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 cycle.
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.
Weil
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Institut National de la Recherche Agronomique/Station Commune de Recherches en Ichtyophysiologie, Biodiversité et Environnement, Equipe Croissance et Qualité de la Chair des Poissons, France. weil@beaulieu.rennes.inra.fr
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Breton
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D000818:Animals; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D006065:Gonadotropins, Pituitary / Q000378:metabolism*; D013006:Growth Hormone / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008297:Male; D017686:Oncorhynchus mykiss / Q000502:physiology*; D012098:Reproduction / Q000502:physiology*
D006065:Gonadotropins, Pituitary; C057763:gonadotropin- pituitary, beta-subunit I, salmon; D007987:Gonadotropin-Releasing Hormone; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone