title: In vivo characterization of vasodilating muscarinic-receptor subtypes in humans.
abstract: The role of muscarinic (M)-receptor subtypes in the regulation of vascular tone has not yet been defined in humans. To analyze the role of M-receptor subtypes in the forearm resistance vasculature of normotensive volunteers (n = 20), we infused acetylcholine (ACh) and methacholine (MCh) in the presence of saline and the antagonists atropine (nonselective), pirenzepine (M1 selective), and AF-DX 116 (M2 selective), using automated R-wave-triggered venous occlusion plethysmography. Schild analysis was applied by calculating plasma concentrations of the infused compounds and determining EC50 values. ACh and MCh both caused dose-dependent vasodilation, with EC50 values of 537 and 52 nmol/L, respectively. The apparent 10-fold higher potency of MCh compared with ACh may be explained by rapid degradation of ACh in plasma. The concentration-response curve of MCh was shifted to the right by atropine, pirenzepine, and AF-DX 116, with apparent pA2 values of 8.03 +/- 0.03, 6.71 +/- 0.08, and 5.32 +/- 0.05, respectively, and slopes not different from unity. The present technique enabled us to perform M-receptor characterization by Schild analysis in humans. The affinity constants and rank order of potency--atropine > pirenzepine > AF-DX 116-suggest that cholinergic vasodilation in this vascular bed is predominantly mediated by the M3-receptor subtype. The EC50 value of MCh and the pA2 values of pirenzepine and AF-DX 116 are comparable to values reported for in vitro experiments.
authors.affiliation: Department of Nephrology, University Hospital Leiden, The Netherlands.
authors.forename: T A
authors.initials: TA
authors.lastname: Bruning
authors.forename: M G
authors.initials: MG
authors.lastname: Hendriks
authors.forename: P C
authors.initials: PC
authors.lastname: Chang
authors.forename: E A
authors.initials: EA
authors.lastname: Kuypers
authors.forename: P A
authors.initials: PA
authors.lastname: van Zwieten
mesh_terms: D000109:Acetylcholine / Q000097:blood / Q000494:pharmacology; D000328:Adult; D001285:Atropine / Q000097:blood / Q000494:pharmacology; D002985:Clinical Protocols; D005542:Forearm; D006801:Humans; D008297:Male; D016210:Methacholine Chloride / Q000097:blood / Q000494:pharmacology; D010890:Pirenzepine / Q000031:analogs & derivatives / Q000097:blood / Q000494:pharmacology; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D014664:Vasodilation / Q000187:drug effects / Q000502:physiology*
chemical_list: D011976:Receptors, Muscarinic; D016210:Methacholine Chloride; D010890:Pirenzepine; D001285:Atropine; D000109:Acetylcholine; C048593:otenzepad