Pituitary control of branchial NCC, NKCC and Na+, K+-ATPase α-subunit gene expression in Nile tilapia, Oreochromis niloticus

This study investigated endocrine control of branchial ionoregulatory function in Nile tilapia (Oreochromis niloticus) by prolactin (Prl(188) and Prl(177)), growth hormone (Gh) and cortisol. Branchial expression of Na+/Cl- cotransporter (ncc) and Na+/K+/2Cl(-) cotransporter (nkcc) genes were employed as specific markers for freshwater- and seawater-type ionocytes, respectively. We further investigated whether Prl, Gh and cortisol direct expression of two Na+, K+-ATPase (nka)-alpha 1 subunit genes, denoted nka-alpha 1a and nka-alpha 1b. Tilapia transferred to fresh water following hypophysectomy failed to adequately activate gill ncc expression; ncc expression was subsequently restored by Prl replacement. Prl(188) and Prl(177) stimulated ncc expression in cultured gill filaments in a concentration-related manner, suggesting that ncc is regulated by Prl in a gill-autonomous fashion. Tilapia transferred to brackish water (23 aEuro degrees) following hypophysectomy exhibited a reduced capacity to up-regulate nka-alpha 1b expression. However, Gh and cortisol failed to affect nka-alpha 1b expression in vivo. Similarly, we found no clear effects of Gh or cortisol on nkcc expression both in vivo and in vitro. When considered with patterns previously described in euryhaline Mozambique tilapia (O. mossambicus), the current study suggests that ncc is a conserved target of Prl in tilapiine cichlids. In addition, we revealed contrasting dependencies upon the pituitary to direct nka-alpha 1b expression in hyperosmotic environments between Nile and Mozambique tilapia.