Figure 6. Conductance properties of I_Ks in guinea-pig SA node cells.

A, cell was depolarized from a holding potential of −50 mV to +40 mV for 4 s to activate I_Ks, and then repolarized to various test potentials between +30 and −110 mV in 10 mV steps (left-hand panel). The same cell was repolarized from the −50 mV holding potential to membrane potentials between −60 and −110 mV without the depolarizing pulse (middle panel). I_Ks tail current elicited on return to the test potentials between −60 and −110 mV after a 4 s depolarizing pulse to +40 mV was determined by subtracting the membrane current without the depolarizing pulse to +40 mV from that with the depolarizing pulse (right-hand panel). A schematic diagram of the voltage protocol is given above the current traces. These recordings were conducted in the presence of 5 μm E-4031 to inhibit I_Kr. The dashed line indicates the zero current level. B, amplitude of the I_Ks tail current at each test potential was divided by the expected decrease in the activation variable and then plotted as a function of membrane potential.