Structure of the axolemma of frog myelinated nerve: relaxation experiments with a lipophilic probe ion.

Asymmetrical displacement currents are measured in the absence and in the presence of the lipophilic anion dipicrylamine (DPA) in the extracellular solution of nerve fibers of the frog Rana esculenta. DPA (30 nM--3 microM) enhances the current by a component that has the properties expected for a translocation current of DPA ion across the lipid membrane. Analysis in terms of a single-barrier model yields the translocation rate constant (k), the total surface density of DPA absorbed to the membrane (Nt), and the equidistribution voltage (psi). The value of kappa of about 10(4) s-1 is similar to that for a solvent-free artificial bilayer formed by the Montal-Mueller method. The surface density Nt varies with the DPA concentration as it does in the artificial bilayer, but is about tenfold smaller at all concentrations. The DPA ions sense an intrinsic electric field that is offset by a transmembrane voltage between 0 and 30 mV (inside positive). The part of the axolemma probed by the DPA ion appears as a thin ( less than 2.5 nm), fluid bilayer of lipids. DPA ions seem, however, to be excluded from the major part of the axolemma as if this area is occupied by integral proteins or negative charges.