EFFECTS OF MEDETOMIDINE ON INTESTINAL AND COLONIC MOTILITY IN THE DOG

The motor responses of the jejunum and colon to stimulation of alpha2- adrenoceptors by medetomidine and clonidine were investigated in four dogs. In fasting dogs, medetomidine, at a dose rate of 30 mug/kg i.v., disrupted the migrating myoelectric complex (MMC) pattern of the smal l intestine for about 2 h. Similar, but shorter-lasting effects were a lso induced by clonidine (30 mug/kg i.v.) on the jejunum. The administ ration of alpha2-agonists inhibited colonic motility in fasting dogs, although medetomidine-induced inhibition was preceded by a short perio d of increased muscle tone. All these effects were reversed by the alp ha2-antagonists atipamezole (0.15 mg/kg i.v.) and yohimbine (0.20 mg/k g i.v.). In fed dogs, medetomidine (30 mug/kg i.v.) induced a strong i ncrease of the tone on the proximal colon, while the activity of the m edium and distal colon was completely suppressed. Yohimbine (0.50 mg/k g i.v.) immediately restored the activity of the colon and induced a p ropagated giant contraction and defaecation by the animal. These data confirm the importance Of alpha2-adrenergic receptors in the control o f intestinal and colonic motility in the dog.