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.


RESULTS
100 μM methyllycaconitine has no inhibitory effects on relaxations, eliminating homomeric α7 subtypes. Subtypes composed of α4β2 subunits are also eliminated because choline acts as an agonist and dihydro-beta-erythroidine is ineffective.


CONCLUSIONS
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 α3β4β2, α2β4, or α4β4 subunits.
Department of Anatomy and Cell Biology, Temple University School of Medicine, Philadelphia, PA, USA.
M R
MR
Ruggieri
A S
AS
Braverman
A K
AK
Vegesna
L S
LS
Miller
D000328:Adult; D018723:Bethanechol / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D049630:Esophageal Sphincter, Lower / Q000187:drug effects* / Q000378:metabolism; D005260:Female; D005753:Gastric Mucosa / Q000378:metabolism; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism; D018722:Nicotinic Agonists / Q000494:pharmacology; D018733:Nicotinic Antagonists / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000378:metabolism*; D013270:Stomach / Q000187:drug effects*
D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D011978:Receptors, Nicotinic; D018723:Bethanechol; D002217:Carbachol
enteric nerves; gastric clasp fibers; gastric sling fibers; gastro-esophageal reflux; nicotinic receptors; smooth muscle