Effects of extracellular cations and mutations in the pore region on the inward rectifier K+ channel IRK1.

It is known that the rectification property of the inward rectifier K+ channel, IRK1, is caused by a block of outward current by cytoplasmic Mg2+ and polyamines, and that the voltage dependence of rectification shifts according to the equilibrium potential of K+. Here it is shown that extracellular K+ (K+ o) but not intracellular K+ (K+ i) affects the activation kinetics of IRK1. A mutant in which the conserved positively charged residue arginine was replaced by a tyrosine (R148Y) exhibited slower activation and a negative shift of the conductance-voltage relationship. In addition, the conductance did not saturate at negative potentials as was observed for the wild type. When using T1+o instead of K+ o as permeant ion, the differences between the wild type and the mutant were qualitatively similar but less prominent. These results suggest that extracellular cations (e.g., K+ o or T1+ o) play a role in the activation of IRK1. Since the effects of K+ o or T1+ o were altered in the mutant, the site R148 presumably is involved in channel regulation by extracellular cations.