The action of capsaicin on type I delayed rectifier K⁺ currents in rabbit Schwann cells

Capsaicin, a lipophilic alkaloid, blocked type I K⁺ currents in rabbit cultured Schwann cells when applied by superfusion. The concentration—response relation at equilibrium was well described by a rectangular hyperbola, with a K_D of 8.7 μM. The kinetics of block resembled an ‘inactivation’, the rate of blockade increasing with increasing concentrations of capsaicin (1—100 μm). Unlike internal tetraethylammonium (TEA) ions (5-10 mm), which preferentially reduced outward current in symmetrical high [K⁺], capsaicin reduced both inward and outward type I current by the same proportion. The block achieved by capsaicin during a voltage-clamp step that activated the current was relieved by subsequent hyperpolarization, and the rate of relief from block at —70 mV and —100 mV could be reasonably accounted for on the assumption that capsaicin had to unbind to allow the channels to close.