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 longit udinal smooth muscle layer (conjoined longitudinal coat) of the human anal canal to cholinergic and adrenergic stimulation, and to compare t his with similar features of the internal anal sphincter (IAS) and rec tal smooth muscle. Tissue was obtained from abdominoperineal and anter ior resection specimens, and strips of muscle mounted for isometric te nsion recording in superfusion organ baths. Both conjoined longitudina l coat and IAS strips generated and maintained spontaneous myogenic te nsion (mean(s.e.m.) 0.10(0.01) and 0.48(0.04) g per mg tissue respecti vely), 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 d ose-dependent manner (10(-7)-10(-4) mol/l); however, IAS strips relaxe d (10(-6)-10(-4) mol/l). Activation of alpha-adrenergic receptors in c onjoined longitudinal coat and IAS strips produced concentration-depen dent contraction (10(-7)-10(-4) mol/l). In comparison, rectal smooth m uscle relaxed. All muscle layers relaxed in response to beta-adrenocep tor stimulation. These data indicate that in the anal canal both the c onjoined longitudinal coat and IAS are specialized sphincteric smooth muscles.