Muscarinic receptor subtypes in human and rabbit bladder

Subtypes of muscarinic receptors have been proposed by several investigators, based on differences in the potency of muscarinic agonists in various tissues. Development of selective muscarinic agonists and antagonists for the urinary bladder would be extremely useful since side effects of the presently used drugs often limit their clinical usefulness. Direct investigation of muscarinic receptor subtypes by radioligand binding techniques has recently become possible. The muscarinic antagonist pirenzepine distinguishes between a high affinity and low affinity binding site in several tissues. Results of the present investigation indicate that only the low affinity pirenzepine binding site is present in both human and rabbit bladder. Muscarinic receptor linked biochemical responses (increased phosphatidyl inositol turnover and inhibition of adenyl cyclase) were also consistent with the presence of only the low affinity pirenzepine binding receptor subtype in both human and rabbit bladder.