TY - JOUR T1 - A model of the block of Kv4.2 ionic currents by 4-aminopyridine JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther DO - 10.1124/jpet.117.243097 SP - jpet.117.243097 AU - Steven James Kehl Y1 - 2017/01/01 UR - http://jpet.aspetjournals.org/content/early/2017/09/01/jpet.117.243097.abstract N2 - Voltage clamp recordings of macroscopic currents were made from rat Kv4.2 channels expressed in HEK293 cells. The main goals of the study were to quantify the concentration-, time- and voltage dependence of the block and to generate a state model that replicated 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 Kd 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 ~40 ms. 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. ER -