Effect of guanethidine in revealing cholinergic sympathetic fibres.

Guanethidine abolished the inhibitory response of segments of rabbit intestine to stimulation of the sympathetic nerves which accompany the mesenteric arteries. In the majority of experiments a motor response of the intestinal segment was then revealed; it was more readily observed in intestinal segments from young than from adult rabbits. The motor response of the intestine to sympathetic stimulation after guanethidine was blocked by atropine. It was not blocked by hexamethonium and was present in rabbits in which the vagal innervation to the small intestine had been sectioned. In the cat isolated atria, guanethidine blocked the accelerator response to sympathetic nerve stimulation and revealed a response resembling that to stimulation of the vagus. It was concluded that guanethidine blocked the release of noradrenaline and thus revealed the response to the direct action of acetylcholine released from cholinergic sympathetic nerves.