Gastrointestinal tract muscarinic receptor antagonists

The following antagonists act only at the muscarinic receptor and therefore affect nerve transmissions to the smooth muscle of the gastrointestinal tract, urinary tract, and glands. The clinical effects and uses of these antagonists reflect this fact. 

GASTROINTESTINAL TRACT In the management of acid-peptic disease, antisecretory doses of muscarinic antagonists produce hmiting side effects (Table 7-2) and, consequently, poor patient compliance. Pirenzepine has selectivity for Mj over and M3 receptors. However, piren-zepine s affinities for Mj and receptors are comparable, so it does not possess total Mj selectivity. 

S)-Hyoscyamine and (6)-scopolamine are esters of the amino alcohols tropine and scopine with ( S)-tropic acid, which is derived from phenylalanine (Fig. 3). The two alkaloids occur exclusively in the Solanaceae family. They act as antagonists of muscarinic acetylcholine receptors (parasympatholytics) and lead to an increase in pulse rate, relaxation of smooth muscles, e.g., in the gastrointestine and the bronchial tract, reduction of salivary, bronchial, gastric, and sweat gland secretion. While hyoscyamine is a central stimulant, scopolamine depresses the central nervous system. 

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