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1. The effects of intracellular pH (pHi) were investigated in inside-out giant patches from Xenopus oocytes expressing the inward rectifier K+ channel IRK1. 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 pHi 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. 5. The voltage dependence and kinetics of this gating mechanism were well described by a sequential two open- and one closed-state model in which the rate constants for transitions between the open states were voltage dependent, and those between the open and closed state were pH dependent. 6. In the absence of intracellular Mg2+ and polyamines, reduced pHi blocked inward and outward current through IRK1 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.
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