Acetylcholine content and release in denervated or botulinum poisoned rat skeletal muscle

Abstract

The acetylcholine (ACh) content and spontaneous and evoked release of ACh in rat extensor digitorum longus (EDL) muscles were determined by pyrolysis-mass fragmentography. The determinations were made on muscles paralyzed by local application of botulinum toxin (BoTx) type A, on unpoisoned muscles, surgically denervated or reinnervated muscles. The ACh content of unpoisoned control muscles was nearly uniform between animals and varied in the experimental series between 36-50 pmol. BoTx failed to affect the ACh content after 2 days of poisoning and caused a slight increase in content after 8 days. Surgical denervation reduced the ACh content within 24 h to < 10% of innervated muscles; upon reinnervation, the ACh content was restored. Following cholinesterase inhibition, the ACh content of innervated and denervated muscles increased somewhat, about equally with time. Spontaneous release of ACh varied in normal innervated muscles between 40-100 pmol/min. In the presence of 25 mM-KCl the rate of release increased .apprx. 4-fold. In BoTx poisoned muscles spontaneous release was reduced by up to 60% of control; high K failed to accelerate the release at 2 days after poisoning and caused only a small increase at 8 days. Denervated muscles released ACh at a rate which was < 20% of controls; it was not accelerated by high K. Thus, 90% of total ACh in the innervated EDL muscle is evidently present in the nerve and its terminals. The remaining ACh is apparently formed and stored in the muscle tissue. BoTx caused a larger reduction in ACh release than can be accounted for by assuming a selective blockade of quantal release of transmitter. Evidently, BoTx has an inhibitory effect also on non-quantal ACh release.