To determine the direct effects of estrogen and dopamine on primate PRL secretion, cultures of dispersed monkey pituitary cells were maintained in seum-free medium on an extracellular matrix secreted by bovine corneal endothelial cells, and female monkeys with pituitary stalk-transections were prepared. Dispersed pituitary cells from male and female monkeys were cultured in a 1: 1 mixture of Dulbecco’s Modified Eagle’s Medium H-16 and Ham’s F-12 medium containing insulin, transferrin, selenium, and cadmium with or without 10.0 nm estradiol. When possible, a sister culture was established in 10% charcoal-treated fetal calf serum with or without estrogen. On days 4, 8, and 12 after plating, the incubation medium was replaced with identical medium containing increasing concentrations of dopamine with ascorbate for 6 h. Medium PRL concentrations were determined by RIA. Half-maximal inhibition (IC60) of PRL secretion occurred at about 10.0 nm dopamine with all treatments. Dopamine suppressed PRL secretion to a greater extent on day 4 when estrogen was present in the serumfree medium. There was no difference in the percent inhibition of PRL on days 8 and 12, as the serum-free cultures without estrogen appeared to increase in sensitivity. In cultures maintained in 10% characoal-treated serum, estrogen had little effect on the dose-response inhibition of PRL on days 4 and 8, but by day 12, there was significantly less inhibition at the higher doses of dopamine. Dopamine infusion in pituitary stalk-transected (SS) monkeys caused a dose-related inhibition of PRL secretion. Serum PRL levels decreased 80% with 100 nm dopamine, and the IC50 occurred with 10 nm dopamine. In SS monkeys receiving 2 months of estrogen treatment, dopamine infusion inhibited PRL secretion to a greater extent at the lowest dose of dopamine, although the IC50 did not change. These experiments suggest that estrogen does not decrease the effectiveness of dopamine as an inhibitor of PRL secretion in SS monkeys or from primate pituitary cells cultured for 12 days in serum-free medium. Extended maintenance of cells in medium containing serum may alter this response to dopamine.
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