Modulation of aminopyridine block of potassium currents in squid axon

Aminopyridines are known to block potassium (K) currents in excitable membranes in a manner dependent upon membrane potential, such that the block is relieved by depolarization and restored upon repolarization. In the present study, the effects of aminopyridines on voltage-dependent potassium (K) channels were examined in internally perfused, voltage-clamped squid giant axons. The time course of block restoration after conditioning depolarization was found to be modulated by membrane electric field, K-channel gating, and external cations. Depolarized holding potentials accelerated block restoration without altering steady-state block levels, suggesting that the voltage dependence of block restoration may be related to K channel gating rather than drug binding per se. In support of this notion, low external calcium concentration, which shifts the voltage dependence of K-channel gating to more negative potentials, also accelerated block restoration. Conversely, the relationship between the rate of block restoration and membrane holding potential was shifted in the depolarizing direction by phloretin, an agent that shifts the dependence of K-channel opening on membrane potential in a similar manner. Modification of K-channel gating also was found to alter the rate of block restoration. Addition of internal zinc or internal treatment with glutaraldehyde slowed the time course of both K-channel activation and aminopyridine block restoration. Aminopyridines also were found to interact in the K channel with external Cs+, NH4+, and Rb+, each of which slowed aminopyridine block restoration. Our results suggest that aminopyridines enter and occlude K channels, and that the availability of the binding site may be modulated by channel gating such that access is limited by the probability of the channel reaching an intermediate closed state at the resting potential.