Sodium transport through the amiloride-sensitive Na-Mg pathway of hamster red cells

Previous work showed that in hamster red cells the amiloride-sensitive (AS) Na+ influx of 0.8 mmol/liter cells/hr is not mediated by Na-H exchange as in other red cells, but depends upon intracellular Mg2+ and can be increased by 40-fold by loading cells with Mg2+ to 10 mM. The purpose of this study was to verify the connection of AS Na+ influx with Na-dependent, amiloride-sensitive Mg2+ efflux and to utilize AS Na+ influx to explore that pathway. Determination of unidirectional influx of Na+ and net loss of Mg2+ in parallel sets of cells showed that activation by extracellular [Na+] follows a simple Michaelis-Menten relationship for both processes with a Km of 105-107 mM and that activation of both processes is sigmoidally dependent upon cytoplasmic [Mg2+] with a [Mg2+]0.5 of 2.1-2.3 mM and a Hill coefficient of 1.8. Comparison of Vmax for both sets of experiments indicated a stoichiometry of 2 Na:1 Mg. Amiloride inhibits Na+ influx and Mg2+ extrusion in parallel (Ki = 0.3 mM). Like Mg2+ extrusion, amiloride-sensitive Na+ influx shows an absolute requirement for cytoplasmic ATP and is increased by cell swelling. Hence, amiloride-sensitive Na+ influx in hamster red cells appears to be through the Na-Mg exchange pathway. There was no amiloride-sensitive Na+ efflux in hamster red cells loaded with Na+ and incubated with high [Mg2+] in the medium with or without external Na+, nor with ATP depletion. Hence, this is not a simple Na-Mg exchange carrier.