Disopyramide and quinidine bind with inverse selectivity to muscarinic receptors in cardiac and extracardiac rat tissues

We investigated the interactions of disopyramide and quinidine with the muscarinic receptor in tissue homogenates from rat atrium, ventricle, cortex, submandibular gland, and urinary bladder by means of competition binding experiments, using the tritium-labeled antagonist N-methyl-4-piperidyl benzilate. The drugs displayed heterogeneous characteristics of binding to the muscarinic receptors in the different tissues. The binding affinity of quinidine to the muscarinic receptor in atrial tissue was five to 10 times greater than in the other tissues studied, whereas the affinity of disopyramide to the muscarinic receptor in the heart was five times lower than in the other tissues. This inverse selectivity shown by the two drugs in their binding to cardiac and to non-cardiac tissues may explain the extracardiac antimuscarinic side effects of treatment with disopyramide and their absence with quinidine.