Na+/Na+ exchange and Na+/H+ antiport in rabbit erythrocytes: two 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-ethymaleimide (NEM) were used. Na+/Na+ exchange activity, determined as the Na+o-dependent 22Na efflux or Na+i-induced 22Na 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 pHi = 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 pHi, is mediated by 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Escobales
N
N
Department of Physiology and Biophysics, University of Puerto Rico School of Medicine, San Juan 00936.
Figueroa
J
J
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002352:Carrier Proteins / Q000097:blood*; D004912:Erythrocytes / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D010042:Ouabain / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000097:blood*; D017923:Sodium-Hydrogen Exchangers; D013054:Spectrophotometry, Atomic; D014634:Valinomycin / Q000494:pharmacology
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D014634:Valinomycin; D010042:Ouabain; D000584:Amiloride; D012964:Sodium; D005033:Ethylmaleimide