Lipoprotein uptake by cultured human arterial smooth muscle cells.

Human arterial smooth muscle cells growing in tissue culture, in contrast to rat cells, preferentially bind and take up large, lipid-rich lipoproteins (125I-labeled low density and very low density lipoproteins) in comparison to the known difference in the propensity of these two species to develop atherosclerosis.     

2-Deoxy-D-glucose uptake in cultured human muscle cells

Hexose uptake was studied with cultured human muscle cells using 2-deoxy-D-[1-3H]glucose. At a concentration of 0.25 and 4 mM, phosphorylation rather than transport was the rate-limiting step in the uptake of 2-deoxy-D-glucose. This was not due to inhibition of the hexokinase activity by either ATP depletion or 2-deoxyglucose 6-phosphate accumulation. In cellular homogenates, hexokinase showed a lower Km value for glucose as compared to 2-deoxyglucose. Intact cells preferentially phosphorylated glucose instead of 2-deoxyglucose. Therefore, transport instead of phosphorylation may be rate limiting in the uptake of glucose by cultured human muscle cells. These data suggest caution in using 2-deoxyglucose for measuring glucose transport.     

Glucose uptake in human and animal muscle cells in culture

Human muscle cells were grown in culture from satellite cells present in muscle biopsies and fusion-competent clones were identified. Hexose uptake was studied in fused myotubes of human muscle cells in culture and compared with hexose uptake in myotubes of the rat L6 and mouse C2C12 muscle cell lines. Uptake of 2-deoxyglucose was saturable and showed an apparent Km of about 1.5 mM in myotubes of all three cell types. The Vmax of uptake was about 6000 pmol/(min.mg protein) in human cells, 4000 pmol/(min.mg protein) in mouse C2C12 muscle cells, and 500 pmol/(min.mg protein) in L6 cells. Hexose uptake was inhibited approximately 90% by cytochalasin B in human, rat, and mouse muscle cell cultures. Insulin stimulated 2-deoxyglucose uptake in all three cultures. The hormone also stimulated transport of 3-O-methylglucose. The sensitivity to insulin was higher in human and C2C12 mouse myotubes (half-maximal stimulation observed at 3.5 X 10(-9) M) than in rat L6 myotubes (half-maximal stimulation observed at 2.5 X 10(-8) M). However, insulin (10(-6) M) stimulated hexose uptake to a larger extent (2.37-fold) in L6 than in either human (1.58-fold) or mouse (1.39-fold) myotubes. It is concluded that human muscle cells grown in culture display carrier-mediated glucose uptake, with qualitatively similar characteristics to those of other muscle cells, and that insulin stimulates hexose uptake in human cells. These cultures will be instrumental in the study of human insulin resistance and in investigations on the mechanism of action of antidiabetic drugs.     

Substance P and neurokinin A metabolism by cultured human skeletal muscle myocytes and fibroblasts

A recent study determined that cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts degraded angiotensins and kinins via neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11) and aminopeptidase N (APN; EC 3.4.11.2). Due to the possible importance of other peptides to skeletal muscle blood flow and function, the present study looked specifically at the metabolism of the neurokinins substance P (SP) and neurokinin A (NKA) by skeletal muscle peptidases. The results show that SP is degraded not only by NEP-24.11, but also sequentially by dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5)/APN. NKA is unaffected by DAP IV but is metabolized by NEP-24.11 and APN. NEP-24.11 was inhibited by phosphoramidon (IC₅₀ = 80 nM), thiorphan and ZINCOV, DAP IV by diprotin A (IC₅₀ = 8 μM), and APN by amastatin (IC₅₀ = 50 nM) and bestatin (IC₅₀ = 100 μM). Skeletal muscle myocyte and fibroblast metabolism of SP and NKA may regulate local skeletal muscle vascular and extravascular functions including SP- and NKA-mediated nerve-induced vasodilation. Inhibition of both NEP-24.11 and DAP IV/APN may increase skeletal muscle blood flow and decrease peripheral vascular resistance via potentiation of local neurokinin levels.     

Purine and pyrimidine metabolism in human muscle and cultured muscle cells

Using radiochemical methods, we determined the activities of various enzymes of purine and pyrimidine metabolism in homogenates of human skeletal muscle and of cultured human muscle cells. Results show a large discrepancy between the enzyme activities in muscle and cultured cells. With regard to purine metabolism, adenylate (AMP) deaminase activity was only 1-3% in cultured cells compared to that in muscle, whereas the activity of adenosine deaminase, purine-nucleoside phosphorylase, adenosine kinase, adenine phosphoribosyltransferase and hypoxanthine phosphoribosyltransferase was 7-15-fold higher in the cultured cells. The enzymes of pyrimidine metabolism, orotate phosphoribosyltransferase, orotidine 5'-monophosphate decarboxylase and uridine kinase showed activity of 100-200-fold higher in cultured cells than in adult muscle. The differences in enzyme activity are probably related to the low differentiation stage and the absence of contractile activity in the cultured muscle cells. Care must be taken when using these cells as a model for studying purine and pyrimidine metabolism of adult myofibers.     

Glucose deprivation accelerates VLDL receptor-mediated TG-rich lipoprotein uptake by AMPK activation in skeletal muscle cells

Glucose and fatty acids are major energy sources in skeletal muscle. Very low-density lipoprotein receptor (VLDL-R), which is highly expressed in heart, skeletal muscle and adipose tissue, plays a crucial role in metabolism of triglyceride (TG)-rich lipoproteins. To explore energy switching between glucose and fatty acids, we studied expression of VLDL-R and lipoprotein uptake in rat L6 myoblasts. l-Glucose or d-glucose deprivation in the medium noticeably induced the AMPK (AMP-activated protein kinase) activation and VLDL-R expression. Dose-dependent induction of VLDL-R expression was observed when d-glucose was less than 4.2 mM. The same phenomenon was also observed in rat primary skeletal myoblasts and cultured vascular smooth muscle cells. The uptake of β-VLDL but not LDL was accompanied by induction of VLDL-R expression. Our study suggests that the VLDL-R-mediated uptake of TG-rich lipoproteins might compensate for glucose shortfall through AMPK activation in skeletal muscle.     

Transmembrane electrical characteristics of cultured human skeletal muscle cells

Skeletal muscle explants from normal subjects were established from biopsy material on collagen. Cellular outgrowth appeared within 3-4 days, and fusion of myoblasts was observed in 5-10 days. Multinucleated myotubes were impaled under high optical magnification, at 37 degrees C, with conventional glass microelectrodes. The mean resting potential was -44.4 mV +/- 2.4 (n = 399); -33 +/- 2.3 mV at 9 days (n = 10) vs -48 +/- 2.5 mV (n = 15) at 27 days. The average input resistance (Rin) was 9.7 M omega (n = 83). Action potentials could be elicited by electrical stimulation and had a mean amplitude of 55.9 +/- 2.1 mV with a mean maximum rate of rise (Vmax) of 72.1 +/- 7.5 V/s. The mean overshoot was 13.9 +/- 2.3 mV, and the action potential duration determined at 50% of repolarization (APD50) was 8.0 msec (n = 7). The resting membrane potential showed a depolarization of 23 mV/decade for extracellular potassium ion concentration ([K]o) between 4.5-100 mM. Thus, we have established the normal resting potential and maximum rate of rise of the action potential for human myotubes in culture. We have shown that the values for these are less than those previously reported in cultured avian and rodent cells. In addition, we have shown that the response in our system of the resting potential to change in extracellular potassium concentration is blunted compared to studies using isolated muscle, suggesting an increase in ratio of sodium to potassium permeability. Cultured human muscle cells depolarized in the presence of ouabain.     

Angiotensin and bradykinin metabolism by peptidases identified in cultured human skeletal muscle myocytes and fibroblasts

Angiotensin (ANG) and kinin metabolizing enzymes, angiotensin-converting enzyme (ACE; EC 3.4.15.1), neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11), and aminopeptidase M (AmM; EC 3.4.11.2), have recently been identified in a purified skeletal muscle glycoprotein fraction. We have analyzed the cellular localization of these enzymes. In cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts, kinins and angiotensins were metabolized by NEP-24.11 and AmM but not by ACE. NEP-24.11 degraded ANG II, ANG III, and bradykinin (BK) and converted ANG I to the active metabolite ANG(1–7). ANG III was converted to the novel ANG IV metabolite [des-Arg¹]ANG III by AmM. These data suggest that, due to their abundance in the body, skeletal muscle myocytes and fibroblasts may play a major role in modulation of the systemic and local effects of angiotensins and kinins. This role could be particularly important in individuals receiving treatment with ACE inhibitors.     

Glucose and amino acid metabolism in an established line of skeletal muscle cells

The suitability of an established myogenic line (L₆) for the study of skeletal muscle intermediary metabolism was investigated. Myoblasts were grown in tissue culture for ten days at which time they had differentiated into multinucleated myotubes. Myotube preparations were then incubated for up to 96 hours in 10 ml of Dulbecco's modified Eagle medium containing 10% fetal calf serum. Glucose was utilized at a nearly linear rate, 3.0 nmol/min/mg protein. Intracellular glucose was detectable throughout the incubation, even when medium glucose was as low as 16 mg%. During the initial 28 hours of incubation, when net lactate production was observed, only 35% of the glucose utilized was converted to lactate. Alanine was produced in parallel to lactate at an average rate of 0.6 nmol/min/mg protein. In concert with active glutamine utilization, high rates of ammoniagenesis were observed as medium glutamine decreased from 3.3 mM to 0.49 mM and medium ammonia increased from 2.3 mM to 6.2 mM, between zero time and 96 hours of incubation, respectively. The cells maintained stable ATP and citrate levels, and physiologic intracellular lactate/pyruvate ratios (10–24) throughout 96 hours of incubation. These results suggest (1) glucose utilization by skeletal muscle in tissue culture is limited by phosphorylation, not transport; (2) as much as 50% of glucose-derived pyruvate enters mitochondrial pathways; (3) glutamine carbon may be utilized simultaneously with glucose consumption and this process accounts for high rates of ammoniagenesis.     

Regulation of fatty acid uptake and metabolism in L6 skeletal muscle cells by resistin

Resistin has been proposed as a potential link between obesity and insulin resistance. It is also well established that altered metabolism of fatty acids by skeletal muscle can lead to insulin resistance and lipotoxicity. However, little is known about the effect of resistin on long chain fatty acid uptake and metabolism in skeletal muscle. Here we show that treating rat skeletal muscle cells with recombinant resistin (50 nM, 24 h) decreased uptake of palmitate. This correlated with reduced cell surface CD36 content and lower expression of FATP1, but no change in FATP4 or CD36 expression. We also found that resistin decreased fatty acid oxidation by measuring 14CO2 production from [1-14C] oleate and an increase in intracellular lipid accumulation was detected in response to resistin. Decreased AMPK and ACC phosphorylation were observed in response to resistin while expression of ACC and AMPK isoforms was unaltered. Resistin mediated these effects without altering cell viability. In summary, our results demonstrate that chronic incubation of skeletal muscle cells with resistin decreased fatty acid uptake and metabolism via a mechanism involving decreased cell surface CD36 content, FATP1 expression and a decrease in phosphorylation of AMPK and ACC.     

Glucose metabolism in perfused skeletal muscle. Interaction of insulin and exercise on glucose uptake.

1. The interaction of insulin and isometric exercise on glucose uptake by skeletal muscle was studied in the isolated perfused rat hindquarter. 2. Insulin, 10 m-i.u./ml, added to the perfusate, increased glucose uptake more than 10-fold, from 0.3-0.5 to 5.2-5.4 mumol/min per 30g of muscle in hindquarters of fed and 48h-starved rats respectively. In contrast, it did not stimulate glucose uptake in hindquarters from rats in diabetic ketoacidosis. 3. In the absence of added insulin, isometric exercise, induced by sciatic-nerve stimulation, increased glucose uptake to 4 and 3.4 mumol/min per 30g of muscle in fed and starved rats respectively. It had a similar effect in rats with moderately severe diabetes, but it did not increase glucose uptake in rats with diabetic ketoacidosis or in hindquarters of fed rats that had been "washed out" with an insulin-free perfusate. Insulin, at concentrations which did not stimulate glucose uptake in resting muscle, restored the stimulatory effect of exercise in these situations. 4. The stimulation of glucose uptake by exercise was independent of blood flow and the degree of tissue hypoxia; also it could not be reproduced by perfusing resting muscle with a medium previously used in an exercise experiment. 5. At rest glucose was not detectable in muscle cell water of fed and starved rats even when perfused with insulin. In the presence of insulin, a small accumulation of glucose, 0.25 mM, was noted in the muscle of ketoacidotic diabetic rats, suggesting inhibition of glucose phosphorylation, as well as of transport. 6. During exercise, the calculated intracellular concentration of glucose in the contracting muscle increased to 1.1-1.6mM in the fed, starved and moderately diabetic groups. Insulin significantly increased the already high rates of glucose uptake by the hindquarters of these animals but it did not alter the elevated intracellular concentration of glucose. 7. In severely diabetic rats, exercise did not cause glucose to accumulate in the cell in the absence of insulin. In the presence of insulin, it increased glucose uptake to 6.1 mumol/min per 30g of muscle and intracellular glucose to 0.72 mM. 8. The data indicate that the stimulatory effect of exercise on glucose uptake requires the presence of insulin. They suggest that in the absence of insulin, glucose uptake is not enhanced by exercise owing to inhibition of glucose transport into the cell.     

Taurine uptake by cultured human lymphoblastoid cells

Cultured human lymphoblastoid cells take up taurine from the medium by two processes: 1) a temperature-dependent, Na⁺-dependent, saturable “active”-transport system and 2) diffusion. The active transport has properties similar to those reported for taurine transport by other tissues. Apparent K_m is about 25 μM and V_max about 7.2 pmol/min/10⁶ cells; saturation occurs at 100 μM taurine. Uptake is competitively inhibited by the β-amino acids hypotaurine (50% inhibition at 44 μM) and β-alanine (50% at 152 μM), as measured at 50 μM taurine. Taurocyamine inhibits 50% at 260 μM. Chlorpromazine and imipramine are strong uncompetitive inhibitors, giving 50% inhibition at 26 μM and 115 μM, respectively; at these concentrations cellular viability per se is not affected. Ouabain inhibits 40–50% over a concentration range of 4–500 μM. Diffusion of taurine into the cells is proportional to concentration up to 20 mM. However, at the concentration of taurine in human plasma, 40–100 μM, active transport would provide 90% of the taurine taken up.     

Adenosine triphosphatases during maturation of cultured human skeletal muscle cells and in adult human muscle.

Na+/K(+)-ATPase, Mg(2+)-ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase are examined in cultured human skeletal muscle cells of different maturation grade and in human skeletal muscle. Na+/K(+)-ATPase is investigated by measuring ouabain binding and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase). SR Ca(2+)-ATPase is examined by ELISA, Ca(2+)-dependent phosphorylation and its activities on ATP and 3-O-methylfluorescein phosphate. Na+/K(+)-ATPase and SR Ca(2+)-ATPase are localized by immunocytochemistry. The activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase show a good correlation with the other assayed parameters of these ion pumps. All ATPase parameters investigated increase with the maturation grade of the cultured muscle cells. The number of ouabain-binding sites and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-MFPase are significantly higher in cultured muscle cells than in muscle. The Mg(2+)-ATPase activity, the content of SR Ca(2+)-ATPase and the activities of SR Ca(2+)-ATPase and Ca(2+)-dependent 3-O-MFPase remain significantly lower in cultured cells than in muscle. The ouabain-binding constant and the molecular activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase are equal in muscle and cultured cells. During ageing of human muscle the activity as well as the concentration of SR Ca(2+)-ATPase decrease. Thus the changes of the activities of the ATPases are caused by variations of the number of their molecules. Na+/K(+)-ATPase is localized in the periphery of fast- and slow-twitch muscle fibers and at the sarcomeric I-band. SR Ca(2+)-ATPase is predominantly confined to the I-band, whereas fast-twitch fibers are much more immunoreactive than slow-twitch fibers. The presence of cross-striation for Na+/K(+)-ATPase and SR Ca(2+)-ATPase in highly matured cultured muscle cells indicate the development and subcellular organization of a transverse tubular system and SR, respectively, which resembles the in vivo situation.     

Glucose uptake stimulatory effect of 4-hydroxypipecolic acid by increased GLUT 4 translocation in skeletal muscle cells

Peganum harmala Linn, commonly known as 'harmal' belonging to the family Zygophyllaceae, is one of the most important medicinal plants of India. In continuation of our drug development program on Indian medicinal plants we discovered antihyperglycemic activity in 4-hydroxypipecolic acid (4-HPA), isolated from the seed of P. harmala. Effect of 4-HPA on glucose uptake and glucose transporter-4 (GLUT-4) translocation was investigated in L6 skeletal muscle cell lines. Treatment with 4-HPA stimulated both glucose uptake and GLUT4 translocation from intracellular to cell surface in skeletal muscle cells in a concentration-dependent manner, which might be leading to antihyperglycemic effect.     

Eicosapentaenoic acid (20:5 n-3) increases fatty acid and glucose uptake in cultured human skeletal muscle cells

This study was conducted to evaluate the chronic effects of eicosapentaenoic acid (EPA) on fatty acid and glucose metabolism in human skeletal muscle cells. Uptake of [14C]oleate was increased >2-fold after preincubation of myotubes with 0.6 mM EPA for 24 h, and incorporation into various lipid classes showed that cellular triacylgycerol (TAG) and phospholipids were increased 2- to 3-fold compared with control cells. After exposure to oleic acid (OA), TAG was increased 2-fold. Insulin (100 nM) further increased the incorporation of [14C]oleate into all lipid classes for EPA-treated myotubes. Fatty acid beta-oxidation was unchanged, and complete oxidation (CO2) decreased in EPA-treated cells. Basal glucose transport and oxidation (CO2) were increased 2-fold after EPA, and insulin (100 nM) stimulated glucose transport and oxidation similarly in control and EPA-treated myotubes, whereas these responses to insulin were abolished after OA treatment. Lower concentrations of EPA (0.1 mM) also increased fatty acid and glucose uptake. CD36/FAT (fatty acid transporter) mRNA expression was increased after EPA and OA treatment compared with control cells. Moreover, GLUT1 expression was increased 2.5-fold by EPA, whereas GLUT4 expression was unchanged, and activities of the mitogen-activated protein kinase p38 and extracellular signal-regulated kinase were decreased after treatment with OA compared with EPA. Together, our data show that chronic exposure of myotubes to EPA promotes increased uptake and oxidation of glucose despite a markedly increased fatty acid uptake and synthesis of complex lipids.     

Glucose transport and metabolism in cultured human skin fibroblasts

Human skin fibroblast cultures, seeded at $\text{10}{}^5\text{cells}\text{5}\text{cm}$ plate and allowed to grow to confluence at approx. $\text{10}{}^6\text{cells}\text{5}\text{cm}$ plate, utilized a glycolytic mode of metabolism where the ratio of glucose utilized to lactate produced wa 0.62±0.05 (Zielke, R.H., Ozand, P.T., Tyldon, J.I., Sevdalian, D.A. and Cornblath, M. (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 4110–4114) (mean±S.E.). When the glucose in the medium was exhausted, the lactate produced during the highly glycolytic phase was then reutilized. In monolayer cultures that had been washed with phosphate-buffered saline, rates of glucose utilization were measured at 0.25 and 2 mM glucose by monitoring the appearance of ³H₂O from [5-³H]glucose. Rate of utilization for each concentration of glucose decreased markedly as the cultures became more confluent. This decrease also correlated with a reduced ability to transport glucose as measured by 2-deoxy-[³H]glucose uptake in washed monolayer cultures. In washed confluent culture of fibroblasts, glucose utilization was markedly decreased by the presence of pyruvate and lactate but not by glutamine. The respiratory inhibitors, rotenone and antimycin, did not increase the rate of glucose utilization except when added in combination with pyruvate. We conclude that cultured skin fibroblasts posses a highly glycolytic mode of metabolism but that this mode can become more oxidative in the presence of sufficient quantities of pyruvate and lactate.     

Glucose transporter expression in human skeletal muscle fibers

The present study was initiated to investigate GLUT-1 through -5 expression in developing and mature human skeletal muscle. To bypass the problems inherent in techniques using tissue homogenates, we applied an immunocytochemical approach, employing the sensitive enhanced tyramide signal amplification (TSA) technique to detect the localization of glucose transporter expression in human skeletal muscle. We found expression of GLUT-1, GLUT-3, and GLUT-4 in developing human muscle fibers showing a distinct expression pattern. 1) GLUT-1 is expressed in human skeletal muscle cells during gestation, but its expression is markedly reduced around birth and is further reduced to undetectable levels within the first year of life; 2) GLUT-3 protein expression appears at 18 wk of gestation and disappears after birth; and 3) GLUT-4 protein is diffusely expressed in muscle cells throughout gestation, whereas after birth, the characteristic subcellular localization is as seen in adult muscle fibers. Our results show that GLUT-1, GLUT-3, and GLUT-4 seem to be of importance during muscle fiber growth and development. GLUT-5 protein was undetectable in fetal and adult skeletal muscle fibers. In adult muscle fibers, only GLUT-4 was expressed at significant levels. GLUT-1 immunoreactivity was below the detection limit in muscle fibers, indicating that this glucose transporter is of minor importance for muscle glucose supply. Thus we hypothesize that GLUT-4 also mediates basal glucose transport in muscle fibers, possibly through constant exposure to tonal contraction and basal insulin levels.     

Effects of troglitazone on cellular differentiation, insulin signaling, and glucose metabolism in cultured human skeletal muscle cells

To determine the immediate effect of thiazolidinediones on human skeletal muscle, differentiated human myotubes were acutely (1 day) and myoblasts chronically (during the differentiation process) treated with troglitazone (TGZ). Chronic TGZ treatment resulted in loss of the typical multinucleated phenotype. The increase of muscle markers typically observed during differentiation was suppressed, while adipocyte markers increased markedly. Chronic TGZ treatment increased insulin-stimulated phosphatidylinositol (PI) 3-kinase activity and membranous protein kinase B/Akt (PKB/Akt) Ser-473 phosphorylation more than 4-fold. Phosphorylation of p42/44 mitogen-activated protein kinase (42/44 MAPK/ERK) was unaltered. Basal glucose uptake as well as both basal and insulin-stimulated glycogen synthesis increased approximately 1.6- and approximately 2.5-fold after chronic TGZ treatment, respectively. A 2-fold stimulation of PI 3-kinase but no other significant TGZ effect was found after acute TGZ treatment. In conclusion, chronic TGZ treatment inhibited myogenic differentiation of that human muscle while inducing adipocyte-specific gene expression. The effects of chronic TGZ treatment on basal glucose transport may in part be secondary to this transdifferentiation. The enhancing effect on PI 3-kinase and PKB/Akt involved in both differentiation and glycogen synthesis appears to be pivotal in the cellular action of TGZ.     

Glucose uptake after immobilization in fast and slow skeletal muscle in rats

1. 3-O-methylglucose uptake was studied after immobilization in rat extensor digitorum longus (EDL) and soleus (Sol) muscles. 2. The immobilization of the ankle was done in one of extreme positions by plaster casts. 3. In both positions, 3-O-methylglucose uptake in EDL increased and that in Sol decreased after immobilization. 4. When immobilization was released uptake returned to control level. 5. The change in uptake after immobilization and after release of immobilization was earlier in Sol.     

Glucose uptake and glucose transporter proteins in skeletal muscle from undernourished rats

Undernutrition in rats impairs secretion of insulin but maintains glucose normotolerance, because muscle tissue presents an increased insulin-induced glucose uptake. We studied glucose transporters in gastrocnemius muscles from food-restricted and control anesthetized rats under basal and euglycemic hyperinsulinemic conditions. Muscle membranes were prepared by subcellular fractionation in sucrose gradients. Insulin-induced glucose uptake, estimated by a 2-deoxyglucose technique, was increased 4- and 12-fold in control and food-restricted rats, respectively. Muscle insulin receptor was increased, but phosphotyrosine-associated phosphatidylinositol 3-kinase activity stimulated by insulin was lower in undernourished rats, whereas insulin receptor substrate-1 content remained unaltered. The main glucose transporter in the muscle, GLUT-4, was severely reduced albeit more efficiently translocated in response to insulin in food-deprived rats. GLUT-1, GLUT-3, and GLUT-5, minor isoforms in skeletal muscle, were found increased in food-deprived rats. The rise in these minor glucose carriers, as well as the improvement in GLUT-4 recruitment, is probably insufficient to account for the insulin-induced increase in the uptake of glucose in undernourished rats, thereby suggesting possible changes in other steps required for glucose metabolism.