State-dependent inhibition of BK channels by the opioid agonist loperamide

Area of Science:

• Molecular biology
• Ion channel physiology
• Pharmacology

Background:

• Large-conductance Ca2+-activated K+ (BK) channels are crucial for physiological functions.
• Dysfunction of BK channels is implicated in various human diseases.
• Loperamide (LOP) is a widely used drug whose interaction with BK channels requires mechanistic clarification.

Purpose of the Study:

• To elucidate the molecular mechanism by which loperamide (LOP) inhibits large-conductance Ca2+-activated K+ (BK) channels.
• To investigate the effects of LOP on BK channel kinetics and gating properties.
• To determine if LOP acts as a state-dependent pore blocker.

Main Methods:

• Electrophysiological recordings of BK channel currents (whole-cell and single-channel).
• Application of LOP to the cytosolic side of the membrane.
• Analysis of LOP's effects on BK channel activation, voltage dependence, and open probability.
• Use-dependent inhibition assays.

Main Results:

• LOP rapidly and reversibly inhibited BK channel activity when applied to the cytosolic side.
• LOP's inhibitory effect was voltage-dependent, consistent with blocking open channels.
• LOP decreased BK channel open probability and mean open times without altering unitary current.
• LOP demonstrated use-dependent inhibition, accumulating with repeated depolarizing steps.
• A mechanism of state-dependent pore blockade was proposed for LOP action.

Conclusions:

• Loperamide (LOP) inhibits BK channels by acting as a state-dependent pore blocker, primarily affecting channel open probability and duration.
• The observed inhibition is use-dependent and occurs from the cytosolic side of the membrane.
• Therapeutic doses of LOP may exert their effects, in part, by inhibiting K+ efflux through intestinal BK channels.