Is the Second Sodium Pump Electrogenic

Transepithelial sodium transport is a process that involves active Na+ transport at the basolateral membrane of the epithelial cell. This process is mediated by the Na+/K+ pump, which exchanges 3 internal Na+ by 2 external K+ inducing a net charge movement and the second Na+ pump, which transports Na+ accompanied by Cl− and water. It has been suggested that this pump could also be electrogenic. Herein, we evaluated, in MDCK cells, the short-circuit current (Isc) generated by these Na+ pumps at the basolateral membrane of the epithelial cells, using amphotericin B as an apical permeabilizing agent. In Cl−-containing media, Isc induced by amphotericin B is totally inhibited by ouabain, indicating that only the electrogenic Na+/K+ pump is detectable in the presence of Cl−. Electrogenicity of the second Na+ pump can be demonstrated in Cl−-free media. The existence of a furosemide-sensitive component of Isc, in addition to an ouabain-sensitive one, was identified in absence of chloride. Passive Cl− movement associated with the function of the second Na+ pump seems to be regulated by the pump itself. These results demonstrate that the second Na+ pump is an electroneutral mechanism result from the stoichiometric movement of Na+ and Cl− across the basolateral plasma membrane of the epithelial cell.