Muscarinic receptors in gastric mucosa are increased in peptic ulcer disease.

Muscarinic receptors stimulate the secretion of acid pepsinogen and mucous in gastric mucosa. Whether muscarinic receptors are involved in the pathogenesis of benign gastric disease is unknown. Receptor changes in these conditions were therefore sought. An autoradiographic technique was developed to determine quantitatively muscarinic receptors in microtome sections of biopsy specimens obtained during gastroscopy. Muscarinic receptor density was mean (SEM) 18.4 (1.2) fmol/mg protein in the corpus and 8.9 (0.7) fmol/mg protein in the antrum (n = 53). Neither chronic nor active gastritis was associated with receptor changes in the antrum but chronic gastritis was associated with a receptor loss in the corpus. Patients with acute or recent duodenal or antral ulcers (n = 23) had significantly higher levels of muscarinic receptors in the corpus than controls (n = 25) (22.2 (1.5) v 16.9 (1.7) fmol/mg protein respectively (p < 0.025). These results suggest that muscarinic M3 receptor is overexpressed in duodenal ulcer disease and may play a part in its pathogenesis.

Human gastric mucosa expresses glandular M3 subtype of muscarinic receptors.

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.

Different binding properties of muscarinic M2-receptor subtypes for agonists and antagonists in porcine gastric smooth muscle and mucosa.

The hypothesis that muscarinic receptors on smooth muscle differ from those in epithelial glands was tested by comparing the properties of muscarinic binding sites in porcine fundic smooth muscle with those in mucosal membranes. The binding of agonists and of antagonists was assessed by displacement of [3H]N-methylscopolamine. Pirenzepine (M1-antagonist) labeled low-affinity binding sites in smooth muscle (KD = 229 nM) and in mucosa (KD = 124 nM) consistent with the presence of M2 sites. Carbachol interacted with a high-affinity (KD = 164 nM) and a low-affinity (KD = 18.2 microM) state of binding sites in smooth muscle. Guanyl 5'-yl-imidodiphosphate converted all sites to the low-affinity state. N-ethylmaleimide pretreatment increased the affinity of carbachol and the proportion of high-affinity sites. In clear contrast, only low-affinity sites of carbachol were detectable in mucosa (KD = 17 microM) that were not modulated by N-ethylmaleimide or guanyl 5'-yl-imidodiphosphate. The cardioselective antagonist AF-DX 116 displayed low affinity to mucosal binding sites (KD = 3.4 microM), whereas its affinity to smooth muscle was 503 nM. The antagonist hexahydro-sila-difenidol had a very high affinity (KD = 2.9 nM) to mucosal receptors, whereas its affinity to smooth muscle sites was 88 nM. These data show that muscarinic M2 binding sites in mucosa and smooth muscle can be distinguished by both agonist and antagonist binding experiments, and suggest the existence of different subtypes of M2-binding sites in these tissues.