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Selective inactivation of muscarinic M 2 and M 3 receptors in guinea‐pig ileum and atria in vitro
Author(s) -
Eglen R.M.,
Harris G.C.
Publication year - 1993
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1993.tb13712.x
Subject(s) - muscarinic acetylcholine receptor , guinea pig , ileum , in vitro , receptor , chemistry , endocrinology , medicine , biophysics , biology , anatomy , biochemistry
1 The role of muscarinic M 2 and M 3 receptors in ileal smooth muscle has been evaluated by use of selective receptor alkylation. The alkylating agents, 4‐diphenylacetoxy‐ N ‐(2‐chloroethyl)‐piperidine (4‐DAMP mustard) was studied for effects against (+)‐ cis ‐dioxolane, at muscarinic M 2 and M 3 receptors in guinea‐pig atria or ileum, respectively. 4‐DAMP mustard (10 n m , 40 min exposure) did not discriminate between these muscarinic receptors. In ileum, 4‐DAMP mustard, at 100 n m , resulted in a large dextral shift (197 fold) and depression in maxima. In atria there was a smaller dextral shift (14 fold) but no depression in maxima. 2 The muscarinic antagonists, atropine (non‐selective), methoctramine (M 2 ‐selective) and para ‐fluoro‐hexahydro‐siladiphenidol (pFHHSiD; M 3 selective) were studied in protection studies against alkylation by phenoxybenzamine. Washout studies following equilibration of the tissues with atropine (30 n m ), methoctramine (0.3 μ m ) or pFHHSiD (3 μ m ), showed the compounds to be reversible. No temporal changes in sensitivity to (+)‐ cis ‐dioxolane were observed. 3 Exposure, for 20 min, of atria and ileum to phenoxybenzamine (3 and 10 μ m respectively) caused dextral shifts and depressions in the maxima of the concentration‐response curve to (+)‐ cis ‐dioxolane. These effects were inhibited by prior equilibration with atropine (30 n m ) and methoctramine (0.1 μ m ) in atria or atropine (30 n m ) and pFHHSiD (3 μ m ) in ileum. Similar results in ileum were obtained when pilocarpine was used as the agonist. 4 These data were consistent with muscarinic M 2 receptors mediating responses in atria and M 3 receptors mediating responses in ileum. No evidence was provided for a direct role of muscarinic M 2 receptors in ileal contraction. 5 It is concluded that receptor protection by reversible antagonists for muscarinic M 2 or M 3 receptors provides a means to isolate pharmacologically a single subtype in a tissue possessing heterogeneous populations. This technique may prove useful in defining the role of the respective subtypes in smooth muscle contraction.