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.