Effect of extracellular cations on the inward rectifying K+ channels Kir2.1 and Kir3.1/Kir3.4.
The effects of Ba2+, Mg2+, Ca2+ and Na+ as blocking ions were investigated in 90 and 10 mM extracellular K+ solutions on the cloned inward rectifying K+ channel Kir2.1 expressed in Xenopus oocytes. Some data were also obtained using another inward rectifying K+ channel Kir3.1/Kir3.4. The addition of Ba2+ caused a concentration-, voltage- and time-dependent block of both channels. Decreasing the extracellular K+ concentration augmented the block. The data suggest that Ba2+ blocks the channels by binding to a site within the channel pore and that the electrical binding distance, delta, of the site is significantly different for Kir2.1 and Kir3. 1/Kir3.4 (0.38 and 0.22, respectively). Mg2+ and Ca2+ caused an instantaneous concentration- and voltage-dependent block of both channels. With Kir2.1, decreasing the K+ concentration augmented the block. The voltage dependence of the block was less than that of Ba2+ ([delta], 0.1), indicating a more superficial binding site for these ions within the channel pore. The affinity of the channels for Mg2+ and Ca2+ was 1000-fold lower than that for Ba2+. Addition of Na+ resulted in a concentration-, voltage- and time-dependent block of Kir2.1, similar to that observed with Ba2+. The competition between the blocking cations (for Kir2.1: Ba2+, Mg2+, Ca2+; for Kir3. 1/Kir3.4: Ba2+) and extracellular K+ suggests that the binding sites for the blocking cations may be sites to which K+ binds as part of the normal passage of K+ through the channels. It is possible that under normal physiological conditions naturally occurring extracellular cations may partly block the two inward rectifying K+ channels.
Owen
J M
JM
Department of Physiology, University of Leeds, Leeds LS2 9JT, UK.
Quinn
C C
CC
Leach
R
R
Findlay
J B
JB
Boyett
M R
MR
D000818:Animals; D001464:Barium / Q000494:pharmacology; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology; D005260:Female; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009865:Oocytes / Q000378:metabolism; D015221:Potassium Channels / Q000235:genetics / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology; D014982:Xenopus laevis
D002412:Cations; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D011994:Recombinant Proteins; D001464:Barium; D012964:Sodium; D008274:Magnesium; D002118:Calcium