IN-VIVO CHARACTERIZATION OF VASODILATING MUSCARINIC-RECEPTOR SUBTYPESIN HUMANS

The role of muscarinic (M)-receptor subtypes in the regulation of vasc ular tone has not yet been defined in humans. To analyze the role of M -receptor subtypes in the forearm resistance vasculature of normotensi ve volunteers (n = 20), we infused acetylcholine (ACh) and methacholin e (MCh) in the presence of saline and the antagonists atropine (nonsel ective), pirenzepine (M, selective), and AF-DX 116 (M(2) selective), u sing automated R-wave-triggered venous occlusion plethysmography. Schi ld analysis was applied by calculating plasma concentrations of the in fused compounds and determining EC(50) values. ACh and MCh both caused dose-dependent vasodilatation, with EC(50) values of 537 and 52 nmol/ L, respectively. The apparent 10-fold higher potency of MCh compared w ith ACh may be explained by rapid degradation of ACh in plasma. The co ncentration-response curve of MCh was shifted to the right by atropine , pirenzepine, and AF-DX 116, with apparent pA(2) values of 8.03+/-0.0 3, 6.71+/-0.08, and 5.32+/-0.05, respectively, and slopes not differen t from unity. The present technique enabled us to perform M-receptor c haracterization by Schild analysis in humans. The affinity constants a nd rank order of potency - atropine >pirenzepine> AF-DX 116 - suggest that cholinergic vasodilatation in this vascular bed is predominantly mediated by the M(3)-receptor subtype. The EC(50) value of MCh and the pA(2) values of pirenzepine and AF-DX 116 are comparable to values re ported for in vitro experiments.