Alpha2-adrenoceptor agonists inhibit the migrating myoelectric complexes of the small intestine in rats

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Effects of medetomidine on intestinal and colonic motility in the dog

The motor responses of the jejunum and colon to stimulation of alpha 2-adrenoceptors by medetomidine and clonidine were investigated in four dogs. In fasting dogs, medetomidine, at a dose rate of 30 micrograms/kg i.v., disrupted the migrating myoelectric complex (MMC) pattern of the small intestine for about 2 h. Similar, but shorter-lasting effects were also induced by clonidine (30 micrograms/kg i.v.) on the jejunum. The administration of alpha 2-agonists inhibited colonic motility in fasting dogs, although medetomidine-induced inhibition was preceded by a short period of increased muscle tone. All these effects were reversed by the alpha 2-antagonists atipamezole (0.15 mg/kg i.v.) and yohimbine (0.20 mg/kg i.v.). In fed dogs, medetomidine (30 micrograms/kg i.v.) induced a strong increase of the tone on the proximal colon, while the activity of the medium and distal colon was completely suppressed. Yohimbine (0.50 mg/kg i.v.) immediately restored the activity of the colon and induced a propagated giant contraction and defaecation by the animal. These data confirm the importance of alpha 2-adrenergic receptors in the control of intestinal and colonic motility in the dog.

Suppression of small intestinal motility and morphine withdrawal diarrhoea by clonidine: peripheral site of action

The effects of systemic administration of morphine (4.0 mg kg-1) and clonidine (2.5-10.0 micrograms kg-1), as well as the peripherally active alpha 2-adrenoceptor agonist oxymetazoline (2.8-11.2 micrograms kg-1), were studied on the migrating myo-electric complexes (MMCs) of the small intestine in conscious, naive rats. Furthermore, the effects of naloxone (1.0 mg kg-1) and the peripherally acting opioid antagonist, methylbromide naloxone (1.0-2.0 mg kg-1) were studied on the MMCs in morphine-dependent animals. Similar doses of clonidine or oxymetazoline inhibited the MMCs of the small intestine, which suggests a peripheral site of action of clonidine. Since naloxone (1.0 mg kg-1) did not antagonize the effect of clonidine, and yohimbine (1.0 mg kg-1) failed to antagonize the effect of morphine on the MMC, the inhibitory effects on intestinal motility of clonidine and morphine are mediated through different receptors. Morphine-dependent rats showed a prolonged interval between activity fronts and decreased propagation velocity of the activity fronts. Both naloxone (1.0 mg kg-1) and methylbromide naloxone (1.0-2.0 mg kg-1) induced intense spiking activity and profuse diarrhoea. Clonidine (5.0-10.0 micrograms kg-1) as well as oxymetazoline (5.6-11.2 micrograms kg-1) given prior to naloxone prevented the intense spiking as well as the concomitant diarrhoea. We conclude that the potent inhibition of small intestinal myoelectric activity by alpha 2-adrenoceptor agonists is mainly executed via peripheral mechanisms. This effect may contribute to their beneficial action in morphine withdrawal diarrhoea, and partly underlie a general antidiarrhoeal action.(ABSTRACT TRUNCATED AT 250 WORDS)