Effects of in vitro antagonism of endocannabinoid-1 receptors on the glucose transport system in normal and insulin-resistant rat skeletal muscle

OBJECTIVE

We determined the direct effects of modulating the endocannabinoid-1 (CB1) receptor on the glucose transport system in isolated skeletal muscle from insulin-sensitive lean Zucker and insulin-resistant obese Zucker rats.

METHODS

Soleus strips were incubated in the absence or presence of insulin, without or with various concentrations of the CB1 receptor antagonist SR141716 or with the CB1 receptor agonist arachidonyl-2-chloroethylamide (ACEA).

RESULTS

CB1 receptor protein expression in visceral adipose (57%), soleus (40%) and myocardial (36%) tissue was significantly (p < 0.05) decreased in obese compared to lean animals, with a trend for a reduction (17%, p = 0.079) in the liver. In isolated soleus muscle from both lean and obese Zucker rats, CB1 receptor antagonism directly improved glucose transport activity in a dose-dependent manner. Basal glucose transport activity was maximally enhanced between 100 and 200 nM SR141716 in lean (26-28%) and obese (22-31%) soleus. The maximal increase in insulin-stimulated glucose transport for lean muscle ( approximately 30%) was achieved at 50 nM SR141716 and for obese muscle ( approximately 30%) at 100 nM SR141716. In contrast, CB1 receptor antagonism did not alter hypoxia-stimulated glucose transport activity. CB1 receptor agonism (1 mM ACEA) significantly decreased both basal (15%) and insulin-stimulated (22%) glucose transport activity in isolated lean soleus. This effect was reversed by 200 nM SR141716. In both lean and obese muscle, the functionality of key signalling proteins (insulin receptor beta-subunit, Akt, glycogen synthase kinase-3beta (GSK-3beta), AMP-dependent protein kinase (AMPK), p38 mitogen-activated protein kinase (p38 MAPK)) was not altered by either CB1 receptor agonism or antagonism.

CONCLUSION

These results indicate that the engagement of CB1 receptor can negatively modulate both basal and insulin-dependent glucose transport activity in lean and obese skeletal muscles, and that these effects are not mediated by the engagement of elements of the canonical pathways regulating this process in mammalian skeletal muscle.

Improvement of insulin-stimulated glucose-disposal in type 2 diabetes after repeated parenteral administration of thioctic acid

Insulin resistance of skeletal muscle glucose uptake is a prominent feature of Type II diabetes (NIDDM); therefore, pharmacological intervention should aim to improve insulin sensitivity. Thioctic acid (TA), a naturally occurring compound, was shown to enhance glucose utilization in various experimental models after acute and chronic administration. It also increased insulin-stimulated glucose disposal in patients with NIDDM after acute administration. This pilot study was initiated to see whether this compound also augments glucose disposal in humans after repeated treatment. Twenty patients with NIDDM received TA (500 mg/ 500 ml NaCl, 0.9%) as daily infusions over a period of ten days. A hyperinsulinaemic, isoglycaemic glucose-clamp was done on day 0 and day 11. Parenteral administration of TA resulted in a significant increase of insulin-stimulated glucose-disposal by about 30% (metabolic clearance rate for glucose, 2.5 +/- 0.3 vs. 3.2 +/- 0.4 ml/kg/min and insulin-sensitivity-index: 3.5 +/- 0.5 vs. 4.7 +/- 0.4 mg/kg/microU/ml; p < 0.05, Wilcoxon-Rank-Sum-Test). There were no changes in fasting plasma levels for glucose or insulin; this can be explained, however, by the short period of treatment and observation. This is the first clinical study to show that a ten day administration of TA is able to improve resistance of insulin-stimulated glucose disposal in NIDDM. Experimental data suggest several mechanisms in the mode of action. As the present investigation was an uncontrolled pilot trial, the encouraging results call for controlled studies to further elucidate the clinical relevance of the findings and the mode of action of this compound.

Selective angiotensin II receptor antagonism reduces insulin resistance in obese Zucker rats

Effects of oral administration of the angiotensin II receptor antagonist (selective AT(1)-subtype) irbesartan on glucose tolerance and insulin action on skeletal-muscle glucose transport were assessed in the insulin-resistant obese Zucker rat. In the acute study, obese rats received either vehicle (water) or irbesartan 1 hour before the experiment. Although irbesartan had no effect on glucose transport (2-deoxyglucose uptake) in the epitrochlearis muscle, which consists mainly of type IIb fibers, acute angiotensin II receptor antagonism led to a dose-dependent increase in insulin action in the predominantly type I soleus muscle. Irbesartan at 25 and 50 mg/kg induced significant increases (41% and 50%, respectively; P<0.05) in insulin-mediated glucose transport. Moreover, these acute irbesartan-induced improvements in soleus-muscle glucose transport were associated with enhancements in whole-body insulin sensitivity (r=-0.732; P<0.05), as assessed during an oral glucose tolerance test. After chronic administration of irbesartan (21 days at 50 mg. kg(-1). d(-1)), glucose tolerance was enhanced further, and insulin-mediated glucose transport was significantly elevated in both epitrochlearis (32%) and soleus (73%) muscle. Chronic angiotensin II receptor antagonism was associated with significant increases in glucose transporter-4 (GLUT-4) protein expression in soleus (22%) and plantaris (20%) muscle and myocardium (15%). Chronic irbesartan-induced increases in whole-body insulin sensitivity were associated with increased insulin-mediated glucose transport in both epitrochlearis (r=-0.677; P<0.05) and soleus (r=-0.892; P<0.05) muscle. In summary, angiotensin II receptor (AT(1)-subtype) antagonism, either acutely or chronically, improves glucose tolerance, at least in part because of an enhancement in skeletal-muscle glucose transport, and the effect of chronic angiotensin II receptor antagonism on type I skeletal-muscle glucose uptake is associated with an increase in GLUT-4 protein expression.

Interactions of exercise training and lipoic acid on skeletal muscle glucose transport in obese Zucker rats

Exercise training (ET) or the antioxidant R(+)-alpha-lipoic acid (R-ALA) individually increases insulin action in the insulin-resistant obese Zucker rat. The purpose of the present study was to determine the interactions of ET and R-ALA on insulin action and oxidative stress in skeletal muscle of the obese Zucker rat. Animals either remained sedentary, received R-ALA (30 mg x kg body wt(-1) x day(-1)), performed ET (treadmill running), or underwent both R-ALA treatment and ET for 6 wk. During an oral glucose tolerance test, ET alone or in combination with R-ALA resulted in a significant lowering of the glucose (26-32%) and insulin (29-30%) responses compared with sedentary controls. R-ALA alone decreased (19%) the glucose-insulin index (indicative of increased insulin sensitivity), and this parameter was reduced (48-52%) to the greatest extent in the ET and combined treatment groups. ET or R-ALA individually increased insulin-mediated glucose transport activity in isolated epitrochlearis (44-48%) and soleus (37-57%) muscles. The greatest increases in insulin action in these muscles (80 and 99%, respectively) were observed in the combined treatment group. Whereas the improvement in insulin-mediated glucose transport in soleus due to R-ALA was associated with decreased protein carbonyl levels (an index of oxidative stress), improvement because of ET was associated with decreased protein carbonyls as well as enhanced GLUT-4 protein. However, there was no interactive effect of ET and R-ALA on GLUT-4 protein or protein carbonyl levels. These results indicate that ET and R-ALA interact in an additive fashion to improve insulin action in insulin-resistant skeletal muscle. Because the further improvement in muscle glucose transport in the combined group was not associated with additional upregulation of GLUT-4 protein or a further reduction in oxidative stress, the mechanism for this interaction must be due to additional, as yet unidentified, factors.

Antioxidant alpha-lipoic acid and protein turnover in insulin-resistant rat muscle

We have shown previously that the antioxidant alpha-lipoic acid (ALA) can stimulate glucose transport and can enhance the stimulation of this process by insulin in skeletal muscle from insulin-resistant obese Zucker rats. As insulin can also acutely activate general protein synthesis and inhibit net protein degradation in skeletal muscle, we hypothesized that ALA could directly affect protein turnover and also increase the effect of insulin on protein turnover in isolated skeletal muscle from developing obese Zucker rats. In epitrochlearis muscles isolated from obese Zucker rats, insulin (2 mU/ml) significantly (p < 0.05) increased in vitro protein synthesis (phenylalanine incorporation into protein) and decreased net protein degradation (tyrosine release), whereas a racemic mixture of ALA (2 mM) had no effect on either process. Interestingly, rates of protein synthesis in muscle from obese Zucker rats were substantially lower compared to those values observed in age-matched insulin-sensitive Wistar rats, whereas rates of protein degradation were comparable. Obese Zucker rats were also treated chronically with either vehicle or ALA (50 mg/kg/d for 10 d). Again, insulin significantly increased net protein synthesis and decreased net protein degradation in epitrochlearis muscles isolated from vehicle-treated obese Zucker rats; however, this stimulatory effect of insulin was not improved by prior in vivo ALA treatment. These results indicate that the previously described effect of the antioxidant ALA to increase insulin-stimulated glucose transport in skeletal muscle of obese, insulin-resistant rats does not apply to another important insulin-regulatable process, protein turnover. These findings imply that the cellular mode of action for ALA is restricted to signaling factors unique to the activation of glucose transport, and does not involve the pathway of stimulation of general protein synthesis and net protein degradation.

Effects of exercise training and ACE inhibition on insulin action in rat skeletal muscle

Our laboratory has demonstrated (Steen MS, Foianini KR, Youngblood EB, Kinnick TR, Jacob S, and Henriksen EJ, J Appl Physiol 86: 2044-2051, 1999) that exercise training and treatment with the angiotensin-converting enzyme (ACE) inhibitor trandolapril interact to improve insulin action in insulin-resistant obese Zucker rats. The present study was undertaken to determine whether a similar interactive effect of these interventions is manifest in an animal model of normal insulin sensitivity. Lean Zucker (Fa/-) rats were assigned to either a sedentary, trandolapril-treated (1 mg. kg(-1). day(-1) for 6 wk), exercise-trained (treadmill running for 6 wk), or combined trandolapril-treated and exercise-trained group. Exercise training alone or in combination with trandolapril significantly (P < 0.05) increased peak oxygen consumption by 26-32%. Compared with sedentary controls, exercise training alone or in combination with ACE inhibitor caused smaller areas under the curve for glucose (27-37%) and insulin (41-44%) responses during an oral glucose tolerance test. Exercise training alone or in combination with trandolapril also improved insulin-stimulated glucose transport in isolated epitrochlearis (33-50%) and soleus (58-66%) muscles. The increases due to exercise training alone or in combination with trandolapril were associated with enhanced muscle GLUT-4 protein levels and total hexokinase activities. However, there was no interactive effect of exercise training and ACE inhibition observed on insulin action. These results indicate that, in rats with normal insulin sensitivity, exercise training improves oral glucose tolerance and insulin-stimulated muscle glucose transport, whereas ACE inhibition has no effect. Moreover, the beneficial interactive effects of exercise training and ACE inhibition on these parameters are not apparent in lean Zucker rats and, therefore, are restricted to conditions of insulin resistance.

Beta-blocking agents in patients with insulin resistance: effects of vasodilating beta-blockers

Essential hypertension is--at least in many subjects--associated with a decrease in insulin sensitivity, while glycaemic control is (still) normal. It seems that in hypertensive patients, two major functions of insulin are impaired: there is insulin resistance of peripheral glucose uptake (primarily skeletal muscle) and insulin resistance of insulin-stimulated vasodilation. In view of some retrospective data and meta-analyses, which showed a less than expected reduction in coronary events (coronary paradox), the metabolic side effects of the antihypertensive treatment have received more attention. Many groups have shown that conventional antihypertensive treatment, both with beta-blockers and/or diuretics, decreases insulin sensitivity by various mechanisms. While low-dose diuretics seem to be free of these metabolic effects, there is no evidence for this in the beta-adrenergic blockers. However, recent metabolic studies evaluated the effects of vasodilating beta-blockers, such as dilevalol, carvedilol and celiprolol, on insulin sensitivity and the atherogenic risk factors. None of them decreased insulin sensitivity, as has been described for the beta-blockers with and without beta1 selectivity. This supports the idea that peripheral vascular resistance and peripheral blood flow play a central role in mediating the metabolic side effects of the beta-blocking agents, as the vasodilating action (either via beta2 stimulation or alpha1-blockade) seems to more than offset the detrimental effects of the blockade of beta (or beta1) receptors. Further studies are needed to elucidate the relevance of the radical scavenging properties of these agents and their connection to their metabolic effects. Therefore, the beneficial characteristics of these newer beta-adrenoreceptor blockers suggest that the vasodilating beta-blocking agents could be advantageous for hypertensive patients with insulin resistance or type 2 diabetes.

Effects of a unique conjugate of alpha-lipoic acid and gamma-linolenic acid on insulin action in obese Zucker rats

The purpose of this study was to assess the individual and interactive effects of the antioxidant alpha-lipoic acid (LPA) and the n-6 essential fatty acid gamma-linolenic acid (GLA) on insulin action in insulin-resistant obese Zucker rats. LPA, GLA, and a unique conjugate consisting of equimolar parts of LPA and GLA (LPA-GLA) were administered for 14 days at 10, 30, or 50 mg. kg body wt(-1). day(-1). Whereas LPA was without effect at 10 mg/kg, at 30 and 50 mg/kg it elicited 23% reductions (P < 0.05) in the glucose-insulin index (the product of glucose and insulin areas under the curve during an oral glucose tolerance test and an index of peripheral insulin action) that were associated with significant increases in insulin-mediated (2 mU/ml) glucose transport activity in isolated epitrochlearis (63-65%) and soleus (33-41%) muscles. GLA at 10 and 30 mg/kg caused 21-25% reductions in the glucose-insulin index and 23-35% improvements in insulin-mediated glucose transport in epitrochlearis muscle. The beneficial effects of GLA disappeared at 50 mg/kg. At 10 and 30 mg/kg, the LPA-GLA conjugate elicited 29 and 38% reductions in the glucose-insulin index. These LPA-GLA-induced improvements in whole body insulin action were accompanied by 28-63 and 38-57% increases in insulin-mediated glucose transport in epitrochlearis and soleus muscles and resulted from the additive effects of LPA and GLA. At 50 mg/kg, the metabolic improvements due to LPA-GLA were substantially reduced. In summary, these results indicate that the conjugate of the antioxidant LPA and the n-6 essential fatty acid GLA elicits significant dose-dependent improvements in whole body and skeletal muscle insulin action on glucose disposal in insulin-resistant obese Zucker rats. Moreover, these actions of LPA-GLA are due to the additive effects of its individual components.

ACE inhibition and glucose transport in insulinresistant muscle: roles of bradykinin and nitric oxide

Acute administration of the angiotensin-converting enzyme (ACE) inhibitor captopril enhances insulin-stimulated glucose transport activity in skeletal muscle of the insulin-resistant obese Zucker rat. The present study was designed to assess whether this effect is mediated by an increase in the nonapeptide bradykinin (BK), by a decrease in action of ANG II, or both. Obese Zucker rats (8-9 wk old) were treated for 2 h with either captopril (50 mg/kg orally), bradykinin (200 micrograms/kg ip), or the ANG II receptor (AT(1) subtype) antagonist eprosartan (20 mg/kg orally). Captopril treatment enhanced in vitro insulin-stimulated (2 mU/ml) 2-deoxyglucose uptake in the epitrochlearis muscle by 22% (251 +/- 7 vs. 205 +/- 9 pmol. mg(-1). 20 min(-1); P < 0.05), whereas BK treatment enhanced this variable by 18% (249 +/- 15 vs. 215 +/- 7 pmol. mg(-1). 20 min(-1); P < 0.05). Eprosartan did not significantly modify insulin action. The BK-mediated increase in insulin action was completely abolished by pretreatment with either the specific BK-B(2) receptor antagonist HOE 140 (200 micrograms/kg ip) or the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (50 mg/kg ip). Collectively, these results indicate that the modulation of insulin action by BK likely underlies the metabolic effects of ACE inhibitors in the insulin-resistant obese Zucker rat. Moreover, this modulation of insulin action by BK is likely mediated through B(2) receptors and by an increase in nitric oxide production and/or action in skeletal muscle tissue.

The beta2-adrenergic modulator celiprolol reduces insulin resistance in obese Zucker rats

Essential hypertension is associated with an increased incidence of insulin resistance of skeletal muscle glucose transport. The present study determined if celiprolol, an antihypertensive agent with selective beta1-adrenoceptor antagonist and additional beta2-agonistic properties, administered by gavage either acutely (3 hr) or chronically (14 d), had a direct effect on improving glucose tolerance and insulin-stimulated glucose transport activity (using 2-deoxyglucose (2-DG) uptake) in isolated epitrochlearis muscles of the insulin-resistant obese Zucker rat. The effects of a selective beta1-blocker, metoprolol, were also assessed. Acute administration of celiprolol, but not metoprolol, increased insulin-stimulated 2-DG uptake in muscle by 22% (p<0.05). Chronic celiprolol treatment significantly lowered fasting plasma insulin (22%) and free fatty acids (40%) in comparison to obese control values. Moreover, chronic celiprolol administration decreased the glucose-insulin index (calculated as the product of the glucose and insulin areas under the curve during an oral glucose tolerance test), by 32% (p<0.05) compared to obese controls, indicating that peripheral insulin action was increased. Indeed, insulin-stimulated skeletal muscle 2-DG uptake was enhanced by 49% (p<0.05) in these celiprolol-treated obese animals. Metoprolol was without significant effect on any of these variables following chronic administration. These findings indicate that, in this animal model of insulin resistance, the beta1-antagonist/beta2-agonist celiprolol has a specific effect of improving insulin-stimulated skeletal muscle glucose transport that is independent of any hemodynamic alterations.

Interactions of exercise training and ACE inhibition on insulin action in obese Zucker rats

Exercise training or chronic treatment with angiotensin-converting enzyme (ACE) inhibitors can ameliorate glucose intolerance, insulin resistance of muscle glucose metabolism, and dyslipidemia associated with the obese Zucker rat. The purpose of the present study was to determine the interactions of exercise training and ACE inhibition (trandolapril) on these parameters in the obese Zucker rat. Animals were assigned to a sedentary control, a trandolapril-treated (1 mg. kg-1. day-1 for 6 wk), an exercise-trained (treadmill running for 6 wk), or a combined trandolapril-treated and exercise-trained group. Exercise training, alone or with trandolapril, significantly (P < 0. 05) increased peak O2 consumption by 31-34%. Similar decreases in fasting plasma insulin (34%) and free fatty acids (31%) occurred with exercise training alone or in combination with trandolapril. Compared with control, exercise training or trandolapril alone caused smaller areas under the curve (AUC) for glucose (12-14%) and insulin (28-33%) during an oral glucose tolerance test. The largest decreases in the glucose AUC (40%) and insulin AUC (53%) were observed in the combined group. Similarly, whereas exercise training or trandolapril alone improved maximally activated insulin-stimulated glucose transport in isolated epitrochlearis (26-34%) or soleus (39-41%) muscles, the greatest improvements in insulin action (67 and 107%, respectively) were seen in the combined group and were associated with similarly enhanced muscle GLUT-4 protein and total hexokinase levels. In conclusion, these results indicate combined exercise training and ACE inhibition improve oral glucose tolerance and insulin-stimulated muscle glucose transport to a greater extent than does either intervention alone.

Antihypertensive therapy and insulin sensitivity: do we have to redefine the role of beta-blocking agents?

Essential hypertension is, at least in many subjects, associated with a decrease in insulin sensitivity, whereas glycemic control is (still) normal. Metaanalyses of hypertension intervention studies revealed different efficacy of treatment on cerebral (cerebrovascular accidents [CVA]) and cardiac (coronary heart disease [CHD]) morbidity and mortality. Although CVA were reduced to an extent similar to that anticipated, the decrease in CHD was less than expected. These differences are likely to be caused by the different impact of concomitant cardiovascular risk factors, such as dyslipidemia, impaired glucose tolerance, and non-insulin-dependent diabetes mellitus on CHD and CVA. Frequently these cardiovascular risk factors are ineffectively controlled in hypertensive patients, and moreover, some of the widely used antihypertensive agents have unfavorable side effects and further deteriorate these particular metabolic risk factors. Therefore, the metabolic side effects of antihypertensive treatment have received more attention. During the past few years, studies demonstrated that most antihypertensive agents modify insulin sensitivity in parallel with alterations in the atherogenic lipid profile. Alpha1-blockers and angiotensin converting enzyme inhibitors were shown to either have no impact on or even improve insulin resistance and the profile of atherogenic lipids, whereas most of the calcium channel blockers were found to be metabolically inert. The diuretics and beta-adrenoreceptor antagonists further decrease insulin sensitivity and worsen dyslipidemia. The mechanisms by which beta-adrenoreceptor antagonist treatment exert its disadvantageous effects are not fully understood, but several possibilities exist: significant body weight gain, reduction in enzyme activities (muscle lipoprotein lipase and lecithin cholesterol acyltransferase), alterations in insulin clearance and insulin secretion, and, probably most important, reduced peripheral blood flow due to increase in total peripheral vascular resistance. Recent metabolic studies found beneficial effects of the newer vasodilating beta-blockers, such as dilevalol, carvedilol and celiprolol, on insulin sensitivity and the atherogenic risk factors. In many hypertensive patients, elevated sympathetic nerve activity and insulin resistance are a deleterious combination. Although conventional beta-blocker treatment was able to take care of the former, the latter got worse; the newer vasodilating beta-blocker generation seems to be capable of successfully treating both of them.

Interactions of captopril and verapamil on glucose tolerance and insulin action in an animal model of insulin resistance

We have shown previously that the combination of a long-acting, non-sulfhydryl-containing angiotensin-converting enzyme (ACE) inhibitor (trandolapril) and the Ca2+ channel blocker verapamil improve insulin-stimulated glucose transport in skeletal muscle of the obese Zucker rat, a model of insulin resistance, hyperinsulinemia, and dyslipidemia. In the present study, we investigated the interactions of chronic treatment (28 days) with verapamil (20 mg/kg) and a short-acting, sulfhydryl-containing ACE inhibitor (captopril, 50 mg/kg) in combination on insulinemia, lipidemia, glucose tolerance, and insulin action on skeletal muscle glucose transport (2-deoxyglucose uptake in epitrochlearis) in lean and obese Zucker rats. In lean animals, verapamil alone and in combination with captopril actually increased (P < .05) plasma insulin, whereas in obese animals, verapamil alone worsened the hyperinsulinemia already present, and this effect was abolished by cotreatment with captopril. Captopril alone or in combination with verapamil reduced plasma free fatty acid (FFA) levels in obese rats, but not in lean rats. Captopril alone reduced the glucose-insulin index in obese animals given an oral glucose load, and this was associated with a significant increase in insulin-mediated muscle glucose transport. The greatest improvement in these responses was elicited in obese animals receiving combined captopril and verapamil treatment, and was associated with increases in muscle GLUT-4 glucose transporter protein and hexokinase and citrate synthase activities. In conclusion, these findings indicate that the short-acting, sulfhydryl-containing ACE inhibitor captopril can elicit beneficial metabolic effects on the hyperinsulinemia, dyslipidemia, glucose intolerance, and insulin resistance of muscle glucose transport of the obese Zucker rat. Moreover, there is a positive interactive effect on these pathophysiological parameters between captopril and verapamil in this animal model of insulin resistance.

Effect of chronic bradykinin administration on insulin action in an animal model of insulin resistance

The nonapeptide bradykinin (BK) has been implicated as the mediator of the beneficial effect of angiotensin-converting enzyme inhibitors on insulin-stimulated glucose transport in insulin-resistant skeletal muscle. In the present study, the effects of chronic in vivo BK treatment of obese Zucker (fa/fa) rats, a model of glucose intolerance and severe insulin resistance, on whole body glucose tolerance and skeletal muscle glucose transport activity stimulated by insulin or contractions were investigated. BK was administered subcutaneously (twice daily at 40 microg/kg body wt) for 14 consecutive days. Compared with a saline-treated obese group, the BK-treated obese animals had significantly (P < 0.05) lower fasting plasma levels of insulin (20%) and free fatty acids (26%), whereas plasma glucose was not different. During a 1 g/kg body wt oral glucose tolerance test, the glucose and insulin responses [incremental areas under the curve (AUC)] were 21 and 29% lower, respectively, in the BK-treated obese group. The glucose-insulin index, the product of the glucose and insulin AUCs and an indirect index of in vivo insulin action, was 52% lower in the BK-treated obese group compared with the obese control group. Moreover, 2-deoxyglucose uptake in the isolated epitrochlearis muscle stimulated by a maximally effective dose of insulin (2 mU/ml) was 52% greater in the BK-treated obese group. Contraction-stimulated (10 tetani) 2-deoxyglucose uptake was also enhanced by 35% as a result of the BK treatment. In conclusion, these findings indicate that in the severely insulin-resistant obese Zucker rat, chronic in vivo treatment with BK can significantly improve whole body glucose tolerance, possibly as a result of the enhanced insulin-stimulated skeletal muscle glucose transport activity observed in these animals.

Antihypertensive agent moxonidine enhances muscle glucose transport in insulin-resistant rats

The sympatholytic antihypertensive agent moxonidine, a centrally acting selective I1-imidazoline receptor modulator (putative agonist), may be beneficial in hypertensive patients with insulin resistance. In the present study, the effects of chronic in vivo moxonidine treatment of obese Zucker rats--a model of severe glucose intolerance, hyperinsulinemia and insulin resistance, and dyslipidemia--on whole-body glucose tolerance, plasma lipids, and insulin-stimulated skeletal muscle glucose transport activity (2-deoxyglucose uptake) were investigated. Moxonidine was administered by gavage for 21 consecutive days at 2, 6, or 10 mg/kg body weight. Body weights in control and moxonidine-treated groups were matched, except at the highest dose, at which final body weight was 17% lower in the moxonidine-treated animals compared with controls. The moxonidine-treated (6 and 10 mg/kg) obese animals had significantly lower fasting plasma levels of insulin (17% and 19%, respectively) and free fatty acids (36% and 28%, respectively), whereas plasma glucose was not altered. During an oral glucose tolerance test, the glucose response (area under the curve) was 47% and 67% lower, respectively, in the two highest moxonidine-treated obese groups. Moreover, glucose transport activity in the isolated epitrochlearis muscle stimulated by a maximally effective insulin dose (13.3 nmol/L) was 39% and 70% greater in the 6 and 10 mg/kg moxonidine-treated groups, respectively (P<.05 for all effects). No significant alterations in muscle glucose transport were elicited by 2 mg/kg moxonidine. These findings indicate that in the severely insulin-resistant and dyslipidemic obese Zucker rat, chronic in vivo treatment with moxonidine can significantly improve, in a dose-dependent manner, whole-body glucose tolerance, possibly as a result of enhanced insulin-stimulated skeletal muscle glucose transport activity and reduced circulating free fatty acids.

Differential effects of lipoic acid stereoisomers on glucose metabolism in insulin-resistant skeletal muscle

The racemic mixture of the antioxidant alpha-lipoic acid (ALA) enhances insulin-stimulated glucose metabolism in insulin-resistant humans and animals. We determined the individual effects of the pure R-(+) and S-(-) enantiomers of ALA on glucose metabolism in skeletal muscle of an animal model of insulin resistance, hyperinsulinemia, and dyslipidemia: the obese Zucker (fa/fa) rat. Obese rats were treated intraperitoneally acutely (100 mg/kg body wt for 1 h) or chronically [10 days with 30 mg/kg of R-(+)-ALA or 50 mg/kg of S-(-)-ALA]. Glucose transport [2-deoxyglucose (2-DG) uptake], glycogen synthesis, and glucose oxidation were determined in the epitrochlearis muscles in the absence or presence of insulin (13.3 nM). Acutely, R-(+)-ALA increased insulin-mediated 2-DG-uptake by 64% (P < 0.05), whereas S-(-)-ALA had no significant effect. Although chronic R-(+)-ALA treatment significantly reduced plasma insulin (17%) and free fatty acids (FFA; 35%) relative to vehicle-treated obese animals, S-(-)-ALA treatment further increased insulin (15%) and had no effect on FFA. Insulin-stimulated 2-DG uptake was increased by 65% by chronic R-(+)-ALA treatment, whereas S-(-)-ALA administration resulted in only a 29% improvement. Chronic R-(+)-ALA treatment elicited a 26% increase in insulin-stimulated glycogen synthesis and a 33% enhancement of insulin-stimulated glucose oxidation. No significant increase in these parameters was observed after S-(-)-ALA treatment. Glucose transporter (GLUT-4) protein was unchanged after chronic R-(+)-ALA treatment but was reduced to 81 +/- 6% of obese control with S-(-)-ALA treatment. Therefore, chronic parenteral treatment with the antioxidant ALA enhances insulin-stimulated glucose transport and non-oxidative and oxidative glucose metabolism in insulin-resistant rat skeletal muscle, with the R-(+) enantiomer being much more effective than the S-(-) enantiomer.

Muscle adaptations to hindlimb suspension in mature and old Fischer 344 rats

We examined skeletal and cardiac muscle responses of mature (8 mo) and old (23 mo) male Fischer 344 rats to 14 days of hindlimb suspension. Hexokinase (HK) and citrate synthase (CS) activities and GLUT-4 glucose transporter protein level, which are coregulated in many instances of altered neuromuscular activity, were analyzed in soleus (Sol), plantaris (PI), tibialis anterior (TA), extensor digitorum longus (EDL), and left ventricle. Protein content was significantly (P < 0.05) lower in all four hindlimb muscles after suspension compared with controls in both mature (21-44%) and old (17-43%) rats. Old rats exhibited significantly lower CS activities than mature rats for the Sol, Pl, and TA. HK activities were significantly lower in the old rats for the Pl (19%) and TA (33%), and GLUT-4 levels were lower in the old rats for the TA (38%) and EDL (24%) compared with the mature rats. Old age was also associated with a decrease in CS activity (12%) and an increase in HK activity (14%) in cardiac muscle. CS activities were lower in the Sol (20%) and EDL (18%) muscles from mature suspended rats and in the Sol (25%), Pl (27%), and EDL (25%) muscles from old suspended rats compared with corresponding controls. However, suspension was associated with significantly higher HK activities for all four hindlimb muscles examined, in both old (16-57%) and mature (10-43%) rats, and higher GLUT-4 concentrations in the TA muscles of the old rats (68%) but not the mature rats. These results indicate that old age is associated with decreased CS and HK activities and GLUT-4 protein concentration for several rat hindlimb muscles, and these variables are not coregulated during suspension. Finally, old rat skeletal muscle appears to respond to suspension to a similar or greater degree than mature rat muscle responds.

Insulin attenuates atrophy of unweighted soleus muscle by amplified inhibition of protein degradation

Unweighting atrophy of immature soleus muscle occurs rapidly over the first several days, followed by slower atrophy coinciding with increased sensitivity to insulin of in vitro protein metabolism. This study determined whether this increased sensitivity might account for the diminution of atrophy after 3 days of tall-cast hindlimb suspension. The physiological significance of the increased response to insulin in unweighted muscle was evaluated by analyzing in vivo protein metabolism for day 3 (48 to 72 hours) and day 4 (72 to 96 hours) of unweighting in diabetic animals either injected with insulin or not treated. Soleus from nontreated diabetic animals showed a similar loss of protein during day 3 (-16.2%) and day 4 (-14.5%) of unweighting, whereas muscle from insulin-treated animals showed rapid atrophy (-14.5%) during day 3 only, declining to just -3.1% the next day. Since fractional protein synthesis was similar for both day 3 (8.6%/d) and day 4 (7.0%/d) of unweighting in insulin-treated animals, the reduction in protein loss must be accounted for by a slowing of protein degradation due to circulating insulin. Intramuscular (IM) injection of insulin (600 nmol/L) stimulated in situ protein synthesis similarly in 4-day unweighted (+56%) and weight-bearing (+90%) soleus, even though unweighted muscle showed a greater in situ response of 2-deoxy-[3H]glucose uptake to IM injection of either insulin (133 nmol/L) or insulin-like growth factor-I (IGF-I) (200 nmol/L) than control muscle. These findings suggest that unweighted muscle is selectively more responsive in vivo to insulin, and that the slower atrophy after 3 days of unweighting was due to an increased effect of insulin on inhibiting protein degradation.

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

Skeletal muscle glucose transport can be regulated by hormonal factors such as insulin and insulin-like growth factor I (IGF-I). Although it is well established that exercise training increases insulin action on muscle glucose transport, it is currently unknown whether exercise training leads to an enhancement of IGF-I-stimulated glucose transport in skeletal muscle. Therefore, we measured glucose transport activity [by using 2-deoxy-D-glucose glucose (2-DG) uptake] in the isolated rat epitrochlearis muscle stimulated by submaximally and maximally effective concentrations of insulin (0.2 and 13.3 nM) or IGF-I (5 and 50 nM) after 1, 2, and 3 wk of voluntary wheel running (WR). After 1 wk of WR, both submaximal and maximal insulin-stimulated 2-DG uptake rates were significantly (P < 0.05) enhanced (43 and 31%) compared with those of sedentary controls, and these variables were further increased after 2 (86 and 57%) and 3 wk (71 and 70%) of WR. Submaximal and maximal IGF-I-stimulated 2-DG uptake rates were significantly enhanced after 1 wk of WR (82 and 61%, and these increases did not expand substantially after 2 (71 and 58%) and 3 wk (96 and 70%) of WR. This enhancement of hormone-stimulated 2-DG uptake 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 citrate synthase. These results indicate that exercise training can enhance both insulin-stimulated and IGF-I-stimulated muscle glucose transport activity and that these improvements can develop without an increase in GLUT-4 protein.

Stimulation by alpha-lipoic acid of glucose transport activity in skeletal muscle of lean and obese Zucker rats

Alpha-lipoic acid (ALA), a potent biological antioxidant, improves insulin action of skeletal muscle glucose transport and metabolism in both human and animal models of insulin resistance. In order to obtain further insight into the potential intracellular mechanisms for the action of ALA on insulin-stimulated glucose transport in skeletal muscle, we investigated the effects of direct incubation with ALA (2 mM) on 2-deoxyglucose (2-DG) uptake by epitrochlearis muscle from either insulin-sensitive lean (Fa/-) or insulin-resistant obese (fa/fa) Zucker rats. ALA stimulated 2-DG uptake in muscle of lean animals by 76%, whereas ALA stimulated 2-DG uptake by only 48% in muscle from obese animals. The stimulation of 2-DG uptake due to ALA was enhanced 30-55% in the presence of insulin. In contrast, ALA action on 2-DG uptake was not additive with the effects of electrically-stimulated muscle contractions in either insulin-sensitive or insulin-resistant muscle. Wortmannin (1 microM), an inhibitor of phosphotidylinositol-3-kinase, completely inhibited insulin action on 2-DG uptake, but inhibited ALA action by only 25%. Collectively, these results indicate that although a portion of ALA action on glucose transport in mammalian skeletal muscle is mediated via the insulin signal transduction pathway, the majority of the direct effect of ALA on skeletal muscle glucose transport is insulin-independent.

GLUT-4 protein and citrate synthase activity in distally or proximally denervated rat soleus muscle

The potential role of neurotrophic factors in the decline of glucose transporter (GLUT-4) protein levels and citrate synthase (CS) activity was studied by comparing distally with proximally denervated juvenile rat soleus muscle. Severing of the tibial nerve produced distal (long stump) or proximal (short stump) denervation. GLUT-4 levels and CS activities were measured at 24-h intervals for up to 96 h after denervation. No differences were observed in GLUT-4 or CS activity between soleus muscles left with short or long nerve stumps at any time point. However, within just 24 h, denervation decreased (P < 0.05). GLUT-4 and CS (67.4 +/- 3.3 and 63.4 +/- 1.7% of innervated control values, respectively). Both parameters continued to decline up to 96 h (44.4 +/- 3.1 and 48.7 +/- 4.0%, respectively). There was a significant correlation between the GLUT-4 protein level and CS activity over this 96-h period of denervation (r = 0.653, P < 0.001). A similar response in the 24-h denervated soleus of adult rats was observed. In contrast, 24-h denervation of red gastrocnemius (type IIa fibers) left with a long nerve stump resulted in a prevention of the decline of GLUT-4 and CS seen in red gastrocnemius left with a short nerve stump in both juvenile and adult animals. These results suggest that unlike type IIa muscles, the decline in GLUT-4 level and CS activity in type I soleus muscle after denervation results from a lack of coordinated electrical activity but likely does not involve a neurotrophic agent. These results also support the hypothesis that there is coregulation of decreased expression of GLUT-4 protein and CS activity in this model of reduced neuromuscular activity.

The antioxidant alpha-lipoic acid enhances insulin-stimulated glucose metabolism in insulin-resistant rat skeletal muscle

Insulin resistance of muscle glucose metabolism is a hallmark of NIDDM. The obese Zucker (fa/fa) rat--an animal model of muscle insulin resistance--was used to test whether acute (100 mg/kg body wt for 1 h) and chronic (5-100 mg/kg for 10 days) parenteral treatments with a racemic mixture of the antioxidant alpha-lipoic acid (ALA) could improve glucose metabolism in insulin-resistant skeletal muscle. Glucose transport activity (assessed by net 2-deoxyglucose [2-DG] uptake), net glycogen synthesis, and glucose oxidation were determined in the isolated epitrochlearis muscles in the absence or presence of insulin (13.3 nmol/l). Severe insulin resistance of 2-DG uptake, glycogen synthesis, and glucose oxidation was observed in muscle from the vehicle-treated obese rats compared with muscle from vehicle-treated lean (Fa/-) rats. Acute and chronic treatments (30 mg.kg-1.day-1, a maximally effective dose) with ALA significantly (P < 0.05) improved insulin-mediated 2-DG uptake in epitrochlearis muscles from the obese rats by 62 and 64%, respectively. Chronic ALA treatment increased both insulin-stimulated glucose oxidation (33%) and glycogen synthesis (38%) and was associated with a significantly greater (21%) in vivo muscle glycogen concentration. These adaptive responses after chronic ALA administration were also associated with significantly lower (15-17%) plasma levels of insulin and free fatty acids. No significant effects on glucose transporter (GLUT4) protein level or on the activities of hexokinase and citrate synthase were observed. Collectively, these findings indicate that parenteral administration of the antioxidant ALA significantly enhances the capacity of the insulin-stimulatable glucose transport system and of both oxidative and nonoxidative pathways of glucose metabolism in insulin-resistant rat skeletal muscle.

Effects of trandolapril and verapamil on glucose transport in insulin-resistant rat skeletal muscle

We have used an animal model of insulin resistance-the obese Zucker (fa/fa) rat-to test whether oral administration of the non-sulfhydryl-containing angiotensin-converting enzyme (ACE) inhibitor, trandolapril, alone or in combination with the Ca2+-channel blocker, verapamil, can induce a beneficial effect on insulin-stimulated glucose transport and metabolism in skeletal muscle. Insulin-stimulated 2-deoxyglucose (2-DG) uptake in the isolated epitrochlearis muscle was less than 50% as great in obese animals compared with lean (Fa/-) controls (P < .05), but was significantly improved in the obese group by both short-term (6 hours, +33%) and long-term (14 days,+70%) oral treatment with trandolapril. Verapamil treatment alone did not alter insulin-stimulated 2-DG uptake in muscle, but simultaneous administration of verapamil and trandolapril resulted in the most pronounced effect on insulin-stimulated 2-DG uptake (+106%). Long-term treatment with trandolapril alone and in combination with verapamil significantly increased muscle glycogen (+26% to 27%), glucose transporter GLUT-4 protein (+27% to 31%), and hexokinase activity (+21% to 49%), and decreased plasma insulin levels (-23% to -29%). Muscle citrate synthase activity was enhanced only when trandolapril and verapamil were administered in combination (+24%). We conclude that the long-acting, non-sulfhydryl-containing ACE inhibitor, trandolapril, alone and in combination with the Ca2+-channel blocker, verapamil, can significantly improve insulin-stimulated glucose transport activity in skeletal muscle of the insulin-resistant obese Zucker rat, and that this improvement is associated with favorable adaptive responses in GLUT-4 protein levels, glycogen storage, and activities of relevant intracellular enzymes of glucose catabolism.

Role of glucose transport in glycogen supercompensation in reweighted rat skeletal muscle

Hindlimb weight bearing after a 3-day period of hindlimb suspension (reweighting) of juvenile rats results in a marked transient elevation in soleus glycogen concentration that cannot be explained on the basis of the activities of glycogen synthase and phosphorylase. We have hypothesized that enhanced glucose transport activity could underlie this response. We directly tested this hypothesis by assessing the response of insulin-dependent and insulin-independent glucose transport activity (in vitro 2-[1,2-3H]deoxy-D-glucose uptake) as well as glucose transporter (GLUT-4) protein levels during a 48-h reweighting period. After a net glycogen loss (from 29 +/- 2 to 16 +/- 1 nmol/mg muscle; P < 0.05) during the first 2 h of reweighting, glycogen accumulated at an average rate of 1.4 nmol.mg-1.h-1 up to 18 h, reaching an apex of 38 +/- 1 nmol/mg. During this same reweighting period, insulin-independent, but not insulin-dependent, glucose transport activity was significantly enhanced (P < 0.05 vs. weight-bearing control values) and was associated with an elevated level of GLUT-4 protein and the specific activity of total hexokinase. The specific activity of citrate synthase was also increased. By 24 h of reweighting, although insulin-independent glucose transport activity and GLUT-4 protein remained elevated, glycogen accumulation had ceased, likely due to enhanced phosphorylase activity at this time point. These results are consistent with the interpretation that the glycogen supercompensation seen during reweighting of the rat soleus may be regulated in part by an enhanced glucose flux arising from an increase in insulin-independent glucose transport activity and hexokinase activity.

Glucose transport activity in insulin-resistant rat muscle. Effects of angiotensin-converting enzyme inhibitors and bradykinin antagonism

Insulin resistance of skeletal muscle glucose disposal underlies the pathogenesis of NIDDM and is associated with hypertension, obesity, and dyslipidemia. Angiotensin-converting enzyme (ACE) inhibitors are used primarily in antihypertensive therapy but also are known to improve whole-body insulin-mediated glucose disposal. However, the exact site of action is not well characterized. We have used the isolated epitrochlearis muscle from a well-established animal model of skeletal muscle insulin resistance, the obese Zucker rat, to test the effect of oral administration of ACE inhibitors on insulin-sensitive muscle glucose transport activity. Both acute and chronic administration of a sulfhydryl-containing ACE inhibitor (captopril) or a non-sulfhydryl-containing ACE inhibitor (tran-dolapril) significantly enhanced in vitro insulin-mediated muscle glucose transport activity. In addition, the acute effect of oral captopril administration was completely abolished by pretreatment of the animal with a bradykinin B2 receptor antagonist (HOE 140). These findings indicate that ACE inhibitors may improve whole-body glucose metabolism by acting on the insulin-sensitive skeletal muscle glucose transport system. In addition, bradykinin or one of its metabolites may be involved in the action of the ACE inhibitor captopril on insulin-resistant muscle.

Potential role of bradykinin in forearm muscle metabolism in humans

Using the euglycemic-hyperinsulinemic glucose clamp and the human forearm technique, we have demonstrated that the improved glucose disposal rate observed after the administration of an angiotensin-converting enzyme (ACE) inhibitor such as captopril may be primarily due to increased muscle glucose uptake (MGU). These results are not surprising because ACE, which is identical to the bradykinin (BK)-degrading kininase II, is abundantly present in muscle tissue, and its inhibition has been observed to elicit the observed metabolic actions via elevated tissue concentrations of BK and through a BK B2 receptor site in muscle and/or endothelial tissue. These findings are supported by several previous studies. Exogenous BK applied into the brachial artery of the human forearm not only augmented muscle blood flow (MBF) but also enhanced the rate of MGU. In another investigation, during rhythmic voluntary contraction, both MBF and MGU increased in response to the higher energy expenditure, and the release of BK rose in the blood vessel, draining the working muscle tissue. Inhibition of the activity of the BK-generating protease in muscle tissue (kallikrein) with aprotinin significantly diminished these functional responses during contraction. Applying the same kallikrein inhibitor during the infusion of insulin into the brachial artery significantly reduced the effect of insulin on glucose uptake into forearm muscle. This is of interest, because in recent studies insulin has been suggested to elicit its actions on MBF and MGU via the accelerated release of endothelium-derived nitric oxide, the generation of which is also stimulated by BK in a concentration-dependent manner. This new evidence obtained from in vitro and in vivo studies sheds new light on the discussion of whether BK may play a role in energy metabolism of skeletal muscle tissue.

Metabolic responses of rat respiratory muscles to voluntary exercise training

Voluntary wheel running for 4 or 8 wk was used to assess whether a volitional training stimulus would induce adaptations in the oxidative capacity [citrate synthase activity (CS)], glucose phosphorylation capacity [hexokinase activity (HK)], and glucose transporter protein level (GLUT-4) of rat respiratory muscles. Running distances averaged approximately 10-13 km/day over the final 5 wk of training. Peak oxygen consumption by the trained animals was 17% greater (P < 0.05) than by age-matched sedentary control animals after 8 wk. CS, HK, and GLUT-4 in soleus and plantaris muscles all increased because of exercise training. CS increased in the rectus abdominis (+17%), external oblique (+28%), and internal oblique (+17%) but not in the costal or crural diaphragm after 4 wk of training. However, after 8 wk, CS in the costal diaphragm was 39% greater than control but was unchanged in the crural diaphragm. Whereas HK was significantly greater than control in the costal diaphragm (+18%) and rectus abdominis (+54%) after 4 wk, 8 wk of running were required for increases in HK in the external oblique (+17%) and internal oblique (+14%). HK in the crural diaphragm was not significantly altered by the exercise training. GLUT-4 did not change significantly in any of the respiratory muscles studied. These results indicate that significant adaptations in the glucose phosphorylation capacity and oxidative capacity of both inspiratory and expiratory muscles can take place in response to voluntary exercise. However, this same stimulus is not sufficient to cause an adaptive response in GLUT-4 protein level in these respiratory muscles.

Enhancement of glucose disposal in patients with type 2 diabetes by alpha-lipoic acid

Insulin resistance of skeletal muscle glucose uptake is a prominent feature of Type II diabetes (NIDDM); therefore pharmacological interventions should aim to improve insulin sensitivity. Alpha-lipoic acid (CAS 62-46-4, thioctic acid, ALA), a natural occurring compound frequently used for treatment of diabetic polyneuropathy, enhances glucose utilization in various experimental models. To see whether this compound also augments insulin mediated glucose disposal in NIDDM, 13 patients received either ALA (1000 mg/Thioctacid/500 ml NaCl, n = 7) or vehicle only (500 ml NaCl, n = 6) during a glucose-clamp study. Both groups were comparable in age, body-mass index and duration of diabetes and had a similar degree of insulin resistance at baseline. Acute parenteral administration of ALA resulted in a significant increase of insulin-stimulated glucose disposal; metabolic clearance rate (MCR) for glucose rose by about 50% (3.76 ml/kg/min = pre vs. 5.82 ml/kg/min = post, p < 0.05), whereas the control group did not show any significant change (3.57 ml/kg/min = pre vs. 3.91 ml/kg/min = post). This is the first clinical study to show that alpha-lipoic acid increases insulin stimulated glucose disposal in NIDDM. The mode of action of ALA and its potential use as an antihyperglycemic agent require further investigation.

Effects of captopril on glucose transport activity in skeletal muscle of obese Zucker rats

This study tested whether the angiotensin-converting enzyme (ACE) inhibitor captopril can modify the glucose transport system in insulin-resistant skeletal muscle. Obese Zucker (fa/fa) rats (approximately 300 g)--a model of insulin resistance--were administered by gavage either a single dose (50 mg/kg body weight) or repeated doses (50 mg/kg/d for 14 consecutive days) of captopril. Corresponding groups of age-matched, vehicle-treated lean (Fa/-) littermates (approximately 170 g) were also studied. Glucose transport activity in the epitrochlearis muscle was assessed by in vitro 2-deoxyglucose (2-DG) uptake. The increase in 2-DG uptake due to insulin (2 mU/mL) in muscles from vehicle-treated obese rats was less than 50% (P < .05) of the increase observed in muscles from lean rats. Short-term captopril treatment improved insulin-stimulable 2-DG uptake in muscles from obese rats by 46% (P < .05), and this enhanced insulin action due to captopril was completely abolished by pretreatment with the bradykinin antagonist HOE 140 (100 micrograms/kg). Long-term treatment with captopril produced a 60% improvement in insulin-stimulated 2-DG uptake (P < .05). Contraction-stimulated 2-DG uptake was significantly impaired (-31%, P < .05) in the obese rat, but was not altered by long-term captopril treatment. These findings indicate that both short- and long-term treatments with captopril significantly improve insulin-stimulated glucose transport activity in skeletal muscle of the obese Zucker rat, and that this improvement involves bradykinin metabolism. These data therefore support the hypothesis that captopril-induced improvements in glucose disposal result in part from an enhancement of the skeletal muscle glucose transport system.

Adaptive responses of GLUT-4 and citrate synthase in fast-twitch muscle of voluntary running rats

Glucose transporter (GLUT-4) protein, hexokinase, and citrate synthase (proteins involved in oxidative energy production from blood glucose catabolism) increase in response to chronically elevated neuromuscular activity. It is currently unclear whether these proteins increase in a coordinated manner in response to this stimulus. Therefore, voluntary wheel running (WR) was used to chronically overload the fast-twitch rat plantaris muscle and the myocardium, and the early time courses of adaptative responses of GLUT-4 protein and the activities of hexokinase and citrate synthase were characterized and compared. Plantaris hexokinase activity increased 51% after just 1 wk of WR, whereas GLUT-4 and citrate synthase were increased by 51 and 40%, respectively, only after 2 wk of WR. All three variables remained comparably elevated (+50-64%) through 4 wk of WR. Despite the overload of the myocardium with this protocol, no substantial elevations in these variables were observed. These findings are consistent with a coordinated upregulation of GLUT-4 and citrate synthase in the fast-twitch plantaris, but not in the myocardium, in response to this increased neuromuscular activity. Regulation of hexokinase in fast-twitch muscle appears to be uncoupled from regulation of GLUT-4 and citrate synthase, as increases in the former are detectable well before increases in the latter.

Interactions between effects of W-7, insulin, and hypoxia on glucose transport in skeletal muscle

The calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) stimulates glucose transport in skeletal muscle, apparently by raising cytosolic Ca2+ (P. Palade. J. Biol. Chem. 262: 6142-6148, 1987; J.H. Youn, E.A. Gulve, and J.O. Holloszy. Am. J. Physiol. 260 (Cell Physiol. 29): C555-C561, 1991). This study was performed to describe the interactions between the effects of W-7 and those of hypoxia and of insulin on glucose transport. The effect on 3-O-methylglucose (3-MG) transport of 50 microM W-7 was additive to the effect of a maximal insulin stimulus (2,000 microU/ml) but not to the effect of maximal (60 min) hypoxic stimulus, suggesting that W-7 stimulates glucose transport via the same pathway as hypoxia, independent of the pathway activated by insulin. The effect of 50 microM W-7 was additive to that of a submaximal (20 min) hypoxia stimulus, indicating that W-7 does not interfere with the stimulation of glucose transport by hypoxia. In contrast, 50 microM W-7 had an inhibitory effect on stimulation of 3-MG transport by submaximally effective insulin levels, causing a fivefold increase in the concentration of insulin needed to produce a half-maximal stimulation of 3-MG transport, from approximately 70 to approximately 350 microU/ml (P < 0.05). Thus these data demonstrate that W-7 selectively inhibits insulin stimulation of glucose transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle protein content and synthesis after voluntary running and subsequent unweighting

The effects of exercise training with or without subsequent unweighting on wet weight, protein content, and in vivo fractional protein synthesis were studied in soleus and plantaris muscles of juvenile female Sprague-Dawley rats under the following four conditions: normal weight bearing (N), voluntary-activity wheel running (WR) for up to 4 weeks, mechanical unweighting for 7 days via hindlimb suspension (HS), or wheel running followed by 7 days of hindlimb suspension (WR-HS). Fractional protein synthesis was determined by the 3H-phenylalanine flooding method. Increases (P < .05) in wet weight and protein content were detected in the soleus after just 1 week of running, with no increase in fractional protein synthesis. Two weeks of running were required for an increase in protein synthesis in this muscle. Significant increases in these parameters were first observed in the plantaris after 2 weeks of running. Maximal increases occurred by 3 weeks in both muscles. Reductions (P < .05) in soleus and plantaris parameters were observed in both HS and WR-HS groups compared with N and WR groups, respectively. However, protein content and fractional synthesis were maintained at significantly higher levels in WR-HS muscles compared with HS muscles. These results indicate that (1) wheel training represents a noninvasive method for inducing rapid hypertrophy of the skeletal muscles studied, in part by increasing fractional protein synthesis; (2) unweighting decreases protein content and synthesis to the same extent whether the muscles are trained; and (3) previously hypertrophied muscles display higher protein contents and fractional protein synthesis following unweighting compared with unweighted muscles of untrained animals.

Cardiac protein content and synthesis in vivo after voluntary running or head-down suspension

The adaptive responses of myocardial protein metabolism to chronic increases in work load were evaluated in juvenile female Sprague-Dawley rats. Rats were studied under four conditions: normal weight bearing (N), voluntary wheel running (WR) for < or = 4 wk, head-down-tilt suspension for 7 days (HS), or wheel running (2 or 3 wk) followed by 7 days of suspension (WR-HS). WR activity plateaued after 2 wk at 16 km/day and was maintained through week 4. WR did not affect normal whole body growth. Protein metabolism was studied by measuring heart protein content and in vivo fractional rate of protein synthesis with the [3H]phenylalanine "flooding dose" method. Two weeks of WR increased (P < 0.05) absolute heart protein content (22%) and protein synthesis (21%) relative to age-matched N group values. These differences in protein content and synthesis were maintained for > or = 4 wk. Rats failed to gain significant body weight during suspension. Heart protein content increased (P < 0.05) by 12% to 26% as did protein synthesis (14% to 22%) in HS compared with N group. In WR-HS group, cardiac protein content and protein synthesis were maintained at significantly elevated levels. These findings indicate that 1) high-volume WR by young rats provides a convenient noninvasive method for producing rapid and substantial cardiac hypertrophy, which results, at least in part, from enhanced cardiac protein synthesis; and 2) head-down suspension of sedentary juvenile rats leads to increased cardiac protein synthesis, which helps to increase cardiac protein content despite a lack of whole body growth.

Early alterations in soleus GLUT-4, glucose transport, and glycogen in voluntary running rats

Voluntary wheel running (WR) by juvenile female rats was used as a noninterventional model of soleus muscle functional overload to study the regulation of insulin-stimulated glucose transport activity by the glucose transporter (GLUT-4 isoform) protein level and glycogen concentration. Soleus total protein content was significantly greater (+18%; P < 0.05) than in age-matched controls after 1 wk of WR, and this hypertrophic response continued in weeks 2-4 (+24-32%). GLUT-4 protein was 39% greater than in controls in 1-wk WR soleus, and this adaptation was accompanied by a similar increase in in vitro insulin-stimulated glucose transport activity (+29%). After 2 and 4 wk of WR, however, insulin-stimulated glucose transport activity had returned to control levels, despite a continued elevation (+25-28%) of GLUT-4 protein. At these two time points, glycogen concentration was significantly enhanced in WR soleus (+21-42%), which coincided with significant reductions in glycogen synthase activity ratios (-23 to -41%). These results indicate that, in this model of soleus muscle functional overload, the GLUT-4 protein level may initially regulate insulin-stimulated glucose transport activity in the absence of changes in other modifying factors. However, this regulation of glucose transport activity by GLUT-4 protein may be subsequently overridden by elevated glycogen concentration.

Experimental approaches in muscle metabolism: hindlimb perfusion and isolated muscle incubations

The perfusion of rat hindlimb muscles and the isolated in vitro muscle preparation are usually the preferred methods for investigating muscle metabolism. In light of recent concerns about the incubated muscle preparation, we have examined the problems, the advantages, and the viability of these two experimental techniques, with focus on glucose metabolism. A major advantage of the hindlimb perfusion system is that it maintains its metabolic viability very well, and perfusions in resting muscles can be achieved successfully with cell-free media. However, variations in the perfused rat hindlimb procedures result in considerable differences in perfusate flow among muscles, making quantitative comparisons among perfusion procedures difficult. Metabolic viability has been identified as a problem in some isolated in vitro muscle preparations. We have provided criteria to avoid muscle hypoxia. Minimum levels of insulin seem to be a key requirement to maintaining the muscle's viability, and essential amino acids are required to retard an increase in the basal rate of glucose and amino acid uptake. Under such conditions metabolic viability can be maintained during prolonged incubations (9-30 h). Both the isolated in vitro muscle preparation and the hindlimb perfusion preparation are viable models for the study of muscle metabolism.

Histone H4 stimulates glucose transport activity in rat skeletal muscle

We investigated the effects of purified histone H4 on glucose transport activity in rat soleus and flexor digitorum brevis muscles. Histone H4, at concentrations up to 11.8 microM, increased 2-deoxyglucose (2-DG) uptake in a dose-dependent fashion. However, at concentrations higher than 11.8 microM, H4 caused a decrease in 2-DG uptake from the maximum, suggesting a secondary inhibitory action of this compound. The maximal effect of H4 on 2-DG uptake was not additive to the maximal effect of insulin. Moreover, 2-DG uptake in the presence of both H4 and insulin was significantly lower than the 2-DG uptake in the presence of insulin alone. The maximal effect of H4 on stimulation of 2-DG uptake was neither additive nor inhibitory to the maximal effects of the intracellularly acting insulin mimetics sodium vanadate or H2O2. It was, on the other hand, additive to the maximal effects of muscle contractions. Also, in contrast with the effects of H4 on insulin-stimulated 2-DG uptake, H4 did not inhibit insulin-like growth factor-I (IGF-I)-stimulated 2-DG uptake, as the maximal effects of H4 and IGF-I were additive. Scatchard analysis of the binding of 125I-insulin in the absence or presence of histone H4 revealed that H4 increased the specific binding of insulin without affecting receptor affinity. These data suggest that H4 interacts with the insulin, rather than the hypoxia/contraction, pathway for activation of glucose transport in muscle tissue, and that H4 acts either directly or indirectly to increase the number of insulin receptors at the surface of the muscle cell. This interaction does not appear to occur with the similar, although distinct, IGF-I receptor. These studies may provide additional insight into the complex signal-transduction systems of insulin action.

Elevated interstitial fluid volume in soleus muscles unweighted by spaceflight or suspension

Recent evidence by Kandarian et al. (J. Appl. Physiol. 71: 910-914, 1991) indicates that prolonged (28 days) unweighting of the rat soleus by hindlimb suspension results in a substantial increase in interstitial fluid volume (IFV), as defined by the inulin space. The lack of any significant difference in absolute IFV values between unweighted and control groups suggested that this elevated IFV was a consequence of muscle atrophy. Using young female rats, we directly tested this hypothesis by comparing the early responses of soleus muscle weight and IFV with unweighting by tail-cast suspension or actual exposure to microgravity during spaceflight. Significant differences from control were first observed after 3 days of suspension unweighting for soleus wet weight (-14%; P < 0.01) and IFV (+35%; P < 0.01) and increased further after 6 days (-32% and +53%, respectively; both P < 0.001). After 5.4 days of spaceflight, soleus wet weight was 38% less and IFV was 52% greater than control (both P < 0.001). A highly significant negative correlation between soleus wet weight and IFV for all groups was observed (r = -0.70, P < 0.001). These data indicate that elevated soleus IFV develops at an early time point during unweighting and that there is a direct relationship between the magnitude of this increase in IFV and the extent of muscle atrophy. This relationship also exists in soleus muscles unweighted by exposure to a microgravity environment.

Effect of insulin-like factors on glucose transport activity in unweighted rat skeletal muscle

We have previously demonstrated that mechanical unweighting of the soleus muscle by hindlimb suspension leads to increases in insulin receptor binding, muscle/fat-specific glucose transporter (GLUT-4) protein levels, and insulin-stimulated glucose transport activity. The present study used a novel approach to further evaluate the potential role of postreceptor binding mechanisms in this enhanced insulin effect after unweighting. Insulin-like growth factor I (IGF-I), vanadate, and phospholipase C were used to stimulate glucose transport activity independently of insulin receptor binding. Soleus glucose transport activity (assessed by 2-deoxyglucose uptake) was evaluated in vitro with soleus strips (approximately 18 mg). Progressively increased responses to maximally effective doses of insulin or IGF-I were observed after 3 and 6 days of unweighting compared with weight-matched control strips. Enhanced maximal responses to vanadate (6 days only) and phospholipase C (3 and 6 days) for 2-deoxyglucose uptake were also observed. The results of this study 1) provide evidence that post-insulin receptor binding mechanisms also play a role in the enhanced response of the insulin-dependent pathway for stimulation of glucose transport in unweighted skeletal muscle and 2) indicate that IGF-I action on glucose transport is included in this enhanced response in unweighted muscle.

Spaceflight on STS-48 and earth-based unweighting produce similar effects on skeletal muscle of young rats

Our knowledge of the effects of unweighting on skeletal muscle of juvenile rapidly growing rats has been obtained entirely by using hindlimb-suspension models. No spaceflight data on juvenile animals are available to validate these models of simulated weightlessness. Therefore, eight 26-day-old female Sprague-Dawley albino rats were exposed to 5.4 days of weightlessness aboard the space shuttle Discovery (mission STS-48, September 1991). An asynchronous ground control experiment mimicked the flight cage condition, ambient shuttle temperatures, and mission duration for a second group of rats. A third group of animals underwent hindlimb suspension for 5.4 days at ambient temperatures. Although all groups consumed food at a similar rate, flight animals gained a greater percentage of body mass per day (P < 0.05). Mass and protein data showed weight-bearing hindlimb muscles were most affected, with atrophy of the soleus and reduced growth of the plantaris and gastrocnemius in both the flight and suspended animals. In contrast, the non-weight-bearing extensor digitorum longus and tibialis anterior muscles grew normally. Earlier suspension studies showed that the soleus develops an increased sensitivity to insulin during unweighting atrophy, particularly for the uptake of 2-[1,2-3H]deoxyglucose. Therefore, this characteristic was studied in isolated muscles within 2 h after cessation of spaceflight or suspension. Insulin increased uptake 2.5- and 2.7-fold in soleus of flight and suspended animals, respectively, whereas it increased only 1.6-fold in control animals. In contrast, the effect of insulin was similar among the three groups for the extensor digitorum longus, which provides a control for potential systemic differences in the animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of soleus unweighting on stimulation of insulin-independent glucose transport activity

Unweighting of the rat soleus by tail-cast suspension results in increased insulin action on stimulation of glucose transport, which can be explained, at least in part, by increased insulin binding and enhanced glucose transporter protein levels. Glucose transport is also activated by an insulin-independent mechanism stimulated by in vitro muscle contractions or hypoxia. Therefore, the purpose of this study was to determine if soleus unweighting leads to an enhanced response of the insulin-independent pathway for stimulation of glucose transport. The hindlimbs of juvenile male Wistar rats were suspended by a tail-cast system for 3 or 6 days. Glucose transport activity in isolated soleus strips (approximately 18 mg) was then assessed by using 2-deoxy-[1,2-3H]glucose (2-DG) uptake. Insulin (2 mU/ml) had a progressively enhanced effect on 2-DG uptake after 3 and 6 days of unweighting (+44 and +72% vs. control, respectively; both P < 0.001). At these same times, there was no difference between groups for activation of 2-DG uptake by maximally effective treatments with contractions (10 tetanuses), hypoxia (60 min), or caffeine (5 mM). These results indicate that the enhanced capacity for stimulation of glucose transport after soleus unweighting is restricted to the insulin pathway, with no apparent enhancement of the insulin-independent pathway.

Regulation of contraction-stimulated system A amino acid uptake in skeletal muscle: role of vicinal sulfhydryls

Functional vicinal sulfhydryls are essential for insulin-stimulated system A neutral amino acid uptake in the rat epitrochlearis muscle. In skeletal muscle, system A uptake is also activated by contractile activity. Therefore, the purposes of this study were to characterize the stimulation of system A activity by contractions induced by electrical stimulation in vitro, and to assess the role of vicinal sulfhydryls in this process. System A activity in the isolated epitrochlearis muscle was measured using the nonmetabolizable analogue alpha-(methylamino)isobutyric acid (MeAIB). Contractions increased MeAIB uptake by increasing the apparent maximal velocity (Vmax), with no alteration in the apparent Km. The maximal stimulatory effects of insulin and contractions on MeAIB uptake were completely additive, demonstrating that these two stimuli exert their effects via different mechanisms. Phenylarsine oxide (PAO), a vicinal sulfhydryl antagonist, at greater than 20 mumol/L inhibited basal and contraction-stimulated MeAIB uptake by approximately 50% and 70%, respectively, by decreasing Vmax, with no change in Km. Both inhibitory effects were completely prevented by cotreatment with the vicinal dithiol dimercaptopropanol (DMP), indicating the effects were mediated by interactions with vicinal sulfhydryls. Contraction-stimulated MeAIB uptake was rapidly (half-time, approximately 7 minutes) reversed by the addition of PAO. These results (1) define conditions under which contraction-stimulated system A amino acid uptake can be studied in an isolated mammalian skeletal muscle preparation, and (2) indicate that vicinal sulfhydryls are essential for stimulation of system A activity by muscle contractions.

Glucose transporters and glucose transport in skeletal muscles of 1- to 25-mo-old rats

It is widely thought that aging results in development of insulin resistance in skeletal muscle. In this study, we examined the effects of growth and aging on the concentration of the GLUT-4 glucose transporter and on glucose transport activity in skeletal muscles of female Long-Evans rats. Relative amounts of immunoreactive GLUT-4 protein were measured in muscle homogenates of 1-, 10-, and 25-mo-old rats by immunoblotting with a polyclonal antibody directed against GLUT-4. In the epitrochlearis, plantaris, and the red and white regions of the quadriceps muscles, GLUT-4 immunoreactivity decreased by 14-33% between 1 and 10 mo of age and thereafter remained constant. In flexor digitorum brevis (FDB) and soleus muscles, GLUT-4 concentration was similar at all three ages studied. Glucose transport activity was assessed in epitrochlearis and FDB muscles by incubation with 2-deoxyglucose under the following conditions: basal, submaximal insulin, and either maximal insulin or maximal insulin combined with contractile activity. Glucose transport in the epitrochlearis muscle decreased by approximately 60% between 1 and 4 mo of age and then did not decline further between 4 and 25 mo of age. Transport activity in the FDB assessed with a maximally effective insulin concentration decreased only slightly (< 20%) between 1 and 7 mo of age. Aging, i.e., the transition from young adulthood to old age, was not associated with a decrease in glucose transport activity in either the epitrochlearis or the FDB.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alkaline pH on the stimulation of glucose transport in rat skeletal muscle

Alkaline pH has been reported to cause release of Ca2+ from skeletal muscle sarcoplasmic reticulum (SR). Elevation of sarcoplasmic Ca2+ concentration is thought to stimulate glucose transport in skeletal muscle. In this context, we examined the effect of alkaline pH (extracellular pH of 8.6) on 3-O-methylglucose transport in skeletal muscle. Incubation of rat epitrochlearis muscles at pH 8.6 for 45 min resulted in an approx. 3-fold increase in glucose transport activity, which was not affected by reducing Ca2+ concentration in the incubation medium and essentially completely blocked by 25 microM dantrolene, an inhibitor of SR Ca2+ release. In addition to stimulating glucose transport by itself, alkaline pH may partially inhibit the stimulation of sugar transport by insulin hypoxia and contractions, as the combined effect of alkaline pH and the maximal effect of insulin, contractions, or hypoxia on glucose transport are not different from the maximal effects of insulin, hypoxia, or contractions alone. The maximal effects of insulin and contractions, and of insulin and hypoxia, on glucose transport are normally additive in muscle. Alkaline pH completely prevented this additivity. In summary, our results show that alkaline pH stimulates glucose transport activity in skeletal muscle and provide evidence suggesting that this effect is mediated by Ca2+. They further show that alkaline pH blocks the additivity of the maximal effects of insulin and contractions or hypoxia suggesting that alkaline pH may partially inhibit the stimulation of glucose transport by insulin, contraction and hypoxia.

Effects of wheel running on glucose transporter (GLUT4) concentration in skeletal muscle of young adult and old rats

We examined the effects of voluntary exercise on glucose transporter concentration in skeletal muscle from young adult and old female Long-Evans rats. Rats had free access to voluntary running wheels beginning at 4 months of age or remained sedentary. Exercising rats ran approximately 7.5, 6.2, 5.6 and 5.3 km/day during their 6th, 8th, 9th and 10th month of age, respectively. During the 23rd, 24th and 25th month of age running distance averaged 3.0, 2.8 and 2.4 km/day, respectively. At 10 and 25 months of age, glucose transporter protein concentration was assessed in epitrochlearis and flexor digitorum brevis muscles with a polyclonal antibody directed against the GLUT4 transporter isoform. GLUT4 protein concentration was not altered by the aging process (i.e., comparing 10- and 25-month-old rats) in either muscle type. Wheel running increased GLUT4 protein concentration by 45% in epitrochlearis muscles of 10-month-old rats relative to age-matched sedentary controls. The training-induced adaptation in GLUT4 protein was no longer present at age 25 months, probably because the running distance had declined by 50%. In the flexor digitorum brevis, exercise did not alter GLUT4 concentration at either 10 or 25 months, presumably due to insufficient recruitment of this muscle during wheel running as assessed by measurement of citrate synthase and hexokinase enzyme activities. Wheel running induced cardiac and soleus muscle hypertrophy in 10- and 25-month-old rats. In summary, voluntary wheel running can induce an increase in skeletal muscle GLUT4 protein concentration in adult rats. Older rats that run less exhibit cardiac and soleus muscle hypertrophy, but do not maintain an elevated GLUT4 protein concentration in the epitrochlearis muscle. Aging does not alter GLUT4 protein concentration in the epitrochlearis or FDB muscles.