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Two Frog Melanotrope Cell Subpopulations Exhibiting Distinct Biochemical and Physiological Patterns in Basal Conditions and under Thyrotropin-Releasing Hormone Stimulation¹

Department of Cell Biology, University of Córdoba (J.L.G.d.A., M.M.M., R.M.V.M., F.G.N.), Córdoba, Spain; and European Institute for Peptide Research, Institut Fédératif de Recherches Multidisciplinaires sur les Peptides 23, Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U-413, Unité Affiliée au Centre National de la Recherche Scientifique, University of Rouen (I.L., M.C.T., H.V.), Mont-Saint-Aignan, France

Address all correspondence and requests for reprints to: Dr. Francisco Gracia-Navarro, Department of Cell Biology, Faculty of Sciences, Avda. San Alberto Magno s/n, University of Córdoba, E-14004 Córdoba, Spain. E-mail: bc1grnaf{at}uco.es

Cell heterogeneity designates the phenomenon by which a particular cell type is composed of morphologically and physiologically distinct cell subpopulations. We have previously isolated two subsets of melanotrope cells in the intermediate lobe of the frog pituitary by means of a separation procedure based on a Percoll density gradient. High density (HD) melanotrope cells were found to exhibit a more granulated cytoplasm and a lower secretory rate than low density (LD) cells. In the present study, we have investigated the biochemical and functional characteristics of each melanotrope cell subpopulation by using various approaches, including chromatographic analysis for the measurement of the proportion of acetylated MSH, microfluorimetric measurement of the cytosolic free calcium concentration ([Ca²⁺]_i), and in situ hybridization for quantification of POMC messenger RNA (mRNA). Under basal conditions, LD melanotrope cells showed higher secretory activity, acetylation rate, [Ca²⁺]_i, and POMC mRNA content compared to HD cells. Incubation of the cells with 100 nM TRH for 2 h induced a more pronounced activation of MSH secretion, [Ca²⁺]_i mobilization, and POMC mRNA accumulation in LD than in HD melanotrope cells. Conversely, TRH increased the rate of acetylation of MSH in HD cells, but did not affect acetylation in LD cells. Taken together, these results demonstrate that the frog intermediate lobe is composed of two subsets of endocrine cells with distinct biochemical and functional characteristics. The coexistence of two cell subpopulations in the frog pars intermedia is consistent with the idea of a cell secretory cycle, in which each melanotrope subset represents a specific state of cellular activity.

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