Inflammation Inhibits Muscarinic Signaling in In Vivo Canine Colonic Circular Smooth Muscle Cells

Abstract

We investigated the effects of experimental colitis on the muscarinic signaling properties and contractile behavior of canine colonic circular smooth muscle. The hypotheses that inflammation 1) inhibits in vivo muscarinic receptor mediated contractions, and 2) alters receptor density or receptor-binding affinities were tested. Muscarine was infused close-intra-arterially in seven conscious dogs during normal and experimental colitis states. Colonic circular muscle contractions were recorded via surgically attached strain gauge transducers. Muscarine stimulated phasic contractions in a dose-dependent manner, whereas colitis was inhibited. The inhibitory concentration 50% dose of M3 receptor inhibitor was several times lower than that of M1, M2, and M4 inhibitors during normal and colitis. However, inflammation induced a significant leftward shift in the circular muscle inhibitory dose-response curve of M2 inhibitor. Muscarinic receptor density and binding analyses in isolated circular muscle cells was done in normal and colitis states. Inflammation significantly decreased maximum binding from 4082 fmol/mg to 2708 fmol/mg, whereas affinity constant remained unaffected. The conclusions were that 1) spontaneous and muscarine-activated in vivo phasic contractile activity of colonic circular muscle cells is primarily mediated by M3 receptors; 2) inflammation was associated with a shift in M2 receptor potency, due chiefly to a decrease in receptor density; and 3) this inhibitory effect was seen in normal and inflamed states, suggesting the importance of M2 receptor. These findings suggest that changes in muscarinic response during colitis may contribute to the abnormal motility seen with inflammatory bowel disease.