The Effect of Anticholinesterase Action on Mammalian Skeletal Neuromuscular Transmission

Abstract

A brief outline of skeletal neuromuscular transmission as a sequence of events leading to muscular contraction and the actions of anticholinesterases is presented in the Introduction. The work has been largely concerned with the irreversible inhibition of cholinesterase by ecothiopate (phospholine) at the neuromuscular junction, thereby altering the time course of transmitter action, and the subsequent effect on the contractile response of isolated mammalian diaphragm preparations. Ecothiopate (5 x 10-7M) evokes a complex and, apparently, time —- dependent series of changes in the contractile response, one of which, the prolonged endplate- localised contraction, was believed to be a new observation and was to be the main subject of further study. Electrophysiological, radioisotopic and histological methods have been employed to determine the cause of these prolonged contractions. They have been found to be associated with prolonged currents and with an accumulation of calcium,both at the endplates, It has not been possible to exclude a direct effect of membrane depolarisation or an effect of post ~ junctional calcium entry as the cause of the prolonged localised contraction, It has been concluded that ecothiopate (5 x 10-7M) is unlikely to have a significant direct effect on the changes in contraction and endplate calcium accumulation. A biochemical method to determine cholinesterase activity has been devised which is believed to give results which represent endplate cholinesterase activity. An attempt has been made to correlate the ecothiopate-meaditated changes in contractile response with the degree of cholinesterase inhibition. It has been concluded that the time-dependent changes in the contractile response after ecothiopate are mainly due to a time-dependent inhibition of cholinesterase in non~ perfused muscles. Finally, the waning of the ecothiopate~ mediated changes in contraction have been suggested to be due to a reduction in transmitter action, perhaps, by receptor desensitisation. The association between desensitisation and endplate calcium accumulation has been discussed,