THE EFFECTS OF GAMMA-RADIATION ON INTESTINAL MOTOR-ACTIVITY AND FECALPELLET 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 p ig 18 hr post irradiation. Neurally mediated relaxations of isolated g ut bath preparations were generally unaffected. However, the contracti le responses to direct smooth muscle stimulation with the cholinergic muscarinic agonist carbachol or ganglionic stimulation of intrinsic ch olinergic motor neurones were significantly increased in the duodenum and colon but not the jejunum. This increased sensitivity to cholinerg ic stimulation was reflected in an increased contractility and a shift in the concentration-response curves for carbachol. The specificity o f radiation actions for cholinergic mediated contractions was further supported by the observation that histamine-evoked contractions were u naffected. In a second series of experiments we examined the effects o f 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 expul sion, indicative of increased propulsive motility. Pretreatment of ani mals with 0.5 mg/kg sc of the 5HT(3) receptor antagonist Granisetron p revented the effect of radiation and reduced the pellet expulsion rate to below normal. These results indicate that gastrointestinal motilit y disturbances seen in organ-bath preparations of the intestine from r ats exposed to whole-body gamma-radiation may be related to an increas ed sensitivity of the cholinergic muscarinic system.