Central and Peripheral α‐Adrenergic Control of Ileal, Caecal and Proximal Colonic Myoelectrical Activity in Sheep

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Abstract
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 .mu.g/kg i.v. or 60 .mu.g/kg i.c.v.) induced an RSA phase in the ileum. The .alpha.2-agonist naphazoline (30 .mu.g/kg i.v. or 3 .mu.g/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 .mu.g/kg i.v., but not 60 .mu.g/kg i.c.v.) selectively reduced the frequency of contractions in the cranial caecum and proximal colon. The .alpha.1-agonist phenylephrine (40 .mu.g/kg i.v., but not 4 .mu.g/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.