4-AMINOPYRIDINE BLOCK OF THE NONINACTIVATING CLONED K+ CHANNEL KV1.5 EXPRESSED IN XENOPUS OOCYTES

The blocking action of 4-aminopyridine (4-AP) on the cloned K+ channel Kv1.5 expressed in Xenopus oocytes was studied using the two-microele ctrode voltage-clamp method. Application of 4-AP to the bath solution reversibly suppressed the expressed current in a voltage- and concentr ation-dependent manner decreasing with membrane depolarization and wit h a half-maximal inhibitory concentration of 0.14 mM (at +40 mV). Both block and unblock occurred mainly during a depolarization when channe ls were activated. With successive depolarizations, 4-AP decreased not only the peak amplitudes of the current in successive pulses, but als o the current during a depolarization. Upon washout of 4-AP, the curre nt recovered with successive depolarizations, whereas no recovery of t he current was noted in the absence of depolarizations. The extent of block markedly increased with alkalization of the external solution an d decreased with acidification. External application of 4-amino-pyridi ne methiodide, a charged form of a quaternary 4-AP derivative, did not affect the current, but internal application markedly suppressed the current, indicating the drug gained access to the channel from the cyt oplasmic side. These data suggest that 4-AP crosses the membrane in it s uncharged form and acts from inside of the cell in its charged form, resulting in block of the channels with higher affinity to the open s tate.