DYNAMIC FLUCTUATIONS IN THE SECRETORY ACTIVITY OF INDIVIDUAL LACTOTROPES AS DEMONSTRATED BY A MODIFIED SEQUENTIAL PLAQUE-ASSAY

The existence of differences in the amount of hormone released by indi vidual lactotropes from the same pituitary gland has been unequivocall y established. Such secretory heterogeneity could reflect the existenc e of lactotrope subtypes that differ quantitatively in their capacity to release constant amounts of hormone over time. Alternatively, the s ame pattern could arise from the intermittent secretion of hormone by individual cells. In an attempt to distinguish between these possibili ties, we used a novel version of a sequential plaque assay to monitor daily the amount of PRL secreted by the same individual cells over a p eriod of 4 days. When more than 1200 cells from male and lactating fem ale rats were analyzed in this manner, we found that the average amoun t of hormone released over time was relatively constant within gender and that (as anticipated) cells from females released 2.6-fold more PR L than did those from males. However, assessment of secretory variabil ity by individual lactotropes revealed that male cells were considerab ly more irregular in their pattern of PRL release than were their fema le counterparts. Indeed, only 11.8 +/- 2.5% (mean +/- SEM) of male cel ls secreted relatively constant amounts of hormone over the 4-day peri od, whereas 43.4 +/- 8.4% of the female cells exhibited a similar prof ile (assessed by comparing coefficients of variation for individual ce lls). Interestingly, this variability in secretion seemed to be invers ely related to the amount of hormone released; constant secretors had a much higher average plaque size (2.5-fold greater; P < 0.01) than ir regular secretors. Taken together, our results demonstrate that indivi dual lactotropes from the same animal can release hormone in either a constant or episodic manner, and that the gender of the pituitary dono r can have a strong influence on the relative abundance of each phenot ype. Moreover, our data suggest the former phenotype makes a greater c ontribution to overall PRL secretion than the latter.