Insulin action on muscle protein kinetics and amino acid transport during recovery after resistance exercise.

We have determined the individual and combined effects of insulin and prior exercise on leg muscle protein synthesis and degradation, amino acid transport, glucose uptake, and alanine metabolism. Normal volunteers were studied in the postabsorptive state at rest and about 3 h after a heavy leg resistance exercise routine. The leg arteriovenous balance technique was used in combination with stable isotopic tracers of amino acids and biopsies of the vastus lateralis muscle. Insulin was infused into a femoral artery to increase the leg insulin concentrations to high physiologic levels without substantively affecting the whole-body level. Protein synthesis and degradation were determined as rates of intramuscular phenylalanine utilization and appearance, and muscle fractional synthetic rate (FSR) was also determined. Leg blood flow was greater after exercise than at rest (P<0.05). Insulin accelerated blood flow at rest but not after exercise (P<0.05). The rates of protein synthesis and degradation were greater during the postexercise recovery (65+/-10 and 74+/-10 nmol x min(-1) x 100 ml(-1) leg volume, respectively) than at rest (30+/-7 and 46+/-8 nmol x min(-1) x 100 ml(-1) leg volume, respectively; P<0.05). Insulin infusion increased protein synthesis at rest (51+/-4 nmol x min(-1) x 100 ml(-1) leg volume) but not during the postexercise recovery (64+/-9 nmol x min(-1) x 100 ml(-1) leg volume; P<0.05). Insulin infusion at rest did not change the rate of protein degradation (48+/-3 nmol x min(-1) 100 ml(-1) leg volume). In contrast, insulin infusion after exercise significantly decreased the rate of protein degradation (52+/-9 nmol x min(-1) x 100 ml(-1) leg volume). The insulin stimulatory effects on inward alanine transport and glucose uptake were three times greater during the postexercise recovery than at rest (P<0.05). In contrast, the insulin effects on phenylalanine, leucine, and lysine transport were similar at rest and after exercise. In conclusion, the ability of insulin to stimulate glucose uptake and alanine transport and to suppress protein degradation in skeletal muscle is increased after resistance exercise. Decreased amino acid availability may limit the stimulatory effect of insulin on muscle protein synthesis after exercise.     

GLUT4 and glycogen synthase are key players in bed rest-induced insulin resistance

To elucidate the molecular mechanisms behind physical inactivity-induced insulin resistance in skeletal muscle, 12 young, healthy male subjects completed 7 days of bed rest with vastus lateralis muscle biopsies obtained before and after. In six of the subjects, muscle biopsies were taken from both legs before and after a 3-h hyperinsulinemic euglycemic clamp performed 3 h after a 45-min, one-legged exercise. Blood samples were obtained from one femoral artery and both femoral veins before and during the clamp. Glucose infusion rate and leg glucose extraction during the clamp were lower after than before bed rest. This bed rest-induced insulin resistance occurred together with reduced muscle GLUT4, hexokinase II, protein kinase B/Akt1, and Akt2 protein level, and a tendency for reduced 3-hydroxyacyl-CoA dehydrogenase activity. The ability of insulin to phosphorylate Akt and activate glycogen synthase (GS) was reduced with normal GS site 3 but abnormal GS site 2+2a phosphorylation after bed rest. Exercise enhanced insulin-stimulated leg glucose extraction both before and after bed rest, which was accompanied by higher GS activity in the prior-exercised leg than the rested leg. The present findings demonstrate that physical inactivity-induced insulin resistance in muscle is associated with lower content/activity of key proteins in glucose transport/phosphorylation and storage.     

Impact of 9 Days of Bed Rest on Hepatic and Peripheral Insulin Action, Insulin Secretion, and Whole-Body Lipolysis in Healthy Young Male Offspring of Patients With Type 2 Diabetes

OBJECTIVE

The aim of this study was to investigate the impact of 9 days of bed rest on insulin secretion, insulin action, and whole-body glucose and fat metabolism in first-degree relative (FDR) and matched control (CON) subjects.

RESEARCH DESIGN AND METHODS

A total of 13 FDR and 20 CON subjects participated in the study. All were studied before and after 9 days of bed rest using the clamp technique combined with indirect calorimetry preceded by an intravenous glucose tolerance test. Glucose and glycerol turnover rates were studied using stable isotope kinetics.

RESULTS

Bed rest caused a significant decrease in whole-body insulin sensitivity in both groups. Hepatic insulin resistance was elevated in FDR subjects prior to bed rest and was significantly augmented by bed rest in FDR (P < 0.01) but not in CON (P = NS) subjects. The rate of whole-body lipolysis decreased during bed rest in both FDR and CON subjects, with no significant differences between the groups. Insulin resistance induced by bed rest was fully accounted for by the impairment of nonoxidative glucose metabolism in both groups (overall P < 0.001).

CONCLUSIONS

Whole-body insulin action in both insulin-resistant FDR and healthy CON subjects deteriorates with 9 days of bed rest, converging toward similar degrees of whole-body insulin resistance. FDR subjects exhibit hepatic insulin resistance (HIR), which, in contrast to CON subjects, deteriorates in response to physical inactivity. FDR subjects exhibit reduced insulin secretion when seen in relation to their degree of HIR but not peripheral insulin resistance.Type 2 diabetes is caused by a complicated interplay between genetic and environmental factors that influence defects of peripheral and hepatic insulin action, insulin secretion, adipose tissue metabolism and lipolysis, and possibly a range of additional metabolic defects in various other organs (1). First-degree relatives (FDR subjects) of patients with type 2 diabetes have been characterized by insulin resistance and β-cell dysfunction (2,3).The habitual degree of physical activity is a moderator of glucose and fat metabolism, including insulin action (4,5). Physical inactivity is associated with increased morbidity and mortality (6,7) and has negative effects on lipid metabolism and insulin sensitivity (8–10). Studies of the regulatory mechanisms influencing skeletal muscle lipoprotein lipase (LPL) activity provided proof of the principle that the cellular and molecular mechanisms influencing LPL activity, and therefore fatty acid metabolism, during physical inactivity are distinct from the cellular events influencing LPL during exercise training. Indeed, reducing normal physical activity level has a much greater effect on LPL regulation than adding vigorous exercise training on top of the normal level of nonexercise activity (11). Thus, there are reasons to believe that exercise training versus physical inactivity influences additional molecular mechanisms and metabolic pathways relevant to metabolic health and risk of type 2 diabetes in a differential manner in humans.Previous studies (12,13) documented the detrimental effect of inactivity on insulin action in healthy individuals. The Dallas Bedrest and Training Study showed that 3 weeks of bed rest caused a fall in Vo_2max comparable to 30 years of aging (14). Previous studies have demonstrated reduced Vo_2max in healthy FDR subjects (15). It has been estimated that a sedentary lifestyle accounts for at least 25% of type 2 diabetes incidence (16), and sedentary FDR subjects have about three times the risk of developing type 2 diabetes (17).Although muscle insulin resistance and defective pancreatic insulin secretion may represent the most prominent defects of metabolism in FDR subjects (18,19), defects of metabolism in other organs, including liver (3,20,21), are important for elevating plasma glucose levels in type 2 diabetic patients. Little is known about the response of muscle, liver, pancreas, and adipose tissue metabolism in FDR subjects when exposed to physical inactivity, and there is a need to understand the impact of physical inactivity on mechanisms involved in the development of type 2 diabetes (22).In the present study, we investigated the effects of 9 days of bed rest on in vivo metabolism in FDR and control (CON) subjects. We hypothesized that FDR subjects may be more sensitive to physical inactivity than CON subjects as a result of their well-known defects of insulin action and secretion and their a priori increased risk of developing type 2 diabetes.     

Resistance exercise and insulin regulate AS160 and interaction with 14-3-3 in human skeletal muscle

A single bout of aerobic exercise can enhance insulin action, but whether a similar effect occurs after resistance exercise is unknown. Hyperinsulinemic-euglycemic clamps were performed on eight male subjects at rest and after a single bout and three repeated bouts of resistance exercise over 7 days. Skeletal muscle biopsies were taken before and after the clamp and immediately after a single exercise bout. Whole-body insulin action measured by glucose infusion rate decreased (P < 0.05) after a single exercise bout, whereas in response to repeated bouts of resistance exercise, the glucose infusion rate was similar to the rest trial. In skeletal muscle, Akt substrate of 160 kDa (AS160) phosphorylation, an Akt substrate implicated in the regulation of GLUT4 translocation, and its interaction with 14-3-3 was decreased (P < 0.05) only after a single exercise bout. Insulin increased (P < 0.05) phosphorylation of AS160 and its interaction with 14-3-3, but the insulin response was not influenced by resistance exercise. Phosphorylation of insulin receptor substrate-1 and Akt were similar to changes in AS160 phosphorylation after exercise and/or insulin. In conclusion, a single bout of resistance exercise impairs whole-body insulin action. Regulation of AS160 and interaction with 14-3-3 in skeletal muscle are influenced by resistance exercise and insulin but do not fully explain the effect of resistance exercise on whole-body insulin action.     

The stimulation-induced increase in skeletal muscle glycogen synthase content is impaired in carriers of the glycogen synthase XbaI gene polymorphism

Associations between glycogen synthase gene (GYS1) polymorphism and states of insulin resistance and type 2 diabetes have been reported. The purpose of this study was to establish if the GYS1 genotype impacts on the content of glycogen synthase (GS) protein in muscle measured under basal and stimulated conditions. To examine this, GYS1 XbaI and Met416Val polymorphisms and thigh muscle GYS1 protein content were determined at rest, both before and after several weeks of neuromuscular electrical stimulation in carriers and noncarriers of the mutations. The allelic frequency was 0.086 for the XbaI mutation (A2) and 0.006 for the Met416Val in our cohort of French-Canadian subjects. When measured at rest, the GS protein content in muscle was similar among carriers and noncarriers of the XbaI variant. However, the stimulation-induced increase (23%) in the amount of GS muscle protein normally seen in wildtype individuals was impaired in those carrying the XbaI mutation. These data demonstrate that some individuals, because of their genetic background, are unable to stimulate the process of GS protein accumulation in skeletal muscle. These results could explain why some individuals appear to be genetically predisposed to developing skeletal muscle insulin resistance when exposed to unfavorable metabolic environments.     

Interleukin-6 regulation of AMP-activated protein kinase. Potential role in the systemic response to exercise and prevention of the metabolic syndrome

Interleukin (IL)-6 is a pleiotropic hormone that has both proinflammatory and anti-inflammatory actions. AMP-activated protein kinase (AMPK) is a fuel-sensing enzyme that among its other actions responds to decreases in cellular energy state by enhancing processes that generate ATP and inhibiting others that consume ATP but are not acutely necessary for survival. IL-6 is synthesized and released from skeletal muscle in large amounts during exercise, and in rodents, the resultant increase in its concentration correlates temporally with increases in AMPK activity in multiple tissues. That IL-6 may be responsible in great measure for these increases in AMPK is suggested by the fact it increases AMPK activity both in muscle and adipose tissue in vivo and in incubated muscles and cultured adipocytes. In addition, we have found that AMPK activity is diminished in muscle and adipose tissue of 3-month-old IL-6 knockout (KO) mice at rest and that the absolute increases in AMPK activity in these tissues caused by exercise is diminished compared with control mice. Except for an impaired ability to exercise and to oxidize fatty acids, the IL-6 KO mouse appears normal at 3 months of age. On the other hand, by age 9 months, it manifests many of the abnormalities of the metabolic syndrome including obesity, dyslipidemia, and impaired glucose tolerance. This, plus the association of decreased AMPK activity with similar abnormalities in a number of other rodents, suggests that a decrease in AMPK activity may be a causal factor. Whether increases in IL-6, by virtue of their effects on AMPK, contribute to the reported ability of exercise to diminish the prevalence of type 2 diabetes, coronary heart disease, and other disorders associated with the metabolic syndrome remains to be determined.     

Inhibition of SNAT2 by metabolic acidosis enhances proteolysis in skeletal muscle

Insulin resistance is a major cause of muscle wasting in patients with ESRD. Uremic metabolic acidosis impairs insulin signaling, which normally suppresses proteolysis. The low pH may inhibit the SNAT2 l-Glutamine (L-Gln) transporter, which controls protein synthesis via amino acid-dependent insulin signaling through mammalian target of rapamycin (mTOR). Whether SNAT2 also regulates signaling to pathways that control proteolysis is unknown. In this study, inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate or metabolic acidosis (pH 7.1) depleted intracellular L-Gln and stimulated proteolysis in cultured L6 myotubes. At pH 7.1, inhibition of the proteasome led to greater depletion of L-Gln, indicating that amino acids liberated by proteolysis sustain L-Gln levels when SNAT2 is inhibited by acidosis. Acidosis shifted the dose-response curve for suppression of proteolysis by insulin to the right, confirming that acid increases proteolysis by inducing insulin resistance. Blocking mTOR or phosphatidylinositol-3-kinase (PI3K) increased proteolysis, indicating that both signaling pathways are involved in its regulation. When both mTOR and PI3K were inhibited, methylaminoisobutyrate or acidosis did not stimulate proteolysis further. Moreover, partial silencing of SNAT2 expression in myotubes and myoblasts with small interfering RNA stimulated proteolysis and impaired insulin signaling through PI3K. In conclusion, SNAT2 not only regulates mTOR but also regulates proteolysis through PI3K and provides a link among acidosis, insulin resistance, and protein wasting in skeletal muscle cells.     

Rosiglitazone but not metformin enhances insulin- and exercise-stimulated skeletal muscle glucose uptake in patients with newly diagnosed type 2 diabetes

Rosiglitazone, a thiazolidinedione, enhances peripheral insulin sensitivity in patients with type 2 diabetes. Because the synergic action of insulin and exercise has been shown to be decreased in insulin resistance, the aim of this study was to compare the effects of rosiglitazone and metformin on muscle insulin responsiveness at rest and during exercise in patients with type 2 diabetes. Therefore, 45 patients with newly diagnosed or diet-treated type 2 diabetes were randomized for treatment with rosiglitazone (4 mg b.i.d.), metformin (1 g b.i.d.), or placebo in a 26-week double-blind trial. Skeletal muscle glucose uptake was measured using fluorine-18-labeled fluoro-deoxy-glucose and positron emission tomography (PET) during euglycemic-hyperinsulinemic clamp and one-legged exercise before and after the treatment period. Rosiglitazone (P < 0.05) and metformin (P < 0.0001) treatment lowered the mean glycosylated hemoglobin. The skeletal muscle glucose uptake was increased by 38% (P < 0.01) and whole-body glucose uptake by 44% in the rosiglitazone group. Furthermore, the exercise-induced increment during insulin stimulation was enhanced by 99% (P < 0.0001). No changes were observed in skeletal muscle or whole-body insulin sensitivity in the metformin group. In conclusion, rosiglitazone but not metformin 1) improves insulin responsiveness in resting skeletal muscle and 2) doubles the insulin-stimulated glucose uptake rate during physical exercise in patients with type 2 diabetes. Our results suggest that rosiglitazone improves synergic action of insulin and exercise.     

Synthesis rate of muscle proteins,muscle functions,and amino acid kinetics in type 2 diabetes

Improvement of glycemic status by insulin is associated with profound changes in amino acid metabolism in type 1 diabetes. In contrast, a dissociation of insulin effect on glucose and amino acid metabolism has been reported in type 2 diabetes. Type 2 diabetic patients are reported to have reduced muscle oxidative enzymes and VO(2max). We investigated the effect of 11 days of intensive insulin treatment (T(2)D+) on whole-body amino acid kinetics, muscle protein synthesis rates, and muscle functions in eight type 2 diabetic subjects after withdrawing all treatments for 2 weeks (T(2)D-) and compared the results with those of weight-matched lean control subjects using stable isotopes of the amino acids. Whole-body leucine, phenylalanine and tyrosine fluxes, leucine oxidation, and plasma amino acid levels were similar in all groups, although plasma glucose levels were significantly higher in T(2)D-. Insulin treatment reduced leucine nitrogen flux and transamination rates in subjects with type 2 diabetes. Synthesis rates of muscle mitochondrial, sarcoplasmic, and mixed muscle proteins were not affected by glycemic status or insulin treatment in subjects with type 2 diabetes. Muscle strength was also unaffected by diabetes or glycemic status. In contrast, the diabetic patients showed increased tendency for muscle fatigability. Insulin treatment also failed to stimulate muscle cytochrome C oxidase activity in the diabetic patients, although it modestly elevated citrate synthase. In conclusion, improvement of glycemic status by insulin treatment did not alter whole-body amino acid turnover in type 2 diabetic subjects, but leucine nitrogen flux, transamination rates, and plasma ketoisocaproate level were decreased. Insulin treatments in subjects with type 2 diabetes had no effect on muscle mitochondrial protein synthesis and cytochrome C oxidase, a key enzyme for ATP production.     

Growth hormone and insulin combine to improve whole-body and skeletal muscle protein kinetics.

BACKGROUND. A cooperative effect of exogenous insulin and recombinant human growth hormone (r-hGH) with respect to whole-body and skeletal muscle protein metabolism has not been demonstrated previously. This study examined the effect of r-hGH and insulin administration during euglycemic clamping and concurrent amino acid supplementation. METHODS. Twenty-three normal volunteers in the postabsorptive state were either treated with r-hGH for 3 consecutive days before a metabolic study (GH group; n = 10) or not treated (CTRL group; n = 13). The r-hGH dose was 0.2 mg/kg/day (n = 5) or 0.1 mg/kg/day (n = 5). All subjects then received an infusion of 14C-labeled leucine and tritiated phenylalanine, followed by measurement of baseline protein kinetics (GH and CTRL). Subsequently a euglycemic insulin infusion (1 mU/kg/min) with concurrent amino acid infusion was administered, and protein kinetic measurements were repeated at steady state. RESULTS. GH and insulin separately produced an increase in whole-body and skeletal muscle protein net balance. GH plus insulin was associated with a higher net balance of protein than was insulin alone. CONCLUSIONS. r-hGH and insulin in the presence of amino acids and glucose combine to improve whole-body and skeletal muscle protein kinetics.     

Role of insulin and insulin resistance in androgen excess disorders

Insulin has complex effects on cell growth, metabolism and differentiation, and these effects are mediated by a cell-surface bound receptor and eventually a cascade of intracellular signaling events. Among the several metabolic and growth-promoting effects of insulin, insulin resistance is defined as an attenuated effect of insulin on glucose metabolism, primarily the limited export of blood glucose into skeletal muscle and adipose tissue. On the other hand, not all the signaling pathways and insulin-responsive tissues are equally affected, and some effects other than the metabolic actions of insulin are overexpressed. Ovaries and the adrenal glands are two examples of tissues remaining sensitive to insulin actions where insulin may contribute to increased androgen secretion. Polycystic ovary syndrome (PCOS) is the most common form of androgen excess disorder (AED), and its pathogenesis is closely associated with insulin resistance. Patients with idiopathic hirsutism also exhibit insulin resistance, albeit lower than patients with PCOS. Although it is not as evident as in PCOS, patients with congenital adrenal hyperplasia may have insulin resistance, which may be further exacerbated with glucocorticoid overtreatment and obesity. Among patients with severe insulin resistance syndromes, irrespective of the type of disease, hyperinsulinemia promotes ovarian androgen synthesis independently of gonadotropins. It is highly debated in whom and how insulin resistance should be diagnosed and treated among patients with AEDs, including PCOS. It is not suitable to administer an insulin sensitizer relying on only some mathematical models used for estimating insulin resistance. Instead, the treatment decision should be based on the constellation of the signs, symptoms and presence of obesity; acanthosis nigricans; and some laboratory abnormalities such as impaired glucose tolerance and impaired fasting glucose.     

Resistance exercise augments the acute anabolic effects of intradialytic oral nutritional supplementation.

BACKGROUND: An intriguing strategy to further enhance the anabolic effects of nutritional supplementation is to combine the administration of nutrients with resistance exercise. We hypothesized that the addition of resistance exercise to oral nutrition supplementation would lead to further increases in skeletal muscle protein accretion when compared to nutritional supplementation alone in chronic haemodialysis (CHD) patients. METHODS: We performed stable isotope protein kinetic studies in eight CHD patients during two separate settings: with oral nutritional supplementation alone (PO) and oral nutritional supplementation combined with a single bout of resistance exercise (PO + EX). Metabolic assessment was performed before, during and after haemodialysis. Both interventions resulted in robust protein anabolic response. RESULTS: There were no differences in metabolic hormones, plasma amino acid and whole-body protein balance between the interventions. During the post-HD phase, PO + EX retained a positive total amino acid (TAA) balance (primarily due to essential amino acid) while PO returned to a negative TAA balance although this difference did not reach statistical significance (78 +/- 109 versus -128 +/- 72 nmol/100 ml/min, respectively; P = 0.69). In the post-HD phase, PO + EX had significantly higher net muscle protein balance when compared to PO (19 +/- 16 versus -24 +/- 10 microg/100 ml/min, respectively; P = 0.036) We conclude that a single bout of resistance exercise augments the protein anabolic effects of oral intradialytic nutritional supplementation when examining skeletal muscle protein turnover.     

Insulin Resistance and Protein Energy Metabolism in Patients with Advanced Chronic Kidney Disease

Insulin resistance (IR), the reciprocal of insulin sensitivity is a known complication of advanced chronic kidney disease (CKD) and is associated with a number of metabolic derangements. The complex metabolic abnormalities observed in CKD such as vitamin D deficiency, obesity, metabolic acidosis, inflammation, and accumulation of “uremic toxins” are believed to contribute to the etiology of IR and acquired defects in the insulin‐receptor signaling pathway in this patient population. Only a few investigations have explored the validity of commonly used assessment methods in comparison to gold standard hyperinsulinemic hyperglycemic clamp technique in CKD patients. An important consequence of insulin resistance is its role in the pathogenesis of protein energy wasting, a state of metabolic derangement characterized by loss of somatic and visceral protein stores not entirely accounted for by inadequate nutrient intake. In the general population, insulin resistance has been associated with accelerated protein catabolism. Among end‐stage renal disease (ESRD) patients, enhanced muscle protein breakdown has been observed in patients with Type II diabetes compared to ESRD patients without diabetes. In the absence of diabetes mellitus (DM) or severe obesity, insulin resistance is detectable in dialysis patients and strongly associated with increased muscle protein breakdown, primarily mediated by the ubiquitin‐proteasome pathway. Recent epidemiological data indicate a survival advantage and better nutritional status in insulin‐free Type II DM patients treated with insulin sensitizer thiazolidinediones. Given the high prevalence of protein energy wasting in ESRD and its unequivocal association with adverse clinical outcomes, insulin resistance may represent an important modifiable target for intervention in the ESRD population.     

Activation of protein kinase C-zeta by insulin and phosphatidylinositol-3,4,5-(PO4)3 is defective in muscle in type 2 diabetes and impaired glucose tolerance: amelioration by rosiglitazone and exercise

Insulin resistance in type 2 diabetes is partly due to impaired glucose transport in skeletal muscle. Atypical protein kinase C (aPKC) and protein kinase B (PKB), operating downstream of phosphatidylinositol (PI) 3-kinase and its lipid product, PI-3,4,5-(PO(4))(3) (PIP(3)), apparently mediate insulin effects on glucose transport. We examined these signaling factors during hyperinsulinemic-euglycemic clamp studies in nondiabetic subjects, subjects with impaired glucose tolerance (IGT), and type 2 diabetic subjects. In nondiabetic control subjects, insulin provoked twofold increases in muscle aPKC activity. In both IGT and diabetes, aPKC activation was markedly (70-80%) diminished, most likely reflecting impaired activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase and decreased ability of PIP(3) to directly activate aPKCs; additionally, muscle PKC-zeta levels were diminished by 40%. PKB activation was diminished in patients with IGT but not significantly in diabetic patients. The insulin sensitizer rosiglitazone improved insulin-stimulated IRS-1-dependent PI 3-kinase and aPKC activation, as well as glucose disposal rates. Bicycle exercise, which activates aPKCs and stimulates glucose transport independently of PI 3-kinase, activated aPKCs comparably to insulin in nondiabetic subjects and better than insulin in diabetic patients. Defective aPKC activation contributes to skeletal muscle insulin resistance in IGT and type 2 diabetes, rosiglitazone improves insulin-stimulated aPKC activation, and exercise directly activates aPKCs in diabetic muscle.     

Tissue-specific effects of rosiglitazone and exercise in the treatment of lipid-induced insulin resistance

Both pharmacological intervention (i.e., thiazolidinediones [TZDs]) and lifestyle modification (i.e., exercise training) are clinically effective treatments for improving whole-body insulin sensitivity. However, the mechanism(s) by which these therapies reverse lipid-induced insulin resistance in skeletal muscle is unclear. We determined the effects of 4 weeks of rosiglitazone treatment and exercise training and their combined actions (rosiglitazone treatment and exercise training) on lipid and glucose metabolism in high-fat-fed rats. High-fat feeding resulted in decreased muscle insulin sensitivity, which was associated with increased rates of palmitate uptake and the accumulation of the fatty acid metabolites ceramide and diacylglycerol. Impairments in lipid metabolism were accompanied by defects in the Akt/AS160 signaling pathway. Exercise training, but not rosiglitazone treatment, reversed these impairments, resulting in improved insulin-stimulated glucose transport and increased rates of fatty acid oxidation in skeletal muscle. The improvements to glucose and lipid metabolism observed with exercise training were associated with increased AMP-activated protein kinase alpha1 activity; increased expression of Akt1, peroxisome proliferator-activated receptor gamma coactivator 1, and GLUT4; and a decrease in AS160 expression. In contrast, rosiglitazone treatment exacerbated lipid accumulation and decreased insulin-stimulated glucose transport in skeletal muscle. However, rosiglitazone, but not exercise training, increased adipose tissue GLUT4 and acetyl CoA carboxylase expression. Both exercise training and rosiglitazone decreased liver triacylglycerol content. Although both interventions can improve whole-body insulin sensitivity, our results show that they produce divergent effects on protein expression and triglyceride storage in different tissues. Accordingly, exercise training and rosiglitazone may act as complementary therapies for the treatment of insulin resistance.     

Chronic endothelin-1 treatment leads to insulin resistance in vivo

We determined whether chronic endothelin-1 (ET-1) treatment could lead to in vivo insulin resistance. Like insulin, ET-1 acutely stimulated glucose transport in isolated soleus muscle strips of WKY rats. ET-1 pretreatment (1 h) decreased insulin-stimulated glucose transport in muscle strips (-23%). Both ET-1-mediated effects were generated through ET(A) receptors, because a specific ET(A) receptor antagonist (BQ610) blocked these effects of ET-1. Osmotic minipumps were used to treat normal rats with ET-1 for 5 days. Subsequent hyperinsulinemic-euglycemic clamps showed that ET-1 treatment led to an approximately 30% decrease in insulin-stimulated glucose disposal rates in male and female rats. In addition, ex vivo study of soleus muscle strips showed decreased glucose transport into muscle from ET-1-treated animals. With respect to insulin signaling, chronic in vivo ET-1 treatment led to a 30-40% decrease in IRS-I protein content, IRS-I-associated p110(alpha), and AKT activation. In summary, 1) in vitro ET-1 pretreatment leads to decreased insulin-stimulated glucose transport in skeletal muscle strips; 2) chronic ET-1 administration in vivo leads to whole-body insulin resistance, with decreased skeletal muscle glucose transport and impaired insulin signaling; and 3) elevated ET-1 levels may be a cause of insulin resistance in certain pathophysiologic states.     

Early hormonal changes affect the catabolic response to trauma.

OBJECTIVE: The authors sought to determine how temporary insulin suppression might alter the catabolic effects of cortisol, glucagon, and epinephrine. SUMMARY BACKGROUND DATA: The metabolic responses to injury include hypermetabolism, accelerated net skeletal muscle protein breakdown, glucose intolerance, and insulin resistance. These alterations are associated with increased stress hormone concentrations. Insulin elaboration is usually suppressed immediately after an injury but is abundant later during convalescence. An infusion of hydrocortisone, glucagon, and epinephrine increases both stress hormone concentrations and insulin levels. It induces many of the metabolic alterations seen in critically ill patients, but it does not affect net muscle breakdown. METHODS: Seven healthy adults received a stress hormone infusion for 3 days in two separate studies. During one study they, also received an infusion of the somatostatin analogue, octreotide (0.005 micrograms/kg/min), to suppress insulin elaboration for the first 24 hours. During the other study (control), insulin was permitted to rise unchecked. RESULTS: Stress hormone concentrations, hypermetabolism (+/- 20% above basal), and leukocytosis were similar during both study periods. When insulin elaboration was temporarily suppressed, whole-body nitrogen loss was increased during the first 48 hours, and the efflux of amino acids from the forearm after 72 hours of infusion was 60% greater than the control level. CONCLUSIONS: Temporary insulin suppression during physiologic increases in stress hormone concentrations amplified whole-body nitrogen loss and led to the development of accelerated net skeletal muscle protein breakdown. Early hormonal changes after an injury may affect the development of later catabolic responses.     

Skeletal muscle mitochondrial health and spinal cord injury

Mitochondria are the main source of cellular energy production and are dynamic organelles that undergo biogenesis, remodeling, and degradation. Mitochondrial dysfunction is observed in a number of disease states including acute and chronic central or peripheral nervous system injury by traumatic brain injury, spinal cord injury(SCI), and neurodegenerative disease as well as in metabolic disturbances such as insulin resistance, type Ⅱ diabetes and obesity. Mitochondrial dysfunction is most commonly observed in high energy requiring tissues like the brain and skeletal muscle. In persons with chronic SCI, changes to skeletal muscle may include remarkable atrophy and conversion of muscle fiber type from oxidative to fast glycolytic, combined with increased infiltration of intramuscular adipose tissue. These changes contribute to a proinflammatory environment, glucose intolerance and insulin resistance. The loss of metabolically active muscle combined with inactivity predisposes individuals with SCI to type Ⅱ diabetes and obesity. The contribution of skeletal muscle mitochondrial density and electron transport chain activity to the development of the aforementioned comorbidities following SCI is unclear. A better understanding of the mechanisms involved in skeletal muscle mitochondrial dynamics is imperative to designing and testing effective treatments for this growing population. The current editorial will review ways to study mitochondrial function and the importance of improving skeletal muscle mitochondrial health in clinical populations with a special focus on chronic SCI.     

Innovation in the Treatment of Uremia: Proceedings from the Cleveland Clinic Workshop: Inflammation and Insulin Resistance as Novel Mechanisms of Wasting in Chronic Dialysis Patients

Over the last decade, there have been no proven therapies to lower the mortality and morbidity risk for chronic dialysis patients. One of the most important determinants of this poor clinical outcome is protein energy wasting (PEW), a unique and highly prevalent nutritional and metabolic abnormality primarily characterized by increased protein breakdown in the skeletal muscle compartment. Although the etiology and mechanisms leading to increased protein breakdown in chronic dialysis patients are complex and mostly ill‐defined, two well‐recognized and presumably interrelated metabolic abnormalities, insulin resistance and chronic inflammation, are likely to play a critical role in the pathogenesis of this condition. Multiple studies demonstrate the anabolic effects of insulin that extend beyond simple carbohydrate metabolism. Insulin is a mediator of accelerated protein breakdown in the catabolic condition such as advanced kidney disease. Chronic inflammation, a condition known to cause muscle catabolism in experimental conditions, has a strong association with advanced kidney disease in epidemiologic studies. Chronic inflammation is also known to induce insulin resistance, primarily by the induction of proinflammatory cytokines. The protein catabolic effects of inflammation and insulin resistance involve common cellular pathways. Thus, it is reasonable to speculate that chronic inflammation of advanced kidney disease mediates its protein catabolic effects by inducing insulin resistance of protein metabolism at both the physiologic and cellular levels. Modulating inflammatory response or insulin signaling by pharmacologic interventions could allow us to clarify the mechanisms contributing to the development of PEW in the setting of these particular metabolic derangements.     

Pharmacological inhibition of glucosylceramide synthase enhances insulin sensitivity

A growing body of evidence implicates ceramide and/or its glycosphingolipid metabolites in the pathogenesis of insulin resistance. We have developed a highly specific small molecule inhibitor of glucosylceramide synthase, an enzyme that catalyzes a necessary step in the conversion of ceramide to glycosphingolipids. In cultured 3T3-L1 adipocytes, the iminosugar derivative N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM) counteracted tumor necrosis factor-alpha-induced abnormalities in glycosphingolipid concentrations and concomitantly reversed abnormalities in insulin signal transduction. When administered to mice and rats, AMP-DNM significantly reduced glycosphingolipid but not ceramide concentrations in various tissues. Treatment of ob/ob mice with AMP-DNM normalized their elevated tissue glucosylceramide levels, markedly lowered circulating glucose levels, improved oral glucose tolerance, reduced A1C, and improved insulin sensitivity in muscle and liver. Similarly beneficial metabolic effects were seen in high fat-fed mice and ZDF rats. These findings provide further evidence that glycosphingolipid metabolites of ceramide may be involved in mediating the link between obesity and insulin resistance and that interference with glycosphingolipid biosynthesis might present a novel approach to the therapy of states of impaired insulin action such as type 2 diabetes.