AM reverses pressor response to ET-1 independently of NO in rat coronary circulation

Endothelin-1 (ET-1) elicits a vasoconstrictor response via ETA receptors, w hereas simultaneous activation of ETB receptors triggers the release of nit ric oxide (NO), which may limit the constrictor effect of ET-1. Recently, s timulation of ETB receptors has been shown to increase the secretion of adr enomedullin (AM), a newly identified vasorelaxing peptide. The present stud y was designed to see whether AM can oppose the vasoconstrictor response to ET-1. In the isolated perfused paced rat heart preparation, infusion of ET -1 at concentrations of I nmol/l for 30 min induced a significant coronary vasoconstriction, whereas it had no effect on perfusion pressure at a dose of 0.08 nmol/l. N-omega-nitro-L-arginine methyl ester (L-NAME; 300 mu mol/l ), a potent inhibitor of NO synthase (NOS), did not change the perfusion pr essure when added alone to the perfusion fluid but it unmasked the constric tor effect of ET-1 at both concentrations. In the presence Of L-NAME, AM (0 .03 to 1 nmol/l) markedly reversed the pressor response to ET-1 at both con centrations. Administration of AM (0.03 and 1 nmol/l) alone resulted in a d ose-dependent decrease in perfusion pressure, which was not modified in the presence of L-NAME. In conclusion, the coronary vasoconstrictor response t o ET-1 is markedly augmented in the presence of a NOS inhibitor. This const rictor response is substantially reversed by AM. Our results indicate that AM may serve as a paracrine modulator of ET-1-induced vasoconstriction inde pendently of the NO pathway.