MODULATION OF LIPOLYTIC-ACTIVITY IN ISOLATED CANINE CARDIAC SARCOLEMMA BY ISOPROTERENOL AND PROPRANOLOL

Isolated dog cardiac sarcolemmal preparations were tested for membrane-associated phospholipase A [PLA] and lipoprotein lipase activities. The sarcolemma hydrolyzed 1-acyl 2[14C]-linoleoyl 3-glycero-phosphorylethanolamine at pH 7.0 to form predominantly 14C-lyso phosphorylethanolamine with 5 mM EDTA and 14C-free fatty acid with 5 mM Ca2+; apparently both PLA1 and PLA2 and/or lysophospholipase activities are present in these preparations. Sarcolemmal PLA activity was stimulated 300% by 10-5-10-6 M dl-isoproterenol; this stimulation was blocked by 10-4 M dl-propranolol. Lipoprotein lipase activity associated with the sarcolemmal fraction was enhanced 10-fold by 10-5 M dl-isoproterenol; stimulation was blocked by dl-propranolol. Pharmacologic doses of isoproterenol elevate serum levels of free fatty acids while large or repeated doses with experimental animals lead to damage and necrosis of the myocardium that are indistinguishable from myocardial infarction. The activities of membrane-bound lipolytic enzymes appear to be modulated by .beta.-adrenergic agents in canine cardiac sarcolemma and could affect lipid dependent enzymes and/or membrane permeability.