Inhibition of sustained smooth muscle contraction by PKA and PKG preferentially mediated by phosphorylation of RhoA.

The role of RhoA in myosin light-chain (MLC)(20) dephosphorylation and smooth muscle relaxation by PKA and PKG was examined in freshly dispersed and cultured smooth muscle cells expressing wild-type RhoA, constitutively active Rho(V14), and phosphorylation site-deficient Rho(A188). Activators of PKA (5,6-dichloro-1-beta-ribofuranosyl benzimidazole 3',5'-cyclic monophosphothionate, Sp-isomer; cBIMPS) or PKG [8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (8-pCPT-cGMP), sodium nitroprusside (SNP)] or both PKA and PKG (VIP) induced phosphorylation of constitutively active Rho(V14) and agonist (ACh)- or GTPgammaS-stimulated wild-type RhoA but not Rho(A188). Phosphorylation was accompanied by translocation of membrane-bound wild-type RhoA and Rho(V14) to the cytosol and complete inhibition of ACh-stimulated Rho kinase and phospholipase D activities, RhoA/Rho kinase association, MLC(20) phosphorylation, and sustained muscle contraction. Each of these events was blocked depending on the agent used, by the PKG inhibitor KT5823 or the PKA inhibitor myristoylated PKI. Inhibitors were used at a concentration (1 microM) previously shown by direct measurement of kinase activity to selectively inhibit the corresponding kinase. In muscle cells overexpressing the active phosphorylation site-deficient mutant Rho(A188), MLC(20) phosphorylation was partly inhibited by SNP, VIP, cBIMPS, and 8-pCPT-cGMP, suggesting the existence of an independent inhibitory mechanism downstream of RhoA. Results demonstrate that dephosphorylation of MLC(20) and smooth muscle relaxation are preferentially mediated by PKG- and PKA-dependent phosphorylation and inactivation of RhoA.