2-Methyl-3- (3,5-diiodo-4-carboxymethoxybenzyl) benzofuran (KB130015; KB), a novel compound derived from amiodarone, has been proposed to have antiarrhythmic properties. Its effect on the G protein-coupled inward rectifying K+ current [IK(ACh) or IK(Ado)], ATP-sensitive K+ current [IK(ATP)], and background inward rectifying current (I(K1)) were studied in guinea pig atrial and ventricular myocytes by the whole-cell voltage-clamp technique. Receptor-activated IK(ACh/Ado), induced in atrial myocytes by the stimulation of either muscarinic or Ado receptors was concentration dependently (IC50 value of approximately 0.6-0.8 microM) inhibited by KB. Receptor-independent guanosine 5'-O-(3-thio)triphosphate-induced and background IK(ACh), which contributes to the resting conductance of atrial myocytes, were equally sensitive to KB (IC50 value of approximately 0.9 microM). IK(ATP) induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP) was also suppressed by KB, whereas IK1 measured in ventricular myocytes was insensitive to the drug (KB < or =50 microM). Although being effective when applied from the outside, intracellular application of KB via the patch pipette affected neither IK(ACh) nor IK(ATP). 3,3',5-triodo-L-thyronin, which shares structural groups with KB, did not have an effect on the K+ currents. Consistent with the effects on single myocytes, KB did not depolarize the resting potential but antagonized the shortening of action potential duration by carbamylcholine-chloride or by DNP in multicellular preparations and antagonized the shortening of action potential duration by acetylcholine in single myocytes. It is concluded that KB inhibits IK(ACh) and IK(ATP) by direct drug-channel interaction at a site more easily accessible from extracellular side of the membrane.