ATP-driven Ca2+ pump activity of macrophage and neutrophil plasma membrane

The results of the investigations here described permit us to conclude that macrophages and neutrophils have a peripheral, outwardly directed Ca2+ extrusion system, which is very similar to the well known Ca2+ pump of the red cell, with regard to capacity and mechanism (16, 21, 29). In fact, all the three cell types have similar maximum pumping rates (about 0.1-0.2 microgram-ions Ca2+/min/ml cells) and use ATP for extruding Ca2+. Furthermore, the plasma membrane of all the three cell types catalyzes a Ca2+-dependent ATPase reaction, which is very likely the enzyme manifestation of the Ca2+ pump activity. Further investigation is needed to establish whether the peripheral Ca2+ pump system of macrophages and neutrophils is utilized to restore steady-state levels of cytosolic Ca2+ upon cell stimulation, or is somehow involved in the triggering of cell response to various stimuli. In fact impairment of the pump activity by a cell stimulant would unbalance Ca2+ passive leaks and active cation extrusion, thereby leading to higher steady-state levels of Ca2+ in the cytosol and to stimulation of Ca2+-dependent functions (1-12).