Neuromuscular effects of chronic administration of two organophosphorus insecticides to rats.

Groups of rats were fed chlorfenvinphos (at two dose levels) or dicrotophos. Whole blood and plasma cholinesterase activity was markedly reduced. Compared with control animals there was no change in muscle action potential amplitude (M response) to a single stimulus for periods of up to 3 months. Exposed animals developed a prolonged negative potential at the end of the spike potential, lasting up to 15 ms. Superimposed on this, repetitive activity (RA) developed with latency of approximately 4.5 ms. These abnormalities became more marked with time, even on constant dosing. Double and repetitive stimulation reduced or abolished the prolonged negative potential and RA.