Role of endothelial and smooth muscle cells in the physiopathology and treatment management of pulmonary hypertension.

Pulmonary hypertension can occur either as primary or secondary disease following cardiac or pulmonary illnesses. In either cases, histological lung biopsies reveal vascular remodelling i.e. smooth muscle cells proliferation with medial hypertrophy, arteriolar muscularization and endothelial cell proliferation. Subsequent intimal thickening, fibrosis and in situ thrombosis, altogether lead to vaso-occlusive alterations referred to as plexiform lesions. Theories concerning the detailed physiopathology of pulmonary hypertension have focused on endothelial and smooth muscle cells' chemical factors production and response to different mediators. The endothelium produces vasoconstrictor and growth-promoting factor such as endothelin-1 (ET-1) as well as vasodilator and growth-inhibitor factors like prostacyclin and nitric oxide (NO). ET-1 has been noted in high concentrations in some clinical cases and experimental models of pulmonary hypertension, coupled with ET-1 receptors' modulation and altered endothelin converting enzyme activities, suggesting their active role in both arteriolar vasoconstriction and occlusion. Vascular thrombosis which has been noted by pathologists in pulmonary hypertension, could be related to an imbalance between thrombotic inducing factors (such as anti-phospholipid antibodies, ET-1 and thromboxane) and decreased fibrinolytic activity and antiaggregant endothelial factors (like prostacyclin, NO, thrombomodulin). The discovery of an endothelial cells' monoclonal proliferation in plexiform lesions of patients with primary pulmonary hypertension may reinforce the cellular proliferation hypothesis to understand the histopathology of this disease. In view of these new findings, the treatments available for pulmonary hypertension have been expanded from the previously employed oxygen therapy, calcium-channel blockers and anticoagulants, to intravenous prostacyclin analogues (epoprostenol) and inhaled nitric oxide.