Effects of nutritional status and acute variation in substrate supply on cardiac and skeletal-muscle fructose 2,6-bisphosphate concentrations

Metabolic shifts during aging and pathology

The heart is a very special organ in the body and has a high requirement for metabolism due to its constant workload. As a consequence, to provide a consistent and sufficient energy a high steady-state demand of metabolism is required by the heart. When delicately balanced mechanisms are changed by physiological or pathophysiological conditions, the whole system's homeostasis will be altered to a new balance, which contributes to the pathologic process. So it is no wonder that almost every heart disease is related to metabolic shift. Furthermore, aging is also found to be related to the reduction in mitochondrial function, insulin resistance, and dysregulated intracellular lipid metabolism. Adenosine monophosphate-activated protein kinase (AMPK) functions as an energy sensor to detect intracellular ATP/AMP ratio and plays a pivotal role in intracellular adaptation to energy stress. During different pathology (like myocardial ischemia and hypertension), the activation of cardiac AMPK appears to be essential for repairing cardiomyocyte's function by accelerating ATP generation, attenuating ATP depletion, and protecting the myocardium against cardiac dysfunction and apoptosis. In this overview, we will talk about the normal heart's metabolism, how metabolic shifts during aging and different pathologies, and how AMPK regulates metabolic changes during these conditions.

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.

Substrate interactions in the short- and long-term regulation of renal glucose oxidation

The present study evaluated the substrate competition between fatty acids (FA) and glucose in the kidney in vivo in relation to the operation of the "glucose-FA" and "reverse glucose-FA" cycles. In fed rats, neither inhibition of adipocyte lipolysis by 5-methylpyrazole-3-carboxylic acid (MPCA) nor inhibition of mitochondrial long-chain FA oxidation by 2-tetradecylglycidate (TDG) influenced the renal ratio of free/acylated carnitine or the percentage of total renal pyruvate dehydrogenase complex (PDHC) in the active (dephosphorylated) form (PDHa). The additional provision of glucose, a precursor for the synthesis of malonyl-coenzyme A (coA), did not influence renal PDHa activity or the renal ratio of free to acylated carnitine, implying that FA oxidation is maximally suppressed in the fed state. A reverse glucose-FA cycle may therefore be important in suppressing renal FA oxidation in the fed state. After 48 hours of starvation, MPCA and TDG decreased short- and long-chain acylcarnitine concentrations (40% to 50%, P < .01) and elevated the renal ratio of free/acylated carnitine (2.5-fold, P < .001, and 3.3-fold, P < .001, respectively), indicating that FA oxidation is increased after starvation. Despite suppression of renal FA oxidation, renal PDHa activity in 48-hour starved rats was only partially restored by treatment with MPCA or TDG. The additional administration of glucose did not remedy this. The failure to reverse completely the effects of prolonged starvation in suppressing PDHC activity by acute inhibition of FA oxidation suggests additional regulatory mechanisms that dampen the PDHC response to acute changes in substrate supply. Estimations of PDH kinase (PDK) activity in renal mitochondria showed a significant 1.7-fold stable increase (P < .01) after 48 hours of starvation. Analysis of PDK pyruvate sensitivity in renal mitochondria incubated with respiratory substrate (5 mmol/L 2-oxoglutarate/0.5 mmol/L L-malate) showed that the pyruvate concentration required for 50% activation was substantially decreased by starvation. Enzyme-linked immunosorbent assay (ELISA) analysis over a range of PDHC activities demonstrated that increased PDK activity was concomitant with a significant (at least P < .01) 1.8-fold increase in the protein expression of the ubiquitously expressed PDK isoform, PDK2. We hypothesize that changes in protein expression and activity of individual PDK isoforms may dictate the renal response to incoming FA lesterification v oxidation) through modulation of the relationship between glycolytic flux and PDHC activity, and thus the provision of precursor for malonyl-coA production.

Starvation and diabetes increase the amount of pyruvate dehydrogenase kinase isoenzyme 4 in rat heart

This study investigated whether conditions known to alter the activity and phosphorylation state of the pyruvate dehydrogenase complex have specific effects on the levels of isoenzymes of pyruvate dehydrogenase kinase (PDK) in rat heart. Immunoblot analysis revealed a remarkable increase in the amount of PDK4 in the hearts of rats that had been starved or rendered diabetic with streptozotocin. Re-feeding of starved rats and insulin treatment of diabetic rats very effectively reversed the increase in PDK4 protein and restored PDK enzyme activity to levels of chow-fed control rats. Starvation and diabetes also markedly increased the abundance of PDK4 mRNA, and re-feeding and insulin treatment reduced levels of the message to that of controls. In contrast with the findings for PDK4, little or no changes in the amounts of PDK1 and PDK2 protein and the abundance of their messages occurred in response to starvation and diabetes. The observed shift in the relative abundance of PDK isoenzymes probably explains previous studies of the effects of starvation and diabetes on heart PDK activity. The results indicate that control of the amount of PDK4 is important in long-term regulation of the activity of the pyruvate dehydrogenase complex in rat heart.

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.

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.

Mechanisms regulating cardiac fuel selection in hyperthyroidism

Starvation (48 h) decreases fructose 2,6-bisphosphate (Fru-2,6-P2) concentrations and the ratio of free to acylated carnitine in hearts of euthyroid rats. These decreases, which are indicative of increased lipid fuel oxidation, are accompanied by decreased rates of glucose uptake and phosphorylation, assessed by using radioactive 2-deoxyglucose. Cardiac concentrations of acylated carnitines were increased at the expense of free carnitine even in the fed state in response to experimental hyperthyroidism, but neither Fru-2,6-P2 concentrations nor rates of glucose utilization were suppressed. Starvation (48 h) did not further increase the proportion of acylated carnitine in the heart in hyperthyroidism, and suppression of Fru-2,6-P2 concentrations and glucose utilization rates by starvation was attenuated. Although glucose utilization rates were decreased, starvation did not decrease immunoreactive GLUT 4 protein concentrations. Furthermore, although hyperthyroidism was associated with a statistically significant (30-40%) increase in relative abundance of GLUT 4 mRNA, the amount of GLUT 4 protein was not increased by hyperthyroidism in either the fed or the starved state. The results demonstrate a significant effect of hyperthyroidism to enhance cardiac glucose utilization in starvation by a mechanism which does not involve changes in GLUT 4 expression but may be secondary to changes in glucose-lipid interactions at the tissue level.

An improved assay for pyruvate dehydrogenase in liver and heart

A radiochemical assay was developed to measure pyruvate dehydrogenase complex (PDC) activity in liver and heart without interference by branched-chain 2-oxo acid dehydrogenase (BCODH). Decarboxylation of pyruvate by BCODH was eliminated by using low pyruvate concentration (0.5 mM), a preferred substrate for BCODH (3-methyl-2-oxopentanoate) that is not used by PDC, and a competitive inhibitor of BCODH, dichloroacetate. This method was validated by assaying a combination of both purified enzymes and tissue homogenates with known amounts of added BCODH. The actual percentage of active PDC decreased after 48 h starvation from 13.6 to 3.1 in liver and from 77.1 to 9.0 in heart. Total PDC activity (munits of PDC/units of citrate synthase) in starved rats was increased by 34% in liver and decreased by 23% in heart. Total PDC activity (munits/g wet wt.) in fed- and starved-rat liver was 0.8 and 1.3, and in heart was 6.6 and 5.8, respectively.

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 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.

Fructose 2,6-bisphosphate in liver of Sparus aurata: influence of nutritional state

1. Fructose 2,6-bisphosphate (fru-2,6-P2) has been measured in liver and muscle of gilthead sea bream fish, Sparus aurata. 2. The fru-2,6-P2 levels in liver depend on the diet given to the fish: in fish fed a high carbohydrate diet, the fru-2,6-P2 levels are higher than any one previously reported. These changes are associated with differences in the phosphofructokinase 2 activity. 3. Fru-2,6-P2 levels has also been measured in liver of Sparus aurata after different fasting periods. In starved fish, fru-2,6-P2 did not decrease as sharply as in rat. The values found in fish starved for 20 days were similar to those reported for rats that had been starved for 24 hr.

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.

Fuel selection and carbon flux during the starved-to-fed transition

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

We investigated the capacity for pyruvate oxidation in skeletal muscle, diaphragm and heart after starvation and re-feeding. Starvation for 48 h decreased pyruvate dehydrogenase (PDH) activity in soleus (by 47%), extensor digitorum longus (64%), gastrocnemius (86%), diaphragm (87%), adductor longus (90%), tibialis anterior (92%) and heart (99%). Chow re-feeding increased PDH activity in all muscles to 43-78% of the fed value within 2 h. However, complete re-activation was not observed for at least 4-6 h, during which time hepatic glycogen was replenished. We discuss the importance of muscle PDH activity in relation to sparing carbohydrate for hepatic glycogen synthesis.

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.

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.

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.

The relationship between changes in lipid fuel availability and tissue fructose 2,6-bisphosphate concentrations and pyruvate dehydrogenase complex activities in the fed state

An elevated concentration of non-esterified fatty acids in the fed state elicited inhibition of cardiac, but not hepatic, pyruvate dehydrogenase complex (PDH). There was a modest decline in fructose 2,6-bisphosphate (Fru-2,6-P2) concentration in heart, and, to a lesser extent, in liver. Surgical stress decreased PDH activities and Fru-2,6-P2 concentrations in both heart and liver. Only the former response was abolished if postoperative lipolysis was inhibited. Surgery also decreased the [Fru-2,6-P2] in gastrocnemius: this response was abolished if lipolysis was inhibited.

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.