The intracellular calcium antagonist, TMB-8, inhibits prolactin gene expression in GH3 cells.

We examined the effects of the drug, TMB-8, which promotes sequestration of intracellular Ca2+, on the ability of extracellular Ca2+ to stimulate prolactin gene expression in GH3 cells. TMB-8 inhibited prolactin mRNA levels in a dose-dependent manner in the concentration range of 2.5-10 microM. Prolactin mRNA levels were increased about 18-fold by the addition of 0.1 mM CaCl2, and about 25-fold by the addition of 0.4 mM CaCl2. Addition of 10 microM TMB-8 reduced these levels to about 4-fold and 7-fold, respectively. At 10 microM TMB-8 did not effect total protein synthesis or the Ca2+-induced aggregation of the cells, indicating a selective inhibition by the drug of prolactin gene expression. Both TMB-8 and the calmodulin inhibitor, calmidazolium, reversed the effects of Ca2+ on prolactin mRNA levels in cells that had been pretreated for 2 days with 0.4 mM CaCl2.