Rate Of Glycogen Synthesis Research Articles

Generation of a Dominant‐Negative Glycogen Targeting Subunit for Protein Phosphatase‐1

Modulation of the expression of the protein phosphatase-1 (PP1) glycogen-targeting subunit PTG exerts profound effects on cellular glycogen metabolism in vitro and in vivo. PTG contains three distinct binding domains for glycogen, PP1, and a common site for glycogen synthase and phosphorylase. The impact of disrupting the PP1-binding domain on PTG function was examined in 3T3-L1 adipocytes. A full-length PTG mutant was generated as an adenoviral construct in which the valine and phenylalanine residues in the conserved PP1-binding domain were mutated to alanine (PTG-VF). Infection of fully differentiated 3T3-L1 adipocytes with the PTG-VF adenovirus reduced glycogen stores by over 50%. In vitro, PTG-VF competitively interfered with wild-type PTG action, suggesting that the mutant construct acted as a dominant-negative molecule. The reduction in cellular glycogen storage was due to a significantly increased rate of glycogen turnover. Interestingly, acute basal and insulin-stimulated glucose uptake and glycogen synthesis rates were enhanced in PTG-VF expressing cells vs. control 3T3-L1 adipocytes, likely as a compensatory response to the loss of glycogen stores. These results indicate that the mutation of the PP1-binding domain on PTG resulted in the generation of a dominant-negative molecule that impeded endogenous PTG action and reduced cellular glycogen levels, through enhancement of glycogenolysis rather than impairment of glycogen synthesis.

The effect of acute exercise on glycogen synthesis rate in obese subjects studied by 13C MRS

In obesity, insulin-stimulated glucose uptake in skeletal muscle is decreased. We investigated whether the stimulatory effect of acute exercise on glucose uptake and subsequent glycogen synthesis was normal. The study was performed on 18 healthy volunteers, 9 obese (BMI = 32.6 ± 1.2 kg/m2, mean ± SEM) and 9 lean (BMI = 22.0 ± 0.9 kg/m2), matched for age and gender. All participants underwent a euglycemic hyperinsulinemic clamp, showing reduced glucose uptake in the obese group (P = 0.01), during which they performed a short intense local exercise (single-legged toe lifting). Dynamic glucose incorporation into glycogen in the gastrocnemius muscle before and after exercise was assessed by 13C magnetic resonance spectroscopy combined with infusion of [1-13C]glucose. Blood flow was measured to investigate its potential contribution to glucose uptake. Before exercise, glycogen synthesis rate tended to be lower in obese subjects compared with lean (78 ± 14 vs. 132 ± 24 μmol/kg muscle/min; P = 0.07). Exercise induced highly significant rises in glycogen synthesis rates in both groups, but the increase in obese subjects was reduced compared with lean (112 ± 15 vs. 186 ± 27 μmol/kg muscle/min; P = 0.03), although the relative increase was similar (184 ± 35 vs. 202 ± 51%; P = 0.78). After exercise, blood flow increased equally in both groups, without a temporal relationship with the rate of glycogen synthesis. In conclusion, this study shows a stimulatory effect of a short bout of acute exercise on insulin-induced glycogen synthesis rate that is reduced in absolute values but similar in percentages in obese subjects. These results suggest a shared pathway between insulin- and exercise-induced glucose uptake and subsequent glycogen synthesis.

Exercise: not just a medicine for muscle?

Exercise: not just a medicine for muscle?

Insulin-stimulated glycogen synthesis and glycogen synthase activation after electrical stimulation of epitrochlearis muscles with different initial glycogen contents

Glycogen synthesis increases after muscle contraction and during insulin stimulation, and insulin-stimulated glycogen synthesis is enhanced after contraction. We hypothesized that the initial glycogen content influences the magnitude of additive effect of contraction on insulin-stimulated glycogen synthesis. Contraction and insulin had full additive effect on rate of glycogen synthesis measured after contraction in muscles with normal and high glycogen content. In muscles with low glycogen, contraction increased insulin-stimulated glycogen synthesis nearly as much as in muscles with normal glycogen, but not to the sum of the two stimuli studied separately; still glycogen synthesis was generally highest in muscles with low glycogen. Glycogen synthase fractional activity inversely correlated with glycogen content and contraction increased glycogen synthase fractional activity. Contraction and insulin additively increased glycogen synthase fractional activity at all glycogen contents. In conclusion, after contraction insulin-stimulated glycogen synthesis was increased by rather similar magnitude at all glycogen contents in concert with increased glycogen synthase activation.

Insulin Action in Hyperthyroidism: A Focus on Muscle and Adipose Tissue

Hyperthyroidism leads to an enhanced demand for glucose, which is primarily provided by increased rates of hepatic glucose production due to increased gluconeogenesis (in the fasting state) and increased Cori cycle activity (in the late postprandial and fasting state). Adipose tissue lipolysis is increased in the fasting state, resulting in increased production of glycerol and nonesterified fatty acids. Under these conditions, increased glycerol generated by lipolysis and increased amino acids generated by proteolysis are used as substrates for gluconeogenesis. Increased nonesterified fatty acid levels are necessary to stimulate gluconeogenesis and provide substrate for oxidation in other tissues (such as muscle). In the postprandial period, insulin-stimulated glucose uptake by the skeletal muscle has been found to be normal or increased, mainly due to increased blood flow. Under hyperthyroid conditions, insulin-stimulated rates of glycogen synthesis in skeletal muscle are decreased, whereas there is a preferential increase in the rates of lactate formation vs. glucose oxidation leading to increased Cori cycle activity. In hyperthyroidism, the Cori cycle could be considered as a large substrate cycle; by maintaining a high flux through it, a dynamic buffer of glucose and lactate is provided, which can be used by other tissues as required. Moreover, lipolysis is rapidly suppressed to normal after the meal to facilitate the disposal of glucose by the insulin-resistant muscle. This ensures the preferential use of glucose when available and helps to preserve fat stores.

Naturally occurring R225W mutation of the gene encoding AMP-activated protein kinase (AMPK)γ3 results in increased oxidative capacity and glucose uptake in human primary myotubes

AMP-activated protein kinase (AMPK) has a broad role in the regulation of glucose and lipid metabolism making it a promising target in the treatment of type 2 diabetes mellitus. We therefore sought to characterise for the first time the effects of chronic AMPK activation on skeletal muscle carbohydrate metabolism in carriers of the rare gain-of-function mutation of the gene encoding AMPKgamma(3) subunit, PRKAG3 R225W. Aspects of fuel metabolism were studied in vitro in myocytes isolated from vastus lateralis of PRKAG3 R225W carriers and matched control participants. In vivo, muscular strength and fatigue were evaluated by isokinetic dynamometer and surface electromyography, respectively. Glucose uptake in exercising quadriceps was determined using [(18)F]fluorodeoxyglucose and positron emission tomography. Myotubes from PRKAG3 R225W carriers had threefold higher mitochondrial content (p < 0.01) and oxidative capacity, higher leak-dependent respiration (1.6-fold, p < 0.05), higher basal glucose uptake (twofold, p < 0.01) and higher glycogen synthesis rates (twofold, p < 0.05) than control myotubes. They also had higher levels of intracellular glycogen (p < 0.01) and a trend for lower intramuscular triacylglycerol stores. R225W carriers showed remarkable resistance to muscular fatigue and a trend for increased glucose uptake in exercising muscle in vivo. Through the enhancement of skeletal muscle glucose uptake and increased mitochondrial content, the R225W mutation may significantly enhance exercise performance. These findings are also consistent with the hypothesis that the gamma(3) subunit of AMPK is a promising tissue-specific target for the treatment of type 2 diabetes mellitus, a condition in which glucose uptake and mitochondrial function are impaired.

Effects of exercise on muscle glycogen synthesis signalling and enzyme activities in pigs carrying the PRKAG3 mutation

The dominant RN mutation in pigs results in excessive glycogen storage in skeletal muscle. The mutation is situated in the PRKAG3 gene, which encodes a muscle-specific isoform of the AMP-activated protein kinase (AMPK) gamma3 subunit. AMPK is an important regulator of carbohydrate and fat metabolism in mammalian cells. The aim of the present study was to examine the effect of exercise on glycogen synthesis signalling pathways in muscle and to study enzyme activities of importance in carbohydrate metabolism in pigs with or without the PRKAG3 mutation. Glycogen content, metabolic enzyme activities and expression or phosphorylation of signalling proteins were analysed in skeletal muscle specimens obtained at rest, after a single treadmill exercise bout and after 3 h recovery. The PRKAG3 mutation carriers had higher glycogen content, a tendency for lower expression of AMPK (P < 0.07) and higher hexokinase and phosphorylase activities, whereas citrate synthase, 3-hydroxyacyl-CoA dehydrogenase and glycogen synthase activities did not differ between genotypes. Carriers and non-carriers of the RN mutation showed a similar degradation of glycogen after exercise, whereas the rate of resynthesis was faster in the carriers. Acute exercise stimulated Akt phosphorylation on Ser(473) in both genotypes, and the effect was greater in the carriers than in the non-carriers. Acute exercise also stimulated phosphorylation of Akt substrate of 160 kDA and Glycogen synthase kinase 3 in the carriers and GSK3alpha in the non-carriers. In conclusion, the increased rate of glycogen synthesis following exercise in pigs carrying the PRKAG3 mutation correlates with an increased signalling response of Akt and its substrate, AS160, and a higher activity of hexokinase, indicating an increased glucose influx and phosphorylation of glucose, directed towards glycogen synthesis.

Stimulation of postprandial in vivo glycogenesis and lipogenesis of rats fed high fructose diet with varied phosphate content

It has been reported that increased fructose intake is associated with the development of the metabolic syndrome. The phosphate (P) sequestering capacity of fructose is likely to affect the phosphorylation capacity of different metabolites, and this, in turn, may be the basis for several metabolic derangements, especially in the P requiring reactions, for example, glycogenesis and lipogenesis. We hypothesized that P enrichment of the diet can balance P status and, consequently, affect glycogenesis and lipogenesis. An animal experiment was executed in which adult male Sprague-Dawley rats were maintained for 4 days on high-fructose diets with different P content (0.15%, 0.165%, 0.30%, and 1.65%). At the end of the feeding period, overnight fasted rats were tube fed a test meal, injected with (3)H(2)O and euthanized 1 hour later. Final plasma glucose, insulin, uric acid, and triacylglycerol concentrations, as well as in vivo rates of glycogen and lipid synthesis and hepatic glycogen content, were measured. Results showed that increased P content of the diet was associated with an increase in postprandial epididymal fat pad (P = .007) and hepatic lipogenesis (P = .029), as well as glycogenesis (P = .024). In conclusion, P content of the diet was found to stimulate both glycogenesis and lipogenesis. These alterations in carbohydrate and fat metabolism point to the potential of P in influencing nutritional status.

Signal Transducer and Activator of Transcription 3 (STAT3) Mediates Amino Acid Inhibition of Insulin Signaling through Serine 727 Phosphorylation

Nutrient overload is associated with the development of obesity, insulin resistance, and type II diabetes. High plasma concentrations of amino acids have been found to correlate with insulin resistance. At the cellular level, excess amino acids impair insulin signaling, the mechanisms of which are not fully understood. Here, we report that STAT3 plays a key role in amino acid dampening of insulin signaling in hepatic cells. Excess amino acids inhibited insulin-stimulated Akt phosphorylation and glycogen synthesis in mouse primary hepatocytes as well as in human hepatocarcinoma HepG2 cells. STAT3 knockdown protected insulin sensitivity from inhibition by amino acids. Amino acids stimulated the phosphorylation of STAT3 at Ser(727), but not Tyr(705). Replacement of the endogenous STAT3 with wild-type, but not S727A, recombinant STAT3 restored the ability of amino acids to inhibit insulin signaling, suggesting that Ser(727) phosphorylation was critical for STAT3-mediated amino acid effect. Furthermore, overexpression of STAT3-S727D was sufficient to inhibit insulin signaling in the absence of excess amino acids. Our results also indicated that mammalian target of rapamycin was likely responsible for the phosphorylation of STAT3 at Ser(727) in response to excess amino acids. Finally, we found that STAT3 activity and the expression of its target gene socs3, known to be involved in insulin resistance, were both stimulated by excess amino acids and inhibited by rapamycin. In conclusion, our study reveals STAT3 as a novel mediator of nutrient signals and identifies a Ser(727) phosphorylation-dependent and Tyr(705) phosphorylation-independent STAT3 activation mechanism in the modulation of insulin signaling.

Magnetic resonance spectroscopy shows an inverse correlation between intramyocellular lipid content in human calf muscle and local glycogen synthesis rate

Intramyocellular lipid (IMCL) content of skeletal muscle, as measured with (1)H MRS, is inversely correlated with insulin sensitivity as determined by whole body glucose uptake. The latter, however, does not necessarily represent the actual glucose uptake in the corresponding skeletal muscle. In this study, we examined whether IMCL content in human calf muscle correlated with local glucose uptake assessed by measurement of glycogen synthesis rate within the same muscle compartment. We studied 20 subjects belonging to four subgroups of five persons each: young lean, elderly lean, young obese and elderly obese. IMCL content in the soleus and gastrocnemius muscle was determined using (1)H MR spectroscopic imaging and local glycogen synthesis rate in the calf muscle was measured by (13)C MRS during a euglycaemic hyperinsulinaemic clamp with 20% w/v 30% (13)C-1-labelled glucose infusion. Significantly higher IMCL contents were found in elderly (soleus: p < 0.0001 and gastrocnemius: p < 0.01) and obese subjects (p < 0.01 for both muscles). Local glycogen synthesis rate decreased significantly with obesity (p < 0.01). The principal finding of this study was that the mean IMCL content of the soleus and gastrocnemius muscles was indeed inversely correlated with the local glycogen synthesis rate in the calf muscle (r(s) = -0.50, p < 0.05), with a very similar dependency as the inverse correlation between mean IMCL content and total body glucose uptake (r(s) = -0.54, p < 0.05). We conclude that IMCL content of the soleus and gastrocnemius muscles reflects a measure for local insulin resistance within the same muscle compartment as determined by glycogen synthesis rate. Although the inverse correlation suggests that insulin sensitivity is affected by the local amount of fat present, it remains to be determined if this is a cause or a consequence.

Glycogen content regulates insulin‐ but not contraction‐mediated glycogen synthase activation in the rat slow‐twitch soleus muscles

The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined. Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin. Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser(641) (R = -0.93, P < 0.001) and Ser(645,649,653,657) (R = -0.85, P < 0.001) phosphorylation, but not with Ser(7) phosphorylation. Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser(641) and Ser(645,649,653,657) seems to regulate GS activity in soleus.

Lipid and insulin infusion-induced skeletal muscle insulin resistance is likely due to metabolic feedback and not changes in IRS-1, Akt, or AS160 phosphorylation

Type 2 diabetes is characterized by hyperlipidemia, hyperinsulinemia, and insulin resistance. The aim of this study was to investigate whether acute hyperlipidemia-induced insulin resistance in the presence of hyperinsulinemia was due to defective insulin signaling. Hyperinsulinemia (approximately 300 mU/l) with hyperlipidemia or glycerol (control) was produced in cannulated male Wistar rats for 0.5, 1 h, 3 h, or 5 h. The glucose infusion rate required to maintain euglycemia was significantly reduced by 3 h with lipid infusion and was further reduced after 5 h of infusion, with no difference in plasma insulin levels, indicating development of insulin resistance. Consistent with this finding, in vivo skeletal muscle glucose uptake (31%, P < 0.05) and glycogen synthesis rate (38%, P < 0.02) were significantly reduced after 5 h compared with 3 h of lipid infusion. Despite the development of insulin resistance, there was no difference in the phosphorylation state of multiple insulin-signaling intermediates or muscle diacylglyceride and ceramide content over the same time course. However, there was an increase in cumulative exposure to long-chain acyl-CoA (70%) with lipid infusion. Interestingly, although muscle pyruvate dehydrogenase kinase 4 protein content was decreased in hyperinsulinemic glycerol-infused rats, this decrease was blunted in muscle from hyperinsulinemic lipid-infused rats. Decreased pyruvate dehydrogenase complex activity was also observed in lipid- and insulin-infused animals (43%). Overall, these results suggest that acute reductions in muscle glucose metabolism in rats with hyperlipidemia and hyperinsulinemia are more likely a result of substrate competition than a significant early defect in insulin action or signaling.

The effect of phosphorus manipulation on in vivo glycogenesis and lipogenesis of rats fed a high fructose diet.

ObjectiveHigh phosphorus intake was reported to be associated with the development of metabolic syndrome. This effect is though to be related to its capacity to induce lipogenesis and uric acid production. Fructose is also known to have phosphorus sequestering capacity, but it is not clear whether phosphorus manipulation of the diet would affect fructose metabolism and thus its capacity to induce metabolic syndrome.MethodsAdult male Sprague‐Dawley rats were maintained for four days on high fructose diet with different added quantities of phosphorus (0%, 0.015%, 0.15% and 1.5%). Food intake and body weight were monitored throughout the experiment. At the end of the feeding period, overnight fasted rats were fed a test meal and injected with 3H2O to measure in vivo rates of glycogen and lipid synthesis.ResultsPostprandial lipogenesis of both epididymal fat pad and liver was found to increase with increased phosphorus content of the diet. At the same time, postprandial hepatic glycogenesis increased with increased phosphorus content of the diet. But, plasma uric acid concentration was similar among the groups.ConclusionIncreased phosphorus content of high fructose diet seems to stimulate the lipogenic capacity of both liver and adipose tissue. This in turn may promote the development of hypertriglyceridemia and thus metabolic syndrome.

Branched-Chain Amino Acid-Containing Dipeptides, Identified from Whey Protein Hydrolysates, Stimulate Glucose Uptake Rate in L6 Myotubes and Isolated Skeletal Muscles

In earlier studies we showed that dietary whey protein increased skeletal muscle and liver glycogen content in exercise-trained rats. However, little is known about whether ingredients of whey protein stimulate skeletal muscle glycogen accumulation. The aim of this study was to identify bioactive peptides in whey protein hydrolysates (WPH) which stimulated glucose uptake and glycogen synthesis rate in skeletal muscles. Branched-chain amino acid (BCAA)-containing dipeptides in WPH were identified using LC/MS/MS. L6 myotubes and isolated epitrochlearis muscles were used for the glucose uptake assays. The myotubes and muscles were incubated with or without 1 mM dipeptides, LY294002 a phosphoinositide 3-kinase (PI3-kinase) inhibitor, or GF102903X an atypical protein kinase C (aPKC) inhibitor, followed by measurement of 2-deoxyglucose uptake. Isolated muscles were incubated for 3 h with or without 1 mM Ile-Leu to determine glycogen synthesis rate. The BCAA-containing dipeptides, Ile-Val, Leu-Val, Val-Leu, Ile-Ile, Leu-Ile, Ile-Leu, and Leu-Leu were detected in the WPH by LC/MS/MS. These dipeptides caused significant stimulation in glucose uptake rate in the L6 myotubes. Ile-Leu, the main component in WPH, also stimulated glucose uptake in isolated skeletal muscles. Stimulation of glucose uptake by Ile-Leu was completely inhibited by treatment with either LY294002, or GF109203X in both L6 cells and isolated muscles. Ile-Leu increased glycogen contents in isolated muscles. These results suggest that BCAA-containing bioactive dipeptides in WPH stimulate glucose uptake in skeletal muscles via the PI3-kinase and aPKC pathways, resulting in increased skeletal muscle glycogen contents.

Quantifying hepatic glycogen synthesis by direct and indirect pathways in rats under normal ad libitum feeding conditions

Hepatic glycogen synthesis from intact hexose (direct pathway) relative to that from gluconeogenic precursors (indirect pathway) was quantified in ad libitum-fed rats. Following (2)H(2)O administration and overnight feeding, the livers were removed and glycogen (2)H-enrichment was measured by (2)H NMR. Six controls and six rats rendered hyperglycemic by streptozotocin (STZ; fasting blood glucose = 385 +/- 31 mg/dl) were studied. The indirect pathway contribution, estimated as glycogen hydrogen 5 relative to hydrogen 2 enrichment, was 54% +/- 4% for control rats-similar to values from healthy, meal-fed humans. In STZ-treated rats, the indirect pathway contribution was significantly higher (68% +/- 4%, P < 0.05 vs. controls), similar to that of Type 1 diabetic (T1D) patients. In conclusion, sources of hepatic glycogen synthesis in rats during ad libitum nocturnal feeding were quantified by analysis of glycogen enrichment from (2)H(2)O. STZ caused alterations resembling the pathophysiology of hepatic glycogen synthesis in T1D patients.

Coingestion of protein with carbohydrate during recovery from endurance exercise stimulates skeletal muscle protein synthesis in humans

Coingestion of protein with carbohydrate (CHO) during recovery from exercise can affect muscle glycogen synthesis, particularly if CHO intake is suboptimal. Another potential benefit of protein feeding is an increased synthesis rate of muscle proteins, as is well documented after resistance exercise. In contrast, the effect of nutrient manipulation on muscle protein kinetics after aerobic exercise remains largely unexplored. We tested the hypothesis that ingesting protein with CHO after a standardized 2-h bout of cycle exercise would increase mixed muscle fractional synthetic rate (FSR) and whole body net protein balance (WBNB) vs. trials matched for total CHO or total energy intake. We also examined whether postexercise glycogen synthesis could be enhanced by adding protein or additional CHO to a feeding protocol that provided 1.2 g CHO x kg(-1) x h(-1), which is the rate generally recommended to maximize this process. Six active men ingested drinks during the first 3 h of recovery that provided either 1.2 g CHO.kg(-1).h(-1) (L-CHO), 1.2 g CHO + 0.4 g protein x kg(-1) x h(-1) (PRO-CHO), or 1.6 g CHO x kg(-1) x h(-1) (H-CHO) in random order. Based on a primed constant infusion of l-[ring-(2)H(5)]phenylalanine, analysis of biopsies (vastus lateralis) obtained at 0 and 4 h of recovery showed that muscle FSR was higher (P < 0.05) in PRO-CHO (0.09 +/- 0.01%/h) vs. both L-CHO (0.07 +/- 0.01%/h) and H-CHO (0.06 +/- 0.01%/h). WBNB assessed using [1-(13)C]leucine was positive only during PRO-CHO, and this was mainly attributable to a reduced rate of protein breakdown. Glycogen synthesis rate was not different between trials. We conclude that ingesting protein with CHO during recovery from aerobic exercise increased muscle FSR and improved WBNB, compared with feeding strategies that provided CHO only and were matched for total CHO or total energy intake. However, adding protein or additional CHO to a feeding strategy that provided 1.2 g CHO x kg(-1) x h(-1) did not further enhance glycogen resynthesis during recovery.

Postexercise Muscle Glycogen Synthesis with Combined Glucose and Fructose Ingestion

To evaluate the efficacy of using combined glucose and fructose (GF) ingestion as a means to stimulate short-term (4 h) postexercise muscle glycogen synthesis compared to glucose only (G). On two separate occasions, six endurance-trained men performed an exhaustive glycogen-depleting exercise bout followed by a 4-h recovery period. Muscle biopsy samples were obtained from the vastus lateralis muscle at 0, 1, and 4 h after exercise. Subjects ingested carbohydrate solutions containing G (90 g x h(-1)) or GF (G = 60 g x h(-1); F = 30 g x h(-1)) commencing immediately after exercise and every 30 min thereafter. Immediate postexercise muscle glycogen concentrations were similar in both trials (G = 128 +/- 25 mmol x kg(-1) dry muscle (dm) vs GF = 112 +/- 16 mmol x kg(-1) dm; P > 0.05). Total glycogen storage during the 4-h recovery period was 176 +/- 33 and 155 +/- 31 mmol x kg(-1) dm for G and GF, respectively (G vs GF, P > 0.05). Hence, mean muscle glycogen synthesis rates during the 4-h recovery period did not differ between the two conditions (G = 44 +/- 8 mmol x kg(-1) dm x h(-1) vs GF = 39 +/- 8 mmol x kg(-1) dm x h(-1), P > 0.05). Plasma glucose and serum insulin responses during the recovery period were similar in both conditions, although plasma lactate concentrations were significantly elevated during GF compared to G (by approximately 0.8 mmol x L(-1), P < 0.05). Glucose and glucose/fructose (2:1 ratio) solutions, ingested at a rate of 90 g x h(-1), are equally effective at restoring muscle glycogen in exercised muscles during the recovery from exhaustive exercise.

The acute insulin sensitizing effects of troglitazone are mediated by AMPK activation and inhibition of pyruvate dehyrogenase activity

This study investigated the acute effects of troglitazone (Tro) on glucose and fatty acid (FA) partitioning in skeletal muscle. Exposure of L6 myotubes to Tro for 1h resulted in ∼1.3‐ and ∼1.6‐fold increases in palmitate oxidation and CPT‐1 activity, respectively. Tro inhibited basal (∼25%) and insulin‐stimulated (∼35%) palmitate uptake but significantly increased basal (∼2.2‐fold) and insulin‐stimulated (∼2.7‐fold) glucose uptake. Inhibition of AMPK prevented the effects of Tro on palmitate oxidation and glucose uptake. Although Tro exerted an insulin sensitizing effect, it reduced basal and insulin‐stimulated rates of glycogen synthesis, incorporation of glucose into lipids, and glucose oxidation to values corresponding to ∼30%, ∼60%, and 30% of the controls, respectively. These effects were accompanied by an increase in basal and insulin‐stimulated phosphorylation of AktThr308, AktSer473, and GSK3α/β. Tro also powerfully suppressed pyruvate decarboxylation, which was followed by a significant increase in basal (∼3.5‐fold) and insulin‐stimulated (∼5.5‐fold) rates of lactate production. In summary, we provide novel evidence that Tro exerts acute insulin sensitizing effects by increasing FA oxidation, reducing FA uptake, suppressing pyruvate dehydrogenase activity, and shifting glucose metabolism towards lactate production in muscle cells. Funding was provided by NSERC, CIHR, and CDA.

Influence of acute and chronic administration of benzylamine on glucose tolerance in diabetic and obese mice fed on very high-fat diet

The combination of vanadate plus benzylamine has been reported to stimulate glucose transport in rodent adipocytes and to mimic other insulin actions in diverse studies. However, benzylamine alone activates glucose uptake in human fat cells and increases glucose tolerance in rabbits. The aim of this work was to unravel the benzylamine antihyperglycemic action and to test whether its chronic oral administration could restore the defective glucose handling of mice rendered slightly obese and diabetic by very high-fat diet (VHFD). When VHFD mice were i.p. injected with benzylamine at 0.7 to 700 micromol/kg before glucose tolerance test, they exhibited reduced hyperglycemic response without alteration of insulin secretion. Whole body glucose turnover, as assessed by the glucose isotopic dilution technique, was unchanged in mice perfused with benzylamine (total dose of 75 micromol/kg). However, their in vivo glycogen synthesis rate was increased. Benzylamine appeared therefore to directly facilitate glucose utilisation in peripheral tissues. When given chronically at 2000 or 4000 micromol/kg/d in drinking water, benzylamine elicited a slight reduction of water consumption but did not change body weight or adiposity and did not modify oxidative stress markers. Benzylamine treatment improved glucose tolerance but failed to normalize the elevated glucose fasting plasma levels of VHFD mice. There was no influence of benzylamine ingestion on lipolytic activity, basal and insulin-stimulated glucose uptake, and on inflammatory adipokine expression in adipocytes. The improvement of glucose tolerance and the lack of adverse effects on adipocyte metabolism, reported here in VHFD mice allow to consider orally given benzylamine as a potential antidiabetic strategy which deserves to be further studied in other diabetic models.

Activation of AMP‐activated protein kinase, inhibition of pyruvate dehydrogenase activity, and redistribution of substrate partitioning mediate the acute insulin‐sensitizing effects of troglitazone in skeletal muscle cells

The aim of this study was to investigate the acute effects of troglitazone on several pathways of glucose and fatty acid (FA) partitioning and the molecular mechanisms involved in these processes in skeletal muscle. Exposure of L6 myotubes to troglitazone for 1 h significantly increased phosphorylation of AMPK and ACC, which was followed by approximately 30% and approximately 60% increases in palmitate oxidation and carnitine palmitoyl transferase-1 (CPT-1) activity, respectively. Troglitazone inhibited basal ( approximately 25%) and insulin-stimulated ( approximately 35%) palmitate uptake but significantly increased basal and insulin-stimulated glucose uptake by approximately 2.2- and 2.7-fold, respectively. Pharmacological inhibition of AMPK completely prevented the effects of troglitazone on palmitate oxidation and glucose uptake. Interestingly, even though troglitazone exerted an insulin sensitizing effect, it reduced basal and insulin-stimulated rates of glycogen synthesis, incorporation of glucose into lipids, and glucose oxidation to values corresponding to approximately 30%, approximately 60%, and 30% of the controls, respectively. These effects were accompanied by an increase in basal and insulin-stimulated phosphorylation of Akt(Thr308), Akt(Ser473), and GSK3alpha/beta. Troglitazone also powerfully suppressed pyruvate decarboxylation, which was followed by a significant increase in basal ( approximately 3.5-fold) and insulin-stimulated ( approximately 5.5-fold) rates of lactate production by muscle cells. In summary, we provide novel evidence that troglitazone exerts acute insulin sensitizing effects by increasing FA oxidation, reducing FA uptake, suppressing pyruvate dehydrogenase activity, and shifting glucose metabolism toward lactate production in muscle cells. These effects seem to be at least partially dependent on AMPK activation and may account for potential acute PPAR-gamma-independent anti-diabetic effects of thiazolidinediones in skeletal muscle.