Gene expression of Na+/H+ exchanger in zebrafish H+-ATPase-rich cells during acclimation to low-Na+ and acidic environments

Gene expression of Na+/H+ exchanger in zebrafish H+-ATPase-rich cells during acclimation to low-Na+ and acidic environments. Am J Physiol Cell Physiol 293: C1814-C1823, 2007. First published October 3, 2007; doi:10.1152/ajpcell.00358.2007. -In mammalian nephrons, most of the Na+ and HCO3- is reabsorbed by proximal tubular cells in which the Na+/H (+) exchanger 3 ( NHE3) is the major player. The roles of NHEs in Na+ uptake/ acid- base regulation in freshwater (FW) fish gills are still being debated. In the present study, functional genomic approaches were used to clone and sequence the full-length cDNAs of the nhe family from zebrafish (Danio rerio). A phylogenetic tree analysis of the deduced amino acid sequences showed that zNHE1-8 are homologous to their mammalian counterparts. By RT- PCR analysis and double/ triple in situ hybridization/immunocytochemistry, only zebrafish NHE3b was expressed in zebrafish gills and was colocalized with V-H+-ATPase but not with Na+-K+-ATPase, indicating that H+-ATPase-rich (HR) cells specifically express NHE3b. A subsequent quantitative RT- PCR analysis demonstrated that acclimation to low- Na (+) FW caused upregulation and downregulation of the expressions of znhe3b and zatp6v0c (H+-ATPase C-subunit), respectively, in gill HR cells, whereas acclimation to acidic FW showed reversed effects on the expressions of these two genes. In conclusion, both NHE3b and H+-ATPase are probably involved in Na (+)uptake/acid-base regulation in zebrafish gills, like mammalian kidneys, but the partitioning of these two transporters may be differentially regulated depending on the environmental situation in which fish are acclimatized.