A 31P-NMR study of some metabolic and functional effects of the inotropic agents epinephrine and ouabain,and the ionophore R02-2985 (X537A) in the isolated,perfused rat heart

1. Some metabolic effects of increased mechanical activity by the Langendorff-perfused rat heart have been characterized using ³¹P-NMR. Mechanical activity was increased by infusion of ouabain (0.9?7.0·10^?5 M), the ionophore R02-2985 (1·10^?5 M) or epinephrine (5·10^?8 M). 2. Similar metabolic changes accompanied infusion of each of the positive inotropic agents into hearts perfused with buffer containing 11 mM glucose as the substrate. In each case phosphocreatine concentrations decreased. During the period of epinephrine infusion the phosphocreatine began to recover its original concentration, although there were no significant changes in mechanical activity. 3. Comparisons of the metabolic changes accompanying the positive inotropic and chronotropic effects of epinephrine were made between hearts perfused with either glucose (11 mM), acetate (5 mM) or lactate (5 mM). A time-dependent decrease in phosphocreatine concentrations also accompanied infusion of epinephrine into hearts perfused with lactate as the sole exogenous substrate, but no statistically significant metabolite changes were observed after identical epinephrine infusions with acetate as the substrate. 4. Calculation of the concentration of free ADP assuming equilibrium in the creatine phosphokinase reaction allows estimation of the cytosolic phosphate potential ($\text{ATP}\text{]}\text{ADP]P}{}_{\mathrm{<mtext>i</mtext>}}\text{]}$), which appears to be dependent on a number of factors, including the nature of the exogenous substrate and the level of mechanical activity. 5. Thus, we conclude that there is no general correlation between the phosphate potential and the mitochondrial respiratory rate in the perfused rat heart.