Abstract

An isolated rat heart perfused with substratefree medium failed to generate contractile force after a period of two hours. In hearts taken from animals pretreated with ³H-norepinephrine, endogenous norepinephrine in the substrate-depleted heart declined to about 60% of the control value without changes in the specific activity of ³H-norepinephrine. ³H-norepinephrine in the granular fraction decreased and ³H-norepinephrine in the soluble fraction increased after perfusing the heart with substrate-free medium. Perfusion of hearts with medium containing glucose maintained contractile force during a two-hour period and completely prevented the changes in the subcellular distribution of ³H-norepinephrine. The leakage of total ³H- and decrease in the endogenous norepinephrine levels in the hearts perfused with medium containing glucose were less in comparison to the hearts perfused with substrate-free medium. The ability of tyramine to exert positive inotropic action was markedly reduced on perfusing the hearts with substrate-free medium. The data presented in this study demonstrate the effects of a substrate-free medium on cardiac norepinephrine stores; in addition these results fail to reveal any relationship between catecholamine stores and cardiac contractile force in this experimental model.