Figure 4.

Voltage dependence and kinetics of recovery from use-dependent block. (A) Mean fractional current (I₁₂/I₁) was plotted as a function of conditioning pulse voltage (pulses 2–11) for three cells that contained 1.5 mM ac-KIFMK-NH₂ (•), or no peptide (○, n = 16). The conditioning pulses (V) varied in voltage from −50 to 200 mV from a holding potential of −80 mV. A voltage profile of the conditioning pulse protocol is shown in the inset. Inspection of the sodium currents recorded during the train of conditioning pulses showed that steady state block was reached at each conditioning potential, although the small size of the currents at 50 mV made assessment of steady state less certain at that potential. The smooth line represents fit of the data to Eq. 4 using values for the voltage for half-maximal block and zδ from the means of individual fits from three cells. The mean voltage at which half the channels were blocked was 91.7 ± 3.3 mV, with a zδ value of 1.0 ± 0.05, n = 3. (B) The fractional current (I₁₂/I₁) remaining after 10 conditioning pulses was measured for unmodified channels with no peptide, sodium channels modified by LqTx with no peptide, wild-type channels with 1 mM ac-KIFMK-NH₂, and LqTx-modified channels with 1 mM ac-KIFMK-NH₂ in response to conditional pulses to 200 mV. The mean fractional current for the control with no additions was 0.99 ± 0.01 (n = 14); for LqTx-modified, 0.79 (n = 2); for 1 mM ac-KIFMK-NH₂, 0.04 ± 0.02 (n = 3); and for 1 mM ac-KIFMK-NH₂ with LqTx, 0.03 ± 0.02 (n = 6). (C) The fractional current (I₁₂/I₁) remaining after 10 conditioning pulses measured for channels blocked by 1 mM ac-KIFMK-NH₂ in the absence or presence of 100 nM LqTx applied to the extracellular side. The conditioning pulses were to 200 mV and the final interval at −80 mV between pulse 11 and 12 was lengthened to 91 s. The mean fractional recovery from use-dependent block by 1 mM ac-KIFMK-NH₂ in the absence of LqTx was 0.26 ± 0.05 (n = 8) and in the presence of 100 nM LqTx was 0.56 ± 0.04 (n = 3).