We studied the effects of NE-10064 (azimilide), a new antiarrhythmic agent reported to be a selective blocker of the slowly activating component of the delayed rectifier, IKs. In ferret papillary muscles, NE-10064 increased effective refractory period (ERP) and decreased isometric twitch tension in a concentration-dependent manner (0.3-30 microM). Increases in ERP showed reverse use-dependence, and were greater at 1 than at 3 Hz. In contrast, changes in tension were use dependent, with larger decreases observed at 3 than at 1 Hz. In guinea pig ventricular myocytes, NE-10064 (0.3-3 microM) significantly prolonged action potential duration (APD) at 1 Hz. At 3 Hz, NE-10064 (0.3-1 microM) increased APD only slightly, and at 10 microM decreased APD and the plateau potential. NE-10064 potently blocked the rapidly activating component of the delayed rectifier, IKr (IC50 0.4 microM), and inhibited IKs (IC50 3 microM) with nearly 10-fold less potency. NE-10064 (10 microM) did not block the inward rectifier potassium current (IKl). NE-10064 (10 microM) blocked the L-type calcium current (ICa) in a use-dependent manner; block was greater at 3 than at 1 Hz. We conclude that (a) NE-10064's block of potassium currents is relatively selective for IKr over IKs, (b) NE-10064 inhibits ICa in a use-dependent fashion, and (c) NE-10064's effects on ERP and tension in papillary muscle as well as APD and action potential plateau level in myocytes may be explained by its potassium and calcium channel blocking properties.