Regulation of Glucose Uptake in Muscle

Sodium hydrosulfide improves the protective potential of the cardioplegic histidine buffer solution

Since H(2)S has an emerging role as a cardioprotector, we hypothesized that NaHS addition to the new cardioplegic histidine buffer solution (HBS) could improve its cardioprotective potential. Male Wistar-Han rat hearts were divided in 4 groups: i) control, ii) perfusion control (perfusion only), iii) 6h ischemia in HBS or in a modified-HBS with 100 μM of NaHS, a H(2)S donor, (HBSM) and iv) as iii followed by 30 min reperfusion. During ischemia, aliquots of the cardioplegic solution were collected for NMR analysis. Heart mitochondria respiration and transmembrane potential were measured after ischemia or after ischemia followed by reperfusion. Proteins involved in the apoptotic signaling pathway were also quantified in both mitochondrial and tissue samples. Cardiac mechanic performance was evaluated by measuring the heart rate and the left ventricular pressure. In HBSM-preserved hearts, a) glucose consumption increased as well as lactate and alanine production during ischemia, b) heart mitochondria presented an improved phosphorylative efficiency, including decreased phosphorylative lag phase for complex I and complex II substrates, c) mitochondrial and tissue p53, Bax and caspase-9 were lower and d) there was a more positive atrial chronotropic response than in HBS-preserved hearts. We concluded that the addition of NaHS to HBS enhances glycolysis during ischemia, decreases mitochondrial dysfunction, especially by preserving the phosphorylative system, prevents apoptosis and during ischemia/reperfusion.

Regulation of glucose uptake in rat slow and fast skeletal muscles

1. Regulation of glucose uptake was compared between extensor digitorum longus (EDL) and soleus (Sol) muscles in rats. 2. Insulin stimulated glucose uptake more in EDL than in Sol. 3. Under high concentrations of insulin, the glucose uptake was higher in EDL than Sol. 4. Inhibition of oxidative phosphorylation by anoxia or an uncoupler stimulated glucose uptake more in EDL than in Sol. 5. Anoxia abolished the effect of insulin on glucose uptake in both EDL and Sol. 6. The blocker to glucose transport system reduced glucose uptake more in Sol than in EDL.

Glucocorticoid stimulation of metabolism and glycerol-3-phosphate dehydrogenase activity in cultured heart cells

The direct effects of the glucocorticoids hydrocortisone and corticosterone on myocardial metabolism were studied in cultured heart cells by assessing several parameters previously unreported. Hormone and growth factor concentrations were carefully controlled by using a serum-free medium, which also allowed maintenance of cells in the absence of glucocorticoids. Heart cell beating rate, glucose uptake rate, and CO2 evolution from radioactively labeled glucose were increased by the addition of 0.03 microM corticosterone to the medium of cells maintained in culture for 11 days. There were no further changes in these parameters as steroid concentration was increased to 14.43 microM. The activity of NAD-linked sn-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) was increased by both corticosteroids and was dose dependent between 0.06 and 1.44 microM corticosterone. The difference between glycerol-3-phosphate dehydrogenase activity in cells maintained with hydrocortisone as compared to cells maintained without hydrocortisone increased with days in culture. The protein and DNA contents of dishes maintained with corticosteroid were depressed, demonstrating an inhibitory effect on cellular replication. Glucocorticoids have numerous direct effects on cardiac cell metabolism, and the nature of these effects suggests that secondary responses of the cell to chronic exposure are significant.

Modulation by adrenalectomy and fasting of insulin effects in perfused hindlimb muscle

Perfused hindlimb muscle from fed adrenalectomized rats accumulated more 2-deoxyglucose at submaximal concentrations of insulin in comparison to muscle from fed normal rats. However, in the fasted state, insulin-stimulated 2-deoxyglucose uptake was largely inhibited by adrenalectomy. Basal 2-deoxyglucose uptake did not differ between fed and fasted normal or adrenalectomized rats. The changes in insulin effects caused by adrenalectomy were due to altered hexose transport as shown by measurements of 3-O-methylglucose uptake and of intracellular free and phosphorylated 2-deoxyglucose. Muscles of fasted normal and fed or fasted adrenalectomized rats showed higher basal glycogen synthase --glucose-6-P/+glucose-6-P activity ratios than those of fed normal rats probably because of decreased glycogen content. However, muscles from fed or fasted adrenalectomized rats did not show any alterations in insulin effects on the activity ratio and half-maximal activation constant (A0.5) for glucose-6-P of glycogen synthase. Because of the dissociation of the effects of insulin on hexose transport and glycogen synthase in muscle of fasted adrenalectomized rats, it is concluded that the impairment in insulin-stimulated hexose transport in these animals is due to a defect lying beyond the interaction of insulin with its receptor.

Regulation of gluconeogenesis by glucocorticoids

1. Regulation of gluconeogenic substrate supply and modulation of the gluconeogenic pathway in the liver are both important in the control of gluconeogenesis by glucocorticoids. 2. Adrenal deficiency decreases the release of gluconeogenic and other amino acids from skeletal muscle during starvation. The effect is reversed by glucocorticoid replacement. The changes in amino acid release are accompanied by similar alterations in tissue amino acid levels and are not explained by alterations in net protein breakdown. Glucocorticoids do not alter protein catabolism and cause a small inhibition of protein synthesis. The biochemical alterations underlying the changes in amino acid metabolism induced by these steroids remain to be elucidated. Glucocorticoids may also regulate the supply of gluconeogenic substrates through permissive effects on the lipolytic action of catecholamines and other hormones in adipose tissue and on the glycogenolytic action of catecholamines on skeletal muscle. 3. Glucocorticoids are required for the increases in gluconeogenesis in starvation and diabetes. Part of their action is exerted directly on the liver and appears to involve modulation of P-enlopyruvate carboxykinase levels. Glucocorticoids increase the synthesis of this enzyme apparently through effects at the level of transcription. 4. Glucocorticoids exert permissive effects on the stimulation of gluconeogenesis in the liver by glucagon and epinephrine. The steroids are not required for cAMP generation or protein kinase activation by these hormones, but appear to act by maintaining the responsiveness of certain enzymes to the effects of the cAMP and alpha-adrenergic systems. It is proposed that this involves the maintenance of a normal intracellular ionic environment.

Glycogen concentration in working and nonworking ventricles of isolated dog hearts

Glycogen utilization in working and nonworking ventricles was studied at high (over 70 mg/100 ml) and low (27 to 61 mg/100 ml) arterial glucose concentrations and after insulin or epinephrine addition in 16 isolated ventricle preparations of dog hearts. Coronary perfusion and hemodynamic determinants of right ventricular work were controlled, and the left ventricle was kept unloaded. Time courses of change in ventricular glycogen concentration were determined during monitoring of heart rate, workload, arterial oxygen saturation, and coronary perfusion pressure. Epicardial samples for glycogen analysis were taken from each ventricle, and glucose uptake from circulating blood was determined. Glycogen loss was greater in working right than in nonworking left ventricles. In spontaneously fibrillating hearts, this difference was not observed, and there was greater glycogenolysis than during coordinated contraction. Insulin administration early in experiments led to equivalent glycogen loss in working right and nonworking left ventricles. There was glycogen preservation in both ventricles of fibrillating hearts. Epinephrine augmented glycogen loss in fibrillating hearts; depletion was never complete. Myocardial glucose uptake, corrected for red cell glycolysis, was proportional to initial arterial glucose concentration.

THE UPTAKE OF GLUCOSE INTO CELLS AND THE ROLE OF INSULIN IN GLUCOSE TRANSPORT

This presentation has been restricted to the role of insulin in glucose transport in muscle cells and deals mainly with experiments using the perfused rat heart. The several possible means for glucose transfer into cells, diffusion, pores, pinocytosis, carriers, and dimerization, have been discussed; and arguments in favor of the carrier theory, namely, specificity, kinetics, inhibition, competition, and counterflow, have been elaborated. Glucose uptake has been considered to consist of three sequential steps: (1) passage of glucose from within the capillary to the cell surface, (2) transport across the cell membrane, and (3) metabolism of glucose within the cell. The first is considered to take place by diffusion and not to be significantly limiting under normal conditions, nor to be influenced by insulin. Transport across the cell membrane is thought to be mainly under the control of insulin and is the major rate-limiting step in glucose uptake when the extracellular glucose levels are in the normal range. Metabolism of glucose within the cell is the major rate-limiting step in glucose uptake when intracellular glucose concentration is so high that its phosphorylation is near saturation.