THE INTERACTION OF NA-GATED POTASSIUM CHANNELS - EVIDENCE FOR CATION-BINDING SITES OF DIFFERENT AFFINITY( AND K+ IN VOLTAGE)

Voltage-gated potassium (K+) channels are multi-ion pores. Recent stud ies suggest that, similar to calcium channels, competition between ion ic species for intrapore binding sites may contribute to ionic selecti vity in at least some K+ channels. Molecular studies suggest that a pu tative constricted region of the pore, which is presumably the site of selectivity, may be as short as one ionic diameter in length. Taken t ogether, these results suggest that selectivity may occur at just a si ngle binding site in the pore. We are studying a chimeric K+ channel t hat is highly selective for K+ over Na+ in physiological solutions, bu t conducts Na+ in the absence of K+. Na+ and K+ currents both display slow (C-type) inactivation, but had markedly differ ent inactivation a nd deactivation kinetics; Na+ currents inactivated more rapidly and de activated more slowly than K+ currents. Currents carried by 160 mM Na were inhibited by external K+ with an apparent IC50 < 30 mu M. K+ als o alter ed both inactivation and deactivation kinetics of Naf currents at these low concentrations. In the complementary experiment, current s carried by 3 mM K+ were inhibited by external Na+, with an apparent IC50 of similar to 100 mM. In contrast to the effects of low [K+] on N a+ current kinetics, Na+ did not affect K+ current kinetics, even at c oncentrations that inhibited K+ currents by 40-50%. These data suggest that Na+ block of K+ currents did not involve displacement of K+ from the high affinity site involved in gating kinetics. We present a mode l that describes the permeation pathway as a single high affinity, cat ion-selective binding site, flanked by low affinity, nonselective site s. This model quantitatively predicts the anomalous mole fraction beha vior observed in two different K+ channels, differential K+ and Na+ co nductance, and the concentration dependence of K+ block of Na+ current s and Na+ block of K+ currents. Based on our results, we hypothesize t hat the permeation pathway contains a single high affinity binding sit e, where selectivity and ionic modulation of gating occur.