We have been able to stimulte the activity of a heart triacyglycerol (TG) lipase with cyclic AMP-inducing agents in perfused rat heart and cardiac myocytes. This activation occurs regardless of whether measurements are made on aqueous homogenates or on delipidated heart powders. The enzyme activity was highly correlated with the amount of TG reduction that occurred in the heart. The pH optimum of this hormone sensitive TG hydrolase is 8.1; therefore, it is referred to as an alkaline TG lipase. We have found that elevating cyclic AMP levles also produces a high correlation between enzyme activity and the reduction in myocardial TG content. Addition to cyclic AMP + Mg-ATP to cell-free extracts of rat heart increases TG lipase activity more than 2-fold. Preliminary experiments indicate that this alkaline TG lipase has the characteristics classically described for lipoprotein (LPL). It is the primary purpose of the proposed experiments to begin to determine the mechanisms involved in the regulation of theis myocardial TG lipase. This will be accomplished by completing the following specific aims: a) determine the effect of cyclic AMP, protein kinase, and alkaline phosphatase on the activity of the cardiac TG lipase in broken cell preparations of rat heart; b) compare and contrast the physical characteristics between rat heart intracellular TG lipase and capillary LPL; c) perform a systematic kinetic analysis of pure TG lipase, LPL, enzyme from crude preparations of rat heart, and lipase from crue preparation of rat heart that has been activated by cyclic AMP + Mg -ATP; d) investigate the possible regulation of purified and partially purified preparations of rat heart hormone-sensitive TG lipse from rat heart by phosphorylation; and e) investigate the regulation of heart TG lipase in heart cardiocytes using various adrenegic agents and antibody to the alkaline TC lipase. The data obtained from these experiments should point out possible mechanisms of the hormonal regulation of the intracellular TG lipase. Moreover, these studies should provide us with fundamental insights into myocardial TG metabolism.
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