Effects of gonadotropin-releasing hormone (GnRH) analogs and sex steroids on growth hormone (GH) secretion and growth in common carp (Cyprinus carpio) and grass carp (Ctenopharyngodon idellus)

The effects of mammalian GnRH analog (D-Ala(6), Pro(9)-NEt-LHRH), salmon GnRH, (Trp(7), Leu(8))-LHRH and its analog, (D-Arg(6), Pro(9)-NEt)-sGnRH on growth hormone (GH) release were examined using a perifusion system for pituitary fragments of the common carp (Cyprinus carpio). Perifusion of 2 min pulses of different concentrations of LHRH-A, sGnRH or sGnRH-A stimulated a rapid and dose-dependent increase in GH release; sGnRH-A showed a significantly greater effect than LHRH-A in stimulating GH release. Administration of LHRH-A in the diet (1 or 10 mu g g(-1) diet) for 5 weeks resulted in a highly significant increase in the growth rate of juvenile grass carp (Ctenopharyngodon idellus). Implantation of oestradiol potentiated basal GH secretion in sexually-mature (pre-spawning) and sexually-regressed fish, whereas testosterone implantation did not affect the serum GH level. Both testosterone and oestradiol treatment potentiated the serum GH response to sGnRH-A; however, testosterone was significantly less potent than oestradiol in this respect. The serum GH response to sGnRH-A after testosterone and oestradiol implantation varied seasonally, with greater response in sexually-mature and sexually-regressed fish than those in early stages of ovarian development. These results suggested that sex steroids probably potentiate GH secretion via the influence on the action of GnRH on GH release.