Acute salinity challenges in Mozambique and Nile tilapia: differential responses of plasma prolactin, growth hormone and branchial expression of ion transporters.

The responses of Mozambique and Nile tilapia acclimated to fresh water (FW) and brackish water (BW; 17 per thousand) were compared following acute salinity challenges. In both species, plasma osmolality increased to above 450 mOsm by 2h after transfer from FW to seawater (SW); these increases in osmolality were accompanied by unexpected increases in plasma prolactin (PRL). Likewise, PRL receptor gene expression in the gill also increased in both species. In Nile tilapia, hyperosmotic transfers (FW to BW and SW) resulted in increased plasma growth hormone (GH) and in branchial GH receptor gene expression, responses that were absent in Mozambique tilapia. Branchial gene expression of osmotic stress transcription factor 1 (OSTF1) increased in both species following transfer from FW to SW, whereas transfer from BW to SW induced OSTF1 expression only in the Nile tilapia. Branchial expression of Na(+)/Cl(-) cotransporter was higher in FW in both species than in BW. Branchial gene expression of Na(+)/K(+)/2Cl(-) cotransporter (NKCC) increased after transfer from BW to SW in Mozambique tilapia, whereas expression was reduced in the Nile tilapia following the same transfer. The difference in the SW adaptability of these species may be related to a limited capacity of Nile tilapia to up-regulate NKCC gene expression, which is likely to be an essential component in the recruitment of SW-type chloride cells. The differential responses of GH and OSTF1 may also be associated with the disparate SW adaptability of these two tilapiine species.