CHOLINESTERASE INHIBITION AND SPONTANEOUS ACTIVITY OF THE FROG RECTUS ABDOMINIS MUSCLE

¹ Department of Pharmacology, Jefferson Medical College, Philadelphia, Pennsylvania

Inhibition of the cholinesterase of the frog rectus abdominis muscle leads to episodes of spontaneous shortening. The constraction may develop during a fraction of a second and engage a major part of the muscle in a single effort, indicating synchronization of a number of motor units.

The effectiveness of various inhibitors in evoking activity generally parallels their ability to sensitize the muscle to acetylcholine, with TEPP most consistently active and neostigmine somewhat inferior. The property is manifested in varying degrees by unsymmetrical diethyl bis (dimethylamido) pyro phosphate (B-6515), pyridostigmine and edrophonium, while OMPA and physostigmine are ineffective. Spontaneous contractions in 10^-6 TEPP or 10^-5 B-6515 occur only after approximately 90% of the cholinesterase of the muscle has been inactivated.

Localization experiments and inhibition by curare and other drugs that depress the responses to acetylcholine indicate that a fully sensitive neuromuscular junction is essential to the development of the activity.

The twitch and tonus components of the rectus are both brought into activity by minimal concentrations of TEPP, while the other inhibitors at comparable levels activate principally twitch fibers.