Calcium extrusion by high-density human red blood cells

: Normal human red blood cells were separated on arabinogalactan density gradients to provide cell populations comprising a very small percentage of the cells (0.4%-1.8%). These very high-density cells were compared to a low-density, mature reference cell population with respect to their ATP content and their capacity to extrude Ca that had been loaded into the cells using the ionophore A23187. Assay of ATP content of freshly drawn and separated cells suggested a decrease of approximately 40% in the ATP concentration of the most dense 0.5%-1% of the cells, but the second most dense percent or so of the cells showed no ATP deficit. When the cells were loaded with millimolar amounts of 45Ca, high and low-density cell populations extruded Ca at the same rate. It appears that even stringently selected cells comprising the highest-density portion of the total cell population have an intact Ca transport system that can rapidly export Ca from the cell when adequate metabolic support is available.