The specificity of ionophore A23187 in cation transport across lipid membranes. Studies with lecithin vesicles.

Lecithin vesicles containing different cations were prepared by sonication and characterized. When the concentration of the divalent cations in the buffer was increased from 1 mM to about 100 mM the cation-concentration inside of the vesicles did not increase proportional to the outside concentration, but showed a saturation behaviour. The efflux of various cations mediated by ionophore A23187 was measured and relative transport-rates were determined. The following sequence was obtained: Zn2+ > Ca2+ > Mg2+ > Sr2+ > Ba2+ approximately Li+ > Na+. The efflux of Ca2+ increased proportional to the square of the A23187-concentration. To lecithin-vesicles containing ethylenediamine tetraacetate (EDTA) inside different cations were added on the outside. In the presence of A23187 divalent cations are transported into the vesicles and bound there by EDTA. During the influx of Me2+-ions into vesicles a H+-efflux was observed. The resulting pH-decrease was measured. The rate of the pH-change depended on the Me2+-ion used. The sequence was: Cd2+ > Zn2+ > Ca2+ > Mn2+ > MG2+ > Sr2+, Ba2+.