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1Department of Physiology and 2Cardiovascular Research Center, Medical College of Wisconsin, and 3Clement Zablocki Veterans Affairs Medical Center, Milwaukee, Wisconsin
Submitted 26 December 2006 ; accepted in final form 28 September 2007
The mechanism of sensing hypoxia and hypoxia-induced activation of cerebral arterial Ca2+-activated K+ (KCa) channel currents and vasodilation is not known. We investigated the roles of the cytochrome P-450 4A (CYP 4A) 
-hydroxylase metabolite of arachidonic acid, 20-hydroxyeicosatetraenoic acid (20-HETE), and generation of superoxide in the hypoxia-evoked activation of the KCa channel current in rat cerebral arterial muscle cells (CAMCs) and cerebral vasodilation. Patch-clamp analysis of K+ channel current identified a voltage- and Ca2+-dependent 238 ± 21-pS unitary K+ currents that are inhibitable by tetraethylammonium (TEA, 1 mM) or iberiotoxin (100 nM). Hypoxia (<2% O2) reversibly enhanced the open-state probability (NPo) of the 238-pS unitary KCa current in cell-attached patches. This effect of hypoxia was not observed on unitary KCa currents recorded from either excised inside-out or outside-out membrane patches. Inhibition of CYP 4A -hydroxylase activity increased the NPo of KCa single-channel current. Hypoxia reduced the basal endogenous level of 20-HETE by 47 ± 3% as well as catalytic formation of 20-HETE in cerebral arterial muscle homogenates as determined by liquid chromatography-mass spectrometry analysis. The concentration of authentic 20-HETE was reduced when incubated with the superoxide donor KO2. Exogenous 20-HETE (100 nM) attenuated the hypoxia-induced activation of the KCa current in CAMCs. Hypoxia did not augment the increase in NPo of KCa channel current induced by suicide inhibition of endogenous CYP 4A -hydroxylase activity with 17-octadecynoic acid. In pressure (80 mmHg)-constricted cerebral arterial segments, hypoxia induced dilation that was partly attenuated by 20-HETE or by the KCa channel blocker TEA. Exposure to hypoxia caused the generation of intracellular superoxide as evidenced by intense staining of arterial muscle with the fluorescent probe hydroethidine, by quantitation using fluorescent HPLC analysis, and by attenuation of the hypoxia-induced activation of the KCa channel current by superoxide dismutation. These results suggest that the exposure of CAMCs to hypoxia results in the generation of superoxide and reduction in endogenous level of 20-HETE that may account for the hypoxia-induced activation of arterial KCa channel currents and cerebral vasodilation.
cytochrome P-450 4A -hydroxylase; superoxide; vasodilation; patch-clamp recording
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