23Na-nuclear magnetic resonance study of ionophore-mediated cation exchange between two populations of liposomes.

A model system to observe and investigate the transfer of Na+ ions between different internal compartments in suspension of liposomes was developed, and the exchange was followed by nuclear magnetic resonance spectroscopy. The experiments were performed under conditions of a Donnan equilibrium. Quantitative analysis of this three-site transmembrane exchange system allowed us to distinguish between direct and indirect exchange between liposomes. It also disclosed a "confining" effect on the exchange between the two populations of liposomes. This confining effect may have been due to an electrostatic field in the presence of a membrane potential. Donnan potentials and ionic compositions at equilibrium for the three-compartment system were calculated numerically. The model system may be used to explore further the effects of membrane potentials, surface potentials, and ionic mobilities on ion transport in biological (model) systems in general.