Reversal of the calcium pump in human red cells

Human red cells containing low ATP and high Pi concentrations were suspended in media with and without 2 mM Ca2+, and the incorporation of (32P)Pi into ATP was measured. There was some incorporation whatever the medium, but in every experiment there was an extra incorporation when the cells were in the Ca2+-containing medium. This extra incorporation was abolished by the ionophore A23187, which collapses the Ca2+ concentration gradient across the membranes, or by LaCl3, which blocks the Ca2+ pump. Starved and phosphate-loaded cells also show an uptake of Ca2+ which is not apparent in fresh cells. Results are consistent with the idea that Ca2+-dependent incorporation of Pi into ATP is catalyzed by the Ca2+ pump using energy derived from the Ca2+ concentration gradient.