Growth regulation in the gobiid teleost, Gillichthys mirabilis: roles of growth hormone, hepatic growth hormone receptors and insulin-like growth factor-I.
Studies of the teleost Gillichthys mirabilis were undertaken to assess the role of GH in regulating hepatic GH receptors, insulin-like growth factor-I (IGF-I) activity and cartilage growth. Hypophysectomized G. mirabilis were injected with saline vehicle or with 10, 100 or 1000 ng tilapia GH (tGH)/g every other day for 2-3 weeks. Growth, judged by increased body weight and length, was inhibited by hypophysectomy and stimulated by increasing tGH doses. Hepatic GH receptors, measured by 125I-labelled tGH binding, were decreased 50% by hypophysectomy. An additional dose-dependent reduction in binding was observed 24 h after tGH injection, but MgCl2 stripping of membranes suggested receptor occupation by exogenous tGH. IGF-I activity, measured by 35SO4 incorporation into oral cartilage explants in vitro, was decreased 50% by hypophysectomy and increased to 200% of intact control levels after injection of 1000 ng tGH/g. In a second experiment, 35SO4 incorporation by oral cartilage from hypophysectomized fish injected with 10, 100 or 1000 ng tGH/g was stimulated to intact control levels. Effects of feeding and tGH injection on in-vitro responsiveness of oral cartilage to recombinant bovine IGF-I (rbIGF-I; 10-1000 ng/ml) were also assessed. Oral cartilage from fed fish showed a parabolic dose-response curve, whereas oral cartilage from starved fish had a lower basal rate of 35SO4 incorporation and a linear dose-response relationship. Oral cartilage from hypophysectomized G. mirabilis showed a significantly attenuated response to rbIGF-I which was restored by tGH injection, suggesting that the GH status of the animal is important for sensitivity of target tissue to IGF-I. Because of its similarity to other vertebrate systems, G. mirabilis presents a good teleost model of growth regulation and of the functions and interactions of GH and IGF-I.
Gray
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Department of Integrative Biology, University of California, Berkeley 94720.
Kelley
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D000818:Animals; D001835:Body Weight / Q000187:drug effects; D002356:Cartilage / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005399:Fishes / Q000502:physiology*; D013006:Growth Hormone / Q000494:pharmacology / Q000502:physiology*; D007016:Hypophysectomy; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000502:physiology*; D008099:Liver / Q000187:drug effects / Q000502:physiology*; D011986:Receptors, Somatotropin / Q000187:drug effects / Q000502:physiology*
D011986:Receptors, Somatotropin; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone