Kir2.x channels form the molecular basis of cardiac I-K1 current and play a major role in cardiac electrophysiology. However, there is a substantial lack of selective Kir2 antagonists. We found the beta(3)-adrenoceptor antagonist SR59230A to be an inhibitor of Kir2.x channels. Therefore, we characterized the effects of SR59230A on Kir2.x and other relevant cardiac potassium channels. Cloned channels were expressed in the Xenopus oocyte expression system and measured with the double-microelectrode voltage clamp technique. SR59230A inhibited homomeric Kir2.1 channels with an IC50 of 33 mu M. Homomeric Kir2.2 and Kir2.3 channels and Kir2.x heteromers were also inhibited by SR59230A with similar potency. In contrast, no relevant inhibitory effects of SR59230A were found in cardiac Kv1.5, Kv4.3 and KyLQT1/minK channels. In hERG channels, SR59230A only induced a weak inhibition at a high concentration. These findings establish SR59230A as a novel inhibitor of Kir2.1-2.3 channels with a favorable profile with respect to additional effects on other cardiac repolarizing potassium channels. (C) 2012 Elsevier Inc. All rights reserved.