M2 and M3 muscarinic receptor‐mediated contractions in longitudinal smooth muscle of the ileum studied with receptor knockout mice

Open Access
Publisher Website
Google Scholar
Abstract
Isometric contractile responses to carbachol were studied in ileal longitudinal smooth muscle strips from wild‐type mice and mice genetically lacking M2 or M3 muscarinic receptors, in order to characterize the mechanisms involved in M2 and M3 receptor‐mediated contractile responses. Single applications of carbachol (0.1–100 μM) produced concentration‐dependent contractions in preparations from M2‐knockout (KO) and M3‐KO mice, mediated via M3 and M2 receptors, respectively, as judged by the sensitivity of contractile responses to blockade by the M2‐preferring antagonist methoctramine (300 nM) or the M3‐preferring antagonist 4‐DAMP (30 nM). The M2‐mediated contractions were mimicked in shape by submaximal stimulation with high K+ concentrations (up to 35 mM), almost abolished by voltage‐dependent Ca2+ channel (VDCC) antagonists or depolarization with 140 mM K+ medium, and greatly reduced by pertussis toxin (PTX) treatment. The M3‐mediated contractions were only partially inhibited by VDCC antagonists or 140 mM K+‐depolarization medium, and remained unaffected by PTX treatment. The contractions observed during high K+ depolarization consisted of different components, either sensitive or insensitive to extracellular Ca2+. The carbachol contractions observed with wild‐type preparations consisted of PTX‐sensitive and ‐insensitive components. The PTX‐sensitive component was functionally significant only at low carbachol concentrations. The results suggest that the M2 receptor, through PTX‐sensitive mechanisms, induces ileal contractions that depend on voltage‐dependent Ca2+ entry, especially associated with action potential discharge, and that the M3 receptor, through PTX‐insensitive mechanisms, induces contractions that depend on voltage‐dependent and ‐independent Ca2+ entry and intracellular Ca2+ release. In intact tissues coexpressing M2 and M3 receptors, M2 receptor activity appears functionally relevant only when fractional receptor occupation is relatively small. British Journal of Pharmacology (2005) 146, 98–108. doi:10.1038/sj.bjp.0706300

This publication has 49 references indexed in Scilit: