Rho-kinase inhibitor inhibits both myosin phosphorylation-dependent and -independent enhancement of myofilament Ca2+ sensitivity in the bovine middle cerebral artery

1 The role of Rho kinase in Ca2+ sensitization of the contractile apparatus in smooth muscle was investigated in the bovine middle cerebral artery. 2 U46619, a thromboxane A(2) analog, induced a greater sustained contraction with a smaller [Ca2+](i) elevation than that seen with 118 mM K+. The level of myosin light chain (MLC) phosphorylation obtained in the initial phase of the contraction was higher than that seen with 118 mM K+; thereafter, it gradually declined to a comparable level in the late phase. 3 During the steady state of the U46619-induced contraction, Y27632 (10 muM), a Rho-kinase inhibitor, partially inhibited [Ca2+](i), although it substantially inhibited tension and MLC phosphorylation. Wortmannin (10 mM), an MLC kinase inhibitor, had no significant effect on [Ca2+](i), but it completely inhibited MLC phosphorylation and partially inhibited tension. The wortmannin-resistant tension development was thus not associated with MLC phosphorylation, and this component was completely inhibited by Y27632. 4 In conclusion, U46619 enhanced Ca2+ sensitivity in a manner both dependent and independent of MLC phosphorylation in the bovine middle cerebral artery. Both mechanisms of Ca2+ sensitization can be inhibited by the Rho-kinase inhibitor.