M1 is a major subtype of muscarinic acetylcholine receptors on mouse colonic epithelial cells

Muscarinic acetylcholine receptors (mAChRs) are major regulators of gut epithelial functions. However, the precise subtype composition has not been clarified. We characterized the pharmacological profile of mAChRs on mouse colonic crypts, employing [H-3]-N-methyl scopolamine chloride as a radioligand and several subtype-selective chemicals, and the functional aspect by measuring short-circuit current (I (sc)) in Ussing chambers and by evaluating MAP kinase phosphorylation in mouse colonic mucosal sheets. The mAChRs were detected on the crypts (K (d) = 163.2 +/- A 32.3 pM, B (max) = 47.3 +/- A 2.6 fmol/mg of total cell protein). Muscarinic toxin 7 (MT-7, M1 subtype selective) gave a displacement curve with high affinity, but there was a part insensitive to MT-7 (18.8 +/- A 0.4 % of the total specific binding). The MT-7-insensitive component was displaced completely by darifenacin (M3 selective) with high affinity. ACh induced an increase in I (sc), which was significantly enhanced by MT-7 but was completely inhibited by darifenacin or atropine. Colitis induction resulted in a significant decrease in the density of mAChRs, which occurred mainly in the MT-7-sensitive component (M1 subtype). Immunological experiments exhibited a reduction of M1 but not of M3 signal after colitis induction. Muscarinic stimulation induced an increase in MAP kinase phosphorylation, which was completely suppressed by MT-7 and was attenuated by inflammation, in mouse colonic epithelium. These results suggest that mAChRs in mouse colonic epithelial cells consist of two subtypes, M1 (80 %) and M3 (20 %). The major M1 subtype was likely to regulate epithelial chloride secretion negatively and was susceptible to inflammation and may be relevant to inflammatory gut dysfunction.