| Abstract: | Lipolysis of intracellular triglycerides in the heart has been shown to be regulated by hormones. However, activation of myocardial triglyceride lipase in a cell-free system has not been directly demonstrated. In the present studies, initial attempts to demonstrate cAMP-dependent activation of triglyceride lipase using the 1,000 X g supernatant fraction (S1) of mouse heart homogenate were unsuccessful, presumably due to the masking effects of high levels of lipoprotein lipase activity even when assayed at pH 7.4 and in the absence of apolipoprotein C-II. Myocardial lipoprotein lipase in the 40,000 X g supernatant fraction was then removed by heparin-Sepharose affinity chromatography. The lipoprotein lipase-free fractions were shown to contain neutral triglyceride lipase and neutral cholesterol esterase of about equal activities. The triglyceride lipase and cholesterol esterase activities fell progressively during preincubation in the presence of 5 mM Mg2+. Additions of cAMP and ATP resulted in 40-70% activation of both triglyceride lipase and cholesterol esterase. The activation was blocked by protein kinase inhibitor and was restored by the addition of exogenous cAMP-dependent protein kinase. Since lipoprotein lipase has no activity toward cholesteryl oleate, activation of cholesterol esterase in untreated S1 was readily demonstrable. Both triglyceride lipase and cholesterol esterase activities were present in homogenates prepared from isolated rat heart myocytes. We conclude that the myocardium contains a hormone-sensitive lipase that is regulated in a fashion similar to that of the adipose tissue enzyme. |
