The present study investigates the effects of human adrenomedullin (ADM) on the pulmonary vascular bed of isolated, blood-perfused rat lung. Because pulmonary blood flow and left atrial pressure were constant, changes in pulmonary arterial pressure directly reflect changes in pulmonary vascular resistance. Under conditions of resting (low) pulmonary vasomotor tone, intra-arterial bolus injections of ADM-(1-52) and two truncated sequences of ADM-(1-52) [ADM-(1-12) and ADM-(13-52)] did not alter pulmonary arterial pressure. When pulmonary vasomotor tone was increased by U-46619, a thromboxane A2 mimic, intra-arterial bolus injections of ADM-(1-52) and ADM-(13-52) at similar doses produced similar, dose-dependent reductions in pulmonary arterial pressure. On a molar basis, ADM-(1-52) had greater pulmonary vasodilator activity than isoproterenol. In contrast, ADM-(1-12) had no activity. When pulmonary vasomotor tone was actively increased to the same level using KCl, the pulmonary vasodilator activity of ADM-(13-52) was decreased 10-fold. The present data demonstrate that ADM-(1-52) dilates the pulmonary vascular bed and suggest that the pulmonary vasodilator activity of ADM is greater on pulmonary blood vessels preconstricted through a receptor-dependent mechanism. Because meclofenamate, nitro-L-arginine methyl ester, methysergide, BW A-1433U83, U-37883A, and calcitonin gene-related peptide [CGRP-(8-37)], a CGRP-receptor antagonist, did not alter the pulmonary vasodilator response to ADM-(1-52), the present data suggest that ADM dilates the pulmonary vascular bed independently of cyclooxygenase products, endothelium-derived relaxation factor, serotoninergic receptors, adenosine1 purinoreceptors, ATP-dependent potassium channels, and CGRP receptors.(ABSTRACT TRUNCATED AT 250 WORDS)