Exercise induces recruitment of the “insulin-responsive glucose transporter”. Evidence for distinct intracellular insulin- and exercise-recruitable transporter pools in skeletal muscle

Role of dihydropyridine sensitive calcium channels in glucose transport in skeletal muscle

Glucose transport in skeletal muscle is a carrier-mediated process activated by insulin and by contractile activity. Since previous evidence suggests a role for calcium influx in the activation of this process, the purpose of this study was to determine if glucose transport is mediated by muscle's voltage dependent (dihydropyridine sensitive) calcium channels. Soleus and extensor digitorum longus (EDL) muscles, isolated from rats, were incubated with the calcium channel blocker nifedipine. Basal glucose transport was decreased in both soleus and EDL by nifedipine. Treatment with nifedipine effectively blocked both insulin and contraction stimulated glucose transport in soleus. Conversely, glucose transport in EDL, although reduced, was still significantly increased over basal by both insulin and contraction, due, perhaps, to a relatively greater number of dihydropyridine receptors in EDL. These results provide evidence that contraction stimulated, as well as insulin stimulated, glucose transport is mediated in-part by dihydropyridine receptors in skeletal muscle.

Gq-coupled receptors transmit the signal for GLUT4 translocation via an insulin-independent pathway

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) induces the translocation of glucose transporter type 4 (GLUT4) from an intracellular pool to the cell surface and increases glucose uptake in adipocytes. The GTP-binding protein(s) responsible for the translocation has remained to be identified. Using a sensitive and quantitative method to assess the translocation of c-MYC epitope-tagged GLUT4, we obtained evidence that the activation of receptor-coupled Gq (neither Gi nor Gs) triggered GLUT4 translocation in cells, independently of insulin signaling pathway(s). Platelet-activating factor (PAF) induced GLUT4 translocation in the cells expressing the Gi- and Gq-coupled PAF receptor, but the translocation was induced even after pretreatment with wortmannin, an islet-activating protein and phorbol 12, 13-dibutyrate. Norepinephrine triggered GLUT4 translocation in cells expressing the Gq-coupled alpha1-adrenergic receptor, but prostaglandin E2 did not cause GLUT4 translocation in cells expressing the Gs-coupled EP4 receptor or the Gi-coupled EP3alpha receptor. The norepinephrine-stimulated GLUT4 translocation and glucose uptake via Gq may possibly contribute to the fuel supply required for thermogenesis in brown adipocytes and for the enhanced contractility in cardiomyocytes, both of which have an abundant endogenous GLUT4.

Glucose transport and cell surface GLUT-4 protein in skeletal muscle of the obese Zucker rat

The relationship between 3-O-methyl-D-glucose transport and 2-N-4-(1-azi-2,2,2-trifluoroethyl)-benzoyl-1, 3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA)-labeled cell surface GLUT-4 protein was assessed in fast-twitch (epitrochlearis) and slow-twitch (soleus) muscles of lean and obese (fa/fa) Zucker rats. In the absence of insulin, glucose transport as well as cell surface GLUT-4 protein was similar in both epitrochlearis and soleus muscles of lean and obese rats. In contrast, insulin-stimulated glucose transport rates were significantly higher for lean than obese rats in both soleus (0.74 +/- 0.05 vs. 0.40 +/- 0.02 mumol.g-1.10 min-1) and epitrochlearis (0.51 +/- 0.05 vs. 0.17 +/- 0.02 mumol.g-1.10 min-1) muscles. The ability of insulin to enhance glucose transport in fast- and slow-twitch muscles from both lean and obese rats corresponded directly with changes in cell surface GLUT-4 protein. Muscle contraction elicited similar increases in glucose transport in lean and obese rats, with the effect being more pronounced in fast-twitch (0.70 +/- 0.07 and 0.77 +/- 0.04 mumol.g-1.10 min-1 for obese and lean, respectively) than in slow-twitch muscle (0.36 +/- 0.03 and 0.40 +/- 0.02 mumol.g-1.10 min-1 for obese and lean, respectively). The contraction-induced changes in glucose transport directly corresponded with the observed changes in cell surface GLUT-4 protein. Thus the reduced glucose transport response to insulin in skeletal muscle of the obese Zucker rat appears to result directly from an inability to effectively enhance cell surface GLUT-4 protein.

The effect of anoxia on cardiomyocyte glucose transport does not involve an adenosine release or a change in energy state

The action of anoxia on glucose transport was investigated in isolated resting rat cardiomyocytes. Incubation of these cells in the absence of oxygen for 30 min resulted in a 4- to 5-fold increase in glucose transport (with a lag period of 5-10 min). Up to 40 min of anoxia failed to alter the cellular concentrations of ATP, phosphocreatine, and creatine. Adenosine deaminase (1.5 U/ml), the A1-adenosine receptor antagonist 1,3-diethyl-8-phenylxanthine (1 microM), or the A2-selective antagonist 3,7-dimethyl-1-propargylxanthine (20 microM) had no effect on anoxia-dependent glucose transport. Moreover, adenosine (10-300 microM, added under normoxia) did not stimulate glucose transport. Wortmannin (1 microM) did not influence the effect of anoxia, but completely suppressed that of insulin. On the other hand, the effects of anoxia and insulin were not additive. These results indicate (i) that the effect of anoxia on cardiomyocyte glucose transport is not mediated by a change in energy metabolism, nor by an adenosine release; (ii) that it probably does not involve a phosphatidylinositol 3-kinase, in contrast to the effect of insulin, and (iii) that the signal chains triggered by anoxia or insulin may converge downstream of this enzyme, or, alternatively, that anoxic conditions may impair the action of the hormone.

Expression and insulin-regulated distribution of caveolin in skeletal muscle. Caveolin does not colocalize with GLUT4 in intracellular membranes

Caveolin is believed to play an important role in sorting processes, vesicular trafficking, transmembrane signaling, and molecular transport across membranes. In this study we have evaluated the expression and distribution of caveolin in skeletal muscle and its interaction with GLUT4 glucose carriers. Caveolin was expressed to substantial levels in muscle and its expression was regulated in muscle; aging and high fat diet enhanced caveolin expression in skeletal muscle and inversely, myogenesis down-regulated caveolin in L6E9 cells. Under fasting conditions, most of caveolin was found in intracellular membranes and the caveolin present in the cell surface was found in both sarcolemma and T-tubules. Insulin administration led to a redistribution of caveolin from intracellular high density membrane fractions to intracellular lighter density fractions and to the cell surface; this pattern of insulin-induced redistribution was different to what was shown by GLUT4. These results suggests that caveolin is a component of an insulin-regulated machinery of vesicular transport in muscle. Quantitative immunoisolation of GLUT4 vesicles obtained from different intracellular GLUT4 populations revealed the absence of caveolin which substantiates the lack of colocalization of intracellular GLUT4 and caveolin. This indicates that caveolin is not involved in intracellular GLUT4 trafficking in skeletal muscle.

Pyruvate dehydrogenase inactivity is not responsible for sepsis-induced insulin resistance

OBJECTIVE

To determine whether activation of pyruvate dehydrogenase with dichloroacetate can reverse sepsis-induced insulin resistance in humans or rats.

DESIGN

Prospective, controlled study.

SETTING

Intensive care unit (ICU) and laboratory at a university medical center.

SUBJECTS

Nine patients were admitted to the ICU with Gram-negative sepsis, confirmed by cultures. In addition, chronically instrumented, Sprague-Dawley rats, either controls or with live Escherichia coli-induced sepsis.

INTERVENTIONS

Hyperinsulinemic euglycemic clamp, with or without coadministration of dichloroacetate.

MEASUREMENTS AND MAIN RESULTS

In humans, a primed, constant infusion of [6,6-2H2]glucose was used to determine endogenous glucose production and whole-body glucose disposal. Septic humans exhibited impaired maximal insulin-stimulated glucose utilization (39.5 +/- 2.7 mumol/kg/min), despite complete suppression of endogenous glucose production. In rats, a primed, constant infusion of [3-3H]glucose was used to determine endogenous glucose production and whole-body glucose disposal. Tissue glucose uptake in vivo was determined by [14C]-2-deoxyglucose uptake. Maximal, whole-body, insulin-stimulated glucose utilization was 205 +/- 11 and 146 +/- 9 mumol/kg/min in control and septic rats, respectively. The defect was specific to skeletal muscle and heart. Stimulation of pyruvate dehydrogenase with dichloroacetate caused a 50% decrease in plasma lactate concentration but failed to improve whole-body insulin-stimulated glucose utilization in either the septic human or rat. Dichloroacetate reversed the impairment of insulin-stimulated myocardial glucose uptake in septic rats, but did not influence skeletal muscle glucose uptake either under basal conditions or during insulin stimulation.

CONCLUSIONS

Activation of pyruvate dehydrogenase with dichloroacetate does not ameliorate the impairment of whole-body, insulin-stimulated glucose uptake in septic humans or rats, or reverse the specific defect in insulin-mediated skeletal muscle glucose uptake by septic rats. Therefore, the decreased pyruvate dehydrogenase activity associated with sepsis does not appear to mediate sepsis-induced insulin resistance during insulin-stimulated glucose uptake at either the whole-body or tissue level.

Exercise-induced increase in glucose transport, GLUT-4, and VAMP-2 in plasma membrane from human muscle

A major effect of muscle contractions is an increase in sarcolemmal glucose transport. We have used a recently developed technique to produce sarcolemmal giant vesicles from human muscle biopsy samples obtained before and after exercise. Six men exercised for 10 min at 50% maximal O2 uptake (Vo2max) and then to fatigue at 100% Vo2max (5.7 +/- 0.2 min). Vesicle glucose transport at 5 mM increased from 3.3 +/- 0.6 pmol.microgram-1.min-1 at rest to 6.6 +/- 1.0 pmol.microgram-1.min-1 at fatigue (mean +/- SE, n = 6, P < 0.05). This increase in glucose transport was associated with a 1.6-fold increase in vesicle GLUT-4 protein content. Glucose transport normalized to GLUT-4 protein content also increased with exercise, suggesting increased intrinsic activity of GLUT-4. Furthermore, exercise resulted in a 1.4-fold increase in sarcolemmal vesicle-associated membrane protein (VAMP-2) content, suggesting that muscle contractions may induce trafficking of GLUT-4-containing vesicles via a mechanism similar to neurotransmitter release. Our results demonstrate for the first time exercise-induced translocation of GLUT-4 and VAMP-2 to the plasma membrane of human muscle and increased sarcolemmal glucose transport.

Dexamethasone-induced impairment in skeletal muscle glucose transport is not reversed by inhibition of free fatty acid oxidation

Our previous studies suggested a possible role for the glucose-free fatty acid (FFA) cycle, ie, preferential utilization of FFA by muscle at the expense of glucose, in dexamethasone (DEX)-induced insulin resistance. To determine whether this resistance could be reversed by inhibiting FFA utilization, we used etomoxir, a potent inhibitor of mitochondrial FFA oxidation. Male Sprague-Dawley rats were injected subcutaneously with 1 mg/kg DEX or the vehicle every other day for 10 days, and half of each group was administered 10 mg/kg etomoxir by gavage once per day and 1 hour before the experiment. As expected, etomoxir treatment increased serum FFA levels and inhibited FFA oxidation by diaphragm in vitro. Administration of etomoxir decreased serum glucose and insulin concentrations under basal conditions in both control and DEX-treated animals, implying enhanced insulin sensitivity. DEX treatment significantly increased endogenous glucose production and decreased whole-body glucose disposal, as well as 2-deoxyglucose (2-DG) uptake by skeletal muscle during euglycemic-hyperinsulinemic clamps. Administration of etomoxir led to small but significant increases in glucose disposal rates of both control (14%) and DEX (23%) groups, but had no effect on residual endogenous glucose production. Thus, DEX-induced insulin resistance was marginally ameliorated but not completely reversed by etomoxir. Depressed 2-DG uptake by individual muscle tissues observed in the present study in conjunction with the absence of free intracellular glucose in muscle tissue following glucose-insulin infusion strongly suggests that the primary defect in glucose metabolism is at the level of transport. Neither overall abundance of the insulin-sensitive glucose transporter (GLUT-4) in skeletal muscle nor its distribution between intracellular stores and plasma membrane were modified by DEX treatment, either, under basal conditions or in response to acute insulin stimulus. These results suggest a defect(s) in the inherent activity of plasma membrane-bound GLUT-4 as the likely mechanism for DEX-induced insulin resistance.

Identification and characterization of an exercise-sensitive pool of glucose transporters in skeletal muscle

Augmentation of glucose transport into skeletal muscle by GLUT4 translocation to the plasma and T-tubule membranes can be mediated independently by insulin and by contraction/exercise. Available data suggest that separable pools of intracellular GLUT4 respond to these two stimuli. To identify and characterize these pools, we fractionated skeletal muscle membranes in a discontinuous sucrose density gradient. Fractions of 32 and 36% sucrose exhibited the highest enrichment of GLUT4 and were independently responsive to insulin and exercise, respectively. The combination of the two stimuli depleted both GLUT4 fractions simultaneously. Both vesicle populations contained the gp160 aminopeptidase, whose expression had previously been shown to be specific to muscle and fat and restricted to GLUT4 vesicles in the latter tissue. In muscle, gp160 translocates exactly as does GLUT4 in response to insulin and exercise. The contraction- and insulin-sensitive GLUT4 pools also contained secretory component-associated membrane protein/glucose transporter vesicle triplet but not GLUT1 and caveolin. Immunoadsorption of the two pools followed by silver staining did not reveal any obvious difference in their major protein components. On the other hand, sedimentational analysis in sucrose velocity gradients revealed that the insulin-sensitive GLUT4 vesicles had a larger sedimentation coefficient than the exercise-sensitive vesicles. Thus, the separation of the two intracellular GLUT4 pools should be useful in dissecting what are likely to be different signal transduction pathways that mediate their translocation to the cell surface.

Benefits of exercise for type II diabetics: convergence of epidemiologic, physiologic, and molecular evidence

In Canada diabetes affects approximately 5% of the population. The economic costs of diabetes and its attendant complications are significant, requiring approximately $1 billion a year from the health care system. Clearly the prevention and alleviation of diabetes is highly desirable. In the past few years there has been a remarkable convergence of physiologic, biochemical, molecular, and epidemiologic data, all of which indicate very strongly that exercise may be used as a therapeutic tool to prevent or alleviate non-insulin-dependent diabetes mellitus (NIDDM), or Type II diabetes. The evidence for this has been reviewed. Recently the significant therapeutic role of exercise for Type II diabetics has been endorsed by the medical community. However, there is virtually no education of exercise professionals in the area of diabetes and the benefits of lifestyle changes in treating Type II diabetics. This deficiency should be remedied. For the research community, the challenge now is to translate the physiologic, biochemical, and epidemiologic knowledge into optimally effective prescriptive exercise programs for Type II diabetic men and women.

Ca(2+)- and GTP[gamma S]-induced translocation of the glucose transporter, GLUT-4, to the plasma membrane of permeabilized cardiomyocytes determined using a novel immunoprecipitation method

In cardiomyocytes glucose transport is activated not only by insulin but also by contractile activity that causes translocation of the glucose transporter, GLUT-4, from intracellular vesicles to the plasma membrane. The latter effect may possibly be mediated by intracellular Ca2+, as suggested by previous studies. To investigate the role of Ca2+, we permeabilized neonatal rat myocytes with alpha-toxin and incubated them for 1 h either at a pCa (i.e.--log10 [Ca2+]) of 8 (control) or at a pCa of 5 in the presence of adenosine 5'-triphosphate (ATP). Translocation of GLUT-4 was then monitored by a novel immunoprecipitation method using a peptide antibody directed against an exofacial (extracellular) loop of GLUT-4 (residues 58-80). Incorporation of GLUT-4 into the plasmalemma was stimulated 1.8-fold by 10 microM Ca2+ and 1.7-fold by insulin (as in the case of intact cells). The insulin effect was Ca2+ independent, i.e. it was identical in the absence and presence of Ca2+ (10 microM). Guanosine 5'-O-(3-thio-triphosphate) (GTP[gamma S]), which was inactive in intact cells, also caused translocation of GLUT-4 in permeabilized cardiomyocytes. Thus, incorporation of GLUT-4 into the plasma membrane was enhanced 2.5-fold by 200 microM GTP[gamma S] in the virtual absence of Ca2+ (pCa 8) and even 3.5-fold at 10 microM free Ca2+. We conclude that an increase in intracellular Ca2+ concentration increases GLUT-4 translocation of (permeabilized) cardiomyocytes to a similar extent as do insulin and GTP[gamma S] in the absence of Ca2+, but that the effects of Ca2+ and GTP[gamma S] may be additive.

The GLUT4 glucose transporter and the alpha 2 subunit of the Na+,K(+)-ATPase do not localize to the same intracellular vesicles in rat skeletal muscle

The GLUT4 glucose transporter and the alpha 2 subunit of the Na+,K(+)-ATPase of rat skeletal muscle are two proteins which redistribute from intracellular membranes to plasma membranes following in vivo insulin stimulation. Here we show that although both proteins co-segregate after subcellular fractionation of unstimulated rat hindlimb muscles, they do not share the same intracellular residence inside the muscle fibre. By immunogold single- and double-labeling on ultrathin muscle cryosections with specific antibodies, the GLUT4 glucose transporter and the Na+,K(+)-ATPase alpha 2 subunit were observed on different vesicular structures within the cell. GLUT4 was detected on subsarcolemmal and perinuclear membranes, and at the junction between myofibrillar A and I bands where triads are localized. The alpha 2 subunit of the Na+,K(+)-ATPase was observed at the plasma membrane and in distinct subsarcolemmal vesicles and intermyofibrillar membranes. Quantitative analysis of double-labeling of GLUT4 and Na+,K(+)-ATPase alpha 2 subunit revealed that less than 6% of the two proteins co-localize in the same continuous vesicular structures. The differential intracellular localization of the two proteins was further confirmed by immunopurification of GLUT4-containing membranes from muscle homogenates, in which the alpha 2 subunit of the Na+,K(+)-ATPase was found only at the same extent as the alpha 1 subunit of the enzyme, a protein exclusively present at the plasma membrane.

The effects of wortmannin on rat skeletal muscle. Dissociation of signaling pathways for insulin- and contraction-activated hexose transport

Both the anabolic hormone insulin and contractile activity stimulate the uptake of glucose into mammalian skeletal muscle. In this study, we examined the role of phosphatidylinositol 3-kinase (PI 3-kinase), a putative mediator of insulin actions, in the stimulation of hexose uptake in response to hormone and contraction. Phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-triphosphate accumulate in skeletal muscle exposed to insulin but not hypoxia, which mimics stimulation of the contractile-dependent pathway of hexose transport activation. The fungal metabolite wortmannin, an inhibitor of PI 3-kinase, completely blocks the appearance of 3'-phospholipids in response to insulin. Moreover, wortmannin entirely prevented the increase in hexose uptake in muscle exposed to insulin but was without effect on muscle stimulated by repetitive contraction or hypoxia. These results support the view that PI 3-kinase is involved in the signaling pathways mediating insulin-responsive glucose transport in skeletal muscle but is not required for stimulation by hypoxia or contraction. Furthermore, these data indicate that there exist at least two signaling pathways leading to activation of glucose transport in skeletal muscle with differential sensitivities to wortmannin.

Translocation of two glucose transporters in heart: effects of rotenone, uncouplers, workload, palmitate, insulin and anoxia

Our previous studies on the acute regulation of glucose transport in perfused rat hearts were extended to explore further the mechanism of regulation by anoxia; to test the effects of palmitate, a transport inhibitor; and to compare the translocation of two glucose transporter isoforms (GLUT1 and GLUT4). Following heart perfusions under various conditions, glucose transporters in intracellular membranes were quantitated by reconstitution of transport activity and by Western blotting. Rotenone stimulated glucose uptake and decreased the intracellular contents of glucose transporters. This indicates that it activates glucose transport via net outward translocation, similarly to anoxia. However, two uncouplers of oxidative phosphorylation produced little or no effect. Increased workload (which stimulates glucose transport) reduced the intracellular contents of transporters, while palmitate increased the contents, indicating that these factors cause net translocation from or to the intracellular pool, respectively. Relative changes in GLUT1 were similar to those in GLUT4 for most factors tested. A plot of changes in total intracellular transporter content vs. changes in glucose uptake was roughly linear, with a slope of -0.18. This indicates that translocation accounts for most of the changes in glucose transport, and the basal pool of intracellular transporters is five times as large as the plasma membrane pool.

Subcellular distribution and immunocytochemical localization of Na,K-ATPase subunit isoforms in human skeletal muscle

The expression of Na,K-ATPase isoforms was investigated in human skeletal muscle membranes isolated by subcellular fractionation. The alpha 1, alpha 2, alpha 3 and beta 1 subunits were detectable in membranes prepared from the human soleus muscle. The alpha 1 subunit was largely detected in a fraction enriched with plasma membranes (PM), its abundance in an intracellular membrane fraction (IM) accounted for only 4% of that in the PM fraction. No alpha 1 subunits were detected in membranes of sarcoplasmic reticulum (SR) origin. The PM and IM fractions were enriched with alpha 2 subunits which were less abundant in the SR-enriched fraction. The abundance of alpha 2 molecules within the IM fraction was about 75% of that in the PM fraction when the total protein content for the two fractions was taken into account. Immunocytochemical studies confirmed the localization of the alpha 1 subunit to the muscle cell surface. The alpha 2 subunit was also found to be present in the cell surface but the observation that alpha 2 immunofluorescence was diffusely dispersed throughout the muscle fibre indicated that it was also present intracellularly, consistent with its biochemical localization in the PM and IM membrane fractions. The alpha 3 subunit was detected largely in the PM fraction but the lack of good antibodies to this isoform precluded an analysis of its immunocytochemical localization. The beta 1 subunit was enriched in the PM fraction but was also detected to a modest extent in the IM.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated GLUT 1 level in crude muscle membranes from diabetic Zucker rats despite a normal GLUT 1 level in perineurial sheaths

Recently, we demonstrated that approximately 60% of GLUT 1 in a crude membrane fraction of rat skeletal muscle originates from perineurial sheaths. To study the in vivo regulation of GLUT 1 expression in different tissues in muscles, we measured the level of GLUT 1 in crude muscle membranes and in perineurial sheaths in diabetic (fa/fa) Zucker rats and lean controls, with and without metformin treatment. The GLUT 1 concentration in perineurial sheaths was identical in all four groups of rats, both when measured by quantitative immunofluorescence and by immunoblotting and densitometry. In a fraction of crude membranes of soleus muscles GLUT 1 expression was more than two-fold higher in (fa/fa) rats than in lean controls (p < 0.005). Metformin treatment significantly elevated GLUT 1 in control rats (p < 0.05) and tended to decrease GLUT 1 in diabetic rats (p < 0.075). The expressions of GLUT 1 and GLUT 4 in crude muscle membranes were inversely correlated (p < 0.01), and GLUT 1 expression correlated positively with fasting glucose (p < 0.05). In conclusion, GLUT 1 expression in perineurial sheaths is unaffected by alterations in glucose homeostasis and by the genes responsible for obesity and diabetes in the Zucker rat. GLUT 1 expression in a crude membrane fraction of soleus muscle is increased in the diabetic animals, likely due to an increased expression in muscle cells proper.

Lactate transport in rat sarcolemmal vesicles and intact skeletal muscle, and after muscle contraction

To determine whether it was possible to measure lactate transport rates into intact skeletal muscles, the transport of lactate (zero-trans) was determined in soleus muscle strips incubated in vitro and compared with lactate transport in sarcolemmal vesicles. In addition, the effects of muscle contractility on lactate transport were investigated in electrically-stimulated soleus muscle strips. In both the intact muscle and the sarcolemmal preparations the rates of transport were saturable, stereospecific, and inhibitable by monocarboxylates (pyruvate, alpha-cyano-4-hydroxycinnamate) and a protein modifier (N-ethylmaleimide; P < 0.05). The anion exchange inhibitor SITS had no effect on lactate uptake (P > 0.05). In both preparations lactate transport followed an inwardly directed proton gradient. Relative comparisons (%) between the preparations indicated a similar slope of increasing transport rates with increasing lactate concentrations and similar responses to a changing pH environment. These characterizations of L-lactate transport into isolated sarcolemmal vesicles and muscle strips revealed that both preparations yielded similar conclusions regarding the transmembrane movement of L-lactate. By using this more physiological muscle preparation, contractile activity, induced by electrical stimulation, did not increase lactate uptake in skeletal muscle in the post-exercise period whereas under similar conditions a marked increase in 2-deoxy-D-glucose uptake occurred (+ 47%; P < 0.05). These data suggest that the transport of glucose and lactate in contracting muscle is regulated differently. These studies also show that the incubated muscle strip preparation may be useful for studying lactate transport in an intact cell system during physiological experiments.

Regulation of glucose transport and expression of GLUT3 transporters in human circulating mononuclear cells: studies in cells from insulin-dependent diabetic and nondiabetic individuals

We have previously shown that human circulating mononuclear cells (CMCs) respond to physiological concentrations of insulin with a rapid increase in glucose transport rate. The responding cells were found to be the monocytes, and cells derived from individuals with insulin-dependent diabetes mellitus (IDDM) had lower basal and insulin-stimulated glucose transport rates. Of interest, both cell types were found to express the GLUT1 but not the typical insulin-responsive GLUT4 transporter isoform. To further study the mechanisms responsible for stimulation of transport in these cells, we investigated (1) the response to insulin-like growth factor-I (IGF-I) and insulin-mimetic agents, and (2) the expression of other glucose transporter isoforms in CMCs of nondiabetic and IDDM individuals. The time course of insulin-stimulated glucose uptake in CMCs was rapid, reaching a plateau within 30 minutes. CMCs showed a dose-dependent and highly sensitive increase in glucose uptake to IGF-I (maximal response reached at 0.1 to 0.5 nmol/L IGF-I). The IGF-I dose-response curve was similar for CMCs of control and IDDM individuals, but both the basal and maximal response to IGF-I were lower in the diabetic group (P < .01). CMCs did not respond to vanadate, lithium, hydrogen peroxide, or short incubation (1 hour) with metformin, but glucose uptake increased in response to peroxides of vanadate and longer-duration (14 hours) metformin incubations. The glucose transporter isoforms of separated monocytes and lymphocytes were further investigated by Northern blotting of total RNA with a GLUT3-specific cDNA probe and by Western blotting of total membranes using GLUT3-specific antiserum.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and cellular localization of glucose transporters (GLUT1, GLUT3, GLUT4) during differentiation of myogenic cells isolated from rat foetuses

Skeletal muscle regeneration is mediated by the proliferation of myoblasts from stem cells located beneath the basal lamina of myofibres, the muscle satellite cells. They are functionally indistinguishable from embryonic myoblasts. The myogenic process includes the fusion of myoblasts into multinucleated myotubes, the biosynthesis of proteins specific for skeletal muscle and proteins that regulates glucose metabolism, the glucose transporters. We find that three isoforms of glucose transporter are expressed during foetal myoblast differentiation: GLUT1, GLUT3 and GLUT4; their relative expression being dependent upon the stage of differentiation of the cells. GLUT1 mRNA and protein were abundant only in myoblasts from 19-day-old rat foetuses or from adult muscles. GLUT3 mRNA and protein, detectable in both cell types, increased markedly during cell fusion, but decreased in contracting myotubes. GLUT4 mRNA and protein were not expressed in myoblasts. They appeared only in spontaneously contracting myotubes cultured on an extracellular matrix. Insulin or IGF-I had no effect on the expression of the three glucose transporter isoforms, even in the absence of glucose. The rate of glucose transport, assessed using 2-[3H]deoxyglucose, was 2-fold higher in myotubes than in myoblasts. Glucose deprivation increased the basal rate of glucose transport by 2-fold in myoblasts, and 4-fold in myotubes. The cellular localization of the glucose transporters was directly examined by immunofluorescence staining. GLUT1 was located on the plasma membrane of myoblasts and myotubes. GLUT3 was located intracellularly in myoblasts and appeared also on the plasma membrane in myotubes. Insulin or IGF-I were unable to target GLUT3 to the plasma membrane. GLUT4, the insulin-regulatable glucose transporter isoform, appeared only in contracting myotubes in small intracellular vesicles. It was translocated to the plasma membrane after a short exposure to insulin, as it is in skeletal muscle in vivo. These results show that there is a switch in glucose transporter isoform expression during myogenic differentiation, dependent upon the energy required by the different stages of the process. GLUT3 seemed to play a role during cell fusion, and could be a marker for the muscle's ability to regenerate.

Facilitative glucose transporters

Facilitative glucose transport is mediated by members of the Glut protein family that belong to a much larger superfamily of 12 transmembrane segment transporters. Six members of the Glut family have been described thus far. These proteins are expressed in a tissue- and cell-specific manner and exhibit distinct kinetic and regulatory properties that reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement. It also plays a special role in transporting glucose across epithelial and endothelial barrier tissues. Glut2 is a high-Km isoform expressed in hepatocytes, pancreatic beta cells, and the basolateral membranes of intestinal and renal epithelial cells. It acts as a high-capacity transport system to allow the uninhibited (non-rate-limiting) flux of glucose into or out of these cell types. Glut3 is a low-Km isoform responsible for glucose uptake into neurons. Glut4 is expressed exclusively in the insulin-sensitive tissues, fat and muscle. It is responsible for increased glucose disposal in these tissues in the postprandial state and is important in whole-body glucose homeostasis. Glut5 is a fructose transporter that is abundant in spermatozoa and the apical membrane of intestinal cells. Glut7 is the transporter present in the endoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphatase on glucose 6-phosphate. This review summarizes recent advances concerning the structure, function, and regulation of the Glut proteins.

The cGMP-inhibitable phosphodiesterase modulates glucose transport activation by insulin

To assess the role of the cGMP-inhibitable phosphodiesterase (cGI-PDE) in the action of insulin on glucose transport, adipocytes from young, lean rats were preincubated for 20 min at 37 degrees C with and without OPC 3911, a specific inhibitor of cGI-PDE, and 3-O-methylglucose uptake was measured. Insulin-stimulated glucose transport was impaired by OPC 3911 (approximately 15%) and this impairment became more pronounced in the presence of the degradable cAMP-analogue 8-bromo-cAMP (approximately 45%). This analogue alone did not significantly decrease glucose transport. Furthermore, insulin sensitivity was impaired by the combination of OPC 3911 and 8-bromo-cAMP. Maximal insulin-stimulated glucose transport in adipocytes from aging, obese rats was affected similarly by OPC 3911 and 8-bromo-cAMP, suggesting that cGI-PDE activity is not markedly altered in this insulin-resistant state. In conclusion, cGI-PDE exerts a modulating effect on the stimulatory action of insulin on glucose transport. This effect is particularly pronounced when the cellular cAMP levels are elevated.

Low-frequency stimulation of rat fast-twitch muscle enhances the expression of hexokinase II and both the translocation and expression of glucose transporter 4 (GLUT-4)

This study followed changes in the capacities of uptake and phosphorylation of glucose in response to contractile activity in low-frequency stimulated (10Hz, 24 h/d) rat fast-twitch muscle. We investigated the intracellular distribution of GLUT-4, the major glucose transporter isoform in muscle, changes in the amounts of its specific mRNA and total cellular protein, as well as changes in its relative synthesis rate. These analyses were complemented by measurements of total hexokinase activity and hexokinase II (HKII) expression at the levels of mRNA content and protein synthesis. Changes in protein synthesis were determined by in vivo labeling with [35S]methionine. Translocation of GLUT-4 into the sarcolemma was an immediate response to contractile activity, whereas changes in its total amount were observed only with ongoing stimulation (5 d and longer). A twofold increase in GLUT-4 content after 5 d and longer stimulation periods was preceded by elevations of its mRNA and by enhanced [35S]methionine incorporation. Conversely, increases in HKII expression with a rise in total hexokinase activity occurred soon after the onset of stimulation (30-fold elevations of HKII mRNA after 12 h and 20-fold increases in [35S]methionine incorporation after 24 h). With ongoing stimulation, HKII mRNA and synthesis returned to lower levels (fivefold elevations). Nevertheless, hexokinase activity continued to rise, stabilizing at fivefold-elevated levels after 3 d. These observation suggested that posttranscriptional mechanisms contributed to the upregulation of HKII, e.g. stabilization by elevated intracellular glucose and mitochondrial binding of the enzyme. This suggestion was supported by experiments with cessation after 24 h where hexokinase activity continued to increase, although the mRNA content and, especially, the [35S]methionine incorporation decayed steeply. The increase in HKII prior to GLUT-4 suggests that phosphorylation may be rate limiting in glucose utilization of glycolytic fibers under conditions of sustained contractile activity. Taken together, the changes in distribution and content of GLUT-4, as well as in HKII represent early metabolic adaptations. In addition, they are related to the overall process of stimulation-induced fiber type transformation.

Different mechanism for insulin induced and contraction induced increases in skeletal muscle glucose uptake

Glucose facilitated diffusion into cells depends on concentration gradients between intracellular and extracellular spaces and can be modified by several factors such as insulin and contractions. Calmodulin participates in the insulin induced recruitment of vesicles containing glucose transporter molecules and its inhibition by trifluoperazine blocks insulin increases in glucose uptake. In the present study we tested if calmodulin inhibition with trifluoperazine blocks hindlimb muscle glucose uptake increase induced by contractions. Trifluoperazine does not inhibit exercise induced increases in glucose uptake; therefore, the mechanisms by which insulin and functional activity increase glucose uptake are different.

Increased insulin-stimulated glucose uptake in athletes: the importance of GLUT4 mRNA, GLUT4 protein and fibre type composition of skeletal muscle

In the present study the expression of GLUT4 and fibre type composition were examined in biopsies from skeletal muscle in seven male athletes and eight male sedentary subjects. Estimated maximal oxygen uptake was increased in the trained group when compared with the sedentary group (74.0 +/- 3.9 vs. 42.9 +/- 5.1 ml kg-1 min-1; P < 0.01). A biopsy of vastus lateralis muscle was taken in the fasting state, 36 h after the last bout of exercise. A second muscle biopsy was obtained following 4 h of a hyperinsulinaemic (2 mU kg-1 min-1), euglycaemic clamp. The rate of insulin-stimulated glucose uptake was increased in the trained subjects (17.34 +/- 0.53 vs. 13.53 +/- 0.79 mg kg-1 min-1, P < 0.01). In parallel, the steady state levels of GLUT4 protein and mRNA per DNA were higher in muscle biopsies obtained in the basal state from athletes than in sedentary controls, 21 and 71% respectively (P < 0.05). In the total group of participants, GLUT4 protein per DNA in the basal state and insulin-stimulated glucose uptake rate correlated positively, (r = 0.51, P = 0.05). In the insulin-stimulated state we did not find any significant correlation between GLUT4 protein per DNA and glucose uptake rate (r = 0.13, n.s.). No significant relationships between GLUT4 protein abundance per DNA and muscle fibre type distribution were observed. A significantly negative correlation was found between type 2B fibre area and insulin-stimulated glucose uptake (r = -0.63, P < 0.05). In conclusion, the abundance of GLUT4 protein and mRNA, respectively, is increased in skeletal muscle from endurance trained subjects compared to sedentary subjects. However, factors other than GLUT4 immunoreactive protein abundance seem to be determinant for the increased insulin-stimulated whole body glucose uptake in endurance trained subjects.

Skeletal muscle glucose uptake during short-term contractile activity in vivo: effect of prior contractions

The goal of the present study was to examine the time course of skeletal muscle glucose uptake and changes in intracellular metabolites occurring with the onset of in situ stimulation, and to assess the effect of a prior period of contractions on subsequent contraction-induced increases in glucose uptake. Hindlimb muscle in anesthetized rabbits was studied noninvasively using the positron-emitting glucose analog 18F-2-fluoro-deoxy-D-glucose (FDG). Fractional rates of FDG phosphorylation were measured on a minute-to-minute basis during rest, 3.5 minutes of priming exercise (PE), 15 or 30 minutes of PE recovery, and a subsequent 15-minute period of contractions. Muscles were electrically stimulated at 2 Hz, and force production was held constant during the contraction period(s). FDG uptake did not differ from control values either during PE or during 60 minutes of recovery from PE. In response to 15 minutes of contractions, muscle stimulated without PE demonstrated increased FDG uptake, but only after a delay of 5.0 +/- 0.7 minutes. Muscle with PE but rested 15 minutes had increased FDG uptake with a delay of 0.5 +/- 0.2 minutes, and muscle with PE but rested 30 minutes had increased FDG uptake after a delay of 8.0 +/- 0.9 minutes (P < .01 all groups). All groups reached similar levels of FDG uptake by the end of 15 minutes of contractions. Both groups with PE had control levels of adenosine triphosphate (ATP), phosphocreatine (PCr), and glucose-6-phosphate (G6P) after PE recovery, but glycogen level was lower than the control value (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise endurance in rats: roles of type I and II corticosteroid receptors

Intact and adrenalectomized (ADX) rats were mildly food deprived and administered dexamethasone (type II agonist), aldosterone (type I agonist), corticosterone (mixed agonist), or vehicle 24 and 2 h prior to forced exercise in a treadmill. The endurance of intact animals was unaffected by hormone treatments. Adrenalectomy greatly advanced the onset of fatigue, and aldosterone exacerbated the effect of adrenalectomy. Corticosterone improved endurance in ADX rats, and dexamethasone was even more potent in this respect. Aldosterone slowed deprivation-induced weight loss in ADXs, while corticosterone and especially dexamethasone accelerated loss. Thus, endurance was directly related to body weight loss, and presumably to the fuels released by such loss. The results extend the type I-type II functional dichotomy to the delivery of utilizable energy for metabolically active tissues.

Metformin ameliorates diabetes but does not normalize the decreased GLUT 4 content in skeletal muscle of obese (fa/fa) Zucker rats

We studied the expression of the glucose transporter GLUT 4 in the soleus and red gastrocnemius muscles from obese, diabetic (fa/fa) Zucker rats compared to their lean littermates (Fa/-), with and without treatment with the antidiabetic drug metformin. In the untreated groups of rats, the GLUT 4 content in a crude membrane fraction of both the soleus and the red gastrocnemius muscles were significantly lower in the obese (fa/fa) rats (3.46 +/- 0.28 vs. 6.04 +/- 0.41, p < 0.001 and 6.0 +/- 0.24 vs. 9.1 +/- 0.48, p < 0.0001, respectively). Differences in GLUT 4 expression in soleus muscle from the same rats were confirmed by quantitative immunofluorescence microscopy, and the results were significantly correlated with the results obtained from quantitative immunoblotting (rho = 0.70, p < 0.0005). The decreased expression of GLUT 4 in fa/fa rats could contribute to the well-established insulin resistance in skeletal muscle of these animals. After 4 weeks of treatment with metformin, weight gain was not affected in either the diabetic (fa/fa) rats or the lean (Fa/-) rats. Improvement of glucose homeostasis by metformin was not associated with normalization of the GLUT 4 expression in the skeletal muscles studied, indicating (1) that the decreased GLUT 4 expression is not directly related to hyperinsulinaemia and diabetes mellitus and (2) that metformin does not normalize the expression of GLUT 4 in skeletal muscle of the diabetic (fa/fa) Zucker rats.

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.

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.

Aerobic versus strength training for risk factor intervention in middle-aged men at high risk for coronary heart disease

To compare the effects of strength training (ST) to those of aerobic training (AT) for coronary heart disease (CHD) risk factor intervention, we studied 37 previously untrained males (aged 50 +/- 9 years, mean +/- SD) before and after 20 weeks of either ST (N = 14), AT (walk/jog, N = 13), or no exercise (inactive controls, N = 10). Lipoprotein and lipid profiles, blood pressure, and glucose and insulin responses to an oral glucose tolerance test (OGTT) were assessed before and after the training period in all three groups. The ST program produced significant reductions in plasma glucose levels at 60, 90, and 120 minutes (P < .05) after glucose ingestion, whereas the AT program resulted in significant reductions only at 90 and 120 minutes (P < .05). ST also decreased insulin levels during fasting (P < .05) and at 90 and 120 minutes (P < .01) after glucose ingestion. AT decreased insulin levels at 90 and 120 minutes (P < .01) after glucose ingestion. Both training programs reduced the total area under the glucose tolerance curve for glucose (both P < .05) and insulin (both P < .05), but there were no significant differences in these changes between the two groups. None of the glucose or insulin values were significantly altered in the control group. There were no significant changes in lipoprotein and lipid profiles or blood pressure in any of the three groups. These results suggest that ST and AT have comparable effects on risk factors for CHD.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction-induced translocation of the glucose transporter Glut4 in isolated ventricular cardiomyocytes

Field stimulation of isolated adult ventricular cardiomyocytes was used to study the effect of contractile activity on 3-O-methylglucose transport and the subcellular distribution of Glut4. Cells contracting at a frequency of 1 Hz for 30 min exhibited unaltered basal and insulin-stimulated rates of glucose transport when compared to resting cells. However, at 5 Hz 3-O-methylglucose transport increased to 224% of control after 5 min. Under these conditions insulin was unable to produce a significant additional stimulation of glucose transport. Immunoblotting with an anti-Glut4 polyclonal antibody showed that both insulin and contraction (5 Hz) increased the amount of Glut4 in a plasma membrane fraction by about 8-fold with a parallel decrease in an intracellular membrane fraction by 60-65%. These data suggest the existence of an identical insulin- and contraction-recruitable Glut4 transporter pool in cardiomyocytes.

Epinephrine administration stimulates GLUT4 translocation but reduces glucose transport in muscle

Epinephrine opposes glucose transport in muscle. Therefore, we investigated the effects of epinephrine administration (25 micrograms/100g body weight) on glucose transport and glucose transporters in rat muscle. Ninety minutes after epinephrine injection 3-O-methyl glucose transport was reduced (approximately 47%) in perfused muscles of the rat hindlimb. Translocation of the insulin-regulatable glucose transporter (GLUT4) in the epinephrine-injected animals was confirmed by the marked increments in the GLUT-4 in the plasma membranes and their concomitant reduction in the intracellular membranes. We speculate a) that it is epinephrine which translocated GLUT4 via a cAMP-linked pathway, and b) that the intrinsic activity reductions are caused either by the glycation of the transporter by the persistent hyperglycemia and/or by epinephrine via the phosphorylation of the GLUT4 transporter protein in muscle.

Detection of the GLUT3 facilitative glucose transporter in rat L6 muscle cells: regulation by cellular differentiation, insulin and insulin-like growth factor-I

The GLUT3 facilitative glucose transporter protein was found to be expressed in rat L6 muscle cells. It was detected at both the myoblast and myotube stage. GLUT3 protein content per mg of total membrane protein increased significantly during L6 cell differentiation. Subcellular fractionation demonstrated that the GLUT3 protein was predominantly localized in plasma membrane-enriched fractions of either myoblasts or myotubes. Short-term exposure of L6 myotubes to IGF-I or insulin caused a redistribution of GLUT3 protein from an intracellular membrane fraction to the plasma membrane, without affecting total membrane GLUT3 protein content. Long-term exposure of L6 myotubes to IGF-I produced an increase of GLUT3 protein in total membranes and all subcellular membrane fractions, especially the plasma membrane. We propose that the GLUT3 glucose transporter may play an important role in glucose metabolism in developing muscle.