NA+/H+ AND NA+/NA+ COUNTERTRANSPORT IN HUMAN, RABBIT, AND RAT ERYTHROCYTES - EVIDENCE FOR 2 INDEPENDENT ION-TRANSPORT SYSTEMS

The relative activities and modes of regulation of Na+/H+ and Na+/Na+ exchanges in human, rabbit, and rat erythrocytes were investigated. No basal Na+/H+-exchange activity (the amiloride-inhibitable component of Na-22(+) influx) in erythrocytes of these species was observed. The inward current of Na-22(+) sharply increased on acidification of erythrocyte cytoplasm and alkalization of the incubation medium (Delta mu(H)+-induced Na+/H+ exchange). The ratios between the rates of Delta mu(H)+-induced Na+/H+ exchange in human, rabbit, and rat erythrocytes are 1.0:1.1:2.3. The ratios of their respective rates of Na+/Na+ exchange (the phloretin-inhibitable component of Na-22(+) influx) were 1.0:4.6:0.2. In rat and rabbit erythrocytes,the rates of Na+/H+ but not Na+/Na+ exchange increased on hyperosmotic shrinkage of the cell. In rat erythrocytes, amiloride (1 mM) inhibited the influx of Na-22(+) induced by hyperosmotic shrinkage and cytoplasm acidification by 95 and 10-20%, respectively. The temperature of inactivation of Na+/H+ exchange induced by either shrinkage or cytoplasm acidification is near the melting temperature of erythrocyte membrane :cytoskeletal proteins (49-51 degrees C). The data suggest that Na+/H+ and Na+/Na+ exchanges are carried out by two independent ion transport systems. The Na+/H+ exchange processes induced by hyperosmotic shrinkage and acidification of cytoplasm may be mediated by different isoforms of the carrier.