A novel K+‐dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

Abstract Aim To determine whether Na + uptake in adult zebrafish ( Danio rerio ) exposed to acidic water adheres to traditional models reliant on Na + /H + Exchangers (NHEs), Na + channels and Na + /Cl − Cotransporters (NCCs) or if it occurs through a novel mechanism. Methods Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0‐12 hours during which 22 Na + uptake (), ammonia excretion, net acidic equivalent flux and net K + flux () were measured. The involvement of NHEs, Na + channels, NCCs, K + ‐channels and K + ‐dependent Na + /Ca 2+ exchangers (NCKXs) was evaluated by exposure to Cl − ‐free or elevated [K + ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT‐PCR. Results was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K + as a counter substrate for Na + uptake during acid exposure. Indeed, elevated environmental [K + ] inhibited during acid exposure in a concentration‐dependent manner, with near‐complete inhibition at 10 mM. Moreover, loss increased approximately fourfold at 8‐10 hours of acid exposure which correlated with recovered in 1:1 fashion, and both and were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms. Conclusions During acid exposure, zebrafish engage a novel Na + uptake mechanism that utilizes the outwardly directed K + gradient as a counter‐substrate for Na + and is sensitive to TEA. NKCXs are promising candidates to mediate this K + ‐dependent Na + uptake, opening new research avenues about Na + uptake in zebrafish and other acid‐tolerant aquatic species.