Permeation and gating properties of a cloned renal K+ channel

Abstract

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.

Secretory modulation of basolateral membrane inwardly rectified K+ channel in guinea pig distal colonic crypts

“…Ion selectivity of the 19-pS gp K ir also supported high preference for K ϩ over Na ϩ or Cl Ϫ (data not shown). These current measurements indicate that the observed channels were K ϩ selective with significant Rb ϩ permeability, suggesting a divergence from other inwardly rectified K ϩ channels (8,49). (Fig.…”

Magnesium Modulates ROMK Channel–Mediated Potassium Secretion

“…With high (100 mM) extracellular (luminal) K ϩ , Mg 2ϩ blocked only outward currents at positive membrane potentials, in agreement with previous results. 8,14 We estimated the apparent K i in the absence of a membrane potential [K i (0)] to be about 5 mM with a voltage dependence (z␦) of approximately 0.6. Earlier studies reported a somewhat lower affinity [K i (0) ϳ 13 mM] and higher voltage dependence (z␦ ϳ 1).…”

“…The results are similar to those reported previously. 8,14 Currents with lower (0.2 mM) and higher (5 mM) Mg 2ϩ are also plotted in Figure 1C. Thus, under conditions of high K ϩ in the lumen and negative membrane potentials, physiologic levels of intracellular Mg 2ϩ (assumed to be Ͻ1 mM, see Discussion) will have little effect on currents through ROMK channels.…”

Aquaporin 4 as a NH3 Channel

“…At physiological pH, its pK a of 9.25 dictates that ϳ1.5% of the ammonia is found as NH 3 , whereas the rest exists as NH 4 ϩ , the latter of which absolutely requires membrane transporters or channels to cross the plasma membrane. Although dedicated NH 4 ϩ transport proteins have been identified (6), NH 4 ϩ is, due to its resemblance to K ϩ , in addition transported by a range of K ϩ transporters and channels, such as the Na ϩ /K ϩ -ATPase, the Na ϩ /K ϩ /2Cl Ϫ cotransporter, the K ϩ /Cl Ϫ cotransporter, and inwardly rectifying, voltage-, and Ca 2ϩ -activated K ϩ channels (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). The permeability of NH 4 ϩ through K ϩ channels usually amounts to ϳ10 -20% of the permeability of K ϩ (7).…”