Upper gastrointestinal porcine smooth muscle expresses M2- and M3-receptors

The existence of 5 different mammalian subtypes of muscarinic receptors, named M1 to M5 has been established. This study was designed to characterize the subtypes of muscarinic receptors occurring in smooth muscle of the upper gastrointestinal tract. Displacement studies of [3H]N-methylscopolamine by the highly M2-selective ligand methoctramine revealed the presence of M2-receptors. The remaining sites had characteristics of M3-receptors. The ratio of M2- to M3-receptors varied from a predominance of M3-receptors in antral tissue over a predominance of M2-receptors in gastric corpus and ileal smooth muscle to the almost exclusive presence of M2-receptors in duodenal smooth muscle. Mucosa from all areas showed characteristics of M3-receptors without evidence for the occurrence of M2-receptors.