Identification of epoxyeicosatrienoic acids as endothelium-derived hyperpolarizing factors

Pentobarbital-sensitive EDHF comediates ACh-induced arteriolar dilation in the hamster microcirculation

It is unclear to what extent the endothelium-derived hyperpolarizing factor (EDHF) contributes to the control of microcirculatory blood flow in vivo. We analyzed, by intravital microscopy in hamster muscles, the potential role of EDHF along the vascular tree under stimulated (ACh) or basal conditions. Experiments were performed in conscious as well as anesthetized (pentobarbital, urethan) animals. Additionally, cellular effects of the potential EDHF were studied in isolated small arteries. In pentobarbital-anesthetized animals, treatment with Nomega-nitro-L-arginine (L-NNA; 30 micromol/l) and indomethacin (3 micromol/l) reduced the dilation in response to 10 micromol/l ACh from 60 +/- 6 to 20 +/- 4%. This nitric oxide/prostaglandin-independent dilation (NPID), which was of a similar magnitude in large and small arterioles, was abolished by potassium depolarization or charybdotoxin (ChTX, 1 micromol/l) but not by glibenclamide. In conscious animals, NPID amounted to 33 +/- 3%. The inhibitor of the P-450 monooxygenase 17-octadecynoic acid (ODYA) reduced NPID further to 9 +/- 4%. ChTX abolished the NPID and also reduced basal diameters (by -11 +/- 3%). The induction of anesthesia with pentobarbital reduced NPID (to 12 +/- 6%), whereas urethan anesthesia was without effect. Pentobarbital also reduced the ACh-induced hyperpolarization of vascular smooth muscle in isolated arteries, whereas ChTX abolished it. This study suggests that a considerable part of the ACh dilation in the microcirculation is mediated by EDHF, which also contributes to the control of basal tone in conscious animals. The direct inhibitory effect of pentobarbital and ODYA supports the idea that "microcirculatory" EDHF is a product of the cytochrome P-450 pathway. The role of EDHF might be underestimated in pentobarbital-anesthetized animals.

Spread of vasodilatation and vasoconstriction along feed arteries and arterioles of hamster skeletal muscle

1. In arterioles of the hamster cheek pouch, vasodilatation and vasoconstriction can spread via the conduction of electrical signals through gap junctions between cells that comprise the vessel wall. However, conduction in resistance networks supplying other tissues has received relatively little attention. In anaesthetized hamsters, we have investigated the spread of dilatation and constriction along feed arteries and arterioles of the retractor muscle, which is contiguous with the cheek pouch. 2. When released from a micropipette, acetylcholine (ACh) triggered vasodilatation that spread rapidly along feed arteries external to the muscle and arterioles within the muscle. Responses were independent of changes in wall shear rate, perivascular nerve activity, or release of nitric oxide, indicating cell-to-cell conduction. 3. Vasodilatation conducted without decrement along unbranched feed arteries, yet decayed markedly in arteriolar networks. Thus, branching of the conduction pathway dissipated the vasodilatation. 4. Noradrenaline (NA) or a depolarizing KCl stimulus evoked constriction of arterioles and feed arteries of the retractor muscle that was constrained to the vicinity of the micropipette. This behaviour contrasts sharply with the conduction of vasodilatation in these microvessels and with the conduction of vasoconstriction elicited by NA and KCl in cheek pouch arterioles. 5. Focal electrical stimulation produced constriction that spread rapidly along feed arteries and arterioles. These responses were inhibited by tetrodotoxin or prazosin, confirming the release of NA along perivascular sympathetic nerves, which are absent from arterioles studied in the cheek pouch. Thus, sympathetic nerve activity co-ordinated the contraction of smooth muscle cells as effectively as the conduction of vasodilatation co-ordinated their relaxation. 6. In the light of previous findings in the cheek pouch, the properties of vasoconstriction and vasodilatation in feed arteries and arterioles of the retractor muscle indicate that substantive differences can exist in the nature of signal transmission along microvessels of tissues that differ in structure and function.

Calcium responses induced by acetylcholine in submucosal arterioles of the guinea-pig small intestine

1. Calcium responses induced by brief stimulation with acetylcholine (ACh) were assessed from the fluorescence changes in fura-2 loaded submucosal arterioles of the guinea-pig small intestine. 2. Initially, 1-1.5 h after loading with fura-2 (fresh tissues), ACh increased [Ca2+]i in a concentration-dependent manner. This response diminished with time, and finally disappeared in 2-3 h (old tissues). 3. Ba2+ elevated [Ca2+]i to a similar extent in both fresh and old tissues. ACh further increased the Ba2+-elevated [Ca2+]i in fresh tissues, but reduced it in old tissues. Responses were not affected by either indomethacin or nitroarginine. 4. In fresh mesenteric arteries, mechanical removal of endothelial cells abolished the ACh-induced increase in [Ca2+]i, with no alteration of [Ca2+]i at rest and during elevation with Ba2+. 5. In the presence of indomethacin and nitroarginine, high-K+ solution elevated [Ca2+]i in both fresh and old tissues. Subsequent addition of ACh further increased [Ca2+]i in fresh tissues without changing it in old tissues. 6. Proadifen, an inhibitor of the enzyme cytochrome P450 mono-oxygenase, inhibited the ACh-induced changes in [Ca2+]i in both fresh and Ba2+-stimulated old tissues. It also inhibited the ACh-induced hyperpolarization. 7. In fresh tissues, the ACh-induced Ca2+ response was not changed by apamin, charybdotoxin (CTX), 4-aminopyridine (4-AP) or glibenclamide. In old tissues in which [Ca2+]i had previously been elevated with Ba2+, the ACh-induced Ca2+ response was inhibited by CTX but not by apamin, 4-AP or glibenclamide. 8. It is concluded that in submucosal arterioles, ACh elevates endothelial [Ca2+]i and reduces muscular [Ca2+]i, probably through the hyperpolarization of endothelial or smooth muscle membrane by activating CTX-sensitive K+ channels.

Endothelial dysfunction in DOCA-salt hypertension: possible involvement of prostaglandin endoperoxides

The effects of the arachidonic acid metabolism inhibitors on the acetylcholine responses of aortae from control (CR) and deoxycorticosterone acetate (DOCA)-salt hypertensive (HR) rats were investigated. The acetylcholine decreased response observed in HR [relaxation (%): CR 95.5+/-2.7, n = 4; HR 52.0+/-6.3, n = 5, p < 0.05] was restored by the cyclooxygenase inhibitor piroxicam [relaxation (%): CR 99.8+/-0.2, n = 4; HR 86.0+/-4.0, n = 5] and by the thromboxane synthetase inhibitor and the thromboxane A2/prostaglandin H2 receptor antagonist ridogrel [relaxation (%): CR 92.1+/-4.4, n = 7; HR 93.1+/-2.0, n = 7] but not by the inhibitors of thromboxane synthetase, prostacyclin synthetase, cytochrome P-450 monooxygenase, and lipoxygenase. So, endoperoxide intermediates seem to be involved in the decreased endothelium-dependent relaxation to acetylcholine in DOCA-salt hypertension.

Transfection of an active cytochrome P450 arachidonic acid epoxygenase indicates that 14,15-epoxyeicosatrienoic acid functions as an intracellular second messenger in response to epidermal growth factor

A common feature of most isolated cell systems is low or undetectable levels of bioactive cytochrome P450. We therefore developed stable transfectants of the renal epithelial cell line, LLCPKcl4, that expressed an active regio- and enantioselective arachidonic acid (AA) epoxygenase. Site-specific mutagenesis was used to convert bacterial P450 BM-3 into an active regio- and stereoselective 14S,15R-epoxygenase (F87V BM-3). In clones expressing F87V BM-3 (F87V BM-3 cells), exogenous AA induced significant 14S,15R-epoxyeicosatrienoic acid (EET) production (241. 82 ng/10(8) cells, >97% of total EETs), whereas no detectable EETs were seen in cells transfected with vector alone. In F87V BM-3 cells, AA stimulated [3H]thymidine incorporation and increased cell proliferation, which was blocked by the tyrosine kinase inhibitor, genistein, by the phosphatidylinositol 3 (PI-3) kinase inhibitors, wortmannin and LY294002, and by the mitogen-activated protein kinase kinase inhibitor, PD98059. AA also induced tyrosine phosphorylation of extracellular signal-regulated kinase (ERK) and PI-3 kinase that was inhibited by the cytochrome P450 BM-3 inhibitor, 17-ODYA. Epidermal growth factor (EGF) increased EET production in F87V BM-3 cells, which was completely abolished by pretreatment with either 17-ODYA or the phospholipase A2 (PLA2) inhibitor, quinacrine. Compared with vector-transfected cells, F87 BM-3 transfected cells demonstrated marked increases in both the extent and sensitivity of DNA synthesis in response to EGF. These changes occurred in the absence of significant differences in EGF receptor expression. As seen with exogenous AA, EGF increased ERK tyrosine phosphorylation to a significantly greater extent in F87V BM-3 cells than in vector-transfected cells. Furthermore, in these control cells, neither 17-ODYA nor quinacrine inhibited EGF-induced ERK tyrosine phosphorylation. On the other hand, in F87V BM-3 cells, both inhibitors reduced ERK tyrosine phosphorylation to levels indistinguishable from that seen in cells transfected with vector alone. These studies provide the first unequivocal evidence for a role for the AA epoxygenase pathway and endogenous EET synthesis in EGF-mediated signaling and mitogenesis and provide compelling evidence for the PLA2-AA-EET pathway as an important intracellular-signaling pathway in cells expressing high levels of cytochrome P450 epoxygenase.

Endothelium-derived hyperpolarizing factor(s): species and tissue heterogeneity

1. Endothelium-derived relaxing factor is almost universally considered to be synonymous with nitric oxide (NO); however, it is now well established that at least two other chemically distinct species (prostacyclin (PGI2) and a hyperpolarizing factor) may also contribute to endothelium-dependent relaxation. 2. Only relatively few studies have provided definitive evidence that an endothelium-derived hyperpolarizing factor (EDHF), which is neither NO nor PGI2, exists as a chemical mediator. 3. There is a lack of agreement as to the likely chemical identity of this putative factor. Some evidence suggests that EDHF may be a cytochrome P450-derived arachidonic acid product, possibly an epoxyeicosatrienoic acid (EET); conflicting evidence supports an endogenous cannabinoid as the mediator and still other studies infer an unknown mediator that is neither a cytochrome P450 nor a cannabinoid. 4. Data from our laboratory with a rabbit carotid artery 'sandwich' preparation have provided evidence that a mediator that meets the pharmacological expectations of a cytochrome P450 product is an EDHF. 5. Data from guinea-pig mesenteric arterioles suggest that EDHF is not a cytochrome P450 product, whereas in guinea-pig middle cerebral arteries, relaxation mediated by the NO/PGI2-independent mediator(s) is sensitive to cytochrome P450 inhibitors. In addition, in the rabbit middle cerebral artery, it is likely that endothelium-dependent hyperpolarization is mediated by both NO and PGI2. 6. In conclusion, these data indicate that EDHF is unlikely to be a single factor and that considerable tissue and species differences exist for the nature and cellular targets of the hyperpolarizing factors.

P450 subfamily CYP2J and their role in the bioactivation of arachidonic acid in extrahepatic tissues

Historically, there has been intense interest in P450 metabolic oxidation, peroxidation, and reduction of xenobiotics. More recently, there has been a growing appreciation for the role of P450s in the oxidation of lipophilic endobiotics, such as bile acids, fat-soluble vitamins, and eicosanoids. This review details the emerging CYP2J subfamily of P450s and their role as catalysts of arachidonic acid metabolism.

Effect of behavioral stress on coronary artery relaxation altered with aging in BHR

Behavioral stress and aging are associated with an increase in vascular disease. This study determined the mechanisms contributing to changes in endothelium-dependent relaxation of isolated coronary arteries (300-350 micrometers) induced by exposure to 10 days of air-jet stress (2 h/day) in young (3 mo) and old (18 mo) male borderline hypertensive rats (BHR). Aging, alone, did not alter endothelium-dependent relaxation to acetylcholine (ACh) quantitatively but did alter the mechanisms contributing to relaxation to ACh, which was largely dependent on nitric oxide synthase (NOS) in vessels from old, but not young, BHR. Behavioral stress resulted in an enhanced relaxation to ACh that was dependent on NOS in vessels from young stressed compared with young control BHR. Conversely, relaxation to ACh was reduced in coronary arteries from old stressed compared with old control BHR. In vessels from old control BHR, there was an NOS-independent component of relaxation mediated by opening of K+ channels that was absent in vessels from old stressed BHR. The superoxide anion scavenger, tiron, partially restored relaxation, and inhibition of cyclooxygenase largely restored relaxation to ACh in vessels from old stressed BHR. In summary, the effect of behavioral stress was age dependent. ACh-induced relaxation of coronary arteries was enhanced in an NOS-dependent manner in young BHR and was impaired in old BHR due to superoxide anions, vasoconstrictor cyclooxygenase products, and a loss of K+ channel-mediated relaxation.

Regulation of vascular tone by endothelium-derived hyperpolarizing factor

1. Endothelium-derived hyperpolarizing factor (EDHF) mediates the nitric oxide (NO)-independent component of the relaxation in rat mesenteric arteries. The relationship between hyperpolarization and vascular tone was studied by simultaneous recording of membrane potential with intracellular microelectrodes and tension in ring segments of rat mesenteric arteries. 2. By depolarizing arteries with high potassium solutions, it was determined that the threshold for contraction is approximately -46 mV. Maximum contraction was attained when the arteries were depolarized to -20 mV. Thus, 1 mV depolarization resulted in an approximate 4% increase in tone. This relationship was not altered in spontaneously hypertensive rats. 3. Noradrenaline (0.3 mumol/L) caused contraction and depolarized arteries by 13 mV. Acetylcholine caused endothelium-dependent relaxation and hyperpolarization up to 14 mV. In the presence of N omega-nitro-L-arginine, the EDHF-mediated relaxation was correlated to hyperpolarization. A hyperpolarization of 1 mV corresponded to a 4.3% decrease of the induced tone. 4. At concentrations (10 mumol/L) causing total relaxation, the maximum hyperpolarization induced by NO was only 7.6 mV. 5. A maximum relaxation of 88% was observed with pinacidil (3 mumol/L), despite a 25 mV hyperpolarization. Relaxations to NO and pinacidil were not correlated with hyperpolarization. At similar levels of hyperpolarization, NO and pinacidil elicited more relaxation than EDHF. 6. These studies show that vascular tone is very sensitive to membrane potential change in the range between -46 and -20 mV in the rat mesenteric artery. The relaxation response to EDHF, unlike that to NO and pinacidil, can be accounted for solely by its effect on the membrane potential.

Endothelium-dependent hyperpolarization and intercellular electrical coupling in guinea-pig mesenteric arterioles

1. Using the conventional whole-cell clamp method, the electrical responses of individual smooth muscle and endothelial cells to acetylcholine (ACh) were observed in multicellular preparations where the two types of cells remained in close apposition. 2. In both types of cells, ACh induced similar hyperpolarizing responses which, when recorded in current clamp mode, had two phases (an initial fast and a second slower phase). 3. After blocking gap junctions, including myoendothelial junctions, with 18beta-glycyrrhetinic acid, ACh induced an outward current with two phases in voltage-clamped endothelial cells. The outward current appeared around -90 mV and increased linearly with the membrane depolarization. 4. In smooth muscle cells, ACh failed to induce a membrane current after gap junctions had been blocked with 18beta-glycyrrhetinic acid. The inhibition of ACh-induced response by 18beta-glycyrrhetinic acid was observed using either sharp or patch electrodes. 5. Nominally Ca2+-free solution reduced the initial phase and abolished the second phase of ACh-induced responses of endothelial cells. Both phases were also reduced by charybdotoxin (CTX). 6. Our results indicate that in guinea-pig mesenteric arterioles, ACh hyperpolarizes endothelial cells by activating Ca2+-activated K+ channels which are sensitive to CTX. On the other hand, hyperpolarizing responses detected in smooth muscle cells seem to originate in endothelial cells and conduct to the muscle layer via myoendothelial gap junctions.

Inhibition of endothelium-dependent hyperpolarization by endothelial prostanoids in guinea-pig coronary artery

1. In smooth muscle of the circumflex coronary artery of guinea-pig, acetylcholine (ACh, 10(-6) M) produced an endothelium-dependent hyperpolarization consisting of two components. An initial component that occurs in the presence of ACh and a slow component that developed after ACh had been withdrawn. Each component of the hyperpolarization was accompanied by an increase in membrane conductance. 2. Indomethacin (5 x 10(-6) M) or diclofenac (10(-6) M), both inhibitors of cyclooxygenase, abolished only the slow hyperpolarization. The initial hyperpolarization was not inhibited by diclofenac nor by nitroarginine, an inhibitor of nitric oxide synthase. 3. Both components of the ACh-induced hyperpolarization were abolished in the presence of atropine (10(-6) M) or high-K solution ([K+]0 = 29.4 mM). 4. The interval between ACh-stimulation required to generate an initial hyperpolarization of reproducible amplitude was 20 min or greater, but it was reduced to less than 5 min after inhibiting cyclooxygenase activity. Conditioning stimulation of the artery with substance P (10(-7) M) also caused a long duration (about 20 min) inhibition of the ACh-response. 5. The amplitude of the hyperpolarization generated by Y-26763, a K+-channel opener, was reproducible within 10 min after withdrawal of ACh. 6. Exogenously applied prostacyclin (PGI2) hyperpolarized the membrane and reduced membrane resistance in concentrations over 2.8 x 10(-9)M. 7. At concentrations below threshold for hyperpolarization and when no alteration of membrane resistance occurred, PGI2 inhibited the initial component of the ACh-induced hyperpolarization. 8. It is concluded that endothelial prostanoids, possibly PGI2, have an inhibitory action on the release of endothelium-derived hyperpolarizing factor.

Lovastatin maintains nitric oxide--but not EDHF-mediated endothelium-dependent relaxation in the hypercholesterolemic rabbit carotid artery

The endothelium contributes to the regulation of vascular tone by producing nitric oxide (NO) and the endothelium-derived hyperpolarising factor (EDHF). In hypercholesterolemia, endothelium-dependent relaxation is impaired but can be restored by treatment with lovastatin (LOVAS). We investigated the effects of LOVAS on NO and EDHF-mediated relaxation. Rabbits were fed 1% cholesterol diet for 4 weeks and 0.5%) cholesterol for the following 12 weeks (CHOL-group). The LOVAS group additionally received 10 mg of lovastatin over the last 12-week period. Experiments were performed in carotid artery rings. Relaxant responses to acetylcholine (ACh) were recorded in the presence of indomethacin. Nitro-L-arginine (NOARG, 100 microM) and potassium chloride (KCl, 35 mM) were used to differentiate between NO- and EDHF-mediated relaxations. Cholesterol impaired ACh-induced relaxations and this effect was prevented by LOVAS (control 100+/-1%, CHOL 81+/-6%, LOVAS 98+/-1%). In the presence of NOARG, relaxations to ACh were not different between the LOVAS and CHOL groups (control 78+/-4%, CHOL 64+/-6%, LOVAS 64+/-5%). When KCl was used, ACh-induced relaxations were similar in the LOVAS and control group (control 75+/-5%, CHOL 49+/-6%, LOVAS 76+/-2%). In arteries treated with NOARG and KCl together, no relaxations were observed. Relaxations of arteries from the control group were not affected by 18 h preincubation with lovastatin (10 microM). Lovastatin selectively maintains nitric oxide-mediated endothelium-dependent relaxation in hypercholesterolemic rabbit carotid arteries.

Calcium influx factor, further evidence it is 5, 6-epoxyeicosatrienoic acid

We present evidence in astrocytes that 5,6-epoxyeicosatrienoic acid, a cytochrome P450 epoxygenase metabolite of arachidonic acid, may be a component of calcium influx factor, the elusive link between release of Ca2+ from intracellular stores and capacitative Ca2+ influx. Capacitative influx of extracellular Ca2+ was inhibited by blockade of the two critical steps in epoxyeicosatrienoic acid synthesis: release of arachidonic acid from phospholipid stores by cytosolic phospholipase A2 and cytochrome P450 metabolism of arachidonic acid. AAOCF3, which inhibits cytosolic phospholipase A2, blocked thapsigargin-stimulated release of arachidonic acid as well as thapsigargin-stimulated elevation of intracellular free calcium. Inhibition of P450 arachidonic acid metabolism with SKF525A, econazole, or N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide, a substrate inhibitor of P450 arachidonic acid metabolism, also blocked thapsigargin-stimulated Ca2+ influx. Nano- to picomolar 5, 6-epoxyeicosatrienoic acid induced [Ca2+]i elevation consistent with capacitative Ca2+ influx. We have previously shown that 5, 6-epoxyeicosatrienoic acid is synthesized and released by astrocytes. When 5,6-epoxyeicosatrienoic acid was applied to the rat brain surface, it induced vasodilation, suggesting that calcium influx factor may also serve a paracrine function. In summary, our results suggest that 5,6-epoxyeicosatrienoic acid may be a component of calcium influx factor and may participate in regulation of cerebral vascular tone.

Exercise training increases L-type calcium current density in coronary smooth muscle

Exercise training produces numerous adaptations in the coronary circulation, including an increase in coronary tone, both in conduit and resistance arteries. On the basis of the importance of voltage-gated Ca2+ channels (VGCC) in regulation of vascular tone, we hypothesized that exercise training would increase VGCC current density in coronary smooth muscle. To test this hypothesis, VGCC current was compared in smooth muscle from conduit arteries (>1.0 mm), small arteries (200-250 micrometer), and large arterioles (75-150 micrometer) from endurance-trained (Ex) or sedentary miniature swine (Sed). After 16-20 wk of treadmill training, VGCC current was determined using whole cell voltage-clamp techniques. In both Ex and Sed, VGCC current density was inversely related to arterial diameter, i.e., large arterioles > small arteries > conduit arteries. Exercise training increased peak inward currents approximately twofold in smooth muscle from all arterial sizes compared with those from Sed (large arteriole, -12.52 +/- 2.05 vs. -5.74 +/- 0.99 pA/pF; small artery, -6.20 +/- 0.97 vs. -3.18 +/- 0.44 pA/pF; and conduit arteries, -4.22 +/- 0.30 vs. -2.41 +/- 0.55 pA/pF; 10 mM Ba2+ external). Dihydropyridine sensitivity, voltage dependence, and inactivation kinetics identified this Ca2+ current to be L-type current in all arterial sizes from both Sed and Ex. Furthermore, peak VGCC current density was correlated with treadmill endurance in all arterial sizes. We conclude that smooth muscle L-type Ca2+ current density is increased within the coronary arterial bed by endurance exercise training. This increased VGCC density may provide an important mechanistic link between functional and cellular adaptations in the coronary circulation to exercise training.

14,15-Epoxyeicosatrienoic acid inhibits prostaglandin E2 production in vascular smooth muscle cells

14,15-Epoxyeicosatrienoic acid (EET), a cytochrome P-450 epoxygenase product of arachidonic acid (AA), reduced PGE2 formation by 40-75% in porcine aortic and murine brain microvascular smooth muscle cells. The inhibition was reversed 6-10 h after removal of 14,15-EET from the medium and was regioisomeric specific; 8,9-EET produced a smaller effect, whereas 11,12- and 5,6-EET were ineffective. Although the cells converted 14,15-EET to 14, 15-dihydroxyeicosatrienoic acid (14,15-DHET), 14,15-DHET did not inhibit PGE2 formation, and the 14,15-EET-induced inhibition was potentiated by 4-phenylchalcone oxide, an epoxide hydrolase inhibitor. The inhibition occurred when substrate amounts of AA were used and was not accompanied by enhanced production of other PGs, suggesting an effect on PGH synthase; however, in murine cells, 14, 15-EET did not reduce PGH synthase mRNA or protein. Moreover, the 14, 15-EET-induced decrease in PGE2 production was overcome by increasing the concentration of AA, but not oleic acid (which is not a substrate for PGH synthase). These findings suggest that 14,15-EET competitively inhibits PGH synthase activity in vascular smooth muscle cells. The 14,15-EET-induced inhibition of PGE2 production resulted in potentiation of platelet-derived growth factor-induced smooth muscle cell proliferation, suggesting that the competitive inhibition of PGH synthase by 14,15-EET can affect growth responses in smooth muscle cells.

Identification of the 11,14,15- and 11,12, 15-trihydroxyeicosatrienoic acids as endothelium-derived relaxing factors of rabbit aorta

A number of endothelium-derived relaxing factors have been identified including nitric oxide, prostacyclin, and the epoxyeicosatrienoic acids. Previous work showed that in rabbit aortic endothelial cells, arachidonic acid was metabolized by a lipoxygenase to vasodilatory eicosanoids. The identity was determined by the present study. Aortic homogenates were incubated in the presence of [U-14C]arachidonic acid, [U-14C]arachidonic acid plus 15-lipoxygenase (soybean lipoxidase), or [U-14C]15-hydroxyeicosatetraenoic acid (15-HPETE) and analyzed by reverse phase high pressure liquid chromatography (RP-HPLC). Under both experimental conditions, there was a radioactive metabolite that migrated at 17.5-18.5 min on RP-HPLC. When the metabolite was isolated from aortic homogenates, it relaxed precontracted aortas in a concentration-dependent manner. Gas chromatography/mass spectrometry (GC/MS) of the derivatized metabolite indicated the presence of two products; 11,12,15-trihydroxyeicosatrienoic acid (THETA) and 11,14,15-THETA. A variety of chemical modifications of the metabolite supported these structures and confirmed the presence of a carboxyl group, double bonds, and hydroxyl groups. With the combination of 15-lipoxygenase, arachidonic acid, and aortic homogenate, an additional major radioactive peak was observed. This fraction was analyzed by GC/MS. The mass spectrum was consistent with this peak, containing both the 11-hydroxy-14, 15-epoxyeicosatrienoic acid (11-H-14,15-EETA) and 15-H-11,12-EETA. The hydroxyepoxyeicosatrienoic acid (HEETA) fraction also relaxed precontracted rabbit aorta. Microsomes derived from rabbit aortas also synthesized 11,12,15- and 11,14,15-THETAs from 15-HPETE, and pretreatment with the cyctochrome P450 inhibitor, miconazole, blocked the formation of these products. The present studies suggest that arachidonic acid is metabolized by 15-lipoxygenase to 15-HPETE, which undergoes an enzymatic rearrangement to 11-H-14,15-EETA and 15-H-11,12-EETA. Hydrolysis of the epoxy group results in the formation of 11,14,15- and 11,12,15-THETA, which relaxed rabbit aorta. Thus, the 15-series THETAs join prostacyclin, nitric oxide, and epoxyeicosatrienoic acids as new members of the family of endothelium-derived relaxing factors.

K+ is an endothelium-derived hyperpolarizing factor in rat arteries

In arteries, muscarinic agonists such as acetylcholine release an unidentified, endothelium-derived hyperpolarizing factor (EDHF) which is neither prostacyclin nor nitric oxide. Here we show that EDHF-induced hyperpolarization of smooth muscle and relaxation of small resistance arteries are inhibited by ouabain plus Ba2+; ouabain is a blocker of Na+/K+ ATPase and Ba2+ blocks inwardly rectifying K+ channels. Small increases in the amount of extracellular K+ mimic these effects of EDHF in a ouabain- and Ba2+-sensitive, but endothelium-independent, manner. Acetylcholine hyperpolarizes endothelial cells and increases the K+ concentration in the myoendothelial space; these effects are abolished by charbdotoxin plus apamin. Hyperpolarization of smooth muscle by EDHF is also abolished by this toxin combination, but these toxins do not affect the hyperpolarizaiton of smooth muscle by added K+. These data show that EDHF is K+ that effluxes through charybdotoxin- and apamin-sensitive K+ channels on endothelial cells. The resulting increase in myoendothelial K+ concentration hyperpolarizes and relaxes adjacent smooth-muscle cells by activating Ba2+-sensitive K+ channels and Na+/K+ ATPase. These results show that fluctuations in K+ levels originating within the blood vessel itself are important in regulating mammalian blood pressure and flow.

Epoxyeicosatrienoic acids and their sulfonimide derivatives stimulate tyrosine phosphorylation and induce mitogenesis in renal epithelial cells

In our present studies utilizing a well characterized proximal tubule cell line, LLCPKcl4, we determined that all four EET regioisomers (5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET) stimulated [3H]thymidine incorporation, with 14,15-EET being the most potent. In contrast, no mitogenic effects were seen with arachidonic acid, other cP450 arachidonate metabolites (12R-hydroxyeicosatetraenoic acid (12R-HETE), 14,15-dihydroxyeicosatrienoic acid (14,15-DHET), or 20-HETE), or lipoxygenase metabolites (5S-HETE, leukotriene B4, or lipoxin A4). We found that their metabolically more stable sulfonimide (SI) analogs (11,12-EET-SI and 14,15-EET-SI) were also potent mitogens. In addition 14,15-EET-SI also increased cell proliferation as well as expression of both c-fos and egr-1 mRNA. The protein kinase C and A inhibitors, H-7 and H-8, or the cyclooxygenase inhibitor, indomethacin, had no effect upon 14, 15-EET-induced [3H]thymidine incorporation, but the selective tyrosine kinase inhibitor, genistein, significantly inhibited it. Immunoprecipitation and immunoblotting demonstrated increased tyrosine phosphorylation of PI3-kinase and epidermal growth factor receptor (EGFR) within 1 min of EET administration. EETs also stimulated association of PI3-kinase with EGFR. PI3-kinase inhibitors, wortmannin and LY 294002, markedly inhibited 14, 15-EET-SI-stimulated [3H]thymidine incorporation. In addition, 14, 15-EET-SI administration stimulated tyrosine phosphorylation of src homologous and collagen-like protein (SHC) and association of SHC with both growth factor receptor-binding protein (GRB2) and EGFR. Mitogen-activated protein kinase was also activated within 5 min. Pretreatment of the cells with the mitogen-activated protein kinase kinase inhibitor, PD98059, inhibited the 14,15-EET-SI-stimulated [3H]thymidine incorporation. Moreover, immunoblotting indicated that 14,15-EET stimulated tyrosine phosphorylation of the specific pp60(c-src) substrate p120 and c-Src association with EGFR. 14, 15-EET increased src kinase activity within 1 min. Our data indicate that EETs are potent mitogens for renal epithelial cells, and the mitogenic effects of the EETs are mediated, at least in part, by the activation of Src kinase and initiation of a tyrosine kinase phosphorylation cascade.

Potassium-channel opener in cardioplegia may restore coronary endothelial function

BACKGROUND

Depolarizing (hyperkalemic) solutions impair the coronary endothelial function through an endothelium-derived hyperpolarizing factor mechanism. I examined the hypothesis that potassium-channel openers may restore the impaired endothelium-derived hyperpolarizing factor-mediated coronary vasorelaxation when added to hyperkalemic cardioplegia.

METHODS

The porcine coronary arteries were exposed to hyperkalemia (potassium, 20 or 50 mmol/L) or hyperkalemia plus the potassium-channel opener aprikalim at 0.1 mmol/L for 1 hour. Endothelium-derived hyperpolarizing factor-mediated relaxation (percentage of 30 nmol/L U46619 precontraction) was induced by calcium ionophore A23187 and bradykinin in the presence of indomethacin (7 micromol/L) and Nomega-nitro-L-arginine (300 micromol/L).

RESULTS

The endothelium-derived hyperpolarizing factor-mediated relaxation was significantly impaired by exposure to hyperkalemia (20 mmol/L: 24.9%+/-14.1% versus 88.0%+/-3.3% in control, p = 0.002 for A23187; 50 mmol/L: 40.5%+/-12.3% versus 76.5%+/-3.8%, p = 0.003 for bradykinin). This reduced relaxation was significantly recovered by addition of aprikalim into the hyperkalemic (20 mmol/L) solution in A23187 experiments (81.2%+/-4.8%, p = 0.002) but only slightly recovered when added into the higher concentration of potassium (50 mmol/L) in bradykinin experiments (56.1%+/-4.7%, p = 0.2).

CONCLUSIONS

Potassium-channel openers may preserve endothelium-derived hyperpolarizing factor-mediated coronary relaxation when added to traditional hyperkalemic cardioplegia. This effect is significant when the potassium concentration is 20 mmol/L but partially lost when it reaches 50 mmol/L. This study may provide new insights into cardioprotection during open heart operations.

Origin and function of epoxyeicosatrienoic acids in vascular endothelial cells: more than just endothelium-derived hyperpolarizing factor?

1. In addition to their contribution to endothelium-derived hyperpolarization, our understanding of the physiological function of epoxyeicosatrienoic acids (EET) within the vascular wall and the actual enzymes involved in the formation of the EET in endothelial cells is very limited. In the present study, the expression of potential cytochrome P450 (CYP) mono/epoxygenases was assessed in endothelial cells isolated from porcine and bovine aortas as well as in the human umbilical vein-derived cell lines EA.hy926 and ECV304. 2. Expression of CYP2B1, CYP2E1 and CYP3A could be found. The latter were inducible by dexamethasone/clofibrate for 72 h, a procedure that also enhanced CYP epoxygenase activity in endothelial cells. 3. Enzyme induction yielded increases in capacitative Ca2+ entry and membrane hyperpolarization in response to autacoids, such as bradykinin and thapsigargin. Thiopentone sodium, an inhibitor of endothelial CYP mono/epoxygenase(s), diminished autacoid-induced capacitative Ca2+ entry and membrane hyperpolarization, while the effect of EET remained unchanged. 4. Epoxyeicosatrienoic acids activated endothelial tyrosine kinase activity in a concentration-dependent manner. Arachidonic acid, at 20-fold higher concentrations, also increased tyrosine kinase activity. Because only the effect of arachidonic acid was inhibited by thiopentone sodium, an inhibitor of CYP mono/epoxygenases, these data suggest that arachidonic acid needs to be converted to the EET in order to stimulate tyrosine kinase. 5. All these data provide clear evidence that the CYP epoxygenase-derived arachidonic acid metabolites (EET) not only serve as potential endothelium-derived hyperpolarizing factors but also constitute highly active intracellular messengers with a physiological role including the control of Ca2+ signalling, membrane potential and tyrosine kinase activity.

Endothelium-derived hyperpolarizing factor activates Ca2+-activated K+ channels in porcine coronary artery smooth muscle cells

Although endothelium-derived hyperpolarizing factor (EDHF) activity has been demonstrated in arteries from various species, EDHF has not been chemically identified, nor its mechanism of action characterized. To elucidate this mechanism, we tested the effect of EDHF on large-conductance Ca2+-activated K+ (K(Ca)) channels in porcine coronary artery smooth muscle cells. By using a patch-clamp technique, single-channel currents were recorded in cultured smooth muscle cells; the organ bath also contained a strip of porcine coronary with endothelium, which served as the source of endothelium-derived relaxing factor(s) including EDHF. Exposure of endothelium to 10(-6) M bradykinin activated K(Ca) channels in cultured smooth muscle cells in cell-attached patches. When the experiment was performed in the presence of 10 microM indomethacin and 30 microM N(G)-nitro-L-arginine (L-NNA), which block the generation of prostaglandin I2 (PGI2) and NO, respectively, K(Ca) channel activity was stimulated by bradykinin, indicating the direct involvement of EDHF in K(Ca) channel stimulation. Neither 10 microM methylene blue nor 25 microM Rp-cAMPS inhibited bradykinin-induced K(Ca) channel activity. In inside-out patches, the addition of bradykinin to the solution was without effect on K(Ca) channel activation. However, in the presence of 0.5 mM guanosine triphosphate (GTP) and 1.0 mM adenosine triphosphate (ATP) in the bath solution, K(Ca) channels was activated by bradykinin. In outside-out patches, the addition of bradykinin also increased K(Ca) channel activity, when GTP and ATP were added to the pipette solution. The addition of GDP-beta-S (100 microM) in the cytosolic solution completely blocked the activation K(Ca) channels induced by bradykinin in inside-out and outside-out patches. Pretreatment with 30 microM quinacrine, a phospholipase A2 inhibitor, or 3 microM 17-octadecynoic acid (17-ODYA), a cytochrome P450 inhibitor, in addition to indomethacin and L-NNA, abolished bradykinin-stimulated K(Ca) channel activity in cell-attached patches. Both 14,15-epoxyeicosatrienoic acid (EET) and 11,12-EET increased the open probabilities of K(Ca) channels in cell-attached patches. These results suggest that EDHF, released from endothelial cells in response to bradykinin, hyperpolarizes smooth muscle cells by opening K(Ca) channels. Furthermore, our data suggest that EDHF is an endothelium-derived cytochrome P450 metabolite of arachidonic acid. The effect of EDHF on K(Ca) channels is not associated with an increase of cAMP and cGMP. The activation of K(Ca) channels appears to be due to the activation of GTP-binding protein.

Direct modulation of tracheal Cl--channel activity by 5,6- and 11,12-EET

Using microelectrode potential measurements, we tested the involvement of Cl- conductances in the hyperpolarization induced by 5,6- and 11,12-epoxyeicosatrienoic acid (EET) in airway smooth muscle (ASM) cells. 5,6-EET and 11,12-EET (0.75 microM) caused -5.4 +/- 1.1- and -3.34 +/- 0.95-mV hyperpolarizations, respectively, of rabbit tracheal cells (from a resting membrane potential of -53.25 +/- 0.44 mV), with significant residual repolarizations remaining after the Ca2+-activated K+ channels had been blocked by 10 nM iberiotoxin. In bilayer reconstitution experiments, we demonstrated that the EETs directly inhibit a Ca2+-insensitive Cl- channel from bovine ASM; 1 microM 5,6-EET and 1.5 microM 11,12-EET lowered the unitary current amplitude by 40 (n = 6 experiments) and 44.7% (n = 4 experiments), respectively. Concentration-dependent decreases in channel open probability were observed, with estimated IC50 values of 0.26 microM for 5,6- and 1.15 microM for 11,12-EET. Furthermore, pharmacomechanical tension measurements showed that both regioisomers induced significant bronchorelaxations in epithelium-denuded ASM strips. These results suggest that 5,6- and 11,12-EET can act in ASM as epithelium-derived hyperpolarizing factors.

Epoxyeicosatrienoic acids relax airway smooth muscles and directly activate reconstituted KCa channels

Epoxyeicosatrienoic acids (EETs) relax various smooth muscles by increasing outward K+ movement, but the molecular mode of action of EET regioisomers remains to be clarified. The effects of EETs were investigated on bovine airway smooth muscle tone and on reconstituted Ca2+-activated K+ (KCa) channels. 5,6-EET and 11, 12-EET induced dose-dependent relaxations of precontracted bronchial spirals. These effects were partly abolished by 10 nM iberiotoxin. Bilayer experiments have shown that 0.1-10 microM 11,12-EET produced up to fourfold increases in the open probability of KCa channels from the cis (extracellular) side by enhancing the mean open time constant and reducing the long closed time constant, without affecting the unitary conductance. EET-induced activations were blocked by 10 nM iberiotoxin. Addition of vehicles or other lipids as well as of GTP and guanosine 5'-O-(3-thiotriphosphate) in the absence of EET had no effect on channel activity. Thus EETs directly activate KCa channels from airway smooth muscle through an interaction with the extracellular face of the channel. We propose that EETs could represent candidate molecules as epithelium-derived hyperpolarizing factors.

Involvement of barium-sensitive K+ channels in endothelium-dependent vasodilation produced by hypercapnia in rat mesenteric vascular beds

1. We examined the vasodilatory effect of hypercapnia in the rat isolated mesenteric vascular bed. The preparation was perfused constantly (5 ml min(-1) with oxygenated Krebs-Ringer solution, and the perfusion pressure was measured. In order to keep the extracellular pH (pHe) constant (around 7.35) against a change in CO2, adequate amounts of NaHCO3 were added to Krebs-Ringer solution. 2. In the endothelium intact preparations, an increase in CO2 from 2.5% to 10% in increments of 2.5% decreased the 10 microM phenylephrine (PE)-produced increase in the perfusion pressure in a concentration-dependent manner. Denudation of the endothelium by CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulphonate) (5 mg l(-1), 90 s perfusion) abolished the vasodilatory effect of hypercapnia. 3. An increase in CO2 from 5% to 10% reduced the increases in the perfusion pressure produced by 10 microM PE and 400 nM U-46619 by 48% and 44%, respectively. NG-monomethyl-L-arginine (100 microM) and indomethacin (10 microM) did not affect the vasodilatory effect of hypercapnia, whereas the vasodilatory response of the preparation to hypercapnia disappeared when the preparation was contracted by 60 mM K+ instead of PE or U-46619. 4. The vasodilatory effect of hypercapnia observed in the PE- or U-46619-precontracted preparation was affected by neither tetraethylammonium (1 mM), apamin (500 microM), glibenclamide (10 microM), nor 4-aminopyridine (1.5 mM). On the other hand, pretreatment with Ba2+ at a concentration of 0.3 mM abolished the hypercapnia-produced vasodilation. 5. An increase in the concentration of K+ in Krebs-Ringer solution from 4.5 mM to 12.5 mM in increments of 2 mM reduced the PE-produced increase in the perfusion pressure in a concentration-dependent manner. Pretreatment of the preparations with not only Ba2+ (0.3 mM) but also CHAPS abolished the vasodilatory effect of K+. 6. The results suggest that an increase in CO2 produces vasodilation by an endothelium-dependent mechanism in the rat mesenteric vascular bed. The membrane hyperpolarization of the endothelial cell by an activation of the inward rectifier K+ channel seems to be the mechanism underlying the hypercapnia-produced vasodilation. Neither nitric oxide nor prostaglandins are involved in this response.

Inhibition of renal arachidonic acid omega-hydroxylase activity with ABT reduces blood pressure in the SHR

The mechanism-based cytochrome P-450 (CYP) inhibitor 1-aminobenzotriazole (ABT) was characterized as an inhibitor of renal arachidonic acid metabolism and administered to spontaneously hypertensive rats (SHRs) to determine the effect of reduced eicosanoid production on mean arterial pressure (MAP). A single intraperitoneal dose of ABT to Sprague-Dawley rats caused a dose-dependent loss of renal CYP content, arachidonic acid metabolism, and CYP4A protein. In the cortex and outer medulla, ABT showed a high degree of selectivity for the CYP4A enzymes, reflected by the potent inhibition of 19- and 20-hydroxyeicosatetraenoic acid (19- and 20-HETE) formation. A 50 mg/kg dose of ABT reduced cortical 20-HETE formation to 16.1 +/- 0.82% of control and outer medullary 20-HETE formation to 23.8 +/- 0.45% of control. In contrast, there was no inhibition of renal epoxygenase activity at this dose. Renal CYP content, arachidonic acid omega- and (omega-1)-hydroxylase activity, and CYP4A protein levels gradually return to control levels by 72 h after a single dose of ABT. Cortical 20-HETE formation recovered from 17.9 +/- 3.15% of control at 6 h to 84.8 +/- 4.67% of control at 72 h after ABT administration. A single injection of ABT to 7-wk-old SHRs caused an acute reduction in MAP, which remained suppressed for at least 12 h. The effect was maximal within 4 h and averaged 17-23 mmHg during the 4- to 12-h period after administration. 20-HETE formation was inhibited 85% in the cortex and 70-80% in the outer medulla during the period when MAP was reduced. A structurally related ABT analog 1-hydroxybenzotriazole had no effect on blood pressure or renal arachidonic acid metabolism. These results identify ABT as a selective inhibitor of renal CYP4A activity and provide further support for a role for 20-HETE in the regulation of blood pressure.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.

Endothelium-derived hyperpolarizing factor--a critical appraisal

Endothelium-derived hyperpolarizing factor is defined as that substance which produces vascular smooth muscle hyperpolarization which cannot be explained by nitric oxide or by a cyclo-oxygenase product such as prostacyclin. The possibility that the factor is an epoxyeicosatrienoic acid or a cannabinoid agonist such as anandamide continues to be investigated, but definitive evidence in favour of either is lacking. The sensitivity of EDHF-mediated responses to charybdotoxin, to apamin or to mixtures of these two toxins may indicate the opening of more than one smooth muscle K-channel, but the possibility that these are located on the vascular endothelium is discussed.

Potassium channels activated in the endothelium-dependent hyperpolarization in guinea-pig coronary artery

1. Properties of endothelium-dependent hyperpolarization evoked by acetylcholine (ACh) in smooth muscle of the guinea-pig coronary artery were investigated using conventional microelectrode techniques. 2. ACh hyperpolarized the membrane in an endothelium-dependent manner. The hyperpolarization comprised two components: an initial and a slow hyperpolarization. The former appeared during application of ACh, while the latter occurred after withdrawal of ACh. 3. Indomethacin and f1p4ofenac, inhibitors of the enzyme cyclo-oxygenase, blocked only the slow hyperpolarization, indicating that this potential was produced by endothelial prostanoids. 4. Clotrimazole and SKF 525a, known inhibitors of the enzyme cytochrome P450, inhibited both the initial and the slow hyperpolarizations, suggesting that these chemicals acted as non-selective inhibitors of arachidonic acid metabolism. Inhibition of the lipoxygenase pathway of arachidonic acid metabolism by nordihydroguaiaretic acid had no effect on either component of the hyperpolarization. 5. The slow hyperpolarization was inhibited by 4-aminopyridine (4-AP; 10(4) 10(-3) M) and glibenclamide (10(-6) M). The initial hyperpolarization was greatly inhibited by charybdotoxin (CTX; 5 x 10(-8) M) and partially inhibited by apamin (10(-7) M), but was not inhibited by glibenclamide (10(-5) M). Ba2+ (10(-4) M) depolarized the membrane and increased the amplitude of both components of the ACh-induced hyperpolarization. 6. Hyperpolarizations produced by Y-26763, a K+ channel opener, were inhibited by glibenclamide, but not by 4-AP. 7. The results indicate that the slow hyperpolarization is produced by endothelial prostanoids through activation of 4-AP-sensitive K+ channels (possibly delayed rectifier type). The initial hyperpolarization is produced mainly through activation of CTX-sensitive K+ channels (possibly Ca(2+)-sensitive type).

Simultaneous analysis of 4- and 5-series lipoxygenase and cytochrome P450 products from different biological sources by reversed-phase high-performance liquid chromatographic technique

Quantification of lipoxygenase and cytochrome P450 products of both arachidonic acid (AA) and eicosapentaenoic acid (EPA) is of broad interest due to the multiple biological activities of these compounds. We developed a method combining (i) solid-phase extraction, (ii) isocratic reversed-phase high-performance liquid chromatographic separation, and (iii) online photodiode array detection with spectrum analysis for identification and measurement of all main 4- and 5-series eicosanoids (leukotrienes, hydroxyeicosatetraenoic acids/hydroxyeicosapentaenoic acids, epoxyeicosatrienoic acids) within one run. With these procedures, standard mixtures of AA- and EPA-derived lipid mediators were recovered from different biological liquids, like lung perfusate, human bronchoalveolar lavage fluid, and cell supernatant with linear characteristics for each compound. Recoveries of the different lipid mediators exceeded 80% showing excellent reproducibility. Application of the method to isolated, perfused, and ventilated human lungs challenged with the calcium ionophore A23187 and to human neutrophils stimulated in the presence of arachidonic acid and eicosapentaenoic acid with N-formyl-methionyl-leucyl-phenylalanine demonstrated the generation of a large array of lipoxygenase and cytochrome P450 products. Thus, convenient quantification of 4- and 5-series eicosanoids in fluids of biological interest is achieved by a technique comprising solid-phase extraction, isocratic reversed-phase high-performance liquid chromatography, and photodiode array-based online spectrum analysis of eluting compounds.

Interactions between endothelium-derived relaxing factors in the rat hepatic artery: focus on regulation of EDHF

1. In rat isolated hepatic arteries contracted with phenylephrine, acetylcholine and the calcium ionophore A23187 each elicit endothelium-dependent relaxations, which involve both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). However, the contribution of prostanoids to these responses, and the potential interaction between EDHF and other endothelium-derived relaxing factors have not been examined. 2. In the presence of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NOARG, 0.3 mM) and a mixture of charybdotoxin (0.3 microM) and apamin (0.3 microM), inhibitors of the target potassium (K) channel(s) for EDHF, acetylcholine and A23187 each induced a concentration-dependent and almost complete relaxation, which was abolished in the additional presence of indomethacin (10 microM). Thus, in addition to EDHF and NO, a relaxing factor(s) generated by cyclo-oxygenase (COX) contributes to endothelium-dependent relaxation in the rat hepatic artery. 3. The resting membrane potentials of endothelium-intact and endothelium-denuded vascular segments were -57 mV and -52 mV, respectively (P>0.05). In intact arteries, the resting membrane potential was not affected by L-NOARG plus indomethacin, but reduced to -47 mV in the presence of charybdotoxin plus apamin. Acetylcholine and A23187 (10 microM each) elicited a hyperpolarization of 13 mV and 15 mV, respectively. The hyperpolarization induced by these agents was not affected by L-NOARG plus indomethacin (12 mV and 14 mV, respectively), but reduced in the presence of charybdotoxin plus apamin (7 mV and 10 mV, respectively), and abolished in the combined presence of charybdotoxin, apamin and indomethacin. 4. The NO donor 3-morpholino-sydnonimine (SIN-1) induced a concentration-dependent relaxation, which was unaffected by charybdotoxin plus apamin, but abolished by the selective soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM). SIN-1 (10 microM) did not alter the resting membrane potential in endothelium-denuded vascular segments. 5. The COX-dependent relaxation induced by acetylcholine was abolished following exposure to 30 mM KCl, but unaffected by glibenclamide (10 microM). The prostacyclin analogue iloprost induced a concentration-dependent relaxation, which was also abolished in 30 mM KCl and unaffected by the combined treatment with glibenclamide, charybdotoxin and apamin. Iloprost (10 microM) induced a glibenclamide-resistant hyperpolarization (8 mV with and 9 mV without glibenclamide) in endothelium-denuded vascular segments. 6. Exposure to SIN-1 or iloprost did not affect the EDHF-mediated relaxation induced by acetylcholine (i.e. in the presence of L-NOARG and indomethacin). Replacement of L-NOARG with the NO scavenger oxyhaemoglobin (10 microM) or the soluble guanylate cyclase inhibitor ODQ (10 microM) or methylene blue (10 microM), which all significantly inhibited responses to endothelium-derived NO, did not affect the acetylcholine-induced relaxation in the presence of indomethacin, indicating that endogenous NO also does not suppress EDHF-mediated responses. 7. These results show that, in addition to EDHF and NO, an endothelium-derived hyperpolarizing factor(s) generated by COX contributes significantly to endothelium-dependent relaxation in the rat heptic artery. Neither this factor nor NO seems to regulate EDHF-mediated responses. Thus, EDHF does not serve simply as a 'back-up' system for NO and prostacyclin in this artery. However, whether EDHF modulates the NO and COX pathways remains to be determined.

Cytochrome P-450-dependent arachidonate metabolites and renal functions in the Lyon hypertensive rat

1. The present work aimed to assess the role of cytochrome P-450 (CP-450) metabolites of arachidonic acid such as epoxy-eicosatrienoic (EET) and hydroxyeicosatetraenoic acids (HETE) in the renal vasoconstriction and decreased natriuresis exhibited by genetically hypertensive (LH) rats of the Lyon strain. 2. The experiment was performed on single-pass isolated perfused kidney preparations from 8-week-old male LH rats and their low blood pressure (LL) controls. The effects of miconazole (an inhibitor of the formation of EET) and of 17-octadecynoic acid (17-ODYA, an inhibitor of both EET and HETE synthesis) were studied before and after stimulation of the kidneys with two noradrenaline (NA) infusions (65 and 110 nmol/L). 3. Unstimulated LH kidneys (n = 12) differed from LL (n = 12) by increased vascular resistance (RVR) and decreased glomerular filtration rate and urinary sodium excretion (UNaV). 4. Miconazole (1 mumol/L) did not change the functions of LH and LL unstimulated kidneys, but blunted the vasoconstrictor response to NA (110 nmol/L), the difference being significant in LH kidneys only (1.7 +/- 0.2 vs 3.6 +/- 1.2 mmHg/mL per min per g; P < 0.05). 5. Addition of 17-ODYA (3 mumol/L) to miconazole did not modify RVR in LH and LL kidneys or the response to NA infusion. On the contrary, it increased UNaV, the differences being significant in LH kidneys only (22.9 +/- 1.4 vs 17.5 +/- 1.4 mumol/min per g; P < 0.05 after NA 110 nmol/L). 6. It is suggested that EET may contribute to the elevated RVR and HETE to the reduced ability to excrete sodium, of LH kidneys.

Endothelium-dependent relaxation and hyperpolarization in guinea-pig coronary artery: role of epoxyeicosatrienoic acid

1. Acetylcholine (ACh) elicits an endothelium-dependent relaxation and hyperpolarization in the absence of nitric oxide (NO) and prostaglandin synthesis in the guinea-pig coronary artery (GPCA). This response has been attributed to a factor termed endothelial-derived hyperpolarizing factor (EDHF). Recently it has been suggested that EDHF may be a cytochrome P450 product of arachidonic acid (AA) i.e., an epoxyeicosatrienoic acid (EET). The present study investigated whether this pathway could account for the response to ACh observed in the GPCA in the presence of 100 microM N(omega)-nitro-L-arginine and 10 microM indomethacin. 2. ACh, AA and 11,12-EET each produced concentration-dependent relaxations in arteries contracted with the H1-receptor agonist AEP (2,2-aminoethylpyridine). The AA-induced relaxation was significantly enhanced in the presence of the cyclo-oxygenase/lipoxygenase inhibitor, eicosatetranynoic acid (30 microM). 3. The cytochrome P450 inhibitors proadifen (10 microM) and clotrimazole (10 microM) inhibited ACh, lemakalim (LEM) and AA-induced relaxation, whereas 17-octadecynoic acid (100 microM) and 7-ethoxyresorufin (10 microM) were without effect on all three vasodilators. Proadifen and clotrimazole also inhibited ACh (1 microM) and LEM (1 microM)-induced hyperpolarization. 4. The ability of various potassium channel blockers to inhibit relaxation responses elicited with ACh, AA and 11,12-EET was also determined. Iberiotoxin (IBTX; 100 nM) was without effect on responses to ACh but significantly reduced responses to both AA and 11,12-EET. In contrast, 4-aminopyridine (4-AP; 5 mM) significantly reduced response to ACh but not responses to AA and 11,12-EET. Combined IBTX plus (4-AP) inhibited the ACh-induced relaxation to a greater extent than 4-AP alone. Apamin (1 microM), glibenclamide (10 microM) and BaCl2 (50 microM) had no significant effect on responses to ACh, AA and 11,12-EET. 5. IBTX (100 nM) significantly reduced both 11,12-EET (33 microM) and AA (30 microM) hyperpolarization without affecting the ACh (1 microM)-induced hyperpolarization. In contrast, 4-AP significantly reduced the ACh-induced hyperpolarization without affecting either AA or 11,12-EET-induced hyperpolarizations. 6. In summary, our results suggest that the coronary endothelium releases a factor upon application of AA which hyperpolarizes the smooth muscle. The similarity of pharmacology between AA and 11,12-EET suggests that this factor is an EET. However, the disparity of pharmacology between responses to ACh versus responses to 11,12-EET do not support the hypothesis that EETs represent the predominant factor which ACh releases from the endothelium that leads to NO- and prostaglandin-independent hyperpolarization and relaxation in the GPCA.

Interaction of endothelial eccrine mechanisms and human adrenomedullin on vascular resistance in canine bone

Adrenomedullin is a novel peptide known to be one of the most potent vascular smooth muscle relaxing agents in vivo. The aim of this study is to investigate the effect of adrenomedullin in relation to nitric oxide, prostaglandins and endothelium-derived hyperpolarized factor (EDHF). A 0.1-ml bolus of 1 nmol human adrenomedullin is a potent inhibitor of the pressor response to exogenous norepinephrine infusion in an ex vivo canine tibia perfusion model for a duration of at least 70 min (P < 0.005). This attenuation of vascular smooth muscle contraction occurs even when nitric oxide production is blocked by NG-monomethyl-L-arginine acetate (L-NMMA) infusion and EDHF is blocked by tetraethylammonium infusion, although the effect is of shorter duration (at least 10 min). Indomethacin as well does not affect the suppression of norepinephrine-induced vascular smooth muscle contraction. Based on these data, human adrenomedullin has both nitric oxide- and EDHF-dependent mechanism as well as a nitric oxide- and EDHF-independent mechanism.

Endothelium-independent relaxation and hyperpolarization to C-type natriuretic peptide in porcine coronary arteries

Endothelial cells produce C-type natriuretic peptide (CNP), which has been proposed as an endothelium-derived hyperpolarizing factor. In porcine coronary arteries, we investigated the vasodilatory effects of CNP and compared them with endothelium-dependent relaxations and hyperpolarizations to bradykinin. Isolated epicardial porcine coronary arteries were studied in organ chambers, and concentration-response curves to CNP and bradykinin were obtained. Membrane potential was measured in endothelial cells and smooth muscle of intact porcine coronary arteries during stimulation with CNP or bradykinin. In precontracted porcine coronary arteries with or without endothelium, CNP (10[-10]-10[-6] M) evoked relaxations (maximum, 42 +/- 4%) smaller than those evoked by bradykinin (100 +/- 1%), blunted in preparations contracted by KCl instead of U46619 (9,11-dideoxy-11a,9a-epoxymethano-prostaglandin F2alpha; p < 0.05) and unaffected by inhibition of NO synthase (NS). CNP evoked hyperpolarization of vascular smooth muscle of similar magnitude in endothelium-intact (-4.4 +/- 1 mV) and endothelium-denuded (-4.6 +/- 1 mV) porcine coronary arteries. Bradykinin (10[-10]-10[-6] M) evoked concentration-dependent relaxations in preparations with endothelium only. Although atrial natriuretic peptide-receptor antagonist HS-142-1 (25 microM) slightly reduced the sensitivity to bradykinin (log shift at IC50, twofold; p < 0.05), it had no effect on the maximal response to bradykinin. Inhibition of NO synthase partially attenuated, whereas high potassium chloride (30 mM) markedly inhibited relaxations to bradykinin (p < 0.05). Hyperpolarization to bradykinin was much more pronounced than that to CNP (-17 +/- 3 mV; p < 0.05 vs. CNP) and was observed in endothelium-intact preparations only and unaffected by HS-142-1. In conclusion, in contrast to bradykinin, CNP induces endothelium-independent and weaker relaxation and hyperpolarization of coronary artery vascular smooth muscle, suggesting that CNP is an unlikely mediator of endothelium-dependent hyperpolarization of porcine coronary arteries.

Roles of calcium-activated and voltage-gated delayed rectifier potassium channels in endothelium-dependent vasorelaxation of the rabbit middle cerebral artery

1. The cellular mechanism(s) of action of endothelium-derived vasodilator substances in the rabbit middle cerebral artery (RMCA) were investigated. Specifically, the subtypes of potassium channels involved in the effects of endothelium-derived relaxing factors (EDRFs) in acetylcholine (ACh)-induced endothelium-dependent vasorelaxation in this vessel were systematically compared. 2. In the endothelium-intact RMCA precontracted with histamine (3 microM), ACh induced a concentration-dependent vasorelaxation, which was sensitive to indomethacin (10 microM) or N(G)-nitro-L-arginine (L-NOARG; 100 microM); pD2 values 8.36 vs 7.40 and 6.38, P < 0.01 for both, n = 6 and abolished by a combination of both agents. ACh caused relaxation in the presence of high K+ PSS (40 mM KCl), which was not affected by indomethacin, but abolished by L-NOARG and a combination of indomethacin and L-NOARG. 3. In the presence of indomethacin, relaxation to ACh in the endothelium-intact RMCA precontracted with histamine was unaffected by either glibenclamide (10 microM), an ATP-sensitive K+ channel (K[ATP]) blocker, 4-aminopyridine (4-AP, 1 mM) or dendrotoxin (DTX, 0.1 microM), delayed rectifier K channel (Kv) blockers. However, relaxation responses to ACh were significantly inhibited by either LY83583 (10 microM) and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 microM), guanylyl cyclase inhibitors, or charybdotoxin (CTX; 0.1 microM), iberiotoxin (ITX, 0.1 microM) and apamin (APA, 0.1 microM), large conductance Ca2+-activated K+ channels (BK[Ca]) blocker and small conductance Ca2+-activated K+ channel (SK[Ca]) blocker, respectively. 4. In the presence of L-NOARG, relaxation to ACh was unaffected by glibenclamide or the cytochrome P450 mono-oxygenase inhibitor, clotrimazole (1 microM), but was significantly inhibited by either 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 10 microM) and 2',3'-dideoxyadenosine (2',3'-DDA, 30 microM), adenylyl cyclase inhibitors, or 4-AP, DTX, CTX, ITX and APA. 5. In the endothelium-denuded RMCA precontracted with histamine, authentic NO-induced relaxation was unaffected by glibenclamide, 4-AP and DTX, but significantly reduced by ODQ, ITX and APA. Authentic prostaglandin I2 (PGI2)-induced relaxation was unaffected by glibenclamide, but significantly reduced by 2',3'-DDA, 4-AP, DTX, ITX and APA. Forskolin-induced relaxation was significantly inhibited by high K+, CTX and 4-AP. 6. These results indicate that: (1) in the RMCA the EDRFs released by ACh are NO and a prostanoid (presumably PGI2), and there is no evidence for the release of a non-NO/PGI2 endothelium-derived hyperpolarizing factor (EDHF), (2) K(Ca) channels are involved in NO-mediated relaxation of the RMCA but both K(Ca) and Kv channels are involved in PGI2-mediated relaxation.

P2U-receptor mediated endothelium-dependent but nitric oxide-independent vascular relaxation

1. The dilator effect of extracellular adenosine triphosphate (ATP) has mainly been characterized as a direct effect on smooth muscle or as an endothelium-dependent effect mediated by nitric oxide (NO) or prostaglandins. We tested the hypothesis that endothelium-derived hyperpolarizing factor (EDHF) may also be involved. Dilator effects were studied in vitro by continuous recording of isomeric tension in cylindrical segments of rat blood vessels precontracted by noradrenaline (NA), in the presence of indomethacin (10 microM). 2. By screening different blood vessels in the rat we found that both acetylcholine (ACh) and ATP dilate mesenteric arteries with a resting tone of 1 mN by an endothelium-dependent non-NO mechanism. With an increased resting tone (4 mN) the dilatation was mediated by NO. Thus by varying the resting tension the different dilator mechanisms could be examined. However, in the carotid artery the dilatation was solely mediated by an endothelium-dependent NO mechanism, even at different resting tones (1 and 4 mN). 3. The N-nitro-L-arginine methyl ester (L-NAME)-resistant dilatation to ACh and ATP was further inhibited by the NO-scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), indicating L-NAME insensitive NO-synthesis. 4. In carotid arteries and mesenteric arteries at high resting tones (4 mN) the ATP-dilatation was totally inhibited by endothelium removal or L-NAME (10(-3) M). In mesenteric arteries at low resting tone (1 mN) the ATP, UTP (uridine-triphosphate) and 2-MeSATP (2methylthioATP)-dilatation was totally inhibited by endothelium removal. However, L-NAME in combination with indomethacin attenuated only 5% of the UTP dilatation, 70% of the ATP dilatation but all of the 2-MeSATP-dilatation. The inhibitors of Ca2+-activated K+ channels charybdotoxin (0.5 x 10(-7) M) together with apamin (10(-6) M), and the cytochrome P450 inhibitor, SKF 525A (10(-4) M), each in combination with indomethacin. L-NAME and PTIO (0.5 x 10(-3) M) totally abolished the remaining ATP and UTP-dilatation. This indicates a dilatation mediated by an endothelium-dependent non-NO factor, probably EDHF. 5. Agonist potency (UTP>ATP>>2-MeSATP), indicates that the EDHF-mediated dilatation was stimulated by a P2U-receptor, possibly by a selective pyrimidine-receptor. In contrast, a P2Y-receptor stimulated NO-mediated dilatation (2-MeSATP=ATP>UTP). 6. In conclusion, the dilator effects of ATP and especially UTP can be mediated by an endothelium-dependent non-NO-mediated mechanism, probably EDHF, mediated by a P2U-receptor, possibly a selective pyrimidine-receptor, while NO-mediated dilatation is stimulated mainly by a P2Y1-receptor. Furthermore, the EDHF-dilatation is dependent on the resting tone of the blood vessel.

Epoxyeicosatrienoic acids, potassium channel blockers and endothelium-dependent hyperpolarization in the guinea-pig carotid artery

1. Using intracellular microelectrodes, we investigated the effects of 17-octadecynoic acid (17-ODYA) on the endothelium-dependent hyperpolarization induced by acetylcholine in the guinea-pig isolated internal carotid artery with endothelium. 2. In the presence of Nomega-nitro-L-arginine (L-NOARG, 100 microM) and indomethacin (5 microM) to inhibit nitric oxide synthase and cyclo-oxygenase, acetylcholine (1 microM) evoked an endothelium-dependent hyperpolarization which averaged -16.4 mV starting from a resting membrane potential of -56.8 mV. There was a negative correlation between the amplitude of the hyperpolarization and the absolute values of the resting membrane potential. 3. The acetylcholine-induced endothelium-dependent hyperpolarization was not altered by charybdotoxin (0.1 microM) or iberiotoxin (30 nM). It was partially but significantly reduced by apamin (0.5 microM) to -12.8+/-1.2 mV (n=10) or the combination of apamin plus iberiotoxin (-14.3+/-3.4mV, n=4). However, the combination of charybdotoxin and apamin abolished the hyperpolarization and under these conditions, acetylcholine evoked a depolarization (+ 7.1+/-3.7 mV, n = 8). 4. 17-ODYA (10 microM) produced a significant hyperpolarization of the resting membrane potential which averaged -59.6 mV and a partial but significant inhibition of the acetylcholine-induced endothelium-dependent hyperpolarization (-10.9 mV). 5. Apamin did not modify the effects of 17-ODYA but in the presence of charybdotoxin or iberiotoxin, 17-ODYA no longer influenced the resting membrane potential or the acetylcholine-induced hyperpolarization. 6. When compared to solvent (ethanol, 1% v/v), epoxyeicosatrienoic acids (EpETrEs) (5,6-, 8,9-, 11,12- and 14,15-EpETrE, 3 microM) did not affect the cell membrane potential and did not relax the guinea-pig isolated internal carotid artery. 7. These results indicate that, in the guinea-pig internal carotid artery, the involvement of metabolites of arachidonic acid through the cytochrome P450 pathway in endothelium-dependent hyperpolarization is unlikely. Furthermore, the hyperpolarization mediated by the endothelium-derived hyperpolarizing factor (EDHF) is probably not due to the opening of BK(Ca) channels.

Endocannabinoids: a new class of vasoactive substances

Endogenous cannabinoids (endocannabinoids) have recently been identified in the CNS and attention has now turned to their cardiovascular actions. The prototypic endocannabinoid, anandamide, derived from arachidonic acid, has been shown to be a vasorelaxant, particularly in the resistance vasculature. This vasorelaxation has been shown to be both endothelium-independent and -dependent, depending on the vascular bed. It has been proposed that an endocannabinoid may mediate the nitric oxide- and prostanoid-independent component of endothelium-dependent relaxations, as these responses are sensitive to a cannabinoid receptor antagonist and show similarities to anandamide-induced relaxations. This hypothesis has generated much controversy and the emerging conflicts in the literature are discussed in this article by Michael Randall and David Kendall. Despite this controversy, it has recently been shown that anandamide is produced by endothelial cells. Clearly, much work is required to adequately define the physiological significance of endocannabinoids in the cardiovascular system.

Effect of endothelium removal on prostaglandin and nitric oxide function in pulmonary resistance arteries in the lamb

We have recently shown that isolated pulmonary resistance arteries of the fetal lamb have prostaglandin (PG) I2 based and nitric oxide (NO) based relaxing mechanisms, which are activated by oxygen (at neonatal levels) and bradykinin. The present study was carried out to ascertain whether these mechanisms remain operational after removal of the endothelium. Endothelium-denuded vessels pre-equilibrated at a neonatal PO2 were not affected by indomethacin (2.8 µM), while they contracted weakly to NG-nitro-L-arginine methyl ester (L-NAME, 100 µM). However, the latter response did not reach significance and resembled that of intact vessels at fetal PO2. Bradykinin (0.1-100 nM) dose dependently (from 1-3 nM upwards) relaxed endothelium-denuded arteries that had been precontracted with a thromboxane (TX) A2 analog (ONO-11113, 0.1 µM) or excess potassium (5 mM Ca2+ in K+-Krebs) at a neonatal PO2. The response was the same under the two conditions, but it was smaller than that of intact arteries. Bradykinin relaxation of ONO-11113-contracted arteries was completely or nearly completely inhibited by indomethacin and L-NAME. We conclude that endothelium-denuded, pulmonary resistance arteries maintain PG (conceivably PGI2) mediated and NO-mediated relaxing mechanisms. These extra-endothelial mechanisms are activated by bradykinin but not by oxygen.Key words: perinatal pulmonary circulation, endothelium, nitric oxide, prostaglandin, oxygen.

ACE inhibition, endothelial function and coronary artery lesions. Role of kinins and nitric oxide

In healthy coronary arteries, the endothelium plays an important role in the regulation of vascular smooth muscle growth and contractility. Furthermore, the endothelium inhibits overt platelet aggregation and prevents the adhesion of white blood cells to, and their infiltration into, the vascular wall. Among the mediators of these functions of endothelial cells, nitric oxide (NO) plays a central role. Moreover, the presence of local kinin-generating enzymatic systems associated with endothelial cells, vascular smooth muscle, platelets, neutrophils and monocytes suggests that bradykinin stimulates endothelial cells to release NO locally. The activation of endothelial cells by bradykinin is inhibited by kininase II, best known as angiotensin converting enzyme (ACE). Hence, ACE inhibitors, in addition to reducing the levels of angiotensin II (a potent stimulus to vascular smooth muscle growth and contraction), cause an amplification of the release of NO and other endothelial mediators that is induced by bradykinin. Independent risk factors for coronary artery disease such as hypertension, diabetes and hypercholesterolaemia reduce the NO-dependent regulation of vascular smooth muscle contractility and growth in otherwise normal coronary arteries. This endothelial dysfunction probably also affects the inhibitory role of NO with regard to platelet aggregation and monocyte infiltration into the vascular wall. In atherosclerotic vessels, the role of NO is severely reduced. In animal models, as well as in patients with coronary artery disease, endothelial dysfunction is improved by treatment with ACE inhibitors. Although in humans the mechanism of the restoration of endothelial function is not known, in animals endogenous kinins and NO are involved. However, it is clear that this process is multifactorial, and thus probably involves both the prevention of the deleterious actions of angiotensin II and the potentiation of bradykinin.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

N-arachidonylethanolamide relaxation of bovine coronary artery is not mediated by CB1 cannabinoid receptor

It has been reported that the endogenous cannabinoid N-arachidonylethanolamide (AEA), commonly referred to as anandamide, has the characteristics of an endothelium-derived hyperpolarizing factor in rat mesenteric artery. We have carried out studies to determine whether AEA affects coronary vascular tone. The vasorelaxant effects of AEA were determined in isolated bovine coronary artery rings precontracted with U-46619 (3 x 10(-9) M). AEA decreased isometric tension, producing a maximal relaxation of 51 +/- 9% at a concentration of 10(-5) M. Endothelium-denuded coronary arteries were not significantly affected by AEA. The CB1 receptor antagonist SR-141716A (10(-6)M) failed to reduce the vasodilatory effects of AEA, suggesting that the CB1 receptor is not involved in this action of AEA. Because AEA is rapidly converted to arachidonic acid and ethanolamine in brain and liver by a fatty acid amide hydrolase (FAAH), we hypothesized that the vasodilatory effect of AEA results from its hydrolysis to arachidonic acid followed by enzymatic conversion to vasodilatory eicosanoids. In support of this hypothesis, bovine coronary arteries incubated with [3H]AEA for 30 min hydrolyzed 15% of added substrate; approximately 9% of the radiolabeled product was free arachidonic acid, and 6% comigrated with the prostaglandins (PGs) and epoxyeicosatrienoic acids (EETs). A similar result was obtained in cultured bovine coronary endothelial cells. Inhibition of the FAAH with diazomethylarachidonyl ketone blocked both the metabolism of [3H]AEA and the relaxations to AEA. Whole vessel and cultured endothelial cells prelabeled with [3H]arachidonic acid synthesized [3H]PGs and [3H]EETs, but not [3H]AEA, in response to A-23187. Furthermore, SR-141716A attenuated A-23187-stimulated release of [3H]arachidonic acid, suggesting that it may have actions other than inhibition of CB1 receptor. These experiments suggest that AEA produces endothelium-dependent vasorelaxation as a result of its catabolism to arachidonic acid followed by conversion to vasodilatory eicosanoids such as prostacyclin or the EETs.

Cytochrome P-450 pathway in acetylcholine-induced canine coronary microvascular vasodilation in vivo

In the canine coronary microcirculation, acetylcholine (ACh)-induced vasodilation of large (> or = 100 microns) epicardial arterioles (LgA), but not small (< 100 microns) epicardial arterioles (SmA), is blocked by nitric oxide (NO) synthase inhibitors in vivo. We hypothesized that the ACh-induced vasodilation of SmA is mediated by a cytochrome P-450 metabolite of arachidonic acid (AA). Epicardial coronary microvascular diameters in dogs were measured at baseline and after treatment with topically applied ACh (1, 10, and 100 microM), AA (1, 5, and 10 microM), or sodium nitroprusside (SNP; 10-100 microM). Coronary microvascular diameters were compared among control dogs (group OO); dogs pretreated with N omega-nitro-L-arginine (L-NNA; 70 microM topically) (group NO); dogs pretreated with L-NNA plus clotrimazole (Clo; 1.6 microM topically) or 17-octadecynoic acid (ODYA; 2 microM topically), cytochrome P-450 monooxygenase inhibitors (groups NC and NY, respectively); dogs pretreated with Clo alone (group OC); and dogs pretreated with L-NNA plus Clo with AA as the agonist (group AA). ACh-induced vasodilation of LgA was abolished by L-NNA alone, whereas in SmA, L-NNA was without effect. Clo alone did not inhibit ACh-induced dilation in either SmA or LgA. However, the combinations of L-NNA plus either Clo or ODYA abolished ACh- and AA-induced dilation of SmA (100 microM ACh: NC, 3 +/- 5%; NY, 8 +/- 2%; 10 microM AA: 6 +/- 3%) but did not affect responses to SNP. These results suggest that the ACh-induced vasodilation of SmA is mediated in part by cytochrome P-450 metabolites of AA and provide the first evidence that the cytochrome P-450 pathway contributes to the regulation of coronary resistance vessels in vivo.

Endothelial and smooth muscle cell conduction in arterioles controlling blood flow

We performed intracellular recording with Lucifer yellow dye microinjection to investigate the cellular pathway(s) by which constriction and dilation are conducted along the wall of arterioles (diameter 47 +/- 1 microns, n = 63) supplying blood flow to the cheek pouch of anesthetized hamsters. At rest, membrane potential (Em) of endothelial (-36 +/- 1 mV) and smooth muscle (-35 +/- 1 mV) cells was not different. Micropipette delivery of norepinephrine (NE) or phenylephrine (PE) produced smooth muscle cell depolarization (5-41 mV) and vasoconstriction (7-49 microns) at the site of release and along the arteriole with no effect on Em of endothelial cells. KCl produced conduction of depolarization and vasoconstriction with similar electrical kinetics in endothelial and smooth muscle cells. Acetylcholine triggered conduction of vasodilation (2-25 microns) and hyperpolarization (3-33 mV) along both cell layers; in smooth muscle, this change in Em was prolonged and followed by a transient depolarization. These cell-specific electrophysiological recordings uniquely illustrate that depolarization and constriction are initiated and conducted along smooth muscle, independent of the endothelium. Furthermore, conduction of vasodilation is explained by the spread of hyperpolarization along homologously coupled endothelial and smooth muscle cells, with distinctive responses between cell layers. The discontinuity between endothelium and smooth muscle indicates that these respective pathways are not electrically coupled during blood flow control.

A comparison of EDHF-mediated and anandamide-induced relaxations in the rat isolated mesenteric artery

1. Relaxation of the methoxamine-precontracted rat small mesenteric artery by endothelium-derived hyperpolarizing factor (EDHF) was compared with relaxation to the cannabinoid, anandamide (arachidonylethanolamide). EDHF was produced in a concentration- and endothelium-dependent fashion in the presence of NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) by either carbachol (pEC50 [negative logarithm of the EC50] = 6.19 +/- 0.01, Rmax [maximum response] = 93.2 +/- 0.4%; n = 14) or calcium ionophore A23187 (pEC50 = 6.46 +/- 0.02, Rmax = 83.6 +/- 3.6%; n = 8). Anandamide responses were independent of the presence of endothelium or L-NAME (control with endothelium: pEC50 = 6.31 +/- 0.06, Rmax = 94.7 +/- 4.6%; n = 10; with L-NAME: pEC50 = 6.33 +/- 0.04, Rmax = 93.4 +/- 6.0%; n = 4). 2. The selective cannabinoid receptor antagonist, SR 141716A (1 microM) caused rightward shifts of the concentration-response curves to both carbachol (2.5 fold) and A23187 (3.3 fold). It also antagonized anandamide relaxations in the presence or absence of endothelium giving a 2 fold shift in each case. SR 141716A (10 microM) greatly reduced the Rmax values for EDHF-mediated relaxations to carbachol (control, 93.2 +/- 0.4%; SR 141716A, 10.7 +/- 2.5%; n = 5; P < 0.001) and A23187 (control, 84.8 +/- 2.1%; SR 141716A, 3.5 +/- 2.3%; n = 6; P < 0.001) but caused a 10 fold parallel shift in the concentration-relaxation curve for anandamide without affecting Rmax. 3. Precontraction with 60 mM KCl significantly reduced (P < 0.01; n = 4 for all) relaxations to 1 microM carbachol (control 68.8 +/- 5.6% versus 17.8 +/- 7.1%), A23187 (control 71.4 +/- 6.1% versus 3.9 +/- 0.45%) and anandamide (control 71.1 +/- 7.0% versus 5.2 +/- 3.6%). Similar effects were seen in the presence of 25 mM K+. Incubation of vessels with pertussis toxin (PTX; 400 ng ml-1, 2 h) also reduced (P < 0.01; n = 4 for all) relaxations to 1 microM carbachol (control 63.5 +/- 7.5% versus 9.0 +/- 3.2%), A23187 (control 77.0 +/- 5.8% versus 16.2 +/- 7.1%) and anandamide (control 89.8 +/- 2.2% versus 17.6 +/- 8.7%). 4. Incubation of vessels with the protease inhibitor phenylmethylsulphonyl fluoride (PMSF; 200 microM) significantly potentiated (P < 0.01), to a similar extent (approximately 2 fold), relaxation to A23187 (pEC50: control, 6.45 +/- 0.04; PMSF, 6.74 +/- 0.10; n = 4) and anandamide (pEC50: control, 6.31 +/- 0.02; PMSF, 6.61 +/- 0.08; n = 8). PMSF also potentiated carbachol responses both in the presence (pEC50: control, 6.25 +/- 0.01; PMSF, 7.00 +/- 0.01; n = 4; P < 0.01) and absence (pEC50: control, 6.41 +/- 0.04; PMSF, 6.88 +/- 0.04; n = 4; P < 0.001) of L-NAME. Responses to the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) were also potentiated by PMSF (pEC50: control, 7.51 +/- 0.06; PMSF, 8.00 +/- 0.05, n = 4, P < 0.001). 5. EDHF-mediated relaxation to carbachol was significantly attenuated by the K+ channel blocker tetraethylammonium (TEA; 1 mM) (pEC50: control, 6.19 +/- 0.01; TEA, 5.61 +/- 0.01; n = 6; P < 0.01). In contrast, TEA (1 mM) had no effect on EDHF-mediated relaxation to A23187 (pEC50: control, 6.47 +/- 0.04; TEA, 6.41 +/- 0.02, n = 4) or on anandamide (pEC50: control, 6.28 +/- 0.06; TEA, 6.09 +/- 0.02; n = 5). TEA (10 mM) significantly (P < 0.01) reduced the Rmax for anandamide (control, 94.3 +/- 4.0%; 10 mM TEA, 60.7 +/- 4.4%; n = 5) but had no effect on the Rmax to carbachol or A23187. 6. BaCl2 (100 microM), considered to be selective for blockade of inward rectifier K+ channels, had no significant effect on relaxations to carbachol or A23187, but caused a small shift in the anandamide concentration-response curve (pEC50: control, 6.39 +/- 0.01; Ba2+, 6.20 +/- 0.01; n = 4; P < 0.01). BaCl2 (1 mM; which causes non-selective block of K+ channels) significantly (P < 0.01) attenuated relaxations to all three agents (pEC50 values: carbachol, 5.65 +/- 0.02; A23187, 5.84 +/- 0.04; anandamide, 5.95 +/- 0.02; n = 4 for each). 7. Apamin (1mu M), a selective blocker of small conductance, Ca2+-activated, K+ channels (SKCa), 4-aminopyridine (1mM), a blocker of delayed rectifier, voltage-dependent, K+ channels (Kv), and ciclazindol (10mu M), an inhibitor of Kv and adenosine 5'-triphosphate (ATP)-sensitive K+ channels (KATP), significantly reduced EDHF-mediated relaxations to carbachol, but had no significant effects on A23187 or anandamide responses. 8. Glibenclamide (10mu M), a KATP inhibitor and charybdotoxin (100 or 300nM), a blocker of several K+ channel subtypes, had no significant effect on relaxations to any of the agents. Iberiotoxin (50nM), an inhibitor of large conductance, Ca2+-activated, K+ channels (BKCa), had no significant effect on the relaxation responses, either alone or in combination with apamin (1muM). Also, a combination of apamin (1muM) with either glibenclamide (10muM) or 4-aminopyridine (1mM) did not inhibit relaxation to carbachol significantly more than apamin alone. Neither combination had any significant effect on relaxation to A23187 or anandamide. 9. A combination of apamin (1muM) with charybdotoxin (100nM) abolished EDHF-mediated relaxation to carbachol, but had no significant effect on that to A23187. Apamin (1muM) and charybdotoxin (300nM) together consistently inhibited the response to A23187, while apamin (1muM) and ciclazindol (10muM) together inhibited relaxations to both carbachol and A23187. None of these toxin combinations had any significant effect on relaxation to anandamide. 10. It was concluded that the differential sensitivity to K+ channel blockers of EDHF-mediated responses to carbachol and A23187 might be due to actions on endothelial generation of EDHF, as well as its actions on the vascular smooth muscle, and suggests care must be taken in choosing the means of generating EDHF when making comparative studies. Also, the relaxations to EDHF and anandamide may involve activation of cannabinoid receptors, coupled via PTX-sensitive G-proteins to activation of K+ conductances. The results support the hypothesis that EDHF is an endocannabinoid but relaxations to EDHF and anandamide show differential sensitivity to K+ channel blockers, therefore it is likely that anandamide is not identical to EDHF in the small rat mesenteric artery.

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.

Possible role of P-450 metabolite of arachidonic acid in vasodilator mechanism of angiotensin II type 2 receptor in the isolated microperfused rabbit afferent arteriole

Although angiotensin II type 2 (AT2) receptor has recently been cloned, its functional role is not well understood. We tested the hypothesis that selective activation of AT2 receptor causes vasodilation in the preglomerular afferent arteriole (Af-Art), a vascular segment that accounts for most of the preglomerular resistance. We microperfused rabbit Af-Arts at 60 mmHg in vitro, and examined the effect of angiotensin II (Ang II; 10(-11)-10(-8) M) on the luminal diameter in the presence or absence of the Ang II type 1 receptor antagonist CV11974 (CV; 10(-8) M). Ang II was added to both the bath and lumen of preconstricted Af-Arts. Ang II further constricted Af-Arts without CV (by 74+/-7% over the preconstricted level at 10(-8) M; P < 0.01, n = 7). In contrast, in the presence of CV, Ang II caused dose-dependent dilation; Ang II at 10(-8) M increased the diameter by 29+/-2% (n = 7, P < 0.01). This dilation was completely abolished by pretreatment with an AT2 receptor antagonist PD123319 (10(-7) M, n = 6), suggesting that activation of AT2 receptor causes vasodilation in Af-Arts. The dilation was unaffected by inhibiting either nitric oxide synthase (n = 7) or cyclooxygenase (n = 7), however, it was abolished by either disrupting the endothelium (n = 10) or inhibiting the cytochrome P-450 pathway, particularly the synthesis of epoxyeicosatrienoic acids (EETs, n = 7). These results suggest that in the Af-Art activation of the AT2 receptor may cause endothelium-dependent vasodilation via a cytochrome P-450 pathway, possibly by EETs.