Electroneutral Na⁺-Cl^- cotransport activity of anguillid eel Slc12a10 expressed in Xenopus oocytes

Slc12a10 is a homolog of the Na⁺-Cl^- cotransporter Ncc (Slc12a3) and is also referred to as Nccβ or Ncc2. The Slc12a10 gene is widely distributed among bony vertebrates; however, it has been lost or pseudogenized in birds, most mammals including humans, and some teleost fishes. Slc12a10 is expressed in the intestinal epithelium of anguillid eels and in ionocytes of the zebrafish gill, where it is thought to contribute to epithelial NaCl transport. Despite these observations, only a limited number of studies have directly evaluated the transport activity of Slc12a10, and its ion-coupling properties and electrical characteristics remain incompletely defined. To address this gap, we examined the transport activity of Slc12a10 from European and Japanese eels (Anguilla anguilla and Anguilla japonica) using a Xenopus laevis oocyte expression system combined with ion-selective microelectrode measurements. When oocytes expressing eel Slc12a10 were exposed to Na⁺-free solution, intracellular Na⁺ and Cl^- activities decreased significantly. Similar decreases in intracellular Na⁺ and Cl^- activities were observed upon exposure to Cl^--free solution. These changes were not observed in control oocytes and were not accompanied by significant changes in membrane potential. Furthermore, under voltage-clamp conditions at -20 mV, exposure of Slc12a10-expressing oocytes to Na⁺-free solution induced a significant decrease in intracellular Na⁺ activity without generating detectable membrane currents. These activities were comparable to those previously reported for zebrafish Slc12a10.1. Together, these results provide new evidence that eel Slc12a10 functions as an electroneutral Na⁺-Cl^- cotransporter and suggest that this transport mechanism is conserved across fish species.