Acetylcholinesterase activity of skeletal muscle in a non-immunogenic model for myasthenia gravis in rats.

Myasthenia gravis is caused by an autoimmune attack to acetylcholine receptors of skeletal muscle. Acetylcholine release from motor nerve terminals is upregulated in patients with myasthenia gravis and also in rat "myasthenic" models, dependent on the reduction of the number of acetylcholine receptors. This study addresses the question as to whether at "myasthenic" endplates there are changes in the activity of acetylcholinesterase. To this end we studied acetylcholinesterase activity in junctional and extrajunctional regions of dilator naris, extensor digitorum longus, and hemidiaphragm muscles from rats with alpha-bungarotoxin-induced myasthenia gravis. In all studied muscles from "myasthenic" rats there was no significant change of junctional acetylcholinesterase activity. In contrast, in dilator naris and extensor digitorum longus muscles, there was a 60% and 30% increase of extrajunctional acetylcholinesterase activity. There was no significant change in the extrajunctional activity in hemidiaphragm muscles. Velocity sedimentation analysis revealed that the increase in extrajunctional activity in extensor digitorum longus muscles could be attributed to an increase of the activity of the G4 form of acetylcholinesterase. Treatment of rats with 6.4 microgh(-1) neostigmine bromide for 29 days had no influence on junctional and extrajunctional acetylcholinesterase activity of extensor digitorum longus muscles from rats with alpha-bungarotoxin-induced myasthenia gravis.