Experimental investigation of the reversed stimulation dependence of ΔKPQ Isus. (a) Cell-attached single-channel activity in a cell expressing ΔKPQ channels. Shown are consecutive sweeps. (b) The dwell time of bursting activity was measured as in Figure 5 and was used to generate the histogram shown (bin size = 500 milliseconds). The smooth curve is the best fit double exponential function best fit to the histogram (see Table 2 for values). (c) TTX-sensitive macroscopic current is shown at high gain in response to the first and 20th pulses (–10 mV, 500 milliseconds, holding potential –100 mV) of a pulse train (20-millisecond interpulse interval). (d) Mean ± SEM ΔKPQ Isus (open circles, n = 6) normalized to Isus1 and plotted as a function of pulse number during the pulse train (interpulse interval, 20 milliseconds). The dotted line replots the experimental data for Y1795C channels shown in Figure 8b for comparison.