Effects of re-feeding after prolonged starvation on pyruvate dehydrogenase activities in heart, diaphragm and selected skeletal muscles of the rat

Nuclear localization of metabolic enzymes in immunity and metastasis

Metabolism is essential to all living organisms that provide cells with energy, regulators, building blocks, enzyme cofactors and signaling molecules, and is in tune with nutritional conditions and the function of cells to make the appropriate developmental decisions or maintain homeostasis. As a fundamental biological process, metabolism state affects the production of multiple metabolites and the activation of various enzymes that participate in regulating gene expression, cell apoptosis, cancer progression and immunoreactions. Previous studies generally focus on the function played by the metabolic enzymes in the cytoplasm and mitochondrion. In this review, we conclude the role of them in the nucleus and their implications for cancer progression, immunity and metastasis.

The pyruvate carboxylase-pyruvate dehydrogenase axis in islet pyruvate metabolism: Going round in circles?

Pyruvate is the major product of glycolysis in pancreatic β-cells, and its ultimate metabolic fate depends on the relative activities of two enzymes. The first, pyruvate carboxylase (PC) replenishes oxaloacetate withdrawn from the tricarboxylic acid (TCA) cycle via the carboxylation of pyruvate to form oxaloacetate. Flux via PC is also involved in the formation of NADPH, one of several important coupling factors for insulin secretion. In most tissues, PC activity is enhanced by increased acetyl-CoA. The alternative fate of pyruvate is its oxidative decarboxylation to form acetyl-CoA via the pyruvate dehydrogenase complex (PDC). The ultimate fate of acetyl-CoA carbon is oxidation to CO2 via the TCA cycle, and so the PDC reaction results of the irreversible loss of glucose-derived carbon. Thus, PDC activity is stringently regulated. The mechanisms controlling PDC activity include end-product inhibition by increased acetyl-CoA, NADH and ATP, and its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDHKs 1-4). Here we review new developments in the regulation of the activities and expression of PC, PDC and the PDHKs in the pancreatic islet in relation to islet pyruvate disposition and glucose-stimulated insulin secretion (GSIS).

Mechanisms underlying regulation of the expression and activities of the mammalian pyruvate dehydrogenase kinases

The mechanisms that control mammalian pyruvate dehydrogenase complex (PDC) activity include its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDKs 1 - 4). Here we review new developments in the regulation of the activities and expression of the PDKs, in particular PDK2 and PDK4, in relation to glucose and lipid homeostasis. This review describes recent advances relating to the acute and long-term modes of regulation of the PDKs, with particular emphasis on the regulatory roles of nuclear receptors including peroxisome proliferator-activated receptor (PPAR) alpha and Liver X receptor (LXR), PPAR gamma coactivator alpha (PGC-1alpha) and insulin, and the impact of changes in PDK activity and expression in glucose and lipid homeostasis. Since PDK4 may assist in lipid clearance when there is an imbalance between lipid delivery and oxidation, it may represent an attractive target for interventions aimed at rectifying abnormal lipid as well as glucose homeostasis in disease states.

Pyruvate dehydrogenase activation and kinase expression in human skeletal muscle during fasting

Fasting forces adaptive changes in whole body and skeletal muscle metabolism that increase fat oxidation and decrease the oxidation of carbohydrate. We tested the hypothesis that 40 h of fasting would decrease pyruvate dehydrogenase (PDH) activity and increase PDH kinase (PDK) isoform mRNA expression in human skeletal muscle. The putative transcriptional activators of PDK isozymes, peroxisome proliferator-activated receptor-alpha (PPAR-alpha) protein, and forkhead homolog in rhabdomyosarcoma (FKHR) mRNA were also measured. Eleven healthy adults fasted after a standard meal (25% fat, 60% carbohydrate, 15% protein) with blood and skeletal muscle samples taken at 3, 15, and 40 h postprandial. Fasting increased plasma free fatty acid, glycerol, and beta-hydroxybutyrate concentrations and decreased glucose and insulin concentrations. PDH activity decreased from 0.88 +/- 0.11 mmol acetyl-CoA. min(-1). kg wet muscle wt(-1) at 3 h to 0.62 +/- 0.10 (P = not significant) and 0.39 +/- 0.06 (P < 0.05) mmol. min(-1). kg wet mass(-1) after 15 and 40 h of fasting. Although all four PDK isoforms were expressed in human skeletal muscle, PDK-2 and -4 mRNA were the most abundant. PDK-1 and -3 mRNA abundance was approximately 1 and 15% of the PDK-2 and -4 levels, respectively. The 40-h fast had no effect on PDK-1, -2, and -3 mRNA expression. PDK-4 mRNA was significantly increased approximately 3-fold after 15 h and approximately 14-fold after 40 h of fasting. Skeletal muscle PPAR-alpha protein and FKHR mRNA abundance were unaffected by the fast. The results suggest that decreased PDH activation after 40 h of fasting may have been a function of the large increase in PDK-4 mRNA expression and possible subsequent increase in PDK protein and activity. The changes in PDK-4 expression and PDH activity did not coincide with increases in the transcriptional activators PPAR-alpha and FKHR.

Recent advances in mechanisms regulating glucose oxidation at the level of the pyruvate dehydrogenase complex by PDKs

The mitochondrial pyruvate dehydrogenase complex (PDC) catalyzes the oxidative decarboxylation of pyruvate, linking glycolysis to the tricarboxylic acid cycle and fatty acid (FA) synthesis. Knowledge of the mechanisms that regulate PDC activity is important, because PDC inactivation is crucial for glucose conservation when glucose is scarce, whereas adequate PDC activity is required to allow both ATP and FA production from glucose. The mechanisms that control mammalian PDC activity include its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDKs 1-4) and its dephosphorylation (activation, reactivation) by the pyruvate dehydrogenase phosphate phosphatases (PDPs 1 and 2). Isoform-specific differences in kinetic parameters, regulation, and phosphorylation site specificity of the PDKs introduce variations in the regulation of PDC activity in differing endocrine and metabolic states. In this review, we summarize recent significant advances in our knowledge of the mechanisms regulating PDC with emphasis on the PDKs, in particular PDK4, whose expression is linked with sustained changes in tissue lipid handling and which may represent an attractive target for pharmacological interventions aimed at modulating whole body glucose, lipid, and lactate homeostasis in disease states.

Insulin activation of pyruvate dehydrogenase complex is enhanced by exercise training

We studied the effects of exercise training on the activity of the pyruvate dehydrogenase (PDH) complex in rat gastrocnemius muscle (experiment 1) and the response of the complex to glucose and insulin infusion (euglycemic clamp) in trained and sedentary rats (experiment 2). In experiment 1, half of the rats were randomly allocated as sedentary animals and the other half were trained by voluntary running exercise for 8 weeks. The total activity of the PDH complex was not affected by exercise training, and the activity state (proportion of the active form) of the PDH complex was decreased from 15.0%+/-2.4% to 7.5%+/-1.1% by exercise training. The activity of 3-hydroxyacyl-coenzyme A (CoA) dehydrogenase ([3-HADH] an enzyme in beta-oxidation) was significantly higher in trained versus sedentary rats. In experiment 2, sedentary and trained rats were starved for 24 hours before performing the euglycemic clamp. Glucose and insulin infusion was performed by a euglycemic clamp (insulin infusion rate, 6 mU/kg/min) for 90 minutes. The PDH complex was inactivated to less than 1% in both sedentary and trained rats after 24 hours of starvation. The glucose infusion rate (GIR) during the euglycemic clamp was higher in trained versus sedentary rats. The euglycemic clamp resulted in activation of the PDH complex in both sedentary and trained rats, but the response of the PDH complex to the euglycemic clamp was significantly higher in trained rats (5.8%+/-0.5%) than in sedentary rats (2.9%+/-0.5%). These results suggest that exercise training promotes fatty acid oxidation in association with suppression of glucose oxidation in skeletal muscle under resting conditions, but increases the rate of carbohydrate oxidation when glucose flux into muscle cells is stimulated by insulin.

pH is regulated differently by glucose in skeletal muscle from fed and starved rats: a study using 31P-NMR spectroscopy

The aim of this study was to determine whether exogenous glucose metabolism influences the pH in superfused EDL muscle from growing rats fed or starved for 48 h (body weight 55 and 45 g, respectively). Energy state and intracellular pH of muscle were repeatedly monitored by 31P-nuclear magnetic resonance spectroscopy (31P-NMRS); glycogen and other energy metabolites were assayed enzymatically in muscle extracts at the end of the experiment. In EDL muscles from starved rats superfused with glucose for 4 h, intracellular pH was elevated (7-7.3), lactate concentration low, glycogen repletion very intense and citrate synthase activity high. We conclude that glucose was routed mainly toward both oxidative phosphorylation and glycogen synthesis in EDL muscles after food deprivation of rats. In contrast, the major pathway in muscles from fed rats may be glycolysis because the glycogen pool remained constant throughout the experiment. The additional and minor pH component (in the range of 6.5 to 6.8) seen in muscles from fed rats, even in the presence of exogenous glucose, might be due to impaired glucose utilization because this component appears also in muscles from starved rats superfused without glucose or with a nonmetabolizable analog of glucose. Consequently, direct pH measurement by 31P-NMR may be considered to be a precise criterion for evaluation of differences in metabolic potentialities of muscle studied ex vivo in relation to the nutritional state of rats.

Differential regulation of glycogen synthase by insulin and glucose in vivo in skeletal muscles of the rat

Glucose 6-phosphate (G-6-P)-independent glycogen synthase (GSa) and glycogen synthase (GS) total activities were measured in muscles from 24-h-starved rats. Intravenous glucose tolerance tests (0.5 g/kg body wt) were used to produce physiological, transient increases in insulin and glucose concentrations. GS activation occurred at approximately 10 min after glucose administration with peak activation at approximately 15 min. GS activation was reversed approximately 15 min after insulin and glucose concentrations had returned to basal. No differences existed between fast- and slow-twitch muscles. Hyperinsulinemia (approximately 160 mU/ml) in the absence of hyperglycemia elicited 1.5-fold activation of GS (P < 0.001) in two of three fast-twitch muscles but did not activate GS in slow-twitch muscles. Glucose infusion (glycemia approximately 8 mM; insulin approximately 40 mU/ml) significantly (P < 0.01) increased the percentage of total GS in the GSa form in four of the five muscles. Hyperglycemia with modest hyperinsulinemia evoked greater enhancement of GSa activity in fast-twitch muscle than insulin alone at a higher concentration (P < 0.01). In summary, hyperinsulinemia without hyperglycemia does not result in maximal activation of GS in fast-twitch muscle, and a rise in glycemia is obligatory for GS activation by insulin in slow-twitch muscle. The data support an important role for glycemia in modulating the response of skeletal muscle GS to insulin and provide further evidence of heterogeneity among skeletal muscle types.

Effects of aging on the activities of pyruvate dehydrogenase complex and its kinase in rat heart

Effects of aging on the activities of heart pyruvate dehydrogenase complex and pyruvate dehydrogenase kinase were examined using 7, 35 and 60 wk old rats. Aging did not affect the total activity of pyruvate dehydrogenase complex but decreased the activity state (percentage of active form) of the complex in rats under the fed condition (52%, 36% and 26% for 7, 35 and 60 wk old rats, respectively). This decrease in the complex activity with aging was suggested to be associated with an age-related decrease in the blood glucose disposal. Starvation for 24 h decreased the activity state to less than 3% in all of the age groups. The activity of pyruvate dehydrogenase kinase associated with the complex was not related to the alteration in the activity state of the complex; the kinase activity was slightly lower in 60 wk old rats than in the younger rats under the fed condition and activation of the kinase by starvation was greater in the younger rats. The mechanism for the decrease in activity of pyruvate dehydrogenase complex was discussed on the basis of glucose and fatty acid utilization of heart muscle cells.

Fuel selection, muscle fibre

The fuel selection of muscle fibres at rest is dependent on substrate availability. Increased lipid availability results in an increase citrate concentration with inhibition of glycolysis. Fat utilization also increases the concentration ratio acetyl-CoA:CoASH, with inhibition of PDH transformation to the active form. The result is an inhibition of carbohydrate utilization in conformity with the classical glucose-fatty acid style. During exercise fuel selection is dependent on the intensity of exercise, the recruitment pattern of fibre type and the availability of fuels. During exercise at maximum intensity the main fuels are PCr and muscle glycogen, the highest energy release occurring with type II fibres. At exercise intensities between 70 and 100% VO2max carbohydrate is the main fuel after the intake of normal mixed or carbohydrate-rich diets. No inhibition of PDHa formation was observed by increased concentration ratio acetyl-CoA:CoASH during the exercise, but the activation and transport of fatty-acyl groups from NEFA may be inhibited by a decrease in the concentration of CoASH. This mechanism may limit the contribution of fat to metabolism during exercise at intensities above 60% VO2max, after an intake of carbohydrate-rich diets. After carbohydrate starvation or an infusion of a fat emulsion, there was a substantial increase in the utilization of fat which, after the infusion, was concomitant with a high PDHa and a high lactate production. This is thought to be due to a decrease in glycolysis and in the catalytic activity of PDHa, especially in type I fibres, while lactate production continues in type II fibres. When exercise intensities fall below 60% VO2max, fat becomes the dominant fuel during prolonged exercise. At the same time the recruitment pattern is shifted toward type I fibres which have the lowest activation threshold and the highest oxidative capacity.

Pyruvate dehydrogenase activity and acetyl group accumulation during exercise after different diets

Pyruvate dehydrogenase activity (PDHa) and acetyl group accumulation were examined in human skeletal muscle at rest and during exercise after different diets. Five males cycled at 75% of maximal O2 uptake (VO2 max) to exhaustion after consuming a low-carbohydrate diet (LCD) for 3 days and again 1-2 wk later for the same duration after consuming a high-carbohydrate diet (HCD) for 3 days. Resting PDHa was lower after a LCD (0.20 +/- 0.04 vs. 0.69 +/- 0.05 mmol.min-1.kg wet wt-1; P < 0.05) and coincided with a greater intramuscular acetyl-CoA-to-CoASH ratio, acetyl-CoA content, and acetylcarnitine content. PDHa increased during exercise in both conditions but at a lower rate in the LCD condition compared with the HCD condition (1.46 +/- 0.25 vs. 2.65 +/- 0.23 mmol.min-1.kg wet wt-1 at 16 min and 1.88 +/- 0.20 vs. 3.11 +/- 0.14 at the end of exercise; P < 0.05). During exercise muscle acetyl-CoA and acetylcarnitine content and the acetyl-CoA-to-CoASH ratio decreased in the LCD condition but increased in the HCD condition. Under resting conditions PDHa was influenced by the availability of fat or carbohydrate fuels acting through changes in the acetyl-CoA-to-CoASH ratio. However, during exercise the activation of PDHa occurred independent of changes in the acetyl-CoA-to-CoASH ratio, suggesting that other factors are more important.

Changes in lipoprotein lipase activities in adipose tissue, heart and skeletal muscle during continuous or interrupted feeding

Lipoprotein lipase (LPL) activities in parametrial and interscapular adipose tissue, soleus and adductor longus muscles and hearts of female rats were measured during progressive starvation, chow re-feeding after 24 h starvation and throughout dark and light phases in rats permitted unrestricted access to chow. Adipose-tissue LPL activities declined by 50% after 6 h starvation and continued to fall as the starvation period was extended to 24 h. Skeletal-muscle LPL activities dramatically increased between 9 and 12 h of starvation. Cardiac LPL activities increased 2.5-fold within 6 h of starvation, reaching a maximum after 12 h of starvation. Adipose-tissue LPL activities increased rapidly within 2 h of re-feeding chow ad libitum after 24 h starvation, achieving 'fed ad libitum' values after 6 h. Oxidative-skeletal-muscle LPL activities also increased after 2 h of refeeding and exceeded 'fed ad libitum' values throughout the 6 h re-feeding period. Cardiac LPL activities remained up-regulated for the 6 h of re-feeding. Adipose-tissue LPL activities exceeded those of cardiac or skeletal muscle throughout both light and dark phases. The lowest adipose-tissue LPL activities were observed at 9 h into the light phase. In contrast, cardiac LPL activity declined throughout the dark phase, with a minimum at 9 h into the dark phase. No such variation was observed for skeletal-muscle LPL activities. A diurnal nadir in plasma triacylglycerol (TG) concentrations coincided with the peak in cardiac LPL activities. The results demonstrate that, during unrestricted feeding and re-feeding after prolonged starvation, changes in skeletal-muscle and adipose-tissue LPL activities are neither reciprocal nor co-ordinate. Regulation of cardiac LPL activity during the diurnal cycle may be an important aspect of both of cardiac fuel selection and whole-body TG metabolism.

Control of muscle pyruvate oxidation during late pregnancy

Despite significant increases in circulating concentrations of lipid fuels (triacylglycerol, non-esterified fatty acids (NEFA) and ketone bodies) in late-pregnant rats sampled in the fed (absorptive) state, cardiac and skeletal muscle active pyruvate dehydrogenase (PDHa) activities remained comparable with those observed in fed, age-matched virgin controls. Cardiac PDHa activity was suppressed in response to acute (6 h) starvation in late-pregnant (as well as virgin) rats: this inactivation was opposed by inhibition of mitochondrial long-chain FA oxidation. Starvation (6 h) also led to PDH inactivation in skeletal muscles of late-pregnant, but not virgin, rats. Starvation for 24 h led to further suppression of cardiac PDHa activity and was associated with significant increases in PDH kinase activities in both virgin and late-pregnant rats. Late pregnancy did not itself influence cardiac PDH kinase activity.

Mechanisms involved in the coordinate regulation of strategic enzymes of glucose metabolism

In this review, we evaluate the relative regulatory importance of specific strategic enzymes (in particular glycogen synthase, acetyl-CoA carboxylase [ACC] and the pyruvate dehydrogenase complex [PDH]) for carbohydrate utilization as an anabolic precursor and as an energy substrate during the nutritional transitions between the fed and fasted states. The involvement of the specific protein kinases contributing to the inactivation of these enzymes by phosphorylation [cyclic AMP-dependent protein kinase, AMP-activated protein kinase and PDH kinase] in achieving each regulatory response is also assessed. We demonstrate a striking temporal correlation between hepatic glycogen mobilization and PDH and ACC inactivation by phosphorylation during the immediate postabsorptive period; in contrast, rates of hepatic glycogen synthesis and PDH and ACC expressed activities do not change in parallel during refeeding. The results are consistent with shifting of the primary sites of control for overall hepatic carbon flux during the fed-to-starved and starved-to-fed nutritional transitions achieved, at least in part, by a complex pattern of regulation by protein phosphorylation and metabolites which is critically dependent on the precise nutritional status. Data are also presented that demonstrate asynchronous suppression of glucose uptake/phosphorylation and pyruvate oxidation in cardiac and skeletal muscle during progressive starvation. Analogous asynchrony is observed in the reactivation of these processes in cardiac and skeletal muscle during refeeding after starvation. We provide evidence in support of the concept that selective suppression of pyruvate oxidation in oxidative muscles during early starvation and during the initial phase of refeeding is achieved because of differential sensitivity of glucose uptake/phosphorylation and pyruvate oxidation to lipid-fuel utilization. We discuss the relative importance of regulatory events governing local fatty acid production and utilization (via lipoprotein lipase and carnitine palmitoyltransferase 1, respectively) or overall fatty acid supply (dictated by events at the adipocyte) for fuel utilization by muscle during nutritional transitions. Finally, we assess the regulatory importance of glycogen synthesis in determining overall rates of glucose clearance by skeletal muscle during alimentary hyperglycemia and hyperinsulinemia.

Effects of exercise and food restriction on rat skeletal muscles

Studies were undertaken to compare the effects of exercise and food restriction on body weight (BW), muscle weight (MW), muscle fiber size, and proportion of muscle fiber types. 20 male Fischer 344 rats were randomly assigned to four equal groups: ad libitum-fed control (AC), ad libitum-fed exercise (AE), food restricted control (RC) and food restricted exercise (RE). From 6 weeks of age, RC and RE rats received 60% of the daily food intake of AC and AE rats, respectively. At 7 months of age, AE and RE rats began 40-50 min of daily treadmill exercise. Running speed increased from 1.2 to 1.6 miles/hour and the grade increased to 15% during the first 2 weeks of training. After 10 weeks of training, rats were weighed, sacrificed, and the soleus (SOL), plantaris (PLN) and extensor digitorum longus (EDL) muscles were removed at in situ rest length, weighed, and quick-frozen. Standard histochemical assays were performed, and muscle fiber cross-sectional area was determined planimetrically. Training had little effect on MW or BW, but food restriction greatly reduced BW. This resulted in greater MW/BW ratio in RC and RE than AC and AE rats, respectively. Exercise also increased SOL muscle fiber area in ad libitum-fed but not food restricted rats resulting in smaller fibers in SOL of RE than AE. No changes in percentage of SOL fiber types occurred with food restriction or exercise. In PLN, the percentage of fast-twitch oxidative fibers of AE and RE was greater than in AC and RC, but there was no effect of food restriction or exercise on fiber area.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased pyruvate dehydrogenase kinase activity in response to sepsis

The effect of sterile inflammation and sepsis on the proportion of active pyruvate dehydrogenase complex (PDH) in mitochondria isolated from skeletal muscle has been investigated. The proportion of active PDH in mitochondria isolated from septic animals was significantly reduced compared with control under all incubation conditions examined, even in the presence of inhibitors of the PDH kinase. There was no significant difference between control and sterile inflammation in any of the incubations examined. The rate constant for ATP-dependent inactivation of the PDH complex in mitochondrial extracts from control animals was -0.42 min-1 (r = 0.993; P less than 0.001) and was not altered in mitochondrial extracts from sterile inflammatory animals (-0.43 min-1; r = 0.999; P less than 0.001). However, rate constants for inactivation in septic animals was significantly increased over twofold to -1.08 min-1 (r = 0.987; P less than 0.001) (P less than 0.001 vs. control or sterile inflammation). In the presence of inhibitors of the PDH kinase reaction (2.5 mM pyruvate or 1 mM dichloroacetate), inactivation of PDH after addition of ATP was significantly greater in mitochondrial extracts from septic than either control or sterile inflammatory animals. These results suggest that sepsis, but not sterile inflammation, induces a stable factor in skeletal muscle mitochondria that increased PDH kinase activity.

Regulation of fatty acid synthesis in lactating rat mammary gland in the fed to starved transition: asynchronous control of pyruvate dehydrogenase, phosphofructokinase and acetyl-CoA carboxylase

1. Withdrawal of food from lactating rats produced a rapid and dramatic decrease in the uptake of glucose by the mammary gland and an inhibition of the rate of fatty acid synthesis that could not be explained alone by decreased substrate supply to the tissue. 2. Within the first 6 hr starvation, fatty acid synthesis and pyruvate dehydrogenase activity were inhibited by 87 and 80%, respectively, but acetyl-CoA carboxylase activity did not change significantly. 3. Between 6 and 24 hr starvation, total and expressed activities of acetyl-CoA carboxylase decreased by 62 and 55%, respectively. 4. The ratio of fructose-6-phosphate/fructose-1,6-bisphosphate concentration in mammary tissue increased 9-fold during the first 6 hr starvation, indicating an inhibition of 6-phosphofructo-1-kinase. However, the major inhibition of this enzyme occurred between 6 and 24 hr starvation when this metabolite ratio increased a further 160-fold in parallel with increased tissue citrate concentration. 5. The increase in citrate concentration between 6 and 24 hr starvation correlated with acetyl-CoA carboxylase inactivation and ketone body accumulation in the mammary gland. 6. This study confirms the asynchronous control of three important regulatory steps in the pathway of glucose utilization and fatty acid synthesis in the lactating rat mammary gland.

Control of respiration in skeletal muscle at rest

The suggestion is made that, under resting conditions in situ, muscle cell respiration is dependent on the way O2 and substrates are distributed to the cells by the microcirculation. (Delivery is measured as arterial-blood concentration multiplied by flow to the organ.) Microscale heterogeneity of this distribution, which is more marked but less stable than the more easily demonstrated larger-scale heterogeneity (0.1 to 0.5-g sampling grain), might indeed ration O2 and substrates in a large population of the cells of a resting organ at any given moment, and microscale heterogeneity of distribution may thus take part in the normal control of cell respiration.

Glucose utilization and disposal in cardiothoracic and skeletal muscles during the starved-to-fed transition in the rat

Glucose utilization indices (GUI) increased to fed values in diaphragm and oxidative skeletal muscles and exceeded fed values in non-oxidative muscles within 2 h of re-feeding chow to 48 h-starved rats. Cardiac GUI reached fed values only after 7 h. Glycogen deposition accounted for most of the glucose phosphorylated in skeletal muscle over the first 2 h in oxidative muscles and over the first 4 h in non-oxidative muscles. In oxidative muscles, the contribution of glycogen deposition to total glucose 6-phosphate disposal diminished as re-feeding was extended from 2 to 6 h.

Glucose utilization by skeletal muscles in vivo in experimental hyperthyroidism in the rat

In the fed state, hyperthyroidism increased glucose utilization indices (GUIs) of skeletal muscles containing a lower proportion of oxidative fibres. Glycogen concentrations were unchanged, but active pyruvate dehydrogenase (PDHa) activities were decreased. Hyperthyroidism attenuated the effects of 48 h of starvation to decrease muscle GUI. Glycogen concentrations and PDHa activities after 48 h of starvation were low and similar in euthyroid and hyperthyroid rats. The increase in glucose uptake and phosphorylation relative to oxidation and storage in skeletal muscle induced by hyperthyroidism may contribute to increased glucose re-cycling in the fed hyperthyroid state and to glucose turnover in the starved hyperthyroid state.

Glucose utilization in heart, diaphragm and skeletal muscle during the fed-to-starved transition

The progressive effects of starvation on muscle glucose utilization were studied in the conscious resting rat. High rates of glucose uptake and phosphorylation in constantly working cardiothoracic (heart, diaphragm) and postural skeletal muscles (soleus, adductor longus) were maintained for at least 9 h of starvation. A rapid decline in cardiac glucose utilization was observed during the period 9-24 h of starvation, but for the other muscles the decline was more gradual. Consequently, even after 24 h, rates of glucose utilization in these muscles remained quantitatively significant. In both cardiothoracic and working (postural) skeletal muscle, glucose uptake and phosphorylation and activity of the active form of pyruvate dehydrogenase exhibited differential sensitivities to starvation and also to acute elevation of fatty acid concentrations during acute (4-9 h) starvation, such that pyruvate oxidation was more rapidly suppressed than glucose uptake and phosphorylation. The results are discussed in relation to the role of the glucose/fatty acid cycle in glucose conservation during the fed-to-starved transition.

Effects of non-esterified fatty acid availability on insulin stimulated glucose utilisation and tissue pyruvate dehydrogenase activity in the rat

Fatty acids in cardiac muscle compete with glucose for oxidation, thereby inhibiting glucose utilisation. It is not clear whether a similar mechanism is important in resting skeletal muscle. We used the hyperinsulinaemic euglycaemic clamp technique in conscious rats fasted for 20 h to examine the effects of increased plasma non-esterified fatty acid levels (approximately 1 mmol/l) on glucose metabolism. Insulin was infused at 75 mU/h (plasma insulin, 2.27 +/- 0.21 micrograms/l) or 300 mU/h (16.41 +/- 0.47 micrograms/l). An increase in non-esterified fatty acid levels decreased clamp glucose requirement and 3-3H-glucose turnover by 35% (p less than 0.001) when the higher insulin dose was used but there was no change at the lower dose. At both insulin infusion rates, clamp blood lactate and pyruvate responses suggested inhibition of muscle glycolysis by elevated plasma non-esterified fatty acid concentrations. Quadriceps muscle glycogen deposition during the clamps was enhanced by increased non-esterified fatty acid availability at the lower insulin dose (p less than 0.001) but not at the higher insulin concentration. Activation of pyruvate dehydrogenase during the clamps was partially inhibited by increased plasma non-esterified fatty acid in the heart, adipose tissue and quadriceps muscle. This was evident at both insulin levels in heart but only at the higher insulin concentration in muscle (p less than 0.002). The findings are consistent with an inhibition of glycolysis in skeletal muscle of mixed fibre type as a result of increased fatty acid availability. At low rates of glucose flux glycogen synthesis may compensate for decreased glycolysis so that glucose turnover is not decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Time courses of the responses of pyruvate dehydrogenase activities to short-term starvation in diaphragm and selected skeletal muscles of the rat

In the fed state, the percentages of the pyruvate dehydrogenase complex (PDH) in the active form (PDHa) in diaphragm and a selection of skeletal muscles (adductor longus, soleus, extensor digitorum longus, tibialis anterior, gastrocnemius) ranged from 8% (soleus) to 38% (gastrocnemius). Major decreases in PDHa activities in all of these muscles were observed after 15 h of starvation, by which time activities were less than 40% of the fed values. In general, the response to starvation was observed more rapidly in muscles of high oxidative capacity. The patterns of changes in skeletal-muscle PDH activities during the fed-to-starved transition are discussed in relation to changes in lipid-fuel supply and oxidation.