piperidines and silahexocyclium

The five muscarinic receptors (m1-m5), although structurally closely related, can be distinguished pharmacologically by the use of subtype-selective ligands. Various tricyclic muscarinic antagonists, including the AF-DX derivative AQ-RA 741 and the alkaloid himbacine, for example, have been shown to display up to 200-fold higher affinities for m2 and m4 than for m5 receptors. On the other hand, antagonists such as sila-hexocyclium and the pirenzepine derivative UH-AH 37 exhibit lower affinities for m2 than for m5 and all other muscarinic receptors. To identify receptor epitopes that contribute to the subtype selectivities of these antagonists, we prepared a series of chimeric m2/m5 muscarinic receptors in which regions of the m5 receptor were systematically replaced with the homologous regions of the m2 receptor. AQ-RA 741, himbacine, and sila-hexocyclium bound to the various chimeric receptors, expressed in COS-7 cells, with affinity profiles indicative of multiple receptor domains contributing to the subtype selectivities of these antagonists. On the other hand, the higher affinity of UH-AH 37 for m5 than for m2 receptors appears to be largely dependent on a short stretch of 31 amino acids comprising most of transmembrane region VI and the third extracellular loop, a region that does not contribute to the subtype selectivity of AQ-RA 741 and himbacine. Our data indicate that different receptor epitopes are involved in conferring subtype selectivity on structurally different muscarinic antagonists.

Topics: Alkaloids; Amino Acid Sequence; Benzodiazepinones; Chimera; Dibenzazepines; Furans; Humans; Molecular Sequence Data; Muscarinic Antagonists; N-Methylscopolamine; Naphthalenes; Parasympatholytics; Piperazines; Piperidines; Receptors, Muscarinic; Scopolamine Derivatives; Sequence Homology, Nucleic Acid

Five subtypes of muscarinic receptors have been distinguished by pharmacological and molecular biological methods. This report characterizes the muscarinic subtype present in human gastric mucosa by radioligand binding studies. The receptor density was 27 +/- 6 fmol/mg protein and the tritiated ligand N-methylscopolamine had an affinity of (KD) 0.39 +/- 0.08 nM (n = 11). The M1 receptor selective antagonist pirenzepine and the M2 receptor selective ligand AF-DX 116 had low affinities of 148 +/- 32 nM (n = 13) and 4043 +/- 1011 nM (n = 3) KD, respectively. The glandular M3 antagonists hexahydrosiladifenidol and silahexocyclium had high affinities of KD 78 +/- 23 nM (n = 5) and 5.6 +/- 1.8 nM (n = 3). The agonist carbachol interacted with a single low-affinity site and binding was insensitive to modulation by guanine nucleotides. Antagonist and agonist binding studies thus showed an affinity profile typical of M3 receptors of the glandular type.

Topics: Carbachol; Gastric Mucosa; Humans; In Vitro Techniques; N-Methylscopolamine; Parasympatholytics; Piperazines; Piperidines; Pirenzepine; Receptors, Muscarinic; Scopolamine Derivatives

Five subtypes of muscarinic receptors have been identified by pharmacological and molecular biological methods. The muscarinic receptor subtype mediating acid secretion at the level of the parietal cell was unknown. Therefore, this study was performed to characterize muscarinic receptors on rat gastric parietal cells using the 3 subtype-selective antagonists hexahydrosiladifenidol and silahexocyclium, which have high affinity for glandular M3 subtypes, and AF-DX 116, which has high affinity to cardiac M2 receptors. The affinity of these antagonists was determined by radioligand binding experiments. In addition, their inhibitory potency on carbachol-stimulated inositol phosphate production was investigated. Inhibition of carbachol-stimulated aminopyrine uptake was used as an indirect measure of proton production. Both M3 antagonists, hexahydrosiladifenidol and silahexocyclium, had nanomolar affinities for parietal cell muscarinic receptors and potently antagonized inositol phosphate production with nanomolar Ki values. Silahexocyclium similarly antagonized aminopyrine accumulation while hexahydrosiladifenidol behaved as a noncompetitive antagonist. AF-DX 116 was a low-affinity ligand and a weak competitive antagonist at parietal-cell muscarinic receptors. It was concluded that muscarinic M3 receptors mediate acid secretion probably by activation of the phosphoinositide second messenger system in rat gastric parietal cells.

Topics: Animals; Gastric Acid; Parasympatholytics; Parietal Cells, Gastric; Piperazines; Piperidines; Radioligand Assay; Rats; Receptors, Muscarinic

1. In an attempt to assess the structural requirements for the muscarinic receptor selectivity of hexahydro-diphenidol (hexahydro-difenidol) and hexahydro-sila-diphenidol (hexahydro-sila-difenidol), a series of structurally related C/Si pairs were investigated, along with atropine, pirenzepine and methoctramine, for their binding affinities in NB-OK 1 cells as well as in rat heart and pancreas. 2. The action of these antagonists at muscarinic receptors mediating negative inotropic responses in guinea-pig atria and ileal contractions has also been assessed. 3. Antagonist binding data indicated that NB-OK 1 cells (M1 type) as well as rat heart (cardiac type) and pancreas (glandular/smooth muscle type) possess different muscarinic receptor subtypes. 4. A highly significant correlation was found between the binding affinities of the antagonists to muscarinic receptors in rat heart and pancreas, respectively, and the affinities to muscarinic receptors in guinea-pig atria and ileum. This implies that the muscarinic binding sites in rat heart and the receptors in guinea-pig atria are essentially similar, but different from those in pancreas and ileum. 5. The antimuscarinic potency of hexahydro-diphenidol and hexahydro-sila-diphenidol at the three subtypes was influenced differently by structural modifications (e.g. quaternization). Different selectivity profiles for the antagonists were obtained, which makes these compounds useful tools to investigate further muscarinic receptor heterogeneity. Indeed, the tertiary analogues hexahydro-diphenidol (HHD) and hexahydro-sila-diphenidol (HHSiD) had an M1 = glandular/smooth muscle greater than cardiac selectivity profile, whereas the quaternary analogues HHD methiodide and HHSiD methiodide were M1 preferring (M1 greater than glandular/smooth muscle, cardiac).

Topics: Animals; Cells, Cultured; Guinea Pigs; Humans; Ileum; In Vitro Techniques; Male; Muscle, Smooth; Myocardium; Pancreas; Parasympatholytics; Piperazines; Piperidines; Rats; Rats, Inbred Strains; Receptors, Muscarinic; Stereoisomerism; Structure-Activity Relationship