The relaxant effect of okadaic acid on canine basilar artery involves activation of PKCα and phosphorylation of the myosin light chain at Thr-9

Vasodilator responses induced by okadaic acid were investigated in canine basilar artery precontracted with 80 mM KCl. Okadaic acid (1 µM) relaxed the artery and this relaxant effect was partially inhibited by Gö6976, a conventional protein kinase C inhibitor, and calphostin C, an inhibitor of conventional and novel PKCs. Rottlerin, a specific inhibitor of PKCδ, did not influence okadaic acid's effect. KCl increased phosphorylation of 20,000-Dalton myosin light chain (MLC20) at Ser-19. Okadaic acid additionally increased MLC20 phosphorylation at Thr-18 and Thr-9, resulting in triphosphorylation of MLC20. This phosphorylation was inhibited by Gö6976. Okadaic acid stimulated phosphorylation of PKCα and 17,000-Dalton PKC-potentiated inhibitory phosphoprotein (CPI-17), and Gö6976 inhibited these phosphorylations. These results suggest that okadaic acid's relaxant effect involves MLC20 triphosphorylation through a direct phosphorylation by PKCα and an indirect phosphorylation by inhibition of myosin light chain phosphatase through PKCα-mediated CPI-17 phosphorylation.