Studies on the involvement of lipolytic enzymes in endogenous lipolysis of the isolated rat heart.

Rat hearts were depleted in vivo from both the heparin-releasable lipoprotein lipase and heparin-resistant tissue neutral triacylglycerol lipase activity by treatment of the animals with cycloheximide (2 mg/kg body weight), intraperitoneally injected 2.5 and 5 h prior to perfusion. The tissue acid lipase, mono- and diacylglycerol lipase activities were not affected by cycloheximide-induced inhibition of protein synthesis. Myocardial basal and glucagon-stimulated lipolysis, determined by the rate of glycerol production and release from the isolated hearts, was not significantly different in control and cycloheximide-treated rats. Tissue triacylglycerols were recovered with the highest relative specific distribution in the lysosomal fraction isolated from heart homogenates. Upon prolongation of the perfusion-duration the relative specific distribution of triacylglycerols in the lysosomal fraction decreased. In addition, the specific lysosomal triacylglycerol content (micrograms/mg protein) dropped significantly, indicating an important role of lysosomes in myocardial triacylglycerol turnover. Our data strongly suggest that the heparin-resistant neutral triacylglycerol lipase activity may not be the only determinant of endogenous lipolysis in the isolated rat heart and indicate that lipolysis may additionally be mediated by the lysosomal, acid lipase in concert with the microsomal mono-and diacylglycerol lipase.