Acid Metabolism In Skeletal Muscle Research Articles

Effects of Metformin and Rosiglitazone Treatment on Insulin Signaling and Glucose Uptake in Patients With Newly Diagnosed Type 2 Diabetes

The effect of metformin or rosiglitazone monotherapy versus placebo on insulin signaling and gene expression in skeletal muscle of patients with newly diagnosed type 2 diabetes was determined. A euglycemic-hyperinsulinemic clamp, combined with skeletal muscle biopsies and glucose uptake measurements over rested and exercised muscle, was performed before and after 26 weeks of metformin (n = 9), rosiglitazone (n = 10), or placebo (n = 11) treatment. Insulin-mediated whole-body and leg muscle glucose uptake was enhanced 36 and 32%, respectively, after rosiglitazone (P < 0.01) but not after metformin or placebo treatment. Insulin increased insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation, IRS-1-associated phosphatidylinositol (PI) 3-kinase activity, and phosphorylation of Akt Ser473 and AS160, a newly described Akt substrate that plays a role in GLUT4 exocytosis, approximately 2.3 fold before treatment. These insulin signaling parameters were unaltered after metformin, rosiglitazone, or placebo treatment. Expression of selected genes involved in glucose and fatty acid metabolism in skeletal muscle was unchanged between the treatment groups. Low-intensity acute exercise increased insulin-mediated glucose uptake but was without effect on insulin signaling. In conclusion, the insulin-sensitizing effects of rosiglitazone are independent of enhanced signaling of IRS-1/PI 3-kinase/Akt/AS160 in patients with newly diagnosed type 2 diabetes.

Impaired expression of NADH dehydrogenase subunit 1 and PPARγ coactivator-1 in skeletal muscle of ZDF rats: restoration by troglitazone

Type 2 diabetes has been related to a decrease of mitochondrial DNA (mtDNA) content. In this study, we show increased expression of the peroxisome proliferator-activated receptor-alpha (PPARalpha) and its target genes involved in fatty acid metabolism in skeletal muscle of Zucker Diabetic Fatty (ZDF) (fa/fa) rats. In contrast, the mRNA levels of genes involved in glucose transport and utilization (GLUT4 and phosphofructokinase) were decreased, whereas the expression of pyruvate dehydrogenase kinase 4 (PDK-4), which suppresses glucose oxidation, was increased. The shift from glucose to fatty acids as the source of energy in skeletal muscle of ZDF rats was accompanied by a reduction of subunit 1 of complex I (NADH dehydrogenase subunit 1, ND1) and subunit II of complex IV (cytochrome c oxidase II, COII), two genes of the electronic transport chain encoded by mtDNA. The transcript levels of PPARgamma Coactivator 1 (PGC-1) showed a significant reduction. Treatment with troglitazone (30 mg/kg/day) for 15 days reduced insulin values and reversed the increase in PDK-4 mRNA levels, suggesting improved insulin sensitivity. In addition, troglitazone treatment restored ND1 and PGC-1 expression in skeletal muscle. These results suggest that troglitazone may avoid mitochondrial metabolic derangement during the development of diabetes mellitus 2 in skeletal muscle.

3P-0708 PPARδ agonist ameliorates obesity and insulin resistance through coordinate regulation of fatty acid metabolism in skeletal muscle

3P-0708 PPARδ agonist ameliorates obesity and insulin resistance through coordinate regulation of fatty acid metabolism in skeletal muscle

Role of leucine in protein metabolism during exercise and recovery.

Exercise produces changes in protein and amino acid metabolism. These changes include degradation of the branched-chain amino acids, production of alanine and glutamine, and changes in protein turnover. One of the amino acid most affected by exercise is the branched-chain amino acid leucine. Recently, there has been an increased understanding of the role of leucine in metabolic regulations and remarkable new findings about the role of leucine in intracellular signaling. Leucine appears to exert a synergistic role with insulin as a regulatory factor in the insulin/ phosphatidylinositol-3 kinase (PI3-K) signal cascade. Insulin serves to activate the signal pathway, while leucine is essential to enhance or amplify the signal for protein synthesis at the level of peptide initiation. Studies feeding amino acids or leucine soon after exercise suggest that post-exercise consumption of amino acids stimulates recovery of muscle protein synthesis via translation regulations. This review focuses on the unique roles of leucine in amino acid metabolism in skeletal muscle during and after exercise.

The response of skeletal muscle to leptin.

There is now compelling evidence that, in addition to signaling to the central nervous system (CNS), leptin also exerts its metabolic effects acting directly on peripheral tissues. It has been demonstrated by in vivo and in vitro studies, that leptin increases glucose and fatty acid metabolism in skeletal muscle. These direct leptin effects are supported by the presence of the long form of the leptin receptor, considered to be capable of performing intracellular signaling, in peripheral tissues, including skeletal muscle. The exposure of soleus muscle to supra-physiological leptin concentrations stimulate the activity of both the pyruvate-dehydrogenase (PDH) complex and Krebs cycle. This could be due to a direct stimulation of PDH and krebs cycle by leptin or a consequence of an indirect effect of this hormone activating the mitochondrial uncoupling process. In addition, in soleus and extensor digitorum longus (EDL) muscles, leptin and insulin had opposite effects on lipid metabolism, with leptin favoring lipid oxidation and insulin favoring lipid storage as triglycerides (TG). The leptin effects on free fatty acid (FFA) oxidation were more pronounced in soleus than in EDL. The differences in response of soleus compared with that of EDL was probably due to differences in fiber type composition and metabolic characteristics. It has been demonstrated that leptin reduces the TG content of skeletal. When tissue TG content is severely depleted by hyperleptinemia in normal rats, there is a dramatic increase in insulin sensitivity. This lipopenic effect of leptin may protect from the development of insulin resistance and diabetes in animals. In humans, obesity is also associated with an increase in insulin resistance and the development of Type II diabetes, however, contrary to rats and mice, there is abundance of leptin, indicating a state of resistance to this hormone in humans. Future studies are necessary to investigate the reasons why lean subjects seem to respond properly to endogenous leptin while obese ones don't. The understanding of the putative direct leptin signaling pathway in skeletal muscle could be an important step towards the utilization of leptin or a leptin receptor agonist as therapeutic tools to treat obesity and its related metabolic disorders.

Expression of genes involved in lipid metabolism correlate with peroxisome proliferator-activated receptor gamma expression in human skeletal muscle.

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) activation in adipose tissue is known to regulate genes involved in adipocyte differentiation and lipid metabolism. However, the role of PPAR-gamma in muscle remains unclear. To examine the potential regulation of genes by PPAR-gamma in human skeletal muscle, we used semiquantitative RT-PCR to determine the expression of PPAR-gamma, lipoprotein lipase (LPL), muscle carnitine palmitoyl transferase-1 (mCPT1), fatty acid-binding protein (FABP), carnitine acylcarnitine transferase (CACT), and glucose transporter-4 (GLUT4) in freeze-dried muscle samples from 14 male subjects. These samples were dissected free of adipose and other tissue contamination, as confirmed by minimal or absent adipsin expression. Between individuals, the messenger ribonucleic acid concentration of PPAR-gamma varied up to 3-fold, whereas LPL varied up to 6.5-fold, mCPT1 13-fold, FABP 4-fold, CACT 4-fold, and GLUT4 up to 3-fold. The expression of LPL (r2 = 0.54; P = 0.003), mCPT1 (r2 = 0.42; P = 0.012), and FABP (r2 = 0.324; P = 0.034) all correlated significantly with PPAR-gamma expression in the same samples. No significant correlation was observed between the expression of CACT and PPAR-gamma or between GLUT4 and PPAR-gamma. These findings demonstrate a relationship between PPAR-gamma expression and the expression of other genes of lipid metabolism in muscle and support the hypothesis that PPAR-gamma activators such as the antidiabetic thiazolidinediones may regulate fatty acid metabolism in skeletal muscle as well as in adipose tissue.

Ammonia and amino acid metabolism in skeletal muscle: human, rodent and canine models.

This review considers four experimental models for studying the dynamics of ammonia and amino acid metabolism in skeletal muscle: the rat hindlimb, the isolated dog gastrocnemius, the leg extensor for humans, and the traditional approach of humans performing two-legged exercise. The rat hindlimb is well suited for studying intense exercise with fast-twitch white fibers, but it is poorly suited for studying prolonged exercise because of rapid fatigue of major portions of the muscle and the restrictions of taking multiple blood samples. The traditional human model is limited because of the inability to quantify accurately the active muscle mass and to determine the true blood flow to the entire active tissue. Despite species differences and the various limitations of the paradigms, there are numerous consistencies in the literature. For example, human muscle and the canine gastrocnemius demonstrate similar magnitudes of efflux of ammonia, glutamine, and alanine (when indexed for the active mass) during prolonged exercise. Muscle has a large ammonia producing capacity during either intense or prolonged exercise. In prolonged exercise this is accompanied by similar productions of alanine and glutamine as well as a large uptake of glutamate. Despite the latter, the intramuscular glutamate concentration rapidly declines by more than 50% and remains constant throughout the exercise period. The leg extensor model and the canine gastrocnemius offer the greatest opportunities to quantify these responses during prolonged exercise.

Stress hormone and amino acid infusion in healthy volunteers: Short-term effects on protein synthesis and amino acid metabolism in skeletal muscle

To study the immediate effects of stress hormones and intravenous amino acid support, healthy male volunteers were administered a stress-hormone infusion including epinephrine, cortisol, and glucagon either alone (Triple, n = 8) or combined with a balanced glutamine-free amino acid solution (Triple AA, n = 8) over a period of 6 hours. The amino acid infusion was started 2 hours after the hormone infusion. A third group (AA, n = 8) received the balanced amino acid solution alone. After 6 hours of the stress-hormone infusion, a decrease was observed in skeletal muscle protein synthesis as measured by the size distribution and concentration of ribosomes. The decrease was prevented by an infusion of the balanced amino acid solution. Following the triple-hormone infusion, a decrease was noted in the content of the total free amino acids in both muscle and plasma. After including amino acids in the infusion solution, the significant decrease in muscle glutamine caused by the triple hormones was not seen. Plasma cortisol, insulin, and glucose increased in response to the triple-hormone infusion alone or in combination with amino acids. In summary, the results show that the signs of muscle protein catabolism elicited by administration of stress hormones can be attenuated by simultaneous administration of a conventional amino acid solution, although it does not contain glutamine.

Effects of changes in cell volume on the rates of glutamine and alanine release from rat skeletal muscle in vitro

The effect of changes in cell volume on the rates of release of glutamine and alanine from muscle and on the concentrations of these amino acids in muscle were investigated by using an isolated preparation of rat skeletal muscle incubated in the presence of hypo- and hyper-osmotic media. Changes in cell volume were associated with changes in the rates of release of glutamine and alanine from muscle: incubation in hypo-osmotic medium decreased the rates of release of glutamine and alanine, and incubation in hyperosmotic medium increased these rates. These changes were rapidly reversed by a change in osmoticity of the medium. Despite marked changes in cell volume, the concentrations of these amino acids in muscle were maintained. It is suggested that cell volume may play a role in the regulation of amino acid metabolism in skeletal muscle.

In-vitro stimulation of the rat epitrochlearis muscle. I. Contractile activity per se affects myofibrillar protein degradation and amino acid metabolism.

The influence of contractile activity on protein degradation and amino acid metabolism in skeletal muscle was investigated by utilizing an in-vitro electrical stimulation model with the rat epitrochlearis muscle preparation. Graded decreases in contraction force and in the muscle content of ATP and PCr, and increases in lactate were recorded with different rates of stimulation (1 h) and with both isometric twitches and tetanic contractions. 3-Methylhistidine and phenylalanine were chosen as indicators of myofibrillar and total protein degradation, respectively. The release of 3-methylhistidine was significantly stimulated by contractile activity, but a significant increase in the total amount of this amino acid (released amount + tissue content) occurred only at the most intense contraction rates. The release rate, tissue content and total amount of phenylalanine were not influenced by the contractions. Glutamate formation was generally inhibited, but its release was increased. Alanine synthesis was increased in moderately and intensely stimulated muscles. Glutamine and glycine were not influenced by the contractions, however. Inhibition of protein synthesis did not significantly influence protein degradation or amino acid release. The data suggest that in the absence of anabolic factors in the medium, myofibrillar protein degradation is increased in heavily activated muscle. This takes place without total protein breakdown being affected.

Validation of transport measurements in skeletal muscle with N-13 amino acids using a rabbit isolated hindlimb model

We are studying the transport of C-11 and N-13 labeled amino acids in tumor-bearing rabbits to determine the role of amino acid transport in the pathogenesis of muscle wasting in cancer. To validate a new, in vivo, method for measuring transport in skeletal muscle with these compounds, an isolated hindlimb model was developed in rabbits. The limb was perfused with a non-recirculating, normothermic, constant pressure system and a cell-free perfusate. Hemodynamic and metabolic parameters were measured during the first 75 min. of pergusion and found to remain normal and stable. Flow varied directly with perfusion pressure over the normal range of resting flows in the intact rabbit hindlimb. Time-activity curves (TAC's) were recorded from the medial thigh following bolus co-injection of L-[amide N-13] glutamine or N-13 L-glutamate with Tc-99m human serum albumin (HSA) into the femoral artery. Regional plasma flow was determined from the Tc-99m data. The N-13 TAC's consistently manifested a three-phased washout with half times of approximately 30 sec., 5 min. and 2 hr. Capillary and cellular transport parameters were computed from the N-13 data using a double barrier, single capillary model of capillary and cellular transport and assuming that the three washout components result, respectively, from tracer throughput, extraction into the interstitial space and extraction into the intracellular space. This interpretation was validated and the sensitivity of the technique to transport processes demonstrated by examining the effects on the N-13 TAC's and computed transport parameters of several factors known to influence cellular transport of amino acids, viz., the insulin concentration, amino acid concentration and pH of the perfusate. Time-activity curves and transport parameters for N-13 L-glutamine in the isolated limb were very similar to those observed in the intact rabbit hindlimb, suggesting that studies in the perfused model are indicative of amino acid transport in vivo. The methodology described here is especially well suited for studying the specific effects on transport of factors which influence amino acid metabolism in skeletal muscle (e.g., hormones and monokines).

Regulation of leucine and alpha-ketoisocaproic acid metabolism in skeletal muscle. Effects of starvation and insulin.

Regulation of leucine and alpha-ketoisocaproic acid metabolism in skeletal muscle. Effects of starvation and insulin.

Glucose and amino acid metabolism during recovery after exercise.

Glucose and amino acid metabolism during recovery after exercise.