The isolated internal anal sphincter from cats shows a myogenic spontaneous rhythmic activity. Basal tension and frequency of contraction waves from internal anal sphincter strips are inhibited by muscarinic and beta-adrenergic receptors and excited by alpha-adrenergic receptors. Isoprenaline in high doses had an alpha-adrenoceptor-stimulating activity in strips from the internal sphincter. The inhibitory muscarinic receptors may be the final effectors in the anorectal reflex. The relaxing effect of acetylcholine on the internal sphincter was sometimes preceded by a temporary contraction due to the interaction of intermingled longitudinal muscle fibres. Rectal circular muscle strips contain excitatory muscarinic receptors and inhibitory alpha- and beta-adrenergic receptors. The same can be said of the strips from the longitudinal muscle layer at the anal canal and at the rectum.
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