The effects of gamma-radiation on intestinal motor activity and faecal pellet expulsion in the guinea pig.

The effects of whole-body gamma-radiation (10 Gy) on intestinal motor activity was examined in the small and large intestine of the guinea pig 18 hr post irradiation. Neurally mediated relaxations of isolated gut bath preparations were generally unaffected. However, the contractile responses to direct smooth muscle stimulation with the cholinergic muscarinic agonist carbachol or ganglionic stimulation of intrinsic cholinergic motor neurones were significantly increased in the duodenum and colon but not the jejunum. This increased sensitivity to cholinergic stimulation was reflected in an increased contractility and a shift in the concentration-response curves for carbachol. The specificity of radiation actions for cholinergic mediated contractions was further supported by the observation that histamine-evoked contractions were unaffected. In a second series of experiments we examined the effects of gamma-radiation on the rate of pellet expulsion from freshly excised colons. Both colons from irradiated animals and nonirradiated colons exposed to carbachol showed significantly faster rates of pellet expulsion, indicative of increased propulsive motility. Pretreatment of animals with 0.5 mg/kg sc of the 5HT3 receptor antagonist Granisetron prevented the effect of radiation and reduced the pellet expulsion rate to below normal. These results indicate that gastrointestinal motility disturbances seen in organ-bath preparations of the intestine from rats exposed to whole-body gamma-radiation may be related to an increased sensitivity of the cholinergic muscarinic system.