A Kinetic Model for the Action of Xylocaine on Receptors for Acetylcholine

A kinetic scheme postulating the rapid formation of a partially active acetylcholine-receptor-drug complex from Xylocaine (or a derivative) and the active acetylcholine-receptor complex can account for the effects of Xylocaine and its derivatives at the neuromuscular junction. Transmembrane currents generated by an analogue computer programmed according to the scheme can exactly match end plate currents produced by nerve stimulation in the presence of the drugs. The scheme also accounts for the qualitatively different effects of the drugs on the end plate potential and on responses to iontophoretically applied acetylcholine. The analysis presented is consistent with very rapid reactions between acetylcholine and receptors, characterized by rate coefficients in the range 10⁴ to 10⁶ sec^-1. It is based on the hypothesis that the activation of receptors by acetylcholine changes the structure of the receptors and thus their affinity for Xylocaine. The analysis does not require pharmacological separability of sodium and potassium conductances during the end plate current.