Immediate X-radiation induced contractions of the isolated guinea pig terminal ileum: the localization of the effect by drugs.

Tone and motility of the isolated guinea pig ileum were increased by irradiation with a dose of 10 krd. The maximal effect corresponds to that induced by 0.001 microng/ml acetylcholine or 0.3 microng/ml nicotine. The pharmacological analysis of this effect performed with acetylcholine and nicotine and several blocking agents including hexamethonium, atropine, tetrodotoxin, diphenhydramine, and verapamil suggests that radiation acts on the postganglionic parasympathetic neuron and the neuromuscular synapse. The mechanism of radiation is likely to consist of both an increased release of acetylcholine from the postganglionic neuron and a sensibilization of the cholinergic receptor site at the smooth muscle cell. The latter effect is thought to result from an increased contractile action induced by acetylcholine or nicotine in the irradiated ileal smooth muscle.