Effect of tolbutamide on myocardial energy metabolism of the ischemic heart.

The oral hypoglycemic agent tolbutamide has been found to protect the ischemic myocardium against irreversible mechanical failure. The possibility that this salutary effect of tolbutamide was related to its ability to alter energy metabolism was examined in ischemic rat hearts perfused with 5 mM glucose, 5mM acetate and 2.5 units/l insulin. In the presence of 0.6 mM tolbutamide, coronary flow and oxygen consumption were unaltered; however, glucose utilization was stimulated by 30%, glycogenolysis was enhanced by 23%, and the drop in ATP content was reduced by 17% after 30 min, of low-flow perfusion. This elevation in glycolytic flux occurred without a parallel rise in the production of inhibitory metabolites; lactate production was unaltered and tissue lactate/pyruvate ratio decreased. Pyruvate dehydrogenase flux measurements reveal that the mechanism by which tolbutamide increases glycolysis without increasing lactate production is by promoting the entry of pyruvate into the mitochondria. The basis for the observed stimulation of anaerobic metabolism and pyruvate oxidation and how this contributes to the increase in ATP content and benefits the ischemic heart is discussed.

A probable mechanism for tolbutamide mediated activation of glycogen phosphorylase in the isolated rat heart.

Previous studies have shown that 0.6 mM tolbutamide stimulates the rate of glycogenolysis and transiently increases % phosphorylase a activity in the isolated rat heart. Since tolbutamide has been reported to activate adenylate cyclase, one possible mechanism for the conversion of phosphorylase b to the a form is through a cAMP mediated process. However, we failed to detect any drug-induced changes in tissue cAMP content of rat and rabbit hearts or in basal, Gpp(NH)p stimulated and isoproterenol-stimulated adenylate cyclase activity of sarcolemma prepared from rat, rabbit or dog ventricles. Since tolbutamide can alter calcium transport across cell membranes, we investigated the possibility that the drug's effects on phosphorylase were linked to an elevation in calcium concentration. It was found that tolbutamide was not able to activate phosphorylase when the calcium channel blocker verapamil was present in the perfusate. In addition, the sulfonylurea increased binding of [3H]-verapamil to isolated sarcolemma suggesting that tolbutamide is able to unmask previously inactive calcium channels and thereby stimulate calcium movement into the cell.

Effect of tolbutamide on myocardial energy metabolism.

Exposure of rat hearts perfused with 5 mM glucose and 5 mM acetate to tolbutamide led to a dramatic stimulation in glucose utilization and glycolytic flux. This effect was concentration dependent, with the largest response occurring at a tolbutamide concentration of 0.6 mM. Measurement of tissue glycogen content revealed that the higher concentrations of tolbutamide also enhanced glycogenolysis, indicating that the increase in glycolysis was caused by an increase in both glucose consumption and glycogen mobilization. On the basis of the determination of key metabolic intermediates, it was concluded that the stimulation of anaerobic metabolism was mediated by an activation of both phosphofructokinase and phosphorylase. The observed increase in glycolytic flux was associated with a rise in lactate production; however, the most pronounced effect of the drug was the stimulation of glucose oxidation. Thus the percentage of oxygen used in the oxidation of glucose was dramatically increased. Calculations also revealed that the contribution of glucose to overall ATP production rose from 8% in the absence of tolbutamide to about 30% in the presence of the sulfonylurea.

Potentiation of the actions of insulin by taurine.

Taurine was found to mediate several changes in myocardial metabolism. In the absence of insulin, only oxygen consumption was significantly elevated by taurine; however, in the presence of 2.5 U/L insulin the amino acid caused the stimulation of glycolysis and glycogenesis, as well as oxygen utilization. These effects of taurine were shown to be dependent on insulin concentration, suggesting a link between the two substances. Measurements of key metabolic intermediates revealed that taurine stimulated glycolysis by enhancing flux through phosphofructokinase. Similarly, it was shown that glycogenesis was promoted because of the increase in glycogen synthase I and decrease in phosphorylase alpha activity. Several possible mechanisms for the observed changes are discussed.

Action of substance P on the working rat heart.

The effect of substance P (SP) on the cardiodynamics of the isolated working rat heart perparation was examined. The peptide over the concentration range of 10(-8) to 10(-6) M was found to have no influence on aortic pressure, cardiac output, or cardiac work. However, a 10-15% reduction in coronary flow was observed at 1 x 10(-6) M substance P. Octapeptide substance P (SP4-11) exhibited a similar vasoconstrictive action. The IC50 of SP4-11 was 2 x 10(-13) M compared to an IC50 of 3.5 x 10(-8) M for substance P. Perfusion of the heart in the presence of bacitracin (1 x 10(-4) M), a protease inhibitor, prevented the reduction in coronary flow observed in the presence of substance P. By contrast, the reduction in coronary flow produced by octapeptide substance P was not altered by the presence of bacitracin. Thus, it appears that a C-terminal fragment such as SP4-11 may be responsible for the observed decrease in coronary flow.