1. A study has been made of the cellular content and movement of Ca across the membrane of human red blood cells. 2. The [Ca] in the cellular contents of fresh red cells is 4·09 × 10−2 m M. The intracellular concentration of free ionic Ca ([Ca2+]) is considered to be less than this value and therefore less than extracellular [Ca2+] under normal conditions. 3. Observation of unidirectional Ca fluxes with 45Ca confirms previous reports of low permeability of the red cell membrane for Ca. After nearly 1 week of loading in the cold, intracellular 45Ca content is 1·8% of extracellular 45Ca content. Appearance in extracellular fluid of 45Ca from coldloaded cells can be considered to arise from two compartments. Efflux of 45Ca from the ‘slower compartment’ is accelerated by the addition of glucose. 4. Starved red cells, incubated at 37° C, after reversible haemolysis for loading with Ca and Mg‐ATP, exhibit an outward net transport of Ca against an electrochemical gradient. The transport is associated with the appearance of inorganic phosphate (Pi). Cells treated similarly, but without ATP show no transport and no appearance of Pi. 5. During the initial phase of transport, 1·3 mole Pi appear per mole Ca transported. 6. The transport of Ca from ATP‐loaded cells is highly temperature‐dependent, with a Q10 of 3·5. 7. Cell membrane adenosine triphosphatase (ATPase) activity of reversibly haemolysed cells is stimulated only by intracellular, and not by extracellular Ca. 8. Neither Ca transport in reversibly haemolysed cells, nor the Ca‐Mg activated ATPase of isolated cell membranes is sensitive to Na, K, ouabain or oligomycin. 9. Mg is not transported under the conditions which reveal Ca transport, but Mg appears to be necessary for Ca transport. 10. Sr is transported from reversibly haemolysed Mg‐ATP‐loaded cells. Sr also can substitute for Ca, but not for Mg, in the activation of membrane ATPase. 11. It is concluded that, in addition to a low passive permeability, an active extrusion mechanism for Ca exists in the human red cell membrane. This extrusion mechanism, in addition to a low passive membrane permeability for Ca, may represent the means by which intracellular Ca content is maintained at a low level. It is suggested that the Ca‐Mg activated membrane ATPase and the active transport of Ca are two manifestations of the same process. © 1969 The Physiological Society
