Identification of a novel, inhibitory action of amiodarone on vesicular monoamine transport.

The benzofuran antiarrhythmic drug, amiodarone, exhibits a wide range of pharmacological properties. Recent in vivo biochemical studies suggest that amiodarone may exert an antiadrenergic action in the heart, which resembles the effects of reserpine. To investigate the cellular basis for this apparent presynaptic, sympatholytic action we used Chinese hamster ovary (CHO) cells expressing the type 2 vesicular monoamine transporter (VMAT2) as a synaptic vesicular model. Amiodarone inhibited the uptake of [3H]norepinephrine in VMAT2-transfected CHO cells in a concentration-dependent manner, with a -log EC50 of 6.44 +/- 0.32. To further identify the site at which amiodarone suppressed vesicular monoamine transport, we examined the ability of amiodarone to displace [3H]reserpine from its binding site in membrane fractions prepared from CHO cells expressing VMAT2. [3H]Reserpine binding was inhibited in a concentration-dependent manner by amiodarone, with an -log EC50 of 6.76 +/- 0.03, reaching 84 +/- 5% inhibition of reserpine binding at 10 microM. A pH-dependent mechanism for this action of amiodarone was excluded in studies using the pH-sensitive fluorescent indicator 2',7'-bis (carboxyethyl)-5,6-carboxyfluorescein (BCECF). These data indicate that amiodarone inhibits the uptake of monoamine into the axoplasmic storage vesicle by inhibiting VMAT. Furthermore, amiodarone competes specifically with reserpine for binding to VMAT. These findings suggest a novel presynaptic site of action for amiodarone.