Simulated action potentials and underlying ionic currents are shown for control conditions and after application of 1 µM isoproterenol at two pacing frequencies 1 Hz (Panels A–E) and 5 Hz (Panels F–J) (Istim = 80 pA/pF, τstim = 1.0 ms). Panels A and F: Simulated action potentials for control conditions (solid line) and after isoproterenol application (dashed line). Panels B, D, G, and I: Currents underlying the AP for control condition. Panel C, E, H, and J: Currents underlying the AP after isoproterenol application. The scales for the relatively large fast Na+ current, INa, are given on the right axis in Panels B, C, G and H. APs and ionic currents are shown after 300 s stimulation. In Panels A, C, E, F, H, and J 1 µM isoproterenol is applied at time t = 0 s.