ABSTRACT
This chapter reviews the distribution and function of muscarinic receptor subtypes in smooth muscle throughout the gastrointestinal tract. It provides a brief description of muscarinic receptors in the associated neurons and muscosa of gastrointestinal smooth muscle because the accessory tissues can often influence the contractile response of the muscle. The most precise methods for identifying receptors in tissue are based on molecular biological techniques. Various methods have been used to identify subtypes of the muscarinic receptor throughout the smooth muscle of the gastrointestinal tract. Muscarinic receptors have been measured directly using immunoprecipitation, RNA hybridization, and radiolig and binding methods. With the development of subtype selective muscarinic antagonists, several investigators examined the binding properties of muscarinic receptors in the ileum, and an enigma soon emerged. Muscarinic receptors are present in the mucosa throughout the gastrointestinal tract where they play an important role in electrolyte transport.
