Distribution of muscarinic receptors in the digestive tract organs.

We have studied the binding of the potent muscarinic antagonist, (3H)-quinuclidinyl benzilate [(3H)-QNB], in order to characterize muscarinic receptors of the digestive tract organs and evaluate their population. In each tissue examined, the binding sites were saturable and stereoselective; (3H)-QNB could also be displaced by known muscarinic agonists and antagonists. Along the digestive tract, the stomach (for stomach, fundus, antrum) and the colon possess the highest number of binding sites while the secretory (parotid, mandibular and pancreas), with a comparable amount of sites, are among those with less receptors. This study presents for the first time a complete distribution of the muscarinic receptors in organs of the digestive system evaluated under identical experimental conditions.