The peptide apelin is localised in the vascular endothelium and highly expressed in pulmonary tissue. The aim of this study was to investigate whether apelin could be a potential lung-derived plasma marker for pulmonary hypertension, and study the effect of apelin in pulmonary arteries.

Apelin protein levels were measured in the lung, right ventricle, and plasma from normoxic and chronic hypoxic rats with pulmonary hypertension. Isolated intrapulmonary arteries were mounted in microvascular myographs and the effect of apelin investigated. Finally, the distribution of apelin receptors in pulmonary tissue was visualised by immunohistochemistry.

Total pulmonary apelin content was not changed by hypoxia. Right ventricular apelin concentrations and content were lower than in the lung, but increased substantially in hypoxia in correlation with right ventricular pressure. Plasma apelin did not reflect pulmonary or right ventricular apelin levels. In pulmonary arteries from normoxic rats, apelin inhibited vasoconstriction to endothelin-1 and angiotensin-II. However, in arteries from hypoxic rats, apelin failed to inhibit contraction to angiotensin-II and endothelin-1. No difference in immunoreaction for apelin receptors was found in lung sections and arteries from normoxic versus chronic hypoxic rats.

Apelin changes in the right ventricle seem more specific for pulmonary hypertension than do changes in pulmonary tissue, which does not speak in favour of apelin as a lung-derived marker for this disease. During normoxic conditions, apelin has a modulating effect on vasoconstriction which is lost in chronic hypoxia. This may reflect alterations in the signal transduction downstream of the apelin receptor.