Pulmonary hypertension is seen in patients in whom there is an increased pulmonary pressure (systolic pulmonary/systemic arterial pressure > 60%; > 15–20 mm Hg). The disease slowly progresses, applying stress on the right heart function leading to heart failure and death. Although the precise mechanism of development of precapillary pulmonary hypertension remains to be defined, understanding of the local factors that control pulmonary vascular tone and structure has increased greatly in the past decade. It is known that the endothelium releases several potent vasoactive mediators, including endothelin-1, prostacyclin, and endothelium derived relaxing factor, nitric oxide (NO). Decreased production of nitric oxide and prostacyclin, and increased release of endothelin, have been shown to promote pulmonary vasoconstriction resulting in pulmonary hypertension.1 The vascular endothelium expresses receptors for ligands that play an important role in maintaining pulmonary vascular tone.2