Interaction of Ba2+ with the pores of the cloned inward rectifier K+ channels Kir2.1 expressed in Xenopus oocytes

Interactions of Ba2+ with K+ and molecules contributing to inward rectification were studied in the cloned inward rectifier K+ channels, Kir2.1. Extracellular Ba2+ blocked Kir2.1 channels with first-order kinetics in a V-m-dependent manner. At V-m more negative than -120 mV, the K-d-V-m relationship became less steep and the dissociation rate constants were larger, suggesting Ba2+ dissociation into the extracellular space. Both depolarization and increasing [K+](i) accelerated the recovery from extracellular Ba2+ blockade. Intracellular KC appears to relieve Ba2+ blockade by competitively slowing the Ba2+ entrance rate, instead of increasing its exit rate by knocking off action. Intracellular spermine (100 mu M) reduced, whereas 1 mM [Mg2+](i) only slightly reduced, the ability of intracellular K+ to repulse Ba2+ from the channel pore. Intracellular Ba2+ also blocked outward I-Kir2.1 in a voltage-dependent fashion. At V-m greater than or equal to +40 mV, where intrinsic inactivation is prominent, intracellular Ba2+ accelerated the inactivation rate of the outward I-Kir2.1 in a V-m-independent manner, suggesting interaction of Ba2+ with the intrinsic gate of Kir2.1 channels.