Osmoregulatory effects of hypophysectomy and homologous prolactin replacement in hybrid striped bass

The effects of ovine prolactin (oPRL) and striped bass prolactin (sbPRL; Morone saxatilis) on plasma osmolality, electrolyte balance, and gill Na+,K+-ATPase activity were investigated in hypophysectomized (Hx), freshwater (FW)-acclimated, hybrid striped bass (M. saxatilis×Morone chrysops). They were kept in dilute (isoosmotic) seawater for about 10 days after surgery. Seven days after transfer to FW, Hx fish had lower plasma osmolality and lower levels of Na+, Cl−, and Ca2+ than sham-operated and intact fish. Fish were injected four times with oPRL (1, 5, or 20 μg/g body mass), sbPRL (10 or 100 ng/g), or hormone vehicle (0.9% NaCl) at 48-h intervals (days 0, 2, 4, and 6) in FW and then sampled for blood plasma 24 h after the fourth injection (day 7). In Hx fish, oPRL (5 and 20 μg/g) and sbPRL (10 and 100 ng/g) were effective in maintaining plasma osmolality and levels of Na+, Cl−, and Ca2+ above values seen in saline-injected controls. Hypophysectomy did not affect branchial Na+,K+-ATPase activity, but enzyme activity was significantly reduced in Hx fish receiving oPRL (20 μg/g) or sbPRL (10 or 100 ng/g). These results indicate that PRL acts to maintain plasma osmotic and ionic balance in FW-adapted hybrid striped bass, and that this may involve downregulation of branchial Na+,K+-ATPase activity.