Permeation and gating properties of a cloned renal K+ channel.
The renal K+ channel (ROMK2) was expressed in Xenopus oocytes, and the patch-clamp technique was used to assess its conducting and gating properties. In cell-attached patches with 110 mM K+ in the bath and pipette, the reversal potential was near zero and the inward conductance (36 pS) was larger than the outward conductance (17 pS). In excised inside-out patches the channels showed rectification in the presence of 5 mM Mg2+ on the cytoplasmic side but not in Mg(2+)-free solution. Inward currents were also observed when K+ was replaced in the pipette by Rb+, NH4+, or thallium (Tl+). The reversal potentials under these conditions yielded a selectivity sequence of Tl+ > K+ > Rb+ > NH4+. On the other hand, the slope conductances for inward current gave a selectivity sequence of K+ = NH4+ > Tl+ > Rb+. The differences in the two sequences can be explained by the presence of cation binding sites within the channel, which interact with Rb+ and Tl+ more strongly and with NH4+ less strongly than with K+. Two other ions, Ba2+ and Cs+, blocked the channel from the outside. The effect of Ba2+ (1 mM) was to reduce the open probability of the channels, whereas Cs+ (10 mM) reduced the apparent single-channel current. The effects of both blockers are enhanced by membrane hyperpolarization. The kinetics of the channel were also studied in cell-attached patches. With K+ in the pipette the distribution of open times could be described by a single exponential (tau 0 = 25 ms), whereas two exponentials (tau 1 = 1 ms, tau 2 = 30 ms) were required to describe the closed-time distribution. Hyperpolarization of the oocyte membrane decreased the open probability and tau 0, and increased tau 1, tau 2, and the number of long closures. The presence of Tl+ in the pipette significantly altered the kinetics, reducing tau 0 and eliminating the long-lived closures. These results suggest that the gating of the channel may depend on the nature of the ion in the pore.
Chepilko
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S
Department of Physiology and Biophysics, Cornell University Medical College, New York, New York 10021.
Zhou
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H
Sackin
H
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Palmer
L G
LG
D000818:Animals; D003001:Cloning, Molecular; D004553:Electric Conductivity; D004594:Electrophysiology; D005260:Female; D015640:Ion Channel Gating; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D010539:Permeability; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000378:metabolism* / Q000502:physiology; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012413:Rubidium / Q000494:pharmacology; D013793:Thallium / Q000494:pharmacology; D014982:Xenopus laevis
D026902:Potassium Channel Blockers; D015221:Potassium Channels; D000644:Quaternary Ammonium Compounds; D013793:Thallium; D012413:Rubidium