Colonic inflammation increases the contribution of muscarinic M2 receptors to carbachol-induced contraction of the rat colon

ABSTRACT

Carbachol-induced contraction of the rat colon is impaired in rats with trinitrobenzene sulfonic acid [TNBS]-induced colitis. The main objective of this study was to examine the effect of colitis on the expression and function of muscarinic [M] receptor subtypes in the rat colon. Rats [n = 80] were treated with TNBS and used 5 days later for measurement of contractility, myeloperoxidase activity, histology and expression of muscarinic receptor isoforms using Western blot analysis. Carbachol produced concentration-dependent contractions of colonic segments from control [n = 40] and TNBS-treated [n = 40] rats with no significant difference in potency. However, the maximum response to carbachol was significantly reduced in colon segments of TNBS-treated rats. The selective muscarinic receptor antagonists 4-diphenylacetoxy-N-methyl piperidine [4-DAMP, M3], pirenzepine [M1] and methoctramine [M2] antagonized carbachol-induced contraction in control [9.1 +/- 0.1, 6.7 +/- 0.3 and 6.0 +/- 0.1, respectively] and TNBS-treated rats [9.2 +/- 0.2, 6.9 +/- 0.2, 6.7 +/- 0.2]. The -logKB values in control rats are consistent with an action of carbachol on muscarinic M3 receptors. There was no significant difference in -logKB values for 4-DAMP and pirenzepine in control and TNBS-treated rats, but methoctramine was fivefold more potent in TNBS-treated rats, possibly indicating an increased contribution of muscarinic M2 receptors to carbachol-induced contraction in the inflamed colon. The expression of M2 receptors was also significantly increased in colon segments from TNBS-treated rats, confirming the increased role of muscarinic M2 receptors in the inflamed colon. The data show that while only M3 receptors appeared to mediate carbachol-induced contraction in control segments, expression of both M2 and M3 receptors was increased in the inflamed rat colon