A, single-sweep responses from an HEK 293 cell expressing hERG channels. Rs was 4.2 MΩ of which 80% was compensated. Cprs of 6.5 pF was fully compensated. Cm was 40 pF. B, the cells were held at 20 mV and stepped to potentials of -130 mV to 50 mV in 10 mV increments for 200 ms. C, the tail currents at +20 mV are plotted vs. Vcom from A. The extracellular solution contained (mM) 117 NaCl, 13 KCl, 10 HEPES, 5 EGTA. The pipette solution contained 160 KCl, 1 EGTA, 10 HEPES. Currents were recoded at 100 kHz and filtered at 10 kHz.