M2 and M3 Muscarinic Receptors Couple, Respectively, With Activation of Nonselective Cationic Channels and Potassium Channels in Intestinal Smooth Muscle Cells

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Abstract
Smooth muscle cells of guinea pig ileum express both M2 and M3 subtypes of muscarinic receptors. Under voltage clamp, activation of the muscarinic receptors with carbachol (CCh) induces Ca2+-activated K+ current (I[K-Ca]) and nonselective cationic current (Icat). Receptor subtypes mediating the current responses were characterized by using pirenzepine, AF-DX116, 4-DAMP and atropine, which have different profiles of the affinity constants for muscarinic receptor subtypes. The muscarinic antagonists inhibited either CCh-evoked I(K-Ca) or Icat with different potencies. Their relative potencies for I(K-Ca) and Icat inhibition resembled the relative affinity constants for M3 and M2 subtypes, respectively. Thus, the I(K-Ca) is mediated via the M3 subtype and the Icat via the M2 subtype.