We used the patch–clamp technique to study apical K+ channels in the split–opened thick ascending limb (TAL) of mouse kidneys. We observed a low–conductance K+ channel in 8 patches and an intermediate–conductance K+ channel in 11 patches from a total of 135 cell–attached patches with GΩ seals in the apical membrane of the mouse TAL. Open probability (Po) of the low–conductance K+ channel was 0.85 and was not voltage–dependent between –60 mV and 0 mV. Po of the intermediate–conductance was 0.6 at a spontaneous cell membrane potential and decreased to 0.4 by 40 mV hyperpolarization. Both the low–conductance and intermediate–conductance K+ channels were an inwardly rectifying K+ channel with inward slope conductances of 26 pS and 74 pS between –20 and 20 mV, respectively. The 74–pS K+ channel was inhibited by 1 mM Ba2+ and 10 mM tetraethylammonium when they were applied to the bath facing the cytosolic surface of inside–out patches. Furthermore, addition of 1 mM Mg–ATP reversibly reduced the activity of the 74–pS K+ channel by 90% within 1 min in inside–out patches. A decrease in bath pH from 7.4 to 6.5 completely blocked the 74–pS K+ channel in inside–out patches. We conclude that two types of K+ channels are present in the apical membrane of the mouse TAL and that the biophysical properties of the apical 74–pS K+ channel are identical to those in the rat TAL.

Keywords:
ATP–sensitive K+ channel, Inwardly rectifying K+ channel, ROMK, Patch–clamp
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2000
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