SUMMARY1. 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 x 10-2 mM.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 370 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 temperaturedependent, 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.

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SUMMARY1. In resealed human red cells loaded with Ca-EGTA buffer solutions it was found that the intracellular free Ca2+ concentration for half saturation of the Ca transport system (which pumps Ca out of the cell) is equal to or smaller than 4 x 10-6 M and thus closely agrees with the dissociation constant of the Ca+ Mg activated membrane ATPase.2. The maximal rate of Ca transport from resealed cells to medium was found to be 0 148 + 0 009 #zmole/ml. cells.min at 280 C.3. The rate of Ca transport was unaffected by a variation of the extracellular Ca2+ concentration from 3-10-7 to 5-10-M.4. Evidence is presented making it probable that the stoichiometric relation between Ca transported and ATP hydrolysed is 1:1 rather than 2:1.5. As the Ca transport is quite rapid even at half saturation and the passive leak for Ca negligible in intact cells it can be predicted that the steady-state cellular Ca2+ concentration must be low, most probably less than 106 /imole/ml. cells. Transport from cells containing 5-10-7

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The molecular architectures of SMP and SRP differ, yet the functional analogies between CAM-associated SMP and SRP are numerous. Most of the divergences observed may have their root in different behavior during dephosphorylation (see Table 1).