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

A renal-cerebral-peripheral sympathetic reflex mediates insulin resistance in chronic kidney disease

Background

Insulin resistance (IR) complicates chronic kidney disease (CKD). We tested the hypothesis that CKD activates a broad reflex response from the kidneys and the white adipose tissue to impair peripheral glucose uptake and investigated the role of salt intake in this process.

Methods

5/6-nephrectomized rats were administered normal- or high-salt for 3 weeks. Conclusions were tested in 100 non-diabetic patients with stage 3–5 CKD.

Findings

High-salt in 5/6-nephrectomized rats decreased insulin-stimulated 2-deoxyglucose uptake >25% via a sympathetic nervous system (SNS) reflex that linked the IR to reactive oxygen species (ROS) and the renin-angiotensin system (RAS) in brain and peripheral tissues. Salt-loading in CKD enhanced inflammation in adipose tissue and skeletal muscle, and enhanced the impairment of insulin signaling and Glut4 trafficking. Denervation of the kidneys or adipose tissue or deafferentation of adipose tissue improved IR >40%. In patients with non-diabetic CKD, IR was positively correlated with salt intake after controlling for cofounders (r = 0.334, P = 0.001) and was linked to activation of the RAS/SNS and to impaired glucose uptake in adipose tissue and skeletal muscle, all of which depended on salt intake.

Interpretation

CKD engages a renal/adipose-cerebral-peripheral sympathetic reflex that activates the RAS/ROS axes to promote IR via local inflammation and impaired Glut4 trafficking that are enhanced by high-salt intake. The findings point to a role for blockade of RAS or α-and-β-adrenergic receptors to reduce IR in patients with CKD.

Fund

National Natural Science Foundation of China.

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.

Maternal high salt diet altered Adenosine-mediated vasodilatation via PKA/BK channel pathway in offspring rats

SCOPE

High salt (HS) diets are related to cardiovascular diseases, and prenatal HS was suggested to increase risks of coronary artery diseases in the offspring. This study tested the hypothesis that prenatal HS may influence Adenosine-induced vasodilatation via protein kinase A (PKA) pathway in coronary arteries.

METHODS AND RESULTS

Sprague-Dawley rats were fed with 8% salt diet for gestation, the control was fed with 0.3% salt diet. Coronary arteries from male adult offspring were tested for K⁺ channels and Adenosine signal pathways. Adenosine-mediated vasodilatation was reduced in coronary arteries in HS. There was no difference in gene expression of A_2A receptors between the two groups. After pretreatment with PKA inhibitor, vasodilatation to Adenosine was decreased to a smaller extent in HS than that in control. Forskolin (activator of adenylate cyclase)-mediated vasodilatation was decreased in HS. Iberiotoxin (large-conductance Ca²⁺ -activated K⁺ channel [BK channel] inhibitor) attenuated Forskolin-induced vasodilatation in control, not in HS group. Currents of BK channels decreased in coronary artery smooth muscle cells, and PKA-modulated BK channel functions were declined. Protein levels of BK β1 and PKA C-subunits in coronary arteries of HS offspring were reduced.

CONCLUSIONS

Prenatal HS diets altered Adenosine-mediated coronary artery vasodilatation in the offspring, which was linked to downregulation of cAMP/PKA/BK channel pathway.

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.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

N-acetyl-Ser-Asp-Lys-Pro inhibits interleukin-1β-mediated matrix metalloproteinase activation in cardiac fibroblasts

Myocardial matrix turnover involves a dynamic balance between collagen synthesis and degradation, which is regulated by matrix metalloproteinases (MMPs). N-acetyl-Ser-Asp-Lys-Pro (Ac-SDKP) is a small peptide that inhibits cardiac inflammation and fibrosis. However, its role in MMP regulation is not known. Thus, we hypothesized that Ac-SDKP promotes MMP activation in cardiac fibroblasts and decreases collagen deposition via this mechanism. To that end, we tested the effects of Ac-SDKP on interleukin-1β (IL-1β; 5 ng/ml)-stimulated adult rat cardiac fibroblasts. We measured total collagenase activity, MMP-2, MMP-9, and MMP-13 expressions, and activity along with their inhibitors, tissue inhibitor of metalloproteinase (TIMP)-1 and TIMP-2. In order to examine the effects of Ac-SDKP on the signaling pathway that controls MMP transcription, we also measured nuclear factor-κB (NFκB) and p42/44 mitogen-activated protein kinase (MAPK) activation. Ac-SDKP did not alter collagenase or gelatinase activity in cardiac fibroblasts under basal conditions, but blunted the IL-1β-induced increase in total collagenase activity. Similarly, Ac-SDKP normalized the IL-1β-mediated increase in MMP-2 and MMP-9 activities and MMP-13 expression. Inhibition of MMPs by Ac-SDKP was associated with increased TIMP-1 and TIMP-2 expressions. Collagen production was not affected by Ac-SDKP, IL-1β, or a combination of both agents. Ac-SDKP blocked IL-1β-induced p42/44 phosphorylation and NFκB activation in cardiac fibroblasts. We concluded that the Ac-SDKP-inhibited collagenase expression and activation was associated with increased expression of TIMP-1 and TIMP-2. These pharmacological effects of Ac-SDKP may be linked to the inhibition of MAPK and NFκB pathway.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

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.

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.

Further evidence supporting a role for gs signal transduction in severe malaria pathogenesis

With the functional demonstration of a role in erythrocyte invasion by Plasmodium falciparum parasites, implications in the aetiology of common conditions that prevail in individuals of African origin, and a wealth of pharmacological knowledge, the stimulatory G protein (Gs) signal transduction pathway presents an exciting target for anti-malarial drug intervention. Having previously demonstrated a role for the G-alpha-s gene, GNAS, in severe malaria disease, we sought to identify other important components of the Gs pathway. Using meta-analysis across case-control and family trio (affected child and parental controls) studies of severe malaria from The Gambia and Malawi, we sought evidence of association in six Gs pathway candidate genes: adenosine receptor 2A (ADORA2A) and 2B (ADORA2B), beta-adrenergic receptor kinase 1 (ADRBK1), adenylyl cyclase 9 (ADCY9), G protein beta subunit 3 (GNB3), and regulator of G protein signalling 2 (RGS2). Our study amassed a total of 2278 cases and 2364 controls. Allele-based models of association were investigated in all genes, and genotype and haplotype-based models were investigated where significant allelic associations were identified. Although no significant associations were observed in the other genes, several were identified in ADORA2A. The most significant association was observed at the rs9624472 locus, where the G allele (∼20% frequency) appeared to confer enhanced risk to severe malaria [OR = 1.22 (1.09–1.37); P = 0.001]. Further investigation of the ADORA2A gene region is required to validate the associations identified here, and to identify and functionally characterize the responsible causal variant(s). Our results provide further evidence supporting a role of the Gs signal transduction pathway in the regulation of severe malaria, and request further exploration of this pathway in future studies.

Inhibition of the adenosine2A receptor-epoxyeicosatrienoic acid pathway renders Dahl salt-resistant rats hypertensive

Adenosine-induced renovasodilation in Dahl rats is mediated via activation of adenosine(2A) receptors (A(2A)Rs) and stimulation of epoxyeicosatrienoic acid (EET) synthesis. Unlike Dahl salt-resistant rats, salt-sensitive rats exhibit an inability to upregulate the A(2A)R-EET pathway with salt loading; therefore, we examined the effect of in vivo inhibition of the A(2A)R-EET pathway on blood pressure and the natriuretic response to salt-loading in Dahl salt-resistant rats. N-Methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH; 20 mg/kg per day), an epoxygenase inhibitor, or ZM241385 (ZM; 5 mg/kg per day), an A(2A)R antagonist, was given daily as an IV bolus dose for 3 days before and after placing rats on high salt intake (2% saline). After 3 days of high salt, systolic blood pressure per 24 hours increased from 108+/-2 mm Hg to 136+/-5 mm Hg and 140+/-4 mm Hg when treated with MS-PPOH or ZM, respectively (P<0.001). Plasma levels of EETs and dihydroxyeicosatrienoic acids during salt loading and MS-PPOH (29.3+/-1.8 ng/mL) or ZM treatment (9.8+/-0.5 ng/mL) did not increase to the same extent as in vehicle-treated rats (59.4+/-1.7 ng/mL; P<0.001), and renal levels of EETs+dihydroxyeicosatrienoic acids were 2-fold lower with MS-PPOH or ZM treatment. On day 3 of the high salt intake, MS-PPOH- and ZM-treated rats exhibited a positive Na(+) balance, and plasma Na(+) levels were significantly increased (163.3+/-1.2 and 158.1+/-4.5 mEq/L, respectively) compared with vehicle-treated rats (142.1+/-1 mEq/L), reflecting a diminished natriuretic capacity. These data support a role for the A(2A)R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading.

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.

Failure to upregulate the adenosine2A receptor-epoxyeicosatrienoic acid pathway contributes to the development of hypertension in Dahl salt-sensitive rats

Adenosine-activated renovascular dilatation in Sprague-Dawley (SD) rats is mediated by stimulating adenosine(2A) receptors (A(2A)R), which is linked to epoxyeicosatrienoic acid (EET) synthesis. The A(2A)R-EET pathway is upregulated by high salt (HS) intake in normotensive SD rats. Because this pathway is antipressor, we examined the role of the A(2A)R-EET pathway in Dahl salt-sensitive (SS) rats. Male Dahl salt-resistant (SR) and SS rats were fed either HS (8.0% NaCl) or normal salt (NS; 0.4% NaCl) diet for 7 days. On day 8, isolated kidneys were perfused with Krebs-Henseleit buffer containing indomethacin and N(G)-nitro-l-arginine methyl ester and preconstricted with phenylephrine. Bolus injections of the stable adenosine analog 2-chloroadenosine (2-CA; 0.1-20 microg) elicited dose-dependent dilation in both Dahl SR and SS rats. Dahl SR rats fed a HS diet demonstrated a greater renal vasodilator response to 10 microg of 2-CA, as measured by the reduction in renal perfusion pressure, than that of Dahl SR rats fed a NS diet (-104 +/- 6 vs. -77 +/- 7 mmHg, respectively; P < 0.05). In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed NS or HS diet (96 +/- 6 vs. 104 +/- 13 mmHg in NS- and HS-fed rats, respectively). In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A)R and the cytochrome P-450 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, and this 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.

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.