Figure 10. Comparison of tail current amplitude of I_Kr and I_Ks.

A, pulse protocol. Depolarizing pulses of 50 ms were applied to +20 mV from a holding potential of −50 mV at a rate of 200 pulses min⁻¹. After an SA node cell was subjected to a train of successive depolarizing pulses for 1 min (200 pulses), 5 μm E-4031 was added to the bath, as marked by the horizontal bar. B, I_K at P₁ (first pulse), P₂ (second pulse), P₃ (third pulse), P₂₀₀ (two hundredth pulse) and P₄₀₀ (four hundredth pulse). The open triangle indicates the current level before applying the pulse train and the arrow denotes the peak level of I_K tail current. The zero current level is indicated by the dashed line. C, I_Kr obtained by digital subtraction of I_K after 1 min exposure to E-4031 (I_K at P₄₀₀) from that before its exposure (I_K at P₂₀₀) (shown in B). D, amplitudes of tail currents of I_Kr and I_Ks (n = 4), determined during successive depolarizations (shown in A). The amplitude of I_Kr tail current was measured from I_Kr obtained as shown in C. The magnitude of I_Ks tail current was determined from I_K at P₄₀₀ by subtracting the current level before applying the pulse train (denoted by open triangle) from the peak of tail current (indicated by arrow). The current amplitudes were normalized with reference to cell capacitance. Columns and bars represent the means and s.e.m., respectively, and the amplitude of I_Ks was significantly larger than the amplitude of I_Kr (Student's paired t test).