Central and peripheral alpha-adrenergic control of ileal, caecal and proximal colonic myoelectrical activity in sheep.

The role of central and peripheral alpha-adrenoceptors in regulation of ileal, caecal and proximal colonic myoelectrical activity was studied in five conscious ewes chronically fitted with intraparietal electrodes and a cannula in a lateral ventricle of the brain. About 5 min after the end of a regular spiking activity (RSA) phase of a migrating motor complex (MMC) in the distal ileum, selective alpha 2- and alpha 1-agonists and antagonists were administered intravenously (i.v.) or intracerebroventricularly (i.c.v.). The alpha 2-antagonist yohimbine (600 micrograms/kg i.v. or 60 micrograms/kg i.c.v.) induced an RSA phase in the ileum. The alpha 2-agonist naphazoline (30 micrograms/kg i.v. or 3 micrograms/kg i.c.v.) stopped or greatly reduced the frequency of caecal and proximal colonic contractions, respectively, and prevented any MMC in the ileum, effects which were abolished when the animals were either i.v. or i.c.v. pretreated with yohimbine. The alpha 1-antagonist prazosin (600 micrograms/kg i.v., but not 60 micrograms/kg i.c.v.) selectively reduced the frequency of contractions in the cranial caecum and proximal colon. The alpha 1-agonist phenylephrine (40 micrograms/kg i.v., but not 4 micrograms/kg i.c.v.) provoked an RSA phase in the ileum, decreased the frequency of contractions in the blind pole and the middle of the caecum and increased that in the cranial caecum and the proximal colon. These effects of phenylephrine were blocked by i.v., but not i.c.v., prazosin. The present results suggest that alpha-adrenoceptors are involved in the control of the intestinal motility as follows: I) central alpha 2-inhibitory and peripheral alpha 1-excitatory receptors for the ileum, II) central and peripheral alpha 2- and peripheral alpha 1-inhibitory receptors for the blind pole and the middle of caecum and III) central and peripheral alpha 2-inhibitory and peripheral alpha 1-excitatory receptors for the cranial caecum and the proximal colon.(ABSTRACT TRUNCATED AT 250 WORDS)