MORPHOFUNCTIONAL ASPECTS OF MELANOTROPE CELL HETEROGENEITY by J.L. GONZALEZ DE AGUILAR, AJ. MARTINEZ-FUENTESa, M.C. TONONa, H. VAUDRYa, M.M. MALAGON and F. GRACIA-NAVARRO (Department of Cell Biology, University of Córdoba, Spain,a European Institute for Peptide Research, Laboratoire de Neuroendocrinologie Cellulaire el Moleculaire, INSERM U413, UA CNRS, Univer- sity of Rouen, France) SUMMARY New data show that differential responses to physiological stimuli by cells of one endocrine cell type depend on differential morphological and physiological characteris- tics of subpopulations of cells. In this work, we present evidence for the existence of cell heterogeneity in the frog intermediate pituitary lobe, in both morphological and physi- ological aspects. KEY WORDS: cell heterogeneity, α-melanocyte stimulating hormone, amphibia, mor- phometry, chromatography, thyrotropin-releasing hormone, calcium. Different approaches have shown that melanotropes are heterogeneous with regard to morphology (DE RiJK et al., 1990), secretory activity (CHILDS, 1990) and expression of proopiomelanocortin mRNA (CHRONWALL Et al., 1987). In the present study, we have characterized two distinct subpopulations of melanotrope cells in the pars intermedia of the frog pituitary gland. After separating dispersed cells with a continuous Percoll density gradient (GONZALEZ DE AGUILAR Et al., 1994) two subsets of melanotropes, referred to as high- and low-density cells, were distinguished, representing 21% and 52%, respectively of all melanotropes recovered from the gradient. Ultrastructural analysis revealed that the main difference between the two cell subpopulations is the content of secretory granules (sg), which is higher in high-density melanotropes 30.00±2.49 in low-density ones). In contrast, the development of the rough endoplasmic reticulum and the Golgi complex is similar in the two cell types. There were also no differences in cell size. The spontaneous release of hormone from dispersed melanotropes during a 48-hour period of culture was quantified by means of a specific radioimmunuoassay for frog oc,-melanocyte stimulating hor- mone (aMSH) (VAUDRY el al., 1978), low-density melanotropes show- ing an almost 2-fold increase in secretory activity compared to high- density ones. In order to evaluate the rate of (XMSH acetylation, a