Dicyclomine, benzhexol and oxybutynine distinguish between subclasses of muscarinic binding sites.

The interactions of various unlabelled antimuscarinic drugs with the muscarinic receptors in the cerebral cortex, heart and urinary bladder were studied by a receptor binding technique, using (-)[3H]QNB as radioligand. In contrast to the other drugs examined, dicyclomine, benzhexol, oxybutynine and pirenzepine were bound with a significantly higher affinity in the cortex than in the heart and bladder. Furthermore, not only pirenzepine, but also dicyclomine and benzhexol were capable of distinguishing between two populations of muscarinic binding sites in the cortex. The low affinity sites for these drugs in the cortex were characterised by dissociation constants which were similar to those determined in the heart and the bladder, respectively. It was concluded that dicyclomine and benzhexol, like pirenzepine, are selective antagonists at the putative M1-receptor. Oxybutynine exhibited the same affinity profile but the tissue selectivity of this drug was less pronounced.