Skeletal muscle contraction stimulates capillary recruitment and glucose uptake in insulin-resistant obese Zucker rats

Exercise and insulin increase muscle glucose uptake by different mechanisms and also increase capillary recruitment, which is proposed to facilitate access for hormones and nutrients. The genetically obese Zucker rat shows impaired insulin- but not contraction-mediated glucose uptake in muscle. Recently, we have shown the genetically obese Zucker rats to have impaired insulin-mediated capillary recruitment and proposed that this contributes to the insulin resistance of muscle in vivo. Because this might imply a general loss of recruitable capillaries, we now assess responses to contraction in muscles of 18 +/- 3-wk-old lean and obese Zucker rats in vivo. Field stimulation (2 Hz, 0.1 ms) was conducted for 1 h on one leg of anesthetized instrumented rats, and measurements were made of femoral blood flow (FBF), heart rate (HR), blood pressure (BP), hindleg metabolism of 1-methylxanthine (a measure of capillary recruitment), hindleg glucose uptake (HGU), and lower leg muscle glucose uptake by 2-deoxyglucose (R'g). Lean animals (311 +/- 9 g) developed tension at 219 +/- 27 g/g muscle with no change in BP but with significant increases in HR, FBF, HGU, 1-MX metabolism, and R'g (P < 0.05), compared with nonstimulated control leans. Obese animals (469 +/- 7 g) developed tension at 265 +/- 31 g/g muscle with no change in HR or BP but with significant increases in FBF, HGU, 1-MX metabolism, and R'g (P < 0.05) compared with nonstimulated control obese rats. Muscle contraction of lean animals led to a greater increase in lower leg R'g, similar responses in HGU and 1-MX, and a smaller increase in FBF than in obese animals. A tight correlation between FBF and capillary recruitment was noted for all data (P < 0.001). It is concluded that contraction-mediated muscle capillary recruitment and glucose uptake are essentially normal in the obese Zucker rat and that control of FBF and capillary recruitment in exercise is closely linked.     

Acute effects of wortmannin on insulin's hemodynamic and metabolic actions in vivo

Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, was systemically infused during a hyperinsulinemic euglycemic clamp to investigate its effects in vivo. Rats were infused under anesthesia with saline, 10 or 20 mU.min-1.kg-1 insulin, wortmannin (1 microg.min-1.kg-1)+saline, or wortmannin+insulin (10 mU.min-1.kg-1); wortmannin was present for 1 h before and throughout the 2-h clamp. Femoral blood flow (FBF), glucose infusion rate to maintain euglycemia (GIR), glucose appearance (Ra), glucose disappearance (Rd), capillary recruitment by 1-methylxanthine metabolism (MXD), hindleg glucose uptake (HLGU), liver, muscle, and aorta Akt phosphorylation (P-Akt/Akt), and plasma insulin concentrations were determined. Plasma insulin increased from 410+/-49 to 1,680+/-430 and 5,060+/-230 pM with 10 and 20 mU.min-1.kg-1 insulin, respectively. Insulin (10 and 20 mU.min-1.kg-1) increased FBF, MXD, GIR, Rd, and HLGU as well as liver, muscle, and aorta P-Akt/Akt and decreased Ra (all P<0.05). Wortmannin alone increased plasma insulin to 5,450+/-770 pM and increased Ra, Rd, HLGU, and muscle P-Akt/Akt without effect on blood glucose, FBF, MXD liver, or aorta P-Akt/Akt. Wortmannin blocked FBF, MXD, and liver P-Akt/Akt increases from 10 mU.min-1.kg-1 insulin. Comparison of wortmannin+10 mU.min-1.kg-1 insulin and 20 mU.min-1.kg-1 insulin alone (both at approximately 5,000 pM PI) showed that wortmannin fully blocked the changes in FBF and Ra and partly those of GIR, Ra, Rd, HLGU, and muscle P-AKT/Akt. In summary, wortmannin in vivo increases plasma insulin and fully inhibits insulin-mediated effects in liver and aorta and partially those of muscle, where the latter may result from inhibition of insulin-mediated increases in blood flow and capillary recruitment.     

Inhibiting NOS blocks microvascular recruitment and blunts muscle glucose uptake in response to insulin

We examined the effects of inhibiting nitric oxide synthase with Nomega-nitro-l-arginine-methyl ester (l-NAME) on total hindlimb blood flow, muscle microvascular recruitment, and hindlimb glucose uptake during euglycemic hyperinsulinemia in vivo in the rat. We used two independent methods to measure microvascular perfusion. In one group of animals, microvascular recruitment was measured using the metabolism of exogenously infused 1-methylxanthine (1-MX), and in a second group contrast-enhanced ultrasound (CEU) was used. Limb glucose uptake was measured by arterial-venous concentration differences after 2 h of insulin infusion. Saline alone did not alter femoral artery flow, glucose uptake, or 1-MX metabolism. Insulin (10 mU.min-1.kg-1) significantly increased hindlimb total blood flow (0.69 +/- 0.02 to 1.22 +/- 0.11 ml/min, P < 0.05), glucose uptake (0.27 +/- 0.05 to 0.95 +/- 0.08 micromol/min, P < 0.05), 1-MX uptake (5.0 +/- 0.5 to 8.5 +/- 1.0 nmol/min, P < 0.05), and skeletal muscle microvascular volume measured by CEU (10.0 +/- 1.6 to 15.0 +/- 1.2 video intensity units, P < 0.05). Addition of l-NAME to insulin completely blocked the effect of insulin on both total limb flow and microvascular recruitment (measured using either 1-MX or CEU) and blunted glucose uptake by 40% (P < 0.05). We conclude that insulin specifically recruits flow to the microvasculture in skeletal muscle via a nitric oxide-dependent pathway and that this may be important to insulin's overall action to regulate glucose disposal.     

Free fatty acid-induced peripheral insulin resistance augments splanchnic glucose uptake in healthy humans

To investigate the effect of elevated plasma free fatty acid (FFA) concentrations on splanchnic glucose uptake (SGU), we measured SGU in nine healthy subjects (age, 44 +/- 4 yr; body mass index, 27.4 +/- 1.2 kg/m(2); fasting plasma glucose, 5.2 +/- 0.1 mmol/l) during an Intralipid-heparin (LIP) infusion and during a saline (Sal) infusion. SGU was estimated by the oral glucose load (OGL)-insulin clamp method: subjects received a 7-h euglycemic insulin (100 mU x m(-2) x min(-1)) clamp, and a 75-g OGL was ingested 3 h after the insulin clamp was started. After glucose ingestion, the steady-state glucose infusion rate (GIR) during the insulin clamp was decreased to maintain euglycemia. SGU was calculated by subtracting the integrated decrease in GIR during the period after glucose ingestion from the ingested glucose load. [3-(3)H]glucose was infused during the initial 3 h of the insulin clamp to determine rates of endogenous glucose production (EGP) and glucose disappearance (R(d)). During the 3-h euglycemic insulin clamp before glucose ingestion, R(d) was decreased (8.8 +/- 0.5 vs. 7.6 +/- 0.5 mg x kg(-1) x min(-1), P < 0.01), and suppression of EGP was impaired (0.2 +/- 0.04 vs. 0.07 +/- 0.03 mg x kg(-1) x min(-1), P < 0.01). During the 4-h period after glucose ingestion, SGU was significantly increased during the LIP vs. Sal infusion study (30 +/- 2 vs. 20 +/- 2%, P < 0.005). In conclusion, an elevation in plasma FFA concentration impairs whole body glucose R(d) and insulin-mediated suppression of EGP in healthy subjects but augments SGU.     

cGMP phosphodiesterase inhibition improves the vascular and metabolic actions of insulin in skeletal muscle

There is considerable support for the concept that insulin-mediated increases in microvascular blood flow to muscle impact significantly on muscle glucose uptake. Since the microvascular blood flow increases with insulin have been shown to be nitric oxide-dependent inhibition of cGMP-degrading phosphodiesterases (cGMP PDEs) is predicted to enhance insulin-mediated increases in microvascular perfusion and muscle glucose uptake. Therefore, we studied the effects of the pan-cGMP PDE inhibitor zaprinast on the metabolic and vascular actions of insulin in muscle. Hyperinsulinemic euglycemic clamps (3 mU·min(-1)·kg(-1)) were performed in anesthetized rats and changes in microvascular blood flow assessed from rates of 1-methylxanthine metabolism across the muscle bed by capillary xanthine oxidase in response to insulin and zaprinast. We also characterized cGMP PDE isoform expression in muscle by real-time PCR and immunostaining of frozen muscle sections. Zaprinast enhanced insulin-mediated microvascular perfusion by 29% and muscle glucose uptake by 89%, while whole body glucose infusion rate during insulin infusion was increased by 33% at 2 h. PDE2, -9, and -10 were the major isoforms expressed at the mRNA level in muscle, while PDE1B, -9A, -10A, and -11A proteins were expressed in blood vessels. Acute administration of the cGMP PDE inhibitor zaprinast enhances muscle microvascular blood flow and glucose uptake response to insulin. The expression of a number of cGMP PDE isoforms in skeletal muscle suggests that targeting specific cGMP PDE isoforms may provide a promising avenue for development of a novel class of therapeutics for enhancing muscle insulin sensitivity.     

Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU x kg(-1) x min(-1)) in eight young (age 24+/-1 yr) healthy sedentary (maximal O(2) consumption, 49.7+/-2.4 ml x kg(-1) x min(-1)) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-1beta. Glucose disposal was 19% lower after ECC (P<0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC (P<0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC (P < 0.05). TNF-alpha, but not IL-6 or IL-1beta production, increased 2.4-fold 24 h after ECC (P<0.05). TNF-alpha production was positively correlated with reduced insulin action on PI 3-kinase (r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-alpha inhibition of insulin action, elevated TNF-alpha production after muscle damage may impair insulin signal transduction.     

Co-administration of etomoxir and RU-486 mitigates insulin resistance in hepatic and muscular tissues of STZ-induced diabetic rats.

Insulin resistance is a condition of central importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary heart disease. Despite its association with numerous health problems, the mechanism responsible for the development of this phenomenon remains to be established. A novel theory has proposed that insulin resistance in diabetes stems, at least in part, from enhanced free fatty acid (FFA) oxidation and/or excessive production of glucocorticoids (GCs). Several key predictions of this premise were subjected to experimental testing using streptozotocin (STZ)-treated rats as a model for insulin-dependent diabetes mellitus and euglycemic-hyperinsulinemic clamp technique for the in vivo measurement of insulin actions. Euglycemic clamp studies with an insulin infusion index of 5 mU/kg/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Post-absorptive basal EGP and plasma levels of glucose and free fatty acids (FFA) were elevated in the STZ diabetic rats compared to their corresponding control values. In contrast, hypoinsulinemia was evident in these animals. Steady-state GIR and GDR during euglycemic-hyperinsulinemic clamp were markedly decreased in the STZ diabetic rats. Similarly, insulin-mediated suppression of EGP and plasma FFA concentration was also impaired in these animals. GUI, a measure of 2-deoxyglucose (2-DG) uptake, was increased in response to insulin in the order of white gastrocnemus (WG), red gastrocnemus (RG), extensor digitorum longus and soleus muscles. This parallels the percentage of red fibers in these muscles. Diabetes interferes with insulin's ability to increase 2-DG uptake in all of the above muscles with the exception of WG. Nullification of the associated hyperlipidemic and hypercortisolemic states of diabetes with etomoxir (hyperlipidemic) and the glucocorticoid receptor blocker RU-486 (hypercortisolemic) ameliorated the diabetes-related impairment of the in vivo insulin action. Overall these results together with those garnered from the literature support the notion that hypercortisolemia and the enhancement of FFA oxidation are involved, at least in part, in the development of hepatic and skeletal muscle insulin resistance in poorly controlled type I diabetes.     

Developmental stage modifies diet-induced peripheral insulin resistance in rats

In the present study, the effects of age and diet on glucose disappearance and tissue-specific glucose uptake (R'g) were examined under basal or hyperinsulinemic, euglycemic conditions in male Sprague-Dawley rats. Rats were equicalorically fed either a high-starch diet (68% of kcal), high-fat diet (HFD; 45% of kcal), or high-sucrose diet (68% of kcal), beginning at either 5 (W; weanling), 10 (Y; young), 18 (M; mature), or 58 wk (O; older) of age for 5 wks (n = 6-9. group(-1) x diet(-1)). Body weight gain was not significantly different among dietary groups within a given age. Significant (P< 0.05) age effects were observed on basal and clamp free fatty acid concentrations. Significant diet effects were observed on basal and clamp triglyceride concentrations. There were significant diet and age effects on basal skeletal muscle R'g. This interaction was primarily due to an age-associated increase in basal R'g microg x g(-1). min(-1)) in HFD (gastrocnemius R'g: 0.9+/-0.2 in W, 1.1+/-0.2 in Y, 1.8+/-0.2 in M, 2.5+/-0.2 in O). Both age and diet significantly decreased insulin-stimulated muscle R'g. However, whereas age-associated reductions in both glucose-6-phosphate concentration and glycogen synthase activity were observed, significant diet effects were observed on glucose-6-phosphate concentrations only. Age significantly reduced basal and clamp adipose tissue R'g when expressed per gram of tissue but significantly increased R'g when expressed per total fat pad mass. These data suggest that diet-induced changes in peripheral glucose metabolism are modulated by age.     

Endothelial Na+-D-glucose cotransporter: no role in insulin-mediated glucose uptake.

A recent report indicates that the Na+-D-glucose cotransporter SGLT1 is present in capillaries of skeletal muscle and is required for insulin-mediated glucose uptake in myocytes. This result is based on the complete inhibition of insulin-mediated muscle glucose uptake by phlorizin, an inhibitor of SGLT1. Using the pump-perfused rat hind limb, we measured glucose uptake, lactate efflux, and radioactive 2-deoxyglucose uptake into individual muscles with saline (control), phlorizin, insulin, and insulin plus phlorizin, as well as with saline and insulin using normal and low Na+ perfusion buffer. Insulin-mediated glucose uptake was not inhibited after correction for phlorizin interference in the glucose assay. Lactate efflux and 2-deoxyglucose uptake by individual muscles were unaffected by phlorizin. Low Na+ buffer did not affect insulin-mediated glucose uptake, lactate efflux, or 2-deoxyglucose uptake. We conclude that endothelial SGLT1 exerts no barrier for glucose delivery to myocytes.     

Insulin does not mediate the attenuation of fatigue associated with glucose infusion in rat plantaris muscle.

Glucose infusion attenuates fatigue in rat plantaris muscle stimulated in situ, and this is associated with a better maintenance of electrical properties of the fiber membrane (Karelis AD, Péronnet F, and Gardiner PF. Exp Physiol 87: 585-592, 2002). The purpose of the present study was to test the hypothesis that elevated plasma insulin concentration due to glucose infusion ( approximately 900 pmol/l), rather than high plasma glucose concentration ( approximately 10-11 mmol/l), could be responsible for this phenomenon, because insulin has been shown to stimulate the Na+-K+ pump. The plantaris muscle was indirectly stimulated (50 Hz, for 200 ms, 5 V, every 2.7 s) via the sciatic nerve to perform concentric contractions for 60 min, while insulin (8 mU x kg-1x min-1: plasma insulin approximately 900 pmol/l) and glucose were infused to maintain plasma glucose concentration between 4 and 6 [6.2 +/- 0.4 mg x kg-1x min-1: hyperinsulinemic-euglycemic (HE)] or 10 and 12 mmol/l [21.7 +/- 1.1 mg. kg-1. min-1: hyperinsulinemic-hyperglycemic clamps (HH)] (6 rats/group). The reduction in submaximal dynamic force was significantly (P < 0.05) less with HH (-53%) than with HE and saline only (-66 and -70%, respectively). M-wave characteristics were also better maintained in the HH than in HE and control groups. These results demonstrate that the increase in insulin concentration is not responsible for the increase in muscle performance observed after the elevation of circulating glucose.     

Neuregulins mediate calcium-induced glucose transport during muscle contraction

Neuregulin, a growth factor involved in myogenesis, has rapid effects on muscle metabolism. In a manner analogous to insulin and exercise, neuregulins stimulate glucose transport through recruitment of glucose transporters to surface membranes in skeletal muscle. Like muscle contraction, neuregulins have additive effects with insulin on glucose uptake. Therefore, we examined whether neuregulins are involved in the mechanism by which muscle contraction regulates glucose transport. We show that caffeine-induced increases in cytosolic Ca2+ mediate a metalloproteinase-dependent release of neuregulins, which stimulates tyrosine phosphorylation of ErbB4 receptors. Activation of ErbB4 is necessary for Ca2+-derived effects on glucose transport. Furthermore, blockage of ErbB4 abruptly impairs contraction-induced glucose uptake in slow twitch muscle fibers, and to a lesser extent, in fast twitch muscle fibers. In conclusion, we provide evidence that contraction-induced activation of neuregulin receptors is necessary for the stimulation of glucose transport and a key element of energetic metabolism during muscle contraction.     

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.     

Effect of metformin on the vascular and glucose metabolic actions of insulin in hypertensive rats

We investigated the long-term effect of metformin treatment on blood pressure, insulin sensitivity, and vascular responses to insulin in conscious spontaneously hypertensive rats (SHR). The rats were instrumented with intravascular catheters and pulsed Doppler flow probes to measure blood pressure, heart rate, and blood flow. Insulin sensitivity was assessed by the euglycemic hyperinsulinemic clamp technique. Two groups of SHR received metformin (100 or 300 mg x kg(-1) x day(-1)) for 3 wk while another group of SHR and a group of Wistar Kyoto (WKY) rats were left untreated. We found that vasodilation of skeletal muscle and renal vasculatures by insulin is impaired in SHR. Moreover, a reduced insulin sensitivity was detected in vivo and in vitro in isolated soleus and extensor digitorum longus muscles from SHR compared with WKY rats. Three weeks of treatment with metformin improves the whole-body insulin-mediated glucose disposal in SHR but has no blood pressure-lowering effect and no influence on vascular responses to insulin (4 mU x kg(-1) x min(-1)). An improvement in insulin-mediated glucose transport activity was detected in isolated muscles from metformin-treated SHR, but in the absence of insulin no changes in basal glucose transport activity were observed. It is suggested that part of the beneficial effect of metformin on insulin resistance results from a potentiation of the hormone-stimulating effect on glucose transport in peripheral tissues (mainly skeletal muscle). The results argue against a significant antihypertensive or vascular effect of metformin in SHR.     

Skeletal muscle capillary responses to insulin are abnormal in late-stage diabetes and are restored by angiotensin-converting enzyme inhibition

Acute physiological hyperinsulinemia increases skeletal muscle capillary blood volume (CBV), presumably to augment glucose and insulin delivery. We hypothesized that insulin-mediated changes in CBV are impaired in type 2 diabetes mellitus (DM) and are improved by angiotensin-converting enzyme inhibition (ACE-I). Zucker obese diabetic rats (ZDF, n = 18) and control rats (n = 9) were studied at 20 wk of age. One-half of the ZDF rats were treated with quinapril (ZDF-Q) for 15 wk prior to study. CBV and capillary flow in hindlimb skeletal muscle were measured by contrast-enhanced ultrasound (CEU) at baseline and at 30 and 120 min after initiation of a euglycemic hyperinsulinemic clamp (3 mU.min(-1).kg(-1)). At baseline, ZDF and ZDF-Q rats were hyperglycemic and hyperinsulinemic vs. controls. Glucose utilization in ZDF rats was 60-70% lower (P < 0.05) than in controls after 30 and 120 min of hyperinsulinemia. In ZDF-Q rats, glucose utilization was impaired at 30 min but similar to controls at 120 min. Basal CBV was lower in ZDF and ZDF-Q rats compared with controls (13 +/- 4, 7 +/- 3, and 9 +/- 2 U, respectively). With hyperinsulinemia, CBV increased by about twofold in control animals at 30 and 120 min, did not change in ZDF animals, and increased in ZDF-Q animals only at 120 min to a level similar to controls. Anatomic capillary density on immunohistology was not different between groups. We conclude that insulin-mediated capillary recruitment in skeletal muscle, which participates in glucose utilization, is impaired in animals with DM and can be partially reversed by chronic ACE-I therapy.     

Contraction-activated glucose uptake is normal in insulin-resistant muscle of the obese Zucker rat.

The rates of muscle glucose uptake of lean and obese Zucker rats were assessed via hindlimb perfusion under basal conditions (no insulin), in the presence of a maximal insulin concentration (10 mU/ml), and after electrically stimulated muscle contraction in the absence of insulin. The perfusate contained 28 mM glucose and 7.5 microCi/mmol of 2-deoxy-D-[3H-(G)]glucose. Glucose uptake rates in the soleus (slow-twitch oxidative fibers), red gastrocnemius (fast-twitch oxidative-glycolytic fibers), and white gastrocnemius (fast-twitch glycolytic fibers) under basal conditions and after electrically stimulated muscle contraction were not significantly different between the lean and obese rats. However, the rate of glucose uptake during insulin stimulation was significantly lower for obese than for lean rats in all three fiber types. Significant correlations were found for insulin-stimulated glucose uptake and glucose transporter protein isoform (GLUT-4) content of soleus, red gastrocnemius, and white gastrocnemius of lean (r = 0.79) and obese (r = 0.65) rats. In contrast, the relationships between contraction-stimulated glucose uptake and muscle GLUT-4 content of lean and obese rats were negligible because of inordinately low contraction-stimulated glucose uptakes by the solei. These results suggest that maximal skeletal muscle glucose uptake of obese Zucker rats is resistant to stimulation by insulin but not to contractile activity. In addition, the relationship between contraction-stimulated glucose uptake and GLUT-4 content appears to be fiber-type specific.     

Cinnamon extract prevents the insulin resistance induced by a high-fructose diet.

The aim of this study was to determine whether cinnamon extract (CE) would improve the glucose utilization in normal male Wistar rats fed a high-fructose diet (HFD) for three weeks with or without CE added to the drinking water (300 mg/kg/day). In vivo glucose utilization was measured by the euglycemic clamp technique. Further analyses on the possible changes in insulin signaling occurring in skeletal muscle were performed afterwards by Western blotting. At 3 mU/kg/min insulin infusions, the decreased glucose infusion rate (GIR) in HFD-fed rats (60 % of controls, p < 0.01) was improved by CE administration to the same level of controls (normal chow diet) and the improving effect of CE on the GIR of HFD-fed rats was blocked by approximately 50 % by N-monometyl-L-arginine. The same tendency was found during the 30 mU/kg/min insulin infusions. There were no differences in skeletal muscle insulin receptor (IR)-beta, IR substrate (IRS)-1, or phosphatidylinositol (PI) 3-kinase protein content in any groups. However, the muscular insulin-stimulated IR-beta and IRS-1 tyrosine phosphorylation levels and IRS-1 associated with PI 3-kinase in HFD-fed rats were only 70 +/- 9 %, 76 +/- 5 %, and 72 +/- 6 % of controls (p < 0.05), respectively, and these decreases were significantly improved by CE treatment. These results suggest that early CE administration to HFD-fed rats would prevent the development of insulin resistance at least in part by enhancing insulin signaling and possibly via the NO pathway in skeletal muscle.     

Effects of daily physical activity on insulin action in the elderly.

The effects of daily physical activity on peripheral insulin action were investigated in aged individuals. Glucose infusion rates (GIR) during the euglycemic insulin clamp procedure in aged bedridden, aged controls, and aged athletes were compared with those in young controls and young athletes at insulin infusion rates of 40 and 400 mU.m-2.min-1 to estimate insulin action at physiological and maximal insulin concentrations, respectively. At both insulin infusion rates, GIR was significantly higher in aged athletes and significantly lower in aged bedridden subjects than in aged controls. Although there was no statistical difference in GIR at 400 mU.m-2 x min-1 between young athletes and young controls, GIR at 40 mU.m-2 x min-1 was higher in young athletes than in young controls. Comparison of the aged and young groups showed that although GIR at 400 mU.m-2 x min-1 was significantly lower in aged controls than in young controls, there was no significant difference between the aged athletes and the young athletes. We conclude that insulin responsiveness (insulin action at the postreceptor binding site) may decrease with the aging process and may be further affected by physical inactivity. Although physical training may improve insulin responsiveness in aged individuals up to levels similar to those in young athletes, physical training in young individuals may improve only insulin sensitivity.     

Nonnutritive flow impairs uptake of fatty acid by white muscles of the perfused rat hindlimb

Triglyceride hydrolysis by the perfused rat hindlimb is enhanced with serotonin-induced nonnutritive flow (NNF) and may be due to the presence of nonnutritive route-associated connective tissue fat cells. Here, we assess whether NNF influences muscle uptake of 0.55 mM palmitate in the perfused hindlimb. Comparisons were made with insulin-mediated glucose uptake. NNF induced during 60 nM insulin infusion inhibited hindlimb oxygen uptake from 22.0 +/- 0.5 to 9.7 +/- 0.8 micromol x g(-1) x h(-1) (P < 0.001), 1-methylxanthine metabolism (indicator of nutritive flow) from 5.8 +/- 0.4 to 3.8 +/- 0.4 nmol x min(-1) x g(-1) (P = 0.004), glucose uptake from 29.2 +/- 1.7 to 23.1 +/- 1.8 micromol x g(-1) x h(-1) (P = 0.005) and muscle 2-deoxyglucose uptake from 82.1 +/- 4.6 to 41.6 +/- 6.7 micromol x g(-1) x h(-1) (P < 0.001). Palmitate uptake, unaffected by insulin alone, was inhibited by NNF in extensor digitorum longus, white gastrocnemius, and tibialis anterior muscles; average inhibition was from 13.9 +/- 1.2 to 6.9 +/- 1.4 micromol x g(-1) x h(-1) (P = 0.02). Thus NNF impairs both fatty acid and glucose uptake by muscle by restricting flow to myocytes but, as shown previously, favors triglyceride hydrolysis and uptake into nearby connective tissue fat cells. The findings have implications for lipid partitioning in limb muscles between myocytes and attendant adipocytes.     

Effects of acute exercise and detraining on insulin action in trained men

Seven endurance-trained subjects [maximal O2 consumption (VO2max) 64 +/- 1 (SE) ml.min-1.kg-1] underwent sequential hyperinsulinemic euglycemic clamps on three occasions: 1) in the "habitual state" 15 h after the last training bout (C), 2) after 60 min of bicycle exercise at 72 +/- 3% of VO2max performed in the habitual state (E), and 3) 5 days after the last ordinary training session (detrained, DT). Sensitivity for insulin-mediated whole-body glucose uptake was not affected by acute exercise [insulin concentrations eliciting 50% of maximal insulin-mediated glucose uptake being 44 +/- 2 (C) vs. 46 +/- 3 (E) microU/ml] but was decreased after detraining (54 +/- 2 microU/ml, P less than 0.05) to levels comparable to those found in untrained subjects [Am. J. Physiol. 254 (Endocrinol. Metab. 17): E248-E259, 1988]. Near-maximal insulin-mediated glucose uptake (responsiveness) was higher than in untrained subjects and not influenced by acute exercise or detraining [13.4 +/- 1.2 (C), 12.2 +/- 0.9 (E), and 12.2 +/- 0.3 (DT) mg.min-1.kg-1]. Calculated by indirect calorimetry, the glucose-to-glycogen conversion was not influenced by E but was reduced during detraining (P less than 0.05) yet remained higher than previously found in untrained subjects (P less than 0.05). However, only on E days did muscle glycogen increase during insulin infusion. Glycogen synthase activity was increased on E and decreased on DT compared with C days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vascular recruitment in skeletal muscle during exercise and hyperinsulinemia assessed by contrast ultrasound

The purpose of this study was to noninvasively quantify the effects of insulin on capillary blood volume (capBV) and RBC velocity (V(RBC)) in skeletal muscle in vivo with the use of contrast-enhanced ultrasound. We performed contrast ultrasound of the rat hindlimb adductor muscles at baseline and after 2-h infusions of either insulin (3 or 40 mU x kg(-1) x min(-1)) or saline. Saline-treated animals were also studied during contractile exercise. V(RBC) and capBV were calculated from the relation between pulsing interval and video intensity. Femoral artery blood flow, measured by a flow probe, increased with both contractile exercise and insulin. Contractile exercise increased capBV more than twofold and V(RBC) fivefold. Insulin also increased capBV more than twofold in a dose-dependent fashion but did not significantly alter V(RBC). Saline infusion did not significantly alter capBV, V(RBC), or femoral artery blood flow. We conclude that physiological changes in skeletal muscle capillary perfusion can be assessed in vivo with the use of contrast-enhanced ultrasound. Exercise increases both V(RBC) and capBV, whereas hyperinsulinemia selectively increases only capBV, which may enhance skeletal muscle glucose uptake.