Calcium dependence of prolactin mRNA accumulation in GH3 rat pituitary tumour cells.

Stimulation of prolactin secretion by TRH probably involves mobilization of intracellular calcium to a greater extent than calcium influx, but less is known about the possible calcium-dependent mechanisms which may control prolactin gene transcription. We have studied this question in the rat pituitary tumour GH3 cell line by measuring prolactin mRNA accumulation by a cytoplasmic dot hybridization assay. Cobalt chloride, an intracellularly acting calcium antagonist, caused marked dose-dependent reductions in prolactin release and mRNA concentrations, whereas the calcium channel-blocking agent verapamil had no effect on prolactin release and had smaller effects on prolactin mRNA. Cobalt chloride abolished the stimulatory effect of TRH on prolactin mRNA levels, while verapamil caused only moderate inhibition. Growth hormone mRNA levels in the same cells were not significantly affected by TRH or verapamil, and only marginally reduced by cobalt. These data suggest that, as for prolactin release from normal rat pituitary lactotrophs, prolactin mRNA accumulation in GH3 cells appears to have a requirement for intracellular calcium which is only partly dependent upon calcium influx.