Near-normalization of glycaemic control with glucagon-like peptide-1 receptor agonist treatment combined with exercise in patients with type 2 diabetes.

AIMS: To investigate the effects of exercise in combination with a glucagon-like peptide-1 receptor agonist (GLP-1RA), liraglutide, or placebo for the treatment of type 2 diabetes. METHODS: Thirty-three overweight, dysregulated and sedentary patients with type 2 diabetes were randomly allocated to 16 weeks of either exercise and liraglutide or exercise and placebo. Both groups had three supervised 60-minute training sessions per week including spinning and resistance training. RESULTS: Glycated haemoglobin (HbA1c) levels dropped by a mean ± standard deviation of 2.0% ± 1.2% (from 8.2% ± 1.4%) in the exercise plus liraglutide group vs 0.3% ± 0.9% (from 8.0% ± 1.2%) in the exercise plus placebo group ( P < .001), and body weight was reduced more with liraglutide (-3.4 ± 2.9 kg vs -1.6 ± 2.3 kg; P < .001). Compared with baseline, similar reductions were seen in body fat (exercise plus liraglutide: -2.5% ± 1.4% [ P < .001]; exercise plus placebo: -2.2% ± 1.9% [ P < .001]) and similar increases were observed in maximum oxygen uptake (exercise plus liraglutide: 0.5 ± 0.5 L O2 /min [ P < .001]; exercise plus placebo: 0.4 ± 0.4 L O2 /min [ P = .002]). Greater reductions in fasting plasma glucose (-3.4 ± 2.3 mM vs -0.3 ± 2.6 mM, P < .001) and systolic blood pressure (-5.4 ± 7.4 mm Hg vs -0.6 ± 11.1 mm Hg, P < .01) were seen with exercise plus liraglutide vs exercise plus placebo. The two groups experienced similar increases in quality of life during the intervention. CONCLUSIONS: In obese patients with type 2 diabetes, exercise combined with GLP-1RA treatment near-normalized HbA1c levels and caused a robust weight loss when compared with placebo. These results suggest that a combination of exercise and GLP-1RA treatment is effective in type 2 diabetes.

Deep muscle-proteomic analysis of freeze-dried human muscle biopsies reveals fiber type-specific adaptations to exercise training.

Skeletal muscle conveys several of the health-promoting effects of exercise; yet the underlying mechanisms are not fully elucidated. Studying skeletal muscle is challenging due to its different fiber types and the presence of non-muscle cells. This can be circumvented by isolation of single muscle fibers. Here, we develop a workflow enabling proteomics analysis of pools of isolated muscle fibers from freeze-dried human muscle biopsies. We identify more than 4000 proteins in slow- and fast-twitch muscle fibers. Exercise training alters expression of 237 and 172 proteins in slow- and fast-twitch muscle fibers, respectively. Interestingly, expression levels of secreted proteins and proteins involved in transcription, mitochondrial metabolism, Ca2+ signaling, and fat and glucose metabolism adapts to training in a fiber type-specific manner. Our data provide a resource to elucidate molecular mechanisms underlying muscle function and health, and our workflow allows fiber type-specific proteomic analyses of snap-frozen non-embedded human muscle biopsies.

Differential aetiology and impact of phosphoinositide 3-kinase (PI3K) and Akt signalling in skeletal muscle on in vivo insulin action.

AIMS/HYPOTHESIS: Insulin resistance in skeletal muscle is a key factor in the development of type 2 diabetes and although some studies indicate that this could be partly attributed to reduced content and activity of various proximal and distal insulin signalling molecules, consensus is lacking. We therefore aimed to investigate the regulation of proximal insulin signalling in skeletal muscle and its effect on glucose metabolism in a large non-diabetic population. METHODS: We examined 184 non-diabetic twins with gold-standard techniques including the euglycaemic-hyperinsulinaemic clamp. Insulin signalling was evaluated at three key levels, i.e. the insulin receptor, IRS-1 and V-akt murine thymoma viral oncogene (Akt) levels, employing kinase assays and phospho-specific western blotting. RESULTS: Proximal insulin signalling was not associated with obesity, age or sex. However, birthweight was positively associated with IRS-1-associated phosphoinositide 3-kinase (PI3K; IRS-1-PI3K) activity (p = 0.04); maximal aerobic capacity (VO2(max)), paradoxically, was negatively associated with IRS-1-PI3K (p = 0.02) and Akt2 activity (p = 0.01). Additionally, we found low heritability estimates for most measures of insulin signalling activity. Glucose disposal was positively associated with Akt-308 phosphorylation (p < 0.001) and Akt2 activity (p = 0.05), but not with insulin receptor tyrosine kinase or IRS-1-PI3K activity. CONCLUSIONS/INTERPRETATION: With the exception of birthweight, 'classical' modifiers of insulin action, including genetics, age, sex, obesity and VO2(max) do not seem to mediate their most central effects on whole-body insulin sensitivity through modulation of proximal insulin signalling in skeletal muscle. We also demonstrated an association between Akt activity and in vivo insulin sensitivity, suggesting a role of Akt in control of in vivo insulin resistance and potentially in type 2 diabetes.

Potential role of TBC1D4 in enhanced post-exercise insulin action in human skeletal muscle.

AIMS/HYPOTHESIS: TBC1 domain family, member 4 (TBC1D4; also known as AS160) is a cellular signalling intermediate to glucose transport regulated by insulin-dependent and -independent mechanisms. Skeletal muscle insulin sensitivity is increased after acute exercise by an unknown mechanism that does not involve modulation at proximal insulin signalling intermediates. We hypothesised that signalling through TBC1D4 is involved in this effect of exercise as it is a common signalling element for insulin and exercise. METHODS: Insulin-regulated glucose metabolism was evaluated in 12 healthy moderately trained young men 4 h after one-legged exercise at basal and during a euglycaemic-hyperinsulinaemic clamp. Vastus lateralis biopsies were taken before and immediately after the clamp. RESULTS: Insulin stimulation increased glucose uptake in both legs, with greater effects (approximately 80%, p < 0.01) in the previously exercised leg. TBC1D4 phosphorylation, assessed using the phospho-AKT (protein kinase B)substrate antibody and phospho- and site-specific antibodies targeting six phosphorylation sites on TBC1D4, increased at similar degrees to insulin stimulation in the previously exercised and rested legs (p < 0.01). However, TBC1D4 phosphorylation on Ser-318, Ser-341, Ser-588 and Ser-751 was higher in the previously exercised leg, both in the absence and in the presence of insulin (p < 0.01; Ser-588, p = 0.09; observed power = 0.39). 14-3-3 binding capacity for TBC1D4 increased equally (p < 0.01) in both legs during insulin stimulation. CONCLUSION/INTERPRETATION: We provide evidence for site-specific phosphorylation of TBC1D4 in human skeletal muscle in response to physiological hyperinsulinaemia. The data support the idea that TBC1D4 is a nexus for insulin- and exercise-responsive signals that may mediate increased insulin action after exercise.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake.

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

Adenosine receptors mediate synergistic stimulation of glucose uptake and transport by insulin and by contractions in rat skeletal muscle.

The role of adenosine receptors in the regulation of muscle glucose uptake by insulin and contractions was studied in isolated rat hindquarters that were perfused with a standard medium containing no insulin or a submaximal concentration of 100 microU/ml. Adenosine receptor antagonism was induced by caffeine or 8-cyclopentyl-1,3-dipropylxantine (CPDPX). Glucose uptake and transport were measured before and during 30 min of electrically induced muscle contractions. Caffeine nor CPDPX affected glucose uptake in resting hindquarters. In contrast, the contraction-induced increase in muscle glucose uptake was inhibited by 30-50% by caffeine, as well as by CPDPX, resulting in a 20-25% decrease in the absolute rate of glucose uptake during contractions, compared with control values. This inhibition was independent of the rate of perfusate flow and only occurred in hindquarters perfused with insulin added to the medium. Thus, adenosine receptor antagonism inhibited glucose uptake during simultaneous exposure to insulin and contractions only. Accordingly, caffeine inhibited 3-O-methylglucose uptake during contractions only in oxidative muscle fibers that are characterized by a high sensitivity to insulin. In conclusion, the present data demonstrate A1 receptors to regulate insulin-mediated glucose transport in contracting skeletal muscle. The findings provide evidence that stimulation of sarcolemmic adenosine receptors during contractions is involved in the synergistic stimulation of muscle glucose transport by insulin and by contractions.

Neutralization of glucagon by antiserum as a tool in glucagon physiology. Lack of depression of basal blood glucose after antiserum treatment in rats.

The method of producing experimental glucagon deficiency by administration of glucagon antiserum was evaluated in rats. A pool of antisera was prepared, the affinity of which exceeded that of the glucagon receptors of liver cell membranes, whereas the binding capacity of the volume used amounted to more than one-third of the total glucagon content in the rat pancreas. That rapid, extensive, and lasting neutralization of glucagon had taken place after antiserum treatment was indicated by the following findings: When examined more than 1 h after the injection and after 60 min of exercise-stimulated glucagon production, all rats had excess free antibodies in plasma. The concentration of free glucagon was lowered to one-third of the concentration in control rats; at 37 degrees C plasma samples could bind 25% of additional 300 pmol/liter of glucagon in 10 s, and 69% in 120 s; the glycemic response to exogenous glucagon was abolished. Antiserum treatment, however, had no effect on blood glucose in rats fasted for 3 and 10 h, in chemically sympathectomized and adrenomedullectomized rats, and in 48-h-fasted, acutely adrenalectomized rats. The antiserum was found to contain 460 nmol/liter of antibody-bound glucagon, originating in the rabbit in which the antiserum was raised. However, antibody preparations from which the bound glucagon had been effectively removed were equally ineffective in lowering the basal blood glucose in rats, although in three-fourths of the rats the concentration of free glucagon was lowered beyond detection limit. The data indicate that the absolute concentration of glucagon in plasma is of minor importance for the maintenance of basal blood glucose in the rat.

Effects of insulin and exercise on muscle lipoprotein lipase activity in man and its relation to insulin action.

The effects of exercise and a physiological increase in plasma insulin concentration on muscle lipoprotein lipase activity (mLPLA), leg exchange of glucose, and serum lipoprotein levels were investigated in healthy young men. During euglycemic hyperinsulinemia (n = 7) at 44 mU.liter-1, m-LPLA in non-exercised muscle decreased from 30 +/- 7.4 mU.g-1 wet weight (w.w.) (mean +/- SE) to 19 +/- 3.3 (P less than 0.05). Furthermore, the decrease in m-LPLA correlated closely (r = 0.97, P less than 0.05) with the increase in leg glucose uptake. Moreover, basal m-LPLA correlated with the insulin-induced increase in leg glucose uptake (r = 0.93, P less than 0.05). In the control group (n = 6) in which saline was infused in place of insulin and glucose, m-LPLA in nonexercised muscle did not change with time. No change in m-LPLA was observed immediately after one-legged knee extension exercise, but 4 h after exercise m-LPLA was higher (P less than 0.05) in the exercised thigh (47 +/- 17.8 mU.g-1 w.w.) compared with the contralateral nonexercised thigh (29 +/- 6.3 mU.g-1 w.w.). This difference was not found 8 h after exercise. The triacylglycerol content of serum lipoproteins decreased during insulin infusion. It is concluded that in contrast to the effect on adipose tissue, physiological concentrations of insulin decrease m-LPLA in proportion to the effect of insulin on muscle glucose uptake, while muscle contractions cause a local, delayed, and transient increase in m-LPLA. Further-more, basal m-LPLA is an indicator of muscle insulin sensitivity.

Muscle glucose metabolism following exercise in the rat: increased sensitivity to insulin.

Muscle glycogen stores are depleted during exercise and are rapidly repleted during the recovery period. To investigate the mechanism for this phenomenon, untrained male rats were run for 45 min on a motor-driven treadmill and the ability of their muscles to utilize glucose was then assessed during perfusion of their isolated hindquarters. Glucose utilization by the hindquarter was the same in exercised and control rats perfused in the absence of added insulin; however, when insulin (30-40,000 muU/ml) was added to the perfusate, glucose utilization was greater after exercise. Prior exercise lowered both, the concentration of insulin that half-maximally stimulated glucose utilization (exercise, 150 muU/ml; control, 480 muU/ml) and modestly increased its maximum effect. The increase in insulin sensitivity persisted for 4 h following exercise, but was not present after 24 h. The rate-limiting step in glucose utilization enhanced by prior exercise appeared to be glucose transport across the cell membrane, as in neither control nor exercised rats did free glucose accumulate in the muscle cell. Following exercise, the ability of insulin to stimulate the release of lactate into the perfusate was unaltered; however its ability to stimulate the incorporation of [(14)C]glucose into glycogen in certain muscles was enhanced. Thus at a concentration of 75 muU/ml insulin stimulated glycogen synthesis eightfold more in the fast-twitch red fibers of the red gastrocnemius than it did in the same muscle of nonexercised rats. In contrast, insulin only minimally increased glycogen synthesis in the fast-twitch white fibers of the gastrocnemius, which were not glycogen-depleted. The uptake of 2-deoxyglucose by these muscles followed a similar pattern suggesting that glucose transport was also differentially enhanced. Prior exercise did not enhance the ability of insulin to convert glycogen synthase from its glucose-6-phosphate-dependent (D) to its glucose-6-phosphate-independent (1) form. On the other hand, following exercise, insulin prevented a marked decrease in muscle glucose-6-phosphate, which could have diminished synthase activity in situ. The possibility that exercise enhanced the ability of insulin to convert glycogen synthase D to an intermediate form of the enzyme, more sensitive to glucose-6-phosphate, remains to be explored. These results suggest that following exercise, glucose transport and glycogen synthesis in skeletal muscle are enhanced due at least in part to an increase in insulin sensitivity. They also suggest that this increase in insulin sensitivity occurs predominantly in muscle fibers that are deglycogenated during exercise.

Effect of the antilipolytic nicotinic acid analogue acipimox on whole-body and skeletal muscle glucose metabolism in patients with non-insulin-dependent diabetes mellitus.

Increased nonesterified fatty acid (NEFA) levels may be important in causing insulin resistance in skeletal muscles in patients with non-insulin-dependent diabetes mellitus (NIDDM). The acute effect of the antilipolytic nicotinic acid analogue Acipimox (2 X 250 mg) on basal and insulin-stimulated (3 h, 40 mU/m2 per min) glucose metabolism was therefore studied in 12 patients with NIDDM. Whole-body glucose metabolism was assessed using [3-3H]glucose and indirect calorimetry. Biopsies were taken from the vastus lateralis muscle during basal and insulin-stimulated steady-state periods. Acipimox reduced NEFA in the basal state and during insulin stimulation. Lipid oxidation was inhibited by Acipimox in all patients in the basal state (20 +/- 2 vs. 33 +/- 3 mg/m2 per min, P less than 0.01) and during insulin infusion (8 +/- 2 vs. 17 +/- 2 mg/m2 per min, P less than 0.01). Acipimox increased the insulin-stimulated glucose disposal rate (369 +/- 49 vs. 262 +/- 31 mg/m2 per min, P less than 0.01), whereas the glucose disposal rate was unaffected by Acipimox in the basal state. Acipimox increased glucose oxidation in the basal state (76 +/- 4 vs. 50 +/- 4 mg/m2 per min, P less than 0.01). During insulin infusion Acipimox increased both glucose oxidation (121 +/- 7 vs. 95 +/- 4 mg/m2 per min, P less than 0.01) and nonoxidative glucose disposal (248 +/- 47 vs. 167 +/- 29 mg/m2 per min, P less than 0.01). Acipimox enhanced basal and insulin-stimulated muscle fractional glycogen synthase activities (32 +/- 2 vs. 25 +/- 3%, P less than 0.05, and 50 +/- 5 vs. 41 +/- 4%, P less than 0.05). Activities of muscle pyruvate dehydrogenase and phosphofructokinase were unaffected by Acipimox. In conclusion, Acipimox acutely improved insulin action in patients with NIDDM by increasing both glucose oxidation and nonoxidative glucose disposal. This supports the hypothesis that elevated NEFA concentrations may be important for the insulin resistance in NIDDM. The mechanism responsible for the increased insulin-stimulated nonoxidative glucose disposal may be a stimulatory effect of Acipimox on glycogen synthase activity in skeletal muscles.

Fiber type-specific muscle glycogen sparing due to carbohydrate intake before and during exercise.

The effect of carbohydrate intake before and during exercise on muscle glycogen content was investigated. According to a randomized crossover study design, eight young healthy volunteers (n = 8) participated in two experimental sessions with an interval of 3 wk. In each session subjects performed 2 h of constant-load bicycle exercise ( approximately 75% maximal oxygen uptake). On one occasion (CHO), they received carbohydrates before ( approximately 150 g) and during (1 g.kg body weight(-1).h(-1)) exercise. On the other occasion they exercised after an overnight fast (F). Fiber type-specific relative glycogen content was determined by periodic acid Schiff staining combined with immunofluorescence in needle biopsies from the vastus lateralis muscle before and immediately after exercise. Preexercise glycogen content was higher in type IIa fibers [9.1 +/- 1 x 10(-2) optical density (OD)/microm(2)] than in type I fibers (8.0 +/- 1 x 10(-2) OD/microm(2); P < 0.0001). Type IIa fiber glycogen content decreased during F from 9.6 +/- 1 x 10(-2) OD/microm(2) to 4.5 +/- 1 x 10(-2) OD/microm(2) (P = 0.001), but it did not significantly change during CHO (P = 0.29). Conversely, in type I fibers during CHO and F the exercise bout decreased glycogen content to the same degree. We conclude that the combination of carbohydrate intake both before and during moderate- to high-intensity endurance exercise results in glycogen sparing in type IIa muscle fibers.

Effect of vanadate on glucose transporter (GLUT4) intrinsic activity in skeletal muscle plasma membrane giant vesicles.

Maximally effective concentrations of vanadate (a phosphotyrosine phosphatase inhibitor) increase glucose transport in muscle less than maximal insulin stimulation. This might be due to vanadate-induced decreased intrinsic activity of GLUT4 accompanying GLUT4 translocation. Thus, the effect of vanadate (NaVO3) on glucose transporter (GLUT4) intrinsic activity (V(max) = intrinsic activity x [GLUT4 protein]) was studied in muscle plasma membrane giant vesicles. Giant vesicles (average diameter 7.6 microns) were produced by collagenase treatment of rat skeletal muscle. The vesicles were incubated for 1.5 h with concentrations of vanadate ranging from 3 to 40 mmol l-1 at 34 degrees C before being used for determination of glucose transport. The dose-response curve showed that vanadate decreased the specific D-glucose uptake by a maximum of 70% compared with a control preparation. The vanadate-induced decrease in glucose uptake was not due to a decrease in number of vesicles. To further verify the apparent vanadate-induced decrease in GLUT4 intrinsic activity, the kinetics of glucose transport were also examined. In the presence of 10 mmol l-1 vanadate the V(max) and K(m) were decreased (P < 0.05, n = 6) 55% and 60%, respectively, compared with control. The plasma membrane GLUT4 protein content was not changed in response to vanadate. It is concluded that vanadate decreased glucose transport per GLUT4 (intrinsic activity). This finding suggests that regulation of glucose transport in skeletal muscle can involve changes in GLUT4 intrinsic activity.

Hypoxia and contractions do not utilize the same signaling mechanism in stimulating skeletal muscle glucose transport.

We have investigated whether hypoxia and muscle contractions stimulate glucose transport in perfused rat muscle to the same extent, additively and with the same sensitivity to the microbial products calphostin C and wortmannin. Hindlimb glucose uptake increased gradually from 3.4+/-0.5 to a maximal level of 12.7+/-0.6 micromol g-1 h-1 (n=11) after 50 min of hypoxia. Compared with hypoxia, the effect of maximal electrical stimulation of the sciatic nerve on muscle glucose uptake was more than two-fold higher (27+/-2 micromol g-1 h-1 (n=14)). This was due to a higher contraction- vs. hypoxia-induced glucose transport rate in oxidative fibers. The stimulatory effect of hypoxia and electrical stimulation was not additive. Contraction-induced muscle glucose transport was inhibitable by both calphostin C and wortmannin in the micromolar range, whereas the effect of hypoxia was totally insensitive to these drugs. Our data suggest that diacylglycerol/phorbol ester-sensitive protein kinase C is involved in stimulation of muscle glucose transport by contractions and that in contrast to the prevailing concept, hypoxia and contractions do not stimulate muscle glucose transport by the same signaling mechanism.

Training increases the concentration of [3H]ouabain-binding sites in rat skeletal muscle.

Exercise is associated with a net loss of K+ from the working muscles and an increased plasma K+ concentration, indicating that the capacity for intracellular reaccumulation of K+ is exceeded. Training reduces the exercise-induced rise in plasma K+, and an increased plasma [K+] may interfere with physical performance. Since the clearing of K+ from the extracellular space depends on the capacity for active K+ uptake in skeletal muscle, the effects of training and inactivity on the total concentration of (Na+ + K+)-ATPase was determined. Following 6 weeks of swim training, the concentration of [3H]ouabain-binding sites in rat hindlimb muscles was up to 46% (P less than 0.001) higher than in those obtained from age-matched controls. Whereas muscle Na+, K+ contents remained unchanged, the concentration of citrate synthase increased by up to 76% (P less than 0.001). Training induced no change in the [3H]ouabain-binding-site concentration in the diaphragm, but in the heart ventricles, the K+-dependent 3-O-methylfluorescein phosphatase activity increased by 20% (P less than 0.001). Muscle inactivity induced by denervation, plaster immobilisation or tenotomy reduced the [3H]ouabain-binding-site concentration by 20-30% (P less than 0.02-0.001) within 1 week. In conclusion, training leads to a significant and reversible rise in the concentration of (Na+ + K+)-ATPase in muscle cells. This may be of importance for the beneficial effects on physical performance by improving the maximum capacity for K+ clearance.

Increased muscle glucose uptake after exercise. No need for insulin during exercise.

It has recently been shown that insulin sensitivity of skeletal muscle glucose uptake and glycogen synthesis is increased after a single exercise session. The present study was designed to determine whether insulin is necessary during exercise for development of these changes found after exercise. Diabetic rats and controls ran on a treadmill and their isolated hindquarters were subsequently perfused at insulin concentrations of 0, 100, and 20,000 microU/ml. Exercise increased insulin sensitivity of glucose uptake and glycogen synthesis equally in diabetic and control rats, but insulin responsiveness of glucose uptake was noted only in controls. Analysis of intracellular glucose-6-phosphate, glucose, glycogen synthesis, and glucose transport suggested that the exercise effect on responsiveness might be due to enhancement of glucose disposal. After electrical stimulation of diabetic hindquarters in the presence of insulin antiserum, insulin sensitivity of 3-O-methylglucose transport was increased to the same extent as in muscle from healthy rats stimulated in the presence of insulin at 50 microU/ml. Furthermore, in muscle depleted of glycogen by contractions, transport of 3-O-methylglucose was increased in the presence of insulin antiserum and in the absence of increased regional perfusate flow. It is concluded that after exercise, increased sensitivity of muscle glucose metabolism to insulin can be found in the absence of insulin during exercise, but still involves increased membrane transport of glucose. At maximal insulin concentrations, the enhancing effect of exercise on glucose uptake may involve enhancement of glucose disposal, an effect that is probably less in muscle from diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Important role of insulin and flow in stimulating glucose uptake in contracting skeletal muscle.

The relative role of contractions, insulin, and increased supply of glucose and insulin, via an increase in blood flow, in stimulating glucose uptake in skeletal muscle during contractions was studied in isolated perfused rat hindlimbs. Hindlimbs were perfused with a standard perfusate medium containing 6 mmol/l glucose and four different insulin concentrations (0, 100, 500, and 20,000 microU/ml). Contractions were induced by supramaximal intermittent electrical stimulation of the sciatic nerve. Three different perfusion protocols were used: 1) muscles were stimulated to contract without concomitantly increasing perfusate flow; 2) flow was increased in the absence of electrical stimulation; and 3) muscles were stimulated to contract together with a flow increase. Both contractions and increased flow of perfusate, applied as separate stimuli, increased (P < 0.05) glucose uptake in the absence of insulin. Yet when submaximal insulin concentrations were added to the perfusate, the stimulatory action of both contractions and increased blood flow on muscle glucose uptake was augmented. The higher the submaximal insulin concentration, the greater the increment (P < 0.05). This effect, however, faded at supramaximal insulin concentration. Electrical stimulation associated with an increase in perfusion flow rate produced a greater (P < 0.05) rise in glucose uptake than did contractions alone. In fact, stimulation of muscle glucose uptake by contractions and increased flow proved to be additive at any insulin concentration. We conclude that contractions and increased blood flow act as additional stimuli to muscle glucose uptake at any insulin concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin signaling in human skeletal muscle: time course and effect of exercise.

Activation of early steps in the insulin signaling cascade in human skeletal muscle was investigated using a one-step euglycemic-hyperinsulinemic (approximately 100 pU/ml) clamp in seven healthy young men 3 h after one-legged exercise. Concomitant insulin stimulation (three- to six-fold [P < 0.05]) of thigh glucose clearance, muscle insulin receptor tyrosine kinase (IRTK), insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) was observed in the rested leg. Twenty minutes after cessation of insulin infusion, the level of these parameters returned toward basal. A twofold higher insulin-stimulated glucose clearance in the exercised compared with the rested thigh was accompanied by unaltered maximal IRTK activation and IRS-1 tyrosine phosphorylation, and by a decreased (approximately 50%, P < 0.05) maximal IRS-1 associated PI 3-kinase activation. Prior exercise caused significantly faster insulin-stimulated tyrosine phosphorylation of IRS-1, PI 3-kinase activity, and glucose clearance compared with those in the rested thigh. In conclusion, physiological hyperinsulinemia activates IRTK, IRS-1 tyrosine phosphorylation, and PI 3-kinase in human skeletal muscle. However, increased insulin action after exercise is not caused by potentiation of these steps in the insulin signaling cascade. In contrast, at steady state, paradoxically decreased insulin-stimulated IRS-1-associated PI 3-kinase activity was observed in exercised muscle. Thus, the activity of IRS-1-associated PI 3-kinase and glucose uptake may not always be tightly coupled during insulin stimulation in human muscle.

Significance of insulin for glucose metabolism in skeletal muscle during contractions.

Glucose uptake rate in active skeletal muscles is markedly increased during exercise. This increase reflects a multifactorial process involving both local and systemic mechanisms that cooperate to stimulate glucose extraction and glucose delivery to the muscle cells. Increased glucose extraction is effected primarily via mechanisms exerted within the muscle cell related to the contractile activity per se. Yet contractions become a more potent stimulus of muscle glucose uptake as the plasma insulin level is increased. In addition, enhanced glucose delivery to muscle, which during exercise is essentially effected via increased blood flow, significantly contributes to stimulate glucose uptake. Again, however, increased glucose delivery appears to be a more potent stimulus of muscle glucose uptake as the circulating insulin level is increased. Furthermore, contractions and elevated flow prove to be additive stimuli of muscle glucose uptake at any plasma insulin level. In conclusion, the extent to which muscle glucose uptake is stimulated during exercise depends on various factors, including 1) the intensity of the contractile activity, 2) the magnitude of the exercise-associated increase in muscle blood flow, and 3) the circulating insulin level.

Dissociation of AMP-activated protein kinase activation and glucose transport in contracting slow-twitch muscle.

5'AMP-activated protein kinase (AMPK) has been suggested to be a key regulatory protein in exercise signaling of muscle glucose transport. To test this hypothesis, we investigated whether muscle glycogen levels affect AMPK activation and glucose transport stimulation similarly during contractions. Rats were preconditioned by a combination of swimming exercise and diet to obtain a glycogen-supercompensated group (high muscle glycogen content [HG]) with approximately 3-fold higher muscle glycogen levels than a glycogen-depleted group (low muscle glycogen content [LG]). In perfused fast-twitch muscles, contractions induced significant increases in AMPK activity and glucose transport and decreases in acetyl-CoA carboxylase (ACC) activity in both HG and LG groups. Contraction-induced glucose transport was nearly 2-fold (P < 0.05) and AMPK activation was 3-fold (P < 0.05) higher in the LG group compared with the HG group, whereas ACC deactivation was not different between groups. Thus, there was a significant positive correlation between AMPK activity and glucose transport in contracting fast-twitch muscles (r = 0.80, P < 0.01). However, in slow-twitch muscles with HG, glucose transport was increased 6-fold (P < 0.05) during contractions, whereas AMPK activity did not increase. In contracting slow-twitch muscles with LG, the increase in AMPK activity (315%) and the decrease in ACC activity (54 vs. 34% at 0.2 mmol/l citrate, LG vs. HG) was higher (P < 0.05) compared with HG muscles, whereas the increase in glucose transport was identical in HG and LG. In conclusion, in slow-twitch muscles, high glycogen levels inhibit contraction-induced AMPK activation without affecting glucose transport. This observation suggests that AMPK activation is not an essential signaling step in glucose transport stimulation in skeletal muscle.