[Effects of metacaine and its decomposition products on the excitation mechanism of isolated myelinated nerve fibers].

The blocking effects of metacaine-methanesulfonate (MS-222; abbreviated MMS) and of the corresponding hydrochloride (MHC) on the ionic currents in potential clamped myelinated nerve fibres were investigated. The dose-response relationships of MMS for blocking the Na-currents and K-currents could be satisfactorily fitted to the Langmuir adsorption isotherm with KNa = 0.7 mmol/l and KK = 1.8 mmol/l. The observed loss of efficacy in blocking ionic currents of 3-day-old solutions containing MMS was assumed to be due to a partial decomposition of the drug. We separated the water-soluble decomposition products by extraction of the metacaine base. The remaining aqueous solution showed an anti-blocking effect on metacaine blocked Na-currents. From corresponding experiments on the blocking effect of MHC we conclude that the anti-blocking effect is due to yet unknown decomposition products, formed in MMS but not in MHC solutions.