Inhibitors of cytochrome P-450 attenuate the myogenic response of dog renal arcuate arteries

The potential mechanisms underlying the modulating effect of perirenal adipose tissue on hypertension: Physical compression, paracrine, and neurogenic regulation

Hypertension, a prevalent global cardiovascular disease, affects approximately 45.4 % of adults worldwide. Despite advances in therapy, hypertension continues to pose a significant health risk due to inadequate management. It has been established that excessive adiposity contributes majorly to hypertension, accounting for 65 to 75 % of primary cases. Fat depots can be categorised into subcutaneous and visceral adipose tissue based on anatomical and physiological characteristics. The metabolic impact and the risk of hypertension are determined more significantly by visceral fat. Perirenal adipose tissue (PRAT), a viscera enveloping the kidney, is known for its superior vascularisation and abundant innervation. Although traditionally deemed as a mechanical support tissue, recent studies have indicated its contributing potential to hypertension. Hypertensive patients tend to have increased PRAT thickness compared to those without, and there is a positive correlation between PRAT thickness and elevated systolic blood pressure. This review encapsulates the anatomical characteristics and biogenesis of PRAT. We provide an overview of the potential mechanisms where PRAT may modulate blood pressure, including physical compression, paracrine effects, and neurogenic regulation. PRAT has become a promising target for hypertension management, and continuous effort is required to further explore the underlying mechanisms.

20-HETE in the regulation of vascular and cardiac function

20-HETE, the ω-hydroxylation product of arachidonic acid catalyzed by enzymes of the cytochrome P450 (CYP) 4A and 4F gene families, is a bioactive lipid mediator with potent effects on the vasculature including stimulation of smooth muscle cell contractility, migration and proliferation as well as activation of endothelial cell dysfunction and inflammation. Clinical studies have shown elevated levels of plasma and urinary 20-HETE in human diseases and conditions such as hypertension, obesity and metabolic syndrome, myocardial infarction, stroke, and chronic kidney diseases. Studies of polymorphic associations also suggest an important role for 20-HETE in hypertension, stroke and myocardial infarction. Animal models of increased 20-HETE production are hypertensive and are more susceptible to cardiovascular injury. The current review summarizes recent findings that focus on the role of 20-HETE in the regulation of vascular and cardiac function and its contribution to the pathology of vascular and cardiac diseases.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Clinical Implications of 20-Hydroxyeicosatetraenoic Acid in the Kidney, Liver, Lung and Brain: An Emerging Therapeutic Target

Cytochrome P450-mediated metabolism of arachidonic acid (AA) is an important pathway for the formation of eicosanoids. The ω-hydroxylation of AA generates significant levels of 20-hydroxyeicosatetraenoic acid (20-HETE) in various tissues. In the current review, we discussed the role of 20-HETE in the kidney, liver, lung, and brain during physiological and pathophysiological states. Moreover, we discussed the role of 20-HETE in tumor formation, metabolic syndrome and diabetes. In the kidney, 20-HETE is involved in modulation of preglomerular vascular tone and tubular ion transport. Furthermore, 20-HETE is involved in renal ischemia/reperfusion (I/R) injury and polycystic kidney diseases. The role of 20-HETE in the liver is not clearly understood although it represents 50%-75% of liver CYP-dependent AA metabolism, and it is associated with liver cirrhotic ascites. In the respiratory system, 20-HETE plays a role in pulmonary cell survival, pulmonary vascular tone and tone of the airways. As for the brain, 20-HETE is involved in cerebral I/R injury. Moreover, 20-HETE has angiogenic and mitogenic properties and thus helps in tumor promotion. Several inhibitors and inducers of the synthesis of 20-HETE as well as 20-HETE analogues and antagonists are recently available and could be promising therapeutic options for the treatment of many disease states in the future.

Elevated 20-HETE impairs coronary collateral growth in metabolic syndrome via endothelial dysfunction

Coronary collateral growth (CCG) is impaired in metabolic syndrome (MetS). microRNA-145 (miR-145-Adv) delivery to our rat model of MetS (JCR) completely restored and neutrophil depletion significantly improved CCG. We determined whether low endogenous levels of miR-145 in MetS allowed for elevated production of 20-hydroxyeicosatetraenoic acid (20-HETE), which, in turn, resulted in excessive neutrophil accumulation and endothelial dysfunction leading to impaired CCG. Rats underwent 0-9 days of repetitive ischemia (RI). RI-induced cardiac CYP4F (neutrophil-specific 20-HETE synthase) expression and 20-HETE levels were increased (4-fold) in JCR vs. normal rats. miR-145-Adv and 20-HETE antagonists abolished and neutrophil depletion (blocking antibodies) reduced (~60%) RI-induced increases in CYP4F expression and 20-HETE production in JCR rats. Impaired CCG in JCR rats (collateral-dependent blood flow using microspheres) was completely restored by 20-HETE antagonists [collateral-dependent zone (CZ)/normal zone (NZ) flow ratio was 0.76 ± 0.07 in JCR + 20-SOLA, 0.84 ± 0.05 in JCR + 20-HEDGE vs. 0.11 ± 0.02 in JCR vs. 0.84 ± 0.03 in normal rats]. In JCR rats, elevated 20-HETE was associated with excessive expression of endothelial adhesion molecules and neutrophil infiltration, which were reversed by miR-145-Adv. Endothelium-dependent vasodilation of coronary arteries, endothelial nitric oxide synthase (eNOS) Ser1179 phosphorylation, eNOS-dependent NO^·- production and endothelial cell survival were compromised in JCR rats. These parameters of endothelial dysfunction were completely reversed by 20-HETE antagonism or miR-145-Adv delivery, whereas neutrophil depletion resulted in partial reversal (~70%). We conclude that low miR-145 in MetS allows for increased 20-HETE, mainly from neutrophils, which compromises endothelial cell survival and function leading to impaired CCG. 20-HETE antagonists could provide viable therapy for restoration of CCG in MetS.NEW & NOTEWORTHY Elevated 20-hydroxyeicosatetraenoic acid (20-HETE) impairs coronary collateral growth (CCG) in metabolic syndrome by eliciting endothelial dysfunction and apoptosis via excessive neutrophil infiltration. 20-HETE antagonists completely restore coronary collateral growth in metabolic syndrome. microRNA-145 (miR-145) is an upstream regulator of 20-HETE production in metabolic syndrome; low expression of miR-145 in metabolic syndrome promotes elevated production of 20-HETE.

The role of mid-chain hydroxyeicosatetraenoic acids in the pathogenesis of hypertension and cardiac hypertrophy

The incidence, prevalence, and hospitalization rates associated with cardiovascular diseases (CVDs) are projected to increase substantially in the world. Understanding of the biological and pathophysiological mechanisms of survival can help the researchers to develop new management modalities. Numerous experimental studies have demonstrated that mid-chain HETEs are strongly involved in the pathogenesis of the CVDs. Mid-chain HETEs are biologically active eicosanoids that result from the metabolism of arachidonic acid (AA) by both lipoxygenase and CYP1B1 (lipoxygenase-like reaction). Therefore, identifying the localizations and expressions of the lipoxygenase and CYP1B1 and their associated AA metabolites in the cardiovascular system is of major importance in understanding their pathological roles. Generally, the expression of these enzymes is shown to be induced during several CVDs, including hypertension and cardiac hypertrophy. The induction of these enzymes is associated with the generation of mid-chain HETEs and subsequently causation of cardiovascular events. Of interest, inhibiting the formation of mid-chain HETEs has been reported to confer a protection against different cardiac hypertrophy and hypertension models such as angiotensin II, Goldblatt, spontaneously hypertensive rat and deoxycorticosterone acetate (DOCA)-salt-induced models. Although the exact mechanisms of mid-chain HETEs-mediated cardiovascular dysfunction are not fully understood, the present review proposes several mechanisms which include activating G-protein-coupled receptor, protein kinase C, mitogen-activated protein kinases, and nuclear factor kappa B. This review provides a clear understanding of the role of mid-chain HETEs in the pathogenesis of cardiovascular diseases and their importance as novel targets in the treatment for hypertension and cardiac hypertrophy.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

In vivo radioactive metabolite analysis for individualized medicine: a basic study of a new method of CYP activity assay using (123)I-IMP

INTRODUCTION

(123)I-N-isopropyl-p-iodoamphetamine ((123)I-IMP) is metabolized and converted to (123)I-p-iodoamphetamine ((123)I-PIA) by CYP2C19 in humans. Since variations in (123)I-PIA levels reflect variations in CYP2C19 activity, CYP2C19 activity can be estimated by quantitative analysis of (123)I-PIA levels. Thus, (123)I-IMP administration can provide diagnostic information not only regarding cerebral blood flow (rCBF) but also regarding metabolic function. The aim of the present study was to detect variations in CYP activity in mice using metabolite analysis.

METHODS

Metabolism of (125)I-IMP in pooled homogenates of mouse liver (MLH) was analyzed by high-performance liquid chromatography (HPLC) in the presence or absence of NADPH. The amount of (125)I-PIA generated was calculated as the normalized peak area of the chromatogram. Inhibition of (125)I-IMP metabolism was evaluated using the inhibitor SKF-525A. A biodistribution study of (125)I-IMP was performed to determine the organ distribution of (125)I-IMP/(125)I-IMP metabolites and the effect of SKF-525A. Variations in CYP activity in vivo were detected by administration of (123)I-IMP and/or SKF-525A to mice. The liver and the kidney were then excised, homogenized and analyzed using HPLC.

RESULTS

(125)I-IMP was metabolized by MLH in the presence of NADPH, and the production of (125)I-PIA was inhibited by SKF-525A. SKF-525A did not greatly affect the biodistribution of (125)I-IMP/(125)I-IMP metabolites in vivo. Both (123)I-IMP and (123)I-PIA were detected in organs of control mice. However, (123)I-PIA was not detected in the livers or kidneys of mice treated with SKF-525A.

CONCLUSIONS

CYP activity in vivo was inhibited by SKF-525A treatment. Variations in CYP activity could be detected in the blood, liver and kidney as changes in the peak area of (123)I-PIA.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

(123)I-IMP metabolite analysis has the potential to provide beneficial information for prediction of the effect of medicines, in addition to its contribution to more accurate rCBF diagnosis that reflects individual CYP activity.

Mechanism of impaired afferent arteriole myogenic response in Dahl salt-sensitive rats: role of 20-HETE

The afferent arteriole (Af-Art) controls glomerular capillary pressure, an important determinant of glomerular injury. Af-Art myogenic response is mediated by ATP, and ATP signaling is in turn mediated by 20-HETE. Dahl salt-sensitive rats (Dahl SS) have decreased renal 20-HETE production. We hypothesized that Dahl SS have an impaired myogenic response and constrictor response to ATP, due to decreased 20-HETE. Af-Arts from Dahl SS or Dahl salt-resistant rats (Dahl SR) were microdissected and perfused. When myogenic response was induced by increasing Af-Art perfusion pressure from 60 to 140 mmHg, luminal Af-Art diameter decreased in Dahl SR but not in Dahl SS (-3.1 ± 0.8 vs. 0.5 ± 0.8 μm, P < 0.01). The 20-HETE antagonist 20-HEDE (10(-6) M) blocked the myogenic response in Dahl SR but had no effect in Dahl SS. Addition of a subconstrictor concentration of 20-HETE (but not a subconstrictor concentration of norepinephrine) restored the myogenic response in Dahl SS. We then perfused Af-Arts at 60 mmHg and tested the effects of the ATP analog α,β-methylene-ATP (10(-6) M). Maximum ATP-induced constriction was attenuated in Dahl SS compared with Dahl SR (1.5 ± 0.5 vs. 7.4 ± 0.8 μm, P < 0.001). 20-HEDE attenuated ATP-induced Af-Art constriction in Dahl SR but not in Dahl SS, and consequently, ATP-induced constriction was no longer different between strains. In conclusion, Dahl SS have an impaired myogenic response and ATP-induced Af-Art constriction due to a decrease in Af-Art 20-HETE. The impaired myogenic responses may contribute to the nephrosclerosis that develops in Dahl SS.

Role of cytochrome P450-mediated arachidonic acid metabolites in the pathogenesis of cardiac hypertrophy

A plethora of studies have demonstrated the expression of cytochrome P450 (CYP) and soluble epoxide hydrolase (sEH) enzymes in the heart and other cardiovascular tissues. In addition, the expression of these enzymes is altered during several cardiovascular diseases (CVDs), including cardiac hypertrophy (CH). The alteration in CYP and sEH expression results in derailed CYP-mediated arachidonic acid (AA) metabolism. In animal models of CH, it has been reported that there is an increase in 20-hydroxyeicosatetraenoic acid (20-HETE) and a decrease in epoxyeicosatrienoic acids (EETs). Further, inhibiting 20-HETE production by CYP ω-hydroxylase inhibitors and increasing EET stability by sEH inhibitors have been proven to protect against CH as well as other CVDs. Therefore, CYP-mediated AA metabolites 20-HETE and EETs are potential key players in the pathogenesis of CH. Some studies have investigated the molecular mechanisms by which these metabolites mediate their effects on cardiomyocytes and vasculature leading to pathological CH. Activation of several intracellular signaling cascades, such as nuclear factor of activated T cells, nuclear factor kappa B, mitogen-activated protein kinases, Rho-kinases, Gp130/signal transducer and activator of transcription, extracellular matrix degradation, apoptotic cascades, inflammatory cytokines, and oxidative stress, has been linked to the pathogenesis of CH. In this review, we discuss how 20-HETE and EETs can affect these signaling pathways to result in, or protect from, CH, respectively. However, further understanding of these metabolites and their effects on intracellular cascades will be required to assess their potential translation to therapeutic approaches for the prevention and/or treatment of CH and heart failure.

Capitalizing on diversity: an integrative approach towards the multiplicity of cellular mechanisms underlying myogenic responsiveness

The intrinsic ability of resistance arteries to respond to transmural pressure is the single most important determinant of their function. Despite an ever-growing catalogue of signalling pathways that underlie the myogenic response, it remains an enigmatic mechanism. The myogenic response's mechanistic diversity has largely been attributed to 'hard-wired' differences across species and vascular beds; however, emerging evidence suggests that the mechanistic basis for the myogenic mechanism is, in fact, 'plastic'. This means that the myogenic response can change quantitatively (i.e. change in magnitude) and qualitatively (i.e. change in mechanistic basis) in response to environmental challenges (e.g. disease conditions). Consequently, understanding the dynamics of how the myogenic response capitalizes on its mechanistic diversity is key to unlocking clinically viable interventions. Using myogenic sphingosine-1-phosphate (S1P) signalling as an example, this review illustrates the remarkable plasticity of the myogenic response. We propose that currently unidentified 'organizational programmes' dictate the contribution of individual signalling pathways to the myogenic response and introduce the concept that certain signalling elements act as 'divergence points' (i.e. as the potential higher level regulatory sites). In the context of pressure-induced S1P signalling, the S1P-generating enzyme sphingosine kinase 1 serves as a divergence point, by orchestrating the calcium-dependent and -independent signalling pathways underlying microvascular myogenic responsiveness. By acting on divergence points, the proposed 'organizational programmes' could form the basis for the flexible recruitment and fine-tuning of separate signalling streams that underlie adaptive changes to the myogenic response and its distinctiveness across species and vascular beds.

Transduction of physical force by the vascular wall Role of phospholipase C and cytochrome P450 metabolites of arachidonic acid

The blood vessel wall responds actively to an elevation in transmural pressure. This pressure-induced myogenic response is thought to set the basal level of vascular tone upon which metabolic and neural influences operate in concert to regulate organ blood flow. The cellular mechanisms that mediate the vascular muscle response to mechanical deformation via a changing transmural pressure include membrane depolarization, activation of phospholipase C, and a rise in intracellular [Ca(2+)](i), which appear to be nonadapting-remaining active as long as the pressure stimulus is applied. This brief review addresses some of the cellular events mediating transduction of transmural pressure by the vessel wall. Two possible mechanisms that are responsible for the nonadapting nature of pressure-induced myogenic tone are also explored, namely, formation of a P450 metabolite of arachidonic acid, which acts to buffer activation of K(+) channels as intracellular Ca(2+) rises, and direct activation of Ca(2+) channels by diacylglycerol. Evidence is provided suggesting that activation of phospholipase C is responsible for both the release of the arachidonic acid substrate for P450 enzymes and for the formation of diacylglycerol via its action on membrane-bound phospholipids.

Pressure-induced myogenic tone and role of 20-HETE in mediating autoregulation of cerebral blood flow

While myogenic force in response to a changing arterial pressure has been described early in the 20th century, it was not until 1984 that the effect of a sequential increase in intraluminal pressure on cannulated cerebral arterial preparations was found to result in pressure-dependent membrane depolarization associated with spike generation and reduction in lumen diameter. Despite a great deal of effort by different laboratories and investigators, the identification of the existence of a mediator of the pressure-induced myogenic constriction in arterial muscle remained a challenge. It was the original finding by our laboratory that demonstrated the capacity of cerebral arterial muscle cells to express the cytochrome P-450 4A enzyme that catalyzes the formation of the potent vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE) from arachidonic acid, the production of which in cerebral arterial muscle cells increases with the elevation in intravascular pressure. 20-HETE activates protein kinase C and causes the inhibition of Ca(²+)-activated K(+) channels, depolarizes arterial muscle cell membrane, and activates L-type Ca(²+) channel to increase intracellular Ca(²+) levels and evoke vasoconstriction. The inhibition of 20-HETE formation attenuates pressure-induced arterial myogenic constriction in vitro and blunts the autoregulation of cerebral blood flow in vivo. We suggest that the formation and action of cytochrome P-450-derived 20-HETE in cerebral arterial muscle could play a critically important role in the control of cerebral arterial tone and the autoregulation of cerebral blood flow under physiological conditions.

20-hydroxyeicosatetraeonic acid: a new target for the treatment of hypertension

Arachidonic acid is metabolized by enzymes of the CYP4A and 4F families to 20-hydroxyeicosatetraeonic acid (20-HETE), which plays an important role in the regulation of renal function, vascular tone, and the long-term control of arterial pressure. In the vasculature, 20-HETE is a potent vasoconstrictor, and upregulation of the production of this compound contributes to the elevation in oxidative stress and endothelial dysfunction and the increase in peripheral vascular resistance associated with some forms of hypertension. In kidney, 20-HETE inhibits Na transport in the proximal tubule and thick ascending loop of Henle, and deficiencies in the renal formation of 20-HETE contributes to sodium retention and development of some salt-sensitive forms of hypertension. 20-HETE also has renoprotective actions and opposes the effects of transforming growth factor β to promote proteinuria and renal end organ damage in hypertension. Several new inhibitors of the synthesis of 20-HETE and 20-HETE agonists and antagonists have recently been developed. These compounds along with peroxisome proliferator-activated receptor-α agonists that induce the renal formation of 20-HETE seem to have promise as antihypertensive agents. This review summarizes the rationale for the development of drugs that target the 20-HETE pathway for the treatment of hypertension and associated cardiovascular complications.

Endothelial-specific CYP4A2 overexpression leads to renal injury and hypertension via increased production of 20-HETE

We have previously reported that adenoviral-mediated delivery of cytochrome P-450 (CYP) 4A2, which catalyzes the synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE), results in endothelial dysfunction and hypertension in Sprague-Dawley (SD) rats (Wang JS, Singh H, Zhang F, Ishizuka T, Deng H, Kemp R, Wolin MS, Hintze TH, Abraham NG, Nasjletti A, Laniado-Schwartzman M. Circ Res 98: 962-969, 2006). In this study, we targeted the vascular endothelium by using a lentivirus construct expressing CYP4A2 under the control of the endothelium-specific promoter VE-cadherin (VECAD-4A2) and examined the effect of long-term CYP4A2 overexpression on blood pressure and kidney function in SD rats. A bolus injection of VECAD-4A2 increased blood pressure (P < 0.001) by 26, 36, and 30 mmHg 10, 20, and 30 days postinjection, respectively. Arteries from VECAD-4A2-transduced rats produced increased levels of 20-HETE (P < 0.01), expressed lower levels of endothelial nitric oxide synthase (eNOS) and phosphorylated eNOS (p-eNOS) (P < 0.05), generated higher levels of superoxide anion, and displayed decreased relaxing responsiveness to acetylcholine (P < 0.05). Proteinuria increased by twofold in VECAD-4A2-transduced rats compared with controls. Treatment of VECAD-4A2-transduced rats with HET0016, an inhibitor of 20-HETE biosynthesis, not only attenuated the increase in blood pressure (P < 0.05) but also improved vascular function (acetylcholine-induced relaxations) and reduced plasma creatinine and proteinuria. HET0016 treatment decreased oxidative stress and increased the phosphorylated state of key proteins that regulate endothelial function, including eNOS, AKT, and AMPK. Collectively, these findings demonstrate that augmentation of vascular endothelial 20-HETE levels results in hypertension, endothelial dysfunction, and renal injury, which is offset by HET0016 through a reduction in vascular 20-HETE coupled with a lessening of oxidative stress and the amplification of pAKT, pAMPK, and p-eNOS levels leading to normalization of endothelial responses.

Deficient renal 20-HETE release in the diabetic rat is not the result of oxidative stress

We confirmed that release of 20-hydroxyeicosatetraenoic acid (20-HETE) from the isolated perfused kidney of diabetic rats is greatly reduced compared with age-matched control rats. The present studies were undertaken to examine potential mechanisms for the deficit in renal 20-HETE in rats with streptozotocin-induced diabetes of 3-4 wk duration. A role for oxidative stress was excluded, inasmuch as treatment of diabetic rats with tempol, an SOD mimetic, for 4 wk did not affect the renal release of 20-HETE. Similarly, chronic inhibition of nitric oxide formation with nitro-l-arginine methyl ester or aldose reductase with zopolrestat failed to alter the release of 20-HETE from the diabetic rat kidney. Inasmuch as 20-HETE may be metabolized by cyclooxygenase (COX), the expression/activity of which is increased in diabetes, we included indomethacin in the perfusate of the isolated kidney to inhibit COX but found no effect on 20-HETE release. Diabetic rats were treated for 3 wk with fenofibrate to increase expression of cytochrome P-450 (CYP4A) in an attempt to find an intervention that would restore release of 20-HETE from the diabetic rat kidney. However, fenofibrate reduced 20-HETE release in diabetic and control rat kidneys but increased expression of CYP4A. Only insulin treatment of diabetic rats for 2 wk to reverse the hyperglycemia and maintain blood glucose levels at <200 mg/dl reversed the renal deficit in 20-HETE. We conclude that oxidative stress, increased aldose reductase activity, or increased COX activity does not contribute to the renal deficit of 20-HETE in diabetes, which may be directly related to insulin deficiency.

The T8590C polymorphism of CYP4A11 and 20-hydroxyeicosatetraenoic acid in essential hypertension

A role for a deficit in transport actions of 20-hydroxyeicosatetraenoic acid (20-HETE) in hypertension is supported by the following: (1) diminished renal 20-HETE in Dahl-S rats; (2) altered salt- and furosemide-induced 20-HETE responses in salt-sensitive hypertensive subjects; and (3) increased population risk for hypertension in C allele carriers of the T8590C polymorphism of CYP4A11, which encodes an enzyme with reduced catalytic activity. We determined T8590C genotypes in 32 hypertensive subjects, 25 of whom were phenotyped for salt sensitivity of blood pressure and insulin sensitivity. Urine 20-HETE was lowest in insulin-resistant, salt-sensitive subjects (F=5.56; P<0.02). Genotypes were 13 TT, 2 CC, and 17 CT. C allele frequency was 32.8% (blacks: 38.9%; whites: 25.0%). C carriers (CC+CT) and TT subjects were similarly distributed among salt- and insulin-sensitivity phenotypes. C carriers had higher diastolic blood pressures and aldosterone:renin and waist:hip ratios but lower furosemide-induced fractional excretions of Na and K than TT. The T8590C genotype did not relate to sodium balance or pressure natriuresis. However, C carriers, compared with TT, had diminished 20-HETE responses to salt loading after adjustment for serum insulin concentration and resetting of the negative relationship between serum insulin and urine 20-HETE to a 1-microg/h lower level of 20-HETE. The effect of C was insulin independent and equipotent to 18 microU/mL of insulin (Delta20-HETE= 2.84-0.054xinsulin-0.98xC; r(2)=0.53; F=11.1; P<0.001). Hence, genetic (T8590C) and environmental (insulin) factors impair 20-HETE responses to salt in human hypertension. We propose that genotype analyses with sufficient homozygous CC will establish definitive relationships among 20-HETE, salt sensitivity of blood pressure, and insulin resistance.

Physiologic and pathophysiologic roles of lipid mediators in the kidney

Small lipids such as eicosanoids exert diverse and complex functions. In addition to their role in regulating normal kidney function, these lipids also play important roles in the pathogenesis of kidney diseases. Cyclooxygenase (COX)-derived prostanoids play important role in maintaining renal function, body fluid homeostasis, and blood pressure. Renal cortical COX2-derived prostanoids, particularly (PGI2) and PGE2 play critical roles in maintaining blood pressure and renal function in volume contracted states. Renal medullary COX2-derived prostanoids appear to have antihypertensive effect in individuals challenged with a high salt diet. 5-Lipoxygenase (LO)-derived leukotrienes are involved in inflammatory glomerular injury. LO product 12-hydroxyeicosatetraenoic acid (12-HETE) is associated with pathogenesis of hypertension, and may mediate angiotensin II and TGFbeta induced mesengial cell abnormality in diabetic nephropathy. P450 hydroxylase-derived 20-HETE is a potent vasoconstrictor and is involved in the pathogenesis of hypertension. P450 epoxygenase derived epoxyeicosatrienoic acids (EETs) have vasodilator and natriuretic effect. Blockade of EET formation is associated with salt-sensitive hypertension. Ceramide has also been demonstrated to be an important signaling molecule, which is involved in pathogenesis of acute kidney injury caused by ischemia/reperfusion, and toxic insults. Those pathways should provide fruitful targets for intervention in the pharmacologic treatment of renal disease.

Reactive oxygen species cerebral autoregulation in health and disease

Reactive oxygen species (ROS) are a family of oxygen-derived free radicals that are produced in mammalian cells under normal and pathologic conditions. Many ROS, such as the superoxide anion (O2-) and hydrogen peroxide (H2O2), act as cellular signaling molecules within blood vessels, altering mechanisms mediating mechanical signal transduction and autoregulation of cerebral blood flow. This article focuses on the actions of ROS, such as O2.- and H2O2, and how they influence mechanisms responsible for the modulation of pressure-induced myogenic tone in the cerebral circulation and blood flow autoregulation in response to elevated arterial pressure. ROS may be a key target for therapeutic interventions in pediatric patients who have hypoxic injury or altered cerebral metabolism induced by trauma or infection.

Oxygenation of omega-3 fatty acids by human cytochrome P450 4F3B: effect on 20-hydroxyeicosatetraenoic acid production

Cytochrome P450 (CYP) omega-oxidases convert arachidonic acid (AA) to 20-hydroxyeicosatetraenoic acid (20-HETE), a lipid mediator that modulates vascular tone. We observed that a microsomal preparation containing recombinant human CYP4F3B, which converts AA to 20-HETE, converted eicosapentaenoic acid (EPA) to 20-OH-EPA. Likewise, docosahexaenoic acid (DHA) was converted to 22-OH-DHA, indicating that human CYP4F3B also can oxidize 22-carbon omega-3 fatty acids. Consistent with these findings, addition of 0.5-5 microM EPA, DHA or omega-3 docosapentaenoic acid (DPA) to incubations containing 0.5 microM [3H]AA inhibited [3H]20-HETE production by 15-65%. [3H]20-OH-EPA was rapidly taken up by COS-7 cells, and almost all of the incorporated radioactivity remained as unmodified 20-OH-EPA. The 20-OH-EPA stimulated luciferase activity in COS-7 cells that express peroxisome proliferator-activated receptor alpha, indicating that this EPA metabolite may function as a lipid mediator. These findings suggest that some functional effects of omega-3 fatty acid supplementation may be due to inhibition of 20-HETE formation or the conversion of EPA to the corresponding omega-oxidized product.

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.

From endothelium-derived hyperpolarizing factor (EDHF) to angiogenesis: Epoxyeicosatrienoic acids (EETs) and cell signaling

Epoxyeicosatrienoic acids (EETs) are generated from arachidonic acid by cytochrome P450 (CYP) epoxygenases. The expression of CYP epoxygenases in endothelial cells is determined by a number of physical (fluid shear stress and cyclic stretch) and pharmacological stimuli as well as by hypoxia. The activation of CYP epoxygenases in endothelial cells is an important step in the nitric oxide and prostacyclin (PGI2)-independent vasodilatation of several vascular beds and EETs have been identified as endothelium-derived hyperpolarizing factors (EDHFs). However, in addition to regulating vascular tone, EETs modulate several signaling cascades and affect cell proliferation, cell migration, and angiogenesis. Signaling molecules modulated by EETs include tyrosine kinases and phosphatases, mitogen-activated protein kinases, protein kinase A (PKA), cyclooxygenase (COX)-2, and several transcription factors. This review summarizes the role of CYP-derived EETs in cell signaling and focuses particularly on their role as intracellular amplifiers of endothelial cell hyperpolarization as well as in cell proliferation and angiogenesis. The angiogenic properties of CYP epoxygenases and CYP-derived EETs implicate that these enzymes may well be accessible targets for anti-angiogenic as well as angiogenic therapies.

Role of Ca2+-independent phospholipase A2 and cytochrome P-450 in store-operated calcium entry in 3T6 fibroblasts

Store-operated calcium (SOC) channels and capacitative Ca2+ entry play a key role in cellular functions, but their mechanism of activation remains unclear. Here, we show that thapsigargin induces [3H] arachidonic acid (AA) release, 45Ca2+ influx and a subsequent enhancement of intracellular calcium concentration ([Ca2+]i. Thapsigargin-induced elevation of [Ca2+]i was inhibited by cytochrome P-450 inhibitors and by cytochrome P-450 epoxygenase inhibitor and was reverted by 11,12 EET addition. However, cyclooxygenase and lipoxygenase inhibitors have no effect. Moreover, we observed that four EETs were able to induce 45Ca2+ influx. Finally, we reported that the effect of 11,12 EET on 45Ca2+ influx was sensible to receptor-operated Ca2+ channel blockers (NiCl2, LaCl3) but not to voltage-dependent Ca2+ channel blocker as verapamil. Thus, AA released by Ca2+-independent phospholipase A2 and AA metabolism through cytochrome P-450 pathway may be crucial molecular determinant in thapsigargin activation of SOC channels and store-operated Ca2+ entry pathway in 3T6 fibroblasts. Moreover, EETs, the main cytochrome P-450 epoxygenase metabolites of AA, are involved in thapsigargin-stimulated Ca2+ influx. In summary, our results suggest that EETs are components of calcium influx factor(s).

Mechanism of Contraction of Rat Isolated Tail Arteries by Hyposmotic Solutions

Contraction induced by hyposmotic swelling was examined in rat tail arteries mounted on a myograph containing a modified Krebs physiological saline solution (PSS) containing 50 mM mannitol (300 mosm/l). Hyposmotic swelling was induced by removing mannitol. In arteries having basal tone or arteries precontracted with K(+) or the thromboxane mimetic U-46619, removal of mannitol caused a concentration dependent contraction of rat tail arteries. Concurrent measurement of tension and intracellular calcium [Ca(2+)](i )in arteries loaded with fura-2 showed that both tension and [Ca(2+)](i) increased on exposure to a hyposmotic solution. Removal of endothelium or inhibition of nitric oxide and cyclooxygenase together did not affect contractile responses. Removal of extracellular Ca(2+) abolished the contractile response to hyposmotic solution and NiCl(2), a nonspecific inhibitor of Ca(2+) influx pathways, blocked the rise in [Ca(2+)](i) and tension in response to a hyposmotic solution. Verapamil and nisoldipine, inhibitors of Ca(v)1.2 (L-type) calcium channels significantly reduced the contractile response to a hyposmotic solution. Addition of NiCl(2) to nisoldipine caused an additional inhibition of the response to a hyposmotic solution. Inhibition of calcium release from the sarcoplasmic reticulum by ryanodine or cyclopiazonic acid (CPA) did not cause any change in the tension response to a hyposmotic solution. CPA did not significantly inhibit the response to a hyposmotic solution in the presence of N(G)-methyl-L-arginine, oxyhaemoglobin and indomethacin. We conclude that contraction induced by a hyposmotic solution is largely due to Ca(v)1.2 calcium channels although other Ca(2+) influx pathways also contribute.

Oxidant stress and constrictor reactivity impair cerebral artery dilation in obese Zucker rats

This study tested the hypothesis that evolution of the metabolic syndrome in obese Zucker rats (OZR) leads to impaired dilator reactivity of cerebral resistance arteries vs. responses determined in lean Zucker rats (LZR). Middle cerebral arteries (MCA) from 17-wk-old male LZR and OZR were isolated and cannulated with glass micropipettes. Vascular reactivity was assessed in response to challenge with ACh, sodium nitroprusside (SNP), reductions and elevations in Po2, 5-HT, and increased intralumenal pressure. Vessels were treated with the free radical scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (tempol) to assess the role of superoxide production in altering reactivity, and passive vascular wall mechanics was assessed in each vessel. Vascular superoxide production was assessed in isolated arteries using fluorescence microscopy. Vessel dilation to ACh and hypoxia was impaired in OZR vs. LZR, although responses to SNP were normal. Vessel constriction to 5-HT, elevated Po2, and elevated intralumenal pressure was enhanced in OZR vs. LZR. Fluorescence microscopy demonstrated an increased superoxide production in arteries of OZR vs. LZR, correctable by incubation with tempol. Although treatment of vessels from OZR with tempol improved dilation to ACh and hypoxia, constrictor responses to 5-HT, elevated Po2, and pressure were not altered by tempol treatment. Indexes of vessel wall mechanics were comparable between groups. These results suggest that vasodilator reactivity of MCA of OZR in response to endothelium-dependent dilator stimuli is impaired vs. LZR and that this may represent a reduced bioavailability of signaling molecules due to oxidant scavenging. However, oxidative stress-independent increases in myogenic tone and constrictor reactivity may contribute to blunted dilator responses of cerebral microvessels.

Glucuronidation of oxidized fatty acids and prostaglandins B1 and E2 by human hepatic and recombinant UDP-glucuronosyltransferases

Arachidonic acid (AA) can be metabolized to various metabolites, which can act as mediators of cellular processes. The objective of this work was to identify whether AA, prostaglandin (PG) B1 and E2, and 15- and 20-hydroxyeicosatetraenoic acids (15- and 20-HETE) are metabolized via glucuronidation. Assays with human recombinant UDP-glucuronosyltransferase 1A (UGT1A) isoforms revealed that AA and 15-HETE were glucuronidated by UGT1A1, 1A3, 1A4, 1A9, and 1A10, whereas 20-HETE was glucuronidated by UGT1A1 and 1A4 and PGB1 was glucuronidated by UGT1A1, 1A9, and 1A10. All substrates were glucuronidated by recombinant UGT2B7, with AA and 20-HETE being the best substrates. Kinetic analysis of UGT1A1 and 1A9 with AA resulted in Km values of 37.9 and 45.8 microM, respectively. PGB1 was glucuronidated by UGT1A1 with a Km of 26.3 microM. The Km values for all substrates with UGT2B7 were significantly higher than with the UGT1A isoforms. Liquid chromatography-mass spectrometry of glucuronides biosynthesized from PGB1 and 15-HETE showed that hydroxyl groups were the major target of glucuronidation. This work demonstrates a novel metabolic pathway for HETEs and PGs and the role of UGT1A isoforms in this process. These results indicate that glucuronidation may play a significant role in modulation of the availability of these fatty acid derivatives for cellular processes.

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

Activation of rat adenosine2A receptors (A2A R) dilates preglomerular microvessels (PGMV), an effect mediated by epoxyeicosatrienoic acids (EETs). Incubation of PGMV with a selective A2A R agonist, 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 microM), increased isolated PGMV EET levels to 7.57+/-1.53 ng mg-1 protein from 1.06+/-0.22 ng mg-1 protein in controls (P<0.05), without affecting hydroxyeicosatetraenoic acid (HETE) levels (10.8+/-0.69 vs 11.02+/-0.74 ng mg-1 protein). CGS 21680-stimulated EETs was abolished by preincubation with an A2A R antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385) (100 microM). A selective epoxygenase inhibitor, methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH; 12 microM) prevented CGS 21680-induced increase in EETs, indicating inhibition of de novo synthesis of EETs. In pressurized (80 mmHg) renal arcuate arteries (110-130 microm) preconstricted with phenylephrine (20 nM), superfusion with CGS 21680 (0.01-10 microM) increased the internal diameter (i.d.) concentration-dependently; vasodilation was independent of nitric oxide and cyclooxygenase activity. CGS 21680 (10 microM) increased i.d. by 32+/-6 microm; vasodilation was prevented by inhibition of EET synthesis with MS-PPOH. Addition of 3 nM 5,6-EET, 8,9-EET and 11,12-EET increased i.d. by 53+/-9, 17+/-4 and 53+/-5 microm, respectively, whereas 14,15-EET was inactive. The responses to 5,6-EET were, however, significantly inhibited by indomethacin. We conclude that 11,12-EET is the likely mediator of A2A R-induced dilation of rat PGMV. Activation of A2A R coupled to de novo EET stimulation may represent an important mechanism in regulating preglomerular microvascular tone. British Journal of Pharmacology (2004) 141, 441-448. doi:10.1038/sj.bjp.0705640

Inhibitors of 20-hydroxyeicosatetraenoic acid reduce renal vasoconstrictor responsiveness

20-Hydroxyeicosatetraenoic acid (20-HETE) is a cytochrome P450-derived constrictor eicosanoid produced by the preglomerular vasculature where it contributes to regulation of tone. Removal of the tonic inhibitory influence of nitric oxide (NO) has been reported to increase renal 20-HETE release. Because inhibition of NO synthesis enhances responses to vasoconstrictor agents, we examined a contribution for increased 20-HETE generation. In the rat kidney perfused with Krebs' buffer, responses to U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy PGF2alpha), a thromboxane A2 mimetic, were compared before and after 50 microM L-nitroarginine (L-NA) to inhibit NO synthase. L-NA raised perfusion pressure (PP) from 79 +/- 3 to 190 +/- 7 mm Hg and enhanced constrictor responsiveness to U46619. U46619 (10, 30, 100, and 300 ng) increased PP by 7 +/- 1, 17 +/- 2, 50 +/- 7, and 67 +/- 7 mm Hg, respectively, before L-NA and 15 +/- 1, 37 +/- 7, 68 +/- 10, and 85 +/- 11 mm Hg, respectively, after L-NA, which did not increase 20-HETE efflux from the kidney. Nonetheless, an inhibitor of omega-hydroxylase, dibromododecencyl methylsulfonimide (DDMS), which reduced 20-HETE release, normalized the enhanced responsiveness to U46619. When PP was elevated with phenylephrine, vasoconstrictor responses to U46619 were similarly enhanced, an effect that was also prevented by DDMS. DDMS and an antagonist of 20-HETE, 20-HEDE [20-hydroxyeicosa-6(Z), 15(Z)-dienoic acid], also reduced vasoconstrictor responses to U46619 in the absence of elevation of PP. Because 20-HETE inhibits K+ channels, we examined the effects of K+ channel inhibitors on vasoconstrictor responses and showed that both tetraethylammonium (TEA) and charybdotoxin enhanced renal vasoconstrictor responses to U46619. However, the inhibitory effects of 20-HEDE on vasoconstrictor responses remained after treatment with TEA. These results support a role for 20-HETE vasoconstrictor responses but suggest an action independent of K+ channels.

Transfection of CYP4A1 cDNA increases vascular reactivity in renal interlobar arteries

20-HETE, a cytochrome P-450 4A (CYP4A1)-derived arachidonic acid metabolite, is a major eicosanoid formed in renal and extrarenal microcirculation. 20-HETE inhibits Ca(2+)-activated K(+) channels in vascular smooth muscle cells and thereby may modulate vascular reactivity. We transfected renal interlobar arteries with an expression plasmid containing the cDNA of CYP4A1, the low-K(m) arachidonic acid omega-hydroxylase, and examined the consequences of increasing 20-HETE synthesis on constrictor responses to phenylephrine. CYP4A1-transfected interlobar arteries demonstrated a twofold increase in CYP4A protein levels and 20-HETE production compared with arteries transfected with the empty plasmid; they also showed increased sensitivity to phenylephrine, as evidenced by a decrease in EC(50) from 0.37 +/- 0.04 microM in plasmid-transfected arteries to 0.07 +/- 0.01 microM in CYP4A1-transfected arteries. The increased sensitivity to phenylephrine was greatly attenuated by N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS), a selective inhibitor of 20-HETE synthesis, and by 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid, a specific 20-HETE antagonist. This effect of DDMS was reversed by addition of 20-HETE, further substantiating the notion that increased levels of 20-HETE contribute to the increased sensitivity to phenylephrine in vessels overexpressing CYP4A1. These data suggest that 20-HETE of vascular origin sensitizes renal vascular smooth muscle to phenylephrine.

Interaction of myogenic mechanisms and hypoxic dilation in rat middle cerebral arteries

The goal of this study was to determine how myogenic responses and vascular responses to reduced Po(2) interact to determine vascular smooth muscle (VSM) transmembrane potential and active tone in isolated middle cerebral arteries from Sprague-Dawley rats. Stepwise elevation of transmural pressure led to depolarization of the VSM cells and myogenic constriction, and reduction of the O(2) concentration of the perfusion and superfusion reservoirs from 21% O(2) to 0% O(2) caused vasodilation and VSM hyperpolarization. Myogenic constriction and VSM depolarization in response to transmural pressure elevation still occurred at reduced Po(2). Arterial dilation in response to reduced Po(2) was not impaired by pressure elevation but was significantly reduced at the lowest transmural pressure (60 mmHg). However, the magnitude of VSM hyperpolarization was unaffected by transmural pressure elevation. This study demonstrates that myogenic activation in response to transmural pressure elevation does not override hypoxic relaxation of middle cerebral arteries and that myogenic responses and hypoxic relaxation can independently regulate vessel diameter despite substantial changes in the other variable.

Cytochrome P-450 4A isoform expression and 20-HETE synthesis in renal preglomerular arteries

20-Hydroxyeicosatetraenoic acid (20-HETE), a potent vasoconstrictor and mediator of the myogenic response, is a major arachidonic acid metabolite in the microvasculature of the rat kidney formed primarily by the cytochrome P-450 (CYP) 4A isoforms, CYP4A1, CYP4A2, and CYP4A3. We examined CYP4A isoform expression and 20-HETE synthesis in microdissected interlobar, arcuate, and interlobular arteries; mRNA for all CYP4A isoforms was identified by RT-PCR. Western blot analysis indicated that the levels of CYP4A2/4A3-immunoreactive protein increased with decreased arterial diameter, whereas those of CYP4A1-immunoreactive protein remained unchanged. 20-HETE synthesis was the highest in the interlobular arteries (17 +/- 1.62 nmol. mg(-1). h(-1)) and, like CYP4A2/4A3-immunoreactive protein, decreased with increasing vessel diameter (4.5 +/- 1.21, 2.65 +/- 0.58, and 0.81 +/- 0.14 nmol. mg(-1). h(-1) in the arcuate, interlobar, and segmental arteries, respectively). 20-HETE synthesis in the renal artery and the abdominal aorta was undetectable. The observed decreased immunoreactivity of NADPH-cytochrome P-450 (c) oxidoreductase with increased arterial diameter provided a possible explanation for the decreased capacity to generate 20-HETE in the large arteries. The increase in CYP4A isoform expression and 20-HETE synthesis with decreasing diameter along the preglomerular arteries and the potent biological activity of 20-HETE underscore the significance of 20-HETE as a modulator of renal hemodynamics.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

Initial and sustained phases of myogenic response of rat mesenteric small arteries

A possible role for a metabolite of cytochrome P-450 omega-hydroxylase in the initial and sustained phases of the myogenic response in cannulated rat mesenteric small arteries was studied. With slight preconstriction (norepinephrine and neuropeptide Y), pressure was raised from 60 to 100 mmHg, and both initial (within 2 min) and sustained phases (at 10 min) of the myogenic response were quantified. The myogenic response was fully inhibited by D600 (methoxyverapamil). Ketoconazole and 17-octadecanoic acid did not affect the initial phase but inhibited the sustained phase. In contrast, miconazole did not affect either phase. Charybdotoxin and iberiotoxin potentiated the initial phase but eliminated the sustained phase. Apamin, glibenclamide, 4-aminopyridine, and barium had no effect on either phase. The results demonstrate different mechanisms for the initial and sustained phases of the myogenic response of rat mesenteric small arteries. Only the sustained phase appears mediated through a cytochrome P-450 omega-hydroxylase metabolite and calcium-activated K+ channels. However, both phases of the response are dependent on calcium influx through voltage-dependent calcium channels.

Impaired dilation of skeletal muscle microvessels to reduced oxygen tension in diabetic obese Zucker rats

This study determined alterations to hypoxic dilation of isolated skeletal muscle resistance arteries (gracilis arteries; viewed via television microscopy) from obese Zucker rats (OZR) compared with lean Zucker rats (LZR). Hypoxic dilation was reduced in OZR compared with LZR. Endothelium removal and cyclooxygenase inhibition (indomethacin) severely reduced this response in both groups, although nitric oxide synthase inhibition (N(omega)-nitro-L-arginine methyl ester) reduced dilation in LZR only. Treatment of vessels with a PGH(2)-thromboxane A(2) receptor antagonist had no effect on hypoxic dilation in either group. Arterial dilation to arachidonic acid, iloprost, acetylcholine, and sodium nitroprusside was reduced in OZR versus LZR, although dilation to forskolin and aprikalim was unaltered. Treatment of arteries from OZR with oxidative radical scavengers increased dilation to hypoxia and agonists, with no effect on responses in LZR. The restored hypoxic dilation in OZR was abolished by indomethacin. These results suggest that hypoxic dilation of skeletal muscle microvessels from LZR represents the summated effects of prostanoid and nitric oxide release, whereas the impaired response of vessels in OZR may reflect scavenging of PGI(2) by superoxide anion.

High-salt diet impairs hypoxia-induced cAMP production and hyperpolarization in rat skeletal muscle arteries

This study determined the effects of hypoxia on diameter, vascular smooth muscle (VSM) transmembrane potential (E(m)), and vascular cAMP levels for in vitro cannulated skeletal muscle resistance arteries (gracilis arteries) from Sprague-Dawley rats fed a low-salt (LS) or a high-salt (HS) diet. Arterial diameter and VSM E(m) were measured in response to hypoxia, iloprost, cholera toxin, forskolin, and aprikalim. In HS rats, arterial dilation and VSM hyperpolarization after hypoxia, iloprost, and cholera toxin were impaired versus responses in LS rats, whereas responses to forskolin and aprikalim were unaltered. Blockade of prostaglandin H(2) and thromboxane A(2) receptors had no effect on responses to hypoxia or iloprost in vessels from both rat groups, suggesting that inappropriate activation of these receptors does not contribute to the impaired hypoxic dilation with HS. Hypoxia, cholera toxin, and iloprost increased vascular cAMP levels in vessels of LS rats only, whereas forskolin increased cAMP levels in all vessels. These data suggest that reduced hypoxic dilation of skeletal muscle microvessels in rats on a HS diet may reflect an impaired ability of VSM to produce cAMP after exposure to prostacyclin.

Expressed CYP4A4 metabolism of prostaglandin E(1) and arachidonic acid

Cytochrome P4504A4 (CYP4A4) is a hormonally induced pulmonary cytochrome P450 which metabolizes prostaglandins and arachidonic acid (AA) to their omega-hydroxylated products. Although the physiological function of this enzyme is unknown, prostaglandins play an important role in the regulation of reproductive, vascular, intestinal, and inflammatory systems and 20-hydroxyeicosatetraenoic acid, the omega-hydroxylated product of arachidonate, is a potent vasoconstrictor. Therefore, it is important to obtain sufficient quantities of the protein for kinetic and biophysical characterization. A CYP4A4 construct was prepared and expressed in Escherichia coli. The enzyme was purified, and its activity with substrates prostaglandin E(1) (PGE(1)) and AA was examined in the presence and absence of cytochrome b(5) (cyt b(5)) and with a heme-depleted form of cyt b(5) (apo b(5)). The stimulatory role played by cyt b(5) in this system is not dependent on electron transfer from cyt b(5) to the CYP4A4 as similar stimulation was observed with apo b(5). Rapid kinetic measurement of CYP4A4 electron transfer rates confirmed this result. Both flavin and heme reduction rates were constant in the absence and presence of cyt b(5) or apo b(5). CD spectroscopy demonstrated that a conformational change occurred in CYP4A4 protein upon binding of cyt b(5) or apo b(5). Finally, acetylenic fatty acid inhibitors 17-octadecynoic acid, 12-hydroxy-16-heptadecynoic acid, 15-hexadecynoic acid, and 10-undecynoic acid (10-UDYA) were used to probe the substrate-binding pocket of CYP4A4. The short-chain fatty acid inhibitor 10-UDYA was unable to inhibit either PGE(1) or AA metabolism. All but 10-UDYA were effective inhibitors of CYP4A4.

Involvement of cytochrome P450 metabolites in the vascular action of angiotensin II on the afferent arterioles

Recent studies have demonstrated that cytochrome P450-dependent metabolites of arachidonic acid (CYP450-AA) play important roles in the control of renal vascular resistance (RVR). In the present study, we examined the possible involvement of CYP450-AA in the vasoconstrictor action of angiotensin II (Ang II) on the afferent arterioles (Af-Arts), a vascular segment crucial to the control of RVR. Rabbit Af-Arts were microperfused at 60 mmHg in vitro, and the vasoconstrictor action of Ang II (10(-11)-10(-8) M, added to both the bath and lumen) was examined with or without blocking the activity of CYP450 epoxygenase or hydroxylase. Ang II decreased the luminal diameter of Af-Arts in a dose-dependent manner (34+/-2% of control diameter at 10(-8) M, n=9, p<0.0001). Pretreatment with miconazole, an inhibitor of CYP450 epoxygenase, at 10(-6) M decreased the basal diameter by 14+/-1% (n=6, p<0.01) and augmented the vasoconstrictor action of Ang II (7+/-3% of control diameter at 10(-8) M, p<0.001 vs. without miconazole). This augmentation was abolished by blocking the Ang II type 2 (AT2) receptor with PD 123319 at 10(-7) M. In contrast, pretreatment with 17-octadecynoic acid (17-ODYA, 10(-6) M), which inhibits both epoxygenase and hydroxylase activity, had no effect on the basal diameter but attenuated the vasoconstrictor action of Ang 11(46+/-2% of control diameter at 10(-8) M, p<0.01 vs. without 17-ODYA). Our results demonstrate that in the Af-Art, endogenous CYP450-AA are involved not only in the control of basal tone but also in the action of Ang II. Further, it appears that the CYP450 epoxygenase pathway attenuates Ang II action via AT2 receptors.

Role of guanylyl cyclase and cytochrome P-450 on renal response to nitric oxide

The present study evaluated whether inhibition of guanylyl cyclase (GC) with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one (ODQ) and methylene blue (MB) or inhibition of the renal metabolism of arachidonic acid by cytochrome P-450 (CYP450) enzymes with 1-aminobenzotriazole (ABT) and N-hydroxy-N'-(4 butyl-2-methyl phenyl)formamidine (HET0016) alters the renal tubular and vascular effects of a nitric oxide (NO) donor in vivo. Intrarenal infusion of ODQ or MB at a dose of 170 nmol. kg(-1). min(-1) lowered renal blood flow (RBF) by 30 and 15%, respectively; glomerular filtration rate (GFR) by 26 and 18%, respectively; and sodium and water excretion by approximately 35%. In rats pretreated with nitro-L-arginine methyl ester (37 nmol. kg(-1). min(-1)) to block the endogenous production of NO, intrarenal infusion of the NO donor S-nitroso-N-acetylcysteine (S-NO-NAC; 50 nmol. kg(-1). min(-1)) increased RBF (18%), sodium (73%), and water excretion (61%). ODQ or MB administration blocked the effect of S-NO-NAC on RBF but not the diuretic and natriuretic response. Pretreatment of rats with ABT or HET0016 also abolished the renal vasodilatory response to the NO donor and reduced its diuretic and natriuretic effect. These results indicate that both activation of GC and inhibition of CYP450 enzymes contribute to the renal vascular actions of NO, whereas the natriuretic and diuretic actions of NO appear to be largely CYP450 dependent.

The kinetic and spectral characterization of the E. coli-expressed mammalian CYP4A7: cytochrome b5 effects vary with substrate

The CYP4A gene subfamily is composed of a number of genes that encode cytochromes P450 from various species, including human, which catalyze the hydroxylation of various saturated and unsaturated fatty acids, including arachidonic acid and prostaglandins. CYP4A7, a fatty acid metabolizing cytochrome P450 from rabbit kidney, was expressed in E. coli by adding the first 10 codons of CYP17alpha producing final yields of 20 nmol/L in order to perform detailed kinetic and spectral studies. CYP4A7 metabolized arachidonate, laurate, and myristate, with maximum turnover numbers of 152, 130, and 64.5 min(-1) and corresponding Km values of 74.5, 27, and 16.7 microM, respectively, in the presence of cytochrome b5. In the absence of cytochrome b5, CYP4A7 metabolized laurate and myristate with turnover numbers of 27.4 and 33.6 min(-1) and corresponding Km values of 3.9 and 33 microM, respectively. Arachidonate was not metabolized in the absence of cytochrome b5. Saturation kinetics studies performed with heme-depleted cytochrome b5 (apo cytochrome b5) yielded turnover numbers of 118 and 74 min(-1) and Km values of 74 and 25 microM with laurate and myristate, respectively, indicating that cytochrome b5 is not involved in electron transfer but rather plays a conformational role. Laurate perturbation of the visible absorption spectrum of CYP4A7 allowed for determination of the spectral binding constant (KS) in the absence and presence of cytochrome b5 (13 and 43 microM, respectively). In stopped-flow kinetics experiments, the flavin reduction (approximately 90 s(-1)) and heme reduction (approximately 9 s(-1)) phases of the monooxygenase reaction of CYP4A7 were not altered by the presence of cytochrome b5. Estimations of the rate of CPR (0.3 s(-1)) or cytochrome b5 (9.1 s(-1)) binding with CYP4A7 were also determined.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

20-HETE modulates myogenic response of skeletal muscle resistance arteries from hypertensive Dahl-SS rats

The present study determined the role of 20-hydroxyeicosatetraenoic acid [20-HETE; produced by omega-hydroxylation of arachidonic acid via cytochrome P-450 (CP450) 4A enzymes] in regulating myogenic activation of skeletal muscle resistance arteries from normotensive (NT) and hypertensive (HT) Dahl salt-sensitive (SS) rats. Gracilis arteries (GA) were isolated from each rat and viewed via television microscopy, and changes in vessel diameter with altered transmural pressure were measured with a video micrometer. Under control conditions, GA from both groups exhibited strong, endothelium-independent myogenic activation. Treatment of GA with 17-octadecynoic acid (17-ODYA; inhibitor of CP450 4A enzymes) did not alter myogenic activation in NT rats, but impaired this response in HT animals. Treatment of GA from HT rats with dibromo-dodecynyl-methylsulfimide (DDMS; inhibitor of 20-HETE production) impaired myogenic activation, as did application of 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid, an antagonist for 20-HETE receptors. Application of iberiotoxin, a Ca(2+)-activated potassium (K(Ca)) channel inhibitor, restored myogenic activation from HT rats treated with DDMS. These results suggest that myogenic activation of skeletal muscle resistance arteries from NT Dahl-SS rats does not depend on CP450, whereas myogenic activation of these vessels in HT Dahl-SS rats is partly a function of 20-HETE production, inhibiting K(Ca) channels through a receptor-mediated process.

CYP4A1 antisense oligonucleotide reduces mesenteric vascular reactivity and blood pressure in SHR

The cytochrome P-450 4A (CYP4A)-derived arachidonic acid metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) affects renal tubular and vascular functions and has been implicated in the control of arterial pressure. We examined the effect of antisense oligonucleotide (ODN) to CYP4A1, the low K(m) arachidonic acid omega-hydroxylating isoform, on vascular 20-HETE synthesis, vascular reactivity, and blood pressure in the spontaneously hypertensive rat (SHR). Administration of CYP4A1 antisense ODN decreased mean arterial blood pressure from 137 +/- 3 to 121 +/- 4 mmHg (P < 0.05) after 5 days of treatment, whereas treatment with scrambled antisense ODN had no effect. Treatment with CYP4A1 antisense ODN reduced the level of CYP4A-immunoreactive proteins along with 20-HETE synthesis in mesenteric arterial vessels. Mesenteric arteries from rats treated with antisense ODN exhibited decreased sensitivity to the constrictor action of phenylephrine (EC(50) 0.69 +/- 0.17 vs. 1.77 +/- 0.40 microM). Likewise, mesenteric arterioles from antisense ODN-treated rats revealed attenuation of myogenic constrictor responses to increases of transmural pressure. The decreased vascular reactivity and myogenic responses were reversible with the addition of 20-HETE. These data suggest that CYP4A1-derived 20-HETE facilitates myogenic constrictor responses in the mesenteric microcirculation and contributes to pressor mechanisms in SHR.

Cytochrome P450 metabolites of arachidonic acid in the control of renal function

Recent studies indicate that arachidonic acid is primarily metabolized by cytochrome P450 enzymes of the 4A and 2C families in the kidney to 20-hydroxyeicosatetraenoic acid (HETE), epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids. These compounds play central roles in the regulation of renal tubular and vascular function. 20-HETE is produced by renal vascular smooth muscle (VSM) cells and is a potent constrictor that depolarizes VSM cells by blocking the calcium-activated potassium channel. Inhibition of the formation of 20-HETE blocks the myogenic response of isolated renal arterioles in vitro, and autoregulation of renal blood flow and tubuloglomerular feedback responses in vivo. EETs are products formed in the endothelium and are potent dilators that activate the calcium-activated potassium channel in renal VSM. Endothelial-dependent vasodilators stimulate the release of EETs, and these compounds appear to serve as an endothelial-derived hyperpolarizing factor. EETs and 20-HETE are produced in the proximal tubule. There, they regulate sodium/potassium-ATPase activity and serve as second messengers for the natriuretic effects of dopamine, parathyroid hormone and angiotensin II. 20-HETE is also produced in the thick ascending loop of Henle. It regulates sodium-potassium-chloride transport in this nephron segment. The renal production of cytochrome P450 metabolites of arachidonic acid is altered in hypertension, diabetes, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of cytochrome P450 metabolites of arachidonic acid in the control of renal function, it is likely that changes in this system contribute to the abnormalities in renal function that are associated with many of these conditions.