Obligatory Metabolism of Angiotensin II to Angiotensin III for Zona Glomerulosa Cell-Mediated Relaxations of Bovine Adrenal Cortical Arteries.

ABSTRACT

Hyperaldosteronism is associated with hypertension, cardiac hypertrophy, and congestive heart failure. Steroidogenic factors facilitate aldosterone secretion by increasing adrenal blood flow. Angiotensin (Ang) II decreases adrenal vascular tone through release of zona glomerulosa (ZG) cell-derived vasodilatory eicosanoids. However, ZG cell-mediated relaxation of bovine adrenal cortical arteries to Ang II is not altered by angiotensin type 1 or 2 receptor antagonists. Because traditional Ang II receptors do not mediate these vasorelaxations to Ang II, we investigated the role of Ang II metabolites. Ang III was identified by liquid chromatography-mass spectrometry as the primary ZG cell metabolite of Ang II. Ang III stimulated ZG cell-mediated relaxation of adrenal arteries with greater potency than did Ang II. Furthermore, ZG cell-mediated relaxations of adrenal arteries by Ang II were attenuated by aminopeptidase inhibition, and Ang III-stimulated relaxations persisted. Ang IV had little effect compared with Ang II. Moreover, ZG cell-mediated relaxations of adrenal arteries by Ang II were attenuated by an Ang III antagonist but not by an Ang (1-7) antagonist. In contrast, Ang II and Ang III were equipotent in stimulating aldosterone secretion from ZG cells and were unaffected by aminopeptidase inhibition. Additionally, aspartyl and leucyl aminopeptidases, which convert Ang II to Ang III, are the primary peptidase expressed in ZG cells. This was confirmed by enzyme activity. These data indicate that intra-adrenal metabolism of Ang II to Ang III is required for ZG cell-mediated relaxations of adrenal arteries but not aldosterone secretion. These studies have defined an important role of Ang III in the adrenal gland.