EFFECTS OF ACUTE SALINITY AND ALKALINITY CHANLLENGES ON PLASMA OSMILALITY, ION CONCENTRATION, GILL ION TRANSPORTERS ACTIVITY AND GENE EXPRESSION IN RED TILAPIA  

Fishes possessed the osmoregulatory abilities to maintain homeostasis when encountering unfavorable environments. In this study, Red tilapia were directly transferred from freshwater to saline water (SW), alkaline water (AW) and saline-alkaline water (S&AW), respectively, the responded changes of plasma osmolality, plasma ion ([Na⁺], [K⁺], [Cl^-], [Ca²⁺]), gill ion-transporters (Na⁺/K⁺-ATPase, NKA; Na⁺/K⁺/2Cl^- cotransporter, NKCC; Cl^- channel, cystic fibrosis transmembrane conductance regulator; CFTR; carbonic anhydrase, CA) activity and their mRNA expression in each treatment were determined during 96 h time course.

The results showed plasma osmolality in SW, AW and S&AW treatments showed a similar change with an early increase, peaking at 12 h, and followed by a reduction. Plasma [Na⁺], [Cl^-] and [K⁺] concentration increased in SW at 12 h post transfer, [Ca²⁺] increased significantly in AW, whereas they all increased significantly in S&AW. Ionic concentration change trend in each treatment was consistent to its osmolality change. The increase of NKA (12h), NKCC (24h), CFTR (48h) activity in gill were observed in SW, CA (24 h) activity increase in AW, whereas they all occurred in S&AW. Different activity responses in SW, AW and S&AW clearly showed these transporters were selectively involved in ion transportation. Meanwhile, elevation of mRNA expression of NKA, NKCC and CFTR in SW, CA in AW, and four genes in S&AW supported their activity change. In conclusion, our comparative work suggested there were heavy ionic load and complicated ion compositions in saline-alkaline fish, a multiple osmoregulatory activities including ion transportation and acid-base balance, would be invoked for saline-alkaline adaption.