Absence of the Adenosine A2A Receptor Confers Pulmonary Arterial Hypertension and Increased Pulmonary Vascular Remodeling in Mice

Abstract

Background: Pulmonary arterial hypertension (PAH) is characterized by sustained elevation of pulmonary vascular resistance resulting from endothelial and smooth muscle cell dysfunction and collagen deposition in pulmonary vascular walls. In this study, we investigated the role of the adenosine A_2A receptor (A_2AR) in the development of PAH by determining the effect of genetic inactivation of A_2ARs on pulmonary vascular remodeling in mice. Methods and Results: We characterized hemodynamic, histological and ultrastructural changes in pulmonary vascular remodeling in A_2AR knockout (KO) mice compared with their wild-type (WT) littermates after exposure to normoxia and hypoxic conditions. After exposure to normoxia, compared to WT mice, A_2AR KO mice displayed: (1) increased right ventricular systolic pressures and an elevated ratio of the right ventricle over left ventricle plus septum (Fulton index), (2) increased wall area and thickness as well as enhanced smooth muscle actin immunoreactivity in pulmonary resistance vessels, (3) increased proliferating cell nuclear antigen-positive cells in pulmonary resistance vessels and (4) increased smooth muscle cells hypertrophy and collagen deposition in the adventitia of pulmonary arteriole walls as revealed by electron microscope. By contrast, histological analysis revealed no features of hypertensive nephropathy in A_2AR KO mice and there was no significant difference in systemic blood pressure, and left ventricular masses among the 3 genotypes. Furthermore, following chronic exposure to hypoxia, A_2AR KO mice exhibited exacerbated elevation in right ventricular systolic pressure, hypertrophy of pulmonary resistance vessels and increased cell proliferation in pulmonary resistance vessels, compared to WT littermates. Thus, genetic inactivation of A_2ARs selectively produced PAH and associated increased smooth muscle proliferation and collagen deposition. Conclusions: Extracellular adenosine acting at A_2ARs represents an important regulatory mechanism to control the development of PAH and pulmonary vascular remodeling.