Magnesium transport in magnesium-loaded ferret red blood cells

Mg efflux from ferret red blood cells is stimulated when cells are Mg loaded, but the properties of efflux depend on the loading method. When cell Mg content is altered using A23187, which is subsequently washed away, Mg efflux is minimal until intracellular ionized [Mg] ([Mg2+](i)) is greater than 0.9 mM, it then increases substantially with [Mg2+](i). Efflux from loaded cells falls as external [Na] ([Na](o)) is reduced, and net Mg influx (against an electrochemical gradient) is seen when [Na](o) is sufficiently low. Both influx and efflux are amiloride sensitive. Mg influx from media containing a normal or low [Na] is not affected by reducing [Mg2+](i) to very low levels. When cells are Mg loaded by incubating them in media containing 5 mM Na and Mg, Mg efflux is again minimal until [Mg2+](i) is greater than 0.9 mM and then it increases with [Mg2+](i), but at a rate approximately 4 times faster than in cells loaded using A23187. This efflux is little affected by 1 mM amiloride. Thus Mg-loading using A23187 reveals the [Mg2+](i) dependence of a transporter which is amiloride sensitive, reversible and can operate against an electrochemical gradient, consistent with Na-Mg antiport. Loading by incubation in low-[Na] media activates a high-capacity Mg transporter which obscures the antiporter.