Fig. 2.

Effect of 12 mM [K⁺]₀ in the presence of 4-aminopyridine (4-AP). High [K⁺]_o was perfused in the absence [BCLs = 2,000 (A) and 300 ms (B)] and presence of 1 mM 4-AP [BCLs = 2,000 (C) and 300 ms (D)]. The preferential rate-dependent depression of Epi AP morphology following exposure to high [K⁺]_o persisted, indicating that the differential response between Endo and Epi is unrelated to transmural differences in transient outward K⁺ current. Rather, the data suggest that differences in fast Na⁺ current (I_Na) underlie the excitability disparities between the two tissues. All AP recordings (obtained with floating microelectrodes) are scaled up to the amplitude of control recordings.