ABSENCE OF A TIGH EFFECT ON ADAPTABILITY TO BRACKISH-WATER IN TILAPIA(OREOCHROMIS-NILOTICUS)

The aim of this study was to investigate the possible role of growth h ormone in the adaptation of tilapia (Oreochromis niloticus) to brackis h water and to analyze its interactions with prolactin in this process . Plasma levels of growth hormone do not change upon transfer to brack ish water. Treatment of intact tilapia in fresh water with growth horm one prior to transfer did not enable the fish to preadapt to brackish water: the duration of the hydromineral imbalance after transfer was t he same in treated animals and controls. The major osmoregulatory role of prolactin in fresh water led us to test the hypothesis that prolac tin might antagonize the effect of growth hormone on adaptation to bra ckish water. Growth-hormone-treated hypophysectomized animals, however , exhibited no increased osmoregulatory capacity as compared to hypoph ysectomized controls, confirming the absence of a growth-hormone-relat ed osmoregulatory effect. When prolactin and growth hormone were coinj ected, growth hormone also proved unable to oppose the Na+ retaining e ffect of prolactin, in both brackish and fresh water. Surprisingly, hy pophysectomized animals adapt better to brackish water than do sham-op erated animals. This result is discussed in light of the effects of pr olactin and cortisol on osmoregulation in brackish water and we sugges t that an important event which allows O. niloticus to adapt to hypero smotic environment is the reduction of plasma PRL upon transfer to bra ckish water. (C) 1995 Academic Press, Inc.