The influence of asymmetrically distributed Ca2 on Na permeability of bovine brain phospholipid liposomes

Liposomes prepared from bovine brain phospholipids in the absence of Ca2 spontaneously capture Na from the outside medium down a concentration gradient. When the Na concentration gradient is not yet completely equilibrated, addition of Ca2 causes an apparent efflux of Na against this gradient. This efflux cannot exclusively be ascribed to increased permeability as a consequence of asymmetrical distribution of Ca2. Another mechanism, for example lysis of the liposomes, must be involved. Liposomes prepared from bovine brain phospholipids in the presence of 3.5 mM Ca2 do not capture Na spontaneously. Removal of Ca2 on the outside by adding EDTA, inducing asymmetrical distribution of Ca2 in this way, provides for an influx of Na into the liposomes down a concentration gradient.