Summary
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 -dependent22Na efflux or Na +i -induced22Na 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 39mm. Valinomycin, which at low [K+] o was lower than 39mm. 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 39mm. Altogether, these findings indicate that both transport systems: the Na+/Na+ and Na+/H+ exchangers, are mediated by distinct transport proteins.
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Escobales, N., Figueroa, J. Na+/Na+ exchange and Na+/H+ antiport in rabbit erythrocytes: Two distinct transport systems. J. Membrain Biol. 120, 41–49 (1991). https://doi.org/10.1007/BF01868589
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DOI: https://doi.org/10.1007/BF01868589