Search

Thujone is thought to be the main constituent of medicinal herbs that have antidiabetic properties. Therefore, we examined whether thujone ameliorated palmitate-induced insulin resistance in skeletal muscle. Soleus muscles were incubated for [less than or equal to] 12 h without or with palmitate (2 mM). Thujone (0.01 mg/ml), in the presence of palmitate, was provided in the last 6 h of incubation. Palmitate oxidation, AMPK/ acetyl-CoA carboxylase (ACC) phosphorylation and insulin-stimulated glucose transport, plasmalemmal GLUT4, and AS 160 phosphorylation were examined at 0, 6, and 12 h. Palmitate treatment for 12 h reduced fatty acid oxidation (-47%), and insulin-stimulated glucose transport (-71%), GLUT4 translocation (-40%), and AS160 phosphorylation (-26%), but it increased AMPK (+51%) and ACC phosphorylations (+44%). Thujone (6-12 h) fully rescued palmitate oxidation and insulin-stimulated glucose transport, but only partially restored GLUT4 translocation and AS 160 phosphorylation, raising the possibility that an increased GLUT4 intrinsic activity may also have contributed to the restoration of glucose transport. Thujone also further increased AMPK phosphorylation but had no further effect on ACC phosphorylation. Inhibition of AMPK phosphorylation with adenine 9-[beta]-D-arabinofuranoside (Ara) (2.5 mM) or compound C (50 [micro]M) inhibited the thujone-induced improvement in insulin-stimulated glucose transport, GLUT4 translocation, and AS 160 phosphorylation. In contrast, the thujone-induced improvement in palmitate oxidation was only slightly inhibited ([less than or equal to] 20%) by Ara or compound C. Thus, while thujone, a medicinal herb component, rescues palmitate-induced insulin resistance in muscle, the improvement in fatty acid oxidation cannot account for this thujone-mediated effect. Instead, the rescue of palmitate-induced insulin resistance appears to occur via an AMPK-dependent mechanism involving partial restoration of insulinstimulated GLUT4 translocation. palmitate oxidation; glucose transport; adenosine 5'-monophosphate kinase; acetyl-CoA carboxylase doi: 10.1152/ajpregu.00216.2010.

Agonists targeting the nuclear receptor peroxisome proliferator-activated receptors (PPAR)-[delta] may be potential therapeutic agents for insulin-resistant related conditions, as they may be able to stimulate fatty acid (FA) oxidation and attenuate the accumulation of harmful lipid species in skeletal muscle. Several reports have demonstrated that PPAR-[delta] agonists improve whole body insulin sensitivity. However, whether these agonists exert their direct effects on glucose and FA metabolism in skeletal muscle, and specifically with different fiber types, is unknown. This study was undertaken to determine the effects of oral treatment with the PPAR-[delta] agonist, GW 501516, in conjunction with the administration of a high-saturated-fat diet on insulin-stimulated glucose transport in isolated oxidative (soleus) and glycolytic (epitrochlearis) rodent skeletal muscle in vitro. High-fat feeding significantly decreased maximal insulin-stimulated glucose transport in soleus, but not epitrochlearis muscle, and was associated with increased skeletal muscle diacylglycerol and ceramide content. Unexpectedly, treatment with the PPAR-[delta] agonist significantly reduced insulin-stimulated glucose transport in both soleus and epitrochlearis muscles, regardless of dietary fat content. The reduction in insulin-stimulated glucose transport induced by the agonist was associated with large increases in total muscle fatty acid translocase (FAT)/CD36protein content, but not diacylglycerol or ceramide contents. Agonist treatment did not alter the protein content of PPAR-[delta], GLUT4, or insulin-signaling proteins (IRS-1, p85 PI3-K, Akt). Agonist treatment led to a small, but significant increase, in the oxidative capacity of glycolytic but not oxidative muscle. We propose that chronic treatment with the PPAR-[delta] agonist GW 501516 may induce or worsen insulin resistance in rodent skeletal muscle by increasing the capacity for FA transport across the sarcolemma without a sufficient compensatory increase in FA oxidation. However, an accumulation of diacylglycerol and ceramide, while associated with diet-induced insulin resistance, does not appear to be responsible for the agonist-induced reduction in insulin-stimulated glucose transport. fatty acid transport; oxidative capacity; diacylglycerol; ceramide; insulin resistance; high-fat diet doi: 10.1152/ajpregu.00431.2009.

The peroxisome proliferator-activated receptor-[gamma] (PPAR[gamma]) coactivator-1[alpha] (PGC-l[alpha]) is a major regulator of exercise-induced phenotypic adaptation and substrate utilization. We provide an overview of 1) the role of PGC-1[alpha] in exercise-mediated muscle adaptation and 2) the possible insulin-sensitizing role of PGC-1[alpha]. To these ends, the following questions are addressed. 1) How is PGC-1[alpha] regulated, 2) what adaptations are indeed dependent on PGC-1[alpha] action, 3) is PGC-1[alpha] altered in insulin resistance, and 4) are PGC-1[alpha]knockout and -transgenic mice suitable models for examining therapeutic potential of this coactivator? In skeletal muscle, an orchestrated signaling network, including [Ca.sup.2+]-dependent pathways, reactive oxygen species (ROS), nitric oxide (NO), AMP-dependent protein kinase (AMPK), and p38 MAPK, is involved in the control of contractile protein expression, angiogenesis, mitochondrial biogenesis, and other adaptations. However, the p38[gamma] MAPK/PGC-1[alpha] regulatory axis has been confirmed to be required for exercise-induced angiogenesis and mitochondrial biogenesis but not for fiber type transformation. With respect to a potential insulin-sensitizing role of PGC-1[alpha], human studies on type 2 diabetes suggest that PGC-1[alpha] and its target genes are only modestly downregulated ([less than or equal to] 34%). However, studies in PGC-1[alpha]-knockout or PGC-1[alpha]-transgenic mice have provided unexpected anomalies, which appear to suggest that PGC-1[alpha] does not have an insulin-sensitizing role. In contrast, a modest (~25%) upregulation of PGC-1[alpha], within physiological limits, does improve mitochondrial biogenesis, fatty acid oxidation, and insulin sensitivity in healthy and insulin-resistant skeletal muscle. Taken altogether, there is substantial evidence that the p38[gamma], MAPK-PGC-1[alpha] regulatory axis is critical for exercise-induced metabolic adaptations in skeletal muscle, and strategies that upregulate PGC-1[alpha], within physiological limits, have revealed its insulin-sensitizing effects. endurance exercise; angiogenesis; mitochondria; fatty acid metabolism; glucose doi: 10.1152/ajpendo.00755.2009.

Fatty acid oxidation is highly regulated in skeletal muscle and involves several sites of regulation, including the transport of fatty acids across both the plasma and mitochondrial membranes. Transport across these membranes is recognized to be primarily protein mediated, limited by the abundance of fatty acid transport proteins on the respective membranes. In recent years, evidence has shown that fatty acid transport proteins move in response to acute and chronic perturbations; however, in human skeletal muscle the localization of fatty acid transport proteins in response to training has not been examined. Therefore, we determined whether high-intensity interval training (HIIT) increased total skeletal muscle, sarcolemmal, and mitochondrial membrane fatty acid transport protein contents. Ten untrained females (22 [+ or -] 1 yr, 65 [+ or -] 2 kg; [Vo.sub.2peak]: 2.8 [+ or -] 0.1 l/min) completed 6 wk of HIIT, and biopsies from the vastus lateralis muscle were taken before training, and following 2 and 6 wk of HIIT. Training significantly increased maximal oxygen uptake at 2 and 6 wk (3.1 [+ or -] 0.1, 3.3 [+ or -] 0.1 1/min). Training for 6 wk increased FAT/CD36 at the whole muscle (10%) and mitochondrial levels (51%) without alterations in sarcolemmal content. Whole muscle plasma membrane fatty acid binding protein (FABPpm) also increased (48%) after 6 wk of training, but in contrast to FAT/CD36, sarcolemmal FABPpm increased (23%), whereas mitochondrial FABPpm was unaltered. The changes on sarcolemmal and mitochondrial membranes occurred rapidly, since differences ([less than or equal to] 2 wk) were not observed between 2 and 6 wk. This is the first study to demonstrate that exercise training increases fatty acid transport protein content in whole muscle (FAT/CD36 and FABPpm) and sarcolemmal (FABPpm) and mitochondrial (FAT/CD36) membranes in human skeletal muscle of females. These results suggest that increases in skeletal muscle fatty acid oxidation following training are related in part to changes in fatty acid transport protein content and localization. fatty acid translocase; plasma membrane fatty acid-binding protein; cellular localization; fat oxidation doi: 10.1152/ajpendo.00073.2010.

Insulin and contraction stimulate both cardiac glucose and long-chain fatty acid (LCFA) uptake via translocation of the substrate transporters GLUT4 and CD36, respectively, from intracellular compartments to the sarcolemma. Little is known about the role of vesicular trafficking elements in insulin- and contraction-stimulated glucose and LCFA uptake in the heart, especially whether certain trafficking elements are specifically involved in GLUT4 versus CD36 translocation. Therefore, we studied the role of coat proteins, actin- and microtubule-filaments and endosomal pH on glucose and LCFA uptake into primary cardiomyocytes under basal conditions and during stimulation with insulin or oligomycin (contraction-like AMP-activated protein kinase activator). Inhibition of coat protein targeting to Golgi/endosomes decreased insulin/oligomycin-stimulated glucose (-42%/-51%) and LCFA (-39%/-68%) uptake. Actin disruption decreased insulin/oligomycin-stimulated glucose uptake (-41%/-75%), while not affecting LCFA uptake. Microtubule disruption did not affect substrate uptake under any condition. Endosomal alkalinization increased basal sarcolemmal CD36 (2-fold), but not GLUT4, content, and concomitantly decreased basal intracellular membrane GLUT4 and CD36 content (-60% and -62%, respectively), indicating successful CD36 translocation and incomplete GLUT4 translocation. Additionally, endosomal alkalinization elevated basal LCFA uptake (1.4-fold) in a nonadditive manner to insulin/oligomycin, and decreased insulin/oligomycin-stimulated glucose uptake (-32%/-68%). In conclusion, 1) CD36 translocation, just like GLUT4 translocation, is a vesicle-mediated process depending on coat proteins, and 2) GLUT4 and CD36 trafficking are differentially dependent on endosomal pH and actin filaments. The latter conclusion suggests novel strategies to alter cardiac substrate preference as part of metabolic modulation therapy. cardiac substrate uptake; CD36 translocation; vesicular trafficking; coat proteins doi: 10.1152/ajpcell.00334.2009.

We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance via increased palmitate oxidation, reductions in intramuscular lipids, and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2 mM, 0-18 h) induced insulin resistance in soleus muscle. From 12-18 h, palmitate was removed or AICAR or leptin was provided while 2 mM palmitate was maintained. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulinstimulated glucose transport, plasmalemmal GLUT4, and Akt and AS 160 phosphorylation were examined at 0, 6, 12, and 18 h. Palmitate treatment (12 h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol + 11%, total ceramide +18%, C16:0 ceramide +60%) and AMPK phosphorylation (+118%), whereas it reduced fatty acid oxidation (-60%) and insulin-stimulated glucose transport (-70%), GLUT4 translocation (-50%), and AS 160 phosphorylation (-40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR and leptin treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation and insulin-stimulated glucose transport, GLUT4 translocation, and AS 160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments, AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS160 phosphorylation were highly correlated (r = 0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2 mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, and ceramide), the rapid AICAR- and leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulinstimulated AS 160 phosphorylation and GLUT4 translocation. ceramide; diacylgycerol; triacylglycerol; palmitate; glucose transporter 4; Akt doi: 10.1152/ajpendo.90908.2008.

We examined fatty acid transporters, transport, and metabolism in hearts and red and white muscles of lean and insulin-resistant (week 6) and type 2 diabetic (week 24) Zucker diabetic fatty (ZDF) rats. Cardiac fatty acid transport was similar in lean and ZDF hearts at week 6 but was reduced at week 24 (-40%) in lean but not ZDF hearts. Red muscle of ZDF rats exhibited an early susceptibility to upregulation (+66%) of fatty acid transport at week 6 that was increased by 50% in lean and ZDF rats at week 24 but remained 44% greater in red muscle of ZDF rats. In white muscle, no differences were observed in fatty acid transport between groups or from week 6 to week 24. In all tissues (heart and red and white muscle), FAT/CD36 protein and plasmalemmal content paralleled the changes in fatty acid transport. Triacylglycerol content in red and white muscles, but not heart, in lean and ZDF rats correlated with fatty acid transport (r = 0.91) and sarcolemmal FAT/CD36 (r = 0.98). Red and white muscle fatty acid oxidation by isolated mitochondria was not impaired in ZDF rats but was reduced by 18-24% in red muscle of lean rats at week 24. Thus, in red, but not white, muscle of insulin-resistant and type 2 diabetic animals, a marked upregulation in fatty acid transport and intramuscular triacylglycerol was associated with increased levels of FAT/CD36 expression and plasmalemmal content. In heart, greater rates of fatty acid transport and FAT/CD36 in ZDF rats (week 24) were attributable to the inhibition of age-related reductions in these parameters. However, intramuscular triacylglycerol did not accumulate in hearts of ZDF rats. Thus insulin resistance and type 2 diabetes are accompanied by tissue-specific differences in FAT/CD36 and fatty acid transport and metabolism. Upregulation of fatty acid transport increased red muscle, but not cardiac, triacylglycerol accumulation. White muscle lipid metabolism dysregulation was not observed. plasma membrane-associated fatty acid-binding protein; FAT/CD36; GLUT4; mitochondria; giant vesicles doi: 10.1152/ajpregu.90820.2008

The plasma membrane fatty acid transport protein FAT/CD36 is also present at the mitochondria, where it may contribute to the regulation of fatty acid oxidation, although this has been challenged. Therefore, we have compared enzyme activities and rates of mitochondrial palmitate oxidation in muscles of wild-type (WT) and FAT/CD36 knockout (KO) mice, at rest and after muscle contraction. In WT and KO mice, carnitine palmitoyltransferase-I, citrate synthase, and [beta]-hydroxyacyl-CoA dehydrogenase activities did not differ in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria of WT and FAT/CD36 KO mice. Basal palmitate oxidation rates were lower (P < 0.05) in KO mice (SS -18%; IMF -13%). Muscle contraction increased fatty acid oxidation (+18%) and mitochondrial FAT/CD36 protein (+16%) in WT IMF but not in WT SS, or in either mitochondrial subpopulation in KO mice. This revealed that the difference in IMF mitochondrial fatty acid oxidation between WT and KO mice can be increased ~2.5-fold from 13% under basal conditions to 35% during muscle contraction. The FAT/CD36 inhibitor sulfo-N-succinimidyl oleate (SSO), inhibited palmitate transport across the plasma membrane in WT, but not in KO mice. In contrast, SSO bound to mitochondrial membranes and reduced palmitate oxidation rates to a similar extent in both WT and KO mitochondria (~80%; P < 0.05). In addition, SSO reduced state III respiration with succinate as a substrate, without altering mitochondrial coupling (P/O ratios). Thus, while SSO inhibits FAT/CD36-mediated palmitate transport at the plasma membrane, SSO has undefined effects on mitochondria. Nevertheless, the KO animals reveal that FAT/CD36 contributes to the regulation of mitochondrial fatty acid oxidation, which is especially important for meeting the increased metabolic demands during muscle contraction. subsarcolemmal; intermyofibrillar; muscle contraction doi: 10.1152/ajpregu.91021.2008.

Intramuscular triacylglycerol (IMTG) accumulation in obesity has been attributed to increased fatty acid transport and/or to alterations in mitochondrial fatty acid oxidation. Alternatively, an imbalance in these two processes may channel fatty acids into storage. Therefore, in red and white muscles of lean and obese Zucker rats, we examined whether the increase in IMTG accumulation was attributable to an increased rate of fatty acid transport rather than alterations in subsarcolemmal (SS) or intermyofibrillar (IMF) mitochondrial fatty acid oxidation. In obese animals selected parameters were upregulated, including palmitate transport (red: +100%; white: +51%), plasmalemmal FAT/CD36 (red: +116%; white: +115%; not plasmalemmal FABPpm, FATP1, or FATP4), IMTG concentrations (red: ~2-fold; white: ~4-fold), and mitochondrial content (red +30%). Selected mitochondrial parameters were also greater in obese animals, namely, palmitate oxidation (SS red: +91%; SS white: +26%; not IMF mitochondria), FAT/CD36 (SS: +65%; IMF: +65%), citrate synthase (SS: +19%), and [beta]-hydroxyacyl-CoA dehydrogenase activities (SS: +20%); carnitine palmitoyltransferase-I activity did not differ. A comparison of lean and obese rat muscles revealed that the rate of change in IMTG concentration was eightfold greater than that of fatty acid oxidation (SS mitochondria), when both parameters were expressed relative to fatty transport. Thus fatty acid transport, esterification, and oxidation (SS mitochondria) are upregulated in muscles of obese Zucker rats, with these effects being most pronounced in red muscle. The additional fatty acid taken up is channeled primarily to esterification, suggesting that upregulation in fatty acid transport as opposed to altered fatty acid oxidation is the major determinant of intramuscular lipid accumulation. fatty acid translocase; fatty acid transport proteins 1 and 4; plasma membrane-associated fatty acid-binding protein; glucose transport

High-fat (HF) diets can induce insulin resistance (IR) by altering skeletal muscle lipid metabolism. An imbalance between fatty acid (FA) uptake and oxidation results in intramuscular lipid accumulation, which can impair the insulin-signaling cascade. Adiponectin (Ad) is an insulin-sensitizing adipokine known to stimulate skeletal muscle FA oxidation and reduce lipid accumulation. Evidence of Ad resistance has been shown in obesity and following chronic HF feeding and may contribute to lipid accumulation observed in these conditions. Whether Ad resistance precedes and is associated with the development of IR is unknown. We conducted a time course HF feeding trial for 3 days, 2 wk, or 4 wk to determine the onset of Ad resistance and identify the ensuing changes in lipid metabolism and insulin signaling leading to IR in skeletal muscle. Ad stimulated FA oxidation (+28%, P [less than or equal to] 0.05) and acetyl-CoA carboxylase phosphorylation (+34%, P [less than or equal to] 0.05) in control animals but failed to do so in any HF-fed group (i.e., as early as 3 days). By 2 wk, plasma membrane FA transporters and intramuscular diacylglycerol (DAG) and ceramide were increased, and insulin-stimulated phosphorylation of both protein kinase B and protein kinase B substrate 160 was blunted compared with control animals. After 4 wk of HF feeding, maximal insulin-stimulated glucose transport was impaired compared with control. Taken together, our results demonstrate that an early loss of Ad's stimulatory effect on FA oxidation precedes an increase in plasmalemmal FA transporters and the accumulation of intramuscular DAG and ceramide, blunted insulin signaling, and ultimately impaired maximal insulin-stimulated glucose transport in skeletal muscle induced by HF diets. adipokines; adenosine 5'-monophosphate-activated protein kinase; acetyl-coenzyme A carboxylase; fatty acid translocase/CD36; insulin signaling AdipoRl; diacylglycerol; ceramide

Decreasing muscle phosphagen content through dietary administration of the creatine analog [beta]-guanidinopropionic acid ([beta]-GPA) improves skeletal muscle oxidative capacity and resistance to fatigue during aerobic exercise in rodents, similar to that observed with endurance training. Surprisingly, the effect of [beta]-GPA on muscle substrate metabolism has been relatively unexamined, with only a few reports of increased muscle GLUT4 content and insulin-stimulated glucose uptake/clearance in rodent muscle. The effect of chronically decreasing muscle phophagen content on muscle fatty acid (FA) metabolism (transport, oxidation, esterification) is virtually unknown. The purpose of the present study was to examine changes in muscle substrate metabolism in response to 8 wk feeding of [beta]-GPA. Consistent with other reports, [beta]-GPA feeding decreased muscle ATP and total creatine content by ~50 and 90%, respectively. This decline in energy charge was associated with simultaneous increases in both glucose (GLUT4; +33 to 45%, P < 0.01) and FA (FAT/CD36; +28 to 33%, P < 0.05) transporters in the sarcolemma of red and white muscle. Accordingly, we also observed significant increases in insulin-stimulated glucose transport (+47%, P < 0.05) and AICAR-stimulated palmitate oxidation (+77%, P < 0.01) in the soleus muscle of [beta]-GPA-fed animals. Phosphorylation of AMPK (+20%, P < 0.05), but not total protein, was significantly increased in both fiber types in response to muscle phosphagen reduction. Thus the content of sarcolemmal transporters for both of the major energy substrates for muscle increased in response to a reduced energy charge. Increased phosphorylation of AMPK may be one of the triggers for this response. [beta]-guanidinopropionic acid; rat; skeletal muscle; energy charge; substrate metabolism

Reduced skeletal muscle mitochondrial content and fatty acid oxidation are associated with obesity and insulin resistance. Although the exact mechanisms remain elusive, this may result from impaired mitochondrial biogenesis or reductions in the mitochondrial reticulum network. Therefore, the purpose of this study was to determine whether the protein contents of various transcription factors, including PGC-l[alpha] and PGC-I[beta] and proteins associated with mitochondrial fusion events, were reduced in skeletal muscle of nine obese (BMI = 37.6 [+ or -] 2.2 kg/[m.sup.-2]) compared with nine age-matched lean (BMI = 23.3 [+ or -] 0.7 kg/[m.sup.-2]) women. The protein contents of PGC-1[alpha], PGC-1[beta], PPAR[alpha], and tFAM were not reduced with obesity. In contrast, PPAR[gamma] was increased (+22%, P < 0.05) with obesity, and there was a trend toward an increase (+31%, P = 0.13) in PPAR[delta]/[beta]. In lean individuals, PGC-1[alpha] protein correlated with citrate synthase (CS; r = 0.67) and rates of palmitate oxidation (r = 0.87), whereas PGC-1[beta] correlated with PPAR[gamma] (r = 0.90), PPAR[delta]/beta] (r = 0.63), and cytochrome c oxidase IV (COX-IV; r = 0.63). In obese individuals, the relationship between PGC-l[alpha] and CS was maintained (r = 0.65); however, the associations between PGC-l[alpha] and palmitate oxidation (r = -0.38) and PGC-1[beta] with PPAR[gamma] (r = 0.14), PPAR[delta]/[beta] (r = 0.21), and COX-IV (r = 0.01) were lost. In addition, mitofusin-1 (MFN-1), MFN-2, and dynamin-related protein-1 (DRP-1) total protein contents were not altered with obesity (P > 0.05). These data suggest that altered regulation, and not reductions in the protein contents of transcription factors, is associated with insulin resistance. Also, it does not appear that alterations in the proteins associated with mitochondrial network formation and degradation can account for the observed decrease in mitochondrial content. mitochondria; transcription factors; fatty acid oxidation; mitochondrial fusion/fission doi: 10.1152/ajpendo.00726.2007.

We examined, in soleus muscle, the effects of prolonged palmitate exposure (0, 6, 12, 18 h) on insulin-stimulated glucose transport, intramuscular lipid accumulation and oxidation, activation of selected insulin-signaling proteins, and the insulin-stimulated translocation of GLUT4. Insulin-stimulated glucose transport was progressively reduced after 6 h (-33%), 12 h (-66%), and 18 h (-89%) of palmitate exposure. These decrements were closely associated with concurrent reductions in palmitate oxidation at 6 h (-40%), 12 h (-60%), and 18 h (-67%). In contrast, intramuscular ceramide (+ 24%) and diacylglycerol (+32%) concentrations, insulin-stimulated AS160 (-36%) and PRAS40 (-33%) phosphorylations, and Akt (-40%), PKC0 (-50%), and GLUT4 translocation (-40%) to the plasma membrane were all maximally altered within the first 6 h of palmitate treatment. No further changes were observed in any of these parameters after 12 and 18 h of palmitate exposure. Thus, the intrinsic activity of GLUT4 was markedly reduced after 12 and 18 h of palmitate treatment. During this reduced GLUT4 intrinsic activity phase at 12 and 18 h, the reduction in glucose transport was twofold greater compared with the early phase ([less than or equal to]6 h), when only GLUT4 translocation was impaired. Our study indicates that palmitate-induced insulin resistance is provoked by two distinct mechanisms: 1) an early phase ([less than or equal to]6 h), during which lipid-mediated impairments in insulin signaling and GLUT4 translocation reduce insulin-stimulated glucose transport, followed by 2) a later phase (12 and 18 h), during which the intrinsic activity of GLUT4 is markedly reduced independently of any further alterations in intramuscular lipid accumulation, insulin signaling and GLUT4 translocation. glucose transport; glucose transporter 4; ceramide; diacylglycerol; Akt; AS160; protein kinase C[xi]/[lambda]; protein kinase C[theta]; palmitate oxidation

Leptin-induced increases in insulin sensitivity are well established and may be related to the effects of leptin on lipid metabolism. However, the effects of leptin on the levels of lipid metabolites implicated in pathogenesis of insulin resistance and the effects of leptin on lipid-induced insulin resistance are unknown. The current study addressed in rats the effects of hyperleptinemia (HL) on insulin action and markers of skeletal muscle (SkM) lipid metabolism in the absence or presence of acute hyperlipidemia induced by an infusion of a lipid emulsion. Compared with controls (CONT), HL increased insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamp (~15%), and increased SkM Akt (~30%) and glycogen synthase kinase 3[alpha] (~52%) phosphorylation. These improvements in insulin action were associated with decreased SkM triglycerides (TG; ~61%), elevated ceramides (~50%), and similar diacylglycerol (DAG) levels in HL compared with CONT. Acute hyperlipidemia in CONT decreased insulin sensitivity (~25%) and increased SkM DAG (~33%) and ceramide (~60%) levels. However, hyperlipidemia did not induce insulin resistance or SkM DAG and ceramide accumulation in HL. SkM total fatty acid transporter CD36, plasma membrane fatty acid binding protein, acetyl Co-A carboxylase phosphorylation, and fatty acid oxidation were similar in HL compared with CONT. However, HL decreased SkM protein kinase CO (PKC0), a kinase implicated in mediating the detrimental effects of lipids on insulin action. We conclude that increases in insulin sensitivity induced by HL are associated with decreased levels of SkM TG and PKC[theta] and increased SkM insulin signaling, but not with decreases in other lipid metabolites implicated in altering SkM insulin sensitivity (DAG and ceramide). Furthermore, insulin resistance induced by an acute lipid infusion is prevented by HL. insulin sensitivity; PKC; Akt; diacylglycerol; ceramide; triglyceride

We examined, in muscle of lean and obese Zucker rats, basal, insulin-induced, and contraction-induced fatty acid transporter translocation and fatty acid uptake, esterification, and oxidation. In lean rats, insulin and contraction induced the translocation of the fatty acid transporter FAT/CD36 (43 and 41%, respectively) and plasma membrane-associated fatty acid binding protein (FABPpm; 19 and 60%) and increased fatty acid uptake (63 and 40%, respectively). Insulin and contraction increased lean muscle palmitate esterification and oxidation 72 and 61%, respectively. In obese rat muscle, basal levels of sarcolemmal FAT/CD36 (+33%) and FABPpm (+14%) and fatty acid uptake (+30%) and esterification (+32%) were increased, whereas fatty acid oxidation was reduced (-28%). Insulin stimulation of obese rat muscle increased plasmalemmal FABPpm (+15%) but not plasmalemmal FAT/CD36, blunted fatty acid uptake and esterification, and failed to reduce fatty acid oxidation. In contracting obese rat muscle, the increases in fatty acid uptake and esterification and FABPpm translocation were normal, but FAT/CD36 translocation was impaired and fatty acid oxidation was blunted. There was no relationship between plasmalemmal fatty acid transporters and palmitate partitioning. In conclusion, fatty acid metabolism is impaired at several levels in muscles of obese Zucker rats; specifically, they are 1) insulin resistant with respect to FAT/CD36 translocation and fatty acid uptake, esterification, and oxidation and 2) contraction resistant with respect to fatty acid oxidation and FAT/CD36 translocation, but, conversely, 3) obese muscles are neither insulin nor contraction resistant at the level of FABPpm. Finally, 4) there is no evidence that plasmalemmal fatty acid transporters contribute to the channeling of fatty acids to specific metabolic destinations within the muscle. insulin; contraction; hindlimb perfusion; palmitate; oxidation; esterification

Derangements in skeletal muscle fatty acid (FA) metabolism associated with insulin resistance in obesity appear to involve decreased FA oxidation and increased accumulation of lipids such as ceramides and diacylglycerol (DAG). We investigated potential lipid-related mechanisms of metformin (Met) and/or exercise for blunting the progression of hyperglycemia/ hyperinsulinemia and skeletal muscle insulin resistance in female Zucker diabetic fatty rats (ZDF), a high-fat (HF) diet-induced model of diabetes. Lean and ZDF rats consumed control or HF diet (48 kcal %fat) alone or with Met (500 mg/kg), with treadmill exercise, or with both exercise and Met interventions for 8 wk. HF-fed ZDF rats developed hyperglycemia (mean: 24.4 [+ or -] 2.1 mM), impairments in muscle insulin-stimulated glucose transport, increases in the FA transporter FAT/CD36, and increases in total ceramide and DAG content. The development of hyperglycemia was significantly attenuated with all interventions, as was skeletal muscle FAT/CD36 abundance and ceramide and DAG content. Interestingly, improvements in insulin-stimulated glucose transport and increased GLUT4 transporter expression in isolated muscle were seen only in conditions that included exercise training. Reduced FA oxidation and increased triacylglycerol synthesis in isolated muscle were observed with all ZDF rats compared with lean rats (P < 0.01) and were unaltered by therapeutic intervention. However, exercise did induce modest increases in peroxisome proliferator-activated receptor-[gamma] coactivator-1[alpha], citrate synthase, and [beta]-hydroxyacyl-CoA dehydrogenase activity. Thus reduction of skeletal muscle FAT/CD36 and content of ceramide and DAG may be important mechanisms by which exercise training blunts the progression of diet-induced insulin resistance in skeletal muscle. Zucker diabetic fatty rat; lipid metabolism; glucose transport; diacylglycerol; ceramide; adenosine 5'-monophosphate-activated protein kinase doi:10.1152/ajpendo.00677.2006

Fatty acid translocase (FAT)/CD36 is involved in regulating the uptake of long-chain fatty acids into muscle cells. However, the contribution of FAT/CD36 to fatty acid metabolism remains unknown. We examined the role of FAT/CD36 on fatty acid metabolism in perfused muscles (soleus and red and white gastrocnemius) of wild-type (WT) and FAT/CD36 null (KO) mice. In general, in muscles of KO mice, 1) insulin sensitivity and 5-aminoimidazole-4-carboxamide-1-[beta]-D-ribofuranoside (AICAR) sensitivity were normal, 2) key enzymes involved in fatty acid oxidation were altered minimally or not at all, and 3) except for an increase in soleus muscle FATP1 and FATP4, these fatty acid transporters were not altered in red and white gastrocnemius muscles, whereas plasma membrane-bound fatty acid binding protein was not altered in any muscle. In KO muscles perfused under basal conditions (i.e., no insulin, no AICAR), rates of hindquarter fatty acid oxidation were reduced by 26%. Similarly, in oxidative but not glycolytic muscles, the basal rates of triacylglycerol esterification were reduced by 40%. When muscles were perfused with insulin, the net increase in fatty acid esterification was threefold greater in the oxidative muscles of WT mice compared with the oxidative muscles in KO mice. With AICAR-stimulation, the net increase in fatty acid oxidation by hindquarter muscles was 3.7-fold greater in WT compared with KO mice. In conclusion, the present studies demonstrate that FAT/CD36 has a critical role in regulating fatty acid esterification and oxidation, particularly during stimulation with insulin or AICAR. perfusion; palmitate; esterification; oxidation; fatty transport proteins 1 and 2; plasma membrane-bound fatty acid binding protein; 5-amino-imidazole-4-carboxamide-1-[beta]-D-ribofuranoside doi:10.1152/ajpendo.00579.2006.

A reduction in fatty acid oxidation has been associated with lipid accumulation and insulin resistance in the skeletal muscle of obese individuals. We examined whether this decrease in fatty acid oxidation was attributable to a reduction in muscle mitochondrial content and/or a dysfunction in fatty acid oxidation within mitochondria obtained from skeletal muscle of age-matched, lean [body mass index (BMI) = 23.3 [+ or -] 0.7 kg/[m.sup.2]] and obese women (BMI = 37.6 [+ or -] 2.2 kg/[m.sup.2]). The mitochondrial marker enzymes citrate synthase (-34%), [beta]-hydroxyacyl-CoA dehydrogenase (-17%), and cytochrome c oxidase (-32%) were reduced (P < 0.05) in obese participants, indicating that mitochondrial content was diminished. Obesity did not alter the ability of isolated mitochondria to oxidize palmitate; however, fatty acid oxidation was reduced at the whole muscle level by 28% (P < 0.05) in the obese. Mitochondrial fatty acid translocase (FAT/ CD36) did not differ in lean and obese individuals, but mitochondrial FAT/CD36 was correlated with mitochondrial fatty acid oxidation (r = 0.67, P < 0.05). We conclude that the reduction in fatty acid oxidation in obese individuals is attributable to a decrease in mitochondrial content, not to an intrinsic defect in the mitochondria obtained from skeletal muscle of obese individuals. In addition, it appears that mitochondrial FAT/CD36 may be involved in regulating fatty acid oxidation in human skeletal muscle. obesity; mitochondria; fatty acid translocase/CD36; transport proteins doi:10.1152/ajpendo.00639.2006.

Skeletal muscle primarily relies on carbohydrate (CHO) for energy provision during high-intensity exercise. We hypothesized that sprint interval training (SIT), or repeated sessions of high-intensity exercise, would induce rapid changes in transport proteins associated with CHO metabolism, whereas changes in skeletal muscle fatty acid transporters would occur more slowly. Eight active men (22 [+ or -] 1 yr; peak oxygen uptake = 50 [+ or -] 2 [ml x [kg.sup.-1] x [min.sup.-1]) performed 4-6 x 30 s all-out cycling efforts with 4-min recovery, 3 days/wk for 6 wk. Needle muscle biopsy samples (vastus lateralis) were obtained before training (Pre), after 1 and 6 wk of SIT, and after 1 and 6 wk of detraining. Muscle oxidative capacity, as reflected by the protein content of cytochrome c oxidase subunit 4 (COX4), increased by ~35% after 1 wk of SIT and remained higher compared with Pre, even after 6 wk of detraining (P < 0.05). Muscle GLUT4 content increased after 1 wk of SIT and remained ~20% higher compared with baseline during detraining (P < 0.05). The monocarboxylate tranporter (MCT) 4 was higher after 1 and 6 wk of SIT compared with Pre, whereas MCT1 increased after 6 wk of training and remained higher after 1 wk of detraining (P < 0.05). There was no effect of training or detraining on the muscle content of fatty acid translocase (FAT/CD36) or plasma membrane associated fatty acid binding protein (FABPpm) (P > 0.05). We conclude that short-term SIT induces rapid increases in skeletal muscle oxidative capacity but has divergent effects on proteins associated with glucose, lactate, and fatty acid transport. GLUT4; monocarboxylate transporters; fatty acid translocase; plasma membrane fatty acid binding protein doi:10.1152/ajpregu.00503.2006

An increased rate of fatty acid transport into skeletal muscle has been has been linked to the accumulation of intramuscular lipids and insulin resistance, and red muscles are more susceptible than white muscles in developing fatty acid-mediated insulin resistance. Therefore, we examined in Zucker diabetic fatty (ZDF) rats, relative to lean rats, 1) whether rates of fatty acid transport and transporters (FAT/CD36 and FABPpm) were upregulated in skeletal muscle during the transition from insulin resistance (week 6) to type 2 diabetes (weeks 12 and 24), 2) whether such changes occurred primarily in red skeletal muscle, and 3) whether changes in FAT/CD36 and GLUT4 were correlated. In red muscles of ZDF compared with lean rats, the rates of fatty acid transport were upregulated (+66%) early in life (week 6). Compared with the increase in fatty acid transport in lean red muscle from weeks 12-24 (+57%), the increase in fatty acid transport rate in ZDF red muscle was 50% greater during this same period. In contrast, no differences in fatty acid transport rates were observed in the white muscles of lean and ZDF rats at any time (weeks 6-24). In red muscle only, there was an inverse relationship between FAT/CD36 and GLUT4 protein expression as well as their plasmalemmal content. These studies have shown that, 1) before the onset of diabetes, as well as during diabetes, fatty acid transport and FAT/CD36 expression and plasmalemmal content are upregulated in ZDF rats, but importantly, 2) these changes occurred only in red, not white, muscles of ZDF rats. insulin resistance; diabetes; plasma membrane-associated fatty acid-binding protein; fatty acid translocase messenger ribonucleic acid; plasma membrane fatty acid-binding protein messenger ribonucleic acid; glucose transporter 4; Zucker diabetic fatty rat doi:10.1152/ajpendo.00096.2006

Fatty acid translocase (FAT/ CD36) is a transport protein with a high affinity for long-chain fatty acids (LCFA). It was recently identified on rat skeletal muscle mitochondrial membranes and found to be required for palmitate uptake and oxidation. Our aim was to identify the presence and elucidate the role of FAT/CD36 on human skeletal muscle mitochondrial membranes. We demonstrate that FAT/CD36 is present in highly purified human skeletal mitochondria. Blocking of human muscle mitochondrial FAT/CD36 with the specific inhibitor sulfo-N-succimidyl-oleate (SSO) decreased palmitate oxidation in a dose-dependent manner. At maximal SSO concentrations (200 [micro]M) palmitate oxidation was decreased by 95% (P < 0.01), suggesting an important role for FAT/CD36 in LCFA transport across the mitochondrial membranes. SSO treatment of mitochondria did not affect mitochondrial octanoate oxidation and had no effect on maximal and submaximal carnitine palmitoyltransferase I (CPT I) activity. However, SSO treatment did inhibit palmitoylcarnitine oxidation by 92% (P < 0.001), suggesting that FAT/CD36 may be playing a role downstream of CPT I activity, possibly in the transfer of palmitoylcarnitine from CPT I to carnitine-acylcarnitine translocase. These data provide new insight regarding human skeletal muscle mitochondrial fatty acid (FA) transport, and suggest that FAT/CD36 could be involved in the cellular and mitochondrial adaptations resulting in improved and/or impaired states of FA oxidation. fatty acid translocase; long-chain fatty acid

The goal of this study was to determine whether changes in cardiac metabolism in Type 2 diabetes are associated with contractile dysfunction or impaired response to ischemia. Hearts from Zucker diabetic fatty (ZDF) and lean control rats were isolated and perfused with glucose, lactate, pyruvate, and palmitate. The rates of glucose, lactate, pyruvate, and palmitate oxidation rates and glycolysis were determined during baseline perfusion and low-flow ischemia (LFI: 0.3 ml/min for 30 min) and after LFI and reperfusion. Under all conditions, ATP synthesis from palmitate was increased and synthesis from lactate was decreased in the ZDF group, whereas the contribution from glucose was unchanged. During baseline perfusion, the rate of glycolysis was lower in the ZDF group: however, during LFI and reperfusion, there were no differences between groups. Despite these metabolic shifts, there were no differences in oxygen consumption or ATP production rates between the groups under any perfusion conditions. Cardiac function was slightly depressed before LFI in the ZDF group, but during reperfusion, function was improved relative to the control group despite the increased dependence on fatty acids for energy production. These data suggest that in this model of diabetes, the shift from carbohydrates to fatty acids for oxidative energy production did not increase myocardial oxygen consumption and was not associated with impaired response to ischemia and reperfusion. contractile dysfunction; ischemia; nuclear magnetic resonance spectroscopy; carbohydrate metabolism

Insulin stimulates fatty acid transport by regulating expression of FAT/CD36 but not FABPpm. Am J Physiol Endocrinol Metab 287: E781-E789, 2004. First published May 27, 2004; 10.1152/ajpendo.00573.2003.--Because insulin has been shown to stimulate long-chain fatty acid (LCFA) esterification in skeletal muscle and cardiac myocytes, we investigated whether insulin increased the rate of LCFA transport by altering the expression and the subcellular distribution of the fatty acid transporters FAT/CD36 and FABPpm. In cardiac myocytes, insulin very rapidly increased the expression of FAT/CD36 protein in a time- and dose-dependent manner. During a 2-h period, insulin (10 nM) increased cardiac myocyte FAT/CD36 protein by 25% after 60 min and attained a maximum after 90-120 min (+40-50%). There was a dose-dependent relationship between insulin ([10.sup.-12] to [10.sup.-7] M) and FAT/CD36 expression. The half-maximal increase in FAT/CD36 protein occurred at 0.5 x [10.sup.-9] M insulin, and the maximal increase occurred at [10.sup.-9] to [10.sup.-8] M insulin (+40-50%). There were similar insulin-induced increments in FAT/CD36 protein in cardiac myocytes (+43%) and in Langendorff-perfused hearts (+32%). In contrast to FAT/CD36, insulin did not alter the expression of FABPpm protein in either cardiac myocytes or the perfused heart. By use of specific inhibitors of insulin-signaling pathways, it was shown that insulin-induced expression of FAT/CD36 occurred via the PI 3-kinase/Akt insulin-signaling pathway. Subcellular fractionation of cardiac myocytes revealed that insulin not only increased the expression of FAT/CD36, but this hormone also targeted some of the FAT/CD36 to the plasma membrane while concomitantly lowering the intracellular depot of FAT/ CD36. At the functional level, the insulin-induced increase in FAT/CD36 protein resulted in an increased rate of palmitate transport into giant vesicles (+34%), which paralleled the increase in plasmalemmal FAT/CD36 (+29%). The present studies have shown that insulin regulates protein expression of FAT/CD36, but not FABPpm, via the PI 3-kinase/Akt insulin-signaling pathway. fatty acid translocase; plasma membrane-associated fatty acid-binding protein; cardiac myocytes; transport; plasma membrane; low-density microsomes; perfusion; heart

Adaptation of myocardial energy substrate utilization may contribute to the cardioprotective effects of regular exercise, a possibility supported by evidence showing that pharmacological metabolic modulation is beneficial to ischemic hearts during reperfusion. Thus we tested the hypothesis that the beneficial effect of regular physical exercise on recovery from ischemia-reperfusion is associated with a protective metabolic phenotype. Function, glycolysis, and oxidation of glucose, lactate, and palmitate were measured in isolated working hearts from sedentary control (C) and treadmill-trained (T: 10 wk, 4 days/wk) female Sprague-Dawley rats submitted to 20 min ischemia and 40 min reperfusion. Training resulted in myocardial hypertrophy (1.65 [+ or -] 0.05 vs. 1.30 [+ or -] 0.03 g heart wet wt, P < 0.001) and improved recovery of function after ischemia by nearly 50% (P < 0.05). Glycolysis was 25-30% lower in T hearts before and after ischemia (P < 0.05), whereas rates of glucose oxidation were 45% higher before ischemia (P < 0.01). As a result, the fraction of glucose oxidized before and after ischemia was, respectively, twofold and 25% greater in T hearts (P < 0.05). Palmitate oxidation was 50-65% greater in T than in C before and after ischemia (P < 0.05), whereas lactate oxidation did not differ between groups. Alteration in content of selected enzymes and proteins, as assessed by immunoblot analysis, could not account for the reduction in glycolysis or increase in glucose and palmitate oxidation observed. Combined with the studies on the beneficial effect of pharmacological modulation of energy metabolism, the present results provide support for a role of metabolic adaptations in protecting the trained heart against ischemia-reperfusion injury. exercise training; cardiac hypertrophy; ischemia-reperfusion; energy metabolism

Different mechanisms can alter fatty acid transport when muscle contractile activity is chronically altered. Am J Physiol Endocrinol Metab 286: E1042-E1049, 2004; 10.1152/ajpendo.00531.2003.--We examined whether skeletal muscle transport rates of long-chain fatty acids (LCFAs) were altered when muscle activity was eliminated (denervation) or increased (chronic stimulation). After 7 days of chronically stimulating the hindlimb muscles of female Sprague-Dawley rats, the LCFA transporter proteins fatty acid translocase (FAT)/CD36 (+43%) and plasma membrane-associated fatty acid-binding protein (FABPpm; +30%) were increased (P < 0.05), which resulted in the increased plasmalemmal content of these proteins (FAT/CD36, +42%; FABPpm +13%, P < 0.05) and a concomitant increase in the LCFA transport rate into giant sarcolemmal vesicles (+44%, P < 0.05). Although the total muscle contents of FAT/CD36 and FABPpm were not altered (P > 0.05) after 7 days of denervation, the LCFA transport rate was markedly decreased (-39%). This was associated with reductions in plasmalemmal FAT/CD36 (-24%) and FABPpm (-28%; P < 0.05). These data suggest that these LCFA transporters were resequestered to their intracellular depot(s) within the muscle. Combining the results from these experiments indicated that changes in rates of LCFA transport were correlated with concomitant changes in plasmalemmal FAT/CD36 and FABPpm, but not necessarily with their total muscle content. Thus chronic alterations in muscle activity can alter the rates of LCFA transport via different mechanisms, either l) by increasing the total muscle content of FAT/CD36 and FABPpm, resulting in a concomitant increase at the sarcolemma, or 2) by reducing the plasma membrane content of these proteins in the absence of any changes in their total muscle content. giant vesicles; tibialis anterior; gastrocnemius; denervation; chronic stimulation

We examined the movement of [[sup.3]H]palmitate across giant sarcolemmal vesicles prepared from red and white muscle of rainbow trout (Oncorhynchus mykiss). Red and white muscle fatty acid carriers have similar affinities for palmitate (apparent [K.sub.m] = 26 [+ or -] 6 and 33 [+ or -] 8 nM, respectively); however, red muscle has a higher maximal uptake compared with white muscle ([V.sub.max] = 476 [+ or -] 41 vs. 229 [+ or -] 23 pmol * mg [protein.sup.-1] * [s.sup.-1], respectively). Phloretin (250 [micro]M) inhibited palmitate influx in red and white muscle vesicles by ~40%, Hg[Cl.sub.2] (2.5 mM) inhibited palmitate uptake by 20-30%, and the anion-exchange inhibitor DIDS (250 [micro]M) inhibited palmitate influx in red and white muscle vesicles by ~15 and 30%, respectively. Western blot analysis of red and white muscle vesicles did not detect a mammalian-type fatty acid transporter (FAT); however, preincubation of vesicles with sulfo-N-succinimidyloleate, a specific inhibitor of FAT in rats, reduced palmitate uptake in red and white muscle vesicles by ~15 and 25%, respectively. A mammalian-type plasma membrane fatty acid-binding protein was identified in trout muscle using Western blotting, but the protein differed in size between red and white muscle. At low concentrations of free palmitate (2.5 nM), addition of high concentrations (111 [micro]M total) of oleate (18:0) caused ~50% reduction in palmitate uptake by red and white muscle vesicles, but high concentrations (100 [micro]M) of octanoate (8:0) caused no inhibition of uptake. Five days of aerobic swimming at ~2 body lengths/s and 9 days of chronic cortisol elevation in vivo, both of which stimulate lipid metabolism, had no effect on the rate of palmitate movement in red or white muscle vesicles. long-chain fatty acid; transport; rainbow trout (Oncorhynchus mykiss), sarcolemmal vesicle

Triiodothyronine ([T.sub.3]) regulates the expression of genes involved in muscle metabolism. Therefore, we examined the effects of a 7-day [T.sub.3] treatment on the monocarboxylate transporters (MCT)1 and MCT4 in heart and in red (RG) and white gastrocnemius muscle (WG). We also examined rates of lactate transport into giant sarcolemmal vesicles and the plasmalemmal MCT1 and MCT4 in these vesicles. Ingestion of [T.sub.3] markedly increased circulating serum [T.sub.3] (P < 0.05) and reduced weight gain (P < 0.05). [T.sub.3] upregulated MCT1 mRNA (RG +77, WG +49, heart +114%, P < 0.05) and MCT4 mRNA (RG +300, WG +40%). However, only MCT4 protein expression was increased (RG +43, WG +49%), not MCT1 protein expression. No changes in MCT1 protein were observed in any tissue. [T.sub.3] treatment doubled the rate of lactate transport when vesicles were exposed to 1 mM lactate (P < 0.05). However, plasmalemmal MCT4 was only modestly increased (+13%, P < 0.05). We conclude that [T.sub.3] 1) regulates MCT4, but not MCT1, protein expression and 2) increases lactate transport rates. This latter effect is difficult to explain by the modest changes in plasmalemmal MCT4. We speculate that either the activity of sarcolemmal MCTs has been altered or else other MCTs in muscle may have been upregulated. giant vesicles; heart; monocarboxylate transporter 1 mRNA; monocarboxylate transporter 4 mRNA; monocarboxylate transporter 1 protein; monocarboxylate transporter 4 protein

The low-molecular-mass, cytosolic heart-type fatty acid-binding protein (H-FABP) is thought to be required for shuttling FA through the cytosol. Therefore, we examined the effects of an H-FABP-null mutation on FA and carbohydrate metabolism in isolated soleus muscle at rest and during a period of increased metabolic demand (30-min contraction). There were lower concentrations of creatine phosphate (-41%), ATP (-22%), glycogen (-34%), and lactate (-31%) (P < 0.05) in H-FABP-null soleus muscles, but no differences in citrate synthase and [beta]-3-hydroxyacyl-CoA dehydrogenase activities or in the intramuscular triacylglycerol (TAG) depots. There was a 43% increase in subsarcolemmal mitochondria in H-FABP-null solei. FA transport was reduced by 30% despite normal content of sarcolemmal long-chain fatty acid transporters fatty acid translocase/CD36 and plasma membrane-associated FABP transport proteins. Compared with wild-type soleus muscles, the H-FABP-null muscles at rest hydrolyzed less TAG (-22%), esterified less TAG (-49%), and oxidized less palmitate (-71%). The H-FABP-null soleus muscles retained a substantial capacity to increase FA metabolism during contraction (TAG esterification by +72%, C[O.sub.2] production by + 120%), although these rates remained lower (TAG esterification -26% and C[O.sub.2] production -64%) than in contracting wild-type soleus muscles. Glycogen utilization during 30 min of contraction did not differ, whereas glucose oxidation was lower at rest (-24%) and during contraction (-32%) in H-FABP-null solei. Although these studies demonstrate that the absence of H-FABP alters rates of FA metabolism, it is also apparent that glucose oxidation is downregulated. The substantial increase in FA metabolism in contracting H-FABP-null muscle may indicate that other FABPs are also present, a possibility that we were not able to completely eliminate. palmitate; esterification; oxidation; soleus; glucose

We have examined the effects of streptozotocin (STZ)-induced diabetes (moderate and severe) on fatty acid transport and fatty acid transporter (FAT/CD36) and plasma membrane-bound fatty acid binding protein (FABPpm) expression, at the mRNA and protein level, as well as their plasmalemmal localization. These studies have shown that, with STZ-induced diabetes, 1) fatty acid transport across the plasma membrane is increased in heart, skeletal muscle, and adipose tissue and is reduced in liver; 2) changes in fatty acid transport are generally not associated with changes in fatty acid transporter mRNAs, except in the heart; 3) increases in fatty acid transport in heart and skeletal muscle occurred with concomitant increases in plasma membrane FAT/CD36, whereas in contrast, the increase and decrease in fatty acid transport in adipose tissue and liver, respectively, were accompanied by concomitant increments and reductions in plasma membrane FABPpm; and finally, 4) the increases in plasma membrane transporters (FAT/CD36 in heart and skeletal muscle; FABPpm in adipose tissue) were attributable to their increased expression, whereas in liver, the reduced plasma membrane FABPpm appeared to be due to its relocation within the cell in the face of slightly increased expression. Taken together, STZ-induced changes in fatty acid uptake demonstrate a complex and tissue-specific pattern, involving different fatty acid transporters in different tissues, in combination with different underlying mechanisms to alter their surface abundance. fatty acid transporter CD6; plasma membrane-bound fatty acid binding protein; muscle; heart; liver; adipose tissue

Rates of lactate uptake into giant sarcolemmal vesicles were determined in vesicles collected from rat muscles at rest and immediately after 10 min of intense muscle contraction. This contraction period reduced muscle glycogen rapidly by 37-82% in all muscles examined (P < 0.05) except the soleus muscle (no change P > 0.05). At an external lactate concentration of 1 mM lactate, uptake into giant sarcolemmal vesicles was not altered (P > 0.05), whereas at an external lactate concentration of 20 mM, the rate of lactate uptake was increased by 64% (P < 0.05). Concomitantly, the plasma membrane content of monocarboxylate transporter (MCT)1 was reduced slightly (-10%, P < 0.05), and the plasma membrane content of MCT4 was reduced further (-25%, P < 0.05). In additional studies, the 10-min contraction period increased the plasma membrane GLUT4 (P < 0.05) while again reducing MCT4 (-20%, P < 0.05) but not MCT1 (P > 0.05). These studies have shown that intense muscle contraction can increase the initial rates of lactate uptake, but only when the external lactate concentrations are high (20 mM). We speculate that muscle contraction increases the intrinsic activity of the plasma membrane MCTs, because the increase in lactate uptake occurred while plasma membrane MCT4 was decreased and plasma membrane MCT1 was reduced only minimally, or not at all. lactate transport; GLUT4; giant vesicles

Leptin acutely increases fatty acid (FA) oxidation and triacylglycerol (TG) hydrolysis and decreases TG esterification in oxidative rodent muscle. However, the effects of chronic leptin administration on FA metabolism in skeletal muscle have not been examined. We hypothesized that chronic leptin treatment would enhance TG hydrolysis as well as the capacity to oxidize FA in soleus (SOL) muscle. Female Sprague-Dawley rats were infused for 2 wk with leptin (LEPT; 0.5 mg*[kg.sup.-1]*[day.sup.-1]) by use of subcutaneously implanted miniosmotic pumps. Control (AD-S) and pair-fed (PF-S) animals received saline-filled implants. Subsequently, FA metabolism was monitored for 45 min in isolated, resting, and contracting (20 tetani/min) SOL muscles by means of pulse-chase procedures. Food intake (-33 [+ or -] 2%, P < 0.01) and body mass (-12.5 [+ or -] 4%, P = 0.01) were reduced in both LEPT and PF-S animals. Leptin levels were elevated (+418 [+ or -] 7%, P < 0.001) in treated animals but reduced in PF-S animals (-73 [+ or -] 8%, P < 0.05) relative to controls. At rest, TG hydrolysis was increased in leptin-treated rats (1.8 [+ or -] 2.2, AD-S vs. 23.5 [+ or -] 8.1 nmol/g wet wt, LEPT; P < 0.001). In contracting SOL muscles, TG hydrolysis (1.5 [+ or -] 0.6, AD-S vs. 3.6 [+ or -] 1.0 [micro]mol/g wet wt, LEPT; P = 0.02) and palmitate oxidation (18.3 [+ or -] 6.7, AD-S vs. 45.7 [+ or -] 9.9 nmol/g wet wt, LEPT; P < 0.05) were both significantly increased by leptin treatment. Chronic leptin treatment had no effect on TG esterification either at rest or during contraction. Markers of overall (citrate synthase) and FA (hydroxyacyl-CoA dehydrogenase) oxidative capacity were unchanged with leptin treatment. Protein expression of hormone-sensitive lipase (HSL) was also unaltered following leptin treatment. Thus leptin-induced increases in lipolysis are likely due to HSL activation (i.e., phosphorylation). Increased FA oxidation secondary to chronic leptin treatment is not due to an enhanced oxidative capacity and may be a result of enhanced flux into the mitochondrion (i.e., carnitine palmitoyltransferase I regulation) or electron transport uncoupling (i.e., uncoupling protein-3 expression). pulse chase; hormone-sensitive lipase; citrate synthase; [beta]-hydroxyacyl-coenzyme A dehydrogenase

Insulin induces the translocation of the fatty acid transporter FAT/CD36 to the plasma membrane. Am J Physiol Endocrinol Metab 282: E491-E495, 2002; 10.1152/ajpendo.00419.2001.--It is well known that muscle contraction and insulin can independently translocate GLUT-4 from an intracellular depot to the plasma membrane. Recently, we have shown that the fatty acid transporter FAT/CD36 is translocated from an intracellular depot to the plasma membrane by muscle contraction ( hindlimb perfusion; glucose transporter 4; palmitate; oxidation; triacylglycerol; diacylglycerol; phospholipids

A study was conducted to determine the effects of the rise of oxidate capacity of skeletal muscle to fatty acid (FA) uptake, FA transporter expression and FA metabolism. Results of experiments with male Sprague-Dawley rats indicate that the increase in muscle contractile activity through chronic electrical simulation has a parallel effect to fatty acid metabolism and transport. It was also observed that the rise of oxidate capacity of skeletal muscles steps up the expression of the membrane transporter FAT/CD36.

Dietary fatty acid composition of a high-fat (HF) diet influences the tissue-specific insulin resistance and systemic glucose tolerance in rats. Consistent body weights were observed in the rats belonging to the different dietary groups. However, insulin resistance varied among the rat groups highlighting the need for evaluating HF dietary effects on insulin and glucose metabolism and the requirements of tissue-specific glucose homeostasis.

A study showed that the effects of epinephrine on lipid metabolism in resting skeletal muscle depend on the physiological levels of epinephrine. The finding is based on experiments performed on incubated rat muscles that were pulsed with [1-14C]palmitate and chased with [9,10-3H] palmitate. The epitrochlearis, flexor digitorum brevis and soleus muscles were considered to reflect the range of oxidative capacities. The Western blotting method was used to characterize the muscle lipid hydrolysis potential and to measure total hormone-sensitive lipase content.

Glucose uptake and glucose transporter translocation was investigated in perfused isolated rat muscles in the presence of epinephrine and insulin, alone or in combination. Both epinephrine and insulin enhance glucose uptake and increase transporter translocation. When the two are given in combination, epinephrine antagonizes the glucose uptake induced by insulin but not transporter translocation.

Electrical stimulation induces GLUT-4 translocation to the plasma membrane and transverse tubules of red gastrocnemius (RG) and white gastrocnemius (WG) muscles. Experiments were conducted testing the hypothesis that GLUT-4 translocation to the cell surface may underlie the responsiveness of type II muscle fibers. The greater translocation of GLUT-4 to transverse tubules of WG in contrast to RG may provide an explanation on both muscle types' responsiveness toward supramaximal electrical stimulation.

Research was conducted to examine whether chronic stimulation of red and white rat muscles increased the concentrations of the monocarboxylate transporter MCT1. Red and white tibialis anterior (RTA and WTA, respectively) and extensor digitorum longus (EDL) muscles were chronically stimulated via the peroneal nerve for 7 days. Stimulated and contralateral muscles were analyzed for MCTI content, L-lactate uptake, lactate dehydrogenase (LDH) isoforms and muscle fiber composition. Results show that chronic muscle contraction increases MCT1 in white and red skeletal muscles.

Expression of the glucose transporter isoforms, GLUT-1 and GLUT-4, has no effect on insulin-mediated glucose transport into cells. In a study of rats, chronic stimulation of skeletal muscles overexpressing the GLUT-1 gene but not GLUT-4 stimulated the transport of 2-deoxy-D-glucose. Similar results were observed in muscles overexpressing the GLUT-4 gene only. The greatest increases were observed in the white muscles. Meanwhile, muscles overexpressing both GLUT-1 and GLUT-4 did not impair muscle contraction, suggesting that increases in basal glucose transport have no effect on muscle glucose transport.

The skeletal muscles are the major site of lactate formation. Transport of lactate in and out of these muscles are carried out by lactate transporters such as the monocarboxylate transporter (MCT1). To characterize MCT1, an antibody was constructed against the MCT1 protein and was analyzed for its expression. Results showed higher MCT1 expression in red than in white muscles. Expression was correlated with oxidative fiber content and lactate uptake by the muscles. Lactate dehydrogenase activity in the heart was also closely correlated with MCT1, suggesting that MCT1 may be expressed with heart forms of LDH.

Subjects maintained on short-term exercise had small increases in the number of glucose transporter (GLUT) proteins in the skeletal muscle. In contrast, endurance training produced immediate and progressive increases in GLUT-4. Increases in GLUT-1 were also observed, but only after 31 days of training. Increases in muscle hexokinase (HK) activity were evident after five days of training, but muscle malate dehydrogenase only increased 31 days after training. These results suggest that changes in GLUT-4 and HK were induced during early exercise.

A study conducted on 23 male rats found that reducing muscle activity via denervation reduces the muscles' lactate uptake capacities but not their diffusive abilities. Loss of contractile activity results in a decrease of lactate transport. The alterations in circulating lactate concentrations are mainly due to the fewer lactate carriers at the sarcolemma. This research indicates that lactate transport in skeletal muscle is regulated in part by muscle activity.

Studies on muscle metabolism, incorporating perfused muscle and in vitro muscle incubation techniques, reveal that insulin effects on glucose transport and metabolism are similar. Hindlimb perfusion is an expensive technique with greater approximation to in vivo conditions, while isolated muscle techniques are comparatively inexpensive while being technically less complex. The measures necessary to avoid muscle hypoxia are enumerated.

We examined the postnatal changes (days 10, 36, 84, 160, 365) of monocarboxylate transporters (MCT)1 and MCT4 in rat heart and soleus muscle. In the heart, MCT1 protein and mRNA remained unaltered from day 10 until 1 yr of age. Both MCT4 protein and mRNA in the heart were detected at 10 days of age, but the MCT4 protein and transcript were not detected thereafter. In the soleus muscle, MCT1 protein (+38%) and mRNA (+ 136%) increased during the first 84 days and remained stable until 1 yr of age. In contrast, soleus MCT4 protein decreased by 90% over the course of 1 yr, with the most rapid decrease (-60%) occurring by day 84 (P [is less than] 0.05). At the same time, MCT4 mRNA was increased by 74% from days 10 to 84 (P [is less than] 0.05), remaining stable thereafter. In conclusion, developmental changes in MCT transport proteins are tissue specific and isoform specific. Furthermore, it appears that MCT1 expression in the heart and MCT1 and MCT4 expression in the soleus are regulated by pretranslational processes, whereas posttranscriptional processes regulate MCT4 expression in the soleus muscle. monocarboxylate transporters; protein; messenger ribonucleic acid; phosphofructokinase; citrate synthase; glucose transporter 4

Bonen, Arend, Mio Tonouchi, Dragana Miskovic, Catherine Heddle, John J. Heikkila, and Andrew P. Halestrap. Isoform-specific regulation of the lactate transporters MCT1 and MCT4 by contractile activity. Am J Physiol Endocrinol Metab 279: E1131-E1138, 2000.--We examined the isoform-specific regulation of monocarboxylate transporter (MCT)1 and MCT4 expression by contractile activity in red and white tibialis anterior muscles. After 1 and 3 wk of chronic muscle stimulation (24 h/day), MCT1 protein expression was increased in the red muscles (+78%, P [is less than] 0.05). In the white muscles, MCT1 was increased after 1 wk (+191%) and then was decreased after 3 wk. In the red muscle, MCT1 mRNA accumulation was increased only after 3 wk (+21%; P [is less than] 0.05). In the white muscle, MCT1 mRNA was increased after 1 wk (+30%; P [is less than] 0.05) and 3 wk (+15%; P < 0.05). MCT4 protein was not altered in either the red or white muscles after 1 or 3 wk. MCT4 mRNA was transiently lowered (-15%) in both muscles in the 1st wk, but MCT4 mRNA levels were back to control levels after 3 wk. In conclusion, chronic contractile activity induces the expression of MCT1 but not MCT4. This increase in MCT1 alone was sufficient to increase lactate uptake from the circulation. muscle fiber composition; lactate; perfusion; messenger ribonucleic acid

Wilkes, Jason J., and Arend Bonen. Reduced insulin-stimulated glucose transport in denervated muscle is associated with impaired Akt-[Alpha] activation. Am J Physiol Endocrinol Metab 279: E912-E919, 2000.--Insulin signaling was examined in muscle made insulin resistant by short-term (24-h) denervation. Insulin-stimulated glucose transport in vitro was reduced by 28% (P [is less than] 0.05) in denervated muscle (DEN). In control muscle (SHAM), insulin increased levels of surface-detectable GLUT-4 (i.e., translocated GLUT-4) 1.8-fold (P [is less than] 0.05), whereas DEN surface GLUT-4 was not increased by insulin (P [is less than] 0.05). Insulin treatment in vivo induced a rapid appearance of phospho[[Ser.sup.473]]Akt-[Alpha] in SHAM 3 min after insulin injection. In DEN, phospho[[Ser.sup.473]]Akt-[Alpha] also appeared at 3 min, but [Ser.sup.473]-phosphorylated Akt-[Alpha] was 36% lower than in SHAM (P [is less than] 0.05). In addition, total Akt-[Alpha] protein in DEN was 37% lower than in SHAM (P [is less than] 0.05). Akt-[Alpha] kinase activity was lower in DEN at two insulin levels tested: 0.1 U insulin/rat (-22%, P [is less than] 0.05) and 1 U insulin/rat (-26%, P [is less than] 0.01). These data indicate that short-term (24-h) denervation, which lowers insulin-stimulated glucose transport, is associated with decreased Akt-[Alpha] activation and impaired insulin-stimulated GLUT-4 appearance at the muscle surface. GLUT-4; soleus; tibialis anterior

Bonen, Arend, Dragana Miskovic, Mio Tonouchi, Kathleen Lemieux, Mariengela C. Wilson, Andre Marette, and Andrew P. Halestrap. Abundance and subcellular distribution of MCT1 and MCT4 in heart and fast-twitch skeletal muscles. Am J Physiol Endocrinol Metab 278: E1067-E1077, 2000.--The expression of two monocarboxylate transporters (MCTs) was examined in muscle and heart. MCT1 and MCT4 proteins are coexpressed in rat skeletal muscles, but only MCT1 is expressed in rat hearts. Among six rat fast-twitch muscles (red and white gastrocnemius, plantaris, extensor digitorum longus, red and white tibialis anterior) there was an inverse relationship between MCT1 and MCT4 (r = -0.94). MCT1 protein was correlated with MCT1 mRNA (r = 0.94). There was no relationship between MCT4 mRNA and MCT4 protein. MCT1 (r = -0.97) and MCT4 (r = 0.88) protein contents were correlated with percent fast-twitch glycolytic fiber. When normalized for their mRNAs, MCT1 but not MCT4 was still correlated with the percent fast-twitch glycolytic fiber composition of rat muscles (r = -0.98). MCT1 and MCT4 were also measured in plasma membranes (PM), triads (TR), T tubules (TT), sarcoplasmic reticulum (SR), and intracellular membranes (IM). There was an intracellular pool of MCT4 but not of MCT1. The MCT1 subcellular distribution was as follows: PM (100%) [is greater than] TR (31.6%) [is greater than] SR (15%) = TT (14%) [is greater than] IM (1.7%). The MCT4 subcellular distribution was considerably different [PM (100%) [is greater than] TR (66.5%) [is greater than] TT (36%) = SR (43%) [is greater than] IM (24%)]. These studies have shown that 1) the mechanisms regulating the expression of MCT1 (transcriptional and posttranscriptional) and MCT4 (posttranscriptional) are different and 2) differences in MCT1 and MCT4 expression among muscles, as well as in their subcellular locations, suggest that they may have different roles in muscle. lactate; transport; transporters; fast-twitch fibers; plasma membrane; T tubules, messenger ribonucleic acid; monocarboxylate transporter 1; monocarboxylate transporter 4