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Muscle is a heterogeneous tissue composed of multiple fiber types. Earlier research revealed fiber type-selective postexercise effects on insulin-stimulated glucose uptake (ISGU) from insulin-resistant rats (increased for type IIA, IIB, IIBX, and IIX, but not type I). In whole muscle from insulin-resistant rats, the exercise increase in ISGU is accompanied by an exercise increase in insulin-stimulated AS160 phosphorylation (pAS160), an ISGU-regulating protein. We hypothesized that, in insulin-resistant muscle, the fiber type-selective exercise effects on ISGU would correspond to the fiber type-selective exercise effects on pAS160. Rats were fed a 2-wk high-fat diet (HFD) and remained sedentary (SED) or exercised before epitrochlearis muscles were dissected either immediately postexercise (IPEX) or at 3 h postexercise (3hPEX) using an exercise protocol that previously revealed fiber type-selective effects on ISGU. 3hPEX muscles and SED controls were incubated ± 100μU/mL insulin. Individual myofibers were isolated and pooled on the basis of myosin heavy chain (MHC) expression, and key phosphoproteins were measured. Myofiber glycogen and MHC expression were evaluated in muscles from other SED, IPEX, and 3hPEX rats. Insulin-stimulated pAktSer473 and pAktThr308 were unaltered by exercise in all fiber types. Insulin-stimulated pAS160 was greater for 3hPEX vs. SED on at least one phosphosite (Ser588, Thr642, and/or Ser704) in type IIA, IIBX, and IIB fibers, but not in type I or IIX fibers. Both IPEX and 3hPEX glycogen were decreased versus SED in all fiber types. These results provided evidence that fiber type-specific pAS160 in insulin-resistant muscle may play a role in the previously reported fiber type-specific elevation in ISGU in some, but not all, fiber types. [ABSTRACT FROM AUTHOR]

Earlier research using muscle tissue demonstrated that postexercise elevation in insulin-stimulated glucose uptake (ISGU) occurs concomitant with greater insulin-stimulated Akt substrate of 160 kDa (AS160) phosphorylation (pAS160) on sites that regulate ISGU. Because skeletal muscle is a heterogeneous tissue, we previously isolated myofibers from rat epitrochlearis to assess fiber type-selective ISGU. Exercise induced greater ISGU in type I, IIA, IIB, and IIBX but not IIX fibers. This study tested if exercise effects on pAS160 correspond with previously published fiber type-selective exercise effects on ISGU. Rats were studied immediately postexercise (IPEX) or 3.5 h postexercise (3.5hPEX) with time-matched sedentary controls. Myofibers dissected from the IPEX experiment were analyzed for fiber type (myosin heavy chain isoform expression) and key phosphoproteins. Isolated muscles from the 3.5hPEX experiment were incubated with or without insulin. Myofibers (3.5hPEX) were analyzed for fiber type, key phosphoproteins, and GLUT4 protein abundance. We hypothesized that insulin-stimulated pAS160 at 3.5hPEX would exceed sedentary controls only in fiber types characterized by greater ISGU postexercise. Values for phosphorylation of AMP-activated kinase substrates (acetyl CoA carboxylaseSer79 and AS160Ser704) from IPEX muscles exceeded sedentary values in each fiber type, suggesting exercise recruitment of all fiber types. Values for pAS160Thr642 and pAS160Ser704 from insulin-stimulated muscles 3.5hPEX exceeded sedentary values for type I, IIA, IIB, and IIBX but not IIX fibers. GLUT4 abundance was unaltered 3.5hPEX in any fiber type. These results advanced understanding of exercise-induced insulin sensitization by providing compelling support for the hypothesis that enhanced insulin-stimulated phosphorylation of AS160 is linked to elevated ISGU postexercise at a fiber type-specific level independent of altered GLUT4 expression. [ABSTRACT FROM AUTHOR]

Metabolic dysfunction and Type 2 diabetes are associated with perturbed circadian rhythms. However, exercise appears to ameliorate circadian disturbances, as it can phase-shift or reset the internal clock system. Evidence is emerging that exercise at a distinct time of day can correct misalignments of the circadian clock and influence energy metabolism. This suggests that timing of exercise training can be important for the prevention and management of metabolic dysfunction. In this study, obese, high-fat diet-fed mice were subjected to voluntary wheel running (VWR) at two different periods of the day to determine the effects of time-of-day-restricted VWR on basal and insulin-stimulated glucose disposal. VWR in the late dark phase reduced body weight gain compared with VWR in the beginning of the dark phase. Conversely, time-of-day-restricted VWR did not influence insulin action and glucose disposal, since skeletal muscle and adipose tissue glucose uptake and insulin signaling remained unaffected. Protein abundance of the core clock proteins, brain-muscle arnt-like 1 (BMAL1), and circadian locomotor output control kaput (CLOCK), were increased in skeletal muscle after VWR, independent of whether mice had access to running wheels in the early or late dark phase. Collectively, we provide evidence that VWR in the late dark phase ameliorates diet-induced obesity without altering insulin action or glucose homeostasis. [ABSTRACT FROM AUTHOR]

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca2+, and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease. [ABSTRACT FROM AUTHOR]

The article discusses a review on the role of membrane fatty acid transporters in cellular lipid metabolism. It states that specific fatty acid transporters transfer from intracellular stores to the plasma membrane to help in fatty acid uptake. Furthermore, the regulatory mechanism made by the fatty acid transporters is important to clear lipids from the circulation and helps in providing substrates when the metabolic demands of organs and tissues are increased by contractile activity.

In skeletal muscle, contractions increase glucose uptake due to a translocation of GLUT4 glucose transporters from intracellular storage sites to the surface membrane. Vesicle-associated membrane proteins (VAMPs) are believed to play an important role in docking and fusion of the GLUT4 transporters at the surface membrane. However, knowledge about which VAMP isoforms colocalize with GLUT4 vesicles in mature skeletal muscle and whether they translocate during muscle contractions is incomplete. The aim of the present study was to further identify VAMP isoforms, which are associated with GLUT4 vesicles and examine which VAMP isoforms translocate to surface membranes in skeletal muscles undergoing contractions. VAMP2, VAMP3, VAMP5, and VAMP7 were enriched in immunoprecipitated GLUT4 vesicles. In response to 20 mm of in situ contractions, there was a redistribution of GLUT4 (+64 ± 13%), transferrin receptor (TfR; +75 ± 22%), and insulin-regulated aminopeptidase (IRAP; +70 ± 13%) to fractions enriched in heavy membranes away from low-density membranes (-32 ± 7%; -18 ± 12%; -33 ± 9%; respectively), when compared with the resting contralateral muscle. Similarly, there was a redistribution of VAMP2 (+240 ± 40%), VAMP5 (+79 ± 9%), and VAMP7 (+79 ± 29%), but not VAMP3, to fractions enriched in heavy membranes away from low-density membranes (-49 ± 10%, -54 ± 9%, -14 ± 11%, respectively) in contracted vs. resting muscle. In summary, VAMP2, VAMP3, VAMP5, and VAMP7 coimmunoprecipitate with intracellular GLUT4 vesicles in muscle, and VAMP2, VAMPS, VAMP7, but not VAMP3, translocate to the cell surface membranes similar to GLUT4, TfR, and IRAP in response to muscle contractions. These findings suggest that VAMP2, VAMP5, and VAMP7 may be involved in translocation of GLUT4 during muscle contractions. [ABSTRACT FROM AUTHOR]

This study evaluated whether improved insulin-stimulated glucose uptake in recovery from acute exercise coincides with reduced malonyl-CoA (MCoA) content in human muscle. Furthermore, we investigated whether a high-fat diet [65 energy-% (Fat)] would alter the content of MCoA and insulin action compared with a high-carbohydrate diet [65 energy-% (CHO)]. After 4 days of isocaloric diet on two occasions (Fat/CHO), 12 male subjects performed I h of one-legged knee extensor exercise (~80% peak workload). Four hours after exercise, insulin-stimulated glucose uptake was determined in both legs during a euglycemic-hyperinsulinemic clamp. Muscle biopsies were obtained in both legs before and after the clamp. Four hours after exercise, insulin-stimulated glucose uptake was improved (--70%, P < 0.001) independent of diet com- position and despite normal insulin-stimulated regulation of insulin receptor substrate- 1-associated phosphatidylinositol 3-kinase, Akt, GSK-3, and glycogen synthase. Interestingly, exercise resulted in a sustained reduction (~20%, P < 0.05) in MCoA content 4 h after exercise that correlated (r = 0.65, P < 0.001) with improved insulin- stimulated glucose uptake. Four days of Fat diet resulted in an increased content of intramyocellular triacylglycerol (P < 0.01) but did not influence muscle MCoA content or whole body insulin- stimulated glucose uptake. However, at the muscular level proximal insulin signaling and insulin-stimulated glucose uptake appeared to be compromised, although to a minor extent, by the Fat diet. Collectively, this study indicates that reduced muscle MCoA content in recovery from exercise may be part of the adaptive response leading to improved insulin action on glucose uptake after exercise in human muscle. [ABSTRACT FROM AUTHOR]

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle. Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle. [ABSTRACT FROM AUTHOR]

Homozygous deletion of the gene of the neuronal glucose transporter GLUT3 (Slc2a3) in mice results in embryonic lethality, whereas heterozygotes (Slc2a3+/-) are viable. Here, we describe the characterization of heterozygous mice with regard to neuronal function, glucose homeostasis, and, since GLUT3 might be a component of the neuronal glucose-sensing mechanism, food intake and energy balance. Levels of GLUT3 mRNA and protein in brain were reduced by 50% in Slc2a3+/- mice. Electrographic features examined by electroencephalographic recordings give evidence for slightly but significantly enhanced cerebrocortical activity in Slc2a3+/- mice. In addition, Slc2a3+/- mice were slightly more sensitive to an acoustic startle stimulus (elevated startle amplitude and reduced prepulse inhibition). However, systemic behavioral testing revealed no other functional abnormalities, e.g., in coordination, reflexes, motor abilities, anxiety, learning, and memory. Furthermore, no differences in body weight, blood glucose, and insulin levels were detected between wild-type and Slc2a3+/- littermates. Food intake as monitored randomly or after intracerebroventricular administration of 2-deoxyglucose or D-glucose, or food choice for carbohydrates/fat was not affected in S1c2a3+/- mice. Taken together, our data indicate that, in contrast to Slc2a1, a single allele of Slc2a3 is sufficient for maintenance of neuronal energy supply, motor abilities, learning and memory, and feeding behavior. [ABSTRACT FROM AUTHOR]

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-β-D-ribofuranoside (AlCAR) 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. [ABSTRACT FROM AUTHOR]

Hexokinase is responsible for glucose phosphorylation, a process fundamental to regulating glucose uptake. In some tissues, hexokinase translocates to the mitochondria, thereby increasing its efficiency and decreasing its susceptibility to product inhibition. It may also decrease free radical formation in the mitochondria and prevent apoptosis. Whether hexokinase translocation occurs in the heart is controversial; here, using immunogold labeling for the first time, we provide evidence for this process. Rat hearts (6 groups, n = 6/group), perfused with either glucose- or glucose + oleate (0.4 mmol/l)-containing buffer, were exposed to 30-mm insulin stimulation, ischemia, or control perfusion. Hexokinase I (HK I) and hexokinase II (HK II) distributions were then determined. In glucose-perfused hearts, HK 1-mitochondrial binding increased from 0.41 ± 0.04 golds/mm in control hearts to 0.71 ± 0.10 golds/mm after insulin and to 1.54 ± 0.38 golds/mm after ischemia (P < 0.05). Similarly, HK II-mitochondrial binding increased from 0.16 ± 0.02 to 0.53 ± 0.08 golds/mm with insulin and 0.44 ± 0.07 golds/mm after ischemia (P < 0.05). Under basal conditions, the fraction of HK I that was mitochondrial bound was five times greater than for HK II; insulin and ischemia caused a fourfold increase in HK II binding but only a doubling in HK I binding. Oleate decreased hexokinase-mitochondrial binding and abolished insulin-mediated translocation of HK I. Our data show that mitochondrial-hexokinase binding increases under insulin or ischemic stimulation and that this translocation is modified by oleate. These events are isoform specific, suggesting that HK I and HK II are independently regulated and implying that they perform different roles in cardiac glucose regulation. [ABSTRACT FROM AUTHOR]

The article argues that glucose phosphorylation is a significant barrier to muscle glucose uptake (MGU) by the working muscle. According to the authors, working muscle becomes freely permeable to glucose and therefore offers no barrier to MGU under these conditions, however, it is still difficult to know whether exercise increases the capacity of the muscle to phosphorylate glucose.