[Effect of the quantal composition, membrane potential and cholinoreceptor density on the temporal flow of the end plate current in the rat under conditions of acetylcholinesterase inhibition].

The factors determining the decay of multiquantal end plate currents (EPC) were studied in the diaphragm muscle of rat by the comparison of EPC and miniature EPC (MEPC) amplitude--temporal characteristics. The decay of EPC (quantal content 25-100) was 1.2 times slower than the decay of MEPC when AChE was active. The AChE inhibition by armine or neostigmine made this difference 10-100 times higher. In most synapses the decay of multiquantal EPC can be approximated by a sum of two or three exponents. It depended on the quantal content and 3-exponential EPC could be transformed in 2-exponential and later to monoexponential ones if increasing concentration of magnesium ions. A slow component of EPCs (but not of MEPC) decay was highly sensitive to concentration of magnesium ions and had 3 times higher dependence of the membrane potential value than that one of MEPC. The irreversible blocking of receptors by alpha-bungarotoxin (alpha-BuTX) accelerated the decay of MEPC but the decay of multiquantal EPC changed in two phases: it was prolonged at the beginning of alpha-BuTX action followed by its acceleration, but never the time of the decay of EPC had achieved the apparent open time of ACh-activated ionic channels. It is suggested that during the multiquantal EPC generation not only the synchronization of opening but the kinetic of ACh-activated channels is changed, probably by blocking of this channels by high concentrations of endogenous ACh.