Figure 10.

Inactivation of ScTx-sensitive currents A, current traces elicited by 1–3 s voltage steps to −10 mV from a holding potential of −70 mV (protocol below traces). Shown are a control trace and currents sensitive and insensitive to 600 nm ScTx (note difference in kinetics between ScTx-sensitive and -insensitive currents) (P30). B, box plot summarizing the time constant (τ) for inactivation determined by fitting the current decline (at −10 mV) with an exponential function. The inactivation time constants for the ScTx-sensitive current were between 2 and 26 s, with a median at 5.2 s (n = 11 cells). For the AB-sensitive current, median τ was 4.5 s (n = 5 cells). C, holding potential sensitivity of ScTx-sensitive and -insensitive currents. Currents were elicited by a step from −70 to −10 mV (200 ms; not shown). The stimulus was repeated at 10 s intervals. Holding potential was then changed to −40 mV. The plots show the amplitude of currents elicited by the test stimulus as a function of time after the holding potential change. The peak amplitude of the ScTx-sensitive currents decreased with time at −40 mV. The changes were well fitted by an exponential function, with a time constant of 2.5 ± 0.3 s. The ScTx-sensitive currents elicited from the −40 mV holding potential were much smaller than the currents recorded from −80 mV holding potential. This strong holding potential dependence of the ScTx-sensitive current is similar to the TEA-sensitive currents (Fig. 3). D, representative traces for steady-state inactivation of ScTx-sensitive current after 5 s inactivation at various potentials from −100 mV to −20 mV (P30). Voltage protocol is shown as inset below. A series of 5 s inactivation voltage steps of varied potentials were delivered from a holding potential of −70 mV every 15 s and closely followed by a 400 ms test voltage step to −10 mV. E, steady-state inactivation curves were obtained by fitting the averaged recordings from 10 cells with the Boltzmann equation. The ScTx-sensitive currents (•) had a half-inactivation potential of −60 mV (slope, 12.5 mV). Average data from 6 cells where steady-state inactivation was tested for the polyclonal anti-Kv2.1-sensitive current are illustrated by open squares (P30; V_1/2 = − 62 mV, slope = 11.5 mV). The data are very similar to the ScTx-sensitive currents. Most of the ScTx-insensitive current (▪) did not inactivate with this protocol. The half-inactivation potential for the ScTx-insensitive component that did inactivate was −87 mV. F, scatter plots summarizing the half-inactivation and slope data for ScTx-sensitive and anti-Kv2.1 (AB)-sensitive current. The data are very similar for the two ways of isolating Kv2.1 currents.