title: Muscarinic receptor subtypes in the human colon: lack of evidence for atypical subtypes.
abstract: Characteristics of muscarinic receptors were investigated in circular muscle from normal human colon. In saturation studies (n=18), binding of [3H]quinuclidinyl benzylate (QNB) was of high affinity (K(d) 87.3 pM) and capacity (B(max) 362+/-27 fmol/mg protein), with no differences between ascending and sigmoid colon. Kinetic studies gave a K(d) of 55 pM. Methoctramine and darifenacin displayed biphasic binding profiles, the high affinity components being compatible with a population of approximately 80+/-5% M(2) and 13+/-2% M(3) muscarinic receptors, respectively. Pirenzepine, mamba toxin 1 and mamba toxin 3 were very weak competitors, indicating negligible expression of muscarinic M(1) and M(4) receptors. Six other subtype-preferring antagonists exhibited K(i) values typical of those reported at cloned human muscarinic M(2) receptors. In the presence of methoctramine, pre-treatment with alkylating agent 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (4-DAMP mustard) inhibited [3H]quinuclidinyl benzylate binding to 26% of sites. Following alkylation of muscarinic M(3) receptors, darifenacin bound to a single low affinity site, indicating binding to muscarinic M(2) receptors.
authors.affiliation: Department of Physiology and Pharmacology, University of New South Wales, Sydney 2052, Australia.
authors.forename: Kylie J
authors.initials: KJ
authors.lastname: Mansfield
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Mitchelson
authors.forename: Kate H
authors.initials: KH
authors.lastname: Moore
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Burcher
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018727:Muscarinic Antagonists / Q000378:metabolism / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D011976:Receptors, Muscarinic / Q000145:classification* / Q000378:metabolism*
chemical_list: D018727:Muscarinic Antagonists; D011976:Receptors, Muscarinic