Pyruvate dehydrogenase activities during the fed-to-starved transition and on re-feeding after acute or prolonged starvation.

We investigated the temporal relationship between hepatic glycogen depletion and cardiac and hepatic PDH (pyruvate dehydrogenase complex) activities during the acute phase of starvation. There was a striking correlation between the decline in hepatic glycogen and PDH inactivation during the first 10 h of starvation. Re-feeding after 6 h starvation was associated with complete re-activation of PDH in liver and re-activation to approx. 75% of the fed value in heart, whereas in rats previously starved for 24-48 h re-activation was delayed in liver and diminished in heart. The results are discussed with reference to the fate of dietary carbohydrate after re-feeding.     

Effect of nutritional status on insulin sensitivity in vivo and tissue enzyme activities in the rat.

The hyperinsulinaemic-glucose-clamp technique, in combination with measurement of glucose turnover in conscious unrestrained rats, was used to assess the effects of nutritional status on insulin sensitivity in vivo and glucose metabolism. Liver, heart and quadriceps skeletal-muscle glycogen content and activities of pyruvate dehydrogenase (PDH) and glycogen synthase were measured both basally and at the end of a 2.5 h glucose clamp (insulin 85 munits/h) in rats 6, 24 and 48 h after food withdrawal. Clamp glucose requirement and glucose turnover were unchanged by fasting. Activation of glycogen synthase and glycogen deposition in liver and skeletal muscle during the clamps were also not impaired in rats after a prolonged fast. By contrast with skeletal muscle, activation of cardiac-muscle glycogen synthase and glycogen deposition during the clamps were markedly impaired by 24 h of fasting and were undetectable at 48 h. Skeletal-muscle PDH activity fell with more prolonged fasting (6 h, 15.3 +/- 3.4%; 24 h, 4.7 +/- 0.7%; 48 h, 4.3 +/- 0.6% active; P less than 0.005), but at 24 and 48 h was stimulated by the clamp to values unchanged by the duration of fasting. Stimulation of cardiac PDH activity by the clamp was, however, impaired in rats fasted for 24 or 48 h. Basal hepatic PDH did not change significantly with fasting (6 h, 5.3 +/- 1.1%; 24 h, 4.6 +/- 0.7%; 48 h, 3.9 +/- 0.5%), and, although it could be partly restored at 24 h, very little stimulation occurred at 48 h. Hepatic pyruvate kinase and acetyl-CoA carboxylase activity were both stimulated by the clamps, and this was not impaired with more prolonged fasting. During the glucose clamps, blood concentrations of lactate, pyruvate and alanine were increased to a greater extent in rats fasted for 24 and 48 h than in rats studied 6 h after food withdrawal. The findings suggest that, although sensitivity to insulin of whole-body glucose disposal is unchanged with fasting, there may be qualitative differences in the metabolism of glucose.     

Hepatic pyruvate dehydrogenase kinase activities during the starved-to-fed transition.

Starvation for 48 h elicited a 74% increase in hepatic pyruvate dehydrogenase (PDH) kinase activity, measured directly by 32Pi-incorporation from [gamma-32P]ATP into a synthetic peptide corresponding to the major phosphorylation site on E1. The administration of chow ad libitum to previously-starved rats suppressed hepatic PDH kinase activity by only approx. 20% within 2 h of re-feeding, and the relatively high activity of PDH kinase was associated with continued suppression of PDC complex re-activation. Whereas there was no further decline in PDH kinase activity over the next 2 h, PDC re-activation to the fed value was observed during this time interval. PDH kinase activity decreased to fed values only after 8 h.     

Regulation of renal and hepatic pyruvate dehydrogenase complex on carbohydrate re-feeding after starvation. Possible mechanisms and a regulatory role for thyroid hormone.

The work investigated the mechanisms for modulation of renal and hepatic pyruvate dehydrogenase complex (PDH) activities after carbohydrate re-feeding of 48 h-starved rats, and identified a regulatory role for tri-iodothyronine. Glucose re-feeding decreased blood concentrations of lipid fuels in both euthyroid and hyperthyroid rats. This treatment was not associated with re-activation of hepatic PDH in either group of rats, or of renal PDH in hyperthyroid rats (where activity was already high), but it increased renal PDH in euthyroid rats. Dichloroacetate (DCA), an activator of PDH kinase, increased renal PDH activities in euthyroid rats, but not hyperthyroid rats, and effects of glucose re-feeding or hyperthyroidism were no longer apparent. These treatments therefore exert their effects on renal PDH through changes in PDH kinase. DCA re-activation of hepatic PDH was more marked in hyperthyroid than in euthyroid rats, suggesting that, under conditions of inhibited kinase activity, PDH phosphatase is more active in livers of hyperthyroid rats. The limited effect of DCA on hepatic PDH in euthyroid rats was potentiated by glucose re-feeding or insulin, but not by inhibition of lipolysis, demonstrating a direct effect of insulin to increase hepatic PDH phosphatase. Glucose re-feeding, inhibition of lipolysis or insulin administration did not increase hepatic PDH in DCA-treated hyperthyroid rats, indicating that effects of hyperthyroidism and of insulin on PDH phosphatase are not additive.     

Effect of starvation and insulin in vivo on the activity of the pyruvate dehydrogenase complex in rat skeletal muscles

The in vivo responses of pyruvate dehydrogenase (PDH) complex to starvation and insulin was assessed in heart, diaphragm and red quadriceps muscle. PDH complex activity was decreased by starvation (3.4–10.2-fold), the magnitude of change depending on muscle type. Insulin increased PDH activity in all muscle types. In fed rats, this effect was relatively small (1.25–1.29-fold). In starved rats there were effects in heart (4.3-fold) and red quadriceps (1.7-fold) but no effect in diaphragm. These results demonstrate that PDH complex in different groups of muscle has different insulin sensitivity (particularly in tissues from starved animals).     

Comparison of tissue pyruvate dehydrogenase activities on re-feeding rats fed ad libitum or meal-fed rats with a chow-diet meal.

Meal-fed rats and rats fed ad libitum had similar rates of hepatic glycogenesis at 60 min after the initiation of re-feeding a chow meal after 22 h starvation, but hepatic PDHa (active form of pyruvate dehydrogenase) activities were 4-fold higher in the meal-fed group. In heart, PDHa activities were 3-fold higher before re-feeding and 2-fold higher after re-feeding in the meal-fed group compared with the group fed ad lib. The blood metabolite profile suggested diminished fat oxidation in starved meal-fed rats and accelerated flux through PDH in meal-fed re-fed rats compared with the group fed ad lib.     

Inactivation of pyruvate dehydrogenase complex in heart muscle mitochondria of gold-thioglucose-induced obese mice is not due to a stable increase in activity of pyruvate dehydrogenase kinase.

The proportion of pyruvate dehydrogenase (PDH) complex in the active dephosphorylated form was decreased (compared with fed lean control mice) in heart muscle mitochondria after the induction of obesity with gold-thioglucose (by 54%) or starvation of lean mice for 48 h (by 81%). The effects of obesity to inactivate PDH complex were demonstrable 4 weeks after administration of gold-thioglucose, and occurred despite significant hyperinsulinaemia in obese animals. Phosphorylation and inactivation of PDH complex in mouse heart muscle in starvation was attributed to a stable increase (2.7-fold) in the activity of PDH kinase as measured in extracts of mitochondria mediated by increased specific activity of a protein activator of PDH kinase (KAP) [Denyer, Kerbey & Randle (1986) Biochem. J. 239, 347-354]. In obese mice no such increase in kinase activity was observed, and we conclude that phosphorylation and inactivation of PDH complex in heart muscle in obesity is not mediated by KAP, but rather is a consequence of increased lipid oxidation.     

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.     

Rapid upregulation of pyruvate dehydrogenase kinase activity in human skeletal muscle during prolonged exercise.

Prolonged moderate-intensity exercise is characterized by a progressive reduction in carbohydrate oxidation and concomitant increase in fat oxidation. Pyruvate dehydrogenase (PDH) controls the entry of pyruvate into oxidative pathways and is a rate-limiting enzyme for carbohydrate metabolism. PDH is controlled by the activities of a kinase (PDK, inhibitory) and phosphatase (stimulatory). To test the hypothesis that increased PDK activity was associated with decreased PDH activity and carbohydrate oxidation during an acute exercise bout, seven recreationally active men completed 4 h of cycle exercise at 55% peak oxygen consumption. Muscle samples were obtained before and at 10 min and 4 h of exercise for the measurement of PDH activity and the extraction of intact mitochondria for the measurements of PDK activity and PDK-2 and PDK-4 protein expression. Carbohydrate oxidation was reduced (P < 0.05) with exercise duration. Muscle glycogen content was lower (P < or = 0.05) at 4 h compared with rest and there was no change in muscle pyruvate content from 10 to 240 min during exercise (10 min: 0.28 +/- 0.05; 240 min: 0.35 +/- 0.09 mmol/kg dry muscle). PDH activity increased (P < 0.05) above resting values at 10 min (2.86 +/- 0.26 mmol.min(-1).kg wet muscle(-1)), but was lower than 10 min after 4 h (2.23 +/- 0.24 mmol.min(-1).kg wet muscle(-1)) of exercise. PDK-2 and PDK-4 protein expression was not different from rest at 10 min and 4 h of exercise. PDK activity at rest averaged 0.081 +/- 0.016 min(-1), was similar at 10 min, and increased (P < 0.05) to 0.189 +/- 0.013 min(-1) at 4 h. Although reduced glycolytic flux may have played a role in decreasing carbohydrate oxidation, the results suggest that increased PDK activity contributed to the reduction in PDH activity and carbohydrate oxidation late in prolonged exercise. The increased PDK activity was independent of changes in intra-mitochondrial effectors, and PDK-2 and PDK-4 protein content, suggesting that it was caused by a change in the specific activity of the existing kinases.     

Mechanisms of glycolytic control during hibernation in the ground squirrel Spermophilus lateralis

Summary The mechanisms of glycolytic rate control during hibernation in the ground squirrel Spermophilus lateralis were investigated in four tissues: heart, liver, kidney, and leg muscle. Overall glycogen phosphorylase activity decreased significantly in liver and kidney to give 50% or 75% of the activity found in the corresponding euthermic organs, respectively. The concentration of fructose-2,6-bisphosphate (F-2,6-P₂) decreased significantly in heart and leg muscle during hibernation to 50% and 80% of euthermic tissue concentrations, respectively, but remained constant in liver and kidney. The overall activity of pyruvate dehydrogenase (PDH) in heart and kidney from hibernators was only 4% of the corresponding euthermic values. Measurements of phosphofructokinase (PFK) and pyruvate kinase (PK) kinetic parameters in euthermic and hibernating animals showed that heart and skeletal muscle had typical rabbit skeletal M-type PFK and M₁-type PK. Liver and kidney PFK were similar to the L-type enzyme from rabbit liver, whereas liver and kidney PK were similar to the M₂ isozyme found primarily in rabbit kidney. The kinetic parameters of PFK and PK from euthermic vs hibernating animals were not statistically different. These data indicate that tissue-specific phosphorylation of glycogen phosphorylase and PDH, as well as changes in the concentration of F-2,6-P₂ may be part of a general mechanism to coordinate glycolytic rate reduction in hibernating S. lateralis.Abbreviations ADP adenosine diphosphate - AMP adenosine monophosphate - ATP adenonine triphoshate - EDTA ethylenediaminetetra-acetic acid - EGTA ethylene glycol tetra-acetic acid - F-6-P fructose 6-phosphate - F-1,6-P₂ fructose 1,6-bisphosphate - F-2,6-P₂ fructose-2,6-bisphosphate - K _a activation coefficient - I₅₀ concentration of inhibitor which reduces control activity by 50% - PDH pyruvate dehydrogenase - PEP phosphoenolpyruvate - PFK 6-phosphofructo-1-kinase - PK pyruvate kinase     

Effects of hyperoxia on skeletal muscle carbohydrate metabolism during transient and steady-state exercise.

This study compared the effects of inspiring either a hyperoxic (60% O(2)) or normoxic gas (21% O(2)) while cycling at 70% peak O(2) uptake on 1) the ATP derived from substrate phosphorylation during the initial minute of exercise, as estimated from phosphocreatine degradation and lactate accumulation, and 2) the reliance on carbohydrate utilization and oxidation during steady-state cycling, as estimated from net muscle glycogen use and the activity of pyruvate dehydrogenase (PDH) in the active form (PDH(a)), respectively. We hypothesized that 60% O(2) would decrease substrate phosphorylation at the onset of exercise and that it would not affect steady-state exercise PDH activity, and therefore muscle carbohydrate oxidation would be unaltered. Ten active male subjects cycled for 15 min on two occasions while inspiring 21% or 60% O(2), balance N(2). Blood was obtained throughout and skeletal muscle biopsies were sampled at rest and 1 and 15 min of exercise in each trial. The ATP derived from substrate-level phosphorylation during the initial minute of exercise was unaffected by hyperoxia (21%: 52.2 +/- 11.1; 60%: 54.0 +/- 9.5 mmol ATP/kg dry wt). Net glycogen breakdown during 15 min of cycling was reduced during the 60% O(2) trial vs. 21% O(2) (192.7 +/- 25.3 vs. 138.6 +/- 16.8 mmol glycosyl units/kg dry wt). Hyperoxia had no effect on PDH(a), because it was similar to the 21% O(2) trial at rest and during exercise (21%: 2.20 +/- 0.26; 60%: 2.25 +/- 0.30 mmol.kg wet wt(-1).min(-1)). Blood lactate was lower (6.4 +/- 1.0 vs. 8.9 +/- 1.0 mM) at 15 min of exercise and net muscle lactate accumulation was reduced from 1 to 15 min of exercise in the 60% O(2) trial compared with 21% (8.6 +/- 5.1 vs. 27.3 +/- 5.8 mmol/kg dry wt). We concluded that O(2) availability did not limit oxidative phosphorylation in the initial minute of the normoxic trial, because substrate phosphorylation was unaffected by hyperoxia. Muscle glycogenolysis was reduced by hyperoxia during steady-state exercise, but carbohydrate oxidation (PDH(a)) was unaffected. This closer match between pyruvate production and oxidation during hyperoxia resulted in decreased muscle and blood lactate accumulation. The mechanism responsible for the decreased muscle glycogenolysis during hyperoxia in the present study is not clear.     

Skeletal-muscle glycogen synthesis during the starved-to-fed transition in the rat.

The pattern of glycogen deposition in skeletal muscles of varying fibre composition was examined in rats during the starved-to-fed transition. In all the muscles studied, glycogen concentrations steadily increased during the first 8 h after chow re-feeding, and the fed value was exceeded. Rates of glycogen deposition varied, not with muscle fibre composition, but with the extent of glycogen depletion during starvation. There was no evidence for skeletal-muscle glycogen breakdown during the period of hepatic glycogenesis, making it unlikely that recycling of carbon from muscle glycogen to lactate is quantitatively important for the provision of glycogenic precursors to the liver, but moderate glycogen loss was observed from 8 to 24 h after re-feeding, when the liver is in the lipogenic mode. The factors influencing glucose disposal by skeletal muscle after re-feeding are discussed.     

Enzymatic regulation of glucose disposal in human skeletal muscle after a high-fat, low-carbohydrate diet.

Whole body glucose disposal and skeletal muscle hexokinase, glycogen synthase (GS), pyruvate dehydrogenase (PDH), and PDH kinase (PDK) activities were measured in aerobically trained men after a standardized control diet (Con; 51% carbohydrate, 29% fat, and 20% protein of total energy intake) and a 56-h eucaloric, high-fat, low-carbohydrate diet (HF/LC; 5% carbohydrate, 73% fat, and 22% protein). An oral glucose tolerance test (OGTT; 1 g/kg) was administered after the Con and HF/LC diets with vastus lateralis muscle biopsies sampled pre-OGTT and 75 min after ingestion of the oral glucose load. The 90-min area under the blood glucose and plasma insulin concentration vs. time curves increased by 2-fold and 1.25-fold, respectively, after the HF/LC diet. The pre-OGTT fraction of GS in its active form and the maximal activity of hexokinase were not affected by the HF/LC diet. However, the HF/LC diet increased PDK activity (0.19 +/- 0.05 vs. 0.08 +/- 0.02 min(-1)) and decreased PDH activation (0.38 +/- 0.08 vs. 0.79 +/- 0.10 mmol acetyl-CoA.kg wet muscle(-1).min(-1)) before the OGTT vs. Con. During the OGTT, GS and PDH activation increased by the same magnitude in both diets, such that PDH activation remained lower during the HF/LC OGTT (0.60 +/- 0.11 vs. 1.04 +/- 0.09 mmol acetyl-CoA.kg(-1).min(-1)). These data demonstrate that the decreased glucose disposal during the OGTT after the 56-h HF/LC diet was in part related to decreased oxidative carbohydrate disposal in skeletal muscle and not to decreased glycogen storage. The rapid increase in PDK activity during the HF/LC diet appeared to account for the reduced potential for oxidative carbohydrate disposal.     

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.     

The disposition of carbohydrate between glycogenesis, lipogenesis and oxidation in liver during the starved-to-fed transition.

A comparison was made between the time courses of restoration of pyruvate dehydrogenase activities, fructose 2,6-bisphosphate concentrations and lipogenic rates, together with net hepatic glucose flux and glycogen synthesis/deposition in livers of 48 h-starved rats provided with laboratory chow ad libitum for up to 24 h. Increased glycogenesis, lipogenesis and net glucose uptake were observed after 1 h of re-feeding, preceding re-activation of pyruvate dehydrogenase, which occurred after 3-4 h. Increased concentrations of fructose 2,6-bisphosphate were only observed after 5-6 h. The implication of the temporal relationship between these parameters is discussed.     

Pyruvate overrides inhibition of PDH during exercise after a low-carbohydrate diet

The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O(2) uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 +/- 0.04 vs. 0.63 +/- 0.18 mmol x min(-1) x kg wet wt(-1)), muscle glycogen content (310.2 +/- 36.9 vs. 563.9 +/- 32.6 mmol/kg dry wt), and muscle lactate content (2.6 +/- 0.3 vs. 4.2 +/- 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 +/- 1.9 vs. 7.4 +/- 0.8 micromol/kg dry wt) and acetylcarnitine (5.3 +/- 1.4 vs. 1.6 +/- 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 +/- 0.4 vs. MD 6.9 +/- 0.2 mmol x min(-1) x kg dry wt(-1)), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.     

Hyperoxia decreases muscle glycogenolysis, lactate production, and lactate efflux during steady-state exercise

The aim of this study was to determine whether the decreased muscle and blood lactate during exercise with hyperoxia (60% inspired O2) vs. room air is due to decreased muscle glycogenolysis, leading to decreased pyruvate and lactate production and efflux. We measured pyruvate oxidation via PDH, muscle pyruvate and lactate accumulation, and lactate and pyruvate efflux to estimate total pyruvate and lactate production during exercise. We hypothesized that 60% O2 would decrease muscle glycogenolysis, resulting in decreased pyruvate and lactate contents, leading to decreased muscle pyruvate and lactate release with no change in PDH activity. Seven active male subjects cycled for 40 min at 70% VO2 peak on two occasions when breathing 21 or 60% O2. Arterial and femoral venous blood samples and blood flow measurements were obtained throughout exercise, and muscle biopsies were taken at rest and after 10, 20, and 40 min of exercise. Hyperoxia had no effect on leg O2 delivery, O2 uptake, or RQ during exercise. Muscle glycogenolysis was reduced by 16% with hyperoxia (267 +/- 19 vs. 317 +/- 21 mmol/kg dry wt), translating into a significant, 15% reduction in total pyruvate production over the 40-min exercise period. Decreased pyruvate production during hyperoxia had no effect on PDH activity (pyruvate oxidation) but significantly decreased lactate accumulation (60%: 22.6 +/- 6.4 vs. 21%: 31.3 +/- 8.7 mmol/kg dry wt), lactate efflux, and total lactate production over 40 min of cycling. Decreased glycogenolysis in hyperoxia was related to an approximately 44% lower epinephrine concentration and an attenuated accumulation of potent phosphorylase activators ADPf and AMPf during exercise. Greater phosphorylation potential during hyperoxia was related to a significantly diminished rate of PCr utilization. The tighter metabolic match between pyruvate production and oxidation resulted in a decrease in total lactate production and efflux over 40 min of exercise during hyperoxia.     

Reversible phosphorylation of pyruvate dehydrogenase in rat skeletal-muscle mitochondria. Effects of starvation and diabetes.

The total activity of pyruvate dehydrogenase (PDH) complex in rat hind-limb muscle mitochondria was 76.4 units/g of mitochondrial protein. The proportion of complex in the active form was 34% (as isolated), 8-14% (incubation with respiratory substrates) and greater than 98% (incubation without respiratory substrates). Complex was also inactivated by ATP in the presence of oligomycin B and carbonyl cyanide m-chlorophenylhydrazone. Ca2+ (which activates PDH phosphatase) and pyruvate or dichloroacetate (which inhibit PDH kinase) each increased the concentration of active PDH complex in a concentration-dependent manner in mitochondria oxidizing 2-oxoglutarate/L-malate. Values giving half-maximal activation were 10 nM-Ca2+, 3 mM-pyruvate and 16 microM-dichloroacetate. Activation by Ca2+ was inhibited by Na+ and Mg2+. Mitochondria incubated with [32P]Pi/2-oxoglutarate/L-malate incorporated 32P into three phosphorylation sites in the alpha-chain of PDH; relative rates of phosphorylation were sites 1 greater than 2 greater than 3, and of dephosphorylation, sites 2 greater than 1 greater than 3. Starvation ( 48h ) or induction of alloxan-diabetes had no effect on the total activity of PDH complex in skeletal-muscle mitochondria, but each decreased the concentration of active complex in mitochondria oxidizing 2-oxoglutarate/L-malate and increased the concentrations of Ca2+, pyruvate or dichloracetate required for half-maximal reactivation. In extracts of mitochondria the activity of PDH kinase was increased 2-3-fold by 48 h starvation or alloxan-diabetes, but the activity of PDH phosphatase was unchanged.     

Pyruvate dehydrogenase activity in several tissues of genetically obese hyperglycemic mice

The active form (PDHa) and total activity of pyruvate dehydrogenase (PDH) were measured in homogenates from heart muscle, epididymal fat pads and liver of genetically obese hyperglycemic mice and compared with similar data derived from lean controls or Swiss albino mice. Both PDHa and total PDH activities were similar in heart muscle from all mice with a precipitous decrease in the PDHa upon fasting. Adipose tissue and liver of obese mice had a PDHa level that was almost two-fold higher than either lean control or Swiss albino mice. Fasting for 24 hours decreased the elevated activity of PDHa in adipose tissue and liver in obese mice to a value that was comparable to lean control or Swiss albino mice, fasted similarly. The elevation in both the active form and total activity of pyruvate dehydrogenase in livers from obese mice could explain the increased provision of acetyl-CoA units necessary for the accelerated hepatic lipogenesis observed with this mouse, a model for human obesity and insulin resistance.     

Chronic high glucose lowers pyruvate dehydrogenase activity in islets through enhanced production of long chain acyl-CoA: prevention of impaired glucose oxidation by enhanced pyruvate recycling through the malate-pyruvate shuttle

In islet beta-cells, the high expression of pyruvate carboxylase and the functional importance of the downstream anaplerosis pathways result in a unique characteristic whereby high glucose and fatty acids both increase production of a key fatty acid metabolite, long chain acyl-CoA, for signaling and enzyme regulation in beta-cells. We showed previously in islets that pyruvate dehydrogenase (PDH) activity is lowered by excess fatty acids (the so-called Randle effect). We have now investigated PDH activity and pyruvate metabolism in islets after 48-h culture at 16.7 mmol/liter glucose. Active PDH V(max) was lowered 65% by 48 h of high glucose, and this effect was markedly attenuated by co-culture with triacsin C, which inhibits acyl-CoA synthase. Despite the large reduction in PDH activity, glucose oxidation was twice normal. The reason was continued metabolism of pyruvate through pyruvate carboxylase (V(max), 83% of control) and diversion of flux through the pyruvate-malate shuttle. The result was a 3-fold increase of the pyruvate concentration that overcame the lowered PDH activity by mass action as shown by glucose oxidation measured with [6-(14)C]glucose being twice normal. In addition, glucose-induced insulin secretion was 3-fold increased after 48 h of high glucose, and this effect was totally blocked by co-culture with triacsin C. These results show that a unique feature of islet beta-cells is not only fatty acids but also excess glucose that impairs PDH activity. Also, a specialized trait of beta-cells is a long chain acyl-CoA-mediated defense mechanism that prevents a reduction in glucose oxidation and consequently in insulin secretion.