The effects of alkali metal ions on active Ca2+ transport in reconstituted ghosts from human red cells.

1. K+, Rb+ or Na+ increase from 30 to 90% the maximum rate of Ca2+ transport from resealed ghosts, leaving unaltered the apparent affinity of the Ca2+ pump for Ca2+. Li+ is ineffective as activator of Ca2+ transport. 2. K+ does not change the temperature dependence of Ca2+ transport. 3. The effects of K+ and Na+ are half-maximal at 4.6 mM and 24 mM, respectively. 4. The site(s) at which K+ or Na+ combine are accessible only from the cytoplasmic surface of the cell membrane. 5. Under conditions in which Na+ activates the Ca2+ pump no Na+ efflux coupled to Ca2+ efflux is apparent.