title: Use-dependent 'agonist' effect of azimilide on the HERG channel.
abstract: Azimilide (AZ) is a class III antiarrhythmic drug that has voltage-dependent dual effects on the HERG channel: 1) increasing current amplitude at low-voltage depolarization (agonist effect), and 2) suppressing current at more depolarized voltages (antagonist effect). We examined the mechanism for the agonist effect of AZ on HERG expressed in Xenopus oocytes. The agonist effect resulted from an AZ-induced 'prepulse potentiation: a strong depolarization prepulse increased the rate and degree of channel activation induced by subsequent depolarization to -50 or -40 mV. The potentiated state decayed slowly in an exponential fashion (time constant, 60-80 s). Degrees of potentiation were proportional to degrees of channel activation during prepulses; hence, the agonist effect of AZ was use dependent. AZ exerted its agonist effect from outside the cell membrane, and the effect did not depend on intracellular G-protein or protein kinase activity. Mutations made in the outer mouth or an extracellular loop connecting the S5 and P regions of HERG, which could hinder or modify conformational changes in the pore region during membrane depolarization, reduced or abolished AZ-induced prepulse potentiation. Importantly, these same mutations also increased the rate and degree of channel activation in the negative voltage range, and the degree of change in the activation properties was inversely correlated with the degree of AZ-induced prepulse potentiation. We propose that conformational changes in the outer mouth and neighboring extracellular domain of HERG during membrane depolarization can affect the process of channel activation. In the presence of AZ, channel activation allowed drug modification of these conformational changes, which subsequently facilitated HERG activation by low-voltage depolarization.
authors.affiliation: Department of Pharmacology, Columbia University, New York, New York, USA.
authors.forename: M
authors.initials: M
authors.lastname: Jiang
authors.forename: W
authors.initials: W
authors.lastname: Dun
authors.forename: J S
authors.initials: JS
authors.lastname: Fan
authors.forename: G N
authors.initials: GN
authors.lastname: Tseng
mesh_terms: D000818:Animals; D027682:Cation Transport Proteins; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004252:DNA Mutational Analysis; D004268:DNA-Binding Proteins; D000072237:ERG1 Potassium Channel; D004594:Electrophysiology; D051638:Ether-A-Go-Go Potassium Channels; D019204:GTP-Binding Proteins / Q000378:metabolism; D006801:Humans; D006827:Hydantoins; D007093:Imidazoles / Q000494:pharmacology*; D048289:Imidazolidines; D015640:Ion Channel Gating / Q000187:drug effects / Q000235:genetics; D016297:Mutagenesis, Site-Directed / Q000187:drug effects; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010879:Piperazines / Q000494:pharmacology*; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000819:agonists* / Q000235:genetics; D024642:Potassium Channels, Voltage-Gated; D011487:Protein Conformation; D011494:Protein Kinases / Q000378:metabolism; D015534:Trans-Activators; D014158:Transcription, Genetic; D000071230:Transcriptional Regulator ERG; D014982:Xenopus laevis
chemical_list: D027682:Cation Transport Proteins; D004268:DNA-Binding Proteins; C072312:ERG protein, human; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; D006827:Hydantoins; D007093:Imidazoles; D048289:Imidazolidines; C000606913:KCNH2 protein, human; C092509:KCNH6 protein, human; D010879:Piperazines; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D015534:Trans-Activators; D000071230:Transcriptional Regulator ERG; C086123:azimilide; D011494:Protein Kinases; D019204:GTP-Binding Proteins