On the muscarinic receptors in the urinary bladder and the putative subclassification of muscarinic receptors

Abstract

The muscarinic cholinergic receptors in the urinary bladders of man, guinea pig, rat and rabbit were studied by means of a receptor binding technique, with l-quinuclidinyl [phenyl 4-3H]benzilate, (-)3H-QNB, as radioligand. The potential role of the receptors in the supersensitivity of the rat bladder to muscarinic agonists, following parasympathetic denervation, hypertrophy and urinary diversion, was also investigated. In addition, the binding of various unlabelled antimuscarinic drugs in the guinea pig bladder was compared to that in other tissues in order to study the putative muscarinic receptor subtypes, commonly referred to as M1 and M2. According to this classification the putative M1 receptors prevail in discrete areas of the brain, whereas the M2-receptors predominate in peripheral tissues, such as the exocrine glands and smooth muscles. The receptor density (but not the qualitative properties of the receptors) in the bladder differed between the species. The affinities of various antimuscarinic drugs were virtually identical in the guinea pig and human bladders. In both species, the binding data were found to correlate with functional in vitro data. In the rat bladder, the receptor density was increased after denervation but decreased, below control values, when the denervation was combined with urinary diversion. A decrease was also found after urinary diversion of innervated bladders, whereas the receptor density was unaffected by hypertrophy. These results suggest that the receptors are not involved in the development of supersensitivity and that the receptor levels may be influenced by the functional state of the bladder. Binding studies with classical muscarinic antagonists indicated that the receptors in the guinea pig bladder are indistinguishable from those in the ileum, heart, parotid gland and cerebral cortex. However, four drugs--namely, oxybutynin, dicyclomine, benzhexol and pirenzepine had a much higher affinity for the receptors in the parotid gland and cortex than for those in the other tissues. Moreover, dicyclomine and benzhexol, like pirenzepine, seemed in the cortex to distinguish between two classes of sites exhibiting high and low affinity. The high affinity sites could be selectively labelled with 3H-benzhexol. The ability of oxybutynin, dicyclomine, benzhexol and pirenzepine to discriminate between the receptors in the parotid gland and those in smooth muscle provides further evidence that the M1/M2 concept is inaccurate. The present data indicate that there may be three classes of muscarinic antagonist binding sites.