23Na-NMR studies of Na + interaction with human red cell membranes from normotensives and hypertensives

Abstract

Na + interaction with unsealed human red cell ghosts has been studied by 23Na-NMR relaxation rate ( R 1) measurements. Data on a total of nine subjects including seven volunteer normotensives (NBP) and two untreated hypertensives (HBP) are presented. Qualitative treatment of the data gives information on the dynamic behavior of Na + undergoing fast exchange between the free and bound states. The excess longitudinal relaxation rate (Δ R) −1 plotted against total [Na +], known as the James-Noggle plot, exhibits different behavior for NBP and HBP ghosts, with a relatively low binding constant of approx. 100 M −1 for HBP ( p<0.025) compared to a high constant of 500–1000 M −1 for NBP. To associate our NMR data with membrane-bound (Na ++K +)-ATPase, 23Na relaxation rates were measured in the presence of 5 mM ouabain. James-Noggle plots constructed for ouabain-sensitive excess relaxation rates show the binding for NBP to be even high affinity (10 3 M −1) but low capacity. These data may suggest that for a given amount of intracellular Na +, the binding affinity could determine the distribution of Na + between the membrane and cytoplasm, and that the (Na ++ K +) -ATPase which is primarily responsible for the Na + affinity might assume an abnormal transport mechanism in HBP membranes.