Molecular mechanisms of K⁺ channel blockade: 4-Aminopyridine interaction with a cloned cardiac transient K⁺ (Kv1.4) channel

We studied the blocking effects of 4-aminopyridine (4-AP) on a Kv1.4 K⁺ channel. A permanently charged 4-AP derivative only produced block when applied intracellularly. 4-AP block accumulated from pulse to pulse indicating trapping of 4-AP in deactivated channels. For long trains of depolarizing pulses, 4-AP block increased with decreasing pulse duration. This increase took many pulses (>10) to accumulate and was relieved by two to three subsequent pulses of 500 msec duration. We conclude that the time- and voltage-dependence of 4-AP block can not be accounted for solely by either simple pure open channel or pure closed channel blocking schemes. We propose that the data can be explained by a model in which 4-AP binding is most stable when the channel has a symmetric arrangement in the binding regions.