Asymmetry of the magnesium sodium exchange across the human red cell membrane.

(1) Net Mg2+ inflow into human red cells through the Mg(2+)-Na+ exchange system is slower, for a given driving force defined by the ionic gradients and Em, than outflow for a similar and opposite force. This is not incompatible with an asymmetric, equilibrating exchange carrier. (2) However, the finding that near zero force the rate of outflow does not tend towards zero implies an active component, i.e., direct input of metabolic energy in addition to the energy provided via the Na(+)-concentration gradient by the Na+/K(+)-pump.