Identification and apical membrane localization of an electrogenic Na⁺/Ca²⁺ exchanger NCX2a likely to be involved in renal Ca²⁺ excretion by seawater fish.

Seawater (SW) contains ∼10 mM Ca(2+), yet marine fish must drink seawater as their major water source. Thus marine teleosts fish need to excrete Ca(2+) to maintain whole body Ca(2+) homeostasis. In the intestine, seawater Ca(2+) interreacts with epithelial-secreted HCO(3)(-) by the intestinal epithelium, and the resulting CaCO(3) precipitates, which is rectally excreted. Recently the transporters involved in intestinal HCO(3)(-) secretion were identified. Ca(2+) is also excreted by the kidney, but the protein(s) involved in renal Ca(2+) excretion have not been identified. Here we identified a candidate transporter by using SW pufferfish torafugu (Takifugu rubripes) and its closely related euryhaline species mefugu (Takifugu obscurus), which are becoming useful animal models for studying molecular mechanisms of seawater adaptation. RT-PCR analyses of Na(+)/Ca(2+) exchanger (NCX) family members in various torafugu tissues demonstrated that only NCX2a is highly expressed in the kidney. Renal expression of NCX2a was markedly elevated when mefugu were transferred from freshwater to seawater. In situ hybridization and immunohistochemical analyses indicated that NCX2a is expressed in the proximal tubule at the apical membrane. NCX2a, expressed in Xenopus oocytes, conferred [Ca(2+)](out)- and Na(+)-dependent currents. These results suggest that NCX2a mediates renal Ca(2+) secretion at the apical membrane of renal proximal tubules and has an important role in whole body Ca(2+) homeostasis of marine teleosts.