Thyrotropin-releasing hormone induces opposite effects on Ca2+ channel currents in pituitary cells by two pathways.

Abstract

Thyrotropin-releasing hormone (TRH) stimulates pituitary secretion by steps involving a cytosolic Ca2+ rise. We examined various pathways of Ca2' elevation in pituitary GH3 cells. By using the patch clamp technique in the whole-cell configuration and Ba2+ as divalent charge carrier through Ca2+ channels, TIH (1 uM) reversibly reduced the current by about 55%. This hormonal effect was prevented by infusing guanine 5'-[ji-thioldlphosphate (GDP(IJS]) intracellularly but not by pretreating the cell with pertussis toxin (PT). Since PT-insensitive guanine nucleotide-binding regulatory (G) proteins are known to mediate a hormone-stimulated inositol trisphosphate-mediated Ca2+ release from intracellular stores, we assume that the inhibitory effect of TRH on Ba2+ currents through Ca2+ channels is caused by the increased intracellular Ca2+. To prevent a Ca2+-release-dependent inhibition of Ca2+ channels, we preincubated GH3 cells in a medium free of divalent charge carriers and measured the Na+ current through Ca2+ channels. When fura-2 was used as indicator for the cytosolic Ca2+, TRB induced a release from intracellular stores only once and had no effect on the intracellular Ca2+ concentration during further applications. In line with this observation, TRH initially reduced the Na' current through Ca2+ channels but stimulated it during subsequent applications. The stimulation was sensitive to GDP[.3S] and was abolished by pretreatment with PT, suggesting that the stimulatory action of TRH is mediated by a G protein different from the one that functionally couples the receptor to phosphatidylinositol 4,5-bisphosphate hydrolysis. In conclusion, the rent data suggest that TRH increases the intracellular Ca + concentration by two interacting pathways, that release from intracellular stores causes a secondary blockage of Ca2l channels, and that, especially with empty intracellular Ca2+ stores, Ca2+ channels are stimulated by a PT-sensitive G protein.Pituitary-hormone-releasing factors such as thyrotropinreleasing hormone (TRH), luteinizing-hormone-releasing hormone (LHRH), and angiotensin II (ATII) increase the cytosolic Ca2l in anterior pituitary cells in two successive phases. They first induce an inositol trisphosphatedependent Cal' release (1-6) and then stimulate extracellular Ca2l entry, which is sustained in the presence ofthe releasing factor (7,8). The nature of this retarded and prolonged Ca2+ entry remains unclear (9-12). A biphasic TRH action was also seen in membrane potential recordings. The membrane potential hyperpolarized within the first phase (about 2 min); action potentials induced thereafter occurred at a higher rate than the basal activity (13, 14). The hyperpolarization was attributed to an activation of Ca2l-dependent K+ channels, which may be related to the intracellular Ca2+ mobilization (11,15), and the increased firing pattern was thought to be associated with an inhibition of inwardly rectifying K+ channels (16). However, the TRH-induced inhibition of voltagedependent Ca2l channels previo...

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