Incomplete dissociation of glibenclamide from wild‐type and mutant pancreatic KATP channels limits their recovery from inhibition

Background and purpose:  The antidiabetic sulphonylurea, glibenclamide, acts by inhibiting the pancreatic ATP-sensitive K+ (KATP) channel, a tetradimeric complex of KIR6.2 and sulphonylurea receptor 1 (KIR6.2/SUR1)4. At room temperature, recovery of channel activity following washout of glibenclamide is very slow and cannot be measured. This study investigates the relation between the recovery of channel activity from glibenclamide inhibition and the dissociation rate of [3H]-glibenclamide from the channel at 37°C. Experimental approach:  KIR6.2, KIR6.2ΔN5 or KIR6.2ΔN10 (the latter lacking amino-terminal residues 2–5 or 2–10 respectively) were coexpressed with SUR1 in HEK cells. Dissociation of [3H]-glibenclamide from the channel and recovery of channel activity from glibenclamide inhibition were determined at 37°C. Key results:  The dissociation kinetics of [3H]-glibenclamide from the wild-type channel followed an exponential decay with a dissociation half-time, t1/2(D) = 14 min; however, only limited and slow recovery of channel activity was observed. t1/2(D) for KIR6.2ΔN5/SUR1 channels was 5.3 min and recovery of channel activity exhibited a sluggish sigmoidal time course with a half-time, t1/2(R) = 12 min. t1/2(D) for the ΔN10 channel was 2.3 min; recovery kinetics were again sigmoidal with t1/2(R) ∼4 min. Conclusions and implications:  The dissociation of glibenclamide from the truncated channels is the rate-limiting step of channel recovery. The sigmoidal recovery kinetics are in quantitative agreement with a model where glibenclamide must dissociate from all four (or at least three) sites before the channel reopens. It is argued that these conclusions hold also for the wild-type (pancreatic) KATP channel.