The extracellular K+ concentration dependence of outward currents through Kir2.1 channels is regulated by extracellular Na+ and Ca2+.
It has been known for more than three decades that outward Kir currents (I(K1)) increase with increasing extracellular K(+) concentration ([K(+)](o)). Although this increase in I(K1) can have significant impacts under pathophysiological cardiac conditions, where [K(+)](o) can be as high as 18 mm and thus predispose the heart to re-entrant ventricular arrhythmias, the underlying mechanism has remained unclear. Here, we show that the steep [K(+)](o) dependence of Kir2.1-mediated outward I(K1) was due to [K(+)](o)-dependent inhibition of outward I(K1) by extracellular Na(+) and Ca(2+). This could be accounted for by Na(+)/Ca(2+) inhibition of I(K1) through screening of local negative surface charges. Consistent with this, extracellular Na(+) and Ca(2+) reduced the outward single-channel current and did not increase open-state noise or decrease the mean open time. In addition, neutralizing negative surface charges with a carboxylate esterifying agent inhibited outward I(K1) in a similar [K(+)](o)-dependent manner as Na(+)/Ca(2+). Site-directed mutagenesis studies identified Asp(114) and Glu(153) as the source of surface charges. Reducing K(+) activation and surface electrostatic effects in an R148Y mutant mimicked the action of extracellular Na(+) and Ca(2+), suggesting that in addition to exerting a surface electrostatic effect, Na(+) and Ca(2+) might inhibit outward I(K1) by inhibiting K(+) activation. This study identified interactions of K(+) with Na(+) and Ca(2+) that are important for the [K(+)](o) dependence of Kir2.1-mediated outward I(K1).
Chang
Hsueh-Kai
HK
Institute of Biomedical Sciences, Academia Sinica, Taipei 11529, Taiwan.
Lee
Jay-Ron
JR
Liu
Tai-An
TA
Suen
Ching-Shu
CS
Arreola
Jorge
J
Shieh
Ru-Chi
RC
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004553:Electric Conductivity; D005110:Extracellular Space / Q000378:metabolism*; D042541:Intracellular Space / Q000378:metabolism; D008958:Models, Molecular; D011188:Potassium / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D012964:Sodium / Q000378:metabolism*; D055672:Static Electricity
C489461:Kir2.1 channel; D024661:Potassium Channels, Inwardly Rectifying; D012964:Sodium; D011188:Potassium; D002118:Calcium