Release of phospholipase A2 activity from rat vascular smooth muscle cells mediated by cAMP

Primary cultures of smooth muscle cells (SMC) derived from rat aorta release a phospholipase A2 activity into the culture medium. Phospholipase A2 activity was determined with [1-14C]oleate-labelled Escherichia coli as substrate. The enzyme has a neutral pH optimum and the activity is critically dependent on the free calcium concentration, with significant activity in the micromolar range of free calcium. Treatment of SMC with the beta agonist salbutamol, forskolin or cholera toxin, which all activate adenylate cyclase and increase intracellular cAMP concentration, increase the release of phospholipase A2 activity in a dose-dependent manner. Likewise, the addition of the membrane-permeable cAMP analogues, (Sp)-adenosine 3',5'-[thio]phosphate and N6,O-2'-dibutyryladenosine 3',5'-phosphate, enhance the release of phospholipase A2 activity from SMC in a dose-dependent manner. There is a lag period of about 4 h before a significant secretion of phospholipase A2 can be detected under basal, as well as under stimulated conditions. The forskolin analogue 1,9-dideoxyforskolin, which is inactive as a stimulator of adenylate cyclase, has no effect on phospholipase A2 secretion. Likewise, the potent vasoconstrictive peptide angiotensin II activates inositol phospholipid turnover in SMC, but has no effect on phospholipase A2 release. Pretreatment of SMC with actinomycin D or cycloheximide completely suppresses basal and cAMP-stimulated secretion of phospholipase A2 activity, thus demonstrating that transcription and protein synthesis are necessary for enzyme release.