Fig. 1.

Activation and inactivation characteristics ofI_A and I_K inDrosophila photoreceptors. A, Representative wild-type (WT) whole-cell recordings of photoreceptor potassium currents using the activation (left) and inactivation (right) protocols. Usually, we can clearly differentiate the inactivatingI_A component (arrowheads) separated from the slowly inactivating I_K.B, Representative whole-cell recordings of activation (left) and inactivation (right) protocols of the Shaker mutant alleleSh^KS133. C(left), The current density (pA/pF) is plotted against the voltage steps for WT (■) andSh^KS133 (▪). TheI_K component is characterized (Boltzmann fitting, assuming a reversal potentialV_K = −85 mV) byI_max = 85.5 ± 4.1 pA/pF and 87.8 ± 5.6 pA/pF for WT (n = 11) andSh^KS133 (n = 8), respectively. Right, Steady-state inactivation ofI_K in WT (■) andSh^KS133 (▪). For Boltzmann fitting parameters, see Table 2. D (left), The current density/voltage curve of I_A is presented (Table 1). I_A is characterized (Boltzmann fitting, measured at peak outward current, assuming a reversal potential V_K = −85 mV) byI_max = 92.3 ± 7.8 pA/pF (n = 11). Right, The Boltzmann fitting of I_A steady-state inactivation gave the values V₅₀ = −42.1 ± 1.0 mV and slope = 10.0 ± 0.8 (n = 4). In all voltage-clamp experiments throughout this study, the holding potential was −100 mV. For the activation protocol, cells were stepped from −100 mV to +60 mV in 10 mV increments, during a 100 msec test pulse (left column). In the steady-state inactivation protocol, the cell membrane was subjected to inactivating prepulses of 1 sec duration from −90 mV to +10 mV in 10 mV increments, before 80 msec test pulse to +30 mV (right column).