Local anesthetics noncompetitively inhibit terbium binding to the exterior surface of nerve membrane vesicles

It has previously been shown that terbium binds to membrane vesicles prepared from the walking leg nerve of the lobster (Homarus americanus) with a high affinity Kd of 2.2 microM. Fluorescence of bound Tb3+ occurs via energy transfer from the aromatic residues of proteins (gamma ex = 280 nm; gamma em = 546 nm), and calcium inhibits Tb3+ binding competitively with a Ki of 1.8 mM. Displacement studies with EDTA demonstrate that more than 95% of the bound Tb3+ is at the vesicle exterior and is not being taken up by the vesicles. To investigate the putative role of Ca2+ in the interaction of local anesthetics with axonal membranes, lidocaine and the analogs GX-HCl and QX-314 were tested as inhibitors of Tb3+ binding. Inhibition by lidocaine is seen only at considerably higher doses (25 mM) than are required for conduction block of intact nerves (5 mM). Inhibition by lidocaine and the primary amine analog GX-HCl is entirely noncompetitive, whereas the quaternary ammonium derivative QX-314 appears to be a mixed competitive-noncompetitive inhibitor of Tb3+ binding. These data are not compatible with the hypothesis that there is a functionally essential cation binding site on the axonal membrane surface for which Ca2+ and local anesthetics compete, although local anesthetic action may be modified indirectly by altered calcium concentrations. Evidence is presented for a mechanism by which local anesthetics indirectly displace Tb3+ by altering the physical state of the axonal membrane.