Muscarinic receptor binding in the parotid gland. Different affinities of some anticholinergic drugs between the parotid gland and ileum.

Some properties of the muscarinic receptors in guinea pig parotid gland were studied by receptor binding, using 1-Quinuclidinyl [phenyl 4-3H] benzilate ((-)3H-QNB) as radio-ligand. In equilibrium experiments, (-)3H-QNB was specifically bound with high affinity (KD 1.1 X 10(-10)M) to a limited number of sites (0.37 pmoles/mg protein) in homogenates of parotid gland. The binding could be described by a simple bimolecular association model. Muscarinic antagonists and agonists effectively inhibited the binding, whereas a variety of other drugs had little or no effect. In competition experiments the affinities of five muscarinic antagonists were compared in the guinea pig parotid gland and ileum. Two of these drugs--namely, oxybutynine and dicyclomine--had a very much higher affinity for the binding-sites in the parotid gland than the ileum (24 vs. 34 times). Whether this observation might be caused by heterogeneous binding-sites for oxybutynine and dicyclomine or some other phenomenon is under further investigation.