Correlation of thyrotropin secretion with morphometric data of secretory granules from individual thyrotropes. An ultrastructural study on rat cells identified by reverse hemolytic plaque assay

We investigated the existence of functional subpopulations of thyrotropes in pituitary glands of male rats, also seeking to establish any correlation of thyrotropin secretion with morphometric data of secretory granules. Pituitary cells secreting thyrotropin were detected using a reverse hemolytic plaque assay. Population analysis of the plaque sizes (TSH secreted) of individual thyrotropes in control cultures from male rats revealed a unimodal frequency distribution. TRH added to the medium resulted in a shift to a bimodal distribution of sizes, consisting of small and large modes, but did not alter the fraction of plaque-forming thyrotropes. The plaque size of control cultures was the same as that of the fraction of TRH-stimulated thyrotropes forming small plaques. Because the extent of hemolysis is related to the amount of thyrotropin secreted by each individual thyrotrope, these results suggest that: a) all thyrotropes secrete thyrotropin even in the absence of a stimulatory dose of TRH; b) the thyrotrope subpopulation secreting small amounts of thyrotropin when incubated with TRH may be unresponsive to this stimulatory factor. After performing reverse hemolytic plaque assay, plaque-forming cells were subsequently processed for electron microscopy. Morphometric analysis of the secretory granules correlated well with plaque size (TSH secreted). It is thus concluded that the functional subtypes of thyrotropes can be recognized on the basis of their ultrastructural images.