at rest both ca2+ uptake into rer and ca2+ extrusion from the cell is promoted by ( ca2+ + mg2+ ) -atpases with different characteristics in both types of membranes and by a coupled na+/ca2+ countertransport in the pm which keep cytosolic free ca2+ concentration at a low level of approximately 2 - 4 x 10 ( -7 ) mol/l .
C0007634
cell
C0007603
pm