Cytochrome P450 epoxygenase-derived 5,6-epoxyeicosatrienoic acid relaxes pulmonary arteries in normoxia but promotes sustained pulmonary vasoconstriction in hypoxia.

AIMS: The aim of the study was to investigate the role of cytochrome P450 (CYP) epoxygenase-derived epoxyeicosatrienoic acids (EETs) in sustained hypoxic pulmonary vasoconstriction (HPV). METHODS: Vasomotor responses of isolated mouse intrapulmonary arteries (IPAs) were assessed using wire myography. Key findings were verified by haemodynamic measurements in isolated perfused and ventilated mouse lungs. RESULTS: Pharmacological inhibition of EET synthesis with MS-PPOH, application of the EET antagonist 14,15-EEZE or deficiency of CYP2J isoforms suppressed sustained HPV. In contrast, knockdown of EET-degrading soluble epoxide hydrolase or its inhibition with TPPU augmented sustained HPV almost twofold. All EET regioisomers elicited relaxation in IPAs pre-contracted with thromboxane mimetic U46619. However, in the presence of KCl-induced depolarization, 5,6-EET caused biphasic contraction in IPAs and elevation of pulmonary vascular tone in isolated lungs, whereas other regioisomers had no effect. In patch-clamp experiments, hypoxia elicited depolarization in pulmonary artery smooth muscle cells (PASMCs), and 5,6-EET evoked inward whole cell currents in PASMCs depolarized to the hypoxic level, but not at their resting membrane potential. CONCLUSIONS: The EET pathway substantially contributes to sustained HPV in mouse pulmonary arteries. 5,6-EET specifically appears to be involved in HPV, as it is the only EET regioisomer able to elicit not only relaxation, but also sustained contraction in these vessels. 5,6-EET-induced pulmonary vasoconstriction is enabled by PASMC depolarization, which occurs in hypoxia. The discovery of the dual role of 5,6-EET in the regulation of pulmonary vascular tone may provide a basis for the development of novel therapeutic strategies for treatment of HPV-related diseases.

Hydrogen sulfide contributes to hypoxic inhibition of airway transepithelial sodium absorption.

In lung epithelial cells, hypoxia decreases the expression and activity of sodium-transporting molecules, thereby reducing the rate of transepithelial sodium absorption. The mechanisms underlying the sensing of hypoxia and subsequent coupling to sodium-transporting molecules remain unclear. Hydrogen sulfide (H2S) has recently been recognized as a cellular signaling molecule whose intracellular concentrations critically depend on oxygen levels. Therefore, it was questioned whether endogenously produced H2S contributes to hypoxic inhibition of sodium transport. In electrophysiological Ussing chamber experiments, hypoxia was established by decreasing oxygen concentrations in the chambers. Hypoxia concentration dependently and reversibly decreased amiloride-sensitive sodium absorption by cultured H441 monolayers and freshly dissected porcine tracheal epithelia due to inhibition of basolateral Na(+)/K(+)-ATPase. Exogenous application of H2S by the sulfur salt Na2S mimicked the effect of hypoxia and inhibited amiloride-sensitive sodium absorption by both tissues in an oxygen-dependent manner. Hypoxia increased intracellular concentrations of H2S and decreased the concentration of polysulfides. Pretreatment with the cystathionine-γ-lyase inhibitor d/l-propargylglycine (PAG) decreased hypoxic inhibition of sodium transport by H441 monolayers, whereas inhibition of cystathionine-ß-synthase (with aminooxy-acetic acid; AOAA) or 3-mercaptopyruvate sulfurtransferase (with aspartate) had no effect. Inhibition of all of these H2S-generating enzymes with a combination of AOAA, PAG, and aspartate decreased the hypoxic inhibition of sodium transport by H441 cells and pig tracheae and decreased H2S production by tracheae. These data suggest that airway epithelial cells endogenously produce H2S during hypoxia, and this contributes to hypoxic inhibition of transepithelial sodium absorption.