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.