Use-dependent features of 4-aminopyridine block of transient outward current in rat ventricular myocytes

The inhibition of the calcium-independent transient outward current (Ito) by the widely used blocker 4-aminopyridine (4-AP) was studied in adult Wistar rat ventricular myocytes, enzymatically isolated and voltage-clamped in the whole cell configuration using patch-clamp pipettes. 4-AP at 1 mmol/l concentration caused complete steady-state block of Ito at resting or hyperpolarized voltages. The block was partially removed during repeated depolarizations applied at frequencies above 0.25 Hz. Changes in the level of block during a depolarizing pulse (to + 40 mV) and during a following rest period (at -90 mV) were analyzed. On depolarization, the recovery from 4-AP block (at 0.5 mmol/l) started with a delay approximately corresponding to the time constant of Ito inactivation and then followed a monoexponential time course (time constant of about 44 ms). The restoration of block at a holding voltage of -90 mV, after recovery of Ito from inactivation, followed a monoexponential time course (time constant of about 1.2 s). The results are consistent with the hypothesis that the binding site for 4-AP at the Ito channel is available in the closed and open states but not when inactivated. Unblocking of Ito at stimulation intervals shorter than approximately 1 s may occur at 4-AP concentrations below 4 mmol/l.