Thyrotropin-releasing Hormone Stimulation of Prolactin Release from Clonal Rat Pituitary Cells: EVIDENCE FOR ACTION INDEPENDENT OF EXTRACELLULAR CALCIUM

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Abstract

Thyrotropin-releasing hormone (TRH) stimulates prolactin release and 45Ca2+ efflux from GH3 cells, a clonal strain of rat pituitary cells. Elevation of extracellular K+ also induces prolactin release and increases 45Ca2+ efflux from these cells. In this report, we distinguish between TRH and high K+ as secretagogues and show that TRH-induced release of prolactin and 45Ca2+ is independent of the extracellular Ca2+ concentration, but the effect of high K+ on prolactin release and 45Ca2+ efflux is dependent on the concentration of Ca2+ in the medium. The increment in 45Ca2+ efflux induced by 50 mM K+ during perifusion was reduced in a concentration-dependent manner by lowering extracellular Ca2+ from 1,500 to 0.02 μM (by adding EGTA), whereas 1 μM TRH enhanced 45Ca2+ efflux similarly over the entire range of extracellular Ca2+ concentrations. Although 50 mM K+ caused release of 150 ng prolactin from 40 × 106 GH3 cells exposed to 1,500 μM Ca2+ (control), reduction of extracellular Ca2+ to 2.8 μM decreased prolactin release caused by high K+ to <3% of controls and no prolactin release was detected after exposure to 50 mM K+ in medium with 0.02 μM free Ca2+. In contrast, TRH caused release of 64 ng of prolactin from 40 × 106 GH3 cells exposed to medium with 1,500 μM Ca2+, and release caused by TRH was still 50 and 35% of control in medium with 2.8 and 0.02 μM Ca2+, respectively. Furthermore, TRH transiently increased by 10-fold the fractional efflux of 45Ca2+ from GH3 cells in static incubations with 1,500 or 3.5 μM Ca2+, hereby confirming that the enhanced 45Ca2+ efflux caused by TRH in both low and high Ca2+ medium was not an artifact of the perifusion system.

Authors

Marvin C. Gershengorn, Sylvia T. Hoffstein, Mario J. Rebecchi, Elizabeth Geras, Brian G. Rubin