Interaction between hemicholinium-3 and edrophonium on the isolated taenia of the guinea-pig caecum

Endrophonium produced a contraction of the taenia of the guinea-pig caecum which was inhibited by hyoscine and by hemicholinium-3 (HC-3) but not by hexamethonium and only slightly by tetrodotoxin or cocaine. Reduced concentrations of Ca²⁺ inhibited contractions produced by edrophonium to a similar extent to contractions produced by acetylcholine. These results suggest that edrophonium directly stimulates hyoscine-sensitive receptors on the taenia and that HC-3 can differentiate between the action of edrophonium and acetylcholine. Endrophonium which selectively inhibits acetylcholinesterase augmented contractions produced by acetylcholine or by transmural stimulation, indicating that acetylcholinesterase is concerned in the inactivation of the transmitter at postganglionic cholinergic nerve endings. HC-3 did not inhibit the anticholinesterase activity of edrophonium or the augmentation of responses to acetylcholine and to transmural stimulation produced by edrophonium.