Evidence that Stimulation of Thyrotropin and Prolactin Secretion by Thyrotropin-Releasing Hormone Occur via Different Calcium-Mediated Mechanisms: Studies with Verapamil*

Abstract

TRH stimulation of TSH and PRL secretion requires Ca²⁺. We determined whether influx of extracellular Ca²⁺ was involved in TRH action in TtT cells (mouse pituitary tumor cells in short term culture that produce TSH), in GH₃ cells (clonal rat pituitary tumor cells that produce PRL), and in anterior pituitary cells in vitro derived from normal rats. TRH increased ⁴⁵Ca²⁺ uptake by TtT cells by 40% but had no effect on ⁴⁵Ca²⁺ uptake by GH₃ cells. Verapamil, which totally inhibited ⁴⁵Ca²⁺ uptake by TtT and GH₃ cells induced by depolarization with 50 mM K⁺, was employed to block influx of extracellular Ca²⁺. Verapamil (50 JUM) abolished the increment in ⁴⁵Ca²⁺ uptake by TtT cells caused by TRH. Verapamil had no effect on TRH-stimulated ⁴⁵Ca²⁺ efflux and PRL release from GH₃ cells but partially inhibited TRH-stimulated ⁴⁵Ca²⁺ efflux and TSH release from TtT cells during perifusion. TRH-stimulated PRL release from GH₃ cells was likewise not affected by verapamil during static incubation (7.2 ± 1.1 ng/ml vs. 6.2 ± 1.2 ng), but TRH-stimulated TSH release from TtT cells was inhibited by 64% (from 5.5 ± 0.7 ngeq/ml to 2.0 ± 0.6 ngeq). Similar findings on the effects of verapamil on hormone release were obtained from nonneoplastic anterior pituitary cells; TRH-stimulated PRL release was not affected, but TRH-stimulated TSH release was inhibited by 50%. Hence, TRH did not cause Ca²⁺ influx into mammotropic cells and TRH-stimulated PRL release was not affected when Ca²⁺ influx was blocked by verapamil. In contrast, TRH appeared to cause Ca²⁺ influx into thyrotropic cells, and TRH-stimulated TSH release was partially inhibited when Ca²⁺ influx was blocked by verapamil. We suggest that TRH action in mammotrophs involves only mobilization of cellular Ca²⁺, whereas TRH action in thyrotrophs involves mobilization of cellular Ca²⁺ and influx of extracellular Ca²⁺.