Abstract
The effects of acute exposure to acidic water on Na+ and Cl- homeostasis, and the mechanisms underlying their compensatory regulation, were investigated in the larval zebrafish Danio rerio Exposure to acidic water (pH 4.0; control pH 7.6) for 2 h significantly reduced Na+ uptake and whole body Na+ content. Nevertheless, the capacity for Na+ uptake was substantially increased in fish preexposed to acidic water but measured in control water. Based on the accumulation of the Na+-selective dye, Sodium Green, two ionocyte subtypes exhibited intracellular Na+ enrichment after preexposure to acidic water: H+-ATPase rich (HR) cells, which coexpress the Na+/H+ exchanger isoform 3b (NHE3b), and a non-HR cell population. In fish experiencing Na+-Cl- cotransporter (NCC) knockdown, we observed no Sodium Green accumulation in the latter cell type, suggesting the non-HR cells were NCC cells. Elimination of NHE3b-expressing HR cells did not prevent the increased Na+ uptake following acid exposure. On the other hand, the increased Na+ uptake was abolished when the acidic water was enriched with Na+ and Cl-, but not with Na+ only, indicating that the elevated Na+ uptake after acid exposure was associated with the compensatory regulation of Cl- Further examinations demonstrated that acute acid exposure also reduced whole body Cl- levels and increased the capacity for Cl- uptake. Moreover, knockdown of NCC prevented the increased uptake of both Na+ and Cl- after exposure to acidic water. Together, the results of the present study revealed a novel role of NCC in the compensatory regulation of Na+ and Cl- uptake following acute acidosis.
Highlights
ZEBRAFISH (DANIO RERIO) HAS emerged as an important model for integrative physiological research, including regulation of body fluid homeostasis and acid-base balance (14 –16, 21)
Acid-sensing ion channels were recently proposed to mediate the apical uptake of Naϩ in Hϩ-ATPase-rich cells (HRCs) of adult zebrafish exposed to a low Naϩ environment [6]
Because the results demonstrated that fish lacking HRCs were still able to increase Naϩ uptake after acute acid exposure, we focused on the potential role of NCC and evaluated the impacts of acute acidosis on both Naϩ and ClϪ homeostasis
Summary
ZEBRAFISH (DANIO RERIO) HAS emerged as an important model for integrative physiological research, including regulation of body fluid homeostasis and acid-base balance (14 –16, 21). During larval stages before the gills are fully developed, regulation of epithelial ion transport is mediated primarily by ionocytes found in the skin of the yolk sac These ionocytes are thought to be analogous to various renal tubular cells in mammals, in terms of ion transport mechanisms and solute transporter expression [16]. A distinct type of NHE isoform, NHE3b (SLC9A3B), is expressed in Hϩ-ATPase-rich cells (HRCs), which are thought to be the major ionocyte responsible for Naϩ absorption [8, 17, 20, 26, 30]. NHE3b, in association with Rhcg (a specific apical isoform of Rh protein involved in ammonia excretion), was critically involved in the compensatory uptake of Naϩ in larval zebrafish following chronic acid acclimation [20].
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