Endothelin‐1, phorbol esters and phenylephrine stimulate MAP kinase activities in ventricular cardiomyocytes

ET‐1 stimulated MBP kinase activity in cultured cardiomyocytes. Maximal activation (3.5‐fold) was at 5 min. EC 50 was 0.2 nM. PMA or PE also increased MBP kinase (4‐ or 2.5‐fold, respectively). Pre‐treatment with PMA down‐regulated the subsequent response to ET‐1 or PMA. ET‐1‐ or PMA‐stimulated MBP kinase was resolved into 2 major (peaks II and IV) and 2 minor peaks by FPLC on Mono Q. Peaks II and IV were inactivated by either LAR or PP2A. Renatured MBP kinase activities following SDS‐PAGE in MBP‐containing gels and immunoblot analysis showed that peak II was a p42 MAP kinase and peak IV was a p44 MAP kinase.