1. End-plate currents have been studied in gylcerol-treated frog sartorius nerve-muscle preparations with the voltage-clamp technique. 2. Adding the anticholinesterase prostigmine (3 muM) to the solution bathing the muscle caused a 2-7 (mean 3-3) times increase in the time constant of decay of end-plate currents. The anticholinesterase edrophonium (15 muM) also prolonged the time course of end-plate currents. 3. Pre-treatment of the preparation with collagenase, which leads to the removal of acetylcholinesterase in the synaptic cleft, prolongs the time course of end-plate currents. 4. Curare (1-2 muM), cobratoxin (0-13 muM), or alpha-bungarotoxin (0-13-0-26 muM) decreased the time constant of decay of end-plate currents in the presence of prostigmine. 5. These observations are consistant with the suggestion that repeated binding of acetylcholine (ACh) molecules to receptors as the ACh escapes from the synaptic cleft can contribute to the prolongation of end-plate currents which occurrs when acetylcholinesterase activity is eliminated. 6. Increasing the amount of transmitter released from the presynaptic nerve terminal leads to a prolongation of end-plate currents in the presence of prostigmine. 7. In the presence of prostigmine, the second of two end-plate currents (interval 2-10 msec) decays more slowly than the first. 8. ACh (1-40 muM) or carbachol (40 muM) applied in the solution bathing the muscle prolongs end-plate currents in the presence of prostigmine. 9. It is suggested on the basis of the observations described in paragraphs 6 to 8 that the time constant of decay of end-plate currents in the presence of prostigmine increases with increasing concentrations of ACh in the synaptic cleft. In the absence of prostigmine, increasing the concentration of ACh in the synaptic cleft did not change the time constant for decay of end-plate currents. 10. We interpret these results to suggest that ACh can have a cooperative action on receptors such that the association of ACh with one receptor (defined as binding a single ACh molecule) favours the binding or retention of ACh at other receptors. This implies that receptors can interact.

Factors affecting the time course of decay of end‐plate currents: a possible cooperative action of acetylcholine on receptors at the frog neuromuscular junction.

1. The mechanism of procaine action on post-synaptic receptors for acetylcholine was studied by recording the end-plate current at membrane potentials ranging from about +30 to about −140 mV.



2. It has been found that at resting membrane potential of about −60 to −80 mV the end-plate current has a fast initial and a slow late component. During hyperpolarization of the muscle fibre the amplitude of the slow component is depressed and its half-time lengthened. When the membrane potential is inverted the difference in the time course of both components is much less pronounced or absent.



3. It is suggested that procaine modifies the receptor response induced by acetylcholine, and that this modification is dependent on membrane potential.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1970.sp009186

The effect of procaine on neuromuscular transmission.

In this chapter neuromuscular transmission will be discussed mainly from an electrophysiological standpoint although some specifically pharmacological topics, in particular the properties of the acetylcholine receptors, will also be considered. Since most of what is known about pre-synaptic mechanisms is based on electrical recordings of the muscle response, we have thought it best to discuss the relevant properties of muscle, as well as post-synaptic events during transmission, before considering the details of transmitter release. The main emphasis throughout will be on results obtained from experiments on isolated tissues; information on corresponding in vivo work, and a more broadly based account of the pharmacology of neuromuscular transmission, may be found in Chapter IV.

Transmission of Impulses from Nerve to Muscle

1. Atropine action on the end-plate currents (e.p.c.s) has been analysed at the macroscopic and elementary levels. 2. The shortening effect of atropine on the e.p.c. and m.e.p.c. level can be fully explained by a reduction of the life time of the elementary current: this effect is markedly increased at more hyperpolarized membrane potentials and at higher concentrations of atropine. 3. It is therefore suggested that atropine binds to the open acetylcholine-receptor complex, leading to a state with a null conductance. According to this model, the forward rate constant of atropine binding could be calculated and was of the order of 10(7) M-1 S-1 AT -90 MV and 20-22 degrees C. 4. Although the conductance at the peak of the e.p.c. is reduced by atropine and becomes voltage sensitive, the elementary conductance is affected neither by voltage nor by atropine. 5. The exclusive binding of atropine to the activated ACh-receptor complex, as proposed above, does not appear to explain this phenomenon. Another binding occurring before the channel is open with a dissociation constant of 60 micrometer could account for this effect.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1977.sp011895

M. Kordasˇ

Papers

The effect of atropine and curarine on the time course of the end-plate potential in frog sartorius muscle