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Effect of extracellular cations on the inward rectifying K+ channels Kir2.1 and Kir3.1/Kir3.4

Published online by Cambridge University Press:  03 January 2001

J. M. OWEN
Affiliation:
Department of Physiology, University of Leeds, Leeds LS2 9JT, UK
C. C. QUINN
Affiliation:
Department of Physiology, University of Leeds, Leeds LS2 9JT, UK
R. LEACH
Affiliation:
Department of Biochemistry & Molecular Biology, University of Leeds, Leeds LS2 9JT, UK
J. B. C. FINDLAY
Affiliation:
Department of Biochemistry & Molecular Biology, University of Leeds, Leeds LS2 9JT, UK
M. R. BOYETT
Affiliation:
Department of Physiology, University of Leeds, Leeds LS2 9JT, UK
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Abstract

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, δ, of the site is significantly different for Kir2.1 and Kir3.1/Kir3.4 ([eqv] 0·38 and [eqv] 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+ (δ, [eqv] 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 [eqv] 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.

Type
Research Article
Copyright
The Physiological Society 1999

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