Contribution of epoxyeicosatrienoic acids to the cerebral blood flow response to hypoxemia

Liu X, Gebremedhin D, Harder DR, Koehler RC. Contribution of epoxyeicosatrienoic acids to the cerebral blood flow response to hypoxemia. J Appl Physiol 119: 1202-1209, 2015. First published March 19, 2015; doi:10.1152/japplphysiol.01043.2014.-Adenosine A_2A receptors and ATP-activated K⁺ (K_ATP) channels contribute to part of the cerebral vasodilatory response to systemic hypoxia, but other mediators are likely involved. Epoxyeicosatrienoic acids (EETs) are cerebral vasodilators and are released from astrocytes exposed to hypoxia. Moreover, stimulation of metabotropic glutamate receptors (mGluR) produces vasodilation by an EET-dependent mechanism. Here, we tested the hypothesis that EET signaling and mGluR activation contribute to hypoxic vasodilation. Laser-Doppler flow was measured over cerebral cortex of anesthetized rats subjected to stepwise reductions in arterial oxygen saturation to 50-70%. Hypoxic reactivity was calculated as the slope of the change in laser-Doppler flow vs. the reciprocal of arterial oxygen content. Hypoxic reactivity significantly decreased from 9.2±1.9 (±95% confidence interval) in controls with vehicle treatment to 2.6±1.4 with the EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid, to 3.0±1.5 with the EET synthesis inhibitor MS-PPOH, to 1.9±2.3 with the combined mGluR subtype 1 and 5 antagonists 2-methyl-6-(phenylethynyl)pyridine and LY367385, to 5.6±1.2 with the K_ATP channel inhibitor glibenclamide, and to 5.8±2.3 with the A_2A receptor antagonist SCH58261. However, reactivity was not significantly altered by the A_2B receptor antagonist MRS1754 (6.7±1.8; P = 0.28 Dunnett's test) or by the 20-hydroxyeicosatetraenoic acid synthesis inhibitor HET0016 (7.5±2.3; P = 0.6). These data indicate that, in addition to the known contributions of A_2A receptors and KATP channels to the increase in cerebral blood flow during hypoxia, EETs and mGluRs make a major contribution, possibly by mGluR stimulation and hypoxia-induced release of EETs. In contrast, A_2B receptors do not make a major contribution, and 20-hydroxyeicosatetraenoic acid does not significantly limit hypoxic vasodilation.