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 pl ays 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 reproducibili ty. Both red blood cell (RBC) Na+-H+ and Na+-Li+ exchange are elevated in essential hypertensive patients relative to normotensive individua ls. RBC Na+-Li+ exchange may be a mode of operation of Na+-H+ exchange . Amiloride and its analogue, 5-(N,N-hexamethylene) amiloride (HMA), a re well-known inhibitors of Na+-H+ exchange, whereas phloretin strongl y inhibits Na+-Li+ exchange. In this study, we tested the effects of a miloride, HMA, and phloretin on Na+-Li+ exchange activity in intact RB Cs by using atomic absorption. We investigated by using Li-7 nuclear m agnetic 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 act ivities on both Na+ and Li+ binding to exposed RBC membranes under dif ferent pH conditions were also studied by Na-23 and Li-7 NMR relaxatio n time measurements. We found that Na+-Li+ exchange activity was inhib ited by amiloride, HMA, and phloretin in suspensions of intact RBCs an d of resealed RBC ghosts. Li+ efflux rates across resealed H+- and Li-loaded RBC ghosts were significantly lower when a pH gradient was pre sent, presumably because of the competition between Li+ and H+ for tra nsport by the same transport protein. Amiloride had similar inhibitory constants on both Na+ and Li+ binding to RBC membranes (1021 +/- 48 M (-1) vs 964 +/- 40 M(-1) at PH 8.0; 731 +/- 147 M(-1) 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.