The effects of anticholinesterases on the latencies of action potentials in mouse skeletal muscles.

1. The purpose of this investigation was to determine the long-term effects of a single dose of persistent anticholinesterases on muscle action potentials evoked by nerve stimulation. 2. Action potentials (APs), elicited by stimulation of the phrenic nerve, were recorded intracellularly in muscle fibres of mouse diaphragm. The time between stimulus and AP was measured and the variability of this latency was calculated during trains of APs. At the beginning of trains of APs there was an increase in latency, and this delay was also measured. 3. Within 3 h of subcutaneous injection, a single dose (500 nmol kg-1) of the anticholinesterase, ecothiopate produced about 90% reduction in the acetylcholinesterase activity of homogenates of mouse diaphragm muscle, but five days after injection, this activity was not different from values in untreated animals. The initial delay of APs and the variability of latencies were increased four fold and two fold respectively, remained at these maxima from the 1st to the 5th day after ecothiopate, and returned to the values in untreated animals between 15 and 27 days after ecothiopate. 4. These effects of ecothiopate on AP latency were dose-dependent and were also seen in extensor digitorum longus and soleus muscles. 5. Other anticholinesterases used were BOS (pinacolyl S-(2-trimethylaminoethyl)methylphosphonothioate), a quaternary compound, and diisopropyl fluorophosphate, a tertiary compound, which had effects similar to those of ecothiopate; the greater duration of the effects of this compound may be related to the greater duration of reduction in cholinesterase activity. 6. Ecothiopate had no effect on the delay or variability of latencies of endplate potentials which were recorded in cut-fibre preparations 5 days later. 7. It is concluded that the effects of ecothiopate on the latencies of indirectly-evoked muscle APs are postjunctional, may not be related to the degree of reduction in cholinesterase activity at the time of recording, and are not directly linked to necrosis.