INWARD RECTIFICATION OF THE IRK1 K+ CHANNEL RECONSTITUTED IN LIPID BILAYERS

Inwardly rectifying potassium (K+) channels (IRK1) were incorporated i nto lipid bilayers to test the relative contributions of various mecha nisms to inward rectification. IRK1 channels were expressed in Xenopus laevis oocytes and oocyte membrane vesicles containing the channels w ere fused with lipid bilayers. The major properties of the IRK1 channe l were similar whether measured in the oocyte membrane or lipid bilaye r; the single channel conductance was 21 pS in 140 mM symmetrical [K+] and varied as a square root of external [K+]. Importantly, IRK1 chann els display voltage-dependent inward rectification in the absence of d ivalent ions or charged regulators such as spermine, indicating that t hey possess an intrinsic rectification mechanism. Although rectificati on was significantly increased by either Mg2+ or spermine added to the cytoplasmic face of the channel, their effects could not be explained by simple block of the open pore. The Hille and Schwartz (1978) model , originally proposed to explain inward rectification by singly charge d blocking particles, cannot be used to explain rectification by multi ply charged blocking particles. As an alternative, we propose that in addition to a slow gating mechanism producing long lasting open and cl osed states, there is a distinct, intrinsic fast gating process amplif ied by cytoplasmic Mg2+ and/or polyamine binding to the channel.