Objective: Abnormalities that characterize pulmonary arterial hypertension (PAH) include an impairment in the structure and function of pulmonary vascular endothelial (EC) and smooth muscle cells (SMC). Levels of the hormone aldosterone are elevated in human PAH patients and in experimental pulmonary hypertension (PH). Inhibition of the aldosterone-binding mineralocorticoid receptor (MR) attenuates PH in multiple animal models. This study explored the specific role of MR in pulmonary vascular ECs and SMCs in PH using cell-specific MR knockout mice exposed to sugen/hypoxia-induced PH.
Methods: SMC-MR-KO mice, EC-MR-KO mice and their appropriate controls were exposed to SU5416 (Su)/hypoxia or control normoxia. Some MR-KO mice and their controls were also treated with spironolactone (SP) from day 1 of hypoxia exposure. Hemodynamic measurements were obtained at the end of exposure. RV hypertrophy was expressed as the Fulton index (RV/LV+septum). Heart and lung tissues were harvested for immunohistological analysis. Lungs were stained with Verhoeff-Van Gieson for the degree of inflammation and muscularization. RV sections were stained with Picrosirius Red for collagen deposition and cardiomyocyte size.
Results: MR inhibition with SP, at a dose that does not impact systemic blood pressure, significantly reduces RV systolic pressure. However, the attenuation in right ventricular systolic pressure with spironolactone was not reproduced by deletion of MR in SMC or EC. Similarly, MR inhibition attenuates the increase in RV cardiomyocyte area independent of vascular MR with no effect on RV weight or interstitial fibrosis. RV perivascular fibrosis is significantly decreased by SP and this could be reproduced by specific deletion of MR from ECs. MR inhibition significantly reduces pulmonary arteriolar muscularization, independent of EC-MR or SMC-MR. Finally, the degree of perivascular inflammation in pulmonary vessels is attenuated by MR antagonism and is fully reproduced by SMC-specific MR deletion.
Conclusions: These studies demonstrate that, in the sugen/hypoxia PH model, systemic-MR blockade significantly attenuates the PH phenotype and vascular MR has cell-specific effects with EC-MR contributing to RV perivascular fibrosis and SMC-MR driving pulmonary perivascular inflammation. Lastly, the role of MR in pulmonary vascular muscularization and RV cardiomyocyte hypertrophy is likely mediated by MR in other non-vascular cell types including cardiomyocytes and/or inflammatory cells.