RESUMEN
Amiloride-inhibitable Li+ influx in dog red blood cells is mediated by the Na+/H+ exchanger, NHE. However, there are substantial differences between the properties of Li+ transport and Na+ transport through the NHE. Li+ influx is activated by cell shrinkage, and Na+ influx is not, as we reported previously (Dunham PB, Kelley SJ, and Logue PJ. Am J Physiol Cell Physiol 287: C336-C344, 2004). Li+ influx is a sigmoidal function of its concentration, and Na+ activation is linear at low Na+ concentrations. Li+ does not inhibit its own influx; in contrast, Na+ inhibits Na+ influx. Li+ prevents this inhibition by Na+. Na+ is a mixed or noncompetitive inhibitor of Li+ influx, implying that both a Na+ and a Li+ can be bound at the same time. In contrast, Li+ is a competitive inhibitor of Na+ influx, suggesting Li+ binding at one class of sites on the transporter. Because the properties of Li+ transport and Na+ transport are different, a simple explanation is that Na+ and Li+ are transported by separate sites. The similarities of the properties of Li+ transport and the inhibition of Na+ transport by Na+ suggest that Li+ is transported by the Na+-inhibitory sites.
Asunto(s)
Eritrocitos/fisiología , Litio/metabolismo , Intercambiadores de Sodio-Hidrógeno/metabolismo , Sodio/metabolismo , Amilorida/farmacología , Animales , Sitios de Unión , Tamaño de la Célula/efectos de los fármacos , Diuréticos/farmacología , Perros , Eritrocitos/efectos de los fármacos , Intercambiadores de Sodio-Hidrógeno/efectos de los fármacosRESUMEN
Na+/H+ exchangers (NHE) are ubiquitous transporters participating in regulation of cell volume and pH. Cell shrinkage, acidification, and growth factors activate NHE by increasing its sensitivity to intracellular H+ concentration. In this study, the kinetics were studied in dog red blood cells of Na+ influx through NHE as a function of external Na+ concentration ([Na+](o)). In cells in isotonic media, [Na+](o) inhibited Na+ influx >40 mM. Osmotic shrinkage activated NHE by reducing this inhibition. In cells in isotonic media + 120 mM sucrose, there was no inhibition, and influx was a hyperbolic function of [Na+](o). The kinetics of Na+-inhibited Na+ influx were analyzed at various extents of osmotic shrinkage. The curves for inhibited Na+ fluxes were sigmoid, indicating more than one Na+ inhibitory site associated with each transporter. Shrinkage significantly increased the Na+ concentration at half-maximal velocity of Na+-inhibited Na+ influx, the mechanism by which shrinkage activates NHE.