Mechanism of adrenomedullin 2/intermedin mediated vasorelaxation in rat main pulmonary artery.

Adrenomedullin 2/intermedin (AM2/IMD) is a member of calcitonin related gene peptide family and an important nitric oxide mediated vasorelaxant in various vascular beds. However, the mechanism of post receptor-interaction is not clear and may differ depending on tissue type and species. In this study, we aimed to investigate the exact mechanism and the role of BKCa and calcium channels on the vasorelaxant effect of AM2/IMD in rat PA. Changes in the AM2/IMD-mediated vasorelaxation were evaluated in the presence of various inhibitors. CGRP(8-37) (10-6  M), L-NAME (10-4  M), ODQ (10-5  M), SQ22536 (10-4  M), H89 (10-6  M), TEA (10-2  M), iberiotoxin (3 × 10-7  M), and verapamil (10-5  M), all partly or completely inhibited the vasorelaxation. The relaxation was also abolished by removal of the endothelium, or in KCl precontracted PAs. AM2/IMD did not elicit vasorelaxation in the Ca2+ -free conditions. However, the vasorelaxation was not inhibited with AM(22-52) (10-6  M), 4-AP (3 × 10-3  M), glibenclamide (10-5  M), apamin (3 × 10-7  M), TRAM-34 (10-5  M), and La+3 (10-4  M). AM2/IMD -induced changes in intracellular calcium levels and isometric force were monitored simultaneously in fura-2-loaded, endothelium-intact PAs. The AM2/IMD-induced increase in intracellular Ca2+ concentration was inhibited in the presence of iberiotoxin and verapamil, whereas no change was observed with La3+ incubation. Our data suggest that the cAMP/PKA pathway is one of the important pathways AM2/IMD-induced vasorelaxation. AM2/IMD acts through activation of endothelial BKCa and subsequently causes hyperpolarization of the endothelial cell membrane. The hyperpolarization induces Ca2+ influx, which leads to NO production and subsequent vasorelaxation.