Permeability of axon membranes to local anesthetics

The permeability of the neutral form of tertiary amine local anesthetics across squid axon membranes was studied by utilizing three different experimental methods: (1) narcotic action of axon excitability was measured by monitoring the time derivative of action potential and the results were analyzed in terms of a diffusion reaction equation of local anesthetics to obtain their permeabilities; (2) the influx of local anesthetic into the axon was measured by use of the radioisotope tracer technique; and (3) the desorption rates of the neutral form of local anesthetics from lipid monolayers were measured and the desorption rate was correlated with permeability. The relative permeabilities obtained for procaine, lidocaine and tetracaine by the above three methods were comparable. The order of relative permeabilities was procaine greater than lidocaine greater than tetracaine, and had an inverse correlation with the partition coefficients of anesthetics at oil/water phases. Some discussion concerning the concept of permeability is made when the partition coefficient of a permeant molecule is high.