Constitutive and regulated prolactin secretion: effects of estradiol.

We have utilized a combined reverse hemolytic plaque/in situ hybridization assay to assess the relationship between individual cell secretion of prolactin and the level of prolactin gene expression within the same cell. It is thought that cells utilizing predominantly a constitutive pathway of protein secretion will exhibit a direct relationship between the level of secretion and that of gene expression. In contrast, cells that are secreting protein predominantly from stored pools exhibit no relationship between these two parameters since secretion does not depend upon synthesis of new hormone. In addition, regulated protein secretion depends upon calcium; therefore, regulated secretion is inhibited by calcium channel blockers such as nifedipine. We tested whether the presence of estradiol influences the proportion of hormone secreted by the constitutive and regulated pathway by 1) assessing whether estradiol changes the relationship between gene expression and secretion in individual cells and 2) measuring the effects of nifedipine on prolactin secretion. Our data demonstrate that estradiol priming results in a direct relationship between the amount of hormone secreted by an individual lactotroph and the level of prolactin mRNA in the same cell; the findings also show that prolactin secretion from lactotrophs of estradiol-treated rats is not suppressed by nifedipine. Therefore, we conclude that estradiol enhances the proportion of hormone that is secreted via a constitutive pathway.