Some properties of a system for sodium-dependent outward movement of magnesium from metabolizing human red blood cells.

1. In agreement with the report by Féray & Garay (1986) it is shown that Mg2+ leaves human red cells mainly by a saturable pathway at a maximal rate of some 200 mumol 1(-1) cells h(-1) only if the medium contains Na+.Mg2+0.5 (i.e. the free Mg2+ concentration for half-maximal rate) at the internal membrane surface is 1.3 mM and the dissociation constant for Na+, KNa, at the external surface is 16-17 mM. Mg2+ shows co-operative behaviour. The Na+o-stimulated Mg2+ outflow is sensitive to millimolar amiloride concentrations. Implication of the Na+ or Ca+ pump can be ruled out. 2. Na+i is inhibitory by simple competition with Na+o. The affinity for Na+ inside is the same as outside. There is no detectable competition between Na+i and Mg2+i. 3. At approximately 1 mM-[Mg2+]i the outward Mg2+ movement stimulated by Na+o still proceeds when [Mg2+]o is increased up to 20 mM. Thus the Mg2+ movement is uphill and the apparent Mg2+0.5 at the external surface is larger than 20 mM. 4. Reversing the Na+ gradient (making [Na+]i greater than [Na+]o) does not elicit an inward Mg2+ movement, even if [Mg2+]o is simultaneously made larger than [Mg2+]i. 5. The Na+o-dependent Mg2+ outflow ceases nearly completely (falling to 5% of the control) in metabolically depleted cells. 6. The behaviour observed is compatible with the assumptions that (1) the system possesses distinct binding sites for Na+ and for Mg2+, (2) the ionophoric moiety is passively mobile when loaded with Na+, (3) the movement of the Na+ form is rate limiting, and (4) the Mg2+ form preferentially moves in the outward direction owing to an input of metabolic energy (ATP hydrolysis) and is immobile in starved cells. 7. Mg2+ may be required at a further site(s) not involved in the actual Mg2+ translocation but in the energy input. The simple kinetics suggesting translocation of one Na+ ion in exchange for one Mg2+ ion were found in selected cells of average maximum transport rate (Vmax) and may not hold for all cell specimens. 8. The conclusion is that the system is a Mg2+ extrusion pump driven by metabolic energy directly and not by the inward Na+ gradient, although net inward Na+ movement is necessary to bring the ionophoric part of the system back to the in-position. It appears that in intact cells the system operates far below saturation by Mg2+i and, by compensating for an inward leak of less than 5 mumol,1(-1) cells h(-1), sets the internal free Mg2+ concentration at about 0.5 of the equilibrium value.