Regulation of triglyceride metabolism in the isotopically prelabeled perfused heart

Utilization of 14C-prelabeled endogenous triglycerides was studied in isolated perfused working rat hearts. Lipolysis was estimated by the disappearance of 14C-labeled and total triglycerides. Metabolic 14CO2 production was continuously monitored to evaluate triglyceride fatty acid oxidation. Triglyceride utilization was enhanced by an increase in ventricular pressure development as evidenced by a faster rate of triglyceride mobilization and oxidation. Added catecholamines stimulated lipolysis in hearts perfused with glucose-containing buffer but were without effect in the presence of exogenous fatty acids; the latter were shown to be potent and, possibly, direct inhibitors of myocardial lipolysis. Mediation of catecholamine-induced lipolysis by cyclic AMP has not been settled. Dibutyryl cyclic AMP produced only a slight lipolytic effect, although theophylline, a known phosphodiesterase inhibitor, was a potent lipolytic agent. Theophylline may have exerted its lipolytic effect through an alternative mechanism. Hypoxia per se was a strong inhibitor of heart triglyceride utilization. Furthermore, added epinephrine was without effect on triglyceride lipolysis in hypoxic hearts. Thus, cardiac muscle triglyceride utilization is influenced by such factors as mechanical function, exogenous substrates, hormones, and oxygen availability. The mechanisms involved in these areas of regulation need to be resolved.