Endothelial modulation of pulmonary vascular tone.

Pulmonary endothelial cells normally synthesize prostacyclin (PGI2) and nitric oxide (NO), which are both potent vasodilators. Although PGI2 is largely used to treat patients with severe pulmonary hypertension, its role in the physiology and pathophysiology of the pulmonary circulation is still debated. NO, which is now considered as the endogenous nitrovasodilator, is perhaps more involved than PGI2 in the mechanisms that modulate pulmonary vascular tone in health and disease. There is evidence to suggest that background release of NO contributes to the normally low pulmonary vascular tone in normoxia. Although there are theoretical grounds to hypothesize that hypoxia reduces the synthesis of NO, lack of the latter does not seem to account for the acute hypoxic pulmonary vasoconstriction. Instead, there is evidence to suggest that NO activity is increased in order to modulate the pulmonary vasopressor response to acute alveolar hypoxia. However, more consistent, concerning the role of NO, are data gathered from studies performed in chronic hypoxic conditions. Both experimental data and studies performed in man demonstrate impairment of NO synthesis and/or release in chronic hypoxic pulmonary hypertension. The impaired NO production, whilst reducing the ability of the pulmonary vasculature to relax, also favours the occurrence of excessive pulmonary vasoconstriction. Lack of NO synthesis might also permit mitogenesis and proliferation of various cell types within the vascular wall. We hypothesize that functional alterations of pulmonary endothelium are likely to affect both reactivity and growth of pulmonary vessels. In this respect, NO probably has a pivotal role in modulating pulmonary vascular tone and controlling pulmonary vascular remodelling in health and disease.