BAPTA-AM is a well-known membrane permeable Ca(2+) chelator. The present study found that BAPTA-AM rapidly and reversibly suppressed human ether a-go-go-related gene (hERG or Kv11.1) K(+) current, human Kv1.3 and human Kv1.5 channel currents stably expressed in HEK 293 cells, and the effects were not related to Ca(2+) chelation. The externally applied BAPTA-AM inhibited hERG channels in a concentration-dependent manner (IC(50): 1.3 microM). Blockade of hERG channels was dependent on channel opening, and tonic block was minimal. Steady-state activation V(0.5) of hERG channels was negatively shifted by 8.5 mV (from -3.7+/-2.8 of control to -12.2+/-3.1 mV, P<0.01), while inactivation V(0.5) was negatively shifted by 6.1 mV (from -37.9+/-2.0 mV of control to -44.0+/-1.6 mV, P<0.05) with application of 3 microM BAPTA-AM. The S6 mutant Y652A and the pore helix mutant S631A significantly attenuated blockade by BAPTA-AM at 10 microM causing profound blockade of wild-type hERG channels. In addition, BAPTA-AM inhibited hKv1.3 and hKv1.5 channels in a concentration-dependent manner (IC(50): 1.45 and 1.23 microM, respectively), and the blockade of these two types of channels was also dependent on channel opening. Moreover, EGTA-AM was found to be an open channel blocker of hERG, hKv1.3, hKv1.5 channels, though its efficacy is weaker than that of BAPTA-AM. These results indicate that the membrane permeable Ca(2+) chelator BAPTA-AM (also EGTA-AM) exerts an open channel blocking effect on hERG, hKv1.3 and hKv1.5 channels.