Affinity profile of the novel muscarinic antagonist, guanylpirenzepine

Subtypes of muscarinic receptors in rat duodenum: a comparison with rabbit vas deferens, rat atria, guinea-pig ileum and gallbladder by using imperialine

The specific binding of [3H]QNB to rat duodenum smooth muscle membranes was a saturable process and Scatchard transformation of the saturation curves indicated a linear plot (nH = 1.017+/-0.071). The K(D) and Bmax values were 0.168+/-0.025 nM and 46.7+/-8.6 fmol/mg protein, respectively. Analyses of competition curves using pirenzepine and guanylpirenzepine indicated more than one class of binding site. A minor population of muscarinic binding sites showed high affinity (M1) for both pirenzepine (19.3+/-1.2%; pKi = 8.29+/-0.36) and guanylpirenzepine (29.4+/-2.0%; pKi = 7.28+/-0.11). The antagonistic affinity values of pirenzepine and guanylpirenzepine for the remaining low affinity binding sites, and that of methoctramine indicated the presence of both M2 and M3 subtypes. McN-A-343 produced relaxations in rat duodenum and inhibited twitch contractions of rabbit vas deferens induced by electrical stimulation in a concentration dependent manner. Carbachol (Cch) exerted concentration-dependent negative inotropic effect in rat atria and contractile effects in guinea-pig gallbladder and ileum longitudinal muscle-myenteric plexus preparation. Imperaline displaced the concentration-response curves to McN-A-343 and Cch to the right in parallel, without affecting the maximum responses in all tissues studied. The rank order of the pA2 values was rabbit vas deferens > rat atria > guinea-pig gallbladder = guinea-pig ileum > rat duodenum. The presynaptic muscarinic receptors at the rat duodenum and rabbit vas deferens were concluded to be of M1 and M4 subtypes, respectively.

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

New functionally selective muscarinic agonists

The muscarinic pharmacology of two novel agonists related to McN-A-343, 4-F-PyMcN and 4-F-PyMcN+, has been studied by the use of pharmacological and radioligand binding techniques. Both compounds were potent agonists at M1 receptors in rabbit vas deferens (pEC50 = 6.24 and 6.96) and rat duodenum (pEC50 = 5.47 and 6.38), but very weak partial agonists or competitive antagonists at guinea-pig cardiac M2 and ileal M3 receptors. There was no receptor reserve for 4-F-PyMcN in rabbit vas deferens, for which the potency (pEC50 = 6.24) and apparent affinity (pKA = 5.99 and 6.21) were similar. 4-F-PyMcN+ showed only limited binding selectivity between four muscarinic receptor binding assays with apparent affinity constants (pKi) of 5.8, 5.2, 5.6 and 5.7 for M1, M2, M3 and M4 muscarinic receptor subtypes. The two novel functionally M1-selective agonists may provide useful tools with which to study muscarinic receptor mechanisms. The non-quaternary compound, 4-F-PyMcN, might become a starting point for the development of drugs that selectively affect M1 receptors involved in central cholinergic function.

A muscarinic receptor different from the M1, M2, M3 and M4 subtypes mediates the contraction of the rabbit iris sphincter

In order to analyse the subtype of muscarinic receptors involved in the methacholine-induced contraction of the rabbit iris sphincter we have determined equilibrium dissociation constants (KB) of various antagonists in the sphincter muscle. The values were compared with those observed at M1 (rabbit vas deferens), M2 (heteroreceptors in rat iris) and M3 receptors (guinea-pig ileum), or at the muscarinic receptors in the guinea-pig uterus. The methacholine-induced contraction of the uterus from immature guinea-pigs was competitively antagonized by pirenzepine (6.64, -log KB), 4-DAMP (8.39), hexahydrodifenidol (HHD; 7.00 for the (R)- and 5.40 for the (S)-enantiomer), p-fluoro-hexahydrosiladifenidol (p-F-HHSiD; 6.25) and valethamate bromide (8.04). The affinity of the antagonists is consistent with the presence of an M2 receptor. The -log KB values of the antagonists in the rabbit iris sphincter (6.43, p-F-HHSiD; 6.22, AQ-RA 741; 7.23 and 5.34, (R)- and (S)-trihexyphenidyl) were lower than, or within the lowest range of, estimates in the other experimental models, irrespective of the subtype selectivity of the antagonist. This excludes the presence of an M1, M2, M3 or M4 receptor in this smooth muscle. The affinity of UH-AH 37 in the iris was intermediate between that for M1 or M3, and M2 receptors. The low affinity of AQ-RA 741 and the low enantiomeric ratio of trihexyphenidyl (THP) in the iris (77.6) would be compatible with a presumed M5 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)