Characteristics of the internal anal sphincter and the rectum of the vervet monkey.

Abstract

The physiology of the internal anal sphincter of the vervet monkey was investigated. Strips of sphincter in vitro contracted to noradrenaline [norepinephrine] and adrenaline [epinephrine]; adrenoceptors were mainly .alpha.-excitatory. Strips of rectal circular muscle relaxed to noradrenaline and contained both inhibitory .alpha.- and .beta.-adrenoceptors. All strips contracted to acetylcholine. After hyoscine or atropine, high doses of acetylcholine relaxed all strips by stimulating intramural inhibitory neurons as relaxations were blocked by tetrodotoxin and hexamethonium. Nicotine and DMPP [dimethyl phenyl piperazinium] gave relaxations with similar characteristics. Relaxations to acetylcholine, nicotine and DMPP were not adrenergic as relaxations still occurred in strips from sympathetically denervated or reserpinized animals. The block of these relaxations by propranolol and guanethidine was considered to be unrelated to their actions as adrenergic blocking drugs. All strips relaxed to field electrical stimulation (1-5 Hz) through stimulation of intramural inhibitory neurones as tetrodotoxin blocked these relaxations. Adrenergic blocking drugs, prior reserpinization or prior section of the hypogastric nerves did not block these responses. The relaxations were adrenergic. 5-Hydroxytryptamine [5-HT] relaxed all strips but was not the transmitter in relaxation to acetylcholine, DMPP or nicotine, nor to field electrical stimulation, as desensitization of strips to 5-HT did not alter these responses. The ciccular smooth muscle of the internal anal sphincter had a dense terminal adrenergic innervation which rapidly decreased orad. In vivo, hypogastric nerve stimulation relaxed the rectum but contracted the sphincter. Sacral nerve root stimulation caused an after-contraction in both rectum and sphincter. In vivo, a close arterial injection of adrenaline or noradrenaline inhibited the spontaneous contaction waves of the rectum, but contracted the sphincter. Both these responses were blocked by phentolamine. The internal anal sphincter is apparently a discrete high pressure zone which has excitatory cholinergic and adrenergic innervations and an inhibitory non-adrenergic innervation.