Morphological and functional heterogeneity of frog melanotrope cells

Previous reports have described the heterogeneity of different pituitary cell types on the basis of morphological and physiological criteria. In the present study, we investigated the possible existence of distinct subpopulations of melanotrope cells in the intermediate lobe of the pituitary of the frog, Rana ridibunda. Separation of dispersed pars intermedia cells in a Percoll density gradient made it possible to isolate two fractions of melanotrope cells whose morphological and functional properties were further characterized. Analysis of the relative volume and number of various cellular organelles showed that high-density cells had a larger number of secretory granules than low-density cells. Concurrently, radioimmunoassay quantification revealed that the concentration of alpha-melanocyte-stimulating hormone (alpha-MSH) was 2 times higher in the heavy cell population. The rate of secretion of alpha-MSH from cultured melanotrophs was significantly higher in low-density than in high-density cells. Thyrotropin-releasing hormone (TRH) was more potent in stimulating alpha-MSH release from the low-density than from the high-density cell subset. In contrast, the response to TRH persisted for a longer time in the high-density cell subpopulation. Taken together, these data demonstrate the existence of two subpopulations of melanotrope cells, and indicate that the low-density cells have a secretory rate substantially greater than high-density cells.