Studies on human chorionic gonadotropin secretion: effects of EGTA, lanthanum, and the ionophore A23187

Abstract

Human malignant trophoblast cells that secrete human chorionic gonadotropin (hCG) in culture were employed to assess the calcium requirement for hormone production. Cellular and secreted hCG was measured by radioimmunoassay. Cells cultured for 7 h in Ca2+- and Mg2-free medium or in Ca2+-free medium, secreted less hCG than cells cultured in medium containing Ca2+. Addition of ethylene glycol bis (beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA), ethylenediamine tetraacetic acid (EDTA), or La2+ inhibited hCG release from the cells, but did not affect the amount of hCG in the cells. Inhibition of hCG release was dependent on time of incubation and on concentration of agent added. Inhibition of hCG secretion by EGTA was reversed upon removal of the EGTA from the culture fluid or by addition of equimolar Ca2+ to the fluid. These results demonstrated that divalent cations, probably Ca2+, are required for release and further synthesis of hCG. Addition of the divalent cation ionophore A23187 (0.1 to 10 muM) failed to increase hCG secretion by the malignant trophoblast, in contrast to the stimulatory effect of this agent in other secretory systems. Incubation of the cells for 15 to 60 min with 10 muM A23187 reduced hCG secretion, and this inhibition was reversed upon removal of the ionophore from the culture fluid. The studies with the ionophore supported other evidence indicating basic differences between hormone secretory mechanisms in the trophoblast compared to other endocrine tissues.