Extracellular Na+ electrode for monitoring net Na+ flux in cell suspensions

A computer-linked extracellular sodium-sensitive electrode system is described that is suitable for the routine measurement of net Na+ transport in cell suspensions. The commercially available Na+ electrode exhibited high selectivity for Na+ over other cations and a rapid response time (less than 3 s). This system resolved changes of 0.4 mM in the presence of 147 mM extracellular Na+. Measurements of Na+ transport in suspensions of rabbit proximal tubules showed that ouabain caused a dose-dependent net Na+ influx with an inhibitor constant (Ki) of 2.5 +/- 0.2 microM and a maximal velocity (Vmax) of 229 +/- 7 nmol Na+.min-1.mg protein-1. This compared favorably with the ouabain-induced K+ efflux (Ki = 2.4 microM; Vmax = 160 +/- 3.3 nmol K+.min-1.mg protein-1) and the ouabain-induced inhibition of respiration (Ki = 3.3 microM; Vmax = 11.8 nmol O2.min-1.mg protein-1). In addition, Ba2+, a K+ channel blocker known to depolarize the cell, caused a net Na+ efflux, whereas glucose, a Na+-cotransported solute, promoted a net Na+ influx. This system should be a powerful tool for continuous monitoring of net Na+ fluxes in cell suspensions.