The bilayer stability of inner monolayer lipids from the human erythrocyte.

An increase in the intracellular Ca” concentration of human erythrocytes results in profound morphological changes and loss of plasma membrane in the form of small vesicles [ 1,2]. It is of interest to determine whether this behaviour could be related to a Ca2+-induced instability in the bilayer structure of the membrane lipid. We have therefore examined the influence of Ca2+ on model membrane systems with lipid compositions approximating those of the outer and inner monolayer of the erythrocyte membrane. We show that whereas the outer monolayer lipids are unaffected by Ca2+, a large proportion of the inner monolayer lipids adopt the hexagonal (H,) phase in the presence of Cap. We suggest that this tendency of the inner monolayer to adopt non-bilayer configurations could be directly related to the fusion event involved in vesicle ejection following CaZ* influx.