Nicotinic receptor subtypes mediating relaxation of the normal human clasp and sling fibers of the upper gastric sphincter

Background Proper function of the gastro-esophageal high pressure zone is essential for the integrity of the antireflux barrier. Mechanisms include tonic contractions and the decreased tone during transient lower esophageal sphincter relaxations. Methods We characterized the pharmacology of nicotinic receptors mediating relaxations of the human upper gastric sphincter (clasp and sling fibers) using currently available subtype selective nicotinic antagonists in tissue from organ transplant donors. Donors with either a history of gastro-esophageal reflux disease or histologic evidence of Barrett's esophagus were excluded. Clasp and sling muscle fiber strips were used for one of three paradigms. For paradigm 1, each strip was exposed to carbachol, washed, exposed to nicotinic antagonists then re-exposed to carbachol. In paradigm 2, strips were exposed to a near maximally effective bethanechol concentration then nicotine was added. Strips then were washed, exposed to nicotinic antagonists then re-exposed to bethanechol followed by nicotine. In paradigm 3, strips were exposed to bethanechol then choline or cytisine. Key Results 100M methyllycaconitine has no inhibitory effects on relaxations, eliminating homomeric 7 subtypes. Subtypes composed of 42 subunits are also eliminated because choline acts as an agonist and dihydro-beta-erythroidine is ineffective. Conclusions & Inferences Because mecamylamine blocks the relaxations and both choline and cytisine act as agonists in both clasp and sling fibers, the nicotinic receptor subtypes responsible for these relaxations could be composed of 342, 24, or 44 subunits.