Evidence for Bidirectional Interconversion of Mammotropes and Somatotropes: Rapid Reversion of Acidophilic Cell Types to Pregestational Proportions after Weaning*

The progression from nonpregnancy through late lactation is associated with an increase in the proportion of anterior pituitary cells that secrete PRL and a comparable decrease in the percentage of cells that release GH. These fluctuations result from variations in both the number of cells that release GH or PRL alone and mammosomatotropes, cells that release both hormones concurrently. However, it has not been determined whether this reciprocal shift in PRL and GH secretors during the onset of lactation is readily reversible. In the present study, anterior pituitaries from adult virgin, late lactating, or postweaning female rats (4, 6, or 8 days) were dispersed with trypsin and subsequently assayed for PRL and GH release using reverse hemolytic plaque assays. We found that separating lactating females from their pups for only 4 days induced a reciprocal shift in the proportions of GH and PRL cells back to levels found in virgin females. Simultaneous plaque assays were then performed to determine whether this post-weaning shift in the percentages of GH and PRL secretors was due to changes in the abundance of cells that secrete each hormone alone or in the proportion of mammosomatotropes. The overall changes in GH and PRL cell proportions consisted of variations only in the fraction of cells that secreted either GH or PRL alone; no differences were observed in the percentage of mammosomatotropes or in the overall abundance of acidophils. Our results demonstrate that the reciprocal shifts in the proportions of PRL- and GH-secreting cells associated with lactation are rapidly reversible. Moreover, these results are consistent with our hypothesis that PRL and GH secretors are functionally interconvertible and further suggest that this process is bidirectional.