Nitric Oxide Induces Dilation of Rat Aorta via Inhibition of Rho-Kinase Signaling

Abstract

NO induces vasodilation through cGMP-dependent protein kinase–dependent and –independent mechanisms. A recent study demonstrated that recombinant cGMP-dependent protein kinase can phosphorylate the small G protein, RhoA, thus inhibiting its activity. Additionally, sodium nitroprusside was found to reverse the phenylephrine-induced translocation of RhoA, which is further indicative of the inhibition of RhoA activity. RhoA is known to be involved in the Ca 2+ sensitization of vascular smooth muscle through the actions of one of its downstream effectors, Rho-kinase. This study examined whether NO endogenously induces the relaxation of intact rat aorta via the inhibition of the Rho-kinase–mediated Ca 2+ -sensitizing pathway. Endogenous Rho-kinase inhibitor activity was inhibited by the selective compound Y-27632. Treatment of endothelium-intact rat aorta with Y-27632 (1 μmol/L) resulted in an attenuation of maximal force generated in response to phenylephrine. In endothelium-denuded rings, however, 1 μmol/L Y-27632 was ineffective at inhibiting the phenylephrine-induced contraction. Additionally, 1 μmol/L Y-27632 was significantly less effective at inhibiting the phenylephrine-induced contraction of endothelium-intact rings in the presence of inhibitors of NO synthase or guanylate cyclase ( N ω -nitro- l -arginine and 1 H -[1,2,4]oxadiazolo-[4,3- a ]quinoxalin-1-one, respectively). Interestingly, sodium nitroprusside restored the ability of 1 μmol/L Y-27632 to attenuate phenylephrine-induced contraction. Rho-kinase inhibition was also found to increase the sensitivity of the endothelium-denuded aorta to sodium nitroprusside. These data demonstrate that NO inhibits Rho-kinase activity in the intact rat aorta, supporting the hypothesis that endogenous NO-mediated vasodilation occurs through the inhibition of Rho-kinase constrictor activity in the intact rat aorta.