Osmoregulatory actions of growth hormone and prolactin in an advanced teleost

To date, growth hormone (GH) is known to contribute to seawater adaptation only in salmonid fishes (primitive Euteleostei). Accordingly, the effects of homologous GH and two forms of homologous prolactin (PRL₁₇₇ and PRL₁₈₈) on hypoosmoregulatory ability and gill Na⁺,K⁺-ATPase activity in a more advanced euryhaline cichlid fish, the tilapia (Oreochromis mossambicus), were examined. Following adaptation of hypophysectomized fish to 25% seawater for 3 weeks, fish were given four injections of hormone or vehicle. They were then exposed to 100% seawater for 12 hr and examined for changes in plasma osmolality. Tilapia GH (0.02 and 0.2 μg/g) significantly improved the ability of tilapia to decrease plasma osmolality following transfer to full-strength seawater, in a dose-related manner. Growth hormone treatment also significantly stimulated gill Na⁺,K⁺-ATPase activity (0.5 μg/g). Both tilapia PRLs (PRL₁₇₇ and PRL₁₈₈) increased plasma osmolality in 100% seawater and reduced gill Na⁺,K⁺-ATPase activity, the effects induced by PRL₁₈₈ being more significant than those by PRL₁₇₇. Thus, GH may be involved in seawater adaptation of tilapia, a species belonging to the most advanced teleost superorder (Acanthopterygii), whereas both PRLs in tilapia are not involved in seawater adaptation.