Cholinergic receptor labelling specific irreversible binding of bromoacetylcholine to cholinergic receptors at neuromuscular junctions

This study shows that (a) the slopes of dose-response curves of bromoacetylcholine (BrACh) and acetylcoline (ACh) were the same; (b) the dose-response curves of BrACh to contract superfused chick biventer cervicis and frog rectus abdominis muscles were shifted to the right by d-tubocurarine in a parallel fashio; and (c) the responses of BrACh were not affected by triethylcholine, which paralyzes cholinergic neurons through inhibition of ACh synthesis. These results indicate that BrACh interacts with cholinergic receptors specifically and directly. When the incubation time was prolonged to 15 min or longer in the organ bath, BrACh (2×10−3 M) produced irreversible blockade of the neuromuscular transmission in chick biventer cervicis and rat diaphragm preparations. The latter blockade was prevented partially by ACh (2×10−2 M) and d-tubocurarine (3.4×10−6 M) (25–38%) and markedly by carbachol (1×10−4 M) and decamethonium (6.2×10−4 M)_(76–79%), indicating agani that BrACh binds specifically and irreversibly to the cholinergic receptor. Therefore, 14C-BrACh is recommended for use in mapping, estimation, and isolation of cholinergic receptors. In order to do such experiments, the biological preparations would have to be pretreated with cold organophosphorus compounds in order to prevent irreversible binding of BrACh to cholinesterases.