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.