Cyclic GMP-dependent protein kinase signaling pathway inhibits RhoA-induced Ca2+ sensitization of contraction in vascular smooth muscle

The potent vasodilator action of cyclic GMP-dependent protein kinase (cGK) involves decreasing the Ca2+ sensitivity of contraction of smooth muscle vi a stimulation of myosin light chain phosphatase through unknown mechanisms (Wu, X., Somlyo, A. V., and Somlyo, A. P. (1996) Biochem. Biophys. Res. Com mun. 220, 658-663). Myosin light chain phosphatase activity is controlled b y the small GTPase RhoA and its target Rho kinase. Here we demonstrate cGMP effects mediated by cGK that inhibit RhoA-dependent Ca2+ sensitization of contraction of blood vessels and actin cytoskeleton organization in culture d vascular myocytes. Ca2+ sensitization and actin organization were inhibit ed by both 8-bromo-cGMP and sodium nitroprusside (SNP). SNP also caused tra nslocation of activated RhoA from the membrane to the cytosol, SNP-induced actin disassembly was lost in vascular myocytes in culture after successive passages but was restored by transfection of cells with cGK I. Furthermore , cGK phosphorylated RhoA in vitro, and addition of cGK I inhibited RhoA-in duced Ca2+ sensitization in permeabilized smooth muscle. 8-Bromo-cGMP-induc ed actin disassembly was inhibited in vascular myocytes expressing RhoA(Ala -188), a mutant that could not be phosphorylated, Collectively, these resul ts indicate that cGK phosphorylates and inhibits RhoA and suggest that the consequent inhibition of RhoA-induced Ca2+ sensitization and actin cytoskel eton organization contributes to the vasodilator action of nitric oxide.