External Ba2+ block of human Kv1.5 at neutral and acidic pH: evidence for Ho+-induced constriction of the outer pore mouth at rest.
Previous studies have shown that low pHo accelerates depolarization-induced inactivation and decreases the macroscopic conductance by reducing channel availability. To test the hypothesis that outer pore constriction underlies the decreased conductance at low pHo, external Ba2+ was used to examine the accessibility of the channel pore at rest under neutral and acidic conditions. At pHo 7.4, Ba2+ block of closed channels follows a monoexponential time course and involves a low-affinity superficial site (KD congruent with 1 mM, -80 mV, 0 mM Ko(+)) and a high-affinity site (KD congruent with 4 microM) deeper in the pore. Depolarization promotes Ba2+ dissociation and an analytical model incorporating state-dependent changes of Ba2+ affinity is presented that replicates the frequency dependence of the time course and the extent of block. Open-channel block by Ba2+ is weak. At pHo 5.5, both the access to and exit from the deep site is inhibited. These results are consistent with a low-pHo-induced conformational change in the outer pore that prevents Ba2+ binding at rest or unbinding during depolarization. If a pore constriction is involved, similar to that proposed to occur during P/C-type inactivation, this would imply that closed-state inactivation in Kv1.5 occurs under acidic conditions.
Cheng
Y May
YM
Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver, British Columbia, Canada.
Fedida
David
D
Kehl
Steven J
SJ
D000595:Amino Acid Sequence; D001464:Barium / Q000378:metabolism / Q000494:pharmacology*; D020134:Catalytic Domain; D002460:Cell Line; D006801:Humans; D006859:Hydrogen / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects*; D051666:Kv1.5 Potassium Channel / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D016062:Porosity; D026902:Potassium Channel Blockers / Q000378:metabolism / Q000494:pharmacology*; D011485:Protein Binding; D012146:Rest; D013997:Time Factors
D051666:Kv1.5 Potassium Channel; D026902:Potassium Channel Blockers; D001464:Barium; D006859:Hydrogen