Stacking of safranine in liposomes during valinomycininduced efflux of potassium ions

Abstract

Liposomes were prepared from phosphatidylcholine and cardiolipin in a KCl medium and suspended in a choline chloride medium with safranine. When efflux of K + was induced by valinomycin, spectral shifts characteristic of stacking were observed. Ca 2+ inhibited the rate of stacking in a competitive manner with a K i of about 200 μM, while La 3+ was about 10 times more potent. When liposomes were prepared from phospholipids with a higher ratio of cardiolipin to phosphatidyl-choline the inhibition was more potent. No effect on the stacking phenomena was seen when Ca 2+ was added after the stacking was completed. When Ca 2+ or an organic cation with four charges, spermine, was trapped in the intraliposomal compartment, no significant change in the rate of stacking was seen. However, the extent of stacking was decreased. It is suggested that safranine is driven by a diffusion potential to a site that is inaccessible to Ca 2+ in the medium, presumably to the inner boundaries of the liposomal membranes.