Seiichi Komori, Toshihiro Unno, Takahisa Nakayama and Hidenori Ohashi
Laboratory of Pharmacology, Department of Veterinary Science, Faculty of
Agriculture, Gifu University, 1-1 Yanagido, Gifu 501-11, Japan
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 (IK-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 IK-Ca
or Icat with different potencies. Their relative potencies for
IK-Ca and Icat inhibition resembled the relative affinity
constants for M3 and M2 subtypes, respectively. Thus, the IK-Ca
is mediated via the M3 subtype and the Icat via the M2 subtype.
Keywords: Carbachol, Muscarinic receptor, Smooth muscle, Ca2+-activated
K+ current, Nonselective cationic current