Probing the interaction between inactivation gating and D-sotalol block ofHERG

Potassium channels encoded by HERG underlie I-Kr, a sensitive target for mo st class III antiarrhythmic drugs, including methanesulfonanilides such as D-sotalol. Recently it was shown that these drugs are trapped in the channe l as it closes during hyperpolarization. At the same time, HERG channels ra pidly open and inactivate when depolarized, and methanesulfonanilide block is known to develop in a use-dependent manner, suggesting a potential role for inactivation in drug binding. However, the role of HERG inactivation in class III drug action is uncertain: pore mutations that remove inactivatio n reduce block, yet many of these mutations also modify the channel permeat ion properties and could alter drug affinity through gating-independent mec hanisms. In the present study, we identify a definitive role for inactivati on gating in D-sotalol block of HERG, using interventions complementary to mutagenesis. These interventions (addition of extracellular Cd2+, removal o f extracellular Na+) modify the voltage dependence of inactivation but not activation. In normal extracellular solutions, block of HERG current by 300 mu mol/L D-sotalol reached 80% after a 10-minute period of repetitive depo larization to +20 mV. Maneuvers that impeded steady-state inactivation also reduced D-sotalol block of HERG: 100 mu mol/L Cd2+ reduced steady-state bl ock to 55% at +20 mV (P<0.05); removing extracellular Na+ reduced block to 44% (P<0.05). An inactivation-disabling mutation (G628C-S631C) reduced D-so talol block to only 11% (P<0.05 versus wild type). However, increasing the rate of channel inactivation by depolarizing to +60 mV reduced D-sotalol bl ock to 49% (P<0.05 versus +20 mV), suggesting that the drug does not primar ily bind to the inactivated state. Coexpression of MiRP1 with HERG had no e ffect on inactivation gating and did not modify D-sotalol block. We postula te that D-sotalol accesses its receptor in the open pore, and the drug-rece ptor interaction is then stabilized by inactivation. Whereas deactivation t raps the bound methanesulfonanilide during hyperpolarization, we propose th at HERG inactivation stabilizes the drug-receptor interaction during membra ne depolarization.