In vitro response of the human anal canal longitudinal muscle layer to cholinergic and adrenergic stimulation: evidence of sphincter specialization.

A study was performed to determine the in vitro response of the longitudinal smooth muscle layer (conjoined longitudinal coat) of the human and canal to cholinergic and adrenergic stimulation, and to compare this with similar features of the internal anal sphincter (IAS) and rectal smooth muscle. Tissue was obtained from abdominoperineal and anterior resection specimens, and strips of muscle mounted for isometric tension recording in superfusion organ baths. Both conjoined longitudinal coat and IAS strips generated and maintained spontaneous myogenic tension (mean(s.e.m.) 0.10(0.01) and 0.48(0.04) g per mg tissue respectively), whereas equivalent rectal smooth muscle did not. Stimulation of muscarinic (cholinergic) receptors caused contraction of strips from the conjoined longitudinal coat and rectal smooth muscle layers in a dose-dependent manner (10(-7)-10(-4) mol/l); however, IAS strips relaxed (10(-6)-10(-4) mol/l). Activation of alpha-adrenergic receptors in conjoined longitudinal coat and IAS strips produced concentration-dependent contraction (10(-7)-10(-4) mol/l). In comparison, rectal smooth muscle relaxed. All muscle layers relaxed in response to beta-adrenoceptor stimulation. These data indicate that in the anal canal both the conjoined longitudinal coat and IAS are specialized sphincteric smooth muscles.