Comparison of Cholinergic and Neuromuscular Toxicity Following Acute Exposure to Sarin and VX in Rat

Male Sprague-Dawley rats injected with a sublethal sc dosage of 110 micrograms/kg of sarin (isopropyl methylphosphonofluoridate), or 12 micrograms/kg of VX (S-(2-diisopropylaminoethyl) O-ethyl methylphosphonothioate), developed severe toxic signs within 5-15 min after sarin and 20-50 min after VX lasting for 5 to 7 hr. Myonecrotic lesions were seen in soleus and diaphragm muscles within 1 hr. A maximum number of lesions had developed after 24 hr, and lesions were also present in extensor digitorum longus (EDL) at this time. Regeneration of muscle fibers was slow since lesions were still evident past 7 days of treatment. Within 1 hr following VX, AChE activity was reduced to 8, 12, and 17% of control activity in soleus, diaphragm, and EDL, respectively, whereas with sarin the enzyme activity was reduced to 23, 48, and 82% of control. A still greater inhibition was seen 24 hr after sarin when AChE activity was reduced to 19, 13, and 43% in these muscles. In skeletal muscles the different molecular forms of AChE, such as 16 S, 12 S, 10 S, and 4 S vary in location and functional importance with the 16 S form highly concentrated at the neuromuscular junction. All forms in a given muscle were equally sensitive to the inhibitors. In EDL, sarin was the least effective in reducing AChE or its molecular forms. In the brain structures (cortex, brain stem, striatum, and hippocampus), AChE activity was reduced to 1-6% of control by sarin and VX with the exception that following VX striatal AChE was reduced to only 41% of control activity. AChE activity in the brain cortex following either of the agents was maximally affected (1%). A slow but significant recovery of brain AChE was evident after 24 hr and more so after Day 7. Butyrylcholinesterase (BuChE) activity was less sensitive to inhibition by both inhibitors compared to AChE activity and showed a rapid recovery. Based on the equitoxic doses (toxic signs of similar magnitude), VX was found to be 10 times more toxic than sarin. The mechanisms of this disparity may be due to differences in rate of uptake, circulation, susceptibility to hydrolysis, and reactivity with nonspecific binding sites.