Abstract

Although muscarinic receptors are known to mediate tonic contraction of human gastrointestinal tract smooth muscle, the receptor subtypes that mediate the tonic contractions are not entirely clear. Whole human stomachs with attached esophagus were procured from organ transplant donors. Cholinergic contractile responses of clasp, sling, lower esophageal circular (LEC), midesophageal circular (MEC), and midesophageal longitudinal (MEL) muscle strips were determined. Sling fibers contracted greater than the other fibers. Total, M₂ and M₃ muscarinic receptor density was determined for each of these dissections by immunoprecipitation. M₂ receptor density is greatest in the sling fibers, followed by clasp, LEC, MEC, and then MEL, whereas M₃ density is greatest in LEC, followed by MEL, MEC, sling, and then clasp. The potency of subtype-selective antagonists to inhibit bethanechol-induced contraction was calculated by Schild analysis to determine which muscarinic receptor subtypes contribute to contraction. The results suggest both M₂ and M₃ receptors mediate contraction in clasp and sling fibers. Thus, this type of analysis in which multiple receptors mediate the contractile response is inappropriate, and an analysis method relating dual occupation of M₂ and M₃ receptors to contraction is presented. Using this new method of analysis, it was found that the M₂ muscarinic receptor plays a greater role in mediating contraction of clasp and sling fibers than in LEC, MEC, and MEL muscles in which the M₃ receptor predominantly mediates contraction.