Voltage-dependent calcium channels in pituitary cells in culture. II. Participation in thyrotropin-releasing hormone action on prolactin release.

Depolarization of membrane potential by high external K+ activates Ca2+ influx via voltage-dependent Ca2+ channels in GH4C1 cells (Tan, K.-N., and Tashjian, A. H., Jr. (1983) J. Biol. Chem. 258, 418-426). The involvement of this channel in thyrotropin-releasing hormone (TRH) action on prolactin (PRL) release was assessed by comparing the pharmacological characteristics of TRH-induced PRL release with PRL release due to high K+. Two components of TRH-stimulated PRL release were detected. The major component (approximately equal to 75%) was dependent on external Ca2+ concentration and was inhibited by voltage-dependent Ca2+ channel blockers in a manner quantitatively similar to high K+-stimulated PRL release. The minor component (approximately equal to 25%) of TRH-stimulated PRL release was insensitive to voltage-dependent Ca2+ channel blockers and could occur in the presence of low external Ca2+ (10(-5)-10(-7) M). Neither voltage-dependent Ca2+ channel blockers nor depletion of medium Ca2+ prevented the action of TRH on mobilizing cell-associated 45Ca2+ from GH4C1 cells. Divalent cations that permeate voltage-dependent Ca2+ channels (Sr2+ and Ba2+) substituted for Ca2+ in supporting high K+- and TRH-stimulated PRL release while Mg2+, a nonpermeant cation, did not. We conclude that TRH stimulates PRL release by increasing [Ca2+]i through at least two mechanisms: one requires only low [Ca2+]o, the second involves Ca2+ influx via voltage-dependent Ca2+ channels. This latter mechanism accounts for approximately equal to 75% of maximum TRH-induced PRL release.