T1238 Comparison of Muscarinic Receptor Subtypes Mediating Contraction of Human and Pig Gastric Clasp and Sling Fibers

Abstract

This study determined how closely the contractile physiology of the pig gastroesophageal junction follows the human. We obtained human tissue from organ transplant donors and pig tissue from a slaughterhouse. Total, M-2 and M-3 receptor density was determined by subtype specific immunoprecipitation. Total and M-2 are higher in pig than human. M-3 receptors are 2 fold higher in human than pig sling and over 2 fold lower in human than pig clasp fibers. The methoctramine and darifenacin potency to inhibit bethanechol contractions, calculated by classic Schild analysis, indicates that both M-2 and M-3 receptors cause contraction which violates the assumption of one receptor causing the effect. An analysis method relating dual occupation of M-2 andM-3 receptors to the contractile response was developed based on the published Ka values of M-2 and M-3 receptors for bethanechol of 170 μM and 110 μM respectively, and mass-action binding which, at equilibrium, gives receptor occupation = [A][R] / ([A] + Ka), where [A] denotes the agonist concentration, [R] is the receptor concentration and Ka is the agonist dissociation constant (reciprocal of affinity). Three dimensional plots for M-2 and M-3 occupation and contractile response are shown in the figure. Although the M-3 receptor subtype density is different between human and pig, the physiology of the contractile response is similar. This indicates that the pig may be a good model for human gastroesophageal junction physiology. Muscarinic receptor density (fMol/mg solubile protein)