1. 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 paralysed by local application of botulinum toxin (BoTx) type A, on unpoisoned muscles, surgically denervated or reinnervated muscles.2. The ACh content of unpoisoned control muscles was nearly uniform between animals and varied in the experimental series between 36 and 50 pmol. BoTx failed to affect the ACh content after 2 d of poisoning and caused a slight increase in content after 8 d. Surgical denervation reduced the ACh content within 24 h to less than 10% of innervated muscles and upon reinnervation the ACh content was restored. Following cholinesterase inhibition the ACh content of innervated and denervated muscles increased somewhat, about equally with time.3. Spontaneous release of ACh varied in normal innervated muscles between 40 and 100 fmol/min. In the presence of 25 mm-KCl the rate of release increased about fourfold. In BoTx poisoned muscles spontaneous release was reduced by up to 60% of control and high potassium failed to accelerate the release at 2 d after poisoning and caused only a small increase at 8 d. Denervated muscles released ACh at a rate which was less than 20% of control and it was not accelerated by high potassium.4. The results show that more than 90% of total ACh in the innervated EDL muscle is 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. It suggests that BoTx has an inhibitory effect also on non-quantal ACh release.
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