Cellular electropharmacology of amiodarone.

The complex profile of amiodarone actions on the electrophysiological properties of cardiac cells reviewed in this article may be summarized as follows. As acute effects, amiodarone inhibits both inward and outward currents. The inhibition of inward Na+ and Ca2+ currents is enhanced in a use- and voltage-dependent manner, resulting in suppression of excitability and conductivity in both iNa- and iCa-dependent cardiac tissues. The inhibition is greater in the tissues stimulated at higher frequencies, and in those with less negative resting (or diastolic) membrane potentials. As outward currents, iK (iKr and iKs), iK,ACh and iK,Na are inhibited by acute amiodarone, iKl could also be inhibited at high concentrations of amiodarone. Acute effects of amiodarone on i(to) remain unclear. Previous reports on the acute effects of amiodarone on APD are conflicting, presumably because different ionic currents are responsible for the repolarization of action potential in different animal species, cardiac tissues and experimental conditions. APD would be shortened if the inhibitory action of amiodarone on the inward current is greater than on the outward current, and vice versa in the opposite case. The major and consistent chronic effect of amiodarone is a moderate APD prolongation with minimal frequency-dependence. This prolongation is most likely due to a decrease in the current density of iK and i(to). Chronic effects of amiodarone are modulated by tissue accumulation of amiodarone and DEA. Variable suppression of excitability and conductivity of the heart by chronic amiodarone might reflect direct acute effects of the parent drug and/or its active metabolite (DEA) retained at the sites of action. Chronic amiodarone was shown to cause a down-regulation of Kv1.5 mRNA in rat hearts, suggesting a drug-induced modulation of potassium channel gene expression. Electrophysiological changes in the heart induced by chronic amiodarone resemble those induced by hypothyroidism. Three mechanisms have been proposed to explain this hypothyroid-like action of amiodarone. Amiodarone and/or DEA may inhibit peripheral conversion from T4 to T3, cellular uptake of T4 and T3, and T3 binding to nuclear receptors (TR). The second and third mechanisms are considered to be more important than the first. Amiodarone or DEA could antagonize T3 action on the heart at a cellular or subcellular level. Two distinct characteristics in the cellular electropharmacology or amiodarone are different from those of other antiarrhythmic drugs. First, it acts on many different types of molecular targets including Na+, Ca2+, and K+ channels as well as adrenoceptors. Second, it may cause antiarrhythmic remodeling of cardiac cells, probably through a modulation of gene expression of ion channels and other functional proteins. We hypothesize that this remodeling is mediated most likely by cellular or subcellular T3 antagonism. Nevertheless, much remains to be studied as ot the acute and especially chronic effects of amiodarone on ionic currents, transporters, receptors and other molecules in cardia cells. The role of the cardiac hypothyroid state in the genesis of antiarrhythmic activity is still a matter of considerable controversy among investigators. Recently, two amiodarone analogues (SR 33589 and ATI-2001) showing a potent acute antiarrhythmic activity in animal models, have been developed [37,87,88,131]. These new compounds are not known to exhibit chronic antiarrhythmic potential or cardiac hypothyroidism activity. Unraveling these tissues will be required to understand the exact molecular and cellular mode of action of amiodarone and to find a new direction for the development of the ideal antiarrhythmic drugs of the future.