The efflux of sodium from human red blood cells

1. 1. We have studied the efflux of Na⁺ from human red blood cells loaded with different concentrations of Na⁺, osmolarity being maintained by either K⁺, choline ions or Mg²⁺. The external medium was buffered MgCl₂, with or without the addition of KCl, but with no Na⁺. 2. 2. With K⁺ or choline internally, and with K⁺ externally, the efflux of Na⁺ attained saturation at relatively low levels of internal Na⁺, the half saturation concentration being 15-25 mM. In contrast, when only negligible concentrations of K⁺ were present externally there was no clear evidence for saturation of the Na⁺ efflux in the concentration range studied (up to 140 mM). About half of this flux was ouabain sensitive and this component similarly did not saturate. 3. 3. The active transport of K⁺ was inhibited in cells loaded with concentrations of Mg²⁺ above 30 mM. In such cells the efflux of Na⁺ did not saturate in the range of Na⁺ concentrations studied, both when K⁺ was added externally or in its absence. K⁺ did stimulate a one for one exchange of Na⁺ with K⁺ and this stimulated efflux sensitive to ouabain, and also did not saturate in the concentration of Na⁺ studied. 4. 4. It would appear, therefore, that under conditions in which pumping can occur, Na⁺ efflux is mediated through a high-affinity site while if pumping is prevented, mediated Na⁺ effluxes can occur on a low-affinity site. This low-affinity site on the inner face of the membrane is nevertheless part of the Na⁺ transport system (as indicated by its sensitivity to ouabain) but is only revealed when pumping is prevented. 5. 5. Our results do not contradict the recently proposed internal transfer model for the (Na⁺ - K⁺)-ATPase which predicts that zero trans Na⁺ efflux experiments should reveal the presence of a low-affinity site for Na⁺ at the inner face of the cell membrane, in contrast to the high-affinity site seen under pumping conditions.