Abstract

ATP-sensitive K⁺ (K_ATP) channels are present on the sarcolemma (sarcK_ATP channels) and mitochondria (mitoK_ATP channels) of cardiac myocytes. Amiodarone, a class III antiarrhythmic drug, reduces sudden cardiac death in patients with organic heart disease. The objective of the present study was to investigate the effects of amiodarone on sarcK_ATP and mitoK_ATP channels. Single sarcK_ATP channel current and flavoprotein fluorescence were measured in guinea pig ventricular myocytes to assay sarcK_ATP and mitoK_ATP channel activity, respectively. Amiodarone inhibited the sarcK_ATP channel currents in a concentration-dependent manner without affecting its unitary amplitude. The IC₅₀ values were 0.35 μM in the inside-out patch exposed to an ATP-free solution and 2.8 μMin the cell-attached patch under metabolic inhibition, respectively. Amiodarone (10 μM) alone did not oxidize the flavoprotein. In addition, the oxidative effect of the mitoK_ATP channel opener diazoxide (100 μM) was unaffected by amiodarone. Exposure to ouabain (1 mM) for 30 min produced mitochondrial Ca²⁺ overload, and the intensity of rhod-2 fluorescence increased to 246 ± 16% of baseline (n = 9). Amiodarone did not alter the ouabain-induced mitochondrial Ca²⁺ overload (236 ± 10% of baseline, n = 7). Treatment with diazoxide significantly reduced the ouabain-induced mitochondrial Ca²⁺ overload (158 ± 15% of baseline, n = 8, p < 0.05 versus ouabain); this effect was not abolished by amiodarone (154 ± 10% of baseline, n = 8, p < 0.05 versus ouabain). These results suggest that amiodarone inhibits sarcK_ATP but not mitoK_ATP channels in cardiac myocytes. Such an action of amiodarone may effectively prevent ischemic arrhythmias without causing ischemic damage.