Estradiol-modified prolactin secretion independently of action potentials and Ca2+ and blockade of outward potassium currents in GH3 cells

Estrogens facilitate prolactin (PRL) secretion acting on pituitary cells. In GH(3) cells, estradiol induces acute action potentials and oscillations of intracellular Ca2+ associated with the secretagogue function. Estradiol modulates several ion channels which may affect the action potential rate and the release of PRL in lactotroph cells, which might depend on its concentration. The aims were to characterize the acute effect of supraphysiological concentrations of estradiol on Ca2+ and noninactivating K+ currents and measure the effect on the spontaneous action potentials and PRL release in the somatolactotroph cell line, GH(3). Electrophysiological studies were carried out by voltage- and current-clamp techniques and ELISA determination of PRL secretion. Pharmacological concentrations of estradiol (above 1 mu M), without a latency period, blocked Ca2+ channels and noninactivating K+ currents, including the large-conductance voltage- and Ca2+-activated K+ channels (BK), studied in whole-cell nystatin perforated and in excided inside-out patches of GH(3) and CHO cells, transiently transfected with the human alpha-pore forming subunit of BK. The effect on BK was contrary to the agonist effect associated with the regulatory beta(1)-subunits of the BK, which GH(3) cells lack, but its transient transfection did not modify the noninactivating current blockade, suggesting a different mechanism of regulation. Estradiol, at the same concentration range, acutely decreased the frequency of action potentials, an expected effect as consequence of the Ca2+ channel blockade. Despite this, PRL secretion initially increased, followed by a decrease in long-term incubations. This suggests that, in GH(3) cells, supraphysiological concentrations of estradiol modulating PRL secretion are partially independent of extracellular Ca2+ influx.