Role of membrane-bound Ca in ghost permeability to Na and K

The permeability of red cell ghosts to K is determined by the amount of membrane-bound Mg which, in turn, depends on internal Mg. Contrasting with such effect, an increase in cellular Ca raises K permeability. To test whether this action is due to a competitive displacement of membrane Mg, the free Ca content of human red cell ghosts was altered by means of Ca-EGTA buffers. Net Na and K movements as well as Ca and Mg bindings were assessed after incubation in a Na-medium at 37 degrees C. Raising Ca from 3 X 10(-7) to 1 X 10(-2) M caused a large K efflux with very little Na gain. Under similar conditions, Ca binding was increased without affecting membrane-bound Mg. Both Ca binding and K loss were markedly diminished by either adding ATP to the hemolytic medium or increasing internal Mg at a fixed Ca concentration. A Scatchard analysis showed three Ca binding sites, two of them having high affinity. It is concluded that Ca action does not arise from a displacement of membrane-bound Mg but from binding to different sites in the membrane. Presumably, high affinity sites are involved in the control of K permeability.