Pharmacological Characterization of the Angiotensin Receptor Negatively Coupled with Adenylate Cyclase in Rat Anterior Pituitary Gland*

Abstract

Angiotensin II (AII) inhibited anterior pituitary adenylate cyclase. GTP was necessary to fully express the AII inhibitory effect; Na+ was not required. The magnitude of inhibition (42 .+-. 6%) permitted a pharmacological characterization of the AII receptor involved in adenylate cyclase inhibition. Angiotensin I (AI) was less potent than AII, and deletion of amino acids in the N-terminal position resulted in a progressive reduction of the Ki (peptide concentration producing half-maximal inhibition). The Ki values were 3 .+-. 0.9, 10 and 700 nM for AII, angiotensin III (AIII), and des-Asp1, des-Arg2-AII, respectively. Sarcosine in position 1 ([Sar1,Phe8]AII) increased the potency of inhibition (Ki = 0.12 .+-. 0.12 nM). Different antagonists of the AII receptors appeared to be partial agonists. There was a very close correlation (r = 0.98) between the respective potencies of a series of AII analogs to inhibit adenylate cyclase and the potencies of these analogs to elicit PRL [prolactin] or ACTH release or to bind to AII-binding sites. Dopamine and AII inhibition of anterior pituitary adenylate cyclase were not additive. Apparently, both receptors are on the same cell and likely on lactotrophs. This hypothesis agrees with the observation that vasoactive intestinal peptide stimulation of adenylate cyclase was inhibited by AII; corticotropin-releasing factor stimulation was unaffected. Although dopamine and AII inhibited the same adenylate cyclase, they had opposing effects on PRL release (inhibition and stimulation, respectively). The possible significance of this observation is related to a model implying that PRL release can be elicited through either a Ca2+ or a cAMP pathway.