Epoxyeicosatrienoic acids mediate adenosine-induced vasodilation in rat preglomerular microvessels (PGMV) via A2A receptors

Crosstalk between adenosine receptors and CYP450-derived oxylipins in the modulation of cardiovascular, including coronary reactive hyperemic response

Adenosine is a ubiquitous endogenous nucleoside or autacoid that affects the cardiovascular system through the activation of four G-protein coupled receptors: adenosine A₁ receptor (A₁AR), adenosine A_2A receptor (A_2AAR), adenosine A_2B receptor (A_2BAR), and adenosine A₃ receptor (A₃AR). With the rapid generation of this nucleoside from cellular metabolism and the widespread distribution of its four G-protein coupled receptors in almost all organs and tissues of the body, this autacoid induces multiple physiological as well as pathological effects, not only regulating the cardiovascular system but also the central nervous system, peripheral vascular system, and immune system. Mounting evidence shows the role of CYP450-enzymes in cardiovascular physiology and pathology, and the genetic polymorphisms in CYP450s can increase susceptibility to cardiovascular diseases (CVDs). One of the most important physiological roles of CYP450-epoxygenases (CYP450-2C & CYP2J2) is the metabolism of arachidonic acid (AA) and linoleic acid (LA) into epoxyeicosatrienoic acids (EETs) and epoxyoctadecaenoic acid (EpOMEs) which generally involve in vasodilation. Like an increase in coronary reactive hyperemia (CRH), an increase in anti-inflammation, and cardioprotective effects. Moreover, the genetic polymorphisms in CYP450-epoxygenases will change the beneficial cardiovascular effects of metabolites or oxylipins into detrimental effects. The soluble epoxide hydrolase (sEH) is another crucial enzyme ubiquitously expressed in all living organisms and almost all organs and tissues. However, in contrast to CYP450-epoxygenases, sEH converts EETs into dihydroxyeicosatrienoic acid (DHETs), EpOMEs into dihydroxyoctadecaenoic acid (DiHOMEs), and others and reverses the beneficial effects of epoxy-fatty acids leading to vasoconstriction, reducing CRH, increase in pro-inflammation, increase in pro-thrombotic and become less cardioprotective. Therefore, polymorphisms in the sEH gene (Ephx2) cause the enzyme to become overactive, making it more vulnerable to CVDs, including hypertension. Besides the sEH, ω-hydroxylases (CYP450-4A11 & CYP450-4F2) derived metabolites from AA, ω terminal-hydroxyeicosatetraenoic acids (19-, 20-HETE), lipoxygenase-derived mid-chain hydroxyeicosatetraenoic acids (5-, 11-, 12-, 15-HETEs), and the cyclooxygenase-derived prostanoids (prostaglandins: PGD₂, PGF_2α; thromboxane: Txs, oxylipins) are involved in vasoconstriction, hypertension, reduction in CRH, pro-inflammation and cardiac toxicity. Interestingly, the interactions of adenosine receptors (A_2AAR, A₁AR) with CYP450-epoxygenases, ω-hydroxylases, sEH, and their derived metabolites or oxygenated polyunsaturated fatty acids (PUFAs or oxylipins) is shown in the regulation of the cardiovascular functions. In addition, much evidence demonstrates polymorphisms in CYP450-epoxygenases, ω-hydroxylases, and sEH genes (Ephx2) and adenosine receptor genes (ADORA1 & ADORA2) in the human population with the susceptibility to CVDs, including hypertension. CVDs are the number one cause of death globally, coronary artery disease (CAD) was the leading cause of death in the US in 2019, and hypertension is one of the most potent causes of CVDs. This review summarizes the articles related to the crosstalk between adenosine receptors and CYP450-derived oxylipins in vascular, including the CRH response in regular salt-diet fed and high salt-diet fed mice with the correlation of heart perfusate/plasma oxylipins. By using A_2AAR^-/-, A₁AR^-/-, eNOS^-/-, sEH^-/- or Ephx2^-/-, vascular sEH-overexpressed (Tie2-sEH Tr), vascular CYP2J2-overexpressed (Tie2-CYP2J2 Tr), and wild-type (WT) mice. This review article also summarizes the role of pro-and anti-inflammatory oxylipins in cardiovascular function/dysfunction in mice and humans. Therefore, more studies are needed better to understand the crosstalk between the adenosine receptors and eicosanoids to develop diagnostic and therapeutic tools by using plasma oxylipins profiles in CVDs, including hypertensive cases in the future.

Eicosanoid blood vessel regulation in physiological and pathological states

Arachidonic acid can be metabolized in blood vessels by three primary enzymatic pathways; cyclooxygenase (COX), lipoxygenase (LO), and cytochrome P450 (CYP). These eicosanoid metabolites can influence endothelial and vascular smooth muscle cell function. COX metabolites can cause endothelium-dependent dilation or constriction. Prostaglandin I2 (PGI2) and thromboxane (TXA2) act on their respective receptors exerting opposing actions with regard to vascular tone and platelet aggregation. LO metabolites also influence vascular tone. The 12-LO metabolite 12S-hydroxyeicosatrienoic acid (12S-HETE) is a vasoconstrictor whereas the 15-LO metabolite 11,12,15-trihydroxyeicosatrienoic acid (11,12,15-THETA) is an endothelial-dependent hyperpolarizing factor (EDHF). CYP enzymes produce two types of eicosanoid products: EDHF vasodilator epoxyeicosatrienoic acids (EETs) and the vasoconstrictor 20-HETE. The less-studied cross-metabolites generated from arachidonic acid metabolism by multiple pathways can also impact vascular function. Likewise, COX, LO, and CYP vascular eicosanoids interact with paracrine and hormonal factors such as the renin-angiotensin system and endothelin-1 (ET-1) to maintain vascular homeostasis. Imbalances in endothelial and vascular smooth muscle cell COX, LO, and CYP metabolites in metabolic and cardiovascular diseases result in vascular dysfunction. Restoring the vascular balance of eicosanoids by genetic or pharmacological means can improve vascular function in metabolic and cardiovascular diseases. Nevertheless, future research is necessary to achieve a more complete understanding of how COX, LO, CYP, and cross-metabolites regulate vascular function in physiological and pathological states.

Arachidonic Acid Metabolism and Kidney Inflammation

As a major component of cell membrane lipids, Arachidonic acid (AA), being a major component of the cell membrane lipid content, is mainly metabolized by three kinds of enzymes: cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP450) enzymes. Based on these three metabolic pathways, AA could be converted into various metabolites that trigger different inflammatory responses. In the kidney, prostaglandins (PG), thromboxane (Tx), leukotrienes (LTs) and hydroxyeicosatetraenoic acids (HETEs) are the major metabolites generated from AA. An increased level of prostaglandins (PGs), TxA₂ and leukotriene B4 (LTB₄) results in inflammatory damage to the kidney. Moreover, the LTB₄-leukotriene B4 receptor 1 (BLT1) axis participates in the acute kidney injury via mediating the recruitment of renal neutrophils. In addition, AA can regulate renal ion transport through 19-hydroxystilbenetetraenoic acid (19-HETE) and 20-HETE, both of which are produced by cytochrome P450 monooxygenase. Epoxyeicosatrienoic acids (EETs) generated by the CYP450 enzyme also plays a paramount role in the kidney damage during the inflammation process. For example, 14 and 15-EET mitigated ischemia/reperfusion-caused renal tubular epithelial cell damage. Many drug candidates that target the AA metabolism pathways are being developed to treat kidney inflammation. These observations support an extraordinary interest in a wide range of studies on drug interventions aiming to control AA metabolism and kidney inflammation.

Orally Active Epoxyeicosatrienoic Acid Analogs

Biologically active epoxyeicosatrienoic acid (EET) regioisomers are synthesized from arachidonic acid by cytochrome P450 epoxygenases of endothelial, myocardial, and renal tubular cells. EETs relax vascular smooth muscle and decrease inflammatory cell adhesion and cytokine release. Renal EETs promote sodium excretion and vasodilation to decrease hypertension. Cardiac EETs reduce infarct size after ischemia-reperfusion injury and decrease fibrosis and inflammation in heart failure. In diabetes, EETs improve insulin sensitivity, increase glucose tolerance, and reduce the renal injury. These actions of EETs emphasize their therapeutic potential. To minimize metabolic inactivation, 14,15-EET agonist analogs with stable epoxide bioisosteres and carboxyl surrogates were developed. In preclinical rat models, a subset of agonist analogs, termed EET-A, EET-B, and EET-C22, are orally active with good pharmacokinetic properties. These orally active EET agonists lower blood pressure and reduce cardiac and renal injury in spontaneous and angiotensin hypertension. Other beneficial cardiovascular actions include improved endothelial function and cardiac antiremodeling actions. In rats, EET analogs effectively combat acute and chronic kidney disease including drug- and radiation-induced kidney damage, hypertension and cardiorenal syndrome kidney damage, and metabolic syndrome and diabetes nephropathy. The compelling preclinical efficacy supports the prospect of advancing EET analogs to human clinical trials for kidney and cardiovascular diseases.

Reduced coronary reactive hyperemia in mice was reversed by the soluble epoxide hydrolase inhibitor (t-AUCB): Role of adenosine A2A receptor and plasma oxylipins

Coronary reactive hyperemia (CRH) protects the heart against ischemia. Adenosine A_2AAR-deficient (A_2AAR^-/-) mice have increased expression of soluble epoxide hydrolase (sEH); the enzyme responsible for breaking down the cardioprotective epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs). sEH-inhibition enhances CRH, increases EETs, and modulates oxylipin profiles. We investigated the changes of oxylipins and their impact on CRH in A_2AAR^-/- and wild type (WT) mice. We hypothesized that the attenuated CRH in A_2AAR^-/- mice is mediated by changes in oxylipin profiles, and that it can be reversed by either sEH- or ω-hydroxylases-inhibition. Compared to WT mice, A_2AAR^-/- mice had attenuated CRH and changed oxylipin profiles, which were consistent between plasma and heart perfusate samples, including decreased EET/DHET ratios, and increased hydroxyeicosatetraenoic acids (HETEs). Plasma oxylipns in A_2AAR^-/- mice indicated an increased proinflammatory state including increased ω-terminal HETEs, decreased epoxyoctadecaenoic/dihydroxyoctadecaenoic acids (EpOMEs/DiHOMEs) ratios, increased 9-hydroxyoctadecadienoic acid, and increased prostanoids. Inhibition of either sEH or ω-hydroxylases reversed the reduced CRH in A_2AAR^-/- mice. In WT and sEH^-/- mice, blocking A_2AAR decreased CRH. These data demonstrate that A_2AAR-deletion was associated with changes in oxylipin profiles, which may contribute to the attenuated CRH. Also, inhibition of sEH and ω-hydroxylases reversed the reduction in CRH.

The Role of Adenosine A2A Receptor, CYP450s, and PPARs in the Regulation of Vascular Tone

Adenosine is an endogenous mediator involved in a myriad of physiologic functions, including vascular tone regulation. It is also implicated in some pathologic conditions. Four distinct receptor subtypes mediate the effects of adenosine, such as its role in the regulation of the vascular tone. Vascular tone regulation is a complex and continuous process which involves many mechanisms and mediators that are not fully disclosed. The vascular endothelium plays a pivotal role in regulating blood flow to and from all body organs. Also, the vascular endothelium is not merely a physical barrier; it is a complex tissue with numerous functions. Among adenosine receptors, A2A receptor subtype (A2AAR) stands out as the primary receptor responsible for the vasodilatory effects of adenosine. This review focuses on important effectors of the vascular endothelium, including adenosine, adenosine receptors, EETs (epoxyeicosatrienoic acids), HETEs (hydroxyeicosatetraenoic acids), PPARs (peroxisome proliferator-activated receptors), and KATP channels. Given the impact of vascular tone regulation in cardiovascular physiology and pathophysiology, better understanding of the mechanisms affecting it could have a significant potential for developing therapeutic agents for cardiovascular diseases.

Effects of adenosine stimulation on the mRNA expression of CLCNKB in the basolateral medullary thick ascending limb of the rat kidney

Adenosine is a molecule produced by several organs within the body, including the kidneys, where it acts as an autoregulatory factor. It mediates ion transport in several nephron segments, including the proximal tubule and the thick ascending limb (TAL). Ion transport is dictated in part by anionic chloride channels, which regulate crucial kidney functions, including the reabsorption of Na+ and Cl‑, urine concentration, and establishing and maintaining the corticomedullary osmotic gradient. The present study investigated the effects of adenosine on the mRNA expression of chloride voltage‑gated channel Kb (CLCNKB), a candidate gene involved in hypertension, which encodes for the ClC‑Kb channel. Medullary thick ascending limb (mTAL) tubules were isolated from the rat kidney, and primary cultures of mTAL cells from the mTAL tubules were established. The cells were treated with adenosine and the mRNA expression of CLCNKB was detected by reverse transcription‑quantitative polymerase chain reaction. The cells were also treated with pathways inhibitors (H8 and AACOCF3), and the protein expression of cyclic adenosine 3',5'‑monophosphate (cAMP)‑protein kinase A (PKA) and phospholipase A2 (PLA2) by were analyzed by western blotting. The findings indicated that adenosine increased the mRNA expression of CLCNKB in primary cultures of medullary TAL cells, and this stimulatory effect was regulated by the cAMP‑PKA and PLA2‑arachidonic acid (AA) pathways. The present study showed that adenosine affected the mRNA expression of CLCNKB, initially through the cAMP‑PKA pathway and then the PLA2‑AA pathway.

Epoxyeicosatrienoic Acids and 20-Hydroxyeicosatetraenoic Acid on Endothelial and Vascular Function

Endothelial and vascular smooth cells generate cytochrome P450 (CYP) arachidonic acid metabolites that can impact endothelial cell function and vascular homeostasis. The objective of this review is to focus on the physiology and pharmacology of endothelial CYP metabolites. The CYP pathway produces two types of eicosanoid products: epoxyeicosatrienoic acids (EETs), formed by CYP epoxygenases, and hydroxyeicosatetraenoic acids (HETEs), formed by CYP hydroxylases. Advances in CYP enzymes, EETs, and 20-HETE by pharmacological and genetic means have led to a more complete understanding of how these eicosanoids impact on endothelial cell function. Endothelial-derived EETs were initially described as endothelial-derived hyperpolarizing factors. It is now well recognized that EETs importantly contribute to numerous endothelial cell functions. On the other hand, 20-HETE is the predominant CYP hydroxylase synthesized by vascular smooth muscle cells. Like EETs, 20-HETE acts on endothelial cells and impacts importantly on endothelial and vascular function. An important aspect for EETs and 20-HETE endothelial actions is their interactions with hormonal and paracrine factors. These include interactions with the renin-angiotensin system, adrenergic system, puringeric system, and endothelin. Alterations in CYP enzymes, 20-HETE, or EETs contribute to endothelial dysfunction and cardiovascular diseases such as ischemic injury, hypertension, and atherosclerosis. Recent advances have led to the development of potential therapeutics that target CYP enzymes, 20-HETE, or EETs. Thus, future investigation is required to obtain a more complete understanding of how CYP enzymes, 20-HETE, and EETs regulate endothelial cell function.

Cytochrome P450 and Lipoxygenase Metabolites on Renal Function

Arachidonic acid metabolites have a myriad of biological actions including effects on the kidney to alter renal hemodynamics and tubular transport processes. Cyclooxygenase metabolites are products of an arachidonic acid enzymatic pathway that has been extensively studied in regards to renal function. Two lesser-known enzymatic pathways of arachidonic acid metabolism are the lipoxygenase (LO) and cytochrome P450 (CYP) pathways. The importance of LO and CYP metabolites to renal hemodynamics and tubular transport processes is now being recognized. LO and CYP metabolites have actions to alter renal blood flow and glomerular filtration rate. Proximal and distal tubular sodium transport and fluid and electrolyte homeostasis are also significantly influenced by renal CYP and LO levels. Metabolites of the LO and CYP pathways also have renal actions that influence renal inflammation, proliferation, and apoptotic processes at vascular and epithelial cells. These renal LO and CYP pathway actions occur through generation of specific metabolites and cell-signaling mechanisms. Even though the renal physiological importance and actions for LO and CYP metabolites are readily apparent, major gaps remain in our understanding of these lipid mediators to renal function. Future studies will be needed to fill these major gaps regarding LO and CYP metabolites on renal function.

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

High salt diet modulates vascular response in A2AAR (+/+) and A 2AAR (-/-) mice: role of sEH, PPARγ, and K ATP channels

This study aims to investigate the signaling mechanism involved in HS-induced modulation of adenosine-mediated vascular tone in the presence or absence of adenosine A2A receptor (A2AAR). We hypothesized that HS-induced enhanced vascular relaxation through A2AAR and epoxyeicosatrienoic acid (EETs) is dependent on peroxisome proliferator-activated receptor gamma (PPARγ) and ATP-sensitive potassium channels (KATP channels) in A2AAR(+/+) mice, while HS-induced vascular contraction to adenosine is dependent on soluble epoxide hydrolase (sEH) that degrades EETs in A2AAR(-/-) mice. Organ bath and Western blot techniques were conducted in HS (4 % NaCl) and normal salt (NS, 0.45 % NaCl)-fed A2AAR(+/+) and A2AAR(-/-) mouse aorta. We found that enhanced vasodilation to A2AAR agonist, CGS 21680, in HS-fed A2AAR(+/+) mice was blocked by PPARγ antagonist (T0070907) and KATP channel blocker (Glibenclamide). Also, sEH inhibitor (AUDA)-dependent vascular relaxation was mitigated by PPARγ antagonist. PPARγ agonist (Rosiglitazone)-induced relaxation in HS-A2AAR(+/+) mice was attenuated by KATP channel blocker. Conversely, HS-induced contraction in A2AAR(-/-) mice was attenuated by sEH inhibitor. Overall, findings from this study that implicates the contribution of EETs, PPARγ and KATP channels downstream of A2AAR to mediate enhanced vascular relaxation in response to HS diet while, role of sEH in mediating vascular contraction in HS-fed A2AAR(-/-) mice.

High salt diet exacerbates vascular contraction in the absence of adenosine A₂A receptor

High salt (4% NaCl, HS) diet modulates adenosine-induced vascular response through adenosine A(2A) receptor (A(2A)AR). Evidence suggests that A(2A)AR stimulates cyp450-epoxygenases, leading to epoxyeicosatrienoic acids (EETs) generation. The aim of this study was to understand the vascular reactivity to HS and underlying signaling mechanism in the presence or absence of A(2A)AR. Therefore, we hypothesized that HS enhances adenosine-induced relaxation through EETs in A(2A)AR⁺/⁺, but exaggerates contraction in A(2A)AR⁻/⁻. Organ bath and Western blot experiments were conducted in HS and normal salt (NS, 0.18% NaCl)-fed A(2A)AR⁺/⁺ and A(2A)AR⁻/⁻ mice aorta. HS produced concentration-dependent relaxation to non-selective adenosine analog, NECA in A(2A)AR⁺/⁺, whereas contraction was observed in A(2A)AR⁻/⁻ mice and this was attenuated by A₁AR antagonist (DPCPX). CGS 21680 (selective A(2A)AR agonist) enhanced relaxation in HS-A(2A)AR⁺/⁺ versus NS-A(2A)AR⁺/⁺, which was blocked by EETs antagonist (14,15-EEZE). Compared with NS, HS significantly upregulated the expression of vasodilators A(2A)AR and cyp2c29, whereas vasoconstrictors A₁AR and cyp4a in A(2A)AR⁺/⁺ were downregulated. In A(2A)AR⁻/⁻ mice, however, HS significantly downregulated the expression of cyp2c29, whereas A₁AR and cyp4a were upregulated compared with A(2A)AR⁺/⁺ mice. Hence, our data suggest that in A(2A)AR⁺/⁺, HS enhances A(2A)AR-induced relaxation through increased cyp-expoxygenases-derived EETs and decreased A₁AR levels, whereas in A(2A)AR⁻/⁻, HS exaggerates contraction through decreased cyp-epoxygenases and increased A₁AR levels.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Adenosine A1 receptors link to smooth muscle contraction via CYP4a, protein kinase C-α, and ERK1/2

Adenosine A1 receptor (A1AR) activation contracts smooth muscle, although signaling mechanisms are not thoroughly understood. Activation of A1AR leads to metabolism of arachidonic acid, including the production of 20-hydroxyeicosatetraenoic acid (20-HETE) by cytochrome P4504a (CYP4a). The 20-HETE can activate protein kinase C-α (PKC-α), which crosstalks with extracellular signal-regulated kinase (ERK1/2) pathway. Both these pathways can regulate smooth muscle contraction, we tested the hypothesis that A1AR contracts smooth muscle through a pathway involving CYP4a, PKC-α, and ERK1/2. Experiments included isometric tension recordings of aortic contraction and Western blots of signaling molecules in wild type (WT) and A1AR knockout (A1KO) mice. Contraction to the A1-selective agonist 2-chloro-N cyclopentyladenosine (CCPA) was absent in A1KO mice aortae, indicating the contractile role of A1AR. Inhibition of CYP4a (HET0016) abolished 2-chloro-N cyclopentyladenosine-induced contraction in WT aortae, indicating a critical role for 20-HETE. Both WT and A1KO mice aortae contracted in response to exogenous 20-HETE. Inhibition of PKC-α (Gö6976) or ERK1/2 (PD98059) attenuated 20-HETE-induced contraction equally, suggesting that ERK1/2 is downstream of PKC-α. Contractions to exogenous 20-HETE were significantly less in A1KO mice; reduced protein levels of PKC-α, p-ERK1/2, and total ERK1/2 supported this observation. Our data indicate that A1AR mediates smooth muscle contraction via CYP4a and a PKC-α-ERK1/2 pathway.

Adenosine A1 receptor signaling inhibits BK channels through a PKCα-dependent mechanism in mouse aortic smooth muscle

Adenosine receptors (AR; A₁, A_2A, A_2B, and A₃) contract and relax smooth muscle through different signaling mechanisms. Deciphering these complex responses remains difficult because relationships between AR subtypes and various end-effectors (e.g., enzymes and ion channels) remain to be identified. A₁AR stimulation is associated with the production of 20–hydroxyeicosatetraenoic acid (20–HETE) and activation of protein kinase C (PKC). 20–HETE and PKC can inhibit large conductance Ca²⁺/voltage-sensitive K⁺ (BK) channels that regulate smooth muscle contraction. We tested the hypothesis that activation of A₁AR inhibits BK channels via a PKC-dependent mechanism. Patch clamp recordings and Western blots were performed using aortae of wild type (WT) and A₁AR knockout (A₁KO) mice. There were no differences in whole-cell K⁺ current or α and β1 subunits expression between WT and A₁KO. 20–HETE (100 nmol/L) inhibited BK current similarly in WT and A₁KO mice. NECA (5′–N–ethylcarboxamidoadenosine; 10 μmol/L), a nonselective AR agonist, increased BK current in myocytes from both WT and A₁KO mice, but the increase was greater in A₁KO (52 ± 15 vs. 17 ± 3%; P < 0.05). This suggests that A₁AR signaling negatively regulates BK channel activity. Accordingly, CCPA (2–chloro–N(6)-cyclopentyladenosine; 100 nmol/L), an A₁AR-selective agonist, inhibited BK current in myocytes from WT but not A₁KO mice (81 ± 4 vs. 100 ± 7% of control; P < 0.05). Gö6976 (100 nmol/L), a PKCα inhibitor, abolished the effect of CCPA to inhibit BK current (99 ± 3% of control). These data lead us to conclude that, in aortic smooth muscle, A₁AR inhibits BK channel activity and that this occurs via a mechanism involving PKCα.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.

Epoxyeicosatrienoic acids, 20-hydroxyeicosatetraenoic acid, and renal microvascular function

The development of pharmacological, genetic, and biochemical tools have allowed for detailed studies to determine the contribution of cytochrome P450 (CYP) metabolites of arachidonic acid to renal microvascular function. Renal microvessels can generate CYP hydroxylase metabolites including 20-hydroxyeicosatetraenoic acid (20-HETE) and CYP epoxygenase metabolites, epoxyeicosatrienoic acids (EETs). 20-HETE constricts afferent arterioles and contributes to renal blood flow autoregulation. EETs act as endothelium-dependent hyperpolarizing factors (EDHFs) on the renal microcirculation. 20-HETE inhibits whereas EETs activate renal microvascular smooth muscle cell large-conductance calcium-activated K(+) channels (KCa). Likewise, 20-HETE renal microvascular actions are pro-hypertensive and EET actions are anti-hypertensive. These findings in the renal microvasculature and those of others have provided impetus for the development of enzymatic inhibitors, agonists, and antagonists for 20-HETE and EETs to determine their potential therapeutic value. Initial genetic studies and experimental studies with soluble epoxide hydrolase inhibitors to increase EETs, EET analogs, and 20-HETE inhibitors have demonstrated improved renal microvascular function in hypertension. These findings have demonstrated the important contributions that 20-HETE and EETs play in the regulation of renal microvascular function.

CYP-epoxygenases contribute to A2A receptor-mediated aortic relaxation via sarcolemmal KATP channels

Previously, we have shown that A(2A) adenosine receptor (A(2A)AR) mediates aortic relaxation via cytochrome P-450 (CYP)-epoxygenases. However, the signaling mechanism is not understood properly. We hypothesized that ATP-sensitive K(+) (K(ATP)) channels play an important role in A(2A)AR-mediated relaxation. Organ bath and Western blot experiments were done using isolated aorta from A(2A)KO and corresponding wild-type (WT) mice. Aortic rings from WT and A(2A) knockout (KO) mice were precontracted with submaximal dose of phenylephrine (PE, 10(-6) M), and concentration-response curves for pinacidil, cromakalim (nonselective K(ATP) openers), and diazoxide (mitochondrial K(ATP) opener) were obtained. Diazoxide did not have any relaxation effect on PE-precontracted tissues, whereas relaxation to pinacidil (48.09 ± 5.23% in WT vs. 25.41 ± 2.73% in A(2A)KO; P < 0.05) and cromakalim (51.19 ± 2.05% in WT vs. 38.50 ± 2.26% in A(2A)KO; P < 0.05) was higher in WT than A(2A)KO aorta. This suggested the involvement of sarcolemmal rather than mitochondrial K(ATP) channels. Endothelium removal, treatment with SCH 58651 (A(2A)AR antagonist; 10(-6) M), N(G)-nitro-l-arginine methyl ester (l-NAME, nitric oxide synthase inhibitor) and methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH, CYP-epoxygenases inhibitor; 10(-5) M) significantly reduced pinacidil-induced relaxation in WT compared with controls, whereas these treatments did not have any effect in A(2A)KO aorta. Glibenclamide (K(ATP) channel inhibitor, 10(-5) M) blocked 2-p-(2-carboxyethyl)phenethylamino-5'N-ethylcarboxamido adenosine hydrochloride (CGS 21680, A(2A)AR agonist)-induced relaxation in WT and changed 5'-N-ethylcarboxamide (NECA) (nonselective adenosine analog)-induced response to higher contraction in WT and A(2A)KO. 5-Hydroxydecanoate (5-HD, mitochondrial K(ATP) channel inhibitor, 10(-4) M) had no effect on CGS 21680-mediated response in WT aorta. Our data suggest that A(2A)AR-mediated vasorelaxation occurs through opening of sarcolemmal K(ATP) channels via CYP-epoxygenases and possibly, nitric oxide, contributing to pinacidil-induced responses.

8,9-Epoxyeicosatrienoic acid inhibits antibody production of B lymphocytes in mice

Epoxyeicosatrienoic acids (EETs), synthesized from arachidonic acid by cytochrome P450 epoxygenases, are converted to dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. EETs exert anti-inflammatory effects. However, the effect of EETs on humoral immunity is poorly understood. The present study is to investigate the potential role of EETs on B cell function and mechanisms. We examined the role of EETs on antibody production of splenic B cells from C57BL/6 and apolipoprotein E-deficient (ApoE−/−) mice by means of ELISA. Of the 4 EET regioisomers, 8,9-EET decreased basal and activation-induced B cell antibody secretion. As well, 8,9-EET significantly inhibited B-cell proliferation and survival, plasma cell differentiation and class-switch recombination. Western blot analysis revealed that lipopolysaccharide-induced nuclear translocation of NF-κB could be attenuated by 8,9-EET. Furthermore, germinal center formation was impaired by 8,9-EET in mice in vivo. 8,9-EET may inhibit B-cell function in vitro and in vivo, which suggests a new therapeutic strategy for diseases with excess B cell activation.

Epoxides and soluble epoxide hydrolase in cardiovascular physiology

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites that importantly contribute to vascular and cardiac physiology. The contribution of EETs to vascular and cardiac function is further influenced by soluble epoxide hydrolase (sEH) that degrades EETs to diols. Vascular actions of EETs include dilation and angiogenesis. EETs also decrease inflammation and platelet aggregation and in general act to maintain vascular homeostasis. Myocyte contraction and increased coronary blood flow are the two primary EET actions in the heart. EET cell signaling mechanisms are tissue and organ specific and provide significant evidence for the existence of EET receptors. Additionally, pharmacological and genetic manipulations of EETs and sEH have demonstrated a contribution for this metabolic pathway to cardiovascular diseases. Given the impact of EETs to cardiovascular physiology, there is emerging evidence that development of EET-based therapeutics will be beneficial for cardiovascular diseases.

Role of ω-hydroxylase in adenosine-mediated aortic response through MAP kinase using A2A-receptor knockout mice

Previously, we have shown that A(2A) adenosine receptor (A(2A)AR) knockout mice (KO) have increased contraction to adenosine. The signaling mechanism(s) for A(2A)AR is still not fully understood. In this study, we hypothesize that, in the absence of A(2A)AR, ω-hydroxylase (Cyp4a) induces vasoconstriction through mitogen-activated protein kinase (MAPK) via upregulation of adenosine A(1) receptor (A(1)AR) and protein kinase C (PKC). Organ bath and Western blot experiments were done using isolated aorta from A(2A)KO and corresponding wild-type (WT) mice. Isolated aortic rings from WT and A(2A)KO mice were precontracted with submaximal dose of phenylephrine (10(-6) M), and concentration responses for selective A(1)AR, A(2A)AR agonists, angiotensin II and cytochrome P-450-epoxygenase, 20-hydroxyeicosatrienoic acid (20-HETE) PKC, PKC-α, and ERK1/2 inhibitors were obtained. 2-p-(2-Carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680, A(2A)AR agonist) induced concentration-dependent relaxation in WT, which was blocked by methylsulfonyl-propargyloxyphenylhexanamide (cytochrome P-450-epoxygenase inhibitor; 10(-5) M) and also with removal of endothelium. A(1) agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA) produced higher contraction in A(2A)KO aorta than WT (49.2 ± 8.5 vs. 27 ± 5.9% at 10(-6) M, P < 0.05). 20-HETE produced higher contraction in A(2A)KO than WT (50.6 ± 8.8 vs. 21.1 ± 3.3% at 10(-7) M, P < 0.05). Contraction to CCPA in WT and A(2A)KO aorta was inhibited by PD-98059 (p42/p44 MAPK inhibitor; 10(-6) M), chelerythrine chloride (nonselective PKC blocker; 10(-6) M), Gö-6976 (selective PKC-α inhibitor; 10(-7) M), and HET0016 (20-HETE inhibitor; 10(-5) M). Also, contraction to 20-HETE in WT and A(2A)KO aorta was inhibited by PD-98059 and Gö-6976. Western blot analysis indicated the upregulation of A(1)AR, Cyp4a, PKC-α, and phosphorylated-ERK1/2 in A(2A)KO compared with WT (P < 0.05), while expression of Cyp2c29 was significantly higher in WT. CCPA (10(-6) M) increased the protein expression of PKC-α and phosphorylated-ERK1/2, while HET0016 significantly reduced the CCPA-induced increase in expression of these proteins. These data suggest that, in the absence of A(2A)AR, Cyp4a induces vasoconstriction through MAPK via upregulation of A(1)AR and PKC-α.

Role of the adenosine(2A) receptor-epoxyeicosatrienoic acid pathway in the development of salt-sensitive hypertension

Activation of rat adenosine(2A) receptors (A(2A) R) dilates preglomerular microvessels, an effect mediated by epoxyeicosatrienoic acids (EETs). High salt (HS) intake increases epoxygenase activity and adenosine levels. A greater vasodilator response to a stable adenosine analog, 2-chloroadenosine (2-CA), was seen in kidneys obtained from HS-fed rats which was mediated by increased EET release. Because this pathway is antipressor, we examined the role of the A(2A) R-EET pathway in a genetic model of salt-sensitive hypertension, the Dahl salt-sensitive (SS) rats. Dahl salt resistant (SR) rats fed a HS diet demonstrated a greater renal vasodilator response to 2-CA. In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed normal salt (NS) or HS diet. In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A) R and cytochrome P450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake. In vivo inhibition of the A(2A) R-EET pathway in Dahl SR rats fed a HS diet results in reduced renal EETs levels, diminished natriuretic capacity and hypertension, thus supporting a role for the A(2A) R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading. An inability of Dahl SS rats to upregulate the A(2A) R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

Increases in plasma trans-EETs and blood pressure reduction in spontaneously hypertensive rats

Epoxyeicosatrienoic acids (EETs) are vasodilator, natriuretic, and antiinflammatory lipid mediators. Both cis- and trans-EETs are stored in phospholipids and in red blood cells (RBCs) in the circulation; the maximal velocity (V(max)) of trans-EET hydrolysis by soluble epoxide hydrolase (sEH) is threefold that of cis-EETs. Because RBCs of the spontaneously hypertensive rat (SHR) exhibit increased sEH activity, a deficiency of trans-EETs in the SHR was hypothesized to increase blood pressure (BP). This prediction was fulfilled, since sEH inhibition with cis-4-[4-(3-adamantan-1-ylureido)cyclohexyloxy]benzoic acid (AUCB; 2 mg·kg(-1)·day(-1) for 7 days) in the SHR reduced mean BP from 176 ± 8 to 153 ± 5 mmHg (P < 0.05), whereas BP in the control Wistar-Kyoto rat (WKY) was unaffected. Plasma levels of EETs in the SHR were lower than in the age-matched control WKY (16.4 ± 1.6 vs. 26.1 ± 1.8 ng/ml; P < 0.05). The decrease in BP in the SHR treated with AUCB was associated with an increase in plasma EETs, which was mostly accounted for by increasing trans-EET from 4.1 ± 0.2 to 7.9 ± 1.5 ng/ml (P < 0.05). Consistent with the effect of increased plasma trans-EETs and reduced BP in the SHR, the 14,15-trans-EET was more potent (ED(50) 10(-10) M; maximum dilation 59 ± 15 μm) than the cis-isomer (ED(50) 10(-9) M; maximum dilation 30 ± 11 μm) in relaxing rat preconstricted arcuate arteries. The 11,12-EET cis- and trans-isomers were equipotent dilators as were the 8,9-EET isomers. In summary, inhibition of sEH resulted in a twofold increase in plasma trans-EETs and reduced mean BP in the SHR. The greater vasodilator potency of trans- vs. cis-EETs may contribute to the antihypertensive effects of sEH inhibitors.

Salt modulates vascular response through adenosine A(2A) receptor in eNOS-null mice: role of CYP450 epoxygenase and soluble epoxide hydrolase

High salt (HS) intake can change the arterial tone in mice, and the nitric oxide (NO) acts as a mediator to some of the receptors mediated vascular response. The main aim of this study was to explore the mechanism behind adenosine-induced vascular response in HS-fed eNOS(+/+) and eNOS(-/-) mice The modulation of vascular response by HS was examined using aortas from mice (eNOS(+/+) and eNOS(-/-)) fed 4% (HS) or 0.45% (NS) NaCl-diet through acetylcholine (ACh), NECA (adenosine-analog), CGS 21680 (A(2A) AR-agonist), MS-PPOH (CYP epoxygenase-blocker; 10(-5) M), AUDA (sEH-blocker; 10(-5) M), and DDMS (CYP4A-blocker; 10(-5) M). ACh-response was greater in HS-eNOS(+/+) (+59.3 ± 6.3%) versus NS-eNOS(+/+) (+33.3 ± 8.0%; P < 0.05). However, there was no response in both HS-eNOS(-/-) and NS-eNOS(-/-). NECA-response was greater in HS-eNOS(-/-) (+37.4 ± 3.2%) versus NS-eNOS(-/-) (+7.4.0 ± 3.8%; P < 0.05). CGS 21680-response was also greater in HS-eNOS(-/-) (+45.4 ± 5.2%) versus NS-eNOS(-/-)(+5.1 ± 5.0%; P < 0.05). In HS-eNOS(-/-), the CGS 21680-response was reduced by MS-PPOH (+7.3 ± 3.2%; P < 0.05). In NS-eNOS(-/-), the CGS 21680-response was increased by AUDA (+38.2 ± 3.3%; P < 0.05) and DDMS (+30.1 ± 4.1%; P < 0.05). Compared to NS, HS increased CYP2J2 in eNOS(+/+) (35%; P < 0.05) and eNOS(-/-) (61%; P < 0.05), but decreased sEH in eNOS(+/+) (74%; P < 0.05) and eNOS(-/-) (40%; P < 0.05). Similarly, CYP4A decreased in HS-eNOS(+/+) (35%; P < 0.05) and HS-eNOS(-/-) (34%; P < 0.05). These data suggest that NS causes reduced-vasodilation in both eNOS(+/+) and eNOS(-/-) via sEH and CYP4A. However, HS triggers possible A(2A)AR-induced relaxation through CYP epoxygenase in both eNOS(+/+) and eNOS(-/-).

Afferent arteriolar vasodilator effect of adenosine predominantly involves adenosine A2B receptor activation

Adenosine is an important paracrine agent regulating renal vascular tone via adenosine A(1) and A(2) receptors. While A(2B) receptor message and protein have been localized to preglomerular vessels, functional evidence on the role of A(2B) receptors in mediating the vasodilator action of adenosine on afferent arterioles is not available. The present study determined the role of A(2B) receptors in mediating the afferent arteriolar dilation and compared the effects of A(2B) and A(2A) receptor blockade on afferent arterioles. We used the rat in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Single afferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing adenosine or adenosine A(2) receptor agonist (CV-1808) along with adenosine A(2B) and A(2A) receptor blockers. Adenosine (10 micromol/l) caused modest constriction and subsequent superfusion with SCH-58261 (SCH), an A(2A) receptor blocker, at concentrations up 10 micromol/l elicited only slight additional decreases in afferent arteriolar diameter with maximum effect at a concentration of 1 micromol/l (-11.0 +/- 2.5%, n = 6, P < 0.05). However, superfusion of adenosine-treated vessels with MRS-1754 (MRS), an A(2B) receptor blocker, elicited greater decreases in afferent arteriolar diameter (-26.0 +/- 4.7%, n = 5, P < 0.01). SCH did not significantly augment the adenosine-mediated afferent constriction elicited by MRS; however, adding MRS after SCH caused further significant vasoconstriction. Superfusion with CV-1808 dilated afferent arterioles (17.2 +/- 2.4%, n = 6, P < 0.01). This effect was markedly attenuated by MRS (-22.6 +/- 2.0%, n = 5, P < 0.01) but only slightly reduced by SCH (-9.0 +/- 1.1%, n = 5, P < 0.05) and completely prevented by adding MRS after SCH (-24.7 +/- 1.8%, n = 5, P < 0.01). These results indicate that, while both A(2A) and A(2B) receptors are functionally expressed in juxtamedullary afferent arterioles, the powerful vasodilating action of adenosine predominantly involves A(2B) receptor activation, which counteracts A(1) receptor-mediated vasoconstriction.

Modulation by salt intake of the vascular response mediated through adenosine A(2A) receptor: role of CYP epoxygenase and soluble epoxide hydrolase

High-salt intake can change the effect of adenosine on arterial tone in mice. The aim of this study was to clarify the mechanism by which this occurs. Using aortas from mice fed a 4% NaCl (HS) or 0.45% NaCl (NS) diet for 4-5 wks, concentration-response curves for ACh, 5'-N-ethylcarboxamidoadenosine (NECA; adenosine analog) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate [CGS-21680; A(2A) adenosine receptor (A(2A) AR) agonist] were obtained with N(omega)-nitro-L-arginine methyl ester (L-NAME; nitric oxide inhibitor, 10(-4) M), methylsulfonyl-propargyloxyphenylhexanamide [MS-PPOH; a CYP (cytochrome P-450) epoxygenase blocker, 10(-5) M including CYP2J2], 12-(3-adamantan-1-yl-ureido)dodecanoic acid [AUDA; soluble epoxide hydrolase (sEH) blocker, 10(-5) M], dibromo-dodecenyl-methylsulfimide [DDMS; CYP omega-hydroxylase (CYP4A blocker), 10(-5) M], glibenclamide (K(ATP) channel blocker; 10(-5) M) and 5-hydroxydecanoate (5-HD; mitochondrial-K(ATP) channel blocker, 10(-4) M). HS dose response to ACh (10(-7) - 10(-5) M) was not different from NS (P > 0.05). Relaxation to 10(-6) M NECA was greater in the HS group (28.4 +/- 3.9%) than in the NS group (4.1 +/- 2.3%). Relaxation to 10(-6) M CGS-21680 was also greater in HS (27.9 +/- 4.5%) than in NS (4.9 +/- 2.2%). L-NAME was able to block the dose response of ACh (10(-7) - 10(-5) M) equally in both HS and NS (P > 0.05), whereas L-NAME did not block CGS-21680-induced response in HS. In HS the CGS-21680 response was greatly reduced by MS-PPOH (to 4.7 +/- 2.0%) and 5-HD (to 8.9 +/- 2.2%), and also abolished by glibenclamide (-1.0 +/- 5.9%). In NS, the CGS-21680 response was increased by AUDA (to 26.3 +/- 3.4%) and DDMS (to 27.2 +/- 3.0%). Compared with NS, HS vessels showed increased CYP2J2 and A(2A) AR expression (46 and 74% higher, respectively) but decreased sEH, CYP4A, and A(1) AR expression (75, 30, and 55% lower, respectively). These data suggest that in mice fed NS-containing diet, upregulation of arterial A(1) receptor causes vasoconstriction via increased sEH and CYP4A proteins. However, in mice fed HS-containing diet, upregulation of A(2A) receptor protein triggers vascular relaxation through ATP-sensitive (K(+)) channels via upregulation of CYP2J2 enzyme.

Epoxyeicosatrienoic acid analogs and vascular function

Arachidonic acid metabolites, eicosanoids, are key contributors to vascular function and improper eicosanoid regulation contributes to the progression of cardiovascular diseases. Epoxyeicosatrienoic acids (EETs) are synthesized from arachidonic acid by epoxygenase enzymes to four regioisomers, 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET. These EETs have interesting beneficial effects like vasodilation, anti-inflammation, and anti-platelet aggregation that could combat cardiovascular diseases. There is mounting evidence that each regioisomeric EET may have unique vascular effects and that the contribution of individual EETs to vascular function differs from organ to organ. Over the past decade EET analogs and antagonists have been synthesized to determine EET structure function relationships and define the contribution of each regioisomeric EET. A number of studies have demonstrated that EET analogs induce vasodilation, lower blood pressure and decrease inflammation. EET antagonists have also been used to demonstrate that endogenous EETs contribute importantly to cardiovascular function. This review will discuss EET synthesis, regulation and physiological roles in the cardiovascular system. Next we will focus on the development of EET analogs and what has been learned about their contribution to vascular function. Finally, the development of EET antagonists and how these have been utilized to determine the cardiovascular actions of endogenous epoxides will be discussed. Overall, this review will highlight the important knowledge garnered by the development of EET analogs and their possible value in the treatment of cardiovascular diseases.

High-salt diet enhances mouse aortic relaxation through adenosine A2A receptor via CYP epoxygenases

We hypothesize that A(2A) adenosine receptors (A(2A) AR) promote aortic relaxation in mice through cytochrome P450 (CYP)-epoxygenases and help to avoid salt sensitivity. Aortas from male mice maintained on a high-salt (HS; 7% NaCl) or normal-salt (NS; 0.45% NaCl) diet for 4-5 wks were used. Concentration-response curves (10(-11)-10(-5) M) for 5'-N-ethylcarboxamidoadenosine (NECA; a nonselective adenosine analog) and CGS 21680 (A(2A) AR agonist) were obtained with different antagonists including ZM 241385 (A(2A) AR antagonist; 10(-6) M), SCH 58261 (A(2A) AR antagonist; 10(-6) M), N(omega)-nitro-l-arginine methyl ester (l-NAME; endothelial nitric oxide synthase inhibitor; 10(-4) M) and inhibitors including methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH; CYP epoxygenases inhibitor; 10(-5)M), 14,15-epoxyeicosa-5(z)-enoic acid (14,15-EEZE; EET antagonist; 10(-5)M), dibromo-dodecenyl-methylsulfimide (DDMS; CYP4A inhibitor; 10(-5)M), and HET0016 (20-HETE inhibitor; 10(-5)M). At 10(-7) M of NECA, significant relaxation in HS (+22.58 +/- 3.12%) was observed compared with contraction in NS (-10.62 +/- 6.27%, P < 0.05). ZM 241385 changed the NECA response to contraction (P < 0.05) in HS. At 10(-7) M of CGS 21680, significant relaxation in HS (+32.04 +/- 3.08%) was observed compared with NS (+10.45 +/- 1.34%, P < 0.05). SCH 58261, l-NAME, MS-PPOH, and 14,15-EEZE changed the CGS 21680-induced relaxation to contraction (P < 0.05) in HS. Interestingly, DDMS and HET0016 changed CGS 21680 response to relaxation (P < 0.05) in NS; however, there was no significant difference found between DDMS, HET0016-treated HS and NS vs. nontreated HS group (P > 0.05). CYP2C29 protein was 55% and 74% upregulated in HS vs. NS (P < 0.05) mice aorta and kidney, respectively. CYP4A protein was 30.30% and 35.70% upregulated in NS vs. HS (P < 0.05) mice aorta and kidneys, respectively. A(1) AR was downregulated, whereas A(2A) AR was upregulated in HS compared with NS. These data suggest that HS may activate CYP2C29 via A(2A) AR, causing relaxation, whereas NS may contribute to the upregulation of CYP4A causing contraction.

Epoxyeicosatrienoic acid activates BK channels in the cortical collecting duct

The cortical collecting duct (CCD), which is involved in renal potassium (K) excretion, expresses cytochrome P450 (CYP)-epoxygenase. Here, we examined the effect of high dietary K on renal expression of CYP2C23 and CYP2J2 in the rat, as well as the role of CYP-epoxygenase-dependent metabolism of arachidonic acid in the regulation of Ca(2+)-activated big-conductance K (BK) channels. By Western blot analysis, high dietary K stimulated the expression of CYP2C23 but not CYP2J2 and increased 11,12-epoxyeicosatrienoic acid (11,12-EET) levels in isolated rat CCD tubules. Application of arachidonic acid increased BK channel activity, and this occurred to a greater extent in rats on a high-K diet compared with a normal-K diet. This effect was unlikely due to arachidonic acid-induced changes in membrane fluidity, because 11,14,17-eicosatrienoic acid did not alter BK channel activity. Inhibiting CYP-epoxygenase but not cyclooxygenase- or CYP-omega-hydroxylase-dependent pathways completely abolished the stimulatory effect of arachidonic acid on BK channel activity. In addition, application of 11,12-EET mimicked the effect of arachidonic acid on BK channel activity, even in the presence of CYP-epoxygenase inhibition. This effect seemed specific to 11,12-EET, because both 8,9- and 14,15-EET failed to stimulate BK channels. Finally, inhibition of CYP-epoxygenase abolished iberiotoxin-sensitive and flow-stimulated but not basal net K secretion in isolated microperfused CCD. In conclusion, high dietary K stimulates the renal CYP-epoxygenase pathway, which plays an important role in activating BK channels and flow-stimulated K secretion in the CCD.

Role of CYP epoxygenases in A2A AR-mediated relaxation using A2A AR-null and wild-type mice

We hypothesized that A2A adenosine receptor (A2A AR) activation causes vasorelaxation through cytochrome P-450 (CYP) epoxygenases and endothelium-derived hyperpolarizing factors, whereas lack of A2A AR activation promotes vasoconstriction through Cyp4a in the mouse aorta. Adenosine 5'-N-ethylcarboxamide (NECA; 10(-6) M), an adenosine analog, caused relaxation in wild-type A2A AR (A2A AR+/+; +33.99 +/- 4.70%, P < 0.05) versus contraction in A2A AR knockout (A2A AR(-/-); -27.52 +/- 4.11%) mouse aortae. An A2A AR-specific antagonist (SCH-58261; 1 microM) changed the NECA (10(-6) M) relaxation response to contraction (-35.82 +/- 4.69%, P < 0.05) in A2A AR+/+ aortae, whereas no effect was noted in A2A AR(-/-) aortae. Significant contraction was seen in the absence of the endothelium in A2A AR+/+ (-2.58 +/- 2.25%) aortae compared with endothelium-intact aortae. An endothelial nitric oxide synthase inhibitor (N-nitro-L-arginine methyl ester; 100 microM) and a cyclooxygenase inhibitor (indomethacin; 10 microM) failed to block NECA-induced relaxation in A2A AR+/+ aortae. A selective inhibitor of CYP epoxygenases (methylsulfonyl-propargyloxyphenylhexanamide; 10 microM) changed NECA-mediated relaxation (-22.74 +/- 5.11% at 10(-6) M) and CGS-21680-mediated relaxation (-18.54 +/- 6.06% at 10(-6) M) to contraction in A2A AR+/+ aortae, whereas no response was noted in A2A AR(-/-) aortae. Furthermore, an epoxyeicosatrienoic acid (EET) antagonist [14,15-epoxyeicosa-5(Z)-enoic acid; 10 microM] was able to block NECA-induced relaxation in A2A AR+/+ aortae, whereas omega-hydroxylase inhibitors (10 microM dibromo-dodecenyl-methylsulfimide and 10 microM HET-0016) changed contraction into relaxation in A2A AR(-/-) aorta. Cyp2c29 protein was upregulated in A2A AR+/+ aortae, whereas Cyp4a was upregulated in A2A AR(-/-) aortae. Higher levels of dihydroxyeicosatrienoic acids (DHETs; 14,15-DHET, 11,12-DHET, and 8,9-DHET, P < 0.05) were found in A2A AR+/+ versus A2A AR(-/-) aortae. EET levels were not significantly different between A2A AR+/+ and A2A AR(-/-) aortae. It is concluded that CYP epoxygenases play an important role in A2A AR-mediated relaxation, and the deletion of the A2A AR leads to contraction through Cyp4a.

Afferent arteriolar dilation to 11, 12-EET analogs involves PP2A activity and Ca2+-activated K+ Channels

The epoxygenase metabolite, 11, 12-epoxyeicosatrienoic acid (11, 12-EET), has renal vascular actions. 11, 12-EET analogs have been developed to determine the structure activity relationship for 11, 12-EET and as a tool to investigate signaling mechanisms responsible for afferent arteriolar dilation. We hypothesized that 11, 12-EET mediated afferent arteriolar dilation involves increased phosphoprotein phosphatase 2A (PP2A) and large-conductance calcium activated K+ (KCa) channels. We evaluated the chemically and/or metabolically table 11, 12-EET analogs: 11, 12-EET-N-methylsulfonimide (11, 12-EET-SI), 11-nonyloxy-undec-8(Z)-enoic acid (11, 12-ether-EET-8-ZE), and 11, 12-trans-oxidoeicosa-8(Z)-eonoic acid (11, 12-tetra-EET-8-ZE). Afferent arteriolar responses were assessed. Activation of KCa channels by 11, 12-EET analogs were established by single cell channel recordings in renal myocytes. Assessment of renal vascular responses revealed that 11, 12-EET analogs increased afferent arteriolar diameter. Vasodilator responses to 11, 12-EET analogs were abolished by K+ channel or PP2A inhibition. 11, 12-EET analogs activated renal myocyte large-conductance KCa channels. 11, 12-EET analogs increased cAMP by 2-fold and PP2A activity increased 3-8 fold in renal myocytes. PP2A inhibition did not significantly affect the 11, 12-EET analog mediated increase in cAMP and PP2A increased renal myocyte KCa channel activity to a much greater extent than PKA. These data support the concept that 11, 12-EET utilizes PP2A dependent pathways to activate large-conductance KCa channels and dilate the afferent arteriole.

Adenosine stimulates the basolateral 50 pS K channels in the thick ascending limb of the rat kidney

We used the patch-clamp technique to examine the effect of adenosine on the basolateral K channels in the thick ascending limb (TAL) of the rat kidney. A 50-pS inwardly rectifying K channel was detected in the basolateral membrane, and the channel activity was decreased by hyperpolarization. Application of adenosine (10 microM) increased the activity of basolateral 50 pS K channels, defined by NP(o), from 0.21 to 0.41. The effect of adenosine on the 50 pS K channels was mimicked by cyclohexyladenosine (CHA), which increased channel activity by a dose-dependent manner. However, inhibition of the A1 adenosine receptor with 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) failed to block the effect of CHA. In contrast, application of 8-(3-chlorostyryl) caffeine (CSC), an A2 adenosine antagonist, abolished the stimulatory effect of CHA. The possibility that the effect of adenosine and adenosine analog on the basolateral 50 pS K channel was the result of activation of the A2 adenosine receptor was also suggested by the observation that application of CGS-21680, a selected A(2A) adenosine receptor agonist, increased the channel activity. Also, inhibition of PKA with N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide-2HC1 abolished the stimulatory effect of CHA on the basolateral 50 pS K channel. Moreover, addition of the membrane-permeable cAMP analog increases the activity of 50 pS K channels. We conclude that adenosine activates the 50 pS K channel in the basolateral membrane of the TAL and the stimulatory effect is mainly mediated by a PKA-dependent pathway via the A2 adenosine receptor in the TAL.

Rat mesenteric arterial dilator response to 11,12-epoxyeicosatrienoic acid is mediated by activating heme oxygenase

11,12-Epoxyeicosatrienoic acid (11,12-EET), a potent vasodilator produced by the endothelium, acts on calcium-activated potassium channels and shares biological activities with the heme oxygenase/carbon monoxide (HO/CO) system. We examined whether activation of HO mediates the dilator action of 11,12-EET, and that of the other EETs, on rat mesenteric arteries. Dose-response curves (10(-9) to 10(-6) M) to 5,6-EET, 8,9-EET, 11,12-EET, 14,15-EET, and ACh (10(-9) to 10(-4) M) were evaluated in preconstricted (10(-6) mol/l phenylephrine) mesenteric arteries (<350 microm diameter) in the presence or absence of 1) the cyclooxygenase inhibitor indomethacin (2.8 microM), 2) the HO inhibitor chromium mesoporphyrin (CrMP) (15 microM), 3) the soluble guanylyl cyclase (GC) inhibitor ODQ (10 microM), and 4) the calcium-activated potassium channel inhibitor iberiotoxin (25 nM). The vasodilator response to 11,12-EET was abolished by CrMP and iberiotoxin, whereas indomethacin and ODQ had no effect. In contrast, the effect of ACh was attenuated by ODQ but not by CrMP. The vasodilator effect of 8,9-EET, like that of 11,12-EET, was greatly attenuated by HO inhibition. In contrast, the mesenteric vasodilator response to 5,6-EET was independent of both HO and GC, whereas that to 14,15-EET demonstrated two components, an HO and a GC, of equal magnitude. Incubation of mesenteric microvessels with 11,12-EET caused a 30% increase in CO release, an effect abolished by inhibition of HO. We conclude that the rat mesenteric vasodilator action of 11,12-EET is mediated via an increase in HO activity and an activation of calcium-activated potassium channels.

Adenosine2A receptor vasodilation of rat preglomerular microvessels is mediated by EETs that activate the cAMP/PKA pathway

Dilation of rat preglomerular microvessels (PGMV) by activation of adenosine A2A receptors (A2AR) is coupled to epoxyeicosatrienoic acid (EET) release. We have investigated the commonality of this signal transduction pathway, i.e., sequential inhibition of G(salpha), adenylyl cyclase, PKA, and Ca2+-activated K+ (KCa) channel activity, to the vasoactive responses to A2AR activation by a selective A2A agonist, CGS-21680, compared with those of 11,12-EET. Male Sprague-Dawley rats were anesthetized, and microdissected arcuate arteries (110-130 microm) were cannulated and pressurized to 80 mmHg. Vessels were superfused with Krebs solution containing NG-nitro-L-arginine methyl ester (L-NAME) and indomethacin and preconstricted with phenylephrine. We assessed the effect of 3-aminobenzamide (10 microM), an inhibitor of mono-ADP-ribosyltranferases, on responses to 11,12-EET (3 nM) and CGS-21680 (10 microM) and found that both were inhibited by approximately 70% (P<0.05), whereas the response to SNP (10 microM) was unaffected. Furthermore, 11,12-EET (100 nM), like cholera toxin (100 ng/ml), stimulated ADP-ribose formation in homogenates of arcuate arteries compared with control. SQ-22536 (10 microM), an inhibitor of adenylyl cyclase activity, and myristolated PKI (14-22) amide (5 microM), an inhibitor of PKA, decreased activity of 11,12-EET and CGS-21680. Incubation of 11,12-EET (3 nM-3 microM) with PGMV resulted in an increase in cAMP levels (P<0.05). The responses to both 11,12-EET and CGS-21680 were significantly reduced by superfusion of iberiotoxin (100 nM), an inhibitor of KCa channel activity. Thus in rat PGMV activation of A2AR is coupled to EET release upstream of adenylyl cyclase activation and EETs stimulate mono-ADP-ribosyltransferase, resulting in Gsalpha protein activation.

Red blood cells: reservoirs of cis- and trans-epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are candidate endothelium-derived hyperpolarizing factors that demonstrate a wide range of biological effects. The presence of both cis- and trans-EETs in rat plasma was identified with HPLC-electrospray ionization tandem mass spectrometry in this study. The total EETs in plasma are 38.2 ng/ml with cis-EETs representing 21.4 +/- 0.4 ng/ml and trans-EETs 16.8 +/- 0.4 ng/ml. EETs in RBCs were estimated to be 20.2 ng/10(9) RBCs, which corresponds to 200 ng in RBCs contained in 1 ml blood. RBC incubation with 10 mM tert-butyl hydroperoxide resulted in 4.4-fold increase of total cis-EETs (from 9.2 to 40.2 ng/10(9) RBCs) and 5.5-fold increase of total trans-EETs (from 11.0 to 60.8 ng/10(9) RBCs). EETs were released (2 ng/ml) from RBCs after incubation at 37 degrees C for 10 min even after being washed 3 times, indicating that RBCs are reservoirs of plasma EETs. The identification of cis- and trans-EETs in RBCs and in plasma as well as their release from RBCs suggest a vasoregulatory role of RBCs in view of their potent vasoactivity.

Epoxide hydrolase and epoxygenase metabolites as therapeutic targets for renal diseases

Renal epoxygenase metabolites are involved in blood flow regulation and long-term blood pressure control. One feature of renal and cardiovascular diseases is the inability of the kidney to properly increase epoxyeicosatrienoic acid (EET) levels. Others (Busse R, Edwards G, Félétou M, Fleming I, Vanhoutte PM, and Weston AH. Trends Phamacol Sci 23: 374–380, 2002; Campbell WB, Gebremedhin D, Pratt PF, and Harder DR. Circ Res 78: 415–423, 1996; Capdevila JH and Falck JR. Biochem Biophys Res Commun 285: 571–576, 2001; Roman RJ. Physiol Rev 82: 131–185, 2002; Zeldin DC. J Biol Chem 276: 36059–36062, 2001) and we (Imig JD, Falck JR, Wei S, and Capdevila JH. J Vasc Res 38: 247–255, 2001; Imig JD, Zhao X, Capdevila JH, Morisseau C, and Hammock BD. Hypertension 39: 690–694, 2002; Zhao X, Pollock DM, Inscho EW, Zeldin DC, and Imig JD. Hypertension 41: 709–714, 2003; Zhao X, Pollock DM, Zeldin DC, and Imig JD. Hypertension 42: 775–780, 2003) have provided compelling evidence that cytochrome P-450-derived EETs have antihypertensive properties and are endothelially derived hyperpolarizing factors (EDHFs) in the kidney. EETs also possess anti-inflammatory actions that could protect the kidney vasculature from injury during renal and cardiovascular diseases. A tactic that has been used to increase EET levels has been inhibition of the soluble epoxide hydrolase enzyme. Epoxide hydrolase inhibitors have been demonstrated to be antihypertensive and renal protective. Thus the renal and cardiovascular protective actions of increasing epoxygenase levels could be translated to therapies for preventing end-organ damage.

Arachidonic acid metabolites, hydrogen peroxide, and EDHF in cerebral arteries

We tested the hypotheses that EDHF in rat middle cerebral arteries (MCAs) involves 1) metabolism of arachidonic acid through the epoxygenase pathway, 2) metabolism of arachidonic acid through the lipoxygenase pathway, or 3) reactive oxygen species. EDHF-mediated dilations were elicited in isolated and pressurized rat MCAs by activation of endothelial P2Y2receptors with either UTP or ATP. All studies were conducted after the inhibition of nitric oxide synthase and cyclooxygenase with Nω-nitro-l-arginine methyl ester (10 μM) and indomethacin (10 μM), respectively. The inhibition of epoxygenase with miconazole (30 μM) did not alter EDHF dilations to UTP, whereas the structurally different epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanoic acid (20 or 40 μM) only modestly inhibited EDHF at the highest concentration of UTP. An antagonist of epoxyeicosatrienoic acids, 14,15-epoxyeicosa-5( Z)-enoic acid, had no effect on EDHF dilations to UTP. Chronic inhibition of epoxygenase in the rat with 1-aminobenzotriazol (50 mg/kg twice daily for 5 days) did not alter EDHF dilations. The inhibition of the lipoxygenase pathway with either 10 μM baicalein or 10 μM nordihydroguaiaretic acid produced no major inhibitory effects on EDHF dilations. The combination of superoxide dismutase (200 U/ml) and catalase (140 U/ml) had no effect on EDHF dilations. Neither tiron (10 mM), a cell-permeable scavenger of reactive oxygen species, nor deferoxamine (1 or 10 mM), an iron chelator that blocks the formation of hydroxyl radicals, altered EDHF dilations in rat MCAs. We conclude that EDHF dilations in the rat MCA do not involve the epoxygenase pathway, lipoxygenase pathway, or reactive oxygen species including H2O2.

Exaggerated response to adenosine in kidneys from high salt-fed rats: role of epoxyeicosatrienoic acids

Cytochrome P-450 (CYP)-dependent epoxyeicosatrienoic acids (EETs) dilate rat preglomerular microvessels when adenosine2Areceptors (A2AR) are stimulated. As high salt (HS) intake increases epoxygenase activity and adenosine levels, we hypothesized that renal adenosine responses would be greater in HS-fed rats. Male Sprague-Dawley rats were fed either HS (4.0% NaCl) or normal salt (NS; 0.4% NaCl) diet. On day 8, isolated kidneys were perfused with Krebs' buffer containing indomethacin (10 μM) and l-NAME (200 μM) and preconstricted to ∼150 mmHg with infusion of phenylephrine (10−7M). Renal effluents were extracted for analysis of eicosanoids by gas chromatography-mass spectrometry. Bolus injections of the stable adenosine analog 2-chloroadenosine (2-CA; 0.1–10 μg) resulted in dose-dependent dilation; at 10 μg, perfusion pressure (PP) was lowered to a greater extent in the kidneys of HS rats compared with NS rats (−60 ± 4 vs. −31 ± 8 mmHg; P < 0.05) and the area of response was increased (27 ± 6 vs. 9 ± 4 mm2; P < 0.05), as was EET release (132 ± 23 vs. 38 ± 18 ng; P < 0.05). HS treatment increased A2AR and CYP2C23 protein expression. A selective epoxygenase inhibitor, MS-PPOH (12 μM), significantly reduced the response to 2-CA in HS rats; PP, area of response, and EET release decreased by 40, 70, and 81%, respectively, whereas lesser changes were evident in NS kidneys. Thus the greater vasodilator response to 2-CA seen in kidneys obtained from HS-fed rats was mediated by increased EET release. As EETs are renal vasodilator and natriuretic eicosanoids, interactions between adenosine and EETs may contribute to the adaptive response to HS intake.

Arachidonic acid inhibits epithelial Na channel via cytochrome P450 (CYP) epoxygenase-dependent metabolic pathways

We used the patch-clamp technique to study the effect of arachidonic acid (AA) on epithelial Na channels (ENaC) in the rat cortical collecting duct (CCD). Application of 10 μM AA decreased the ENaC activity defined by NPo from 1.0 to 0.1. The dose–response curve of the AA effect on ENaC shows that 2 μM AA inhibited the ENaC activity by 50%. The effect of AA on ENaC is specific because neither 5,8,11,14-eicosatetraynoic acid (ETYA), a nonmetabolized analogue of AA, nor 11,14,17-eicosatrienoic acid mimicked the inhibitory effect of AA on ENaC. Moreover, inhibition of either cyclooxygenase (COX) with indomethacin or cytochrome P450 (CYP) ω-hydroxylation with N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) failed to abolish the effect of AA on ENaC. In contrast, the inhibitory effect of AA on ENaC was absent in the presence of N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide (MS-PPOH), an agent that inhibits CYP-epoxygenase activity. The notion that the inhibitory effect of AA is mediated by CYP-epoxygenase–dependent metabolites is also supported by the observation that application of 200 nM 11,12-epoxyeicosatrienoic acid (EET) inhibited ENaC in the CCD. In contrast, addition of 5,6-, 8,9-, or 14,15-EET failed to decrease ENaC activity. Also, application of 11,12-EET can still reduce ENaC activity in the presence of MS-PPOH, suggesting that 11,12-EET is a mediator for the AA-induced inhibition of ENaC. Furthermore, gas chromatography mass spectrometry analysis detected the presence of 11,12-EET in the CCD and CYP2C23 is expressed in the principal cells of the CCD. We conclude that AA inhibits ENaC activity in the CCD and that the effect of AA is mediated by a CYP-epoxygenase–dependent metabolite, 11,12-EET.

Dietary K intake regulates the response of apical K channels to adenosine in the thick ascending limb

We used the patch-clamp technique to study the effect of adenosine on the apical 70-pS K channel in the thick ascending limb (TAL) of the rat kidney. Application of 1 μM cyclohexyladenosine (CHA), an adenosine analog, stimulated apical 70-pS K channel activity and increased the product of channel open probability and channel number ( NPo) from 0.34 to 0.7. Also, addition of CGS-21680, a specific A2a adenosine receptor agonist, mimicked the effect of CHA and increased NPo from 0.33 to 0.77. The stimulatory effect of CHA and CGS-21680 was completely blocked by H89, an inhibitor of protein kinase A (PKA), or by inhibition of adenylate cyclase with SQ-22536. This suggests that the stimulatory effect of adenosine analogs is mediated by a PKA-dependent pathway. The effect of adenosine analog was almost absent in the TAL from rats on a K-deficient (KD) diet for 7 days. Application of DDMS, an agent that inhibits cytochrome P-450 hydrolase, not only significantly increased the activity of the 70-pS K channel but also restored the stimulatory effect of CHA on the 70-pS K channel in the TAL from rats on a KD diet. Also, the effect of CHA was absent in the presence of 20-HETE. Inhibition of PKA blocked the stimulatory effect of CHA on the apical 70-pS K channel in the presence of DDMS in the TAL from rats on a KD diet. We conclude that stimulation of adenosine receptor increases the apical 70-pS K channel activity via a PKA-dependent pathway and that the effect of adenosine on the apical 70-pS K channel is suppressed by low-K intake. Moreover, the diminished response to adenosine is the result of increase in 20-HETE formation, which inhibits the cAMP-dependent pathway in the TAL from rats on a KD diet.

Identification of 5,6-trans-Epoxyeicosatrienoic Acid in the Phospholipids of Red Blood Cells

A novel eicosanoid, 5,6-trans-epoxy-8Z,11Z,14Z-eicosatrienoic acid (5,6-trans-EET), was identified in rat red blood cells. Characterization of 5,6-trans-EET in the sn-2 position of the phospholipids was accomplished by hydrolysis with phospholipase A(2) followed by gas chromatography/mass spectrometry as well as electrospray ionization-tandem mass spectrometry analyses. The electron ionization spectrum of 5,6-erythro-dihydroxyeicosatrienoic acid (5,6-erythro-DHET), converted from 5,6-trans-EET in the samples, matches that of the authentic standard. Hydrogenation of the extracted 5,6-erythro-DHET with platinum(IV) oxide/hydrogen resulted in an increase of the molecular mass by 6 daltons and the same retention time shift as an authentic standard in gas chromatography, suggesting the existence of three olefins as well as the 5,6-erythro-dihydroxyl structure in the metabolite. Match of retention times by chromatography indicated identity of the stereochemistry of the red blood cell 5,6-erythro-DHET vis à vis the synthetic standard. High pressure liquid chromatography-electrospray ionization-tandem mass spectrometry analysis of the phospholipase A(2)-hydrolyzed lipid extracts from red blood cells revealed match of the mass spectrum and retention time of the compound with the authentic 5,6-trans-EET standard, providing direct evidence of the existence of 5,6-trans-EET in red blood cells. The presence of other trans-EETs was also demonstrated. The ability of both 5,6-trans-EET and its product 5,6-erythro-DHET to relax preconstricted renal interlobar arteries was significantly greater than that of 5,6-cis-EET. In contrast, 5,6-cis-EET and 5,6-trans-EET were equipotent in their capacity to inhibit collagen-induced rat platelet aggregation, whereas 5,6-erythro-DHET was without effect. We propose that the red blood cells serve as a reservoir for epoxides which on release may act in a vasoregulatory capacity.