Relaxation of porcine coronary artery to bradykinin. Role of arachidonic acid

Coronary vasodilation mediated by T cells expressing choline acetyltransferase

CD4⁺ T cells expressing choline acetyltransferase (ChAT) have recently been shown to cause a drop in systemic blood pressure when infused into mice. The aim of this study was to determine if ChAT-expressing T cells could regulate coronary vascular reactivity. Preconstricted segments of epicardial and intramyocardial porcine coronary arteries relaxed in response to Jurkat T cells (JT) that overexpressed ChAT (JT_ChAT cells). The efficacy of the JT_ChAT cells was similar in epicardial and intramyocardial vessels with a maximum dilator response to 3 × 10⁵ cells/mL of 38.0 ± 6.7% and 38.7 ± 7.25%, respectively. In contrast, nontransfected JT cells elicited a weak dilator response, followed by a weak contraction. The response of JT_ChAT cells was dependent on the presence of the endothelial cells. In addition, the response could be significantly reduced by N^ω-nitro-l-arginine methyl ester (l-NAME) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) in the presence of indomethacin. JT_ChAT cells, but not JT cells, increased the expression of phosphorylated endothelial nitric oxide synthase (eNOS). JT_ChAT cells contained significantly greater levels of acetylcholine compared with JT cells; however, the nonselective muscarinic antagonist atropine and the M₁ receptor antagonist pirenzepine both failed to block the dilator effect of JT_ChAT cells. Exogenously added acetylcholine induced only a weak relaxation (∼10%) at low concentrations, which became a contractile response at higher concentrations. These data illustrate the capacity for cells that express ChAT to regulate coronary vascular reactivity, via mechanisms that are dependent on interaction with the endothelium and in part mediated by the release of nitric oxide.NEW & NOTEWORTHY This study shows ChAT-expressing T cells can induce vasodilation of the blood vessel in the coronary circulation and that this effect relies on a direct interaction between T cells and the coronary vascular endothelium. The study establishes a potential immunomodulatory role for T cells in the coronary circulation. The present findings offer an additional possibility that a deficiency of ChAT-expressing T cells could contribute to reduced coronary blood flow and ischemic events in the myocardium.

Tambulin is a major active compound of a methanolic extract of fruits of Zanthoxylum armatum DC causing endothelium-independent relaxations in porcine coronary artery rings via the cyclic AMP and cyclic GMP relaxing pathways

BACKGROUND

Zanthoxylum armatum DC (Z. armatum), belonging to Rutaceae family, has been traditionally used for the treatment of various diseases such as hypertension, abdominal pain, headache, fever, high altitude sickness, diarrhea, dysentery, and as a tonic, condiment, and an anthelmintic treatment.

HYPOTHESIS

The present study aims to evaluate the vasorelaxant effect of a methanolic extract of the fruits of Z. armatum, isolate the active components and characterize the underlying mechanism.

STUDY DESIGN

A methanolic extract of fruits of Z. armatum was prepared and its vasorelaxant effect was studied using porcine coronary artery rings. Thereafter, the methanolic extract was analyzed, and a major compound was isolated and its structure elucidated (tambulin). Different pharmacological tools were used to characterize the vasorelaxant effect of tambulin.

RESULTS

The methanolic extract and the isolated tambulin caused similar endothelium-independent relaxations of porcine coronary artery rings with and without endothelium indicating a direct relaxing effect at the vascular smooth muscle. Tambulin did not affect the relaxation curves to the endothelium-dependent vasodilators, bradykinin and the calcium ionophore A23187 in rings with endothelium. Tambulin (1 µM) slightly but significantly shifted leftwards the concentration-relaxation curve to the endothelium-independent vasodilators, sodium nitroprusside (SNP), forskolin (FC) and isoproterenol but not those to soluble guanylyl cyclase activators (YC-1 and BAY 41-2272) and K⁺ channel openers (levcromakalim and 1-EBIO). Pretreatment with tambulin inhibited, in a concentration-dependent manner, contractions to KCl, serotonin (5-HT), CaCl₂ and U46619 in coronary artery rings without endothelium. Both the protein kinase A (H-89, 10 µM) and the protein kinase G (Rp-8-br-cyclic GMPS, 30 µM) inhibitors significantly reduced relaxations to tambulin in coronary artery rings without endothelium.

CONCLUSION

The present findings indicate that tambulin isolated from Z. armatum (fruits) is a major active principle inducing vasorelaxation through a direct effect at the vascular smooth muscle and involving both the cyclic AMP and/or cyclic GMP relaxing pathways.

Pulsed ultrasound enhances the delivery of nitric oxide from bubble liposomes to ex vivo porcine carotid tissue

Ultrasound-mediated drug delivery is a novel technique for enhancing the penetration of drugs into diseased tissue beds noninvasively. By encapsulating drugs into microsized and nanosized liposomes, the therapeutic can be shielded from degradation within the vasculature until delivery to a target site by ultrasound exposure. Traditional in vitro or ex vivo techniques to quantify this delivery profile include optical approaches, cell culture, and electrophysiology. Here, we demonstrate an approach to characterize the degree of nitric oxide (NO) delivery to porcine carotid tissue by direct measurement of ex vivo vascular tone. An ex vivo perfusion model was adapted to assess ultrasound-mediated delivery of NO. This potent vasodilator was coencapsulated with inert octafluoropropane gas to produce acoustically active bubble liposomes. Porcine carotid arteries were excised post mortem and mounted in a physiologic buffer solution. Vascular tone was assessed in real time by coupling the artery to an isometric force transducer. NO-loaded bubble liposomes were infused into the lumen of the artery, which was exposed to 1 MHz pulsed ultrasound at a peak-to-peak acoustic pressure amplitude of 0.34 MPa. Acoustic cavitation emissions were monitored passively. Changes in vascular tone were measured and compared with control and sham NO bubble liposome exposures. Our results demonstrate that ultrasound-triggered NO release from bubble liposomes induces potent vasorelaxation within porcine carotid arteries (maximal relaxation 31%±8%), which was significantly stronger than vasorelaxation due to NO release from bubble liposomes in the absence of ultrasound (maximal relaxation 7%±3%), and comparable with relaxation due to 12 μM sodium nitroprusside infusions (maximal relaxation 32%±3%). This approach is a valuable mechanistic tool for assessing the extent of drug release and delivery to the vasculature caused by ultrasound.

Clot retraction affects the extent of ultrasound-enhanced thrombolysis in an ex vivo porcine thrombosis model

We investigated ultrasound-enhanced thrombolysis in two whole-blood clot models using a Food and Drug Administration-approved contrast agent (Definity, Lantheus Medical Imaging; Billerica, MA USA) and thrombolytic drug (recombinant tissue-type plasminogen activator [rt-PA]) (Genentech; South San Francisco, CA USA). Porcine venous blood was collected from donor hogs and coagulated in vials made of two different materials. This method produced clots with differing compositional properties, as determined by routine scanning electron microscopy and histology. Clots were deployed in an ex vivo porcine thrombosis model, and exposed to an intermittent ultrasound scheme previously developed to maximize stable cavitation while acoustic emissions were detected. Exposure to 3.15 μg/mL rt-PA promoted lysis in both clot models, compared with exposure to plasma alone. However, only unretracted clots experienced significant enhancement of thrombolysis in the presence of rt-PA, Definity, and ultrasound, compared with treatment with rt-PA. In these clots, microscopy revealed loose erythrocyte aggregates, a significantly less extensive fibrin network and a higher porosity, which may facilitate increased penetration of thrombolytics by cavitation.

Kaempferol enhances endothelium-independent and dependent relaxation in the porcine coronary artery

The vascular effects of kaempferol were investigated in isolated porcine coronary artery rings. U46619 (9,11-dideoxy-9alpha, 11alpha-methanoepoxy prostaglandin F2alpha, 30 nM) was used to contract porcine coronary arterial rings. Concentration relaxation curve of kaempferol (1 nM - 100 microM) was constructed and kaempferol demonstrated significant relaxation at high concentrations. At low concentration with no significant effect on relaxation, kaempferol (10 microM) enhanced relaxation produced by bradykinin, the calcium ionophore A23187, isoproterenol and sodium nitroprusside in endothelium-intact porcine coronary arteries. In endothelium-disrupt rings, kaempferol (10 microM) also enhanced the relaxation caused by isoproterenol, sodium nitroprusside, levcromakalim and nifedipine. On the other hand, antioxidant agents did not affect bradykinin-induced relaxation or the enhancement effect of kaempferol. In summary, a low concentration of kaempferol (10 microM), devoid of significant vascular effect, has the ability to enhance endothelium-dependent and endothelium-independent relaxations. This action of kaempferol is unrelated to its antioxidant property.

Ginsenosides block HIV protease inhibitor ritonavir-induced vascular dysfunction of porcine coronary arteries

Human immunodeficiency virus (HIV) protease inhibitor ritonavir (RTV) may induce vascular dysfunction through oxidative stress. Ginsenosides have been shown to have potential benefits on the cardiovascular system through diverse mechanisms, including antioxidative property. The objective of this study was to determine whether ginsenosides could prevent coronary arteries from RTV-induced dysfunction. Porcine coronary artery rings were incubated with RTV and ginsenosides Rb1, Rc, and Re for 24 h. Vasomotor function was recorded by a myograph tension system. In response to the thromboxane A(2) analog U-46619, the contraction of the vessel rings was significantly reduced. When cocultured with Rb1, Rc, and Re, the contractility significantly increased. In response to bradykinin at 10(-5) M, the endothelium-dependent relaxation of vessel rings was significantly reduced by 59% for RTV compared with controls (P < 0.05). When cocultured with Rb1, Rc, and Re, the relaxation significantly increased 100%, 90%, and 134%, respectively, compared with the RTV-alone groups (P > 0.05). In response to sodium nitroprusside, RTV significantly reduced vasorelaxation. In addition, the endothelial nitric oxide synthase (eNOS) mRNA levels were significantly reduced by 78% for RTV group (P < 0.05) by real-time PCR analysis. The eNOS protein levels measured by Western blot analysis and nitrite concentrations measured by Griess assay were also decreased, whereas O(2)(-) production by lucigenin-enhanced chemiluminescence was significantly increased in the RTV-treated group. These effects of RTV were effectively blocked by ginsenosides. Thus HIV protease inhibitor RTV significantly impaired the vasomotor function of porcine coronary arteries. This effect may be mediated by the downregulation of eNOS and overproduction of O(2)(-). These results suggest that ginsenosides can effectively block RTV-induced vascular dysfunction.

Reactive oxygen species mediate arachidonic acid-induced dilation in porcine coronary microvessels

Reactive oxygen species (ROS) have been proposed to mediate vasodilation in the microcirculation. We investigated the role of ROS in arachidonic acid (AA)-induced coronary microvascular dilation. Porcine epicardial coronary arterioles (110 +/- 4 microm diameter) were mounted onto pipettes in oxygenated Krebs buffer. Vessels were incubated with vehicle or 1 mM Tiron (a nonselective ROS scavenger), 250 U/ml polyethylene-glycolated (PEG)-superoxide dismutase (SOD; an O2- scavenger), 250 U/ml PEG-catalase (a H2O2 scavenger), or the cyclooxygenase (COX) inhibitors indomethacin (10 microM) or diclofenac (10 microM) for 30 min. After endothelin constriction (30-60% of resting diameter), cumulative concentrations of AA (10(-10)-10(-5)M) were added and internal diameters measured by video microscopy. AA (10-7 M) produced 37 +/- 6% dilation, which was eliminated by the administration of indomethacin (4 +/- 7%, P < 0.05) or diclofenac (-8 +/- 8%, P < 0.05), as well as by Tiron (-4 +/- 5%, P < 0.05), PEG-SOD (-10 +/- 6%, P < 0.05), or PEG-catalase (1 +/- 4%, P < 0.05). Incubation of small coronary arteries with [3H]AA resulted in the formation of prostaglandins, which was blocked by indomethacin. In separate studies in microvessels, AA induced concentration-dependent increases in fluorescence of the oxidant-sensitive probe dichlorodihydrofluorescein diacetate, which was inhibited by pretreatment with indomethacin or by SOD + catalase. We conclude that in porcine coronary microvessels, COX-derived ROS contribute to AA-induced vasodilation.

12-lipoxygenase in porcine coronary microcirculation: implications for coronary vasoregulation

Noncyclooxygenase metabolites of arachidonic acid (AA) have been proposed to mediate endothelium-dependent vasodilation in the coronary microcirculation. Therefore, we examined the formation and bioactivity of AA metabolites in porcine coronary (PC) microvascular endothelial cells and microvessels, respectively. The major noncyclooxygenase metabolite produced by microvascular endothelial cells was 12(S)-hydroxyeicosatetraenoic acid (HETE), a lipoxygenase product. 12(S)-HETE release was markedly increased by pretreatment with 13(S)-hydroperoxyoctadecadienoic acid but not by the reduced congener 13(S)-hydroxyoctadecadienoic acid, suggesting oxidative upregulation of 12(S)-HETE output. 12(S)-HETE produced potent relaxation and hyperpolarization of PC microvessels (EC(50), expressed as -log[M] = 13.5 +/- 0.5). Moreover, 12(S)-HETE potently activated large-conductance Ca(2+)-activated K(+) currents in PC microvascular smooth muscle cells. In contrast, 12(S)-HETE was not a major product of conduit PC endothelial AA metabolism and did not exhibit potent bioactivity in conduit PC arteries. We suggest that, in the coronary microcirculation, 12(S)-HETE can function as a potent hyperpolarizing vasodilator that may contribute to endothelium-dependent relaxation, particularly in the setting of oxidative stress.

Bradykinin-induced vasodilation of human forearm resistance vessels is primarily mediated by endothelium-dependent hyperpolarization

Bradykinin (BK) stimulates endothelial cells to release a number of relaxing factors, such as NO, prostanoids (PGs), and an endothelium-derived hyperpolarizing factor (EDHF). However, the contributions of NO, PG, and EDHF in the vascular relaxation to BK vary with species and anatomic origin of blood vessels used. Therefore, the present study was designed to investigate the contributions of NO, PG, and EDHF in vasodilation caused by BK in human forearm resistance vessels. Forearm blood flow (FBF) was recorded with venous occlusion plethysmography in healthy nonsmoking subjects. At first, studies were performed to validate the NO clamp technique for its ability to inhibit endogenous NO generation. Brachial artery infusion of serotonin (0.6, 1.8, and 6 ng. 100 mL forearm volume [FAV](-1). min(-1)) caused significant forearm vasodilation (2.6 to 4.6 mL. 100 mL FAV(-1). min(-1)), which is known to be NO mediated. Indeed, during the NO clamp, cumulative doses of serotonin caused no vasodilation (2.4 to 2.6 mL. 100 mL FAV(-1). min(-1)), indicating that the generation of endogenous NO was completely blocked. Thereafter, the vasodilative actions of BK were investigated. Brachial artery infusion of BK (50, 100, and 200 ng. 100 mL FAV(-1). min(-1)) caused significant forearm vasodilation in all studies (from 3.1 to 20.4 mL. 100 mL FAV(-1). min(-1)). After the inhibition of cyclooxygenase and NO synthase activity through the use of aspirin and the NO-clamp technique, BK increased FBF in a similar manner (3.9 to 18.9 mL. 100 mL FAV(-1). min(-1)), indicating that the vasodilative actions of BK are independent of NO and PG generation. However, vasodilation caused by the 2 lower doses of BK were significantly attenuated after K(Ca) channel activity was blocked with tetraethylammonium chloride (0.1 mg. 100 mL FAV(-1). min(-1)), suggesting that in the lower dose range, BK mediates vasodilation through the opening of vascular potassium channels. In conclusion, BK is a potent vasodilator peptide in human forearm resistance vessels, causing vasodilation through hyperpolarization of the vascular wall independent of NO and PG production. In addition, the NO-clamp technique is a valid instrument to investigate the contribution of NO in the vasodilative response to different agents.

Epoxide hydrolases regulate epoxyeicosatrienoic acid incorporation into coronary endothelial phospholipids

Cytochrome P-450-derived epoxyeicosatrienoic acids (EETs) are avidly incorporated into and released from endothelial phospholipids, a process that results in potentiation of endothelium-dependent relaxation. EETs are also rapidly converted by epoxide hydrolases to dihydroxyeicosatrienoic acid (DHETs), which are incorporated into phospholipids to a lesser extent than EETs. We hypothesized that epoxide hydrolases functionally regulate EET incorporation into endothelial phospholipids. Porcine coronary artery endothelial cells were treated with an epoxide hydrolase inhibitor, 4-phenylchalcone oxide (4-PCO, 20 micromol/l), before being incubated with (3)H-labeled 14,15-EET (14,15-[(3)H]EET). 4-PCO blocked conversion of 14,15-[(3)H]EET to 14,15-[(3)H]DHET and doubled the amount of radiolabeled products incorporated into cell lipids, with >80% contained in phospholipids. Moreover, pretreatment with 4-PCO before incubation with 14,15-[(3)H]EET enhanced A-23187-induced release of radiolabeled products into the medium. In contrast, 4-PCO did not alter uptake, distribution, or release of [(3)H]arachidonic acid. In porcine coronary arteries, 4-PCO augmented 14,15-EET-induced potentiation of endothelium-dependent relaxation to bradykinin. These data suggest that epoxide hydrolases may play a role in regulating EET incorporation into phospholipids, thereby modulating endothelial function in the coronary vasculature.

Reactive oxygen species: role in the relaxation induced by bradykinin or arachidonic acid via EDHF in isolated porcine coronary arteries

Although endothelium-derived hyperpolarizing factor (EDHF) is thought to be a cytochrome P-450 product (arachidonic acid metabolite) in some tissues, in porcine coronary arteries (PCAs) its nature remains unclear. Because phospholipase A2 and C are involved in the synthesis and/or release of EDHF in the PCA, the arachidonic acid (AA) pathway may be involved. In the presence of the cyclooxygenase inhibitor indomethacin (10(-5) M) and the NOS inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME; 10(-4) M), both bradykinin (BK; 10(-9)-10(-6) M) and AA (10(-7)-10(-4) M) induced dose-dependent relaxation of PGF2alpha-contracted PCA rings, which was blocked by a high extracellular concentration of KCl (30 mM) or pretreatment with ouabain, a Na+/K+-adenosine triphosphatase (ATPase) inhibitor (5 x 10(-7) M). Eicosatetraynoic acid (ETYA; 20 microM), which inhibits all AA pathways, slightly affected the response to BK and AA; however, lipoxygenase or cytochrome P-450 inhibitors had no effect, suggesting that relaxation is independent of these enzymatic pathways. Because endothelial cells can generate reactive oxygen species (ROS) via metabolism of AA and independent of cyclooxygenase activity, we also studied (a) whether ROS can relax the PCA, as well as the mechanism(s) involved, and (b) the role of ROS in BK- and AA-induced relaxation. Xanthine (X; 100 microM) plus xanthine oxidase (XO; 0.02 U/ml) induced time-dependent relaxation of PGF2alpha-contracted PCA rings in the presence of indomethacin and L-NAME. Dilatation was not affected by superoxide dismutase (SOD; 500 U/ml) but was abolished by catalase (300 U/ml), suggesting that hydrogen peroxide (H2O2) is involved. When rings were contracted by depolarizing them with 30 mM KCl, X/XO failed to elicit relaxation. Ouabain abolished the response to X/XO, suggesting that X/XO may induce relaxation by hyperpolarizing vascular smooth muscle cells via stimulation of the Na+/K+-ATPase pump. We therefore questioned whether ROS might be involved in BK- and AA-induced relaxation. Because catalase combined with SOD had little or no effect, we concluded that in the PCA, the relaxation induced by BK via EDHF involves some mechanism independent of NO, AA metabolism, or ROS.

Important role of endothelium-derived hyperpolarizing factor in shear stress--induced endothelium-dependent relaxations in the rat mesenteric artery

Shear stress is one of the most important stimulators for the release of endothelium-derived relaxing factors. Although shear stress-induced release of nitric oxide (NO) has been extensively investigated, it remains to be elucidated whether endothelium-derived hyperpolarizing factor (EDHF) contributes to the endothelium-dependent relaxations to shear stress. This study was designed to address this point in the isolated rat mesenteric artery. Large mesenteric arteries (400-500 microm) and resistance mesenteric arteries (150-250 microm) of the rat were precontracted with phenylephrine (at 80 mm Hg of perfusion pressure), and the changes in vessel diameter in response to variable flow (0-300 microl/min) were continuously examined. The relative contributions of vasodilator prostaglandins, NO, and EDHF were analyzed by the inhibitory effects of indomethacin (10(-5) M), N(G)-nitro-L-arginine (L-NNA, 10(-4) M), and KCl (40 mM), respectively. The shear stress-induced relaxations were totally endothelium dependent in both-sized blood vessels, and the contribution of NO was more prominent in large arteries than in resistance arteries, whereas that of EDHF was noted in both-sized blood vessels. Tetrabutylammonium (a nonselective inhibitor of K channels) almost abolished, whereas the combination of charybdotoxin (an inhibitor of both large- and intermediate-conductance Ca2+ -activated K channels) and apamin (an inhibitor of small-conductance Ca2+ -activated K channels) significantly inhibited the EDHF-mediated component of the shear stress-induced relaxations. These results indicate that EDHF plays an important role in shear stress-induced endothelium-dependent relaxations, where K channels, especially calcium-activated K channels, appear to be involved.

Epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acids are potent vasodilators in the canine coronary microcirculation

Cytochrome P450 epoxygenases convert arachidonic acid into 4 epoxyeicosatrienoic acid (EET) regioisomers, which were recently identified as endothelium-derived hyperpolarizing factors in coronary blood vessels. Both EETs and their dihydroxyeicosatrienoic acid (DHET) metabolites have been shown to relax conduit coronary arteries at micromolar concentrations, whereas the plasma concentrations of EETs are in the nanomolar range. However, the effects of EETs and DHETs on coronary resistance arterioles have not been examined. We administered EETs and DHETs to isolated canine coronary arterioles (diameter, 90.0+/-3.4 microm; distending pressure, 20 mm Hg) preconstricted by 30% to 60% of the resting diameter with endothelin. All 4 EET regioisomers produced potent, concentration-dependent vasodilation (EC50 values ranging from -12.7 to -10.1 log [M]) and were approximately 1000 times more potent than reported in conduit coronary arteries. The vasodilation produced by 14,15-EET was not attenuated by removal of the endothelium and indicated a direct action of 14,15-EET on microvascular smooth muscle. Likewise, 14,15-DHET, 11,12-DHET, 8,9-DHET, and the delta-lactone of 5,6-EET produced extremely potent vasodilation (EC50 values ranging from -15.8 to -13.1 log [M]). The vasodilation produced by these eicosanoids was highly potent in comparison to that produced by other vasodilators, including arachidonic acid (EC50=-7.5 log [M]). The epoxide hydrolase inhibitor, 4-phenylchalone oxide, which blocked the conversion of [3H]14,15-EET to [3H]14,15-DHET by canine coronary arteries, did not alter arteriolar dilation to 11,12-EET; thus, the potent vasodilation induced by EETs does not require formation of DHETs. In contrast, charybdotoxin (a KCa channel inhibitor) and KCl (a depolarizing agent) blocked vasodilation by 11,12-EET and 11,12-DHET. We conclude that EETs and DHETs potently dilate canine coronary arterioles via activation of KCa channels. The preferential ability of these compounds to dilate resistance blood vessels suggests that they may be important regulators of coronary circulation.

Importance of endothelium-derived hyperpolarizing factor in human arteries

The endothelium plays an important role in maintaining the vascular homeostasis by releasing vasodilator substances, including prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). Although the former two substances have been investigated extensively, the importance of EDHF still remains unclear, especially in human arteries. Thus we tested our hypothesis that EDHF plays an important role in human arteries, particularly with reference to the effect of vessel size, its vasodilating mechanism, and the influences of risk factors for atherosclerosis. Isometric tension and membrane potentials were recorded in isolated human gastroepiploic arteries and distal microvessels (100-150 microm in diameter). The contribution of PGI2, NO, and EDHF to endothelium-dependent relaxations was analyzed by inhibitory effects of indomethacin, NG-nitro- L-arginine, and KCl, respectively. The nature of and hyperpolarizing mechanism by EDHF were examined by the inhibitory effects of inhibitors of cytochrome P450 pathway and of various K channels. The effects of atherosclerosis risk factors on EDHF-mediated relaxations were also analyzed. The results showed that (a) the contribution of EDHF to endothelium-dependent relaxations is significantly larger in microvessels than in large arteries; (b) the nature of EDHF may not be a product of cytochrome P450 pathway, while EDHF-induced hyperpolarization is partially mediated by calcium-activated K channels; and (c) aging and hypercholesterolemia significantly impair EDHF-mediated relaxations. These results demonstrate that EDHF also plays an important role in human arteries.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Cytochrome P450 metabolites of arachidonic acid: rapid incorporation and hydration of 14,15-epoxyeicosatrienoic acid in arterial smooth muscle cells

Arachidonic acid is converted to epoxyeicosatrienoic acids (EETs) by cytochrome P450 monooxygenases. EETs produce arterial vasodilatation, and recent evidence suggests that they are endothelium-derived hyperpolarizing factors. In porcine coronary arteries contracted with a thromboxane mimetic agent, we find that relaxation is rapidly initiated by exposure to 14,15-EET. The relaxation slowly increases in magnitude, resulting in a response which is sustained for more than 10 min. Cultured porcine aortic smooth muscle cells rapidly take up [3H]14,15-EET. After 3 min, radioactivity is present in neutral lipids, phosphatidylcholine, and phosphatidylinositol. The cells also convert 14,15-EET to 14,15-dihydroxyeicosatrienoic acid (14,15-DHET), and some DHET is detected in the medium after only 1 min of incubation. Like 14,15-EET, 14,15-DHET produces relaxation of the contracted coronary artery rings. These findings suggest that the incorporation into phospholipids and conversion to 14,15-DHET can occur at a rate that is fast enough to modulate the vasorelaxation produced by 14,15-EET.

Potentiation of endothelium-dependent relaxation by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are potent endothelium-derived vasodilators formed from cytochrome P-450 metabolism of arachidonic acid. EETs and their diol products (DHETs) are also avidly taken up by endothelial cells and incorporated into phospholipids that participate in signal transduction. To investigate the possible functional significance of EET and DHET incorporation into cell lipids, we examined the capacity of EETs and DHETs to relax porcine coronary arterial rings and determined responses to bradykinin (which potently activates endothelial phospholipases) before and after incubating the rings with these eicosanoids. 14,15-EET and 11,12-EET (5 mumol/L) produced 75 +/- 9% and 52 +/- 4% relaxation, respectively, of U46619-contracted rings, whereas 8,9-EET and 5,6-EET did not produce significant relaxation. The corresponding DHET regioisomers produced comparable relaxation responses. Preincubation with 14,15-EET, 11,12-EET, 14,15-DHET, and 11,12-DHET augmented the magnitude and duration of bradykinin-induced relaxation, whereas endothelium-independent relaxations to aprikalim and sodium nitroprusside were not potentiated. Pretreatment with 2 mumol/L triacsin C (an inhibitor of acyl coenzyme A synthases) inhibited [3H]14,15-EET incorporation into endothelial phospholipids and blocked 11,12-EET- and 14,15-DHET-induced potentiation of relaxation to bradykinin. Exposure of [3H]14,15-EET-labeled endothelial cells to the Ca2+ ionophore A23187 (2 mumol/L) resulted in a 4-fold increased release of EET and DHET into the medium. We conclude that incorporation of EETs and DHETs into cell lipids results in potentiation of bradykinin-induced relaxation in porcine coronary arteries, providing the first evidence that incorporated EETs and DHETs are capable of modulating vascular function.

Coronary effects of exogenous and endogenous bradykinin in conscious dogs

The effects of intravenous (iv) bolus administrations of bradykinin (0.1-1 microgram.kg-1) on large and small coronary arteries were investigated in seven chronically instrumented conscious dogs. Bradykinin dose-dependently increased heart rate, left ventricular dP/dt max, coronary blood flow and coronary artery diameter and decreased aortic pressure. Subchronic inhibition of the nitric oxide synthase (NOS) pathway (N omega-nitro-L-arginine, 20 mg.kg-1.d-1 during 7 days) attenuated the systemic and coronary effects of bradykinin. HOE 140, a specific bradykinin B2 receptor antagonist, administered at a dose (30 micrograms/kg) sufficient to completely inhibit the systemic and coronary effects of exogenous bradykinin (1 microgram/kg, iv bolus), had no effect on baseline systemic and coronary hemodynamic parameters. HOE 140 had also no effect on the flow-dependent increase in large coronary artery diameter and on the relationship between flow debt and flow repayment volumes observed during myocardial reactive hyperemia. This lack of effect of HOE 140 persisted when experiments were repeated after NOS inhibition. We conclude that (a) exogenous bradykinin dilates large and small coronary arteries through a partially NO-mediated mechanism, and (b) endogenous bradykinin plays no role in the control of arterial pressure, heart rate, LV dP/dt max, basal and flow-stimulated coronary hemodynamics, both in control conditions and after subchronic inhibition of NOS in the conscious dog.

Vascular endothelium: vasoactive mediators

In most blood vessels, the endothelium generates both vasodilator and growth-stabilizing mediators under normal physiological circumstances. The vasodilator influence of the endothelium modulates the vasoconstriction induced by adrenergic nerves, bloodborne substances, and local autacoids. Nitric oxide (NO) is a major endothelium-derived vasodilator, along with prostacyclin. A third substance called endothelium-derived hyperpolarizing factors (EDHF) mediates vasodilatation in certain conduit arteries and in most resistance vessels. EDHF may be a cytochrome P-450 metabolite of arachidonic acid. NO acts mostly through an elevation of cyclic guanosine monophosphate in vascular smooth muscle, whereas prostacyclin stimulates adenylate cyclase. The mode of action of EDHF involves the activation of K+ channels. The multiplicity of the factors released by the endothelium, as well as the complexity of the interactions among these factors and those with other nonendothelial mediators, determine the extent of vasomotor control exerted locally by the endothelium.

Contribution of nitric oxide to the beneficial effects of enalapril in the fructose-induced hyperinsulinemic rat

We examined accumulating evidence of the positive contribution of nitric oxide to the pharmacological effects of converting enzyme inhibitors in 36 rats rendered hypertensive, hyperinsulinemic, and hypertriglyceridemic by a fructose-enriched diet. We studied the response of blood pressure, insulin, and triglyceride levels to inhibition of either converting enzyme-kininase II, nitric oxide synthase, or both. Two weeks of the converting enzyme inhibitor enalapril (20 mg/kg) reduced blood pressure from 137 +/- 2 to 105 +/- 7 mm Hg, insulin from 7.6 +/- 2.0 to 2.2 +/- 1.1 pg/mL, and triglycerides from 292 +/- 37 to 163 +/- 37 mg/dL. Treatment with NG-nitro-L-arginine methyl ester (100 mg/kg) raised blood pressure from 144 +/- 7 to 170 +/- 8 mm Hg without affecting the other parameters. Two weeks of concomitant treatment with both agents blunted the hypotensive and beneficial metabolic effects of enalapril; thus, final blood pressure (141 +/- 7 mm Hg), insulin (6.4 +/- 2.4 pg/mL), and triglyceride (231 +/- 51 mg/dL) values were no different from those of untreated fructose-fed rats. These data suggest that persistent synthesis of nitric oxide contributes to the vasodilator and metabolic effects of enalapril in the fructose-fed rat model.

Functional implications of a newly characterized pathway of 11,12-epoxyeicosatrienoic acid metabolism in arterial smooth muscle

Epoxyeicosatrienoic acids (EETs) are potent vasodilators derived from cytochrome P-450 metabolism of arachidonic acid. The rapid conversion of EETs to their corresponding dihydroxyeicosatrienoic acids (DHETs) has been proposed as a process whereby EETs are rendered biologically inactive. However, the vascular metabolism of EETs and the vasoactivities of EET metabolites have not been extensively studied. Accordingly, 11,12-EET metabolism was characterized in porcine aortic smooth muscle cells. The cells converted [3H]11,12-EET to 11,12-DHET and to a newly identified metabolite, 7,8-dihydroxy-hexadecadienoic acid (DHHD). 11,12-DHET accumulation in the medium reached a maximum in 2 to 4 hours and then declined, whereas 7,8-DHHD accumulation increased continuously and exceeded the amount of 11,12-DHET by 8 hours. [3H]11,12-EET conversion to radiolabeled 7,8-DHHD was reduced in the presence of unlabeled 11,12-DHET, indicating that 11,12-DHET is an intermediate in the conversion of 11,12-EET to 7,8-DHHD. This is consistent with a pathway whereby 11,12-EET is converted by an epoxide hydrolase to 11,12-DHET, which then undergoes two beta-oxidations to form 7,8-DHHD. In porcine coronary artery rings contracted with a thromboxane mimetic, 11,12-DHET produced relaxation similar in magnitude to that produced by 11,12-EET (77% versus 64% relaxation at 5 mumol/L, respectively). 7,8-DHHD also produced vasorelaxation. Thus, the vasoactivity of 11,12-EET is not eliminated by conversion to 11,12-DHET and 7,8-DHHD. These results suggest that 11,12-DHET and its metabolite, 7,8-DHHD, may contribute to the regulation of vascular tone in the porcine coronary artery and possibly other vascular tissues.

Inhibitors of the cytochrome P450-mono-oxygenase and endothelium-dependent hyperpolarizations in the guinea-pig isolated carotid artery

1. Transmembrane potentials were recorded from isolated carotid arteries of the guinea-pig superfused with modified Krebs-Ringer bicarbonate solution. Smooth muscle cells were impaled with sharp intracellular microelectrodes. 2. Acetylcholine (1 microM) induced an endothelium-dependent hyperpolarization (14.3 +/- 2.8 mV, n = 6) which was not affected (15.1 +/- 1.1 mV, n = 35) by inhibitors of cyclo-oxygenase (indomethacin, 5 microM) and nitric oxide synthase (N omega nitro-L-arginine: L-NOARG, 100 microM). 3. The hyperpolarization produced by acetylcholine was abolished in the presence of elevated potassium (35 mM) in the superfusing physiological saline solution. 4. The acetylcholine-induced hyperpolarization was not affected by the inhibitors of cytochrome P450 mono-oxygenases, SKF525a (10 and 100 microM, 13.9 +/ 2.2 and 15.3 +/- 4.6 mV), metyrapone (100 microM, 13.1 +/- 1.9 mV), clotrimazole (100 microM, 13.5 +/- 2.7 mV), 17-octadecynoic acid (5 microM, 16.5 +/- 1.9 mV), methoxsalen (10 microM, 15.3 +/- 1.6 mV), the inhibitor of phospholipase A2 quinacrine (10 microM 12.8 +/- 2.5 mV) and the non specific lipoxygenases/cyclo-oxygenases/cytochrome P450 inhibitor, eicosatetraynoic acid (50 microM, 15.0 +/- 2.2 mV). However, the muscarinic antagonist, atropine (100 nM), abolished the hyperpolarization. 5. These results suggest that in guinea-pig carotid artery, the metabolism of arachidonic acid, either through cyclo-oxygenase, lipoxygenase or cytochrome p450 mono-oxygenase, is not involved in acetylcholine-induced endothelium-dependent hyperpolarizations.

Vasodilatation to arachidonic acid in humans. An insight into endogenous prostanoids and effects of aspirin

BACKGROUND

Human endothelial and vascular smooth muscle cells synthesize prostanoids. Several of these have been implicated in the physiological and pathophysiological regulation of vascular tone; however, there is no direct evidence that human blood vessels synthesize sufficient prostanoid to alter vessel tone.

METHODS AND RESULTS

We explored the effects of local infusions of arachidonic acid on the tone of preconstricted superficial hand veins in healthy volunteers. Aspirin was used to assess the contribution of prostanoids to the responses seen. Local infusion of arachidonic acid produced a dose-dependent dilatation of preconstricted veins. This was abolished by local infusion of aspirin. Oral aspirin was also effective: a high (anti-inflammatory) dose of aspirin (1 g) taken 2 hours before the experiment blocked the arachidonic acid-induced venodilatation; however, a low (cardioprotective) dose of aspirin (75 mg) did not. Unlike the responses to arachidonic acid, responses to glyceryltrinitrate and bradykinin were unaltered by aspirin (1 g). Ex vivo platelet aggregation was inhibited by aspirin in both high and low doses. Aspirin (1 g) inhibited arachidonic acid-induced venodilatation for up to 5 days. The time course was similar for vascular and platelet effects.

CONCLUSIONS

The present findings demonstrate that local generation of prostanoids in a human vessel in vivo alters vascular tone. The predominant prostanoid synthesized is a dilator and its synthesis can be blocked by an anti-inflammatory but not a cardioprotective dose of aspirin. The results suggest that selective inhibition of platelet aggregation by oral aspirin might be a function of dose rather than the interval between doses.

Relationship of arachidonic acid release to porcine coronary artery relaxation

In porcine coronary artery endothelium-dependent relaxation to bradykinin is in part attributed to a chemically unidentified factor, termed endothelium-derived hyperpolarizing factor (EDHF). We hypothesize that arachidonic acid, acting through a cyclooxygenase-independent mechanism, is responsible for EDHF production. To define the relationship between EDHF production and arachidonic acid release, we investigated the role of phospholipase C in bradykinin-induced relaxation and prostaglandin I2 production (an index of arachidonic acid release) in porcine coronary artery. The phospholipase C inhibitor U73122 (1 mumol/L) abolished bradykinin-induced, nitric oxide-mediated relaxation but did not inhibit either bradykinin-induced, EDHF-mediated relaxation or prostaglandin I2 production. However, when given at a larger dose (20 mumol/L) U73122 abolished both bradykinin-induced, EDHF-mediated relaxation and prostaglandin I2 production. Similarly, the calcium-ATPase inhibitor thapsigargin, given at a dose (1 mumol/L) that abolished bradykinin-induced increases in intracellular calcium concentration in cultured porcine coronary artery endothelial cells, eliminated both bradykinin-induced. EDHF-mediated relaxation and prostaglandin I2 production. Although thapsigargin abolished bradykinin-induced prostaglandin I2 production, the basal production of prostaglandin I2 was enhanced and contraction of endothelium-intact rings was attenuated. These latter responses are most likely related to enhanced basal arachidonic acid release and associated EDHF production. These observations suggest that phospholipase C activation and increased intracellular calcium concentration are required for both bradykinin-induced arachidonic acid release and EDHF production in porcine coronary artery.(ABSTRACT TRUNCATED AT 250 WORDS)