PARTICIPATION OF PROTEIN-KINASE-C IN ENDOTHELIN-1-INDUCED CONTRACTION IN RAT AORTA - STUDIES WITH A NEW TOOL, CALPHOSTIN-C

1 Calphostin C at 10(-6) M was shown to be selective and highly effective in inhibiting contractile responses of rat aortae to 12-o-tetradecanoylphorbol-13-acetate, while it had no effect on contractile responses to elevated KCl. 2 In the rat aorta, endothelin-1 (ET-1) developed a sustained tonic contraction dose-dependently in both normal Ca2+-containing Krebs and Ca2+-free Krebs containing 1 mM EGTA. Calphostin C (10(-6) M ), a selective protein kinase C inhibitor, antagonized the maximal tensions for cumulative addition of 10(-8) M ET-1 by 13.2% in Ca2+-containing medium and 25.8% in Ca2+-free Krebs containing 1 mM EGTA. 3 In both Ca2+-containing medium and Ca2+-free Krebs containing 1 mM EGTA, precontraction with 10(-8) M ET-1 had no effects on the contractile response to subsequently added 10(-6) M 12-o-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C. 4 In Ca2+-free Krebs containing 1 mM EGTA, precontraction with 10(-6) M TPA potentiated the contractile response to subsequently added 10(-8) M ET-1, whereas this potentiation was abolished by pretreatment with 10(-6) M calphostin C. The mechanism of the TPA-induced potentiating effect remains to be determined. 5 These results suggest that the participation of protein kinase C in the 10(-8) M ET-1-induced contraction may be 13.2% and 25.8% in the presence and absence of extracellular Ca2+, respectively, and that mechanisms other than protein kinase C may be predominantly responsible for ET-1-induced tonic contraction.