Clonidine delays small intestinal transit in the rat

Subcutaneous clonidine (0.01--1.0 mg/kg) was found to delay small intestinal transit but not gastric emptying in the unanesthetized rat, with a maximal effect seen at 0.1 mg/kg. Gastric emptying was expressed as the percentage of intragastrically administered 51Cr emptied into the small intestine after 45 min. Small intestinal transit was the percentage of the small intestinal length traveled 45 min after oral or duodenal administration of black ink. The depression of small intestinal transit by clonidine to 20 to 30% of control values was blocked by phentolamine and yohimbine, but not by prazosin or phenoxybenzamine, suggesting a presynaptic (alpha-2) agonist action of clonidine. Pretreatment of rats with 6-hydroxydopamine, propranolol, atropine, methysergide, naloxone, mepyramine or metiamide failed to alter the effects of clonidine. These results suggest that an alpha adrenergic receptor, possibly presynaptic, regulates small bowel propulsion in rat without involvement of acetylcholine, norepinephrine, endorphins, histamine or serotonin.