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

Abstract

SUMMARY1. End-plate currents have been studied in glycerol-treated frog sartorius nerve-muscle preparations with the voltage-clamp technique.2. Adding the anticholinesterase prostigmine (3/gM) 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 /tM) 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 /tM), cobratoxin (0.13 ,sm), or oc-bungarotoxin (0.13-0*26 /SM) decreased the time constant of decay of end-plate currents in the presence of prostigmine.5. These observations are consistent 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 occurs 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 AM) or carbachol (40 AM) applied in the solution bathing the muscle prolongs end-plate currents in the presence of prostigmine.

Mechanisms of the inhibition by neostigmine of tetanic contraction in the mouse diaphragm

“…What is the consequence of the acetylcholine accumulation in the synaptic cleft? The time-course of e.p.ps was prolonged markedly (Katz & Miledi, 1973;Hartzell et al, 1975;Magleby & Terrar, 1975), so that during repetitive stimulation the membrane potential could not recover before the next pulse. This resulted in a progressive depolarization of the endplate area, with the shorter the interval between pulses the more marked the depolarization.…”

The effect of (+)‐tubocurarine on neuromuscular transmission during repetitive stimulation in the rat, mouse, and frog.

“…The voltage-clamping apparatus was similar to that described previously (Connor & Stevens, 1971;Magleby & Terrar, 1975). Microelectrodes were filled with 3 M-KCI and had resistances in the range 5-20 MCI.…”

Nickel and calcium ions modify the characteristics of the acetylcholine receptor‐channel complex at the frog neuromuscular junction.

“…Two possible reasons for the slower decay under these conditions are: The individual channels opened by ACh could have enhanced lifetimes in the presence of Ni2+; alternatively, Ni2+ could inhibit the cholinesterase that degrades ACh so that the time course of the m.e.p.c. would then be partially controlled by the time course of escape of ACh from the synaptic cleft (Katz & Miledi, 1973b;Magleby & Terrar, 1975).…”