Participation of protein kinase C in endothelin‐1‐induced contraction in rat aorta: studies with a new tool, calphostin C

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Abstract
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.