Consequences of the interaction of calcium with dioleoylphosphatidate-containing model membranes: changes in membrane...

The permeability behaviour of dioleoylphosphatidate/dioleoylphosphatidylcholine (20:80, mol%) large unilamellar vesicles at low millimolar calcium concentrations is different for various solutes. Between 0.5 mM and 2.5 mM of calcium a selective influx of calcium and efflux of enclosed calcium chelating anions is observed. At higher calcium concentrations the membrane loses its barrier function for a large variety of solutes. These permeability increases are a specific consequence of calcium phosphatidate interactions, because control experiments in which calcium was replaced by magnesium or in which dioleoylphosphatidate was replaced by dioleoylphosphatidylglycerol showed under the same conditions no permeability changes. These results are discussed on the basis of various putative mechanistic models for phosphatidate-mediated calcium translocation across membranes. Furthermore a kinetical model is presented by which the observed selective calcium and calcium-chelator translocation can be explained.