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Figure 4 Sensitivity of I_Ks in the epicardial (Epi), midmyocardial (M), and endocardial (Endo) cells to inhibition by chromanol 293B. (A) Representative superimposed current traces elicited by 3-s depolarizing voltage-clamp steps applied from a holding potential of –50 mV to +50 mV in an epicardial cell, before (control) and during exposure to 293B at a concentration of 0.1, 1, 5, 10, and 30 µmol/l. The I_Kr inhibitor E-4031 (3 µmol/l) was present throughout. Tail currents were demonstrated on an expanded scale. (B) The percent block of I_Ks in the Epi (open circles), M (open squares), and Endo (open triangles) cells. The degree of I_Ks inhibition was measured as the fraction of the tail current reduced by each concentration of 293B with reference to the control amplitude of the tail current. Smooth curves through the data points represent a least-squares fit of a Hill equation: percent block = 100/(1 + (IC₅₀/[293B])^nH), yielding the concentration required for the half-maximal block (IC₅₀) and the Hill coefficient (n_H). pA = pico (x 10^–12) Ampere.