Prostaglandin I2 contributes to the vasodepressor effect of baicalein in hypertensive rats

Bioactive lipids in hypertension

Hypertension is a major healthcare issue that afflicts one in every three adults worldwide and contributes to cardiovascular diseases, morbidity and mortality. Bioactive lipids contribute importantly to blood pressure regulation via actions on the vasculature, kidney, and inflammation. Vascular actions of bioactive lipids include blood pressure lowering vasodilation and blood pressure elevating vasoconstriction. Increased renin release by bioactive lipids in the kidney is pro-hypertensive whereas anti-hypertensive bioactive lipid actions result in increased sodium excretion. Bioactive lipids have pro-inflammatory and anti-inflammatory actions that increase or decrease reactive oxygen species and impact vascular and kidney function in hypertension. Human studies provide evidence that fatty acid metabolism and bioactive lipids contribute to sodium and blood pressure regulation in hypertension. Genetic changes identified in humans that impact arachidonic acid metabolism have been associated with hypertension. Arachidonic acid cyclooxygenase, lipoxygenase and cytochrome P450 metabolites have pro-hypertensive and anti-hypertensive actions. Omega-3 fish oil fatty acids eicosapentaenoic acid and docosahexaenoic acid are known to be anti-hypertensive and cardiovascular protective. Lastly, emerging fatty acid research areas include blood pressure regulation by isolevuglandins, nitrated fatty acids, and short chain fatty acids. Taken together, bioactive lipids are key contributors to blood pressure regulation and hypertension and their manipulation could decrease cardiovascular disease and associated morbidity and mortality.

Eicosanoid blood vessel regulation in physiological and pathological states

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

Baicalein alleviates tubular-interstitial nephritis in vivo and in vitro by down-regulating NF-κB and MAPK pathways

Tubular-interstitial nephritis (TIN) is characterized by tubular cell damage and inflammatory lesions of kidneys. Baicalein (BAI) is a flavonoid compound found in the roots of Scutellaria baicalensis Georgi. The present study was undertaken to explore the anti-inflammatory and anti-oxidative effects of BAI on TIN patients and a lipopolysaccharide (LPS)-induced TIN cell model. The expression levels of interleukin-6 (IL-6), IL-10, and tumor necrosis factor α in serum samples of TIN patients and culture supernatants of renal proximal tubular epithelial cells (RPTECs) were evaluated using enzyme-linked immunosorbent assay. Creatinine clearance was calculated using the Cockcroft-Gault equation. Activities of malondialdehyde, superoxide dismutase, and glutathione peroxidase were also determined. Viability and apoptosis of RPTECs were measured using MTT assay and Guava Nexin assay, respectively. qRT-PCR was performed to determine the expressions of Bax, Bcl-2, nuclear factor kappa B (IκBα), and p65. Protein levels of Bax, Bcl-2, IκBα, p65, c-Jun N-terminal kinase, extracellular regulated protein kinases, and p38 were analyzed using western blotting. We found that BAI reduced inflammation and oxidative stress in vivo and in vitro. Moreover, BAI alleviated the LPS-induced RPTECs viability inhibition and apoptosis enhancement, as well as nuclear factor kappa B (NF-κB), and mitogen-activated protein kinase (MAPK) activation. Phorbol ester, an activator of NF-κB, attenuated the effects of BAI on LPS-induced inflammatory cytokine expressions in RPTECs. In conclusion, BAI had anti-inflammatory and anti-oxidative effects on TIN patients and LPS-induced RPTECs by down-regulating NF-κB and MAPK pathways.

Baicalein attenuates angiotensin II-induced cardiac remodeling via inhibition of AKT/mTOR, ERK1/2, NF-κB, and calcineurin signaling pathways in mice

BACKGROUND

Baicalein, a specific lipoxygenase (LOX) inhibitor, has anti-inflammatory and antioxidant effects. However, the functional role of baicalein in angiotensin II (Ang II)-induced hypertension and cardiac remodeling remains unclear. Here we investigated the effect of baicalein on cardiac hypertrophy and fibrosis and the underlying mechanism.

METHODS

Wild-type (WT) mice were injected with Ang II (1,200ng/kg/min) alone or together with 12/15-LOX inhibitor baicalein (25mg/kg) for 14 days. Histological examinations were performed on heart sections with hematoxylin and eosin, Masson's trichrome, wheat germ agglutinin staining, and immunohistochemistry. The messenger RNA (mRNA) expression of cytokines and protein levels were detected by real-time polymerase chain reaction (PCR) and western blot analysis respectively.

RESULTS

Ang II infusion significantly increased blood pressure but decreased cardiac contractile function reflected by fractional shortening% and ejection fraction% compared with saline-treated mice. Moreover, Ang II infusion resulted in marked cardiac hypertrophy and fibrosis, promoted accumulation of macrophages and T cells, the expression of proinflammatory cytokines and malondialdehyde (MDA) production. However, these actions were markedly reversed by administration of baicalein in mice. Mechanistically, the protective effects of baicalein were associated with the inhibition of inflammation, oxidative stress, and multiple signaling pathways (AKT/mTOR, ERK1/2, nuclear factor-κB (NF-κB), and calcineurin) in the Ang II-treated mice.

CONCLUSIONS

This study demonstrates that baicalein can significantly ameliorate Ang II-induced hypertension and cardiac remodeling, and may be a novel therapeutic drug for prevention of hypertensive heart diseases.

Baicalein protects against hypertension associated with diabetes: effect on vascular reactivity and stiffness

The present work investigated the possible protective effect of baicalein, a natural lipoxygenase enzyme inhibitor, on both insulin deficiency (ID) and insulin resistance (IR)-induced macro-vascular impairment. ID and IR were induced by STZ or fructose for 8 or 12 weeks respectively while baicalein was administered in the last six weeks. Blood pressure (BP) was recorded and isolated aorta reactivity to phenylephrine (PE) and acetylcholine (ACh) were studied. Blood levels of glucose, insulin, advanced glycation end products (AGEs) and tumour necrosis factor-α (TNF-α) were determined. Aortic nuclear transcription factor-κB (NF-κB) activation was assessed. Both models resulted in elevated BP, increased vasoconstriction and impaired relaxation KCl, elevated TNF-α and AGEs, NF-κB activation, marked infiltration of leukocytes in the adventitia, pyknosis of endothelial cells and marked collagen deposition. Baicalein ameliorated elevations in BP in models, prevented exaggerated vasoconstriction IR model and improved relaxation in ID model. Baicalein reduced AGEs and TNF-α level, decreased NF-κB activation and inhibited histopathological changes in both models. Baicalein offsets the hypertensive and the vascular impairment associated with both diabetic models via ameliorating functional and structural derangements of blood vessels.

Inhibitory activities of baicalin against renin and angiotensin-converting enzyme

CONTEXT

Baicalin has been characterized as the active compound and quality control marker in Scutellaria baicalensis Georgi, traditionally used as a hypotensive herb.

OBJECTIVES

To investigate the inhibitory activities of baicalin against renin and angiotensin-I converting enzyme (ACE) and their molecule mechanism of interactions.

METHODS

The fluorescence method using renin substrate 1(R-2932) and the spectroscopy method by Cushman were used to determine renin and ACE activities, respectively. The fluorescence quench techniques were used to characterize their interactions.

RESULTS

The results showed that baicalin inhibited renin activity with an IC(50) value of 120.36 µM and inhibited ACE activity with an IC(50) value of 2.24 mM in vitro. The fluorescence emission of both renin and ACE were efficiently quenched by baicalin and a complete quenching was achieved at a high concentration of baicalin. Furthermore, baicalin was more effective in quenching the fluorescence of renin (K(SV) = 60 × 10(3) M(-1)) than ACE (K(SV) = 17.1 × 10(3) M(-1)). The quenching of fluorescence of renin and ACE involved static interactions, which was characterized by the formation of quencher-enzyme complex. The baicalin-renin complex formed through three-sites binding including the active site with a binding constant of 796.15 × 10(13) M(-1), but there was only one binding site for the baicalin-ACE complex with a much smaller binding constant of 6.8 × 10(5) M(-1).

CONCLUSION

The inhibition activity of baicalin against renin was a result of the formation of stable complex through multisites binding including the active site, which could explain the higher inhibitory efficiency.

Nitric oxide dilates rat retinal blood vessels by cyclooxygenase-dependent mechanisms

It has been suggested that nitric oxide (NO) stimulates the cyclooxygenase (COX)-dependent mechanisms in the ocular vasculature; however, the importance of the pathway in regulating retinal circulation in vivo remains to be elucidated. Therefore, we investigated the role of COX-dependent mechanisms in NO-induced vasodilation of retinal blood vessels in thiobutabarbital-anesthetized rats with and without neuronal blockade (tetrodotoxin treatment). Fundus images were captured with a digital camera that was equipped with a special objective lens. The retinal vascular response was assessed by measuring changes in diameter of the retinal blood vessel. The localization of COX and soluble guanylyl cyclase in rat retina was examined using immunohistochemistry. The NO donors (sodium nitroprusside and NOR3) increased the diameter of the retinal blood vessels but decreased systemic blood pressure in a dose-dependent manner. Treatment of rats with indomethacin, a nonselective COX inhibitor, or SC-560, a selective COX-1 inhibitor, markedly attenuated the vasodilation of retinal arterioles, but not the depressor response, to the NO donors. However, both the vascular responses to NO donors were unaffected by the selective COX-2 inhibitors NS-398 and nimesulide. Indomethacin did not change the retinal vascular and depressor responses to hydralazine, 8-(4-chlorophenylthio)-guanosine-3′, 5′-cyclic monophosphate (a membrane-permeable cGMP analog) and 8-(4-chlorophenylthio)-adenosine-3′, 5′-cyclic monophosphate (a membrane-permeable cAMP analog). Treatment with SQ 22536, an adenylyl cyclase inhibitor, but not ODQ, a soluble guanylyl cyclase inhibitor, significantly attenuated the NOR3-induced vasodilation of retinal arterioles. The COX-1 immunoreactivity was found in retinal blood vessels. The retinal blood vessel was faintly stained for soluble guanylyl cyclase, although the apparent immunoreactivities on mesenteric and choroidal blood vessels were observed. These results suggest that NO exerts a substantial part of its dilatory effect via a mechanism that involves COX-1-dependent pathway in rat retinal vasculature.

Endothelial function and cardiovascular disease: Effects of quercetin and wine polyphenols

Endothelial dysfunction is an early pathophysiological feature and independent predictor of poor prognosis in most forms of cardiovascular diseases. Epidemiological studies report an inverse association between dietary flavonoid consumption and mortality from cardiovascular diseases. In the present paper, we review the effects of flavonoids, especially quercetin and wine polyphenols, on endothelial function and dysfunction and its potential protective role in hypertension, ischemic heart disease and stroke. In vitro studies show that flavonoids may exert multiple actions on the NO-guanylyl cyclase pathway, endothelium-derived hyperpolarizing factor(s) and endothelin-1 and protect endothelial cells against apoptosis. In vivo, flavonoids prevent endothelial dysfunction and reduce blood pressure, oxidative stress and end-organ damage in hypertensive animals. Moreover, some clinical studies have shown that flavonoid-rich foods can improve endothelial function in patients with hypertension and ischemic heart disease. Altogether, the available evidence indicates that quercetin and wine polyphenols might be of therapeutic benefit in cardiovascular diseases even though prospective controlled clinical studies are still lacking.

Vasodilator effect of nicorandil on retinal blood vessels in rats

We examined the effect of nicorandil on retinal blood vessels in rats in vivo. Male Wistar rats (8 to 10 weeks old) were anaesthetised with thiobutabarbital (120 mg/kg, intraperitoneal). Fundus images were captured with a digital camera that was equipped with a special objective lens. Diameters of retinal blood vessels were measured with a personal computer. Nicorandil (1-300 microg kg(-1) min(-1), intravenous [i.v.]) increased diameters of retinal blood vessels and decreased systemic blood pressure in a dose-dependent manner. Both responses to nicorandil were attenuated by glibenclamide (20 mg/kg, i.v.), an adenosine triphosphate (ATP)-dependent K(+) (K(ATP)) channel blocker. On the other hand, indomethacin (5 mg/kg, i.v.), a cyclooxygenase inhibitor, attenuated the vasodilation of retinal blood vessels, but not depressor response, to nicorandil and sodium nitroprusside. Pinacidil (1-300 microg kg(-1) min(-1), i.v.), a K(ATP) channel opener, also dilated retinal blood vessels and decreased systemic blood pressure. The responses to pinacidil were prevented by glibenclamide, but not by indomethacin. The vasodilation of retinal arteriole, but not depressor response, to sodium nitroprusside (1-30 microg kg(-1) min(-1), i.v.), a nitric oxide donor, was attenuated by indomethacin. These results suggest that nicorandil dilates retinal blood vessels through opening of K(ATP) channels and production of prostaglandins that are probably generated by nitric oxide.

Baicalein impairs vascular tone in normal rat aortas: role of superoxide anions

Acute exposure to the flavonoid baicalein inhibited endothelium-dependent relaxation in physiological arteries, although the mechanisms are not fully understood. We investigated the effect of baicalein on vascular tone in Wistar-Kyoto (WKY) rat isolated aortic rings in the presence and absence of oxidative stress to further determine the underlying mechanisms. Exposure to baicalein (10 microM) completely abolished endothelium-dependent relaxation induced by acetylcholine and attenuated significantly the endothelium-independent relaxation induced by sodium nitroprusside. Baicalein, similar to Nomega-nitro-L-arginine methyl ester (L-NAME, 10 microM), potentiated significantly the contractile response of aortic rings to alpha1-adrenoceptor agonist phenylephrine. In the presence of L-NAME the baicalein effect on phenylphrine contraction or acetylcholine relaxation was unaltered, suggesting that these effects of baicalein are (like L-NAME effect) endothelial nitric oxide synthase (eNOS)/endothelium-derived nitric oxide-dependent. Inhibition of cyclooxygenase activity with indomethacin (10 microM) or scavenging of superoxide anions with superoxide dismutase (150 units/ml), but not scavenging of hydrogen peroxide with catalase (800 units/ml), enhanced significantly by an essentially similar extent the relaxation to acetylcholine in baicalein-pretreated aortic rings. Relaxant effect to acetylcholine was significantly attenuated in control aortic rings, but was completely abolished in baicalein-pretreated aortic rings in the presence of reduced form of beta-nicotinamide adenine di-nucleotide (beta-NADH, 300 microM). Baicalein blocked beta-NADH (300 microM)-induced transient contractions, suggesting that baicalein may have inhibited activity of NADH/NADPH-oxidase. Baicalein did not alter the failure of acetylcholine to induce relaxation in the presence of pyrogallol (300 microM). In summary, acute exposure to baicalein impairs eNOS/endothelium-derived nitric oxide-mediated vascular tone in rat aortas through the inhibition of endothelium-derived nitric oxide bioavailability coupled to reduced bioactivity of endothelium-derived nitric oxide and to cyclooxygenase-mediated release of superoxide anions.

Comparison of high-speed counter-current chromatography instruments for the separation of the extracts of the seeds of Oroxylum indicum

Analytical Milli high-speed counter-current chromatography (HSCCC) was used for the selection and optimization of the two-phase solvent system to separate flavonoids from the extracts of the seeds of Oroxylum indicum. The optimum solvent system obtained from Milli-CCC was also the best solvent system for preparative HSCCC and led to the successful separation of two crude flavonoids from the seeds of O. indicum by Lab/Prep (laboratory preparative) HSCCC using different sized coils. Four flavonoids were isolated by preparative HSCCC: baicalein-7-O-diglucoside (25.0 mg, 92% purity), baicalein-7-o-glucoside (50.4 mg; 95% purity), baicalein (75 mg; purity 98%) and chrysin (100 mg; purity 98%).

Prostaglandin I2 does not contribute to the hypotensive effect of the superoxide dismutase mimetic Tempo in rats with aortic coarctation-induced hypertension

This study was designed to investigate the contribution of prostaglandins to the vasodepressor effect of the superoxide dismutase mimetic Tempo in rats made hypertensive by ligation of the abdominal aorta at a point between the left and right renal arteries. Rings of thoracic aorta taken from rats with aortic coarctation released more 6-keto-PGF1alpha (a non-enzymatic product of PGI2 degradation) in the presence than in the absence of Tempo (1 mmol/L; 35.3 +/- 10.1 versus 13.6 +/- 2.6 pg/mg tissue). However, Tempo administered intravenously (2 mg/kg bolus injection plus infusion at 3 mg/kg/h) to rats with aortic coarctation did not increase significantly the concentration of 6-keto-PGF1alpha in vena cava blood. Treatment with Tempo did not affect the arterial pressure of un-operated normotensive rats but promptly decreased the arterial pressure of rats with aortic coarctation-induced hypertension (from 178 +/- 2 to 125 +/- 6 mmHg). The vasodepressor effect of Tempo in hypertensive animals was not affected by pretreatment with indomethacin to inhibit prostaglandin synthesis. These data argue against the hypothesis that PGI2 contributes to the acute hypotensive effect of Tempo in rats with aortic coarctation.

Cross Talk between Angiotensin II and Alpha 1 Adrenergic Receptors in Rabbit Aorta: Role of Endothelium

Interaction between the renin-angiotensin system and the sympathetic nervous system has been proposed to be like a physiological regulation mechanism. The present work was designed to study the cross talk between angiotensin II and adrenergic receptors on the smooth muscle contractile response and the endothelium influence in this phenomenon. Homologous and endothelium independent desensitization of angiotensin II-contractile response was observed. Treatment with noradrenaline between two cumulative doses response curves (CDRC) to angiotensin II caused a rightward shift of the second CDRC in unrubbed arteries and increased the maximal response in rubbed arteries. Prazosin blocked these effects. No homologous desensitization of noradrenaline contractile response was found. Treatment with angiotensin II between two CDRC to noradrenaline caused a loss of affinity in the second CDRC in unrubbed arteries. Losartan was able to avoid this phenomenon. Maximal response was enhanced both in arteries with and without endothelium treated or not with angiotensin II. Results demonstrate homologous and endothelium-independent desensitization of the contractile response to angiotensin II but not to noradrenaline. In addition, heterologous and endothelium-dependent desensitization induced by noradrenaline and angiotensin II on the contractile response to each other was found. Furthermore, results provided the first evidence that there is an endothelium-dependent cross talk between alpha1-adrenergic and angiotensin II receptors in smooth muscle of rabbit aorta.

Isolation and identification of four flavonoid constituents from the seeds of Oroxylum indicum by high-speed counter-current chromatography

Four flavonoids, chrysin, baicalein, baicalein-7-O-glucoside, baicalein-7-O-diglucoside (Oroxylin B) and one unknown flavonoid have been isolated and purified for the first time in the seeds of Oroxylum indicum by high-speed counter-current chromatography with a two-phase solvent system composed of chloroform-methanol-water (8:10:5, v/v). Then, 50 mg baicalein-7-O-glucoside, 10.5 mg baicalein-7-O-diglucoside, 4.5 mg chrysin-7-O-diglucoside, 25 mg baicalein and 45 mg chrysin could be obtained after injecting 20 mg/ml sample extract ten times and their purities were 96, 90, 85, 95 and 98%, respectively. All these constituents were identified by high-performance liquid chromatography-mass spectrometry and nuclear magnetic resonance.

Comparison of metabolic pharmacokinetics of baicalin and baicalein in rats

Baicalin and baicalein, a flavone glucuronide and its aglycone, are bioactive constituents of Scutellariae Radix with various beneficial activities. We have characterized and compared the metabolic pharmacokinetics of baicalin and baicalein in rats. Baicalein was administered intravenously and orally to rats, and baicalin was orally administered. An HPLC method was used to determine the concentration of baicalein before and after hydrolysis using beta-glucuronidase/sulfatase. The pharmacokinetic parameters were calculated by using WINNONLIN. Unpaired Student's t-test was used for statistical comparison. The result showed that after intravenous administration of baicalein, 75.7% of the dose was circulating as its conjugated metabolites. After oral administration of baicalein, absorption of baicalein itself was negligible, whereas the glucuronides/sulfates of baicalein were predominant in the plasma. When compared with intravenous bolus administration with dose correction, the absolute absorption was 40%. When baicalin was administered orally, glucuronides and sulfates of baicalein were exclusively circulating in the plasma. The relative absorption for baicalin was 65% when compared with baicalein. Profound differences of serum profile and pharmacokinetics were observed between oral baicalein and baicalin. Baicalin demonstrated significantly later time to peak concentration (t(max)) and lower peak serum concentration (C(max)) of baicalein conjugated metabolites than baicalein, indicating baicalin was absorbed more slowly and to a lesser extent than baicalein.

Effects of San'o-shashin-to and the constituent herbal medicines on theophylline-induced increase in arterial blood pressure of rats

San'o-shashin-to, composed of Scutellariae Radix, Coptidis Rhizoma and Rhei Rhizoma (volume ratio = 1:1:1), reduced an increase in arterial blood pressure of anesthetized rats induced by theophylline (5 mg/kg, i.v.). The hypotensive effect of San'o-shashin-to was produced in a dose dependent manner and was maximum at its 0.5 g/kg. Then the constituent herbal medicines were examined for their possible hypotensive effect. Scutellariae Radix of 0.2 g/kg slightly decreased in the blood presure. Rhei Rhizoma of 0.2 g/kg decreased in the blood pressure and the hypotensive effect was significantly produced even at the dose of 0.05 g/kg, while Coptidis Rhizoma had little effect. Among fractions of San'o-shashin-to separated by Diaion HP-20 column chromatography, the 50% methanol-eluted fraction had a large hypotensive effect. The 50% methanol-eluted fraction of Scutellariae Radix and Rhei Rhizoma were also effective and, especially, that of Rhei Rhizoma had a large hypotensive effect. In isometric tension study, Scutellariae Radix and Rhei Rhizoma (10-30 microg/ml) slightly exerted contractile and relaxant effects, respectively, on the phenylephrine-contracted endothelium-intact rat thoracic aorta. Coptidis Rhizoma (1-10 microg/ml) caused both endothelium-dependent and -independent relaxantion. These results suggest that the hypotensive effect of San'o-shashin-to is not mediated by the direct action on blood vessel but by other actions. Some components in Scutellariae Radix and Rhei Rhizoma, especially in the latter may play a main role in the hypotensive effect.

Endothelium-dependent desensitization to angiotensin II in rabbit aorta: the mechanisms involved

The aim of this study was to characterize the role of the endothelium in angiotensin II-desensitization and its mechanisms of action. Rabbit aortic rings were exposed to increasing doses of angiotensin II (Ang II, 10–9 to 2.5 × 10–6) to generate two cumulative dose-response curves (CDRC I and II). A 50-min interval separated CDRC I and II. Desensitization was observed at all doses in unrubbed aortic tissue and at lower doses in rubbed aortic tissue. Tachyphylaxis was greater in arteries with endothelium. Treatment of intact rings with L-NG-nitroarginine methyl ester (L-NAME, 10–4 M) did not prevent this phenomenon. However, indomethacin (10–5 M) and miconazol (10–6 M) attenuated Ang II-desensitization. Treatment of unrubbed rings with nifedipine (10–6 M) and cromakalim (10–6 M) inhibited the effect of indomethacin. To confirm the involvement of K+ channels, unrubbed and rubbed aortic rings were treated with the KCa2+ blockers apamin (10–7 M), tetraethylammonium (TEA, 10–3 M), and iberiotoxin (10–8 M), and the KATP blocker glibenclamide (10–5 M). In both arteries apamin, TEA, and glibenclamide abolished the tachyphylaxis without changes in the maximal response. Iberiotoxin diminished Ang II-desensitization in rubbed but not unrubbed arteries. Results from this study suggest that Ang II-desensitization involves endothelium-dependent and -independent mechanisms. Endothelium-dependent desensitization could be mediated by a cyclooxygenase-cytochrome P450 product, which could act by increasing KCa2+ channel activity.Key words: angiotensin II, rabbit aorta, desensitization, endothelium, cyclooxygenase products.

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

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

Molecular structure and activity toward DNA of baicalein, a flavone constituent of the Asian herbal medicine "Sho-saiko-to"

Baicalein (5,6,7-trihydroxyflavone, 1) is of interest because of its broad spectrum of biological activity. It is a constituent of the east Asian herbal remedy, "Sho-saiko-to". The 3D structure of 1 was determined using X-ray diffraction. The compound exists in an almost planar conformation with a C-2-C-1' bond distance of 1.476(5) A. Hydrogen-bonding interactions predominate in the crystal structure. The position of the three hydroxyl groups maximizes intramolecular hydrogen bonding, and each of the hydroxyl hydrogen atoms is a donor in a three-center hydrogen bond. The carbonyl oxygen, O-4, is an acceptor in an intramolecular hydrogen bond (with OH-5). Two molecules of 1 exist as hydrogen-bonded dimers related by inversion center (-x + 1, -y, -z + 1). O-4 is also an acceptor in an intermolecular hydrogen bond with OH-6. The planarity of the flavone framework is dependent on structural and/or electronic forces that stabilize the negative charge on the exocyclic oxygen atom, O-4. Compound 1, therefore, is planar in any situation where forces can stabilize the negative charge on O-4. Consistent with this, UV absorbance studies performed on 1-DNA complexes with varying concentrations of 1 strongly suggest intercalation of 1 within the double helix, followed by possible interstrand cross-links.

Potentiating effects on contractions by purified baicalin and baicalein in the rat mesenteric artery

The effects of purified baicalin and baicalein from the traditional Chinese herb, Huangqin, on contractions induced by phenylephrine, U46619, and high extracellular K+ were investigated in isolated rat mesenteric arteries. Both baicalin (1-100 microM) and baicalein (1-50 microM) potentiated the contractile response to phenylephrine in a concentration-related manner. Both flavonoids (10 microM) also enhanced the U46619- or 40 mM K+-induced contractions. Baicalein (100-300 microM) reduced the phenylephrine-induced tone. Prazosin at 1 microM did not affect U46619-induced contraction in the absence and presence of baicalein or baicalin. Neither baicalin (1-100 microM) nor baicalein (1-100 microM) affected the basal tension. Removal of the functional endothelium abolished the potentiating effects of baicalin and baicalein in arteries preconstricted by both constrictors. Pretreatment of endothelium-intact rings with 100 microM N(G)-nitro-L-arginine also potentiated phenylephrine- or U46619-induced contraction but completely inhibited the effects of baicalin and baicalein. Pretreatment with 1 mM L-arginine reversed the enhancing effect of baicalin but not of baicalein on phenylephrine-evoked contraction. Pretreatment with 10 microM baicalin or 10 microM baicalein significantly reduced the endothelium-dependent relaxation induced by acetylcholine or ionomycin. These results indicate that both baicalin and baicalein potentiated the evoked contractile response, likely through inhibition of nitric oxide formation and/or release in the endothelium.

Arachidonic acid metabolic pathways regulating activity of renal Na(+)-K(+)-ATPase are age dependent

Locally formed arachidonic acid (AA) metabolites are important as modulators of many aspects of renal tubular function, including regulation of the activity of tubular Na(+)-K(+)-ATPase. Here we examined the ontogeny of the AA metabolic pathways regulating proximal convoluted tubular (PCT) Na(+)-K(+)-ATPase activity in infant and adult rats. Eicosatetraynoic acid, an inhibitor of all AA-metabolizing pathways, abolished this effect. AA inhibition of PCT Na(+)-K(+)-ATPase was blocked by the 12-lipoxygenase inhibitor baicalein in infant but not in adult rats and by the specific cytochrome P-450 fatty acid omega-hydroxylase inhibitor 17-octadecynoic acid in adult but not in infant rats. The lipoxygenase metabolite 12(S)-hydroxyeicosatetraenoic acid (HETE) and the cytochrome P-450 metabolite 20-HETE both inhibited PCT Na(+)-K(+)-ATPase in a protein kinase C-dependent manner, but the effect was significantly more pronounced in infant PCT. Lipoxygenase mRNA was only detected in infant cortex. Expression of renal isoforms of cytochrome P-450 mRNA was more prominent in adult cortex. In summary, the AA metabolic pathways that modulated the activity of rat renal proximal tubular Na(+)-K(+)-ATPase are age dependent.

Lipoxygenase-dependent mechanisms in hypertension

This study was designed to examine the contribution of lipoxygenase products to mechanisms of vascular contraction and elevated blood pressure in rats with aortic coarctation-induced hypertension. In cytosolic fractions of aortae taken from hypertensive rats, 12-lipoxygenase protein was increased as compared to normotensive controls. Aortic rings from hypertensive, but not from normotensive rats, exhibited a basal tone which was reduced 74+/-12 and 71+/-22%, respectively, by the lipoxygenase inhibitors cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate (CDC, 10(-5) mol/L) and 5,8,11-eicosatriynoic acid (ETI, 10(-5) mol/L). CDC (8 mg/kg s.c.) did not affect the blood pressure of normotensive rats but decreased that of hypertensive rats from 182+/-6 to 151+/-10 mm Hg. The blood pressure lowering effect of CDC was blunted in hypertensive rats pretreated with indomethacin or antibodies against 5,6-dihydro-prostaglandin I2. These data suggest contribution of lipoxygenase-derived products to mechanisms underlying aortic smooth muscle basal tone and elevated blood pressure in rats with aortic coarctation-induced hypertension. The vasodepressor effect of CDC depends on a mechanism involving vasodilatory prostaglandins.