Chronic Estrogen Treatment Reduces Angiotensin II Receptors in the Anterior Pituitary*

Abstract

We have demonstrated previously that angiotensin II (ANG II) receptors in the rat pituitary fluctuate with the stage of the estrous cycle, with the highest binding level found in diestrus and the lowest found in estrus. These findings were extended in the present study by examining the role of estrogens in variations of ANG II binding. Ovariectomized rats were treated with a Silastic implant containing estradiol benzoate for 4 or 14 days before death, and pituitary [125I]ANG II binding was measured. Compared to that in ovariectomized controls, ANG II binding in the anterior pituitary was found to be significantly reduced after either 4 or 14 days of estrogen treatment. This reduction was mainly an effect on receptor density rather than receptor affinity. The observed apparent Kd and receptor density were as follows: ovariectomized controls, 0.8 nM and 92 fmol/mg protein; 4 days of estrogen treatment, 1.0 nM and 25 fmol/mg protein; 14 days of estrogen treatment, 0.8 nM and 12 fmol/mg protein. We conclude that estrogen treatment leads to a reduction in ANG II-binding sites in the anterior pituitary, and this effect may account for the reduction of pituitary ANG II receptors at estrus.