Erythromycin administration has been associated with a prolongation of cardiac repolarization in certain clinical settings. This could be due to blockade of voltage-dependent K+ channels in the human heart. For this reason we examined the effects of erythromycin on a rapidly activating delayed rectifier K+ channel (Kv1.5) cloned from human heart and stably expressed in human embryonic kidney cells. When examined using the whole-cell patch clamp technique, erythromycin (100 microM) blocked Kv1.5 current in a time-dependent manner but required prolonged exposure to do so. However, when we examined Kv1.5 current using inside-out macro-patches, erythromycin applied to the cytoplasmic surface rapidly (within 1-2 min) inhibited Kv1.5 current with an IC50 value of 2.6 x 10(-5)M (1.7 - 3.9 x 10(-5)M, 95% C.L.). The main effect of erythromycin was to accelerate the rate of Kv1.5 current decay thereby reducing the current at the end of a prolonged voltage-clamp pulse. Erythromycin also blocked Kv1.5 current in both a voltage- and frequency-dependent manner but had little effect on the activation kinetics, deactivation kinetics, or the steady-state inactivation properties of Kv1.5. These data suggest that erythromycin acts as a blocker of an activated state of the Kv1.5 channel and that it may access its binding site from the intracellular face of the channel. This study is the first to examine the effects of erythromycin on a cloned human cardiac K+ channel. It is concluded that erythromycin blocks Kv1.5 at clinically relevant concentrations. Blockade of voltage-dependent K+ channels in the heart could contribute to the alterations in cardiac repolarization that have been observed with erythromycin.