Angiotensin II (AII) and adrenocorticotropin release: modulation by estradiol of the AII biological activity and binding characteristics in anterior pituitary dispersed cells.

The present studies were designed to test if estrogens influence basal and angiotensin II (AII)-stimulated ACTH release and the binding characteristics of AII receptors in isolated anterior pituitary (AP) cells from estrogen-deplete and estrogen-replete rats incubated in vitro. AP cells were obtained from various adult donors: randomly cycling (rc), 10-day ovariectomized (Ovx), Ovx with estradiol restored at a circulating physiological level (Ovx + lEB), Ovx with estradiol at a supraphysiological circulating level (Ovx + hEB); and male (m) rats. The amount of ACTH released under basal conditions was similar in the rc, Ovx + lEB, and m groups, although this amount was significantly greater (P less than 0.02) than that in the Ovx and Ovx + hEB groups. The ACTH-releasing activity (CRA) of AII was concentration dependent (10(-9)-10(-6) M) in all cells. The rank order of the CRA of AII in the groups varied as follows: rc = Ovx + lEB greater than Ovx = Ovx + hEB = m. The slopes of the AII responses were similar. Estradiol addition in vitro did not modify either basal or AII-stimulated ACTH release in any group of cells. AII binding studies indicated that AP cells from donors with different circulating estradiol levels had similar apparent equilibrium dissociation constants (Kd, 16-30 nM). However, the maximum binding capacities in AP cells from the m, Ovx, and Ovx + hEB groups (40, 51, and 49 fmol/10(6) cells, respectively) were significantly lower (P less than 0.05) than those in the rc and Ovx + lEB groups (77 and 81 fmol/10(6) cells, respectively). In summary, these studies indicate that 1) circulating levels of estradiol are able to modulate spontaneous ACTH release by dispersed AP cells; and 2) circulating estradiol, at a lower or higher level than that during the estrous cycle, decreases the in vitro ACTH-releasing activity of AII, possibly through a reduction in the number of AP AII receptors. These results further suggest that estrogen is likely to have a physiological role in corticotropic function.