Amiodarone blocks L-type calcium current in single myocytes isolated from the rabbit atrioventricular node

1. Single myocytes were isolated from the rabbit atrioventricular node (AVN) and whole cell patch clamped, using a Cs(+)-based internal dialysis solution. Depolarizing voltage clamps from a holding potential of -40 mV were applied to investigate the effects of the antiarrhythmic agent amiodarone on L-type calcium current (ICa,L). 2. The current-voltage (I-V) relation for ICa,L was bell shaped in normal Tyrode's solution, with a peak at +10 mV. After a 1-min exposure to 10 microM amiodarone, ICa,L at all potentials between -30 mV and +60 mV was largely and reversibly blocked (n = 4). In 13 cells, peak ICa,L was blocked by 85% after amiodarone application. 3. The time course of the blocking effect was monitored during continuous pulsing (at 0.33 Hz) from -40 mV to +10 mV to observe the time course of ICa,L blockade. This could be described by a single exponential function with a rate constant of 0.21 per second. In other cells, amiodarone was applied for 30 sec without any stimulation; and, when stimulation was resumed, the ICa,L amplitude with the first pulse was 22% of the control amplitude (n = 4), indicating that a substantial portion of the blockade of ICa,L by amiodarone was tonic in nature. 4. In addition to the effect of amiodarone on the ICa,L channel, when the beta adrenergic agonist isoprenaline (1 microM) was applied to cells that had been pretreated with amiodarone, ICa,L amplitude was increased by 44.5 +/- 10.8% (n = 4). In cells that had received no such pretreatment with amiodarone, isoprenaline increased the ICa,L amplitude by 101.5 +/- 5.9% (n = 4). Thus, isoprenaline produced a larger increase in ICa,L in the absence of amiodarone than in its presence (P < 0.001). 5. It is concluded that, in single AVN cells, amiodarone exerts a direct ICa,L blocking action. In addition, the attenuated increase in ICa,L with isoprenaline in amiodarone pretreated cells appears consistent with an antagonism of beta adrenergic stimulation of ICa,L.