EFFECT OF THE FLAVOID PHLORETIN ON CA2+-ACTIVATED K+ CHANNELS IN MYELINATED NERVE-FIBERS OF XENOPUS-LAEVIS

The effects of the flavonoid phloretin on K+ channels in amphibian myelinated nerve were studied by patch clamping. The open probability of Ca2+-acrivated K+ channels was greatly increased by external phloretin (10-200 mu M) due to a shift of the membrane potential for half-maximal activation, E(50), of -63.9 mV (80 mu M phloretin). Open times were prolonged and closed limes shortened. Channel activation by phloretin developed slowly (tau(on) = 33.4 s) and its washout was even slower (tau(off,1) = 4.7 and tau(off,2) = 183.2 s). In contrast, submillimolar phloretin blocked two delayed rectifier K+ channels (I and F) whereas the gating of the ATP-sensitive and the flickering K+ channel were unaffected. Phloretin may directly affect the voltage sensor of K+ channels.