Autocrine Role for the Endothelin-B Receptor in the Secretion of Adrenomedullin

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Abstract
—Adrenomedullin, originally discovered in human pheochromocytoma, is a vasodilating and natriuretic peptide of vascular endothelial and smooth muscle cell origin. Although endothelin-1 (ET-1) has been implicated as a vasoconstricting and growth-promoting peptide of endothelial origin, it may more importantly function as an autocrine factor and release vasodilatory substances such as nitric oxide by mechanisms linked to the endothelin-B (ET B ) receptor subtype. The present study was designed to establish that the ET B receptor stimulates the secretion of adrenomedullin from cultured canine aortic endothelial cells. We first sought to determine the presence and production of adrenomedullin in canine aortic endothelial cells using immunohistochemistry and Northern blot analysis, which revealed that adrenomedullin immunoreactivity and adrenomedullin mRNA were present in canine aortic endothelial cells. Second, 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 ET B receptor in canine aortic endothelial cells, and ET B receptor stimulation by sarafotoxin S6c increased adrenomedullin production and secretion from canine aortic endothelial cells. Such actions were blocked with the ET B receptor antagonist IRL-2500 but not with ET A receptor antagonist FR-139317. These studies are the first to report an additional autocrine role of the ET B receptor in the release of vasodilating and natriuretic peptide adrenomedullin, and they suggest another important vasoactive system regulated by the ET receptor subtype.