A Model of the Block of Voltage-Gated Potassium Kv4.2 Ionic Currents by 4-Aminopyridine

Voltage clamp recordings of macroscopic currents were made from rat potassium-gated potassium 4.2(Kv4.2) channels expressed in human embryonic kidney (HEK293) cells with the main goals of quantifying the concentration, time, and voltage dependence of the block and to generate a state model that replicates the features of the block. When applied either externally or internally, the block of Kv4.2 currents by 4-aminopyridine (4AP) occurs at the holding potential (-80 mV), is affected by the stimulus frequency, and is relieved by membrane depolarization. The K-d for the tonic block at -80 mV was 0.9 +/- 0.07 mM and was consistent with 1:1 binding. Relief of block during a step to 50 mV was well fitted by a single exponential with a time constant of similar to 40 milliseconds. At -80 mV, the association rate constant was 0.08 mM(-1) s(-1), and the off-rate was 0.08 s(-1). The state model replicates the features of the experimental data reasonably well by assuming that 4AP binds only to closed states, that 4AP binding and inactivation are mutually exclusive processes, and that the activation of closed-bound channels is the same as for closed channels. Since the open channel has a very low or no affinity for 4AP, channel opening promotes the unbinding of 4AP, which accounts for the reverse use dependence of the block.