Allosteric Modulation of [3H]Nitrendipine Binding to Cardiac and Cerebral Cortex Membranes by Amiodarone

The possible interaction between the antianginal and antiarrhythmic drug amiodarone and the slow calcium channel was investigated by competition binding experiments in guinea-pig cerebral cortex and rat heart membranes using [H]nitrendipine as radioligand. Amiodarone displaced specifically bound [H]nitrendipine from cerebral cortex and cardiac membranes in an apparently competitive manner. In saturation binding experiments, apparent affinity for [H]nitrendipine progressively decreased with increasing concentrations of amiodarone, whereas maximal binding capacity (Bmax remained unchanged. Both diltiazem and verapamil reversed the inhibitory effect of amiodarone on [H]nitrendipine binding to cerebral cortex membranes. Together these results suggest that amiodarone exerts a pseudocompetitive inhibition on [H]nitrendipine binding by acting at a site in allosteric interaction with the 1,4 dihydropyridine binding site associated with the calcium channel. The data are compatible with the existence of a common binding site for diltiazem, verapamil, and amiodarone. These observations are discussed in connection with the pharmacological properties of the drug.