Competitive adsorption of Ca2+ and local anesthetics on lipid membrane surfaces

Adsorption of tertriary amine local anesthetics and Ca2+ onto lipid membranes having various negative surface charge densities was studied by measuring lipid vesicle electrophoretic mobility. As the surface charge density of the membrane was reduced, the adsorption of the local anesthetics dominated that of the divalent cation. For a relatively high negatively charged membrane, the adsorption of both local anesthetic and Ca2+ became comparable and competitive. It is deduced that the major factor for the adsorption of local anesthetic onto lipid membranes is due to simple physical partitioning between aqueous and membrane phases, and not due to ionic type of binding as seen for divalent cations with membranes. However, the adsorption of anesthetics is influenced by the surface potential of membranes which is in turn related to the surface concentration of local anesthetics near the membrane. The amounts of competitive adsorption of divalent cations and local anesthetics are analyzed with respect to their bulk concentrations and various surface change densities of the membranes. With the results of the above studies, a possible interpretation for the interaction site as well as the mode of adsorption of local anesthetics onto axon membranes is made in relation to divalent cation concentrations in the bulk phases.