Effects of acute and chronic treatments with amiodarone, both in the presence and the absence of exogenous triiodothyronine (T3), on repolarizing outward K+ currents were investigated by patch-clamp technique in cultured newborn rat ventricular cells. Acute exposure to amiodarone dose-dependently inhibited the transient outward (Ito IC50 = 4.9 microM) and the steady-state outward (IK, IC50 = 6.3 microM) K4 currents. The dose-response curve of this acute inhibitory action was unaffected by the presence of T3. When amiodarone was applied chronically. 72-h exposure to a low dose of the drug (1 microM) significantly decreased the current densities of Ito and I kappa for the cells cultured in a serum-supplemented medium containing 0.12 nM T3. In a serum-free medium without T3o chronic amiodarone treatment revealed null effect on either Ito or IK. In addition, 72-h in-vitro treatment with Ti enhanced the current densities of both Ito (EC50 = 0.13 nM) and I kappa (EC50 = 0.33 nM). Concentration-response analysis indicated that amiodarone (1 microM) showed competitive inhibition towards the action of T3 on Ito but noncompetitive inhibition towards the action of T3 on IK. These results suggest that different ionic mechanisms are produced by acute and long-term treatments with amiodarone. The latter showed T3-dependent inhibition of cardiac Ito and IK. When chronically administered, amiodarone may antagonize T3 and thereby counteract its hormonal effect on K+ channels. This implies that, at the myocyte level, antagonism of the action of thyroid hormones in K+ channel activities may contribute to the cardiac effects of chronic amiodarone therapy.