities. These results suggest that HFD and exercise training affect insulin sensitivity by acting predominantly on different steps of intracellular glucose metabolism. High-fat feeding appears to induce insulin resistance by affecting predominantly steps distal to G-6-P (e.g., glycolysis and glycogen synthesis). Exercise training affected multiple steps of glucose metabolism both proximal and distal to G-6-P. However, increased muscle G-6-P levels in the face of increased glucose metabolic fluxes suggest that the effect of exercise training is quantitatively more prominent on the steps proximal to G-6-P (i.e., glucose transport and phosphorylation).     

More tetanic contractions are required for activating glucose transport maximally in trained muscle

Kawanaka, Kentaro, Izumi Tabata, and MitsuruHiguchi. More tetanic contractions are requiredfor activating glucose transport maximally in trained muscle.J. Appl. Physiol. 83(2): 429-433, 1997.Exercise training increases contraction-stimulated maximalglucose transport and muscle glycogen level in skeletal muscle.However, there is a possibility that more muscle contractions arerequired to maximally activate glucose transport in trained than inuntrained muscle, because increased glycogen level after training mayinhibit glucose transport. Therefore, the purpose of this study was toinvestigate the relationship between the increase in glucose transportand the number of tetanic contractions in trained and untrained muscle.Male rats swam 2 h/day for 15 days. In untrained epitrochlearis muscle,resting glycogen was 26.6 µmol glucose/g muscle. Ten, 10-s-longtetani at a rate of 1 contraction/min decreased glycogen level to 15.4 µmol glucose/g muscle and maximally increased2-deoxy-D-glucose(2-DG) transport. Training increasedcontraction-stimulated maximal 2-DG transport (+71%;P < 0.01), GLUT-4 protein content(+78%; P < 0.01), and restingglycogen level (to 39.3 µmol glucose/g muscle;P < 0.01) on the next day after thetraining ended, although this training effect might be due, at least inpart, to last bout of exercise. In trained muscle, 20 tetani werenecessary to maximally activate glucose transport. Twenty tetanidecreased muscle glycogen to a lower level than 10 tetani (18.9 vs.24.0 µmol glucose/g muscle; P < 0.01). Contraction-stimulated 2-DG transport was negatively correlatedwith postcontraction muscle glycogen level in trained (r = 0.60;P < 0.01) and untrained muscle(r = 0.57;P < 0.01).

     

Postexercise glucose uptake and glycogen synthesis in skeletal muscle from GLUT4-deficient mice.

To determine the role of GLUT4 on postexercise glucose transport and glycogen resynthesis in skeletal muscle, GLUT4-deficient and wild-type mice were studied after a 3 h swim exercise. In wild-type mice, insulin and swimming each increased 2-deoxyglucose uptake by twofold in extensor digitorum longus muscle. In contrast, insulin did not increase 2-deoxyglucose glucose uptake in muscle from GLUT4-null mice. Swimming increased glucose transport twofold in muscle from fed GLUT4-null mice, with no effect noted in fasted GLUT4-null mice. This exercise-associated 2-deoxyglucose glucose uptake was not accompanied by increased cell surface GLUT1 content. Glucose transport in GLUT4-null muscle was increased 1.6-fold over basal levels after electrical stimulation. Contraction-induced glucose transport activity was fourfold greater in wild-type vs. GLUT4-null muscle. Glycogen content in gastrocnemius muscle was similar between wild-type and GLUT4-null mice and was reduced approximately 50% after exercise. After 5 h carbohydrate refeeding, muscle glycogen content was fully restored in wild-type, with no change in GLUT4-null mice. After 24 h carbohydrate refeeding, muscle glycogen in GLUT4-null mice was restored to fed levels. In conclusion, GLUT4 is the major transporter responsible for exercise-induced glucose transport. Also, postexercise glycogen resynthesis in muscle was greatly delayed; unlike wild-type mice, glycogen supercompensation was not found. GLUT4 it is not essential for glycogen repletion since muscle glycogen levels in previously exercised GLUT4-null mice were totally restored after 24 h carbohydrate refeeding.-Ryder, J. W., Kawano, Y., Galuska, D., Fahlman, R., Wallberg-Henriksson, H., Charron, M. J., Zierath, J. R. Postexercise glucose uptake and glycogen synthesis in skeletal muscle from GLUT4-deficient mice.     

Muscle glycogen content affects insulin-stimulated glucose transport and protein kinase B activity

We investigated the possible regulatory role of glycogen in insulin-stimulated glucose transport and insulin signaling in skeletal muscle. Rats were preconditioned to obtain low (LG), normal, or high (HG) muscle glycogen content, and perfused isolated hindlimbs were exposed to 0, 100, or 10,000 microU/ml insulin. In the fast-twitch white gastrocnemius, insulin-stimulated glucose transport was significantly higher in LG compared with HG. This difference was less pronounced in the mixed-fiber red gastrocnemius and was absent in the slow-twitch soleus. In the white gastrocnemius, insulin activation of insulin receptor tyrosine kinase and phosphoinositide 3-kinase was unaffected by glycogen levels, whereas protein kinase B activity was significantly higher in LG compared with HG. In additional incubation experiments on fast-twitch epitrochlearis muscles, insulin-stimulated cell surface GLUT-4 content was significantly higher in LG compared with HG. The data indicate that, in fast-twitch muscle, the effect of insulin on glucose transport and cell surface GLUT-4 content is modulated by glycogen content, which does not involve initial but possibly more downstream signaling events.     

Glycogen overload by postexercise insulin administration abolished the exercise-induced increase in GLUT4 protein

Summary To elucidate the role of muscle glycogen storage on regulation of GLUT4 protein expression and whole-body glucose tolerance, muscle glycogen level was manipulated by exercise and insulin administration. Sixty Sprague-Dawley rats were evenly separated into three groups: control (CON), immediately after exercise (EX0), and 16 h after exercise (EX16). Rats from each group were further divided into two groups: saline- and insulin-injected. The 2-day exercise protocol consisted of 2 bouts of 3-h swimming with 45-min rest for each day, which effectively depleted glycogen in both red gastrocnemius (RG) and plantaris muscles. EX0 rats were sacrificed immediately after the last bout of exercise on second day. CON and EX16 rats were intubated with 1 g/kg glucose solution following exercise and recovery for 16 h before muscle tissue collection. Insulin (0.5 μU/kg) or saline was injected daily at the time when glucose was intubated. Insulin injection elevated muscle glycogen levels substantially in both muscles above saline-injected group at CON and EX16. With previous day insulin injection, EX0 preserved greater amount of postexercise glycogen above their saline-injected control. In the saline-injected rats, EX16 significantly increased GLUT4 protein level above CON, concurrent with muscle glycogen supercompensation. Insulin injection for EX16 rats significantly enhanced muscle glycogen level above their saline-injected control, but the increases in muscle GLUT4 protein and whole-body glucose tolerance were attenuated. In conclusion, the new finding of the study was that glycogen overload by postexercise insulin administration significantly abolished the exercise-induced increases in GLUT4 protein and glucose tolerance.     

Effects of high-intensity swimming training on GLUT-4 and glucose transport activity in rat skeletal muscle.

This study was performed to assess the effects of short-term, extremely high-intensity intermittent exercise training on the GLUT-4 content of rat skeletal muscle. Three- to four-week-old male Sprague-Dawley rats with an initial body weight ranging from 45 to 55 g were used for this study. These rats were randomly assigned to an 8-day period of high-intensity intermittent exercise training (HIT), relatively high-intensity intermittent prolonged exercise training (RHT), or low-intensity prolonged exercise training (LIT). Age-matched sedentary rats were used as a control. In the HIT group, the rats repeated fourteen 20-s swimming bouts with a weight equivalent to 14, 15, and 16% of body weight for the first 2, the next 4, and the last 2 days, respectively. Between exercise bouts, a 10-s pause was allowed. RHT consisted of five 17-min swimming bouts with a 3-min rest between bouts. During the first bout, the rat swam without weight, whereas during the following four bouts, the rat was attached to a weight equivalent to 4 and 5% of its body weight for the first 5 days and the following 3 days, respectively. Rats in the LIT group swam 6 h/day for 8 days in two 3-h bouts separated by 45 min of rest. In the first experiment, the HIT, LIT, and control rats were compared. GLUT-4 content in the epitrochlearis muscle in the HIT and LIT groups after training was significantly higher than that in the control rats by 83 and 91%, respectively. Furthermore, glucose transport activity, stimulated maximally by both insulin (2 mU/ml) (HIT: 48%, LIT: 75%) and contractions (25 10-s tetani) (HIT: 55%, LIT: 69%), was higher in the training groups than in the control rats. However, no significant differences in GLUT-4 content or in maximal glucose transport activity in response to both insulin and contractions were observed between the two training groups. The second experiment demonstrated that GLUT-4 content after HIT did not differ from that after RHT (66% higher in trained rats than in control). In conclusion, the present investigation demonstrated that 8 days of HIT lasting only 280 s elevated both GLUT-4 content and maximal glucose transport activity in rat skeletal muscle to a level similar to that attained after LIT, which has been considered a tool to increase GLUT-4 content maximally.     

Voluntary exercise training enhances glucose transport in muscle stimulated by insulin-like growth factor I

Hokama, Jason Y., Ryan S. Streeper, and Erik J. Henriksen.Voluntary exercise training enhances glucose transport in muscle stimulated by insulin-like growth factor I. J. Appl. Physiol. 82(2): 508-512, 1997.Skeletal muscle glucosetransport can be regulated by hormonal factors such as insulin andinsulin-like growth factor I (IGF-I). Although it is well establishedthat exercise training increases insulin action on muscle glucosetransport, it is currently unknown whether exercise training leads toan enhancement of IGF-I-stimulated glucose transport in skeletal muscle. Therefore, we measured glucose transport activity [by using 2-deoxy-D-glucose (2-DG)uptake] in the isolated rat epitrochlearis muscle stimulated bysubmaximally and maximally effective concentrations of insulin (0.2 and13.3 nM) or IGF-I (5 and 50 nM) after 1, 2, and 3 wk of voluntary wheelrunning (WR). After 1 wk of WR, both submaximal andmaximal insulin-stimulated 2-DG uptake rates were significantly(P < 0.05) enhanced (43 and 31%)compared with those of sedentary controls, and these variables werefurther increased after 2 (86 and 57%) and 3 wk (71 and 70%) ofWR. Submaximal and maximal IGF-I-stimulated 2-DG uptakerates were significantly enhanced after 1 wk of WR (82 and 61%), andthese increases did not expand substantially after 2 (71 and 58%) and3 wk (96 and 70%) of WR. This enhancement of hormone-stimulated 2-DGuptake in WR muscles preceded any alteration in glucose transporter(GLUT-4) protein level, which increased only after 2 (24%) and 3 wk(54%) of WR. Increases in GLUT-4 protein were significantly correlated (r = 0.844) with increases in citratesynthase. These results indicate that exercise training can enhanceboth insulin-stimulated and IGF-I-stimulated muscle glucose transportactivity and that these improvements can develop without an increase inGLUT-4 protein.

     

Whey Protein Hydrolysate Increases Translocation of GLUT-4 to the Plasma Membrane Independent of Insulin in Wistar Rats

Whey protein (WP) and whey protein hydrolysate (WPH) have the recognized capacity to increase glycogen stores. The objective of this study was to verify if consuming WP and WPH could also increase the concentration of the glucose transporters GLUT-1 and GLUT-4 in the plasma membrane (PM) of the muscle cells of sedentary and exercised animals. Forty-eight Wistar rats were divided into 6 groups (n = 8 per group), were treated and fed with experimental diets for 9 days as follows: a) control casein (CAS); b) WP; c) WPH; d) CAS exercised; e) WP exercised; and f) WPH exercised. After the experimental period, the animals were sacrificed, muscle GLUT-1 and GLUT-4, p85, Akt and phosphorylated Akt were analyzed by western blotting, and the glycogen, blood amino acids, insulin levels and biochemical health indicators were analyzed using standard methods. Consumption of WPH significantly increased the concentrations of GLUT-4 in the PM and glycogen, whereas the GLUT-1 and insulin levels and the health indicators showed no alterations. The physical exercise associated with consumption of WPH had favorable effects on glucose transport into muscle. These results should encourage new studies dealing with the potential of both WP and WPH for the treatment or prevention of type II diabetes, a disease in which there is reduced translocation of GLUT-4 to the plasma membrane.     

Role of insulin on exercise-induced GLUT-4 protein expression and glycogen supercompensation in rat skeletal muscle.

The purpose of this study was to investigate the role of insulin on skeletal muscle GLUT-4 protein expression and glycogen storage after postexercise carbohydrate supplementation. Male Sprague-Dawley rats were randomly assigned to one of six treatment groups: sedentary control (Con), Con with streptozocin (Stz/C), immediately postexercise (Ex0), Ex0 with Stz (Stz/Ex0), 5-h postexercise (Ex5), and Ex5 with Stz (Stz/Ex5). Rats were exercised by swimming (2 bouts of 3 h) and carbohydrate supplemented immediately after each exercise session by glucose intubation (1 ml of a 50% wt/vol). Stz was administered 72-h before exercise, which resulted in hyperglycemia and elimination of the insulin response to the carbohydrate supplement. GLUT-4 protein of Ex0 rats was 30% above Con in fast-twitch (FT) red and 21% above Con in FT white muscle. In Ex5, GLUT-4 protein was 52% above Con in FT red and 47% above Con in FT white muscle. Muscle glycogen in FT red and white muscle was also increased above Con in Ex5 rats. Neither GLUT-4 protein nor muscle glycogen was increased above Con in Stz/Ex0 or Stz/Ex5 rats. GLUT-4 mRNA in FT red muscle of Ex0 rats was 61% above Con but only 33% above Con in Ex5 rats. GLUT-4 mRNA in FT red muscle of Stz/C and Stz/Ex0 rats was similar but significantly elevated in Ex5/Stz rats. These results suggest that insulin is essential for the increase in GLUT-4 protein expression following postexercise carbohydrate supplementation.     

Postexercise muscle glycogen resynthesis in obese insulin-resistant Zucker rats.

We determined the effect of an acute bout of swimming (8 x 30 min) followed by either carbohydrate administration (0.5 mg/g glucose ip and ad libitum access to chow; CHO) or fasting (Fast) on postexercise glycogen resynthesis in soleus muscle and liver from female lean (ZL) and obese insulin-resistant (ZO) Zucker rats. Resting soleus muscle glycogen concentration ([glycogen]) was similar between genotypes and was reduced by 73 (ZL) and 63% (ZO) after exercise (P < 0.05). Liver [glycogen] at rest was greater in ZO than ZL (334 +/- 31 vs. 247 +/- 16 micromol/g wet wt; P < 0.01) and fell by 44 and 94% after exercise (P < 0.05). The fractional activity of glycogen synthase (active/total) increased immediately after exercise (from 0.22 +/- 0.05 and 0.32 +/- 0.04 to 0.63 +/- 0.08 vs. 0.57 +/- 0.05; P < 0.01 for ZL and ZO rats, respectively) and remained elevated above resting values after 30 min of recovery. During this time, muscle [glycogen] in ZO increased 68% with CHO (P < 0.05) but did not change in Fast. Muscle [glycogen] was unchanged in ZL from postexercise values after both treatments. After 6 h recovery, GLUT-4 protein concentration was increased above resting levels by a similar extent for both genotypes in both fasted (approximately 45%) and CHO-supplemented (approximately 115%) rats. Accordingly, during this time CHO refeeding resulted in supercompensation in both genotypes (68% vs. 44% for ZL and ZO). With CHO, liver [glycogen] was restored to resting levels in ZL but remained at postexercise values for ZO after both treatments. We conclude that the increased glucose availability with carbohydrate refeeding after glycogen-depleting exercise resulted in glycogen supercompensation, even in the face of muscle insulin-resistance.     

Effect of endurance exercise training on muscle glycogen supercompensation in rats

Nakatani, Akira, Dong-Ho Han, Polly A. Hansen, Lorraine A. Nolte, Helen H. Host, Robert C. Hickner, and John O. Holloszy. Effect of endurance exercise training on muscle glycogensupercompensation in rats. J. Appl.Physiol. 82(2): 711-715, 1997.The purpose of this study was to test the hypothesis that the rate and extent ofglycogen supercompensation in skeletal muscle are increased byendurance exercise training. Rats were trained by using a 5-wk-long swimming program in which the duration of swimming was gradually increased to 6 h/day over 3 wk and then maintained at 6 h/day for anadditional 2 wk. Glycogen repletion was measured in trained anduntrained rats after a glycogen-depleting bout of exercise. The ratswere given a rodent chow diet plus 5% sucrose in their drinking waterad libitum during the recovery period. There were remarkabledifferences in both the rates of glycogen accumulation and the glycogenconcentrations attained in the two groups. The concentration ofglycogen in epitrochlearis muscle averaged 13.1 ± 0.9 mg/g wet wtin the untrained group and 31.7 ± 2.7 mg/g in the trained group(P < 0.001) 24 h after the exercise.This difference could not be explained by a training effect on glycogensynthase. The training induced ~50% increases in muscle GLUT-4glucose transporter protein and in hexokinase activity inepitrochlearis muscles. We conclude that endurance exercise trainingresults in increases in both the rate and magnitude of muscle glycogensupercompensation in rats.

     

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

Kawanaka, Kentaro, Izumi Tabata, Shigeru Katsuta, andMitsuru Higuchi. Changes in insulin-stimulated glucose transport and GLUT-4 protein in rat skeletal muscle after training.J. Appl. Physiol. 83(6):2043-2047, 1997.After running training, which increased GLUT-4protein content in rat skeletal muscle by <40% compared with controlrats, the training effect on insulin-stimulated maximal glucosetransport (insulin responsiveness) in skeletal muscle was short lived(24 h). A recent study reported that GLUT-4 protein content in ratepitrochlearis muscle increased dramatically (~2-fold) after swimmingtraining (J.-M. Ren, C. F. Semenkovich, E. A. Gulve, J. Gao, andJ. O. Holloszy. J. Biol.Chem. 269, 14396-14401, 1994).Because GLUT-4 protein content is known to be closely related toskeletal muscle insulin responsiveness, we thought it possible that thetraining effect on insulin responsiveness may remain for >24 h afterswimming training if GLUT-4 protein content decreases gradually fromthe relatively high level and still remains higher than control levelfor >24 h after swimming training. Therefore, we examined thispossibility. Male Sprague-Dawley rats swam 2 h a day for 5 days with aweight equal to 2% of body mass. Approximately 18, 42, and 90 h aftercessation of training, GLUT-4 protein concentration and2-[1,2-³H]deoxy-D-glucosetransport in the presence of a maximally stimulating concentration ofinsulin (2 mU/ml) were examined by using incubated epitrochlearismuscle preparation. Swimming training increased GLUT-4 proteinconcentration 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 andinsulin responsiveness were still higher by 52 and 51%, respectively,in muscle from trained rats compared with control. GLUT-4 proteinconcentration and insulin responsiveness in trained muscle returned tosedentary control level within 90 h after training. We conclude that1) the change in insulinresponsiveness during detraining is directly related to muscle GLUT-4protein content, and 2)consequently, the greater the increase in GLUT-4 protein content thatis induced by training, the longer an effect on insulin responsivenesspersists after the training.