Transbilayer and Lateral Motions of Fluorescent Analogs of Phosphatidylcholine and Phosphatidylethanolamine in the Plasma Membrane of Bovine Aortic Endothelial Cells

Abstract

The apical plasma membranes of vascular endothelial cells present a non thrombogenic surface to the blood vessel, in vivo, or to the culture medium, in vitro. In the view of the catalytic role attributed to the anionic phospholipids in the blood coagulation cascade, we have checked if exogenous added fluorescent analog of phosphatidylethanolamine in the outer leaflet of the apical plasma membrane of these cells have different transmembrane and/or lateral movements than the same fluorescent analog of phosphatidylcholine. We have found that the fluorescent phosphatidylcholine derivative remains located at least one hour on the outer monolayer of the plasma membrane at 0°C as well as at 20°C, while the phosphatidylethanolamine analog is rapidly “internalized” at 20°C (half time approximatively 30 min) but not at 0°C. Evidences are presented suggesting that this “internalization” corresponds mainly to the transfert of the phosphatidylethanolamine from the outer to the inner leaflet of the membrane lipid bilayer, without labeling of intracellular particles. Furthermore, the lateral diffusion rate of the phosphatidylethanolamine probe was found to raise fourfold after these fluorescent probes have “flip” from the outer to the inner leaflet of the plasma membrane, demonstrating that the lateral motion of the lipids are considerably different on the two leaflets of the apical plasma membrane of aortic endothelial cells.