23Na NMR measurement of the maximal rate of active sodium efflux from human red blood cells.

The method for 23Na NMR measurement of the maximal rate of active Na+ efflux from human red blood cells (RBC) is proposed. The nonpenetrating paramagnetic shift reagent (SR) bis(tripolyphosphate)dysprosium(III) complex is used to distinguish extracellular Na+ ions from intracellular. RBC are proved to retain their physiological activity in the presence of SR. Intracellular Na+ is shown to be 100% NMR visible. The levels of intracellular and extracellular Na+ and K+ ions are changed to decrease their concentration gradients across the erythrocyte membrane to make active Na+ efflux the only 23Na NMR measurable process; so the integrated areas of intra- and extracellular Na+ peaks remain invariant throughout the incubation period in the presence of 0.25 mM ouabain, a specific inhibitor of Na+, K+-ATPase. The accuracy of the proposed technique is evaluated to be 10%. The maximal Na+ efflux is determined to be 10.1 +/- 1.0 mM/h/liter of cells.