Regulation of rates of lipolysis in isolated myocardial cells from rat heart.

Rates of lipolysis in isolated myocardial cells (myocytes) from rat heart, as measured by the release of glycerol and a reduction in endogenous triacylglycerols, can be stimulated by isoproterenol. Myocyte preparations were calcium-tolerant and were quiescent, even in the presence of isoproterenol, so the stimulation of lipolysis by isoproterenol cannot be secondary to a physiological (inotropic) response. N6-Phenylisopropyladenosine did not reduce isoproterenol-stimulated rates of lipolysis. Increasing the calcium concentration in the incubation medium from 0.75 to 3 mM did not increase the basal output of glycerol. Furthermore, incubation of calcium-tolerant myocytes in the absence of calcium had no effect on either basal or isoproterenol-stimulated rates of lipolysis. Therefore, calcium ions must not influence the lipolytic process directly, and so the calcium dependency for lipolysis observed with perfused heart preparations must reflect the effect of calcium on the contractile performance of the heart, which only secondarily produced a change in rates of lipolysis.