The adrenomedullin receptor acts as clearance receptor in pulmonary circulation

Adrenomedullin (AM) is a powerful pulmonary vasodilator with antimitogenic properties. We investigated the role of the AM receptor (AMR) and the calcitonin gene-related peptide type-1 receptor (CGRP1R) in regulating pulmonary vascular AM levels. The AMR antagonist hAM(22-52) (120 nmol/L) significantly elevated AM release compared with controls to 250% after 2 h in isolated rat lungs and to 830% after 4 h in pulmonary artery endothelial cells (PAEC). CGRP1R blockade had no effect. AMR blockade did not influence prepro-AM mRNA levels nor did inhibition of protein synthesis by cycloheximide (0.01 mg/mL) abolish the effect of the AMR antagonist. Radioligand-binding studies with PAEC membranes revealed a decrease by 44% of the AMR density in response to AMR antagonism. Altogether, the pulmonary vascular AMR represents not only a functionally active, but also a clearance receptor; its expression is constitutively stimulated by basal AM. This identifies a novel mechanism for controlling pulmonary AM levels.