Short- and long-term amiodarone treatments regulate Cav3.2 low-voltage-activated T-type Ca2+ channel through distinct mechanisms.

Low-voltage-activated T-type Ca2+ channels have been recognized recently in the mechanisms underlying atrial arrhythmias. However, the pharmacological effects of amiodarone on the T-type Ca2+ channel remain unclear. We investigated short- and long-term effects of amiodarone on the T-type (Cav 3.2) Ca2+ channel. The Cav3.2 α1H subunit derived from human heart was stably transfected into cells [human embryonic kidney (HEK)-Cav3.2] cultured with or without 5 μM amiodarone. Patch-clamp recordings in the conventional whole-cell configuration were used to evaluate the actions of amiodarone on the T-type Ca2+ channel current ( I Ca.T). Amiodarone blockade of ICa.T occurred in a dose- and holding potential-dependent manner, shifting the activation and the steady-state inactivation curves in the hyperpolarization direction, when amiodarone was applied immediately to the bath solution. However, when the HEK-Cav3.2 cells were incubated with 5 μM amiodarone for 72 h, I Ca.T density was significantly decreased by 31.7 ± 2.3% for control,-93.1 ± 4.3 pA/pF ( n = 8), versus amiodarone,-56.5 ± 3.2 pA/pF ( n = 13), P < 0.001. After the prolonged administration of amiodarone, the activation and the steadystate inactivation curves were shifted in the depolarization direction by -7.1 ( n = 41) and -5.5 mV ( n = 37), respectively, and current inactivation was significantly delayed [time constant (τ): control, 13.3 ± 1.1 ms ( n = 6) versus amiodarone, 39.6 ± 5.5 ms ( n = 6) at -30 mV, P < 0.001)]. Nevertheless, short-term inhibitory effects of amiodarone on the modified T-type Cav3.2 Ca2+ channel created by long-term amiodarone treatment were functionally maintained. We conclude that amiodarone exerts its short- and long-term inhibitory actions on I Ca.T via distinct blocking mechanisms.