Voltage- and Use-Dependent Modulation of Calcium Channel Current in Guinea Pig Ventricular Cells by Amiodarone and Des-Oxo-Amiodarone

Amiodarone is an effective antiarrhythmic drug handicapped by serious side effects. The mechanism of its antiarrhythmic activity is not known but is presumed to involve inhibition of current flowing through ion channels. Des-oxo-amiodarone, a close structural analogue of amiodarone, was synthesized based on the hypothesis that the toxic and therapeutic properties reside in different parts of the molecule and that chemical modification could result in a less toxic agent that yet preserved amiodarone's antiarrhythmic efficacy. We compared the effects of amiodarone and des-oxo-amiodarone on Ca current in enzymatically dispersed guinea pig ventricular myocytes using the whole-cell patch-clamp method. Amiodarone caused both a tonic and a phasic (use-dependent) reduction of the Ca current. The relationship between membrane potential and the availability for channel opening upon depolarization (inactivation curve) was shifted toward more negative membrane potentials by amiodarone (delta - 10.6 +/- 2.2 mV, n = 7). The use-dependent reduction of the Ca current was also dependent on the frequency of the voltage clamp steps (0.5 Hz, 40.2 +/- 7.9%; 1.0 Hz, 50.0 +/- 6.7%). Dex-oxo-amiodarone had a dual effect on the Ca current: After maintaining the membrane potential for several seconds at negative membrane potentials (less than -45 mV), the Ca current was increased by des-oxo-amiodarone. Des-oxo-amiodarone also shifted the Ca channel inactivation curve to more negative membrane potentials up to 16 mV. Consequently, Ca current could be increased or decreased depending on the experimental conditions. Enhancement of Ca current by des-oxo-amiodarone was transient and was supplanted entirely by the antagonistic effects of the drug after approximately 5 min. The antagonistic effects of des-oxo-amiodarone on Ca current were also use- and frequency-dependent.