Growth hormone receptors in ovary and liver during gametogenesis in female rainbow trout (Oncorhynchus mykiss)
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Changes of growth hormone receptivity in the ovary during the reproductive cycle were studied in rainbow trout (Oncorhynchus mykiss). A method for characterizing growth hormone receptors in crude ovary homogenate was required for this. Binding of radiolabelled recombinant rainbow trout growth hormone (125I-labelled rtGH) to crude ovary preparation was dependent on ovarian tissue concentration. The sites were specific to growth hormone, with no affinity for prolactins and gonadotrophins. Similar high affinities for125I-labelled rtGH were obtained with crude ovary (4.2 × 109 ± 0.3 mol l−1) and crude liver preparations (4.9 × 109 ± 0.1 mol l−1) at all stages of ovogenesis, and with ovarian membrane preparations (8.2 × 109 mol l−1) tested at the beginning of vitellogenesis. Ovarian growth hormone receptor concentration was highest during the early phases of follicular development (endogenous vitellogenesis: 315–310 fmol g−1 ovary) and decreased regularly during oocyte and follicular growth (exogenous vitellogenesis) to reach a minimal value at oocyte maturation (42 fmol g−1 ovary). In postovulated fish, binding was at a similar level (297 fmol g−1 ovary) to that found in endogenous vitellogenesis. Conversely, the absolute binding capacity of the whole ovary was low from immaturity to early exogenous vitellogenesis (0.1–0.6 pmol per pair of gonads), increased slowly during vitellogenesis and more markedly during rapid oocyte growth and at the time of final maturation (10.8 pmol per pair of gonads). In postovulated fish, the absolute binding capacity decreased partially (4.4 pmol per pair of gonads). Mean hepatic growth hormone receptor concentration did not vary with the reproductive stage for most of the cycle (3.0–4.5 pmol g−1 liver) except in endogenous vitellogenesis where significantly higher concentrations were observed (6.7 pmol g−1 liver). Individual ovarian growth hormone receptor concentrations were correlated with hepatic growth hormone receptor concentrations, indicating that they are regulated in a similar way. We conclude that growth hormone receptors are present in the ovary during the entire ovarian cycle in rainbow trout, probably mainly in somatic cells as indicated by the same concentration of binding sites in immature and in postovulated fish. Growth hormone is potentially important during oocyte recruitment in vitellogenesis and initiation of growth and during final follicular maturation.
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