Abrogation of calcium exclusion by erythrocytes under hypotonic stress

Normal human erythrocytes in isotonic suspension maintain a high gradient of calcium ion concentration across the membrane, with the cytosol [Ca2+] several orders of magnitude below plasma concentration. Despite the high degree of exclusion maintained against the entry of Ca2+, the osmotic resistance of red cells in hypotonic saline is significantly augmented by the presence in the medium of Ca2+ in concentrations greater than 0.07 mmol/l. Resistive particle spectroscopy showed this to occur without alteration of the mean critical haemolytic volume; furthermore, in the presence of Ca2+ there was a reduction in cell volume at prolytic osmolalities when compared with cells in calcium-free sodium chloride solutions. This is shown to be due to augmentation of the prolytic loss of K+ in the presence of Ca2+ through calcium-sensitive channels, and indicates that the pathway for passive K+ efflux uncovered in the prolytic state is indistinguishable from that of normal passive K+ diffusion. It is also demonstrated that the prolytic red cell membrane is permeable to Ca2+ and admits the divalent cation to the inner aspect of the plasma membrane. These changes are comparable to those of cells prior to haemolysis in vivo, as in the irreversibly sickled cell, and are therefore not specific to the latter condition.