The nature of the prolonged endplate depolarization in anti-esterase treated muscle

In anti-esterase treated muscle, repeated nerve impulses give rise to a cumulative depolarization of the endplate which persists for several seconds after the end of the tetanus. It has been suggested that this phenomenon may be due to a side-effect of the esterase inhibitors, namely a stabilization of opened ionic channels in the endplate membrane per­sisting after the transmitter has diffused away, rather than - as pre­viously supposed - to an accumulated residue of acetylcholine which continues to act during the slow depolarization. The question has been studied by intracellular recording of endplate potential and current, in glycerol-pretreated frog sartorius muscle. Noise analysis has been used to distinguish between the alternative explanations. The results show that the post-tetanic depolarization of anti-esterase treated endplates is accompanied by membrane fluctuations which resemble typical ‘acetyl­choline noise’, in size and time characteristics. It is concluded that at least a large part of the prolonged depolarization arises from continued action of a residue of acetylcholine which has accumulated during the tetanus and disappears slowly thereafter.