Evidence for the presence and characterization of angiotensin II receptors in rat anterior pituitary membranes

Previous work from this laboratory (1) indicates that angiotensin II (AII) can affect release of several anterior pituitary hormones, both in vivo and in vitro. To ascertain whether specific receptors mediate the effects of AII on the anterior pituitary, specific binding as well as the kinetics of [¹²⁵I] AII binding to rat anterior pituitary membranes were analyzed. Binding of [¹²⁵I] AII was rapid, reaching equilibrium within 4 min at 37 C. Specific binding was ∼ 90%. Increasing concentrations of ligand resulted in saturation of binding, with equilibrium attained at [¹²⁵I] AII = 2nM. Scatchard analysis of the data indicated a single class of binding sites, with an equilibrium dissociation constant, Kd = 0.49 nM, and a maximum binding capacity of 40 fmol/mg protein. Specific binding was directly proportional to membrane protein, concentration (range 20-240 μg protein). Binding was competitively inhibited on an equimolar basis by (Sar¹, Ala) AII (Saralasin), a specific AII receptor antagonist. The decapeptide Angiotensin I was about 10-20-fold less potent in inhibiting specific AII binding. These studies demonstrate and characterize specific receptor sites for AII in the anterior pituitary gland and offer additional evidence for a role of AII in the regulation of anterior pituitary hormone secretion.