NA+/NA+ EXCHANGE AND NA+/H+ ANTIPORT IN RABBIT ERYTHROCYTES - 2 DISTINCT TRANSPORT-SYSTEMS

Rabbit erythrocytes are well known for possessing highly active Na+/Na+ and Na+/H+ countertransport systems. Since these two transport systems share many similar properties, the possibility exists that they represent different transport modes of a single transport molecule. Therefore, we evaluated this hypothesis by measuring Na+ transport through these exchangers in acid-loaded cells. In addition, selective inhibitors of these transport systems such as ethylisopropyl-amiloride (EIPA) and N-ethylmaleimide (NEM) were used. Na+/Na+ exchange activity, determined as the Na(o)+-dependent Na-22 efflux or Na(i)+-induced Na-22 entry was completely abolished by NEM. This inhibitor, however, did not affect the H(i)+-induced Na+ entry sensitive to amiloride (Na+/H+ exchange activity). Similarly, EIPA, a strong inhibitor of the Na+/H+ exchanger, did not inhibit Na+/Na+ countertransport, suggesting the independent nature of both transport systems. The possibility that the NEM-sensitive Na+/Na+ exchanger could be involved in Na+/H+ countertransport was suggested by studies in which the net Na+ transport sensitive to NEM was determined. As expected, net Na+ transport through this transport system was zero at different [Na+]i/[Na+]o ratios when intracellular pH was 7.2. However, at pH(i) = 6.1, net Na+ influx occurred when [Na+]i, was lower than 39 mM. Valinomycin, which at low [K+]o clamps the membrane potential close to the K+ equilibrium potential, did not affect the net NEM-sensitive Na+ entry but markedly stimulated the EIPA- and NEM-resistant Na+ uptake. This suggest that the net Na+ entry through the NEM-sensitive pathway at low pH(i), is mediated by an electroneutral process possibly involving Na+/H+ exchange. In contrast, the EIPA-sensitive Na+/H+ exchanger is not involved in Na+/Na+ countertransport, because Na+ transport through this mechanism is not affected by an increase in cell Na+ from 0.4 to 39 mM. Altogether, these findings indicate that both transport systems: the Na+/Na+ and Na+/H+ exchangers, are mediated by distinct transport proteins.