[Blocking effects of extracellular Ba(2+) on the inward rectifier potassium channel].
Two-microelectrode voltage clamp (TEV) method was used to study the blocking effects of extracellular Ba(2+) on the inward rectifier potassium channel (IRK1) expressed in Xenopus oocytes. Blockage of Ba(2+) on IRK1 (1 ms after voltage applied) is Ba(2+) concentration (0,1,3,10 or 100 micromol/L) dependent with 10 or 90 mmol/L potassium and also voltage-dependent. Ba(2+) almost has no effect on the open/close of IRK1. IRK1 is not permeable to Ba(2+). Three exponential fitting analysis indicates that Ba(2+) and K(+) compete the same binding site in IRK1 when external Ba(2+) concentration is lower (1 or 3 micromol/L). The time constant of IRK1 does not increase, but the concentration dependency of the weights of the fittings increases with the increase of external Ba(2+) concentration. As a result, the inactivation becomes faster and faster as the external Ba(2+) concentration increases. Moreover, since the time constant of the channel decreases and the weights of the fittings concentration dependently increase with the increase of external Ba(2+) concentration (10 or 100 micromol/L), the inactivation becomes faster and faster. It is demonstrated that Ba(2+) can contact with deeper binding sites in IRK1 as external Ba(2+) concentration increases. It is suggested that two different mechanisms may underlie the external Ba(2+) blocking effect. External Mn(2+) or Mg(2+) can compete with external Ba(2+) at the IRK1 binding site at an external Ba(2+) concentration of 30 mol/L and K(+) concentration of 90 mmol/L. Inactivation becomes slower and slower and Ba(2+) is repelled from the IRK1 binding site when Mg(2+) or Mn(2+) con-centration is further increased. Mg(2+), but not Mn(2+), can contact with deeper binding sites of IRK1 to block the channel, suggesting that multiple-ion blockage may exist in IRK1.
Xie
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A
Department of Physiology, Basic Science Institute, The Fourth Military Medical University, Xi an 710032, China. xiean@fmmu.edu.cn
Zang
Y M
YM
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D054426:Chemokine CXCL2; D005110:Extracellular Space / Q000378:metabolism*; D008839:Microelectrodes; D015846:Monokines / Q000037:antagonists & inhibitors*; D009865:Oocytes / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D014981:Xenopus
D054426:Chemokine CXCL2; D015846:Monokines; D015221:Potassium Channels; D001464:Barium