End-plate currents evoked by paired stimuli in frog muscle fibres

Abstract

Using voltage-clamp techniques spontaneously occuring miniature end-plate currents (mepc) and nerve-evoked end-plate currents (epc) were recorded in frog glycerol-treated or cut muscle preparations. Epcs were induced by pairs of stimuli (the delay of the 2nd stimulus, Δt being 6 ms–30 s; one pair was delivered every 60–90 s). The decay time constant of the epc (τ_epc) was longer, the larger its quantal content despite the presence of active acetylcholinesterase (AChE). After treatment with anticholinesterases (prostigmine or armin, an irreversible inhibitor) this increase in τ_epc became more pronounced. When AChE was fully active the decay of the 1st epc τ₁ was slightly faster than the decay of the 2nd epc τ₂ only when the interstimulus interval was rather short (Δt<20 ms). Following treatment with anticholinesterases this difference between τ₂ and τ₁ could be determined even when Δt was as long as 30 s. In anticholinesterase-treated preparations Δτ was found to be inversely proportional to log Δt: a 50% increase in the decay time-constant of the 2nd epc occurred with Δt=120 ms. During continuous stimulation (10 impulses/s) τ_epc increased from the 1st to the 5–6th responses, but then decreased in parallel with the fall in the epc amplidude. The phenomenon of postsynaptic potentiation we observed could be readily abolished when quantal content was decreased by the presence of magnesium ions, but it was relatively unaffected when the receptor density was decreased by α-bungarotoxin (α-BuTX).

The possible existence is discussed of two kinds of repetitive binding of ACh molecules, first, to free cholinoreceptors (a process which could be inhibited by α-BuTX) and, second, to a complex of the cholinoreceptor plus one molecule of ACh (a process which is less sensitive to α-BuTX blocking action).