Interaction of Permeant and Blocking Ions in Cloned Inward-Rectifier K+ Channels

Abstract Blocking cloned inward-rectifier potassium (K ir ) channels from the cytoplasmic side was analyzed with a rapid application system exchanging the intracellular solution on giant inside-out patches from Xenopus oocytes in ir 1.1 (ROMK1) channels blocked by ammonium derivatives and in K ir 4.1 (BIR10) channels blocked by spermine. The blocking reaction in both systems showed first-order kinetics and allowed separate determination of on- and off-rates. The off-rates of block were strongly dependent on the concentration of internal and external bulk ions, but almost independent of the ion species at the cytoplasmic side of the membrane. With K + as the only cation on both sides of the membrane, off-rates exhibited strong coupling to the K + reversal potential ( E K ) and increased and decreased with reduction in intra and extracellular K + concentration, respectively. The on-rates showed significant dependence on concentration and species of internal bulk ions. This control of rate-constants by interaction of permeant and impermeant internal and external ions governs the steady-state current-voltage relation ( I-V ) of K ir channels and determines their physiological function under various conditions.