Modulation of aminopyridine block of potassium currents in squid axon

Abstract

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.

Interaction of Riluzole with the Closed Inactivated State of Kv4.3 Channels

“…Thus, riluzole must unbind from the closed channel during the late phase of the depolarizing pulse, and the channel enters into the open state after the dissociation of the drug and inactivates at a normal rate, which led to a crossover phenomenon. Similar results have been reported for 4-AP on Kv4.2 (Kirsch et al, 1986) and (Ϫ)-epigallocatechin-3-gallate on Kv1.5 (Choi et al, 2001): the closed state inhibition can be relieved during membrane depolarization due to the unbinding of the drug from the channel at depolarized potentials. Because Kv4.3 channels can enter the inactivated state directly from the closed state, bypassing the open state during subthreshold depolarization (Beck and Covarrubias, 2001;Wang et al, 2005), riluzole interacts with both the closed and closed inactivated states of the channel.…”

Mechanism of inhibition of delayed rectifier K+ current by 4‐aminopyridine in rabbit coronary myocytes.

“…The behaviour of IK(V) in the presence of 4-AP was similar to that reported for the delayed rectifier K+ current in squid axon (Yeh, Oxford, Wu & Narahashi, 1976;Kirsch, Yeh & Oxford, 1986) Campbell, Qu, Rasmusson & Strauss, 1993;Jahnel, Klemm & Nawrath, 1994) and melanotroph cells (Kehl, 1990). In squid axon (Kirsch et al 1986) and ventricular myocytes (Castle & Slawsky, 1992;Campbell et al 1993), the concentration and voltage dependence of block by 4-AP was explained by assuming that the sequential closed states through which the channel passes during activation would exhibit successively lower affinity toward 4-AP, thus leading to time-dependent unblocking. the closed state, and its rapid release by membrane depolarization, could be reproduced well by our computer model, which assumed an extremely slow dissociation constant of 4-AP at negative potentials (0O001 s-'), and rapid unblocking due to state-dependent unbinding.…”

Effects of Ranolazine on Cloned Cardiac Kv4.3 Potassium Channels

“…After the dissociation of the drug from the binding site of the Kv4.3 channel, Kv4.3 became inactivated at a normal rate and increased during the late phase of the depolarizing pulse, which resulted in the crossover phenomenon. Similar results have been reported for 4-aminopyrine on K ϩ currents in squid giant axons (Yeh et al, 1976;Kirsch et al, 1986) and for riluzole on the Kv4.3 channel (Ahn et al, 2006). However, bupivacaine and ropivacaine accelerated the decline of the macroscopic current of Kv4.3 channels, which is a characteristic feature of open channel block, but increased the slow time constants of inactivation, resulting in the crossover of the currents (Friederich and Solth, 2004;Solth et al, 2005).…”