Influence of monovalent ions on the activity of the (Ca2+ + Mg2+)-ATPase and Ca2+ -transport of human red blood cells.

In reconstituted human red blood cells a difference was found in (Ca2+ + Mg2+)-ATPase activity and in Ca2+ efflux at 37 degrees C, depending on the side of the membrane at which the monovalent cations K+ and Na+ were placed. Under the conditions used, (Ca2+ + Mg2+)-ATPase activity and Ca2+ efflux was highest when K+ (35 +/- 0.5 mM (+/- S.E.), mean of four experiments) was at the inside and Na+ (130 mM) at the outside of the ghost membrane.