Agonist-dependent difference in the mechanisms involved in Ca2+ sensitization of smooth muscle of porcine coronary artery.

This study was undertaken to explore possible signal-transduction mechanisms involved in the Ca 2+ -sensitizing effects of carbachol and endothelin-1 (ET-1) by using β-escin-skinned smooth muscle of porcine coronary artery. Pretreatment with C3 exoenzyme of Clostridium botulinum, which selectively inactivates rho p21 by adenosine diphosphate (ADP) ribosylation, resulted in a significant inhibition of ET-1-induced Ca 2+ sensitization, but had no effect on carbachol-induced Ca 2+ sensitization. Whereas the protein kinase C (PKC) inhibitors calphostin C and staurosporine did not affect the Ca 2+ -sensitizing effect of carbachol, the tyrosine kinase inhibitors genistein and tyrphostin 25 greatly but incompletely suppressed it. In contrast, the Ca 2+ -sensitizing effect of ET-1 was significantly inhibited by either calphostin C or genistein. Although the inhibitory effect of calphostin C on ET-1-induced Ca 2+ sensitization was less than that of genistein, the effects of calphostin C and genistein were additive. The genistein-sensitive component of ET-1-induced Ca 2+ sensitization appeared to include the C3-sensitive one. However, a substantial enhancement by ET-1 of the Ca 2+ -induced contraction was observed even in the presence of the two inhibitors. In β-escin-skinned smooth muscle of rabbit mesenteric artery, ET-1-induced Ca 2+ sensitization was marginally affected by C3 pretreatment, calphostin C, and genistein. We conclude that, although PKC activation and rho p21 protein-dependent and -independent tyrosine phosphorylation each plays an important role in an increase in myofilament Ca 2+ sensitivity, the contributions of these signaling pathways to Ca 2+ sensitization are different depending on receptor agonists and tissues used. Furthermore, these data suggest the existence of an as yet undefined signal-transduction mechanism involved in Ca 2+ sensitization caused by receptor agonists.