Therefore, ‘fast’ ramps (slope ± 40 mV) had to be imposed to make sure that a sufficiently large fraction of pores prevailed while the membrane potential passed through the physiological voltage range. Background currents (including those due to an imperfect seal and ion transporters in the membrane, compare Fig. 1) were taken into account by a preceding ramp experiment that started with a prepulse to ± 150 mV (that would not induce pore formation, compare Fig. 2). Current voltage relations obtained from such a pair of ramp experiments were superimposed (Fig. 3).