Comparison of human and porcine gastric clasp and sling fiber contraction by M2 and M3 muscarinic receptors

To compare the gastroesophageal junction of the human with the pig, M2 and M3 receptor densities and the potencies of M2 and M3 muscarinic receptor subtype selective antagonists were determined in gastric clasp and sling smooth muscle fibers. Total muscarinic and M2 receptors are higher in pig than human clasp and sling fibers. M3 receptors are higher in human compared with pig sling fibers but lower in human compared with pig clasp fibers. Clasp fibers have fewer M3 receptors than sling fibers in both humans and pigs. Similar to human clasp fibers, pig clasp fibers contract significantly less than pig sling fibers. Analysis of the methoctramine Schild plot suggests that M2 receptors are involved in mediating contraction in pig clasp and sling fibers. Darifenacin potency suggests that M3 receptors mediate contraction in pig sling fibers and that M2 and M3 receptors mediate contraction in pig clasp fibers. Taken together, the data suggest that both M2 and M3 muscarinic receptors mediate the contraction in both pig clasp and sling fibers similar to human clasp and sling fibers.