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. Whe n cell Mg content is altered using A23187, which is subsequently washe d 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 red uced, 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 no t 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 effl ux 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) depe ndence of a transporter which is amiloride sensitive, reversible and c an operate against an electrochemical gradient, consistent with Na-Mg antiport. Loading by incubation in low-[Na] media activates a high-cap acity Mg transporter which obscures the antiporter.