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

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 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 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.