Inhibitors of 1,2-diacylglycerol kinase potentiate the TRH-induced stimulation of Ca2+-activated K+ current.

Transient activation of the outward K+ current caused by a rise in the cytosolic free Ca2+ concentration, [Ca2+]i was the predominant change in plasma membrane ion flux during the first phase of thyrotropin-releasing hormone (TRH) action on pituitary cells. Following the intracellular application of inhibitors of 1,2-diacylglycerol (DG) kinase, R59022 and 1-oleyl-2-acetyl glycerol (OAG) the outward K+ current response to TRH in cells of the pituitary line GH3B6 was potentiated. This potentiation was analyzed further with the combination of microfluorimetric and electrophysiological recording techniques. Receptor-induced changes in [Ca2+]i and ion channel activation were monitored simultaneously in the same cell. It was found that R59022 and OAG altered in parallel the TRH-induced transient rise in [Ca2+]i and outward K+ current. This resulted in a significant correlation between the kinetic parameters (speed of onset, duration) of the [Ca2+]i and the K+ current responses to TRH. Intracellular application of vanadate abolished the rapid start of the TRH response presumably by its block of Ca2+ uptake into the endoplasmic reticulum, leading to depletion of a Ca2+ pool mobilizable by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). The use of vanadate unmasked a slowly developing response to TRH, which was still potentiated by OAG and R59022. Together, these observations suggest that Ca2+ mobilization during the first phase of TRH action is mediated by two distinct processes, one of which is linked to receptor stimulation of DG production.