Na⁺-H⁺ and Na⁺-Li⁺ exchange are mediated by the same membrane transport protein in human red blood cells: An NMR investigation

Na⁺-H⁺ exchange is a transport system present in erythrocytes which plays an important role in the regulation of intracellular pH, cellular volume, and transmembrane ion transport. Na⁺-Li⁺ exchange has received much attention and has been investigated in more detail than have any of the other ion transport systems, because of its high reproducibility. Both red blood cell (RBC) Na⁺-H⁺ and Na⁺-Li⁺ exchange are elevated in essential hypertensive patients relative to normotensive individuals. RBC Na⁺-Li⁺ exchange may be a mode of operation of Na⁺-H⁺ exchange. Amiloride and its analogue, 5-(N,N-hexamethylene)amiloride (HMA), are well-known inhibitors of Na⁺- H⁺ exchange, whereas phloretin strongly inhibits Na⁺-Li⁺ exchange. In this study, we tested the effects of amiloride, HMA, and phloretin on Na⁺-Li⁺ exchange activity in intact RBCs by using atomic absorption. We investigated by using ⁷Li nuclear magnetic resonance (NMR) spectroscopy the effects of HMA and phloretin inhibition on Li⁺ efflux across resealed H⁺- and Li⁺-loaded RBC ghosts in the absence and presence of pH gradients. Amiloride inhibitory activities on both Na⁺ and Li⁺ binding to exposed RBC membranes under different pH conditions were also studied by ²³Na and ⁷Li NMR relaxation time measurements. We found that Na⁺-Li⁺ exchange activity was inhibited by amiloride, HMA, and phloretin in suspensions of intact RBCs and of resealed RBC ghosts. Li⁺ efflux rates across resealed H⁺- and Li⁺- loaded RBC ghosts were significantly lower when a pH gradient was present, presumably because of the competition between Li⁺ and H⁺ for transport by the same transport protein. Amiloride had similar inhibitory constants on both Na⁺ and Li⁺ binding to RBC membranes (102 ± 48 M^-1 vs 964 ± 40M^{- 1} at pH 8.0; 731 ± 147 M^-1 m vs 716 ± 27 M^-1 at pH 7.0). These results suggest that Na⁺-H⁺ exchange and Na⁺-Li⁺ exchange are mediated by the same RBC membrane transport protein.