INHIBITION OF PHORBOL ESTER-INDUCED CONTRACTION BY CALMODULIN ANTAGONISTS IN RAT AORTA

The purpose of the present study was to investigate the relative roles of protein kinase C (PKC) and myosin light chain kinase (MLCK) in phorbol ester-induced contraction of vascular smooth muscle through the use of PKC and calmodulin antagonists. Prior exposure to PKC antagonists staurosporine (0.03-mu-M) and H-7 (10-mu-M) had relatively little effect on contractions to phorbol 12-myristate 13-acetate (PMA), while contractions to norepinephrine and KCl were greatly inhibited. Prior exposure to the calmodulin antagonists calmidazolium (3 and 10-mu-M) and W-7 (10-mu-M) inhibited contractions to PMA in the presence and absence of extracellular Ca2+, while contractions to norepinephrine and KCl remained relatively unaffected. Calmidazolium and W-7 were relatively weak relaxants when applied during the PMA contraction, and the magnitudes of relaxation were similar to those observed in norepinephrine- and KCl-contracted tissues. Calmidazolium partially inhibited the PMA-induced translocation of PKC. These results suggest that 1) the calmodulin antagonists inhibit the development of PMA-induced contraction, at least in part, through inhibition of PKC translocation; 2) the mechanisms of phorbol ester- and agonist-induced translocation of PKC are distinct; 3) the potencies and inhibitory mechanisms of these agents depend on whether the agents are added before or during the contraction; and 4) the selectivity of these agents, as evaluated in enzyme preparations, may not be consistent with their cellular actions.