Effects of 17 beta-estradiol on angiotensin II receptors and prolactin release in cultured pituitary cells.

Abstract

Angiotensin II (AII) binds to specific receptors in the lactotroph and stimulates PRL secretion from isolated rat pituitary cells. Since estrogens exert major regulatory actions on PRL secretion, the effects of estradiol (E2) on pituitary AII receptors and PRL responses were studied in vivo and in cultured rat anterior pituitary cells. In female rats, treatment with E2-containing Silastic capsules for 4 days caused a significant increase in PRA from 1.3 to 3 ng/ml X min and a 38% decrease in the binding of [125I]AII to anterior pituitary membrane-rich fractions (P less than 0.01). In vitro studies showed that treatment of cultured anterior pituitary cells with 1 nM E2 for 4 days caused a 57 +/- 6% decrease in AII receptor concentration with no change in binding affinity. Reduction of AII receptors by E2 in 4-day cultures was dose dependent and was demonstrable with E2 concentrations that occur in plasma during the estrous cycle (0.01-1 nM). The decrease in AII receptors in cells incubated with 1 nM E2 was near maximum after 24 h of culture, and results were similar when receptor concentrations were calculated per unit protein or per cell. Despite the substantial decrease in AII receptors, E2 treatment did not specifically decrease the responsiveness of the pituitary cells to AII stimulation. Thus, PRL responses to AII (10 nM) or TRH (100 nM) were unchanged after 1 day of E2 treatment and were increased after 4 days of treatment. These findings demonstrate that E2 has a direct inhibitory action on expression of pituitary AII receptors that is not accompanied by a decrease in AII-stimulated PRL secretion. In the rat pituitary, estrogen modulation of postreceptor events is the predominant determinant of lactotroph responsiveness during stimulation of PRL release by AII.