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 K _m 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 V_max for both sets of experiments indicated a stoichiometry of 2 Na: l Mg. Amiloride inhibits Na⁺ influx and Mg²⁺ extrusion in parallel (K _i = 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.