Ionophore-mediated loading of Ca ²⁺ into large unilamellar vesicles in response to transmembrane pH gradients

Summary

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.

Key Words:

• A23187
• ionomycin
• transmembrane pH gradient
• liposome