Regulation of dopamine receptors in the MtT/W15 transplantable pituitary tumor by estrogen.

We investigated the effects of estrogens on the regulation of dopamine receptors in the MtT/W15 transplantable rat pituitary tumor. Diethylstilbestrol (DES) and 17beta-estradiol treatment in female rats significantly decreased the number of dopamine binding sites (B max) from 85 +/- 3.9 fmol/mg protein in untreated rats to 9.2 +/- 1.2 and 8.2 +/- 2.8 fmol/mg protein in DES and 17beta-estradiol-treated rats, respectively, while the binding affinities (Kd) did not change significantly. Testosterone treatment did not change the B max, while ovariectomy resulted in a significant increase in the B max (146.3 +/- 6.7 fmol/mg protein). The effects of DES on the B max were reversible, since removal of the DES for one week before sacrificing the animals led to a marked increase in the B max (54.9 +/- 3.1 fmol/mg protein). Pituitaries from normal female rats treated with DES for 6 and 9 weeks had a significant decrease in the B max. These results show that the number of dopamine binding sites in the membranes of MtT/W15 tumors is decreased by estrogen treatment and that this effect is reversible after removal of the estrogenic stimulus.