Ionophore-mediated loading of Ca2+into large unilamellar vesicles in response to transmembrane pH gradients

The Ca2+ translocating properties of the carboxylic ionophores A23187, ionomycin and lasolocid A (X537A) have been investigated by employing large unilamellar vesicles that exhibit a pH gradient (acidic interior). An analysis of Ca2+ uptake at equilibrium reveals that Ca2+ accumulation is an electroneutral process, whereby one Ca2+ ion is transported in for every two H+ ions transported out. A kinetic analysis shows that both A23187 and ionomycin transport Ca2+ in the form of a 1:1 cation:ionophore complex, whereas a 1:2 complex is observed for lasolocid A. The specificity of the ionophores for transporting Ca2+ is reflected by the influence of exterior Na+ ions that inhibit Ca2+ uptake for lasolocid A but do not influence ionomycin-mediated uptake.