PHOSPHORYLATION OF PHOSPHOLAMBAN CORRELATES WITH RELAXATION OF CORONARY-ARTERY INDUCED BY NITRIC-OXIDE, ADENOSINE, AND PROSTACYCLIN IN THE PIG

The intracellular mechanisms underlying the action of the endogenous v asodilators such as NO/EDRF, adenosine, and prostacyclin acting throug h cGMP and cAMP, respectively, are not well understood. One important action of cyclic nucleotides in smooth muscle relaxation is to lower t he cytosolic Ca2+ concentration by enhanced sequestration into the sar coplasmic reticulum. The present study was undertaken to elucidate the potential role of phosphorylation of phospholamban, the regulator of sarcoplasmic reticulum Ca2+ pump, for the control of coronary vascular tone by NO/EDRF, adenosine, and prostacyclin. Phospholamban was ident ified in pig coronary artery preparations by immunofluorescence micros copy, Western blotting and in vitro phosphorylation. Segments of pig c oronary artery, with either intact or denuded endothelium, were precon tracted with prostaglandin F-2 alpha (PGF(2 alpha)). in endothelium-de nuded preparations 3-morpholinosydnonimine (SIN-1), 5'-N-ethylcarboxia midoadenosine (NECA), and iloprost (ILO) caused both relaxation and ph ospholamban phosphorylation with the potency: SIN-1 > NECA > ILO. The regulatory myosin light chain was significantly dephosphorylated only by SIN-1. In endothelium-intact pig coronary artery L-NAME caused addi tional vasoconstriction and a decrease in phospholamban phosphorylatio n, while phosphorylation of myosin light chain remained unchanged. An inverse relationship between phospholamban phosphorylation and vessel tone was obtained. Our findings demonstrate significant phospholamban phosphorylation during coronary artery relaxation evoked by NO, prosta cyclin, and adenosine receptor activation. Because of the close correl ation between phosphorylation of phospholamban and vessel relaxation, we propose that phospholamban phosphorylation is an important mechanis m by which endogenous vasodilators, especially endothelial NO/EDRF, co ntrol coronary vascular smooth muscle tone. (C) 1998 Wiley-Liss, Inc.