Vascular dysfunction in human and rat cirrhosis: Role of receptor-desensitizing and calcium-sensitizing proteins

Abstract

In cirrhosis, vascular hypocontractility leads to vasodilation and contributes to portal hypertension. Impaired activation of contractile pathways contributes to vascular hypocontractility. Angiotensin II type 1 receptors (AT₁‐Rs) are coupled to the contraction‐mediating RhoA/Rho‐kinase pathway and may be desensitized by phosphorylation through G‐protein‐coupled receptor kinases (GRKs) and binding of β‐arrestin‐2. In the present study, we analyzed vascular hypocontractility to angiotensin II in cirrhosis. Human hepatic arteries were obtained during liver transplantation. In rats, cirrhosis was induced by bile duct ligation (BDL). Contractility of rat aortic rings was measured myographically. Protein expression and phosphorylation were analyzed by Western blot analysis. Immunoprecipitation was performed with protein A–coupled Sepharose beads. Myosin light chain (MLC) phosphatase activity was assessed as dephosphorylation of MLCs. Aortas from BDL rats were hyporeactive to angiotensin II and extracellular Ca²⁺. Expression of AT₁‐R and Gα_q/11,12,13 remained unchanged in hypocontractile rat and human vessels, whereas GRK‐2 and β‐arrestin‐2 were up‐regulated. The binding of β‐arrestin‐2 to the AT₁‐R was increased in hypocontractile rat and human vessels. Inhibition of angiotensin II–induced aortic contraction by the Rho‐kinase inhibitor Y‐27632 was pronounced in BDL rats. Basal phosphorylation of the ROK‐2 substrate moesin was reduced in vessels from rats and patients with cirrhosis. Analysis of the expression and phosphorylation of Ca²⁺‐sensitizing proteins (MYPT1 and CPI‐17) in vessels from rats and patients with cirrhosis suggested decreased Ca²⁺ sensitivity. Angiotensin II–stimulated moesin phosphorylation was decreased in aortas from BDL rats. MLC phosphatase activity was elevated in aortas from BDL rats. Conclusion: Vascular hypocontractility to angiotensin II in cirrhosis does not result from changes in expression of AT₁‐Rs or G‐proteins. Our data suggest that in cirrhosis‐induced vasodilation, the AT₁‐R is desensitized by GRK‐2 and β‐arrestin‐2 and that changed patterns of phosphorylated Ca²⁺‐sensitizing proteins decrease Ca²⁺ sensitivity. (HEPATOLOGY 2007;45:495–506.)