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Characterization of an atypical muscarinic cholinoceptor mediating contraction of the guinea‐pig isolated uterus
Author(s) -
Boxall Donna K,
Ford Anthony P D W,
Choppin Agnes,
Nahorski Stefan R,
Challiss R A John,
Eglen Richard M
Publication year - 1998
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 - 10.1038/sj.bjp.0702002
Subject(s) - methoctramine , carbachol , muscarinic acetylcholine receptor , endocrinology , medicine , agonist , population , guinea pig , biology , receptor , chemistry , muscarinic acetylcholine receptor m2 , stimulation , environmental health
In many smooth muscle tissues a minor M 3 ‐muscarinic acetylcholine (mACh) receptor population mediates contraction, despite the presence of a larger M2‐mACh receptor population. However, this is not the case for guinea‐pig uterus where radioligand binding and functional studies exclude a dominant role for M 3 ‐mACh receptors. Using tissue from animals pre‐treated with diethylstilboestrol, estimates of antagonist affinity were made before and after selective alkylation procedures, together with estimates of agonist affinity to characterise the mACh receptor population mediating carbachol‐induced contraction of guinea‐pig isolated uterus. Antagonist affinity estimates made at `protected' receptors were not significantly different from those made in untreated tissues. However all estimations were significantly different from those reported in guinea‐pig ileum and atria. The rank order of affinities were atropine>zamifenacin=tripitramine>methoctramine. Carbachol‐induced contractions were insensitive to the M 4 ‐selective muscarinic toxin MTx‐3, or PD102807 (0.1 μ M ) ruling out a role for M 4 ‐mACh receptors. The agonist affinity value for L‐660,863, a putative `M 2 ‐selective' agonist of 5.44±0.30 ( n =6) was significantly different from that reported in guinea‐pig atria. In contrast, the pK A value for carbachol (4.22±0.17; n =8) agrees with that reported for guinea‐pig ileum. Carbachol‐induced contractions were insensitive to pertussis toxin although carbachol‐induced inhibition of forskolin‐stimulated cyclic AMP production was attenuated, ruling out the involvement of G i ‐proteins in contraction. Radioligand binding studies revealed a K D for N‐[ 3 H]‐methylscopolamine of 0.12±0.05 n M and a B max of 147±18 fmol mg protein −1 . Antagonist affinity estimates made using competition binding studies supported previous data suggesting the presence of a homogenous population of M 2 ‐mACh receptors. These data suggest a small population of mACh receptors with an atypical operational profile which can not be distinguished using radioligand binding studies may mediate carbachol‐induced contraction of guinea‐pig isolated uterus.British Journal of Pharmacology (1998) 124 , 1615–1622; doi: 10.1038/sj.bjp.0702002