Effects of catecholamines on myocardial endogenous substrates and contractility

The effects of epinephrine and norepinephrine on triglyceride and glycogen metabolism and contractility were studied in isolated perfused working rat hearts. Hearts with lipids prelabeled in vivo with [1-14C]palmitate were perfused with bicarbonate buffer containing 5.5 mM glucose, with or without 0.6 mM palmitate (3% albumin), and varying concentrations of catecholamines. Direct evidence for catecholamine-stimulated myocardial triglyceride lipolysis was obtained and, for the first time, was shown to be concentration-dependent. Also, catecholamines enhanced heart triglyceride fatty acid oxidation in concentration-dependent fashion. Stimulation of lipolysis and oxidation was observed only in hearts perfused with buffer containing glucose as the sole substrate, and was inhibited in the presence of 0.6 mM palmitate. Palmitate inhibited net glycogenolysis in the absence of catecholamines, but had little effect on epinephrine-stimulated glycogenolysis. Therefore, with free fatty acids present, mobilization of endogenous triglycerides to meet the increased metabolic demands of catecholamine stimulation is minimized; this is due, possibly, to enhanced utilization of exogenous free fatty acids and to inhibition of net lipolysis.