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 Ca 2+ ‐containing Krebs and Ca 2+ ‐free Krebs containing 1 m m 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 Ca 2+ ‐containing medium and 25.8% in Ca 2+ ‐free Krebs containing 1 m m EGTA. 3 In both Ca 2+ ‐containing medium and Ca 2+ ‐free Krebs containing 1 m m 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 Ca 2+ ‐free Krebs containing 1 m m 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 Ca 2+ , respectively, and that mechanisms other than protein kinase C may be predominantly responsible for ET‐1‐induced tonic contraction.