Fig. 4.

A: family of currents elicited by holding the cell at −80 mV and stepping through a series of voltages from −80 mV to +50 mV in normal Hanks' solution. K⁺ ions in the pipette solution were replaced with Cs⁺ to block K⁺ currents. At voltages positive to −20 mV, a transient inward current was evoked that peaked within 5 ms and relaxed to 0 within 200 ms. This current was partially offset by an outward current that activated over the same voltage range and was likely to be due to cesium ions being carried through potassium channels. B: same protocol as in A applied to the same cell but this time in Ca²⁺ -free Hanks' solution. The transient inward current was abolished, leaving only the outward current carried by Cs⁺. C: current-voltage (I-V) relationship of the inward current is summarized in the plot of the mean results of six such experiments. Peak inward current (■) developed at voltages positive to −20 mV, reached a maximum at 0 mV, and reversed at just under +30 mV. In Ca²⁺-free Hanks' solution (▲) the inward current was abolished at all voltages. D: voltage dependence of activation (■) and inactivation (▲) of the inward current. The mean results of 14 experiments were fitted with a Boltzmann equation giving a half-maximal voltage (V_1/2) of activation of −14 ± 1.7 mV. The current activated at voltages positive to −10 mV and was maximally active at voltages positive to 0 mV. Voltage-dependent inactivation was investigated by holding cells at a series of conditioning potentials from −100 mV to +10 mV for 2 s before stepping to a test potential of 0 mV for 500 ms. For analysis, the peak current at 0 mV was measured at each conditioning potential, normalized to the maximum current (I_max), and plotted against the appropriate potential. Mean data points in 5 such experiments were again fitted with a Boltzmann equation, yielding a V_1/2 of inactivation of −26 ± 1.68 mV. It is clear from this plot that availability of the inward current was high over the measured range of resting membrane potential. G_max, maximum conductance.