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 (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 Nain 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.

Dining on the dead in the deep: Active NH4+ excretion via Na+/H+(NH4+) exchange in the highly ammonia tolerant Pacific hagfish, Eptatretus stoutii

“…NHEs are bi-directional transporters whose direction is dictated by environmental and intra-ionocyte concentration gradients of [Na + ] and [H + ], such that Na + uptake and H + excretion take place when is true. [44][45][46] ] in both control and HEA hagfish, critical features of this model, such as relative affinity constants for each of the substrates remain to be elucidated through the use of two substrate kinetics. 47 To investigate the expression of Rh channels in active ammonium excretion and [T amm ] p regulation during HEA exposure, we quantified Rh channel protein abundance by Western blotting and further determined the sub-cellular localization of Rh channels in hagfish gills using immunohistochemistry, confocal resolution microscopy, and Z-stack image reconstructions.…”

A new model for sodium uptake in the zebrafish gill

“…In the current issue of Acta Physiologica, Clifford et al provide evidence of a novel ion transporter (Na + /Ca 2+ -K + exchanger) in the zebrafish gill that will leave readers re-evaluating existing models of ion movement in fresh water (FW) fish. 1 The fish gill forms a vast interface separating the internal environment from the surrounding waters, providing a large surface for gases and salts to diffuse. Thus, the gill is the principal site of salt loss in FW and of salt accumulation in sea water.…”

Physiology and aquaculture: A review of ion and acid‐base regulation by the gills of fishes

“…In most species studied to date, a specific subtype of ionocyte is responsible for Na + /H + exchange, through apical Na + /H + -exchanger (NHE) and/ or H + -ATPase coupled to a cation channel, while a separate subtype expresses apical Na + -Cl-co-transporter (NCC). Furthermore, an additional low pH-activated mechanism for Na + uptake has been suggested in zebrafish whereby Na + is taken up across the apical membrane in exchange for K + , possibly via a K + -dependent Na + / Ca 2+ exchanger (NCKX) (Clifford et al, 2022). In all of these pathways, basolateral exit of Na + into the blood is facilitated by Na + / K + -ATPase (NKA) and/or Na + -HCO − 3 -co-transporter (NBC) (Figure 3; Table 1).…”

“…Na + uptake under so-called control FW conditions remains unclear, however, because treatment with the NCC inhibitors hydrochlorothiazide or metolazone has yielded inconsistent results in studies examining zebrafish (Clifford et al, 2022;Esaki et al, 2007;Wang et al, 2009) and other species (Brix et al, 2018;Wood et al, 2014). In larval zebrafish, NCC may play a critical role in Na + uptake following acute exposure to acidic FW or in fishes reared under Na + -poor conditions .…”