Is the Second Sodium Pump Electrogenic?

Abstract

Transepithelial sodium transport is a process that involves Na+ entry across the apical membrane of the epithelial cell following its electrochemical gradient and its active extrusion at the basolateral plasma membrane by two sodium pumps: The Na/K-pump, which depends on K+, is inhibited by ouabain and insensitive to furosemide; and the Second Sodium pump, which is K+-independent, insensitive to ouabain but inhibited by furosemide (J. Gen. Physiol. 51:303s‑314s, 1968; Pflugers Archiv Eur J Physiol 316:1-25, 1970; Biochim Biophys Acta 394:281-292, 1975). Both transport mechanisms are associated with two ATPases present in the basolateral plasma membrane of the epithelial cells with similar functional characteristics, the Na+/K+-ATPase and the Na+-ATPase, respectively (Biochim Biophys Acta 1808:1684-1700, 2011; Pflugers Archiv Eur J Physio. 316:1-25, 2012). The Na+/K+-pump is electrogenic; it exchanges 3 internal Na+ by 2 external K+ producing a net charge movement. The Second pump transports Na+ with Cl− and water. It has been suggested that this pump could be electrogenic. The Na+ charge transfer would induce Cl− movement and osmotic movement of water following the movement of both Na+ and Cl− across the membrane. We have evaluated electrical parameters of the basolateral plasma membranes of MDCK cells cultured on Transwell in Ussing chambers, using amphotericin B as apical permeabilizing agent. Our results confirm that the Na+/K+-pump is electrogenic and that the Second sodium pump is electroneutral, coupling the active Na+ transport to a specific Cl− movement through a particular conductive pathway.