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

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.

Data obtained with chlortetracycline (CTC), a probe of membrane-bound Ca²⁺, were concordant with those obtained by measuring ⁴⁵Ca²⁺ efflux. Cellular fluorescence of CTC varied with the extracellular Ca²⁺ concentration and the duration of incubation. TRH decreased the fluorescence of cell-associated CTC in a manner strongly suggesting stimulus-induced mobilization of Ca²⁺, and this effect was still demonstrable in GH₃ cells incubated in 50 mM K⁺.

These data suggest that TRH acts to mobilize sequestered cell-associated Ca²⁺ reflected as a ⁴⁵Ca²⁺ efflux which is independent of the extracellular Ca²⁺ concentration. Mobilization of sequestered Ca²⁺ into the cytoplasm may elevate free intracellular Ca²⁺ and serve to couple stimulation by TRH to secretion of prolactin.

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