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-22(+) uptake (JNain), ammonia excretion, net acidic equivalent flux and net K+ flux (JHnet) were measured. The involvement of NHEs, Na+ channels, NCCs, K+-channels and K+-dependent Na+/Ca2+ 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 JNain 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 JNain during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, JHnet loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered JNain in 1:1 fashion, and both JNain and JHnet 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.

Automated summary

During exposure to acidic water, zebrafish utilize a novel mechanism of Na+ uptake that relies on the outwardly directed K+ gradient as a counter-substrate for Na+ and is sensitive to tetraethylammonium (TEA).