Effect of diisopropylfluorophosphate and soman on rat skeletal muscle contracture during tetanic stimulation.

A number of reports have documented the myopathy that accompanies exposure to irreversible acetylcholinesterase inhibitors. However, less information is available on the functional consequences of the myopathy. The purpose of this study was to examine the effects of diisopropylfluorophosphate (DFP) and soman on skeletal muscle contracture. Rats were given daily doses of DFP or soman, and the tetanic contracture of the triceps surae muscles was recorded at various exposure periods up to 20 d. DFP decreased tetanic contracture during 5-Hz stimulation. Increased stimulation up to 50 Hz did not enhance the effect. The muscles showed the greatest deficit on d 5. Continued dosing produced a slightly attenuated effect. Soman produced a frequency-dependent increase in muscle contracture. The increase was greatest on d 2 and 4, but remained elevated over the 20-d study. These results show that two drugs that presumably both raised levels of transmitter in the synaptic cleft as a result of esterase inhibition can produce either enhanced or decreased muscle contraction.