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Differential Effect of Insulin-Like Growth Factor I on in Vitro Gonadotropin (I and II) and Growth Hormone Secretions in Rainbow Trout (Oncorhynchus mykiss) at Different Stages of the Reproductive Cycle¹

Institut National de la Recherche Agronomique/Station Commune de Recherches en Ichtyophysiologie, Biodiversité et Environnement, Equipe Croissance et Qualité de la Chair des Poissons, Campus de Beaulieu, 35042 Rennes cedex, France

Address all correspondence and requests for reprints to: Dr. Claudine Weil, INRA/SCRIBE, Equipe Croissance et Qualité de la Chair des Poissons, Campus de Beaulieu, 35042 Rennes cedex, France. E-mail: weil{at}beaulieu.rennes.inra.fr

The short-term effect of insulin-like growth factor I (IGF-I) on GTH I (FSH-like), GTH II (LH-like), and GH production by cultured rainbow trout pituitary cells was studied in immature fish of both sexes, at early gametogenesis and in spermiating and periovulatory animals. IGF-I had no effect on basal GTH I and GTH II release, whereas it always inhibited basal GH, showing decreasing intensity with the gonad maturation. In absence of IGF-I, GTH I and GTH II cells were always responsive to GnRH, whereas no response was observed for GH cells whatever the sexual stage. The action of IGF-I on the sensitivity to GnRH differs between GTH and GH cells. The former requires a coincubation with IGF-I (10^-6 M)/GnRH to show an increase in sensitivity, independent of the sexual stage. To be responsive to GnRH, the GH cells require longer exposure to IGF-I, the efficiency of which decreases with gonad maturation. The action of IGF-I (10^-6 M) on GTH cell sensitivity to GnRH does not seem to be related to a mitogenic effect or to an improvement in cell survival. It seems to be IGF-I specific, not passing via the insulin receptor. Certain hypotheses on the putative role of IGF-I and GnRH as a link between growth and puberty are suggested.

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