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 ²² Na ⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K ⁺ flux ( J H net ) 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: J Na in 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 J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net 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.
(© 2022 Scandinavian Physiological Society. Published by John Wiley & Sons Ltd.)