EFFECTS OF DIVALENT-CATIONS ON LIPID FLIP-FLOP IN THE HUMAN ERYTHROCYTE-MEMBRANE

Treatment of human erythrocytes with ionophore A23187 (10 μmol · 1-1) and Ca2+ (0.05-0.5 mmol · 1-1) or Sr2+ (0.2-1 mmol · 1-1) in results in a concentration-dependent acceleration of the transmembrane reorientation (flip) of the lipid probes lysophosphatidylcholine and palmitoylcarnitine to the inner membrane leaflet after their primary insertion into the outer leaflet. Mg2+, Mn2+, Zn2+ and La3+ do not accelerate flip. Ca2+-induced flip acceleration depends also on the ionophore concentration. It is reversed by removal of Ca2+ with EDTA. A causal role of Ca2+-induced membrane protein degradation and decrease of the polyphosphoinositide level in flip acceleration could be excluded. Likewise, calmodulin-dependent processes are probably not involved since the calmodulin antagonist calmidazolium (2-10 μmol · 1-1) does not suppress but even enhances the Ca2+-induced flip acceleration. The same is true for the Ca2+ antagonist flunarizine. These drugs do not alter flip rate in the absence of Ca2+. At high Ca2+ (1-5 mmol · 1-1) an initial flip acceleration is followed by flip normalization. High concentrations of Mn2+ and Mg2+ slow down flip rates. The selective acceleration of flip by Ca2+ and Sr2+ is discussed to be due to a local detachment of the membrane skeleton from the bilayer, whereas the unselective slow down of flip by divalent cations might be due to a stabilization of the membrane bilayer by the cations. After loading of cells with Ca2+ (but not with Mn2+) the inner membrane leaflet phospholipid phosphatidylserine becomes rapidly exposed to the outer membrane surface, as detectable by its accessibility to phospholipase A2 (5 min). This fast reorientation of phosphatidylserine is, however, not tightly coupled to the flip acceleration indicated by the lipid probes. © 1990.