Thyroliberin stimulates rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate by a phosphodiesterase in rat mammotropic pituitary cells. Evidence for an early Ca2+-independent action

Thyrotropin-releasing hormone (TRH; thyroliberin) stimulated rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate PtdIns(4,5)P2 by a phosphodiesterase (phospholipase C) in GH3 cells, a prolactin-secreting rat pituitary tumour cell line. TRH caused a rapid decrease in the level of PtdIns(4,5)P2 to 60% of control and stimulated a marked transient increase in inositol 1,4,5-trisphosphate, the unique product of phosphodiesteratic hydrolysis of PtdIns(4,5)P2, to a peak of 410% of control at 15 s. TRH also caused decreases in phosphatidylinositol 4-monophosphate (PtdIns4P) and phosphatidylinositol (PtdIns) to 65% and 93% of control at 15 s respectively. Inositol 1,4-bisphosphate was increased to a peak of 450% at 30 s; inositol 1-monophosphate and inositol were not elevated until 30 s and 1 min respectively after TRH addition. To study whether PtdIns(4,5)P2 hydrolysis may be caused by an elevation in cytosolic Ca2+ concentration, the changes induced by TRH in the levels of inositol sugars were compared with the effects of membrane depolarization by high extracellular K+. The elevation in cytosolic Ca2+ induced by K+ depolarization did not change the level of inositol 1,4,5-trisphosphate. These data suggest that phosphodiesteratic hydrolysis of PtdIns(4,5)P2 may be the initial event in TRH stimulation of inositol lipid metabolism in GH3 cells and that PtdIns(4,5)P2 hydrolysis is not stimulated by an elevation in cytosolic Ca2+ concentration. The decreases in PtdIns4P and PtdIns may be due to enhanced conversion of PtdIns into PtdIns4P into PtdIns(4,5)P2 or to their direct hydrolysis by phosphomonoesterases and/or phosphodiesterases. These results are consistent with the hypothesis that TRH-stimulated PtdIns(4,5)P2 breakdown causes Ca2+ mobilization leading to prolactin secretion.