Movement of calcium through artificial lipid membranes and the effects of ionophores

The calcium efflux from multi-layered vesicles (liposomes) of different lipid composition has been studied. Liposomes composed of lipids extracted from cattle retinas are compared with liposomes which consist of phosphatidylcholine or a 1 : 1 phosphatidylcholine/phosphatidylserine mixture. The percentages of ⁴⁵Ca capture by these three types of liposomes are 10, 1 and 4% respectively.The efflux rates are 2.5 · 10^?6, 2 · 10^?6 and 4 · 10^?5 s^?1 respectively. The semilogarithmic efflux curves for phosphatidylcholine and phosphatidylcholine/phosphatidylserine liposomes are linear with time, but those for the retinal lipid liposomes are discontinuous. The activation energy for the calcium efflux from the latter liposomes is about 10.5 kcal/mol, both before and after the discontinuity.The ionophores X537A and A23187 enhance the calcium leakage from retinal lipid liposomes, the latter ionophore being much more effective than the former. At high concentrations both ionophores seem to transport calcium as a 1 : 2Ca · ionophore complex. At low ionophore concentrations, however, X537A appears to transport calcium as a 1 : 1 complex, but A23187 as a 2 : 1 complex.