AUTOCRINE ROLE FOR THE ENDOTHELIN-B RECEPTOR IN THE SECRETION OF ADRENOMEDULLIN

Adrenomedullin, originally discovered in human pheochromocytoma, is a vasodilating and natriuretic peptide of vascular endothelial and smoot h muscle cell origin. Although endothelin-1 (ET-1) has been implicated as a vasoconstricting and growth-promoting peptide of endothelial ori gin, it may more importantly function as an autocrine factor and relea se vasodilatory substances such as nitric oxide by mechanisms linked t o the endothelin-B (ETB) receptor subtype. The present study was desig ned to establish that the ETB receptor stimulates the secretion of adr enomedullin from cultured canine aortic endothelial cells. We first so ught to determine the presence and production of adrenomedullin in can ine aortic endothelial cells using immunohistochemistry and Northern b lot analysis, which revealed that adrenomedullin immunoreactivity and adrenomedullin mRNA were present in canine aortic endothelial cells. S econd, adrenomedullin was time-dependently secreted from canine aortic endothelial cells, with a secretion rate of 15.7+/-1.5 pg/10(5) cells per 24 hours. Furthermore, immunohistochemistry revealed the presence of the ETB receptor in canine aortic endothelial cells, and ETB recep tor stimulation by sarafotoxin S6c increased adrenomedullin production and secretion from canine aortic endothelial cells. Such actions were blocked with the ETB receptor antagonist IRL-2500 but not with ETA re ceptor antagonist FR-139317. These studies are the first to report an additional autocrine role of the ETB receptor in the release of vasodi lating and natriuretic peptide adrenomedullin, and they suggest anothe r important vasoactive system regulated by the ET receptor subtype.