Studies on the effects of catecholestrogens on prolactin secretion by cultured normal rat anterior pituitary cells

The catecholestrogens 2-hydroxyesterone (2-OHE1) and 2-hydroxyestradiol (2-OHE2) are shown to directly inhibit PRL release by cultured normal rat anterior pituitary cells if they have been cultured in charcoal-treated estrogen-stripped fetal calf serum. Both catecholestrogens affect PRL release in a dose-dependent bimodal way. 2-OHE1 (1 microM) maximally inhibits PRL release by 38%, but 10 microM of the substance has no effect. In contrast, 0.1 microM 2-OHE2 maximally inhibits PRL release by 37%, while the effect of 1 microM of the compound was significantly less (-26%). The effect of the catecholestrogens on PRL release by cultured pituitary cells was significant after 2 h and was still effective after 48 h. The mechanism of action of 2-OHE1 and 2-OHE2 probably does not involve dopamine receptors, as 50 nM haloperidol did not alter the PRL release inhibitory effect of these substances. Moreover, 2-OHE1 and 2-OHE2 did not affect dopamine-mediated inhibition of PRL release. The total amount of PRL (medium plus cells) present in cultures of normal pituitary cells exposed to 2-OHE1 or 2-OHE2 was identical to that in control cells. TRH (100 nM)-stimulated PRL release from cells exposed to catecholestrogens was significantly higher than that from control cells. Addition of the calcium entry-blocking agent verapamil prevented the inhibitory effect of dopamine on PRL release, but did not prevent the effects of 2-OHE1 and 2-OHE2. Preincubation of cultured normal rat pituitary cells for 30 min with 50 nM estradiol completely prevented the inhibitory effects of 2-OHE1 and 2-OHE2 on PRL release. In conclusion, the acute inhibitory effects of the catecholestrogens 2-OHE1 and 2-OHE2 on PRL release by cultured normal rat pituitary cells do not involve dopamine receptors or calcium transport over the cell membrane. These effects are only demonstrable after culture of the cells in charcoal-treated fetal calf serum and can be completely prevented by short term preincubation with estradiol. Our observations support the suggestion that the action of estradiol on lactotropic cells may be modulated by its metabolites, which have opposite effects on PRL release.