Regional variations in neurokinin receptor subtype contributions to muscularis mucosae and epithelial function in rat colon

It is known that the muscularis mucosae and mucosa are not pharmacologically homogeneous throughout the rat colon. The aim of this study was to simultaneously characterize all three neurokinin (NK) receptors in the muscularis mucosae and mucosa along the length of the rat colon. Strips of proximal, mid, and distal colonic muscularis mucosae were prepared for isometric recording or sheets of muscle-free mucosa were mounted in Ussing chambers for measurement of short-circuit current. In both muscularis mucosae and mucosa the greatest responses to substance P were found in the proximal region. Use of selective agonists revealed the presence of all three NK receptors in both structures, however, selective antagonism suggests that only NK2 receptors in the muscularis mucosae and NK1 receptors in the mucosa are physiologically relevant. In conclusion, substance P-induced responses in the rat colon are region-specific and not mediated by a single NK receptor subtype common to both structures.