Glucose Transport Activity In Muscle Research Articles

20-Hydroxyecdysone ameliorates metabolic and cardiovascular dysfunction in high-fat-high-fructose-fed ovariectomized rats

BackgroundEcdysteroids are polyhydroxylated steroids present in invertebrates and plants. 20-Hydroxyecdysone (20E) is the most common and the main biologically active compound of ecdysteroids. Previous studies have demonstrated anabolic and metabolic effects of 20E in mammals. However, it is unknown whether 20E has a positive effect on all aspects of cardiometabolic syndrome. The aims of this study were to investigate the favorable effect and possible underlying mechanisms of 20E in a rat model of cardiometabolic syndrome (CMS) induced by a high-calorie diet combined with female sex hormone deprivation.Methods20E (5 mg/kg, 10 mg/kg, or 20 mg/kg) or pioglitazone (PIO) (10 mg/kg) was intragastrically administered to sham-operated Sprague-Dawley female rats and ovariectomized rats fed a high-fat-high-fructose diet (OHFFD) for 8 weeks. The phenotypic characteristics of CMS, including central adiposity, blood pressure, serum lipid profile, glucose tolerance, insulin action on skeletal muscle glucose transport activity and hepatic protein expression, were determined.ResultsSome CMS characteristics were improved by 20E treatment. Rats treated with 20E had lower body weight, abdominal fat accumulation than rats treated with vehicle control without changes in total caloric intake and fat-free mass. OHFFD rats exhibited high blood pressure, but 20E-treated rats maintained normal blood pressure with a lower level of low-density lipoprotein (LDL)-cholesterol. Although 20E showed no positive effect on inducing insulin-mediated glucose transport in the skeletal muscle of OHFFD rats, 20E improved whole body glucose homeostasis. Analysis of protein expression in livers from 20E-treated rats revealed significantly increased expression of pAkt Ser473, pFOXO1 Ser256, pAMPKα Thr172, and FGF21.Conclusion20E treatment can alleviate cardiometabolic disorder caused by a high-fat-high-fructose diet and female sex hormone deprivation. In particular, 20E helps improve whole body insulin sensitivity in OHFFD rats, and the mechanisms that underlie this favorable effect are potentially mediated by the activation of AMPK and FGF21. The present study indicates that 20E could be an alternative therapeutic option for the prevention and alleviation of cardiometabolic syndrome.

AVE0991, a Nonpeptide Angiotensin 1-7 Receptor Agonist, Improves Glucose Metabolism in the Skeletal Muscle of Obese Zucker Rats: Possible Involvement of Prooxidant/Antioxidant Mechanisms.

Angiotensin 1-7 (Ang 1-7) enhances insulin signaling and glucose transport activity in the skeletal muscle. The aim of our study was to evaluate the effect of AVE0991, a nonpeptide Mas receptor agonist, on the metabolic parameters, expression of RAS components and markers of oxidative stress, and insulin signaling in the skeletal morbidly obese rats. 33-week-old male obese Zucker rats were treated with vehicle and AVE0991 (0.5 mg/kg BW/day) via osmotic minipumps for two weeks. Gene expressions were determined by qPCR and/or Western blot analysis in musculus quadriceps. The enzymatic activities were detected flourometrically (aminopeptidase A) or by colorimetric assay kit (protein tyrosine phosphatase 1B). Administration of AVE0991 enhanced insulin signaling cascade in the skeletal muscle, reflected by improved whole-body glucose tolerance. It has been shown that reactive oxygen species (ROS) have insulin-mimetic action in muscle. The expression of renin receptor, transcription factor PLZF, and prooxidant genes was upregulated by AVE0991 accompanied by elevated expression of genes coding enzymes with antioxidant action. Our results show that AVE0991 administration activates genes involved in both ROS generation and clearance establishing a new prooxidant/antioxidant balance on a higher level, which might contribute to the improved insulin signaling pathway and glucose tolerance of obese Zucker rats.

Rosmarinic acid improves hypertension and skeletal muscle glucose transport in angiotensin II-treated rats

BackgroundRosmarinic acid (RA) is a natural pure compound from herbs belonging to the Lamiaceae family, such as rosemary, sage, basil, and mint. The antioxidant, angiotensin-converting enzyme inhibitory, and vasodilatory effects of RA have been revealed. Angiotensin II (ANG II) is a potent agent that generates hypertension and oxidative stress. Hypertension and skeletal muscle insulin resistance are strongly related. The aim of this study was to evaluate the effects of acute and chronic RA treatment on blood pressure and skeletal muscle glucose transport in ANG II-induced hypertensive rats.MethodsEight-week-old male Sprague Dawley rats were separated into SHAM and ANG II-infused (250 ng/kg/min) groups. ANG II rats were treated with or without acute or chronic RA at 10, 20, or 40 mg/kg. At the end of the experiment, body weight, liver and heart weights, oral glucose tolerance, skeletal muscle glucose transport activity, and signaling proteins were evaluated.ResultsBoth acute and chronic RA treatment decreased systolic, diastolic, and mean arterial blood pressure. Only acute RA at 40 mg/kg resulted in a reduction of fasting plasma glucose levels and an induction of skeletal muscle glucose transport activity. These effects might involve increased ERK activity in skeletal muscle. Meanwhile, chronic RA treatment with 10, 20, and 40 mg/kg prevented ANG II-induced hyperglycemia.ConclusionsBoth acute and chronic RA treatment attenuated ANG II-induced cardiometabolic abnormalities in rats. Therefore, RA would be an alternative strategy for improving skeletal muscle glucose transport and protecting against ANG II-induced hypertension and hyperglycemia.

Sex differences in the metabolic dysfunction and insulin resistance of skeletal muscle glucose transport following high fructose ingestion.

The role of high fructose ingestion (HFI) in the development of conditions mimicking human metabolic syndrome has mostly been demonstrated in male animals; however, the extent of HFI-induced metabolic alterations in females remains unclear. The present study investigated whether HFI-induced metabolic perturbations differ between sexes and whether HFI aggravates the metabolic disturbances under ovarian hormone deprivation. Male, female, and ovariectomized (OVX) Sprague-Dawley rats were given either water or liquid fructose (10% wt/vol) for 6 wk. Blood pressure, glucose tolerance, insulin-stimulated glucose transport activity and signaling proteins, including insulin receptor (IR), insulin receptor substrate 1 (IRS-1), Akt, Akt substrate of 160 kDa (AS160), AMPKα, JNK, p38 MAPK, angiotensin-converting enzyme (ACE), ANG II type 1 receptor (AT1R), ACE2, and Mas receptor (MasR) in skeletal muscle, were evaluated. We found that HFI led to glucose intolerance and hypertension in male and OVX rats but not in female rats with intact ovaries. Moreover, HFI did not induce insulin resistance in the skeletal muscle of female and OVX rats but impaired the insulin-stimulated glucose transport activity in the skeletal muscle of male rats, which was accompanied by lower insulin-stimulated IRS-1 Tyr989 (44%), Akt Ser473 (30%), and AS160 Ser588 (43%), and increases in insulin-stimulated IRS-1 Ser307 (78%), JNK Thr183/Tyr185 (69%), and p38 MAPK Thr180/Tyr182 (81%). The results from the present study show sex differences in the development of metabolic syndrome-like conditions and indicate the protective role of female sex hormones against HFI-induced cardiometabolic abnormalities.

Akt/PKB activation and insulin signaling: a novel insulin signaling pathway in the treatment of type 2 diabetes

Type 2 diabetes is a metabolic disease categorized primarily by reduced insulin sensitivity, β-cell dysfunction, and elevated hepatic glucose production. Treatments reducing hyperglycemia and the secondary complications that result from these dysfunctions are being sought after. Two distinct pathways encourage glucose transport activity in skeletal muscle, ie, the contraction-stimulated pathway reliant on Ca2+/5′-monophosphate-activated protein kinase (AMPK)-dependent mechanisms and an insulin-dependent pathway activated via upregulation of serine/threonine protein kinase Akt/PKB. Metformin is an established treatment for type 2 diabetes due to its ability to increase peripheral glucose uptake while reducing hepatic glucose production in an AMPK-dependent manner. Peripheral insulin action is reduced in type 2 diabetics whereas AMPK signaling remains largely intact. This paper firstly reviews AMPK and its role in glucose uptake and then focuses on a novel mechanism known to operate via an insulin-dependent pathway. Inositol hexakisphosphate (IP6) kinase 1 (IP6K1) produces a pyrophosphate group at the position of IP6 to generate a further inositol pyrophosphate, ie, diphosphoinositol pentakisphosphate (IP7). IP7 binds with Akt/PKB at its pleckstrin homology domain, preventing interaction with phosphatidylinositol 3,4,5-trisphosphate, and therefore reducing Akt/PKB membrane translocation and insulin-stimulated glucose uptake. Novel evidence suggesting a reduction in IP7 production via IP6K1 inhibition represents an exciting therapeutic avenue in the treatment of insulin resistance. Metformin-induced activation of AMPK is a key current intervention in the management of type 2 diabetes. However, this treatment does not seem to improve peripheral insulin resistance. In light of this evidence, we suggest that inhibition of IP6K1 may increase insulin sensitivity and provide a novel research direction in the treatment of insulin resistance.

The lipid peroxidation end-product and oxidant 4-hydroxynonenal induces insulin resistance in rat slow-twitch skeletal muscle

Context: The lipid peroxidation end-product and oxidant 4-hydroxynonenal (4-HNE) impairs cell function. However, the impact of 4-HNE on the glucose transport system in mammalian slow-twitch skeletal muscle is not known. Objective: We assessed the effects of 4-HNE on insulin signalling and glucose transport activity in slow-twitch muscle by incubating soleus strips from lean Zucker rats with 4-HNE (50 µM) in the absence or presence of insulin (5 mU/ml) for up to 6 hr. Results: Insulin-stimulated glucose transport activity was significantly (p < 0.05) decreased by 4-HNE at 2 hr. AS160 Thr642 phosphorylation was decreased at 2 hr, whereas Akt Ser473 phosphorylation and IRS-1 protein expression were not substantially changed until 4 hr. IRS-2 protein expression was slightly decreased only at 6 hr. Conclusions: The lipid peroxidation end-product and oxidant 4-HNE induces insulin resistance of glucose transport activity in rat slow-twitch skeletal muscle, initially associated with impaired phosphorylation of AS160.

Effects of alternate-day fasting on high-fat diet-induced insulin resistance in rat skeletal muscle

AimsThe purpose of this study was to investigate the effects of alternate-day fasting (ADF) on insulin-stimulated glucose transport activity in skeletal muscle in rats fed a high-fat diet. Main methodsMale Wistar rats were placed on a high-fat diet (n=24) or standard chow diet (Chow, n=12) for 10weeks. Rats fed the high-fat diet were separated into two groups after 4weeks. One group was subjected to ADF for the subsequent 6weeks (HF-ADF, n=12), and the other group was maintained on an ad libitum diet (HF-AL, n=12). After the 10-week dietary intervention, measurements of insulin-stimulated glucose uptake and insulin tolerance test (ITT) were performed. Key findingsWhereas the total intra-abdominal fat mass in the HF-AL group was significantly higher than in the Chow and HF-ADF groups, there was no significant difference between the Chow and HF-ADF groups. However, insulin-stimulated glucose uptake in skeletal muscles was significantly lower in both high-fat fed groups than in the Chow group. Muscle GLUT-4 protein content in HF-AL is significantly lower (~30%) than in Chow, and further reduction (~42%) was observed in the HF-ADF group rats. The HF-ADF and HF-AL group rats had less reduction in glycemia than did the Chow group rats during ITT. SignificanceADF was unable to eliminate high-fat diet-induced muscle insulin resistance, despite a substantial decrease in total intra-abdominal fat mass. This might have resulted from a reduction in GLUT-4 protein in both HF-AL and HF-ADF rats compared to the Chow group.

The lipid peroxidation end‐product and oxidant 4‐ hydroxynonenal causes insulin resistance in rat slow‐twitch skeletal muscle in vitro

The impact of the lipid peroxidation end‐product, reactive aldehyde, and oxidant 4‐hydroxynonenal (4‐HNE) on the glucose transport system in rat slow‐twitch skeletal muscle is currently unknown. Therefore, we assessed the effect of 4‐HNE on insulin signaling (IRS proteins and phosphorylation of Akt Ser473 (pAkt) and AS160 Thr642 (pAS160)) and glucose transport in slow‐twitch muscle. Soleus strips from lean Zucker rats were incubated with 4‐ HNE (50 μM) for up to 6 hr. Insulin‐stimulated (5 mU/ml) glucose transport was decreased by 4‐HNE at 2 hr (30%), 4 hr (26%), and 6 hr (39%) (p&lt;0.05). At 2 hr of 4‐HNE treatment with insulin, pAS160 was decreased by 28%, whereas pAkt was only reduced by 11% and IRS‐1 protein was not changed. At 4 hr, pAS160 and pAkt were decreased by 22% and IRS‐1 was 39% lower. At 6 hr, pAS160 was 47% lower, pAkt was decreased by 26%, and IRS‐1 protein was reduced by 51%. Interestingly, IRS‐2 was slightly decreased (17%) only at 6 hr. These data indicate that the lipid peroxidation end‐product and oxidant 4‐HNE induces insulin resistance of glucose transport activity in rat slow‐twitch skeletal muscle, associated initially with impaired AS160 phosphorylation and subsequently with impaired Akt phosphorylation and selective loss of IRS‐1 protein. These results provide further support for an important role of lipid‐derived oxidative stress in the etiology of skeletal muscle insulin resistance.

The Novel Angiotensin II Receptor Blocker Azilsartan Medoxomil Ameliorates Insulin Resistance Induced by Chronic Angiotensin II Treatment in Rat Skeletal Muscle

Angiotensin receptor (type 1) blockers (ARBs) can reduce both hypertension and insulin resistance induced by local and systemic activation of the renin-angiotensin-aldosterone system. The effectiveness of azilsartan medoxomil (AZIL-M), a novel imidazole-based ARB, to facilitate metabolic improvements in conditions of angiotensin II (Ang II)-associated insulin resistance is currently unknown. The aim of this study was to determine the impact of chronic AZIL-M treatment on glucose transport activity and key insulin signaling elements in red skeletal muscle of Ang II-treated rats. Male Sprague-Dawley rats were treated for 8 weeks with or without Ang II (200 ng/kg/min) combined with either vehicle or AZIL-M (1 mg/kg/day). Ang II induced significant (p < 0.05) increases in blood pressure, which were completely prevented by AZIL-M. Furthermore, Ang II reduced insulin-mediated glucose transport activity in incubated soleus muscle, and AZIL-M co-treatment increased this parameter. Moreover, AZIL-M treatment of Ang II-infused animals increased the absolute phosphorylation of insulin signaling molecules, including Akt [both Ser⁴⁷³ (81%) and Thr³⁰⁸ (23%)] and AS160 Thr⁶⁴² (42%), in red gastrocnemius muscle frozen in situ. Absolute AMPKα (Thr¹⁷²) phosphorylation increased (98%) by AZIL-M treatment, and relative Thr³⁸⁹ phosphorylation of p70 S6K1, a negative regulator of insulin signaling, decreased (51%) with AZIL-M treatment. These results indicate that ARB AZIL-M improves the in vitro insulin action on glucose transport in red soleus muscle and the functionality of the Akt/AS160 axis in red gastrocnemius muscle in situ in Ang II-induced insulin-resistant rats, with the latter modification possibly associated with enhanced AMPKα and suppressed p70 S6K1 activation.

ANG-(1–7) reduces ANG II-induced insulin resistance by enhancing Akt phosphorylation via a Mas receptor-dependent mechanism in rat skeletal muscle

The nonapeptide angiotensin II (ANG II) induces vasoconstriction via the ANG II type I receptor, while its splice product ANG-(1–7) elicits an antihypertensive effect via the Mas receptor. Although a critical role of ANG II in the etiology of skeletal muscle insulin resistance is well documented, the role of the ANG-(1–7)/Mas receptor axis in this context is poorly understood. Therefore, we determined whether ANG-(1–7) is effective in ameliorating the negative effects of ANG II on insulin-stimulated insulin signaling and glucose transport activity in isolated soleus muscle from normotensive lean Zucker rats. ANG II alone (500nM for 2h) decreased insulin-stimulated glucose transport activity by 45% (P<0.05). In the presence of 500–1000nM ANG-(1–7), insulin-stimulated glucose transport activity in muscle exposed to ANG II improved by ∼30% (P<0.05). Moreover, ANG-(1–7) treatment increased Akt Ser473 phosphorylation (47%, P<0.05) without an effect on glycogen synthase kinase-3β Ser9 phosphorylation. The dependence of ANG-(1–7) action on the Mas receptor was assessed using A779 peptide, a selective Mas receptor antagonist. The positive effects of ANG-(1–7) on insulin-stimulated glucose transport activity and Akt Ser473 phosphorylation in soleus muscle were completely prevented in presence of 1000nM A779. In conclusion, the present study demonstrates that ANG-(1–7), via a Mas receptor-dependent mechanism, can ameliorate the inhibitory effect of ANG II on glucose transport activity in mammalian skeletal muscle, associated with enhanced Akt phosphorylation. These results provide further evidence supporting the targeting of the renin–angiotensin system for interventions designed to reduce insulin resistance in skeletal muscle tissue.

Improvements of insulin resistance in ovariectomized rats by a novel phytoestrogen from Curcuma comosa Roxb

BackgroundCurcuma comosa Roxb. (C. comosa) is an indigenous medicinal herb that has been used in Thailand as a dietary supplement to relieve postmenopausal symptoms. Recently, a novel phytoestrogen, (3R)-1,7-diphenyl-(4E,6E)-4,6-heptadien-3-ol or compound 049, has been isolated and no study thus far has investigated the role of C. comosa in preventing metabolic alterations occurring in estrogen-deprived state. The present study investigated the long-term effects (12 weeks) of C. comosa hexane extract and compound 049 on insulin resistance in prolonged estrogen-deprived rats.MethodsFemale Sprague-Dawley rats were ovariectomized (OVX) and treated with C. comosa hexane extract (125 mg, 250 mg, or 500 mg/kg body weight (BW)) and compound 049 (50 mg/kg BW) intraperitoneally three times per week for 12 weeks. Body weight, food intake, visceral fat weight, uterine weight, serum lipid profile, glucose tolerance, insulin action on skeletal muscle glucose transport activity, and GLUT-4 protein expression were determined.ResultsProlonged ovariectomy resulted in dyslipidemia, impaired glucose tolerance and insulin-stimulated skeletal muscle glucose transport, as compared to SHAM. Treatment with C. comosa hexane extract and compound 049, three times per week for 12 weeks, markedly reduced serum total cholesterol and low-density lipoprotein levels, improved insulin sensitivity and partially restored uterine weights in ovariectomized rats. In addition, compound 049 or high doses of C. comosa hexane extract enhanced insulin-mediated glucose uptake in skeletal muscle and increased muscle GLUT-4 protein levels.ConclusionsTreatment with C. comosa and its diarylheptanoid derivative improved glucose and lipid metabolism in estrogen-deprived rats, supporting the traditional use of this natural phytoestrogen as a strategy for relieving insulin resistance and its related metabolic defects in postmenopausal women.

Chronic renin inhibition with aliskiren improves glucose tolerance, insulin sensitivity, and skeletal muscle glucose transport activity in obese Zucker rats

We have demonstrated previously that overactivity of the renin-angiotensin system (RAS) is associated with whole body and skeletal muscle insulin resistance in obese Zucker (fa/fa) rats. Moreover, this obesity-associated insulin resistance is reduced by treatment with angiotensin-converting enzyme inhibitors or angiotensin receptor (type 1) blockers. However, it is currently unknown whether specific inhibition of renin itself, the rate-limiting step in RAS functionality, improves insulin action in obesity-associated insulin resistance. Therefore, the present study assessed the effect of chronic, selective renin inhibition using aliskiren on glucose tolerance, whole body insulin sensitivity, and insulin action on the glucose transport system in skeletal muscle of obese Zucker rats. Obese Zucker rats were treated for 21 days with either vehicle or aliskiren (50 mg/kg body wt ip). Renin inhibition was associated with a significant lowering (10%, P < 0.05) of resting systolic blood pressure and induced reductions in fasting plasma glucose (11%) and free fatty acids (46%) and homeostatic model assessment for insulin resistance (13%). Glucose tolerance (glucose area under the curve) and whole body insulin sensitivity (inverse of the glucose-insulin index) during an oral glucose tolerance test were improved by 15% and 16%, respectively, following chronic renin inhibition. Moreover, insulin-stimulated glucose transport activity in isolated soleus muscle of renin inhibitor-treated animals was increased by 36% and was associated with a 2.2-fold greater Akt Ser(473) phosphorylation. These data provide evidence that chronic selective inhibition of renin activity leads to improvements in glucose tolerance and whole body insulin sensitivity in the insulin-resistant obese Zucker rat. Importantly, chronic renin inhibition is associated with upregulation of insulin action on skeletal muscle glucose transport, and it may involve improved Akt signaling. These data support the strategy of targeting the RAS to improve both blood pressure regulation and insulin action in conditions of insulin resistance.

Critical role of the transient activation of p38 MAPK in the etiology of skeletal muscle insulin resistance induced by low-level in vitro oxidant stress

Increased cellular exposure to oxidants may contribute to the development of insulin resistance and type 2 diabetes. Skeletal muscle is the primary site of insulin-dependent glucose disposal in the body; however, the effects of oxidative stress on insulin signaling and glucose transport activity in mammalian skeletal muscle are not well understood. We therefore studied the effects of a low-level in vitro oxidant stress (30–40 μM H 2O 2) on basal and insulin-stimulated (5 mU/ml) glucose transport activity and insulin signaling at 2, 4, and 6 h in isolated rat soleus muscle. H 2O 2 increased basal glucose transport activity at 2 and 4 h, but not at 6 h. This low-level oxidant stress significantly impaired insulin-stimulated glucose transport activity at all time points, and was associated with inhibition of insulin-stimulated phosphorylation of Akt Ser 473 and GSK-3β Ser 9. In the presence of insulin, H 2O 2 decreased total protein expression of IRS-1 at 6 h and IRS-2 at 4 and 6 h. Phosphorylation of p38 MAPK Thr 180/Tyr 182 was transiently increased by H 2O 2 in the presence and absence of insulin at 2 and 4 h, but not at 6 h. Selective inhibition of p38 MAPK with A304000 partially rescued the H 2O 2-induced reduction in insulin-stimulated glucose transport activity. These results indicate that direct in vitro exposure of isolated mammalian skeletal muscle to a low-level oxidant stress impairs distal insulin signaling and insulin-stimulated glucose transport activity, at least in part, due to a p38 MAPK-dependent mechanism.

Dysregulation of Glycogen Synthase Kinase-3 in Skeletal Muscle and the Etiology of Insulin Resistance and Type 2 Diabetes

Insulin resistance of glucose transport and metabolism in insulin-sensitive tissues is a primary defect leading to the development of type 2 diabetes. While the etiology of insulin resistance is multifactorial, one factor associated with reduced insulin action is enhanced activity of the serine/threonine kinase glycogen synthase kinase-3 (GSK-3) in skeletal muscle, liver, and adipose tissue. GSK-3 is involved in numerous cellular functions, including glycogen synthesis, protein synthesis, gene transcription, and cell differentiation. Evidence from muscle and fat cell lines and in skeletal muscle from a variety of obese rodent models and from type 2 diabetic humans supports a role of GSK-3 overactivity in the development of insulin resistance of glucose transport and glycogenesis. Studies utilizing highly selective GSK-3 inhibitors indicate that GSK-3 overactivity in obesity is associated with enhanced IRS-1 serine phosphorylation and defective IRS-1-dependent signaling, ultimately resulting in reduced GLUT-4 translocation and glucose transport activity in skeletal muscle. A role of GSK-3 overactivity in the exaggerated hepatic glucose production of type 2 diabetes has also been reported. Recent studies have demonstrated that oxidative stress, resulting from enhanced exposure to oxidants, causes impaired insulin signaling and insulin resistance of skeletal muscle glucose transport, in part due to reduced suppression of GSK-3 activity and increased IRS-1 Ser(307) phosphorylation. The evidence to date supports an important role of GSK-3 dysfunction in the multifactorial etiology of insulin resistance in skeletal muscle. GSK-3 remains an important target for interventions designed to improve insulin action in obesity-associated insulin resistance and type 2 diabetes.

The effects of antioxidants treatment on training induced insulin-stimulated glucose transport activity in muscle.

The effects of antioxidants treatment on training induced insulin-stimulated glucose transport activity in muscle.

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

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. 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). 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. 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.

Direct inhibition by angiotensin II of insulin-dependent glucose transport activity in mammalian skeletal muscle involves a ROS-dependent mechanism

No previous study has investigated how the vaso-constrictive peptide Ang II impacts insulin action in isolated mammalian skeletal muscle. We investigated the molecular actions of Ang II on insulin signalling and glucose transport in skeletal muscle from lean Zucker rats. Soleus strips were incubated with insulin (5 mU/ml) and/or Ang II (500 nM) for 2 hours. Ang II caused significant (p < 0.05) inhibition of insulin-stimulated glucose transport (39%) and decreased phosphorylation of Akt Ser473 (37%) and glycogen synthase kinase-3β Ser9 (42%) without affecting phosphorylation of IRS-1 Ser307 or p38 MAPK. We used the superoxide dismutase mimetic, tempol (1 mM), to determine if reactive oxygen species (ROS) contribute to Ang II-mediated insulin resistance. Tempol partially reversed (42%) Ang II-induced inhibition of insulin-stimulated glucose transport. These results indicate that Ang II inhibits distal insulin signalling and insulin-stimulated glucose transport in isolated mammalian skeletal muscle, and that this effect is partially mediated by ROS.

Direct inhibition by angiotensin II of insulin‐dependent glucose transport activity in mammalian skeletal muscle involves a ROS‐dependent mechanism

Angiotensin II (Ang II) is elevated in hypertension and impairs insulin signaling and insulin‐stimulated glucose transport activity in muscle cell lines. No previous study has directly investigated how Ang II impacts insulin action in isolated mammalian skeletal muscle. We investigated the molecular actions of Ang II on insulin signaling and glucose transport activity in isolated skeletal muscle from female lean Zucker rats. Soleus strips were incubated with insulin (5 mU/ml) and/or Ang II (500 nM) for 2 hr. Ang II alone did not affect glucose transport or distal insulin signaling (Akt and glycogen synthase kinase‐3β (GSK‐3β)). Ang II inhibited insulin‐stimulated glucose transport (39%) and decreased phosphorylation of Akt Ser473 (37%) and GSK‐3β Ser9 (42%). Ang II did not affect phosphorylation of IRS‐1 Ser307 or p38 MAPK. We used the superoxide dismutase mimetic, tempol (1 mM), to determine if reactive oxygen species (ROS) contribute to Ang II‐mediated insulin resistance. Tempol partially reversed (42%) the Ang II‐induced inhibition of insulin‐stimulated glucose transport. These results indicate that Ang II inhibits distal insulin signaling and insulin‐stimulated glucose transport activity in isolated mammalian skeletal muscle, and this effect is partially mediated by ROS. Ang II may represent a factor that contributes to the etiology of both hypertension and insulin resistance in skeletal muscle.

The effect of 5 days of aspartate and asparagine supplementation on glucose transport activity in rat muscle

The consumption of protein supplements containing amino acids is increasing around the world. Aspartate (Asp) and asparagine (Asn) are amino acids metabolized by skeletal muscle. This metabolism involves biochemical pathways that are involved in increasing Krebs cycle activity via anaplerotic reactions, resulting in higher glutamine concentrations. A connection between amino acid supplementation, glycogen concentration, and glucose uptake has been previously demonstrated. The purpose of this study was to evaluate the effect of Asp and Asn supplementation on glucose uptake in rats using three different glycogen concentrations. The results indicate that Asp and Asn supplementation in rats with high glycogen concentrations (fed state) further increased the glycogen concentration in the muscle, and decreased in vitro 2-deoxyglucose (a glucose analog) uptake by the muscle at maximal insulin concentrations. When animals had a medium glycogen concentration (consumed lard for 3 days), glucose uptake was higher in the supplemented group at sub-maximal insulin concentrations. We conclude that supplementation of Asp and Asn reduced glucose transport in rat muscle only at higher levels of glycogen. The ingestion of lard for 3 days changed the responsiveness and sensitivity to insulin, and that group had higher levels of insulin sensitivity with Asp and Asn supplementation.

Attenuation of oxidant-induced muscle insulin resistance and p38 MAPK by exercise training

We have recently shown that direct exposure to an oxidant stress induces resistance to insulin in glucose transport activity in intact rat skeletal muscle. In this study, we evaluated the effectiveness of prior exercise training in attenuating oxidative stress-induced insulin resistance. Male Sprague–Dawley rats either remained sedentary or underwent a treadmill-running regimen for 6 weeks. Isolated soleus muscles were incubated in the absence or presence of hydrogen peroxide (H 2O 2) (50–70 μM) with or without insulin for 2 h. In the sedentary animals, H 2O 2 significantly inhibited insulin action on glucose transport activity and phosphorylation of Akt (Ser 473), by 28 and 24%, respectively, and substantially activated the phosphorylation levels of p38 MAPK (Thr 180/Tyr 182) by 43% and SAPK/JNK (Thr 183/Tyr 185) by 111%. Interestingly, the inhibitory effects of H 2O 2 on insulin-stimulated glucose transport and Akt (Ser 473) phosphorylation were attenuated by 43 and 75% in exercise-trained muscles. Additionally, the phosphorylation level of p38 MAPK (Thr 180/Tyr 182) triggered by oxidative stress was reduced by 59% in the exercise-trained muscle. We have demonstrated for the first time in mammalian skeletal muscle that endurance exercise training can partially protect against glucose transport resistance to insulin induced by oxidative stress, and this benefit of exercise training is at least in part mediated through the insulin signaling pathway and stress-activated signaling elements.