Osmotically-induced trapping of terbium ions in axonal membrane vesicles

Abstract

1. Using terbium ions as fluorescence probes of calcium-binding sites and osmotic shock to induce trapping of Tb 3+ in the vesicle interior, direct binding assays have been developed to study the competition between calcium and local anesthetics for binding sites at the cytoplasmic surface of axonal membrane vesicles. 2. Pharmacologically active concentrations of the membrane-permeable local anesthetic, lidocaine, competitively displace bound Tb 3+ in the vesicles, while QX-314, a quaternary ammonium analog of lidocaine that has poor access to the vesicle interior, exhibits no significant displacement of osmotically-loaded, internally-bound Tb 3+. 3. These experiments support the hypothesis that local anesthetics may function by displacing Ca 2+ from a functionally specific binding site in nerve membranes.