Effects of benzocaine and its isomers on sodium permeability and steady state sodium inactivation in the myelinated nerve, obtained by an improved dissection technique.

The blocking effects of benzocaine and its isomers (1 mmol/l) on sodium currents in myelinated nerve fibres were tested. As far as the so-called fast sodium inactivation is concerned, benzocaine shifted the h alpha-curve in negative direction to a stronger extent than did its isomers, while the potency of the isomers did not differ significantly from each other. The drug-induced reductions of maximum sodium permeability PNa were tested at constant test pulses at h alpha = 1. In this kind of experiments all the three isomers had the same potencies. The findings could not be correlated to the lipid solubilities of the drugs as measured by the corresponding octanol/water partition coefficients. In addition, efforts were undertaken to minimize any noxious pull during the isolation of the axon. Some consequences of the improvements introduced are discussed in terms of the reliability of ionic current measurements in Ranvier nodes.