Inward rectification of the IRK1 channel expressed in Xenopus oocytes: effects of intracellular pH reveal an intrinsic gating mechanism.

Abstract

1. The effects of intracellular pH (pH1) were investigated in inside-out giant patches from Xenopus oocytes expressing the inward rectifier K+ channel IRKI. 2. After excising patches into Mg2+-and polyamine-free solution, a residual time-dependent inactivation of outward current during depolarizing voltage-clamp pulses persisted, reaching an apparent steady-state by 5 min. Raising pH, from 7-2 to 9 0 increased the inactivation rate of the outward current.3. In the presence of intracellular Mg2+ or polyamines, however, pHi 9 0 either decreased or did not change the inactivation rate of outward current. 4. These results suggest that the inactivation of outward current remaining after > 5 min in Mg2+_ and polyamine-free solution is not due to slow washout of these substances, but represents a third and probably intrinsic gating mechanism contributing to the inward rectifying property of IRK1. 6. In the absence of intracellular Mg2+ and polyamines, reduced pHi blocked inward and outward current through IRKI channels in a voltage-independent manner without appreciably altering the kinetics. Half-maximal block occurred at pH 6-2-6-4 (Hill coefficient, 1 6). Block of IRK1 by intracellular protons may contribute to membrane depolarization in ischaemic tissue.

Two-sided action of protons on an inward rectifier K + channel (IRK1)

“…Since this paper was submitted a paper has appeared reporting intracellular pH effects on IRK1 channels in inside-out giant patches from Xenopus oocytes [17].…”

K+ Binding Sites and Interactions between Permeating K+ Ions at the External Pore Mouth of an Inward Rectifier K+ Channel (Kir2.1)

“…These findings strongly support that the side chain of histidine at position 148 can be protonated by external H ϩ , and 148ᮍ seems to enhance K ϩ ion permeation through the Kir2.1 channel pore. 6.0 (where the channel is mostly in the protonated and high conductance state) or at pH o 9.0 (where the channel is mostly in the deprotonated and low conductance state) yet is most prominent at pH o 7.4, the pK a of the histidine residue at position 148 in the R148H mutant (Fig. 4) ] ranging from 10 to 1000 mM in inside-out patches exposed to internal Mg 2ϩ -and polyamine-free solutions. ]…”

“…Another important "asymmetrical" feature of the inward rectifier K ϩ channels is that external but not internal K ϩ seems to interact with and thereby activate the channel (1). The mechanisms underlying inward rectification have been ascribed to voltage (V m )-dependent blockade by internal cations such as Mg 2ϩ (2,3) and polyamines (4,5) and to a pH i -dependent intrinsic gate (6). On the other hand, the ion permeating process itself, namely interactions between K ϩ ions and between the permeating K ϩ ion and the inward rectifier K ϩ channel, is less characterized.…”

Mechanosensitivity of the Cardiac Muscarinic Potassium Channel

“…For the experiments measuring reversal potentials, KCl was replaced isotonically by NaCl. Giant cell-attached patches were formed on de-vitellinized oocytes (16,17), and currents were recorded under voltage clamp conditions as described previously (17). The bath and pipette solutions (room temperature) contained 98 mM KCl, 2 mM KOH, 1.8 mM CaCl 2 , 1.0 mM MgCl 2 , and 5.0 mM HEPES, pH 7.2.…”