Epitrochlearis muscle. I. Mechanical performance, energetics, and fiber composition

Distinct mechanisms involving diacylglycerol, ceramides, and inflammation underlie insulin resistance in oxidative and glycolytic muscles from high fat-fed rats

This study investigated whether oxidative and glycolytic rat skeletal muscles respond differently to a high-fat (HF) sucrose-enriched diet with respect to diacylglycerol (DAG) and ceramides accumulation, protein kinase C (PKC) activation, glucose metabolism, and the expression of inflammatory genes. HF diet (8 weeks) suppressed insulin-stimulated glycogen synthesis and glucose oxidation in soleus (Sol), extensor digitorum longus (EDL) and epitrochlearis (Epit) muscles. However, DAG and ceramides levels increased in Sol and EDL, but not in Epit muscles of HF-fed rats. Additionally, membrane-bound PKC-delta and PKC-theta increased in Sol and EDL, whereas in Epit muscles both PKC isoforms were reduced by HF diet. In Epit muscles, HF diet also increased the expression of tumor necrosis factor-α (TNF-α) receptors (CD40 and FAS), toll-like receptor 4 (TLR4), and nuclear factor kappa light polypeptide gene enhancer in B cells (NF-kB), whereas in Sol and EDL muscles the expression of these inflammatory genes remained unchanged upon HF feeding. In conclusion, HF diet caused DAG and ceramides accumulation, PKC activation, and the induction of inflammatory pathways in a fiber type-specific manner. These findings help explain why oxidative and glycolytic muscles similarly develop insulin resistance, despite major differences in their metabolic characteristics and responsiveness to dietary lipid abundance.

Acute bout of exercise downregulates thioredoxin-interacting protein expression in rat contracting skeletal muscles

We previously reported that in rat skeletal muscle, disuse (i.e., decreased muscle contractile activity) rapidly increases thioredoxin-interacting protein (TXNIP), which is implicated in the reduced glucose uptake. Accordingly, we sought herein to (a) determine the effect of exercise (i.e., increased muscle contractile activity) on muscle TXNIP protein expression, and (b) elucidate the mechanisms underlying the changes of TXNIP protein expression in response to exercise. Rat epitrochlearis and soleus muscles were dissected out after an acute bout of 3-hr swimming (without weight loading) or 3-hr treadmill running (15% grade at 9m/min). In a separate protocol, the isolated epitrochlearis and soleus muscles were incubated for 3 hr with AMP-dependent protein kinase activator AICAR. Immediately after the cessation of the 3-hr swimming, the TXNIP protein was decreased in epitrochlearis but not in soleus muscle. Conversely, 3-hr treadmill running decreased the TXNIP protein in soleus but not in epitrochlearis muscle. TXNIP protein was decreased concomitantly with reduced postexercise muscle glycogen, showing that a decrease in TXNIP protein expression occurs in muscles that are recruited during exercise. In addition, 3-hr incubation with AICAR decreased TXNIP protein in both isolated epitrochlearis and soleus muscles. Our results suggest that (a) an acute bout of exercise downregulates TXNIP protein expression in rat contracting skeletal muscles, and (b) the reduction in TXNIP protein expression in contracting muscles is probably mediated by AMPK activation, at least in part.

Vitamin C alleviates acute enterocolitis in Campylobacter jejuni infected mice

Human foodborne infections with the zoonotic pathogen Campylobacter jejuni are on the rise and constitute a significant socioeconomic burden worldwide. The health-beneficial, particularly anti-inflammatory effects of vitamin C (ascorbate) are well known. In our preclinical intervention study, we assessed potential anti-pathogenic and immunomodulatory effects of ascorbate in C. jejuni-infected secondary abiotic IL-10-/- mice developing acute campylobacteriosis similar to humans. Starting 4 days prior peroral C. jejuni-infection, mice received synthetic ascorbate via the drinking water until the end of the experiment. At day 6 post-infection, ascorbate-treated mice harbored slightly lower colonic pathogen loads and suffered from less severe C. jejuni-induced enterocolitis as compared to placebo control animals. Ascorbate treatment did not only alleviate macroscopic sequelae of infection, but also dampened apoptotic and inflammatory immune cell responses in the intestines that were accompanied by less pronounced pro-inflammatory cytokine secretion. Remarkably, the anti-inflammatory effects of ascorbate pretreatment in C. jejuni-infected mice were not restricted to the intestinal tract but could also be observed in extra-intestinal compartments including liver, kidneys and lungs. In conclusion, due to the potent anti-inflammatory effects observed in the clinical murine C. jejuni-infection model, ascorbate constitutes a promising novel option for prophylaxis and treatment of acute campylobacteriosis.

The Effects of Caffeine on Metabolomic Responses to Muscle Contraction in Rat Skeletal Muscle

Exercise has beneficial effects on our health by stimulating metabolic activation of skeletal muscle contraction. Caffeine is a powerful metabolic stimulant in the skeletal muscle that has ergogenic effects, including enhanced muscle power output and endurance capacity. In the present study, we aim to characterize the metabolic signatures of contracting muscles with or without caffeine stimulation using liquid chromatography-mass spectrometry and capillary electrophoresis coupled to mass spectrometry. Isolated rat epitrochlearis muscle was incubated in the presence or absence or of 3 mM caffeine for 30 min. Electrical stimulation (ES) was used to induce tetanic contractions during the final 10 min of incubation. Principal component analysis and hierarchical clustering analysis detected 184 distinct metabolites across three experimental groups—basal, ES, and ES with caffeine (ES + C). Significance Analysis of Microarray identified a total of 50 metabolites with significant changes in expression, and 23 metabolites significantly changed between the ES and ES + C groups. Changes were observed in metabolite levels of various metabolic pathways, including the pentose phosphate, nucleotide synthesis, β-oxidation, tricarboxylic acid cycle, and amino acid metabolism. In particular, D-ribose 5-phosphate, IMP, O-acetylcarnitine, butyrylcarnitine, L-leucine, L-valine, and L-aspartate levels were higher in the ES + C group than in the ES group. These metabolic alterations induced by caffeine suggest that caffeine accelerates contraction-induced metabolic activations, thereby contributing to muscle endurance performance and exercise benefits to our health.

Heat stress acutely activates insulin-independent glucose transport and 5'-AMP-activated protein kinase prior to an increase in HSP72 protein in rat skeletal muscle

Heat stress (HS) stimulates heat shock protein (HSP) 72 mRNA expression, and the period after an increase in HSP72 protein is characterized by enhanced glucose metabolism in skeletal muscle. We have hypothesized that, prior to an increase in the level of HSP72 protein, HS activates glucose metabolism by acutely stimulating 5'-AMP-activated protein kinase (AMPK). Rat epitrochlearis muscle was isolated and incubated either with or without HS (42°C) for 10 and 30 min. HS for 30 min led to an increase in the level of Hspa1a and Hspa1b mRNA but did not change the amount of HSP72 protein. However, HS for both 10 and 30 min led to a significant increase in the rate of 3-O-methyl-d-glucose (3MG) transport, and the stimulatory effect of 3MG transport was completely blocked by cytochalasin B. HS-stimulated 3MG transport was also inhibited by dorsomorphin but not by wortmannin. HS led to a decrease in the concentration of ATP, phosphocreatine, and glycogen, to an increase in the level of phosphorylation of AMPKα Thr(172), and to an increase in the activity of both AMPKα1 and AMPKα2. HS did not affect the phosphorylation status of insulin receptor signaling or Ca(2+)/calmodulin-dependent protein kinase II. These results suggest that HS acts as a rapid stimulator of insulin-independent glucose transport, at least in part by stimulating AMPK via decreased energy status. Although further research is warranted, heat treatment of skeletal muscle might be a promising method to promote glucose metabolism acutely.

Caffeine and contraction synergistically stimulate 5'-AMP-activated protein kinase and insulin-independent glucose transport in rat skeletal muscle

5′-Adenosine monophosphate-activated protein kinase (AMPK) has been identified as a key mediator of contraction-stimulated insulin-independent glucose transport in skeletal muscle. Caffeine acutely stimulates AMPK in resting skeletal muscle, but it is unknown whether caffeine affects AMPK in contracting muscle. Isolated rat epitrochlearis muscle was preincubated and then incubated in the absence or presence of 3 mmol/L caffeine for 30 or 120 min. Electrical stimulation (ES) was used to evoke tetanic contractions during the last 10 min of the incubation period. The combination of caffeine plus contraction had additive effects on AMPKα Thr¹⁷² phosphorylation, α-isoform-specific AMPK activity, and 3-O-methylglucose (3MG) transport. In contrast, caffeine inhibited basal and contraction-stimulated Akt Ser⁴⁷³ phosphorylation. Caffeine significantly delayed muscle fatigue during contraction, and the combination of caffeine and contraction additively decreased ATP and phosphocreatine contents. Caffeine did not affect resting tension. Next, rats were given an intraperitoneal injection of caffeine (60 mg/kg body weight) or saline, and the extensor digitorum longus muscle was dissected 15 min later. ES of the sciatic nerve was performed to evoke tetanic contractions for 5 min before dissection. Similar to the findings from isolated muscles incubated in vitro, the combination of caffeine plus contraction in vivo had additive effects on AMPK phosphorylation, AMPK activity, and 3MG transport. Caffeine also inhibited basal and contraction-stimulated Akt phosphorylation in vivo. These findings suggest that caffeine and contraction synergistically stimulate AMPK activity and insulin-independent glucose transport, at least in part by decreasing muscle fatigue and thereby promoting energy consumption during contraction.

Voluntary physical activity prevents insulin resistance in a tissue specific manner

Physical inactivity and a sedentary lifestyle are risk factors for the development of type 2 diabetes. Here, we identified the effects 8 weeks of voluntary physical activity had on the prevention of insulin resistance in mouse skeletal muscles and liver (a hallmark of T2D). To do this, 8 week old C57BL/6J mice with (RUN) and without (SED) voluntary access to running wheels were fed a standard rodent chow ad libitum for 8 weeks. In the liver, there was a 2.5-fold increase in insulin stimulated Akt^SER473 phosphorylation, and a threefold increase in insulin-stimulated (0.5 U/kg) GSK3β^SER9 phosphorylation in RUN compared to SED mice. Although not induced in skeletal muscles, there was a twofold increase in SOCS3 expression in SED compared to RUN mice in the liver. There was no difference in the glucose tolerance test between groups. This study was the first to show differences in liver insulin sensitivity after 8 weeks of voluntary physical activity, and increased SOCS3 expression in the liver of sedentary mice compared to active mice. These findings demonstrate that even in young mice that would normally be considered healthy, the lack of physical activity leads to insulin resistance representing the initial pathogenesis of impaired glucose metabolism leading to type 2 diabetes.

Vitamin C supplementation does not alter high-intensity endurance training-induced mitochondrial biogenesis in rat epitrochlearis muscle

The purpose of this study was to investigate whether vitamin C supplementation prevents high-intensity intermittent endurance training-induced mitochondrial biogenesis in the skeletal muscle. Male Wistar-strain rats were assigned to one of five groups: a control group, training group, small dose vitamin C supplemented training group, middle dose vitamin C supplemented training group, and large dose vitamin C supplemented training group. The rats of the trained groups were subjected to intense intermittent swimming training. The vitamin C supplemented groups were administrated vitamin C for the pretraining and training periods. High-intensity intermittent swimming training without vitamin C supplementation significantly increased peroxisome proliferator-activated receptor-γ coactivator-1α protein content and citrate synthase activity in the epitrochlearis muscle. The vitamin C supplementation did not alter the training-induced increase of these regardless of the dose of vitamin C supplementation. The results demonstrate that vitamin C supplementation does not prevent high-intensity intermittent training-induced mitochondrial biogenesis in the skeletal muscle.

Possible involvement of AMPK in acute exercise-induced expression of monocarboxylate transporters MCT1 and MCT4 mRNA in fast-twitch skeletal muscle

OBJECTIVE

The regulatory mechanisms responsible for acute exercise-induced expression of monocarboxylate transporters MCT1 and MCT4 mRNA in skeletal muscle remain unclear. 5'-adenosine-activated protein kinase (AMPK) is a key signaling molecule that regulates gene expression at the mRNA level. We examined whether AMPK activation is involved in acute exercise-induced expression of MCT1 and MCT4 mRNA in fast-twitch muscle.

MATERIALS/METHODS

Male Sprague-Dawley rats were subjected to an acute bout of either 5min high-intensity intermittent swimming (HIS) or 6-h low-intensity prolonged swimming (LIS). The effects of acute exercise on the phosphorylation of AMPK (p-AMPK), calcium/calmodulin pendent kinase II (p-CaMKII), p38 mitogen-activated protein kinase (p-p38MAPK), and MCTs mRNA were analyzed in vivo. To observe the direct effects of AMPK activation on MCTs mRNA, the effects of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), caffeine, and dantrolene were analyzed in vitro using an isolated muscle incubation model.

RESULTS

The p-AMPK increased in response to both HIS and LIS, although the p-CaMKII and p-p38MAPK were increased only following HIS. Irrespective of exercise intensity, MCT1 and MCT4 mRNA was also transiently upregulated by both HIS and LIS. Direct exposure of the epitrochlearis muscle to 0.5mmol/L AICAR or 1mmol/L caffeine, which activated p-AMPK increased both MCT1 and MCT4 mRNA levels. When pAMPK was inhibited by dantrolene, neither MCT1 nor MCT4 mRNA was increased.

CONCLUSION

These results suggest that acute exercise-induced increases in MCT1 and MCT4 mRNA expression may be possibly mediated by AMPK activation, at least in part in fast-twitch muscle.

Loss of the anorexic response to systemic 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside administration despite reducing hypothalamic AMP-activated protein kinase phosphorylation in insulin-deficient rats

This study tested whether chronic systemic administration of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) could attenuate hyperphagia, reduce lean and fat mass losses, and improve whole-body energy homeostasis in insulin-deficient rats. Male Wistar rats were first rendered diabetic through streptozotocin (STZ) administration and then intraperitoneally injected with AICAR for 7 consecutive days. Food and water intake, ambulatory activity, and energy expenditure were assessed at the end of the AICAR-treatment period. Blood was collected for circulating leptin measurement and the hypothalami were extracted for the determination of suppressor of cytokine signaling 3 (SOCS3) content, as well as the content and phosphorylation of AMP-kinase (AMPK), acetyl-CoA carboxylase (ACC), and the signal transducer and activator of transcription 3 (STAT3). Rats were thoroughly dissected for adiposity and lean body mass (LBM) determinations. In non-diabetic rats, despite reducing adiposity, AICAR increased (∼1.7-fold) circulating leptin and reduced hypothalamic SOCS3 content and food intake by 67% and 25%, respectively. The anorexic effect of AICAR was lost in diabetic rats, even though hypothalamic AMPK and ACC phosphorylation markedly decreased in these animals. Importantly, hypothalamic SOCS3 and STAT3 levels remained elevated and reduced, respectively, after treatment of insulin-deficient rats with AICAR. Diabetic rats were lethargic and displayed marked losses of fat and LBM. AICAR treatment increased ambulatory activity and whole-body energy expenditure while also attenuating diabetes-induced fat and LBM losses. In conclusion, AICAR did not reverse hyperphagia, but it promoted anti-catabolic effects on skeletal muscle and fat, enhanced spontaneous physical activity, and improved the ability of rats to cope with the diabetes-induced dysfunctional alterations in glucose metabolism and whole-body energy homeostasis.

Chronic treatment with the AMP-kinase activator AICAR increases glycogen storage and fatty acid oxidation in skeletal muscles but does not reduce hyperglucagonemia and hyperglycemia in insulin deficient rats

This study tested whether the glycogen-accumulating effect of chronic in vivo pharmacological 5′AMP-activated protein kinase (AMPK) activation could improve glycemic control under conditions of insulin deficiency. Male Wistar rats were rendered diabetic through the administration of streptozotocin (STZ) and then treated for 7 consecutive days with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR). Subsequently, glycogen content and synthesis, glucose oxidation, and fatty acid oxidation (FAO) were determined in oxidative and glycolytic skeletal muscles. Glycemia, insulinemia, glucagonemia, and circulating triglycerides (TG) and non-esterified fatty acids (NEFAs) were measured after AICAR treatment. Insulin was almost undetectable in STZ rats and these animals were severely hyperglycemic. Glycogen content was markedly low mainly in glycolytic muscles of STZ rats and AICAR treatment restored it to control values. No differences were found among all muscles studied with regards to the content and phosphorylation of Akt/protein kinase B and glycogen synthase kinase 3. Even though glycogen synthase content was reduced in all muscles from STZ rats, insulin-induced dephosphorylation/activation of this enzyme was preserved and unaffected by AICAR treatment. Glucagon and NEFAS were 2- and 7.4-fold fold higher in STZ rats than controls, respectively. AICAR did not affect hyperglycemia and hyperglucagonemia in STZ rats; however, it normalized circulating NEFAs and significantly increased FAO in glycolytic muscles. In conclusion, even though AICAR-induced AMPK activation enhanced glycogen accumulation in glycolytic muscles and normalized circulating NEFAs and TG levels, the hyperglycemic effects of glucagon likely offset the potentially glucose-lowering effects of AICAR, resulting in no improvement of glycemic control in insulin-deficient rats.

Clustering of GLUT4, TUG, and RUVBL2 protein levels correlate with myosin heavy chain isoform pattern in skeletal muscles, but AS160 and TBC1D1 levels do not

Skeletal muscle is a heterogeneous tissue. To further elucidate this heterogeneity, we probed relationships between myosin heavy chain (MHC) isoform composition and abundance of GLUT4 and four other proteins that are established or putative GLUT4 regulators [Akt substrate of 160 kDa (AS160), Tre-2/Bub2/Cdc 16-domain member 1 (TBC1D1), Tethering protein containing an UBX-domain for GLUT4 (TUG), and RuvB-like protein two (RUVBL2)] in 12 skeletal muscles or muscle regions from Wistar rats [adductor longus, extensor digitorum longus, epitrochlearis, gastrocnemius (mixed, red, and white), plantaris, soleus, tibialis anterior (red and white), tensor fasciae latae, and white vastus lateralis]. Key results were 1) significant differences found among the muscles (range of muscle expression values) for GLUT4 (2.5-fold), TUG (1.7-fold), RUVBL2 (2.0-fold), and TBC1D1 (2.7-fold), but not AS160; 2) significant positive correlations for pairs of proteins: GLUT4 vs. TUG (R = 0.699), GLUT4 vs. RUVBL2 (R = 0.613), TUG vs. RUVBL2 (R = 0.564), AS160 vs. TBC1D1 (R = 0.293), and AS160 vs. TUG (R = 0.246); 3) significant positive correlations for %MHC-I: GLUT4 (R = 0.460), TUG (R = 0.538), and RUVBL2 (R = 0.511); 4) significant positive correlations for %MHC-IIa: GLUT4 (R = 0.293) and RUVBL2 (R = 0.204); 5) significant negative correlations for %MHC-IIb vs. GLUT4 (R = -0.642), TUG (R = -0.626), and RUVBL2 (R = -0.692); and 6) neither AS160 nor TBC1D1 significantly correlated with MHC isoforms. In 12 rat muscles, GLUT4 abundance tracked with TUG and RUVBL2 and correlated with MHC isoform expression, but was unrelated to AS160 or TBC1D1. Our working hypothesis is that some of the mechanisms that regulate GLUT4 abundance in rat skeletal muscle also influence TUG and RUVBL2 abundance.

An amino acid mixture enhances insulin-stimulated glucose uptake in isolated rat epitrochlearis muscle

Protein and certain amino acids (AA) have been found to lower blood glucose. Although these glucose-lowering AA are important modulators of skeletal muscle metabolism, their impact on muscle glucose uptake remains unclear. We therefore examined how an AA mixture consisting of 2 mM isoleucine, 0.012 mM cysteine, 0.006 mM methionine, 0.0016 mM valine, and 0.014 mM leucine impacts skeletal muscle glucose uptake in the absence or presence of a submaximal (sINS) or maximal insulin (mINS) concentration. The AA mixture, sINS, and mINS significantly increased 2-[(3)H]deoxyglucose (2-DG) uptake by 63, 79, and 298% above basal, respectively. When the AA mixture was combined with sINS and mINS, 2-DG uptake was further increased significantly by 26% (P = 0.028) and 14% (P = 0.032), respectively. Western blotting analysis revealed that the AA mixture increased basal and sINS Akt substrate of 160 kDa (AS160) phosphorylation, while AA mixture did not change phosphorylation of Akt or mammalian target of rapamycin (mTOR) under these conditions. Interestingly, addition of the AA mixture to mINS increased phosphorylation of mTOR, Akt as well as AS160, compared with mINS alone. These data suggest that certain AA increase glucose uptake in the absence of insulin and augment insulin-stimulated glucose uptake in an additive manner. Furthermore, these effects appear to be mediated via a pathway that is independent from the canonical insulin cascade and therefore may prove effective as an alternative therapeutic treatment for insulin resistance.

Insulin resistance for glucose uptake and Akt2 phosphorylation in the soleus, but not epitrochlearis, muscles of old vs. adult rats

The slow-twitch soleus, but not fast-twitch muscle, of old vs. adult rats has previously been demonstrated to become insulin resistant for in vivo glucose uptake. We probed cellular mechanisms for the age effect by assessing whether insulin resistance for glucose uptake was an intrinsic characteristic of the muscle ex vivo and by analyzing key insulin signaling steps. We hypothesized that isolated soleus and epitrochlearis (fast-twitch) muscles from old (25 mo) vs. adult (9 mo) male Fisher-344 x Brown Norway rats would have insulin resistance for Akt2 Thr308 phosphorylation (pAkt2Thr308), AS160 phosphorylation Thr642 (pAS160Thr642), and atypical PKC (aPKCzeta/lambda) activity corresponding in magnitude to the extent of insulin resistance for [3H]-2-deoxyglucose (2-DG) uptake. Epitrochlearis insulin-stimulated 2-DG uptake above basal values was unaltered by age, and epitrochlearis pAkt2Thr308, pAS160Thr642, and aPKCzeta/lambda activity were not significantly different in adult vs. old rats. Conversely, insulin-stimulated 2-DG uptake by the soleus of old vs. adult rats was reduced with 1.2 nM (42%) and 30 nM (28%) insulin concomitant with an age-related decline in pAkt2Thr308 of the insulin-stimulated soleus. There were no age effects on pAS160Thr642 or aPKCzeta/lambda activity or abundance of Akt2, AS160, GLUT4 or Appl1 protein in either muscle. The results suggest the possibility that an age-related decline in pAkt2Thr308, acting by a mechanism other than reduced pAS160Thr642, may play a role in the insulin resistance in the soleus of old rats. Skeletal muscle insulin resistance in old age is distinctive compared with other insulin-resistant rodent models that are not selective for greater insulin resistance in the soleus vs. the epitrochlearis.

Leucine modulates contraction- and insulin-stimulated glucose transport and upstream signaling events in rat skeletal muscle

Leucine has profound effects on glucose metabolism in muscle; however, the effects of leucine on glucose transport in muscle have not been well documented. We investigated the effects of leucine on contraction- and insulin-stimulated glucose transport in isolated rat epitrochlearis muscle in vitro. In the absence of insulin, tetanic contraction increased 3-O-methyl-D-glucose (3-MG) transport and Thr(172) phosphorylation of the catalytic alpha-subunit of 5'-AMP-activated protein kinase (AMPK), a signaling intermediary leading to insulin-independent glucose transport. Leucine (2 mM, 30 min) significantly enhanced contraction-stimulated 3-MG transport and AMPK phosphorylation, accompanied by increased phosphorylation of p70 S6 kinase (p70S6K) Thr(389). The stimulatory effects of leucine on 3-MG transport and AMPK phosphorylation were canceled by STO-609 blockade of Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK) or rapamycin blockade of p70S6K. On the other hand, leucine blunted insulin-stimulated 3-MG transport and reduced insulin-stimulated Akt Thr(473) phosphorylation. Leucine increased insulin-stimulated p70S6K Thr(389) phosphorylation and enhanced the inhibitory phosphorylation of the insulin receptor substrate 1 (IRS1) Ser(636/639). Furthermore, the effects of leucine on insulin-stimulated 3-MG transport and IRS phosphorylation were abolished by rapamycin. These results indicate that leucine activates contraction-stimulated glucose transport and inhibits insulin-stimulated glucose transport in skeletal muscle by activating mammalian target of rapamycin (mTOR)/p70S6K signaling. Enhanced increases in contraction-stimulated AMPK Thr(172) phosphorylation and insulin-stimulated IRS1 Ser(636/639) phosphorylation might be responsible for these opposing effects of leucine, respectively.

A myosin II ATPase inhibitor reduces force production, glucose transport, and phosphorylation of AMPK and TBC1D1 in electrically stimulated rat skeletal muscle

Contraction-stimulated glucose transport by skeletal muscle appears to be caused by the cumulative effects of multiple inputs [potentially including AMP-activated protein kinase (AMPK), Ca(2+) flux, and force production], making it challenging to isolate the roles of these putative regulatory factors. To distinguish the effects of force production from the direct consequences of Ca(2+) flux, the predominantly type II rat epitrochlearis muscle was incubated without (vehicle) or with N-benzyl-p-toluenesulfonamide (BTS), a highly specific myosin II ATPase inhibitor that prevents force production by electrically stimulated (ES) type II fibers without altering cytosolic Ca(2+). In ES muscles, BTS vs. vehicle had an 84% reduction in force production and a 57% decrement in contraction-stimulated 3-O-methylglucose transport (3MGT). BTS did not alter the ES increase in phosphorylation of CaMKII (indicative of cytosolic Ca(2+)) or the amount of glycogen depletion. ES caused significant reductions in ATP (48%) and phosphocreatine (67%) concentrations for vehicle-treated muscles. For BTS-treated muscles, ES did not reduce ATP and caused only a 42% decrease in phosphocreatine. There was an ES increase in phosphorylation of AMPK, acetyl-CoA carboxylase (an AMPK substrate), and TBC1D1 for vehicle-treated muscles but not for BTS-treated muscles. These results point toward an essential role for tension-related events, including AMPK activation, in the 57% contraction-stimulated increase in 3MGT that was inhibited by BTS and further suggest a possible role for TBC1D1 phosphorylation. Non-tension-related events (e.g., increased cytosolic Ca(2+) rather than increased AMPK and TBC1D1 phosphorylation) are implicated in the contraction-stimulated increase in 3MGT that persisted in the presence of BTS.

A Ca(2+)-calmodulin-eEF2K-eEF2 signalling cascade, but not AMPK, contributes to the suppression of skeletal muscle protein synthesis during contractions

Skeletal muscle protein synthesis rate decreases during contractions but the underlying regulatory mechanisms are poorly understood. It was hypothesized that there would be a coordinated regulation of eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E-binding protein 1 (4EBP1) phosphorylation by signalling cascades downstream of rises in intracellular [Ca(2+)] and decreased energy charge via AMP-activated protein kinase (AMPK) in contracting skeletal muscle. When fast-twitch skeletal muscles were contracted ex vivo using different protocols, the suppression of protein synthesis correlated more closely with changes in eEF2 than 4EBP1 phosphorylation. Using a combination of Ca(2+) release agents and ATPase inhibitors it was shown that the 60-70% suppression of fast-twitch skeletal muscle protein synthesis during contraction was equally distributed between Ca(2+) and energy turnover-related mechanisms. Furthermore, eEF2 kinase (eEF2K) inhibition completely blunted increases in eEF2 phosphorylation and partially blunted (i.e. 30-40%) the suppression of protein synthesis during contractions. The 3- to 5-fold increase in skeletal muscle eEF2 phosphorylation during contractions in situ was rapid and sustained and restricted to working muscle. The increase in eEF2 phosphorylation and eEF2K activation were downstream of Ca(2+)-calmodulin (CaM) but not other putative activating factors such as a fall in intracellular pH or phosphorylation by protein kinases. Furthermore, blunted protein synthesis and 4EBP1 dephosphorylation were unrelated to AMPK activity during contractions, which was exemplified by normal blunting of protein synthesis during contractions in muscles overexpressing kinase-dead AMPK. In summary, in fast-twitch skeletal muscle, the inhibition of eEF2 activity by phosphorylation downstream of Ca(2+)-CaM-eEF2K signalling partially contributes to the suppression of protein synthesis during exercise/contractions.

Age-related differences in skeletal muscle insulin signaling: the role of stress kinases and heat shock proteins

Aging is associated with an increase in insulin resistance in skeletal muscle, yet the underlying mechanism is not well established. We hypothesize that with aging, a chronic increase in stress kinase activation, coupled with a decrease in oxidative capacity, leads to insulin resistance in skeletal muscle. In aged (24 mo old) and young (3 mo old) Fischer 344 rats, 2-deoxyglucose uptake and insulin signaling [as measured by phosphorylation of insulin receptor substrate-1 (IRS-1), Akt (protein kinase B), and Akt substrate of 160 kDa (AS160)] decreased significantly with age. Activation of, c-Jun NH(2)-terminal kinase (JNK), glycogen serine kinase-3beta (GSK-3beta), and degradation of IkappaBalpha by the upstream inhibitor of kappa B kinase (IKKbeta), as measured by Western blot analysis, were increased with age in both soleus and epitrochlearis (Epi) muscles. However, much higher activation of these kinases in Epi muscles from young rats compared with soleus results in a greater effect of these kinases on insulin signaling in fast-twitch muscle with age. Heat shock protein (HSP) 72 expression and phosphorylation of HSP25 were higher in soleus compared with Epi muscles, and both parameters decreased with age. Age and fiber type differences in cytochrome oxidase activity are consistent with observed changes in HSP expression and activation. Our results demonstrate a significant difference in the ability of slow-twitch and fast-twitch muscles to respond to insulin and regulate glucose with age. A greater constitutive HSP expression and lower stress kinase activation may account for the ability of slow-twitch muscles to preserve the capacity to respond to insulin and maintain glucose homeostasis with age.

Effect of acute activation of 5'-AMP-activated protein kinase on glycogen regulation in isolated rat skeletal muscle

5'-AMP-activated protein kinase (AMPK) has been implicated in glycogen metabolism in skeletal muscle. However, the physiological relevance of increased AMPK activity during exercise has not been fully clarified. This study was performed to determine the direct effects of acute AMPK activation on muscle glycogen regulation. For this purpose, we used an isolated rat muscle preparation and pharmacologically activated AMPK with 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR). Tetanic contraction in vitro markedly activated the alpha(1)- and alpha(2)-isoforms of AMPK, with a corresponding increase in the rate of 3-O-methylglucose uptake. Incubation with AICAR elicited similar enhancement of AMPK activity and 3-O-methylglucose uptake in rat epitrochlearis muscle. In contrast, whereas contraction stimulated glycogen synthase (GS), AICAR treatment decreased GS activity. Insulin-stimulated GS activity also decreased after AICAR treatment. Whereas contraction activated glycogen phosphorylase (GP), AICAR did not alter GP activity. The muscle glycogen content decreased in response to contraction but was unchanged by AICAR. Lactate release was markedly increased when muscles were stimulated with AICAR in buffer containing glucose, indicating that the glucose taken up into the muscle was catabolized via glycolysis. Our results suggest that AMPK does not mediate contraction-stimulated glycogen synthesis or glycogenolysis in skeletal muscle and also that acute AMPK activation leads to an increased glycolytic flux by antagonizing contraction-stimulated glycogen synthesis.

Ca2+ and AMPK both mediate stimulation of glucose transport by muscle contractions

It is now generally accepted that activation of AMP-activated protein kinase (AMPK) is involved in the stimulation of glucose transport by muscle contractions. However, earlier studies provided evidence that increases in cytosolic Ca(2+) mediate the effect of muscle contractions on glucose transport. The purpose of this study was to test the hypothesis that both the increase in cytosolic Ca(2+) and the activation of AMPK are involved in the stimulation of glucose transport by muscle contractions. Caffeine causes release of Ca(2+) from the sarcoplasmic reticulum. Incubation of rat epitrochlearis muscles with a concentration of caffeine that raises cytosolic Ca(2+) to levels too low to cause contraction resulted in an approximate threefold increase in glucose transport. Caffeine treatment also resulted in increased phosphorylation of calmodulin-dependent protein kinase (CAMK)-II in epitrochlearis muscle. The stimulation of glucose transport by caffeine was blocked by the Ca(2+)-CAMK inhibitors KN62 and KN93. Activation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) also resulted in an approximate threefold increase in glucose transport in the epitrochlearis. The increases in glucose transport induced by AICAR and caffeine were additive, and their combined effect was not significantly different from that induced by maximally effective contractile activity. KN62 and KN93 caused an approximately 50% inhibition of the stimulation of glucose transport by contractile activity. Our results provide evidence that both Ca(2+) and AMPK are involved in the stimulation of glucose transport by muscle contractions. They also suggest that the stimulation of glucose transport by Ca(2+) involves activation of CAMK.

Effects of acute bouts of running and swimming exercise on PGC-1alpha protein expression in rat epitrochlearis and soleus muscle

The purpose of this study was to elucidate the mechanisms underlying low-intensity exercise-induced peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) protein expression in rat skeletal muscles. Rats (5-6 wk old) swam without a load and ran on the treadmill at a speed of 13 m/min, respectively, in two 3-h sessions separated by 45 min of rest. PGC-1alpha content in epitrochlearis muscle (EPI) was increased by 75 and 95%, immediately and 6 h after swimming, respectively, with no increase in PGC-1alpha content in the soleus (SOL). After running, PGC-1alpha content in EPI was unchanged, whereas a 107% increase in PGC-1alpha content was observed in SOL 6 h after running. Furthermore, in EPI and SOL as well as other muscles (triceps, plantaris, red and white gastrocnemius), PGC-1alpha expression was enhanced concomitant with reduced glycogen postexercise, suggesting that expression of PGC-1alpha occurs in skeletal muscle recruited during exercise. PGC-1alpha content in EPI was increased after 18-h in vitro incubation with 0.5 mM 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) and 4 mM caffeine. However, AICAR incubation did not affect PGC-1alpha content in the SOL, whereas caffeine incubation increased it. These results suggest that exercise-induced PGC-1alpha expression in skeletal muscle may be mediated by at least two exercise-induced signaling factors: AMPK activation and Ca2+ elevation. The number of factors involved (both AMPK and Ca2+, or Ca2+ only) in exercise-induced PGC-1alpha expression may differ among muscles.

Changes in [Ca2+]i induced by several glucose transport-enhancing stimuli in rat epitrochlearis muscle

The purpose of the present investigation was to establish a method for estimating intracellular Ca(2+) concentrations ([Ca(2+)](i)) in isolated rat epitrochlearis muscles. Epitrochlearis muscles excised from 4-wk-old male Sprague-Dawley rats were loaded with a fluorescent Ca(2+) indicator, fura 2-AM, for 60-90 min at 35 degrees C in oxygenated Krebs-Henseleit buffer. After fura 2 loading and subsequent 20-min incubation, the intensities of 500-nm fluorescence, induced by 340- and 380-nm excitation lights (F(total)340 and F(total)380), were measured. The fluorescences specific to fura-2 (F(fura 2)340 and F(fura 2)380) were calculated by subtracting the non-fura 2-specific component from F(total)340 and F(total)380, respectively. The ratio of F(fura 2)340 to F(fura 2)380 was calculated as R, and the change in the ratio from the baseline value (DeltaR) was used as an index of the change in [Ca(2+)](i). In resting muscle, DeltaR was stable for 60 min. Incubation for 20 min with caffeine (3-10 mM) significantly increased DeltaR in a concentration-dependent manner. Incubation with hypoxic Krebs-Henseleit buffer for 10-60 min significantly elevated DeltaR, depending on the duration of the incubation. Incubation with 50 microM N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide for 20 min significantly elevated DeltaR (P < 0.05). No significant increases in DeltaR were observed during incubation for 20 min with 2 mM 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside or with 2 mU/ml insulin. These results demonstrated that, by using the fura 2-AM fluorescence method, the changes in [Ca(2+)](i) can be monitored in the rat epitrochlearis muscle and suggest that the method can be utilized to observe quantitative information regarding [Ca(2+)](i) that may be involved in contraction- and hypoxia-stimulated glucose transport activity in skeletal muscle.

Modulation of muscle insulin resistance by selective inhibition of GSK-3 in Zucker diabetic fatty rats

A role for elevated glycogen synthase kinase-3 (GSK-3) activity in the multifactorial etiology of insulin resistance is now emerging. However, the utility of specific GSK-3 inhibition in modulating insulin resistance of skeletal muscle glucose transport is not yet fully understood. Therefore, we assessed the effects of novel, selective organic inhibitors of GSK-3 (CT-98014 and CT-98023) on glucose transport in insulin-resistant muscles of Zucker diabetic fatty (ZDF) rats. Incubation of type IIb epitrochlearis and type I soleus muscles from ZDF rats with CT-98014 increased glycogen synthase activity (49 and 50%, respectively, P < 0.05) but did not alter basal glucose transport (2-deoxyglucose uptake). In contrast, CT-98014 significantly increased the stimulatory effects of both submaximal and maximal insulin concentrations in epitrochlearis (37 and 24%) and soleus (43 and 26%), and these effects were associated with increased cell-surface GLUT4 protein. Lithium enhanced glycogen synthase activity and both basal and insulin-stimulated glucose transport in muscles from ZDF rats. Acute oral administration (2 x 30 mg/kg) of CT-98023 to ZDF rats caused elevations in GSK-3 inhibitor concentrations in plasma and muscle. The glucose and insulin responses during a subsequent oral glucose tolerance test were reduced by 26 and 34%, respectively, in the GSK-3 inhibitor-treated animals. Thirty minutes after the final GSK-3 inhibitor treatment, insulin-stimulated glucose transport was significantly enhanced in epitrochlearis (57%) and soleus (43%). Two hours after the final treatment, insulin-mediated glucose transport was still significantly elevated (26%) only in the soleus. These results indicate that specific inhibition of GSK-3 enhances insulin action on glucose transport in skeletal muscle of the insulin-resistant ZDF rat. This unique approach may hold promise as a pharmacological treatment against insulin resistance of skeletal muscle glucose disposal.

Effects of AICAR and exercise on insulin-stimulated glucose uptake, signaling, and GLUT-4 content in rat muscles

Physical activity is known to increase insulin action in skeletal muscle, and data have indicated that 5'-AMP-activated protein kinase (AMPK) is involved in the molecular mechanisms behind this beneficial effect. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) can be used as a pharmacological tool to repetitively activate AMPK, and the objective of this study was to explore whether the increase in insulin-stimulated glucose uptake after either long-term exercise or chronic AICAR administration was followed by fiber-type-specific changes in insulin signaling and/or changes in GLUT-4 expression. Wistar rats were allocated into three groups: an exercise group trained on treadmill for 5 days, an AICAR group exposed to daily subcutaneous injections of AICAR, and a sedentary control group. AMPK activity, insulin-stimulated glucose transport, insulin signaling, and GLUT-4 expression were determined in muscles characterized by different fiber type compositions. Both exercised and AICAR-injected animals displayed a fiber-type-specific increase in glucose transport with the most marked increase in muscles with a high content of type IIb fibers. This increase was accompanied by a concomitant increase in GLUT-4 expression. Insulin signaling as assessed by phosphatidylinositol 3-kinase and PKB/Akt activity was enhanced only after AICAR administration and in a non-fiber-type-specific manner. In conclusion, chronic AICAR administration and long-term exercise both improve insulin-stimulated glucose transport in skeletal muscle in a fiber-type-specific way, and this is associated with an increase in GLUT-4 content.

Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.

Glucose uptake and metabolic stress in rat muscles stimulated electrically with different protocols

In the present study, the relationship between the pattern of electrical stimulation and glucose uptake was investigated in slow-twitch muscles (soleus) and fast-twitch muscles (epitrochlearis) from Wistar rats. Muscles were stimulated electrically for 30 min in vitro with either single pulses (frequencies varied between 0.8 and 15 Hz) or with 200-ms trains (0.1-2 Hz). Glucose uptake (measured with tracer amount of 2-[(3)H]deoxyglucose) increased with increasing number of impulses whether delivered as single pulses or as short trains. The highest glucose uptake achieved with short tetanic contractions was similar in soleus and epitrochlearis (10.9 +/- 0.7 and 12.0 +/- 0.8 mmol x kg dry wt(-1) x 30 min(-1), respectively). Single pulses, on the other hand, increased contraction-stimulated glucose uptake less in soleus than in epitrochlearis (7.5 +/- 1.1 and 11.7 +/- 0.5 mmol x kg dry wt(-1) x 30 min(-1), respectively; P < 0.02). Glucose uptake correlated with glycogen breakdown in soleus (r = 0.84, P < 0.0001) and (epitrochlearis: r = 0.91, P < 0.0001). Contraction-stimulated glucose uptake also correlated with breakdown of ATP and PCr and with reduction in force. Our data suggest that metabolic stress mediates contraction-stimulated glucose uptake.

Motor nerve topology reflects myocyte morphology in hamster retractor and epitrochlearis muscles

Neuromuscular activation is a primary determinant of metabolic demand and oxygen transport. The m. retractor and m. epitrochlearis are model systems for studying metabolic control and oxygen transport; however, the organization of muscle fibers and motor nerves in these muscles is unknown. We tested whether the topology of motor innervation was related to the morphology of muscle fibers in m. retractor and m. epitrochlearis of male hamsters ( approximately 100 g). Respective muscles averaged 47 and 12 mm in length 100 and 35 mg in mass. Staining for acetylcholinesterase revealed neuromuscular junctions arranged in clusters throughout m. retractor and as a central band across m. epitrochlearis, suggesting differences in fiber morphology. For both muscles, complete cross-sections contained approximately 1,700 fibers. Fiber cross-sectional areas were distributed nearly normal in m. epitrochlearis (mean = 1,559 +/- 17 microm(2)) and skewed left (P < 0.05) in m. retractor (mean = 973 +/- 15 microm(2)). Single fiber length (Lf) spanned muscle length (Lm) in m. epitrochlearis, while fibers tapered to terminate within m. retractor (Lf/Lm = 0.43 +/- 0. 02). With myelin staining, a single branch of ulnar nerve projected axons across the midregion of m. epitrochlearis. For m. retractor, the spinal accessory nerve branched to give rise to proximal and distal regions of innervation, with intermingling of axons between nerve branches. Nerve bundle cross-sections stained for acetylcholinesterase indicate that each motor axon projects to an average of 65 muscle fibers in m. epitrochlearis and 100 in m. retractor. Differences in fiber morphology, innervation topology, and neuromuscular organization may contribute to the heterogeneity of metabolic demand and oxygen supply in skeletal muscle.

Muscle wasting in insulinopenic rats results from activation of the ATP-dependent, ubiquitin-proteasome proteolytic pathway by a mechanism including gene transcription

In normal subjects and diabetic patients, insulin suppresses whole body proteolysis suggesting that the loss of lean body mass and muscle wasting in insulinopenia is related to increased muscle protein degradation. To document how insulinopenia affects organ weights and to identify the pathway for accelerated proteolysis in muscle, streptozotocin-treated and vehicle-injected, pair-fed control rats were studied. The weights of liver, adipose tissue, and muscle were decreased while muscle protein degradation was increased 75% by insulinopenia. This proteolytic response was not eliminated by blocking lysosomal function and calcium-dependent proteases at 7 or 3 d after streptozotocin. When ATP synthesis in muscle was inhibited, the rates of proteolysis were reduced to the same level in insulinopenic and control rats suggesting that the ATP-dependent, ubiquitin-proteasome pathway is activated. Additional evidence for activation of this pathway in muscle includes: (a) an inhibitor of proteasome activity eliminated the increased protein degradation; (b) mRNAs encoding ubiquitin and proteasome subunits were increased two- to threefold; and (c) there was increased transcription of the ubiquitin gene. We conclude that the mechanism for muscle protein wasting in insulinopenia includes activation of the ubiquitin-proteasome pathway with increased expression of the ubiquitin gene.

The acidosis of chronic renal failure activates muscle proteolysis in rats by augmenting transcription of genes encoding proteins of the ATP-dependent ubiquitin-proteasome pathway

Chronic renal failure (CRF) is associated with negative nitrogen balance and loss of lean body mass. To identify specific proteolytic pathways activated by CRF, protein degradation was measured in incubated epitrochlearis muscles from CRF and sham-operated, pair-fed rats. CRF stimulated muscle proteolysis, and inhibition of lysosomal and calcium-activated proteases did not eliminate this increase. When ATP production was blocked, proteolysis in CRF muscles fell to the same level as that in control muscles. Increased proteolysis was also prevented by feeding CRF rats sodium bicarbonate, suggesting that activation depends on acidification. Evidence that the ATP-dependent ubiquitin-proteasome pathway is stimulated by the acidemia of CRF includes the following findings: (a) An inhibitor of the proteasome eliminated the increase in muscle proteolysis; and (b) there was an increase in mRNAs encoding ubiquitin (324%) and proteasome subunits C3 (137%) and C9 (251%) in muscle. This response involved gene activation since transcription of mRNAs for ubiquitin and the C3 subunit were selectively increased in muscle of CRF rats. We conclude that CRF stimulates muscle proteolysis by activating the ATP-ubiquitin-proteasome-dependent pathway. The mechanism depends on acidification and increased expression of genes encoding components of the system. These responses could contribute to the loss of muscle mass associated with CRF.

Acute effects of phorbol esters on the protein-synthetic rate and carbohydrate metabolism of normal and mdx mouse muscles

1. mdx mice do not express dystrophin, the product of the gene which is defective in Duchenne and Becker muscular dystrophy. We have previously shown that protein-synthetic rates (ks) are increased in mdx mouse muscles [MacLennan & Edwards (1990) Biochem. J. 268, 795-797]. 2. The tumour-promoting stereoisomer of phorbol 12,13-didecanoate (4 beta-PDD) acutely increased the ks of muscles from mdx and wild-type (C57BL/10) mice incubated in vitro in the absence of insulin. The effects of 4 beta-PDD are presumably mediated by activation of protein kinase C (PKC). 3. The muscle glycogen concentrations of mdx mice were higher than those of C57BL/10 mice. Studies performed in vivo and in vitro suggested that the effect might be at least partially due to increased rate of glycogen synthesis in mdx muscle. 4. 4 beta-PDD increased the glycogen-synthetic rates rates of C57BL/10, but not mdx, muscles incubated in vitro in the absence of insulin. 5. In muscles from both species incubated in the absence of insulin, treatment with 4 beta-PDD also induced increased rates of glucose uptake and lactate production. Kinetic studies of C57BL/10 and mdx muscles suggested that 4 beta-PDD raised the Vmax. of glucose uptake, but did not alter the Km for the process. 6. The possible role of PKC in controlling the protein and carbohydrate metabolism of normal and mdx mouse muscles is discussed.

Long-term treatment of isolated rat soleus muscle with phorbol ester leads to loss of contraction-induced glucose transport

Muscle contraction involves mobilization of intracellular Ca2+ and is associated with several metabolic adjustments, including increased glucose transport. In the present study isolated rat soleus muscles were exposed to 12-O-tetradecanoylphorbol 13-acetate, and responses to both insulin and contraction in terms of glucose transport were assessed. Muscles treated with this phorbol ester for 12 h showed an increased basal rate of 3-O-methylglucose uptake, and responded partially to insulin but did not respond to contraction. Phorbol-ester-treated and non-treated (vehicle-only) muscles were indistinguishable in terms of pre-contraction content of adenine nucleotide, phosphocreatine, lactate and glycogen, as well as contractile performance and contraction-induced glycogenolysis. Phorbol ester treatment of isolated solei for 12 h resulted in the loss of 90% of protein kinase C activity as determined with histone IIIs as substrate, and 70% as determined by using phorbol ester binding. It is concluded that treatment of solei with phorbol ester gives rise to a marked loss of contraction-induced glucose transport.

A method to quantify glucose utilization in vivo in skeletal muscle and white adipose tissue of the anaesthetized rat

A quantitative method allowing determination of glucose metabolism in vivo in muscles and white adipose tissue of the anaesthetized rat is presented. A tracer dose of 2-deoxy[3H]glucose was injected intravenously in an anaesthetized rat and the concentration of 2-deoxy[3H]glucose was monitored in arterial blood. After 30-80 min, three muscles, the soleus, the extensor digitorum longus and the epitrochlearis, periovarian white adipose tissue and brain were sampled and analysed for their content of 2-deoxy[3H]glucose 6-phosphate. This content could be related to glucose utilization during the same time period, since (1) the integral of the decrease of 2-deoxy[3H]glucose in arterial blood was known and (2) correction factors for the analogue effect of 2-deoxyglucose compared with glucose in the transport and phosphorylation steps were determined from experiments in vitro. Glucose utilization was then measured by this technique in the tissues of post-absorptive rats in the basal state (0.1 munit of insulin/ml of plasma) or during euglycaemic-hyperinsulinaemic glucose clamp (8 munits of insulin/ml of plasma) and of 48 h-starved rats. Results corresponded qualitatively and quantitatively to the known physiological characteristics of the tissues studied.

Insulin sensitivity and responsiveness of epitrochlearis and soleus muscles from fed and starved rats. Recognition of differential changes in insulin sensitivities of protein synthesis and glucose incorporation into glycogen

The insulin sensitivity of protein synthesis and glucose incorporation into glycogen by the soleus and epitrochlearis muscles from fed rats and 24 h-starved rats was determined in vitro during the first and second hours of incubation after isolation of the muscles. Rates of protein synthesis by both muscles from fed rats in the first hour of incubation were 2-fold higher than in the second hour and were not increased by insulin. Rates of protein synthesis during the first hour in the presence of 6000 microunits of insulin/ml were increased in soleus, but not in epitrochlearis, muscles from starved rats. Rates of protein synthesis in both muscles from fed and starved rats were increased significantly by insulin during the second hour. High concentrations of insulin caused a marked stimulation of the rates of glucose incorporation by both muscles from fed and starved rats in both the first and second hours of incubation. The insulin sensitivity of glucose incorporation during the second hour, defined as the concentration of insulin causing half-maximal stimulation, was increased 10-fold for both muscle types from starved rats (soleus, 65 microunits/ml; epitrochlearis, 45 microunits/ml) relative to muscles from fed rats (soleus, 600 microunits/ml; epitrochlearis, 500 microunits/m). The insulin sensitivity of protein synthesis in the second hour was greater for soleus muscles from starved rats (65 microunits/ml) than from fed rats (500 microunits/ml). In contrast, the insulin sensitivity of protein synthesis in epitrochlearis muscles from starved rats was significantly decreased (225 microunits/ml) compared with fed rats (25 microunits/ml Maximal rates achieved by high concentrations of insulin were not different from those in the same muscle from fed rats. It is suggested that protein synthesis, in distinction to glucose utilization, may be resistant to insulin stimulation during periods of acute starvation in muscles with fibre compositions similar to the epitrochlearis, but not in muscles with fibre compositions similar to the soleus. Partial reversal of the resistance observed in vitro for epitrochlearis muscles from starved rats may be due to the loss of factors which suppress the effect of insulin in vivo.

The rate of substrate cycling between fructose 6-phosphate and fructose 1,6-bisphosphate in skeletal muscle

Substrate cycling of fructose 6-phosphate through reactions catalysed by 6-phosphofructokinase and fructose-1,6-bisphosphatase was measured in skeletal muscles of the rat in vitro. The rate of this cycle was calculated from the steady-state values of the 3H/14C ratio in hexose monophosphates and fructose 1,6-bisphosphate after the metabolism of either [5-3H,6-14C]glucose or [3-3H,2-14C] glucose. Two techniques for the separation of hexose phosphates were studied; t.l.c. chromatography on poly(ethyleneimine)-cellulose sheets or ion-exchange chromatography coupled with enzymic conversion. These two methods gave almost identical results, suggesting that either technique could be used for determination of rates of fructose 6-phosphate/fructose 1,6-bisphosphate cycling. It was found that more than 50% of the 3H was retained in the fructose 1,6-bisphosphate; it is therefore probable that previous measurement of cycling rates, which have assumed complete loss of 3H, have underestimated the rate of this cycle. The effects of insulin, adrenaline and adrenergic agonists and antagonists on rates of fructose 6-phosphate/fructose 1,6-bisphosphate cycling were investigated. In the presence of insulin, adrenaline (1 microM) increased the cycling rate by about 10-fold in epitrochlearis muscle in vitro; the maximum rate under these conditions was about 2.5 mumol/h per g of tissue. The concentration of adrenaline that increased the cycling rate by 50% was about 50 nM. This effect of adrenaline appears to be mediated by the beta-adrenergic receptor, since the rate was increased by beta-adrenergic agonists and blocked by beta-adrenergic antagonists. From the knowledge of the precise rate of this cycle, the possible physiological importance of cycling is discussed.

Muscle protein turnover and glucose uptake in acutely uremic rats. Effects of insulin and the duration of renal insufficiency

Acute renal failure (ARF) in rats is associated with increased amino acid release from peripheral tissues and insulin resistance. To study whether abnormal protein and carbohydrate metabolism are linked in ARF, the effects of insulin on net muscle protein degradation (T) and on glucose uptake were measured in the perfused hindquarters of paired ARF and sham-operated (SO) rats. The basal rate of T increased 40% after 24 and 98% after 48 h of ARF. Insulin was less effective in decreasing T in ARF (-79% SO vs. -22% ARF 24 h and -64% SO vs. -23% ARF 48 h; P less than 0.01). Protein synthesis (PS) and protein degradation (PD) were measured independently in incubated epitrochlearis muscles; the increase in T after 24 h of ARF was due specifically to increased PD, while PS was unchanged. At this stage, insulin was less effective in decreasing PD in ARF (-10% ARF vs. -23% SO; P less than 0.02), although PS responded normally. After 48 h of ARF, the further increment in T was caused by the additional appearance of depressed basal and insulin-stimulated PS. This was confirmed in the perfused hindquarter (26 +/- 3 ARF vs. 38 +/- 3 SO, basal; 54 +/- 5 ARF vs 73 +/- 7 SO, insulin-stimulated, nmol phenylalanine/g per h; P less than 0.05). Although basal glucose uptake by hindquarters of ARF and SO rats was comparable, insulin-stimulated glucose uptake was 33% less at 24 and 44% less after 48 h of ARF. After 48 h of ARF, lactate and alanine release were increased and net glycogen synthesis in muscle was depressed. These abnormalities were even more apparent in the presence of insulin. Inefficient glucose utilization, estimated as the ratio of lactate release to glucose uptake, was correlated with T (r = +0.78; P less than 0.001). In conclusion, after 24 h of ARF, both increased PD and altered glucose utilization could be detected. After 48 h of ARF, T increased further because PS was depressed. At this time, glucose utilization was clearly abnormal and the results suggest that abnormal net protein degradation in ARF may be a consequence of defective glucose utilization.

Comparison of protein synthesis and degradation in incubated and perfused muscle

Rates of muscle protein synthesis and degradation measured in the perfused hindquarter were compared with those in incubated epitrochlearis muscles. With fed or starved mature rats, results without insulin treatment were identical. With insulin treatment, protein synthesis in perfused hindquarters was greater, though protein degradation was the same. Thus rates of muscle protein degradation estimated by these two methods in vitro correspond closely.

Effects of insulin in vitro on protein turnover in rat epitrochlearis muscle

Rates of protein synthesis and degradation were measured in the isolated rat epitrochlearis muscle by radiotracer techniques, by using the specific radioactivity of tRNA-bound amino acid as precursor for protein synthesis. The tissue maintained linear rates of protein synthesis for 3 h of incubation in the presence of amino acids and glucose and in the absence of insulin. Under these conditions, however, the muscles were in negative nitrogen balance, with rates of protein degradation exceeding rates of protein synthesis. Under steady-state conditions of labelling, the specific radioactivities of tRNA-bound leucine, phenylalanine and valine were significantly less than their respective values in the incubation medium, at concentrations in the medium varying from 1 to 10 times those in normal rat serum. Insulin caused a dose- and time-dependent increase in tRNA-based protein synthesis rates, more than doubling rates at 5 and 50 ng of insulin/ml. At the lower, physiological, concentration of insulin, the stimulation of protein synthesis was not observed until the third hour of incubation with the hormone, whereas the rate of protein synthesis at the higher concentration was elevated during the second hour. There were no delays in the stimulation by insulin of glucose conversion into glycogen. The delayed stimulatory effects of insulin on the rate of protein synthesis brought the tissue to a nitrogen balance near zero. The presence of the hormone also prevented the increase in the rate of protein degradation seen in the third hour of incubation in the absence of the hormone. These studies demonstrate the viability of the incubated rat epitrochlearis muscle with respect to protein metabolism and sensitivity to the protein anabolic effects of physiological concentrations of insulin, and indicate that the preparation is a suitable experimental model for the study of the control of protein metabolism in fast-twitch skeletal muscle.