Thapsigargin, but not caffeine, blocks the ability of thyrotropin-releasing hormone to release Ca2+ 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 in 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.
Law
G J
GJ
Department of Pharmacology, Yale University School of Medicine, New Haven, CT 06510.
Pachter
J A
JA
Thastrup
O
O
Hanley
M R
MR
Dannies
P S
PS
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D007295:Inositol Phosphates / Q000378:metabolism; D007700:Kinetics; D009553:Nimodipine / Q000494:pharmacology; D010911:Pituitary Neoplasms; D010946:Plants, Medicinal; D011388:Prolactin / Q000378:metabolism; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin; D013973:Thyrotropin-Releasing Hormone / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism*
D007295:Inositol Phosphates; D013729:Terpenes; D002110:Caffeine; D009553:Nimodipine; D013973:Thyrotropin-Releasing Hormone; D019284:Thapsigargin; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D011388:Prolactin; D002118:Calcium