Hypoxia inhibits human recombinant large conductance, Ca2+-activated K+ (maxi-K) channels by a mechanism which is membrane delimited and Ca2+ 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 α- and β-subunits of human brain maxi-K channels. At +50 mV, and in the presence of 300 n m , 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 (30 and 100 n m ), hypoxic inhibition did not reach significance. In contrast, 300 n m and 1 μ m both sustained significant hypoxic suppression of activity over the entire activating voltage range. At these two levels, hypoxia evoked a positive shift in the activating voltage (by ∼10 mV at 300 n m and ∼25 mV at 1 μ m ). At saturating [Ca 2+ ] i (100 μ m ), hypoxic inhibition was absent. Distinguishing between hypoxia-evoked changes in voltage- and/or -sensitivity was achieved by evoking maximal channel activity using high depolarising potentials (up to +200 mV) in the presence of 300 n m or 100 μ m or in its virtual absence (> 1 n m ). Under these experimental conditions, hypoxia caused significant channel inhibition only in the presence of 300 n m . 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 sensitive.