The effects of phospholipase A2 on beta adrenoceptor function in isolated cardiac cells

The role of phospholipids in the maintenance of beta-adrenoceptor function was investigated in isolated canine myocytes prepared from eight adult mongrel dogs by using collagenase. The characteristics of beta-adrenoceptors were assessed by determining the number and the affinity of receptors by a radioactive ligand binding assay using 125I-iodocyanopindolol. The increase in cyclic AMP content induced by isoproterenol or forskolin was also determined by radioimmunoassay with or without pretreatment with phospholipase (PLase) A2. The amount of free fatty acids released from isolated myocytes by PLase A2 was measured by high-performance liquid chromatography. PLase induced a significant decrease in the number of beta-adrenoceptors but did not affect their affinity. Although the isoproterenol-stimulated increase in cyclic AMP was significantly inhibited by the pretreatment with PLase A2, the forskolin-stimulated increase was not affected. Responsive accumulation of cyclic AMP to isoproterenol was much more impaired than the decrease in beta-adrenoceptor number. These results indicate that PLase A2 deteriorates the function of the adenylate cyclase system linked-beta-adrenoceptor, and suggest that PLase A2 affects both beta-adrenoceptors and the coupling of beta-adrenoceptors with adenylate cyclase.