[K+] dependence of polyamine-induced rectification in inward rectifier potassium channels (IRK1, Kir2.1)

The effects of permeant (K+) ions on polyamine (PA)-induced rectification of cloned strong inwardly rectifying channels (IRK1, Kir2.1) expressed in Xenopus oocytes were examined using patch-clamp techniques. The kinetics of PA-induced rectification depend strongly on external, but not internal, K+ concentration. Increasing external [K+] speeds up ''activation'' kinetics and. shifts rectification to more positive membrane potentials. The shift of rectification is directly proportional to the shift in the K+ reversal potential (E(K)) with slope factors +0.62, +0.81, and +0.91 for 1 mM putrescine (Put), 100 mu M spermidine and 20 mu M spermine (Spm), respectively. The time constant of current activation, resulting from unblock of Spm, also shifts directly in proportion to E(K) with slope factor +1.1. Increasing internal [K+] slows down activation kinetics and has a much weaker relieving effect on block by PA: Spm-induced rectification and time constant of activation (Spm unblock) shift directly in proportion to the corresponding change in E(K) with slope factors -0.15 and +0.31, respectively, for 20 mu M Spm. The speed up of activation kinetics caused by increase of external [K+] cannot be reversed by equal increase of internal [K+]. The data are consistent with the hypothesis that the conduction pathway of strong inward rectifiers is a long and narrow pore dth multiple binding sites for PA and K+.