Hypoxia inhibits human recombinant large conductance, Ca(2+)-activated K(+) (maxi-K) channels by a mechanism which is membrane delimited and Ca(2+) sensitive.
Large conductance, Ca(2+)-activated K(+) (maxi-K ) channel activity was recorded in excised, inside-out patches from HEK 293 cells stably co-expressing the alpha- and beta-subunits of human brain maxi-K channels. At +50 mV, and in the presence of 300 nM Ca2+i, single channel activity was acutely and reversibly suppressed upon reducing P(O(2)) from 150 to > 40 mmHg by over 30 %. The hypoxia-evoked reduction in current was due predominantly to suppression in NP(o), although a minor component was attributable to reduced unitary conductance of 8-12 %. Hypoxia caused an approximate doubling of the time constant for activation but was without effect on deactivation. At lower levels of Ca2+i(30 and 100 nM), hypoxic inhibition did not reach significance. In contrast, 300 nM and 1 microM Ca2+i both sustained significant hypoxic suppression of activity over the entire activating voltage range. At these two Ca2+i levels, hypoxia evoked a positive shift in the activating voltage (by approximately 10 mV at 300 nM and approximately 25 mV at 1 microM). At saturating [Ca(2+)](i) (100 microM), hypoxic inhibition was absent. Distinguishing between hypoxia-evoked changes in voltage- and/or Ca2+i-sensitivity was achieved by evoking maximal channel activity using high depolarising potentials (up to +200 mV) in the presence of 300 nM or 100 microM Ca2+i or in its virtual absence (> 1 nM). Under these experimental conditions, hypoxia caused significant channel inhibition only in the presence of 300 nM Ca2+i. Thus, since regulation was observed in excised patches, maxi-K channel inhibition by hypoxia does not require soluble intracellular components and, mechanistically, is voltage independent and Ca2+i sensitive.
Lewis
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A
School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK.
Peers
C
C
Ashford
M L J
ML
Kemp
P J
PJ
D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D015687:Cell Hypoxia / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology*; D006801:Humans; D007668:Kidney; D007700:Kinetics; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D024681:Potassium Channels, Calcium-Activated / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D014162:Transfection
C494841:KCNMA1 protein, human; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D021122:Protein Subunits; D011994:Recombinant Proteins; D002118:Calcium