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 M 3 receptor inhibitor was several times lower than that of M 1 , M 2 , and M 4 inhibitors during normal and colitis. However, inflammation induced a significant leftward shift in the circular muscle inhibitory dose-response curve of M 2 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 M 3 receptors; 2) inflammation was associated with a shift in M 2 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 M 2 receptor. These findings suggest that changes in muscarinic response during colitis may contribute to the abnormal motility seen with inflammatory bowel disease. ACh, the principal neurotransmitter of the enteric nervous system, is released not only as a result of parasympathetic activation, but also in response to many other gastrointestinal peptides (1). Atropine (nonselective muscarinic antagonist), when administered intravenously or close-intra-arterially (arterial) completely blocks all spontaneous smooth muscle contractions in the gut (1-3). In the gastrointestinal system, muscarinic receptors play a significant role in coordination of peristalsis and propulsion of luminal contents (4).The muscarinic receptors in the gut are localized at presynaptic, postsynaptic, and prejunctional and postjunctional sites (5). These receptors on smooth muscle cells mediate contractions by G-protein coupled mechanisms, whereas those at presynaptic and prejunctional sites modulate the release of ACh by negative feedback. The activation of muscarinic receptors at postsynaptic sites stimulates postsynaptic excitatory and inhibitory neurons to release neurotransmitters. The physiologic mechanisms by which muscarinic receptors stimulate in vivo colonic circular muscle contractions are not completely understood.

Does Decreased c-KIT Expression in Myenteric Interstitial Cells of Cajal Cause Decreased Spontaneous Contraction in Murine Proximal Colon?

“…7 In this paper, the decreased contractile response to carbachol in IL-10 knockout mice with the reduced expression of M 2 receptor in the inflamed proximal colon is different from some other literature. 6,7 To clarify this difference, further study on M 3 receptor is necessary. Finally, concern can be raised about the electrical field stimulation (EFS) experiments reported in this article.…”

“…1,5 In addition, this study's investigation of the function and expression level of muscarinic type 2 (M 2 ) receptors is worthwhile in that the M 2 receptors may play a greater role in the inflamed colon. 6,7 These considerations make the article a strong one, and its conclusions can serve as a stepping stone to investigate several unanswered problems in gastrointestinal (GI) smooth muscle biology as follows. One such problem is the causality of the relationship between loss of c-KIT expression in specific ICC such as myenteric ICC (ICC-MY) or ICC associated with the submuscular plexus (ICC-SMP) and loss of spontaneous contraction in the proximal colon of mouse.…”

“…[13][14][15] However, it has been reported that the number and sensitivity of the muscarinic receptors can be altered 14,15 and that M 2 receptors may play a greater role in some situations such as in the inflamed rat and canine colon. 6,7 Thus, inflammation seems to modulate M 2 receptor function 16 and to be associated with a shift in M 2 receptor potency, due chiefly to a decrease in receptor density. 7 In this paper, the decreased contractile response to carbachol in IL-10 knockout mice with the reduced expression of M 2 receptor in the inflamed proximal colon is different from some other literature.…”

“…6,7 Thus, inflammation seems to modulate M 2 receptor function 16 and to be associated with a shift in M 2 receptor potency, due chiefly to a decrease in receptor density. 7 In this paper, the decreased contractile response to carbachol in IL-10 knockout mice with the reduced expression of M 2 receptor in the inflamed proximal colon is different from some other literature. 6,7 To clarify this difference, further study on M 3 receptor is necessary.…”

Donepezil Abolishes Anticholinergic Activity in a Patient with Amnesia

“…In fact, Nazarov et al [29] reported that CRP, which increases during inflammatory states, may result in AA by capturing ACh or binding to the ACh receptor, whereas Tsuruta et al [30] reported that plasma AA was related to CRP in critically ill and injured patients. Furthermore, Horiuchi et al [31] reported that an endogenous immunologically active peptide (apelin) inhibited cholinergic activity by binding to the ACh receptor, and Jadcherla [32] found that inflammation inhibited muscarinic receptormediated signaling. Therefore, the downregulation of ACh in the central nervous system causes AA in both the central and peripheral systems by way of inflammation.…”