IDENTIFICATION OF METABOLIC PATHWAYS OF BRAIN ANGIOTENSIN-II AND ANGIOTENSIN-III USING SPECIFIC AMINOPEPTIDASE INHIBITORS - PREDOMINANT ROLE OF ANGIOTENSIN-III IN THE CONTROL OF VASOPRESSIN RELEASE

Angiotensin (Ang) LT and Ang III are two peptide effecters of the brai n renin-angiotensin system that participate in the control of blood pr essure and increase water consumption and vasopressin release. In an a ttempt to delineate the respective roles of these peptides in the regu lation of vasopressin secretion, their metabolic pathways and their ef fects on vasopressin release were identified in vivo. For this purpose , we used recently developed selective inhibitors of aminopeptidase A (APA) and aminopeptidase N (APN), two enzymes that are believed to be responsible for the N-terminal cleavage of Ang II and Ang III, respect ively. Mice received [H-3] Ang II intracerebroventricularly (i.c.v.) i n the presence or absence of the APA inhibitor, EC33 (3-amino-4-thio-b utyl sulfonate) or the APN inhibitor, EC27 (2-amino-pentan-1,5-dithiol ). [H-3]Ang II and [H-3]Ang III levels were evaluated from hypothalamu s homogenates by HPLC. EC33 increased the half-life of [H-3] Ang II 2. 6-fold and completely blocked the formation of [H-3]Ang III, whereas E C27 increased the half-life of [H-3]Ang III 2.3-fold. In addition, the effects of EC33 and EC27 on Ang-induced vasopressin release were stud ied in mice. Ang II was injected i.c.v. in the presence or absence of EC33, and plasma vasopressin levels were estimated by RIA. While vasop ressin levels were increased 2-fold by Ang II (5 ng), EC33 inhibited A ng II-induced vasopressin release in a dose-dependent manner. In contr ast, EC27 injected alone increased in a dose-dependent manner vasopres sin levels. The EC27-induced vasopressin release was completely blocke d by the coadministration of the Ang receptor antagonist (Sar(1)-Ala(8 )) Ang II. These results demonstrate for the first time that (i) APA a nd APN are involved in vivo in the metabolism of brain Ang II and Ang III, respectively, and that (ii) the action of Ang II on vasopressin r elease depends upon the prior conversion of Ang II to Ang III. This sh ows that Ang III behaves as one of the main effector peptides of the b rain renin-angiotensin system in the control of vasopressin release.