Cell age-dependent changes in deformability and calcium accumulation of human erythrocytes

The deformability of human erythrocytes was measured in a rheoscope, as a function of intracellular calcium content (varied with ionophore (A23187) and CaCl2) without complete ATP depletion and echinocytic transformation. Loading calcium into intact erythrocytes (calcium content: 16.8 mumol/1 packed cells = 1.48 amol per cell), the cell volume and energy charge gradually decreased. Further, the membrane fluidity of the lipid portion decreased without crosslinking of membrane proteins. A distinct transition from deformable to undeformable cells was observed by the rheoscope technique: i.e., 50% transition occurred at 40-50 mumol calcium/1 packed cells (= 3.5-4.0 amol per cell) and more than 90% above 100 mumol/1 packed cells (= 6.5 amol per cell) at a shear stress of 140 dyn/cm2. The deformable cells maintained their deformability to ellipsoidal disks independent of the average calcium content. The underformable cells, separated as high-density cells by density gradient centrifugation after calcium-loading, showed lower glucose-6-phosphate dehydrogenase activity than low-density-deformable cells; thus, the calcium-loaded, undeformable cells were presumably in vivo aged cells. The younger cells, fractionated as low-density cells from intact erythrocytes, were more deformable than aged cells. Upon calcium-loading, the younger cells restored their cell volume and deformability, while the aged cells, containing originally more calcium and less ATP, decreased their volume and became undeformable. Therefore, calcium accumulation by ionophore-CaCl2 takes place in preference to aged cells of lower energy metabolism, and leads to cellular dehydration and loss of deformability, due to condensed hemoglobin and altered membrane organization.