Variation in pH, HCO3−, carbonic anhydrases, and HCO3− transporters in Nile tilapia during carbonate alkalinity stress

Metabolite changes in Nile tilapia in response to carbonate alkalinity stress were investigated by transferring the fish directly from freshwater into different carbonate alkaline water. Levels of plasma pH/HCO 3 − concentration, the mRNA and protein expression of two carbonic anhydrases (CAhz and CAIV), and two HCO 3 − transporters (Na + /HCO 3 − cotransporter and Cl − /HCO 3 – exchanger) in the gill, kidney, and intestine were determined using a pH meter, UV spectrophotometer, quantitative real-time PCR, and western blotting within 192 h of exposure. Plasma pH showed an “up-peak-down” variation, whereas HCO 3 − concentration decreased at first and then increased in all alkaline water groups. The overall mRNA expression was regulated in an alkalinity- and time-dependent manner. Western blot results showed that the Cl − /HCO 3 − exchanger protein was detected in all tissues examined, whereas the two carbonic anhydrases and Na + /HCO 3 − cotransporter proteins were only expressed in the gill and kidney. Therefore, the studied carbonic anhydrases and HCO 3 − transporters are involved in the HCO 3 − metabolism and transport to maintain acid–base balance in Nile tilapia under carbonate alkalinity stress.