THAPSIGARGIN, BUT NOT CAFFEINE, BLOCKS THE ABILITY OF THYROTROPIN-RELEASING-HORMONE TO RELEASE CA-2+ FROM AN INTRACELLULAR STORE IN GH4C1 PITUITARY-CELLS

Thapsigargin stimulates an increase of cytosolic free Ca2+ concentration ([Ca2+](c)) in, and 45Ca2+ efflux from, a clone of GH4C1 pituitary cells. This increase in [Ca2+](c) was followed by a lower sustained elevation of [Ca2+](c), which required the presence of extracellular Ca2+, and was not inhibited by a Ca2+-channel blocker, nimodipine. Thapsigargin had no effect on inositol phosphate generation. We used thyrotropin-releasing hormone (TRH) to mobilize Ca2+ from an InsP3-sensitive store. Pretreatment with thapsigargin blocked the ability of TRH to cause a transient increase in both [Ca2+](c) and 45Ca2+ efflux. The block of TRH-induced Ca2+ mobilization was not caused by a block at the receptor level, because TRH stimulation of InsP3 was not affected by thapsigargin. Rundown of the TRH-releasable store by Ca2+-induced Ca2+ release does not appear to account for the action of thapsigargin on the TRH-induced spike in [Ca2+](c), because BAY K 8644, which causes a sustained rise in [Ca2+](c), did not block Ca2+ release caused by TRH. In addition, caffeine, which releases Ca2+ from intracellular stores in other cell types, caused an increase in [Ca2+](c) in GH4C1 cells, but had no effect on a subsequent spike in [Ca2+](c) induced by TRH or thapsigargin. TRH caused a substantial decrease in the amount of intracellular Ca2+ released by thapsigargin. We conclude that GH4C1 cells thapsigargin actively discharges an InsP3-releasable pool of Ca2+ and that this mechanism alone causes the block of the TRH-induced increase in [Ca2+](c).