Discordance of prolactin gene transcription, mRNA storage, and hormone release in individual mammotropes.

The mammotrope has traditionally been a favored model for studies of hormonal gene expression, biosynthesis, and release. However, the primary site(s) at which these processes are coordinated and integrated remains to be established. Because there is considerable indirect evidence to suggest that the rate of prolactin (PRL) secretion is dictated, in large part, at the level of transcription, the relative contribution of other putative regulatory foci has received less attention. The purpose of the present study was to test the primacy of transcriptional regulation at the single-cell level. To this end, we quantified within individual mammotropes the relationship between PRL gene transcription, mRNA storage, and hormone release. This was accomplished by the combined application of "real-time" measurement of gene expression, in situ hybridization cytochemistry, and reverse hemolytic plaque assay, respectively. Our results demonstrate a quantitative dissociation among these variables, suggesting that control mechanisms besides transcription play a primary role in integrating and coordinating flow through the PRL secretory pathway.