Differences between binding affinities of some antimuscarinic drugs in the parotid gland and those in the urinary bladder and ileum.

Possible differences between the muscarinic receptors in the guinea pig urinary bladder and those in the ileum and the parotid gland were investigated, using a receptor binding technique. The affinities of 18 antimuscarinic drugs were indirectly derived from the ability to inhibit the receptor-specific binding of the radioligand (-)3H-QNB. The Hill coefficients were close to unity which indicated that the drugs were bound to apparently uniform populations of receptors within each tissue. In contrast to traditional muscarinic antagonists, four drugs - namely, oxybutynine, dicyclomine, benzhexol and pirenzepine - bound with a significantly higher affinity in the parotid gland than in the urinary bladder and ileum. A tendency towards reversed selectivity was found for secoverine. Thus, the present results further support the hypothesis that differences in muscarinic receptors between tissues exist, e.g. smooth muscle compared with parotid gland, which can be detected only by certain antimuscarinic drugs.