Effects of calcium-EGTA buffers on active calcium transport in inside-out red cell membrane vesicles

In inside-out red cell membrane vesicles, the free calcium concentration half-maximally stimulating active calcium uptake is about 2 orders of magnitude smaller in a calcium-EGTA buffer than in media containing unbuffered calcium. In calcium-EGTA buffer, the maximum rate of calcium uptake is determined by the total calcium concentration present. A possible model for explaining these findings is presented.