Direct effects of quercetin on impaired reactivity of spontaneously hypertensive rat aortae: comparative study with ascorbic acid

1. There is a growing interest in the anti-oxidant characteristics and use of flavonoids in the management of cardiovascular diseases. The cardiovascular mechanism of action of these plant derivatives remains controversial. This study compared the effects of the flavonoid quercetin with those of the anti-oxidant vitamin ascorbic acid (vitamin C) on the reactivity of aortic rings from spontaneously hypertensive rats (SHR). 2. The phenylephrine (PE)-induced contractile and the endothelium-dependent and independent relaxant responses of aortic rings from 21 to 22 week old SHR and age-matched normotensive Wistar (WKY) rats were observed in the presence of quercetin or ascorbic acid. All the experiments were performed in the presence of the cyclooxygenase inhibitor, indomethacin (10 micromol/L). 3. The endothelium-dependent and independent relaxations to acetylcholine (ACh) and sodium nitroprusside (SNP), respectively, were significantly lesser in the SHR compared to the WKY tissues whereas the contractile responses to PE were similar in both tissues. Pretreatment of WKY rings with quercetin or ascorbic acid had no effect on the responses to ACh or PE. In the SHR tissues, however, quercetin or ascorbic acid significantly improved the relaxation responses to ACh and reduced the contractions to PE with greater potency for quercetin. Both compounds lacked any effects on the responses to SNP in either aortic ring types. N(omega)-nitro-L-arginine methyl ester (l-NAME, 10 micromol/L) significantly attenuated the vasodepressor effects of quercetin and ascorbic acid, raising the responses to PE to a level similar to that observed in the control SHR tissues. In l-NAME pretreated aortic rings, quercetin and ascorbic acid inhibited the contractile responses to PE with the same magnitude in WKY and SHR tissues. 4. The present results suggest that acute exposure to quercetin improves endothelium-dependent relaxation and reduces the contractile responses of hypertensive aortae with a greater potency than ascorbic acid. This suggests a better vascular protection with this flavonoid than ascorbic acid in the SHR model of hypertension and possibly in human cardiovascular diseases.     

Modulation of vascular reactivity in normal, hypertensive and diabetic rat aortae by a non-antioxidant flavonoid.

In this study, we report the effects of a non-antioxidant flavonoid flavone on vascular reactivity in Wistar-Kyoto (WKY) rat isolated aortae. Whether flavone directly modulates vascular reactivity in spontaneously hypertensive rat (SHR) and streptozotocin-induced diabetic-WKY rat isolated aortae was also determined. Thoracic aortic rings were mounted in organ chambers and exposed to various drug treatments in the presence of flavone (10 microM) or its vehicle (DMSO), which served as control. Pretreatment with flavone enhanced relaxant effects to endothelium-dependent vasodilator acetylcholine (ACh) and attenuated contractile effects to alpha(1)-receptor agonist phenylephrine (PE) in WKY aortae compared to those observed in control aortic rings. Flavone had no effect on relaxations to ACh in WKY aortae incubated with either L-NAME or methylene blue, but enhanced relaxations to ACh in WKY aortae incubated with indomethacin or partially depolarized with KCl. Relaxations to ACh are totally abolished in both control or flavone pretreated endothelium-denuded WKY aortae. Flavone attenuated the inhibition by beta-NADH of ACh-induced relaxation in WKY aortae, but it had no significant effect on the transient contractions induced by beta-NADH nor the pyrogallol-induced abolishment of ACh-induced relaxation in WKY aortae. Flavone enhanced endothelium-independent relaxation to sodium nitroprusside (SNP) in both endothelium-intact and -denuded WKY aortae. Flavone enhanced relaxation to ACh and SNP as well as attenuated contractile effects to PE in SHR and diabetic aortae, a finding similar to that observed in normal WKY aortae. From these results, we conclude that flavone modulates vascular reactivity in normal as well as hypertensive and diabetic aortae. These effects of flavone results probably through enhanced bioactivity of nitric oxide released from the endothelium.     

Downregulation of vascular soluble guanylate cyclase induced by high salt intake in spontaneously hypertensive rats

1. Cyclic guanosine monophosphate (cyclic GMP)-mediated mechanism plays an important role in vasodilatation and blood pressure regulation. We investigated the effects of high salt intake on the nitric oxide (NO) - cyclic GMP signal transduction pathway regulating relaxation in aortas of spontaneously hypertensive rats (SHR). 2. Four-week-old SHR and normotensive Wistar-Kyoto rats (WKY) received a normal salt diet (0.3% NaCl) or a high salt diet (8% NaCl) for 4 weeks. 3. In aortic rings from SHR, endothelium-dependent relaxations in response to acetylcholine (ACh), adenosine diphosphate (ADP) and calcium ionophore A23187 were significantly impaired by the high salt intake. The endothelium-independent relaxations in response to sodium nitroprusside (SNP) and nitroglycerin were also impaired, but that to 8-bromo-cyclic GMP remained unchanged. On the other hand, high salt diet had no significant effects on the relaxations of aortic rings from WKY. 4. In aortas from SHR, the release of NO stimulated by ACh was significantly enhanced, whereas the production of cyclic GMP induced by either ACh or SNP was decreased by the high salt intake. 5. Western blot analysis showed that the protein level of endothelial NO synthase (eNOS) was slightly increased, whereas that of soluble guanylate cyclase (sGC) was dramatically reduced by the high salt intake. 6. These results indicate that in SHR, excessive dietary salt can result in downregulation of sGC followed by decreased cyclic GMP production, which leads to impairment of vascular relaxation in responses to NO. It is notable that chronic high salt intake impairs the sGC/cyclic GMP pathway but not the eNOS/NO pathway.     

Acetylcholine-induced contractions in the perforating branch of the human internal mammary artery: protective role of the vascular endothelium

The effect of acetylcholine (ACh) on the isolated, nonprecontracted perforating branch of the human internal mammary artery (HIMA) was investigated. ACh induced concentration-dependent contractions of nonprecontracted rings with denuded endothelium (pEC(50) = 6.72 +/- 0.02, E(max) = 88.8% of contractions induced by phenylephrine, 10(-5) mol/l) and was without effect on arterial segments with intact endothelium. An inhibitor of nitric oxide synthase, N(G)-monomethyl-L-arginine (L-NMMA), or indometacin, a cyclooxygenase inhibitor, had no effect on acetylcholine-induced contractions of rings of the perforating branch of HIMA with denuded endothelium (pEC(50) = 6.76 +/- 0.03 and 6.62 +/- 0.05, respectively). In the presence of indometacin, ACh did not evoke contractions of arterial segments with intact endothelium. In contrast, in the same type of preparations ACh induced contractions in the presence of L-NMMA (E(max) = 34%). The muscarinic receptor antagonists atropine (no selectivity), pirenzepine (M(1)), methoctramine (M(2)), and p-fluoro-hexahydro-sila-difenidol (M(1)/M(3)) competitively antagonized the response to ACh. The pA(2) values were 9.60 +/- 0.10, 6.99 +/- 0.02, 6.37 +/- 0.17, and 8.02 +/- 0.06, respectively. In conclusion, the results obtained indicate that secretion of nitric oxide from vascular endothelium may protect the perforating branch of HIMA against the contractile effects of ACh. On the basis of differential antagonist affinity, it can be suggested that the muscarinic receptors involved in the ACh-induced contractions of the isolated perforating branch of the HIMA are predominantly of the M(3) subtype.     

Apocynin reduces blood pressure and restores the proper function of vascular endothelium in SHR

This study has evaluated how the vascular endothelium of hypertensive rats chronically treated with apocynin affects acetylcholine (ACh), sodium nitroprusside (SNP), and phenylephrine (PE) action on the nitric oxide (NO) signal transduction pathway in endothelial (EC) and vascular smooth muscle cells. Treatment with apocynin significantly reduced the mean arterial pressure in spontaneously hypertensive rats (SHR). In addition, apocynin improved the impaired ACh hypotensive effect on SHR. Although systemic oxidative stress was high in SHR, SHR treated with apocynin and normotensive rats presented similar systemic oxidative stress levels. Endothelium significantly blunted PE contractions in intact aortas of treated SHR. The ACh effect was impaired in resistance arteries and aortas of SHR, but this same effect was improved in treated SHR. The SNP potency was higher in intact resistance arteries of treated SHR than in intact resistance arteries of untreated SHR. NO and calcium concentrations increased, whereas reactive oxygen species levels decreased in EC of treated SHR. Aortas of untreated and treated SHR did not differ in terms of sGC alpha or beta units expression. Aorta of treated SHR expressed higher eNOS levels as compared to aorta of untreated SHR. The study groups did not differ with respect to NOX1, NOXO1, or NOX4 expression. However, treatment with apocynin normalized overexpression of NOX2 and its subunit p47phox in aortas of SHR. Based on all the results presented in this study, we suggest apocynin increases NO biovailability by different mechanisms, restoring the proper function of vascular endothelium in SHR.     

Hypertension and the absence of EDHF-mediated responses favour endothelium-dependent contractions in renal arteries of the rat

Background and purpose:Experiments were designed to determine the modulation by nitric oxide (NO) and endothelium-dependent hyperpolarizations (EDHF-mediated responses) of endothelium-dependent contractions in renal arteries of normotensive and hypertensive rats.Experimental approach:Rings, with or without endothelium, of renal arteries of 8-month-old Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were suspended in myographs for isometric force recording.Key results:ACh evoked relaxations in preparations contracted with phenylephrine. L-NAME (inhibitor of NOS) attenuated (WKY) or abolished (SHR) these relaxations. TRAM-34 plus UCL 1684 (inhibitors of EDHF-mediated responses) did not decrease the relaxation, except in rings of WKY when L-NAME was also present. High concentrations of ACh caused a secondary increase in tension, augmented in rings of WKY by L-NAME or TRAM-34 plus UCL 1684. The increase in tension was prevented by indomethacin. Under baseline tension, ACh induced endothelium-dependent contractions, prevented by indomethacin (COX inhibitor) or terutroban (TP receptor antagonist). The calculated endothelium-dependent contractions were larger in rings of SHR compared with those of WKY. In preparations of SHR, the contractions were augmented by L-NAME in the presence of SC19220 (EP-1 receptor antagonist). In arteries of WKY, the endothelium-dependent contractions were augmented by TRAM-34 plus UCL 1684. The responses were reduced by SC19220.Conclusions and implications:In the renal artery of the rat, EDCF-mediated contractions are augmented by hypertension. The endothelium-dependent contractions are facilitated by NOS inhibition (in the presence of an EP-1 receptor antagonist) and by the withdrawal of EDHF-mediated responses.British Journal of Pharmacology (2008) 155, 217-226; doi:10.1038/bjp.2008.256; published online 23 June 2008.     

Developmental changes in endothelium-dependent pulmonary vasodilatation in pigs.

1. We compared in vitro endothelium-dependent vasorelaxant responses to acetylcholine (ACh) and the endothelium-independent vasodilator response to sodium nitroprusside (SNP) in prostaglandin F2 alpha (PGF2 alpha)-precontracted muscular pulmonary arteries (PA) from pigs aged 5 min to 2 h (neonatal), 3-10 days, 3-8 weeks and adults. 2. In the pulmonary artery (PA) rings from neonatal animals, the vasodilator response to ACh was negligible. However, responses to ACh were present in all PA rings from older animals, being greatest at 3-10 days and then decreasing with age (P less than 0.001, ANOVA). ACh (30 microM) induced a 1 +/- 1%, 92 +/- 9%, 62 +/- 5% and 51 +/- 6% reduction of the PGF2 alpha-generated tension in neonatal, 3-10 days, 3-8 weeks and adult groups, respectively. 3. The relaxant response to SNP was present in the PA rings from all age groups and increased with age (P less than 0.001, ANOVA). SNP (1 microM)-induced relaxation was 55 +/- 9%, 73 +/- 7%, 97 +/- 5% and 93 +/- 6% in neonatal, 3-10 days, 3-8 week and adult groups, respectively. 4. Removal of the vascular endothelium abolished the relaxant response to ACh but had no effect on the response to SNP in any groups. 5. NG-monomethyl-L-arginine (30 microM), a nitric oxide synthesis inhibitor, inhibited the response to ACh but not to SNP. The lipoxygenase inhibitor, nordihydroguaiaretic acid, had no significant effect on responses to ACh or SNP in any group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual dimorphism in the response of thoracic aorta from SHRs to losartan

1. We compared the endothelium-dependent responses of thoracic aortic rings obtained from male and female spontaneously hypertensive rats (SHR) in order to explore gender differences in the normalization of the high blood pressure by antihypertensive drug therapy and in the correction of the endothelial dysfunction found in these animals. 2. Concentration-effect curves to acetylcholine (ACh) and sodium nitroprusside (SNP) were obtained using aortic rings isolated from male and female rats pretreated or not with losartan for 24 h or 15 d. The responses achieved and the EC50s were determined. 3. Losartan, AT(1) receptor antagonist, normalized (around 125 mmHg) the high blood pressure levels in 100% of the females and in 53.3% of males SHR within 24 h of initiating the treatment and remained normal during the remainder of the treatment period (15 d). 4. Losartan (15 d) corrected the decreased response to ACh in male and female SHR, independently of the normalization of blood pressure in male SHR. 5. An increased sensitivity to SNP was observed after chronic treatment with losartan in aortic rings from female SHR. 6. Ridogrel, a TXA(2)/PGH(2) receptor antagonist, restored the decreased response to ACh in aortic rings from male and female SHR. 7. These results suggest that there are gender-related differences in the normalization of the high blood pressure levels by losartan in SHR. The decreased response to ACh observed in male and female is corrected after sustained (15 d) reduction of high blood pressure. In female but not in male SHR, correction seems to involve an increased sensitivity of the smooth muscle to nitric oxide.     

Cocaine-induced relaxation of isolated rat aortic rings and mechanisms of action: possible relation to cocaine-induced aortic dissection and hypotension

Cocaine HCl is well known for its toxic effects on the cardiovascular system, but little is known about its effects on different regional blood vessels. We designed experiments to determine if cocaine HCl could influence the tension of isolated aortic rings, i.e., induce contraction or relaxation. Surprisingly, cocaine HCl (1 x 10(-5) to 6 x 10(-3) M) relaxed isolated aortic rings precontracted by phenylephrine in a concentration-dependent manner. No significant differences were found between intact or denuded isolated aortic rings (P>0.05). The maximal % relaxations of intact vs. denuded isolated aortic rings were 108.9+/-24.3% vs. 99.5+/-8.3% (P>0.05). Cocaine HCl, 2 x 10(-3) M, was found to inhibit contractions by phenylephrine; EC50s were increased (P<0.01) and Emax's were decreased (51.3+/-16.4% vs. 89.8+/-10.6%, P<0.01). A variety of amine antagonists could not inhibit the relaxant effects of cocaine HCl (P>0.05). The cyclooxygenase-1 inhibitor, indomethacin, also failed to inhibit relaxations induced by cocaine HCl (P>0.05). Neither L-arginine, NG-monomethyl-L-arginine (L-NMMA), nor methylene blue could inhibit the relaxations induced by cocaine HCl (P>0.05), suggesting cocaine HCl does not relax isolated aortic rings by inducing the synthesis or release of nitric oxide (NO) or prostanoids from either endothelial or vascular muscle cells. Inhibitors of cAMP, cGMP and protein kinase G (PKG) also failed to inhibit cocaine-induced relaxations. Cocaine HCl (1 x 10(-5) to 6 x 10(-3) M) could also relax isolated aortic rings precontracted by phenylephrine in high K+ depolarizing buffer. Surprisingly, calyculin A, an inhibitor of myosin light chain (MLC) phosphatase, inhibited cocaine-induced relaxations in a concentration-dependent manner, suggesting the probable importance of cocaine-induced MLC phosphatase activation in rat aortic smooth muscle cells. It was also found that cocaine HCl could dose-dependently inhibit Ca2+-induced contractions of isolated aortic rings in high K+-Ca2+-free buffer, suggesting that cocaine HCl may inhibit Ca2+ influx and/or intracellular release.     

Evidence for differential roles of nitric oxide (NO) and hyperpolarization in endothelium-dependent relaxation of pig isolated coronary artery.

1. The possible roles of endothelial and smooth muscle cell hyperpolarization and nitric oxide (NO) in endothelium-dependent relaxation were examined in isolated rings of pig right coronary artery. 2. The effects of hyperpolarization were prevented with high K+ (30-125 mM), isotonic Krebs solutions. Functional antagonism due to high K(+)-induced smooth muscle contraction was prevented with 0.3 microM nifedipine (in all treatments, for consistency). All rings were contracted with the thromboxane-mimetic U46619, (1-100 nM) to bring them to an initial active force of within 30-50% of maximum contraction. 3. High K+ had no effects on the sensitivity (EC50) or time course of endothelium-dependent (substance P, SP; bradykinin, BK; calcimycin, A23187) and -independent (sodium nitroprusside, SNP) agents. Maximum relaxations (Rmax) to SP, BK and A23187 were reduced significantly by approximately 20% but only with 125 mM K+. 4. In normal K+ Krebs solution (5.9 mM), NG-nitro-L-arginine (L-NOARG; 100 microM) caused 40%, 20% and no reduction in Rmax for SP, BK and SNP respectively. EC50s for SP and BK were decreased significantly by approximately two fold whereas that for SNP was increased significantly by approximately ten fold. At all high K+ concentrations (30-125 mM), L-NOARG (100 microM) caused complete inhibition of relaxations to SP and BK but those to SNP were unaffected. 5. High K+ (30 mM) unmasked potent and concentration-dependent inhibition of relaxations of SP by L-NOARG. At 10 microM L-NOARG, all relaxation responses to SP were abolished and at the higher concentrations of SP (1-10 nM) small but significant contractions were observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in vascular reactivity induced by acute hyperthyroidism in isolated rat aortae

Hyperthyroidism was induced by subcutaneous injections of L-thyroxine (T(4)) (500 mg/kg/day) for 3 days in order to study whether adrenergic and muscarinic receptor-mediated vascular responses alter at an early stage of the disease. T(4) treatment was sufficient to induce a significant degree of thyroid weight loss, tachycardia, cardiac hypertrophy, and an elevation in serum T(4) levels. The tension of aortic ring preparations isolated from rats was measured isometrically to investigate the influence of acute hyperthyroidism. The contractions induced by norepinephrine (NE) were significantly suppressed in aortic rings from rats treated with T(4) compared with control rats. N(G)-nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide synthase (NOS), significantly enhanced NE-induced contraction in aortic rings from both control and T(4)-treated rats, and the enhancement was greater in rats treated with T(4) than control rats. The relaxations induced by either acetylcholine (ACh) or sodium nitroprusside (SNP) were also significantly enhanced by T(4) treatment. L-NOARG abolished the relaxation induced by ACh in aortic rings from both control and T(4)-treated rats. L-NOARG shifted SNP-induced relaxation curves of aortic rings from those of control rats to the left, but not with rats treated with T(4). T(4) treatment showed no influence on the amount of endothelial NOS (eNOS) protein. These results suggest that vascular responses alter at an early stage of hyperthyroidism and that it may be due to a modification in the NO system which is independent from the amount of eNOS protein.     

Antihypertensive potential and mechanism of action of astaxanthin: II. Vascular reactivity and hemorheology in spontaneously hypertensive rats

The current study was designed to determine the effects of a dietary astaxanthin (ASX-O) on vascular reactivity in spontaneously hypertensive rats (SHR), in order to verify its antihypertensive action mechanism. We evaluated contractions induced by phenylephrine (Phe), angiotensin II (Ang II) and the xanthine/xanthine oxidase (Xan/XOD) system, and relaxations induced by sodium nitroprusside (SNP) as well as endothelium-dependent relaxations mediated by acetylcholine (ACh) in thoracic aorta of the SHR, with and without ASX-O intervention. We also investigated the effects of ASX-O on blood rheology using a microchannel array system. In this study, ASX-O showed a significant modulatory effect on nitric oxide (NO)-induced vasorelaxation by the NO-donor SNP (p<0.05). However, it did not show significant effects in restoring the impaired endothelium-dependent relaxation to ACh in the SHR. On the other hand, the constrictive effects by Phe, Ang II and Xan/XOD were ameliorated by ASX-O (p<0.05). ASX-O also demonstrated significant hemorheological effect by decreasing the microchannel transit time of whole blood. In conclusion, the results suggest that ASX-O may act in modulating the blood fluidity in hypertension, and that the antihypertensive effects of ASX-O may be exerted through mechanisms including normalization of the sensitivity of the adrenoceptor sympathetic pathway, particularly [alpha]-adrenoceptors, and by restoration of the vascular tone through attenuation of the Ang II- and reactive oxygen species (ROS)-induced vasoconstriction.     

Effects of S-ethylisothiourea, a potent inhibitor of nitric oxide synthase, alone or in combination with a nitric oxide donor in splanchnic artery occlusion shock.

1. The aim of this study was to compare the effects of an intravenous infusion of a potent and non selective nitric oxide synthase inhibitor S-ethylisothiourea (Ethyl-TU) with that of a nitric oxide (NO) donor on the pathological sequelae associated with splanchnic artery occlusion (SAO) shock. In addition the effects of the combination of these two treatments were also investigated. 2. SAO shock was induced in anaesthetized rats by clamping splanchnic arteries for 45 min. Sham operated animals were used as controls. Survival time, white blood cell (WBC) count, mean arterial blood pressure, myeloperoxidase activity (MPO; studied as a quantitative means to evaluate neutrophil accumulation) and the responsiveness of aortic rings to acetylcholine (ACh, 10 nM-10 microM) and to phenylephrine (PE, 1 nM-10 microM) were studied. 3. SAO shocked rats had a decreased survival rate (0% survival 2 h after the release of occlusion) and survival time (76 +/- 10 min), increased MPO activity in the ileum (3.39 +/- 0.8 u x 10(-3) g-1 tissue), a marked leukopenia and a profound hypotension. In addition aortic rings from shocked rats showed a marked hyporeactivity to PE and reduced responsiveness to ACh. Endothelium denuded aortic rings had also a marked hyporeactivity to PE. 4. In vivo administration of Ethyl-TU (0.1 mg kg-1 h-1, beginning 1 min after the onset of reperfusion) significantly increased survival time and rate, improved mean arterial blood pressure, restored the responsiveness to PE, but did not change MPO activity, leukopenia or the impairment in the responsiveness of aortic rings to ACh. Addition of Ethyl-TU (2 microM) to endothelium denuded aortic rings in vitro, restored the marked hyporeactivity to PE. Administration of the NO donor C87-3754 (0.75 mg kg-1 h-1, beginning 1 min after the onset of reperfusion) slightly increased survival time and reduced MPO activity and leukopenia, but did not change survival rate and mean arterial blood pressure. In addition C87-3754 restored the responsiveness of aortic rings to ACh to control values, but did not modify the hyporeactivity to PE. The combination of these two interventions produced a higher degree of protection than either Ethyl-TU or C87-3754 alone. In fact, co-administration of Ethyl-TU plus C87-3754 completely prevented mortality, reduced MPO activity, attenuated leukopenia and the profound hypotension and restored the impaired responsiveness of aortic rings to PE and ACh. 5. Our study suggests that treatment with a nitric oxide synthase inhibitor combined with an NO donor may be a new therapeutic approach to the treatment of splanchnic artery occlusion shock.     

Alpha lipoic acid treatment improved endothelium-dependent relaxation in diabetic rat aorta

The aim of this study was to ascertain the effects of α-lipoic acid (ALA) treatment on relaxant responses of acetylcholine (ACh) and isoprenaline (ISO) in aortic rings precontracted with serotonin (5-HT, 10(-6) M) obtained from streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in the rats by 50 mg/kg streptozotocin (STZ) via an intraperitoneal injection. Rat body and aorta weights were measured. The isometric tension to ACh (10(-9)-3×10(-6) M) and ISO (10(-9)-10(-4) M) of 5-HT-precontracted diabetic and non-diabetic rat (control), diabetic-ALA-treated, and ALA-treated aortas, in organ baths were recorded. Six weeks after STZ treatment blood glucose was elevated compared to control rats. In aortic rings from diabetic rats ACh and ISO-induced relaxations were impaired whereas endothelium-independent relaxation to sodium nitroprusside (SNP) was unaffected. ALA (100 mg/kg/day) treatment for 5 weeks enhanced ACh and ISO-induced relaxation in diabetic aortas. This recovering effect was via NO because prevented by incubating the vessels with N(G)-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor). It may be assumed that ALA treatment in vivo, can protect against impaired vascular responsiveness in STZ-induced diabetic rats.     

Endothelial function in spontaneously hypertensive rats: influence of quinapril treatment.

1. Angiotensin converting enzyme (ACE) inhibition has been shown to restore the impaired endothelial function in hypertension, but the mediators underlying the promoted endothelium-dependent dilatation have not been fully characterized. Therefore, we investigated the effects of 10-week-long quinapril therapy (10 mg kg-1 day-1) on responses of mesenteric arterial rings in vitro from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. 2. Endothelium-dependent relaxations of noradrenaline (NA)-precontracted rings to acetylcholine (ACh) and adenosine 5'-diphosphate (ADP) were similar in WKY rats and quinapril-treated SHR and more pronounced than in untreated SHR. The nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) attenuated the relaxations in both WKY groups and quinapril-treated SHR, and completely inhibited them in untreated SHR. When endothelium-dependent hyperpolarization was prevented by precontraction of the preparations with potassium chloride (KCl), no differences were found in relaxations to ACh and ADP between the study groups. In addition, in NA-precontracted rings the L-NAME- and indomethacin-resistant relaxations to ACh were partially prevented by apamin, an inhibitor of calcium-activated potassium channels. 3. Interestingly, in quinapril-treated SHR but not in the other groups, exogenous bradykinin potentiated the relaxations to ACh in both NA- and KCl-precontracted arterial rings. 4. Contractile sensitivity of endothelium-intact rings to NA was reduced in SHR by quinapril, and was more effectively increased by L-NAME in quinapril-treated than untreated SHR.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic epigallocatechin-gallate improves aortic reactivity of diabetic rats: Underlying mechanisms

Diabetes mellitus is associated with major cardiovascular risk factors which are responsible for excess morbidity and mortality. Green tea catechins including epigallocatechin-3-gallate (EGCG) could exert beneficial health effects to ameliorate cardiovascular and metabolic diseases. Thus, the effect of chronic administration of EGCG was studied on aortic reactivity of streptozotocin (STZ)-diabetic rats. Male diabetic rats received EGCG 25 mg/kg/day for 8 weeks 1 week after diabetes induction. Contractile responses to KCl and phenylephrine (PE) and relaxation responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were obtained from aortic rings. Maximum contractile response of endothelium-intact rings to KCL and PE was significantly lower in EGCG-treated diabetic rats relative to untreated diabetic ones. Endothelium removal abolished the significant difference between EGCG-treated and untreated diabetic groups regarding contractile response to KCl and PE. Meanwhile, endothelium-dependent relaxation to ACh was significantly higher in EGCG-treated diabetic rats as compared to diabetic ones. Pretreatment of rings with N(omega)-L-arginine methyl ester (L-NAME) and indomethacin (INDO) significantly attenuated the observed responses. Meanwhile, two-month diabetes resulted in an elevation of malondialdehyde (MDA) and decreased superoxide dismutase (SOD) activity in aortic tissue and EGCG treatment attenuated the increased MDA content and reduced activity of SOD. Therefore, chronic treatment of diabetic rats with EGCG could prevent the abnormal functional changes in vascular reactivity in diabetic rats through nitric oxide- and prostaglandin-dependent pathways and via attenuation of aortic lipid peroxidation.     

Endothelium-dependent relaxations mediated by inducible B1 and constitutive B2 kinin receptors in the bovine isolated coronary artery.

1. Rings of bovine left anterior descending coronary artery (LAD) were contracted with the thromboxane A2-mimetic, U46619 (1-30 nM), to approximately 40% of their maximum contraction to 125 mM KCl Krebs solution (KPSSmax) for comparison of responses to the B1 and B2 kinin receptor agonists, des-Arg9-bradykinin (des-Arg9-BK) and bradykinin (BK), respectively. Relaxation responses were normalized as percentages of the initial U46619-induced contraction level, while contractile responses were expressed as percentages of KPSSmax. 2. After 6 h of in vitro incubation in Krebs solution at 37 degrees C, des-Arg9-BK (pEC50, 8.00 +/- 0.08; maximum response (Rmax), 93.9 +/- 1.9%) and BK (pEC50, 9.75 +/- 0.07; Rmax, 100.1 +/- 0.7%) caused endothelium-dependent relaxations in precontracted rings of bovine LAD which were competitively and selectively antagonized by the B1 receptor antagonist, des-Arg9-[Leu8]-BK (pA2, 6.27 +/- 0.11) and the B2 receptor antagonist Hoc-140 (pA2, 9.63 +/- 0.14), respectively. 3. At 3 h of in vitro incubation, the sensitivity (pEC50, 7.45 +/- 0.10) and Rmax (84.6 +/- 3.3%) to des-Arg9-BK were significantly less than those obtained in the same tissues at 6 h (pEC50, 7.94 +/- 0.06; Rmax, 91.4 +/- 2.5%), whereas endothelium-dependent relaxations to BK and ACh were unaffected by incubation time. 4. Relaxation responses to des-ARg9-BK, but not BK, at both 3 h and 6 h were significantly attenuated by the protein synthesis inhibitors, cycloheximide (30 and 100 microM) and actinomycin D (2 microM). 5. At 6 h, the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG, 100 microM), caused a significant 2 fold decrease in pEC50 (9.58 +/- 0.03) but had no effect on Rmax for BK. For des-Arg9-BK, L-NOARG (100 microM) caused a marked and significant decrease in both the pEC50 and Rmax and revealed contractions to low concentrations of des-Arg9-BK. In both cases, L-NOARG inhibition was reversed in the presence of L-arginine (10 mM). 6. At 6 h removal of the endothelium abolished relaxation responses to des-Arg9-BK and BK, and for des-Arg9-BK, but not BK, unmasked concentration-dependent contractions (pEC50, 7.57 +/- 0.09; Rmax, 83.4 +/- 9.1%). The sensitivity of contractions to des-Arg9-BK increased slightly from 3 h (pEC50, 7.37 +/- 0.08) to 6 h (pEC50, 7.62 +/- 0.12) of in vitro incubation; however, there was a small but significant depression in the maximum response over this time (Rmax, 126.8 +/- 8.5% and 103.3 +/- 8.6% for 3 h and 6 h of incubation respectively). 7. In conclusion, the bovine LAD contains inducible B1 and constitutive B2 endothelial cell kinin receptors, both of which mediate endothelium-dependent relaxation partly via the release of NO. B1 receptors were also present on the smooth muscle layer of the bovine LAD.     

Effect of celiprolol therapy on arterial dilatation in experimental hypertension.

1. It has recently been suggested that therapy with beta-adrenoceptor blockers reduces peripheral arterial resistance via enhanced vascular dilatation. Therefore, we studied the effects of celiprolol, which is a specific beta 1-antagonist that has a weak beta 2-agonist action, on arterial tone in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. 2. Two doses of celiprolol (5 and 50 mg kg-1 day-1) were administered to the SHR, while the WKY rats received only the higher dose of the drug. During the 12-week treatment period the higher dose attenuated the increase in blood pressure by approximately 20 mmHg in SHR, whereas the lower dose was without significant antihypertensive effect. Celiprolol therapy did not affect blood pressure in the normotensive WKY rats. 3. Responses of mesenteric arterial rings in vitro were examined at the end of the study. Interestingly, endothelium-mediated relaxations of noradrenaline (NA)-precontracted rings to acetylcholine (ACh) in the absence and presence of the cyclo-oxygenase inhibitor, diclofenac, were equally enhanced in both celiprolol-treated SHR groups. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) practically abolished the relaxations to ACh in all SHR irrespective of whether they had received celiprolol, whereas in WKY rats L-NAME only attenuated the responses to ACh. However, no differences were found between the SHR groups in relaxations to ACh when hyperpolarization of smooth muscle was prevented by precontractions induced by 50 mM KCl. Vasorelaxation of NA-precontracted rings to the exogenous nitric oxide donor, nitroprusside, was also moderately augmented in both celiprolol-treated SHR groups, while the relaxation to beta-adrenoceptor agonist, isoprenaline, remained equally impaired in all SHR whether or not they had received celiprolol. No differences were observed between the two WKY groups in the responses to ACh, nitroprusside or isoprenaline. 4. Contractile sensitivity of mesenteric arterial rings to the receptor-mediated agonists, NA and 5-hydroxytryptamine, was comparable in all study groups. 5. In conclusion, SHR treatment with either the low or the higher dose of celiprolol was accompanied by enhancement of both endothelium-dependent and endothelium-independent nitric oxide-mediated arterial relaxation, possibly via a hyperpolarization mechanism. Interestingly, this effect appeared to be independent of the reduction in blood pressure.     

Impairment of pulmonary-artery endothelium-dependent relaxation in chronic obstructive lung disease is not due to dysfunction of endothelial cell membrane receptors nor to L-arginine deficiency.

1. Endothelium-dependent relaxation mediated by endothelium-derived relaxing factor (EDRF) or nitric oxide (NO), is impaired in pulmonary arteries (PA) of hypoxic patients with chronic obstructive lung disease (COLD). To determine the mechanisms responsible for this impairment, we compared the response of rings of isolated PA from 12 COLD patients and 8 controls to the endothelium-dependent vasodilators acetylcholine (ACh), adenosine diphosphate (ADP), and the calcium ionophore, A23187. The response of PA rings to the endothelium-independent nitro-vasodilator sodium nitroprusside (SNP) was also studied in both groups. The PA rings had been pre-contracted by the alpha-adrenoceptor agonist phenylephrine (PE). 2. Endothelium-dependent relaxation was significantly reduced in PA rings from COLD patients as compared with controls when tested with ACh (37.8 +/- 8.8% vs 73.4 +/- 7.9%), ADP (38.4 +/- 6.7% vs 80 +/- 5.6%), and the calcium ionophore, A23187 (35.8 +/- 6.1% vs 87 +/- 6.6%). Relaxation with SNP was, however, significantly greater in PA rings from COLD patients (99.4 +/- 0.6% vs 90.3 +/- 3.1%), as was the contractile response to PE (1.91 +/- 0.21 g vs 1.33 +/- 0.15 g). Pretreatment with the specific inhibitor of NO formation, NG-monomethyl-L-arginine (L-NMMA; 10(-4) M) significantly reduced the relaxation to ACh in all PA rings. This inhibition could be reversed by L-arginine (10(-3) M), the substrate for NO synthesis. Pretreatment with L-arginine alone, however, did not restore the impaired endothelium-dependent relaxation of PA rings from COLD patients.(ABSTRACT TRUNCATED AT 250 WORDS)     

Citrus flavonoid naringenin improves aortic reactivity in streptozotocin-diabetic rats

Background and Objective:

Cardiovascular disorders continue to constitute major causes of morbidity and mortality in diabetic patients. In this study, the effect of chronic administration of naringenin was investigated on aortic reactivity of streptozotocin (STZ)-induced diabetic rats.

Materials and Methods:

Male diabetic rats (n=32) were divided into control, naringenin-treated control, diabetic, and naringenin-treated diabetic groups of eight animals each. The latter group received naringenin for 5 weeks at a dose of 10 mg/kg/day after diabetes induction. The contractile responses to potassium chloride (KCl) and phenylephrine (PE) and relaxation response to acetylcholine (ACh) were obtained from aortic rings. Meanwhile, participation of nitric oxide (NO) and endothelial vasodilator factors in response to ACh were evaluated using N (G)-nitro-l-arginine methyl ester (L-NAME) and indomethacin (INDO), respectively.

Results:

Maximum contractile response of endothelium-intact rings to KCl and PE was significantly (P<0.05) lower in naringenin-treated diabetic rats as compared to untreated diabetics. Endothelium-dependent relaxation to ACh was significantly (P<0.05-0.01) higher in naringenin-treated diabetic rats as compared to diabetic ones and pretreatment of rings with nitric oxide synthase inhibitor N (G)-nitro-l-arginine methyl ester (L-NAME) significantly (P<0.001) attenuated the observed response.

Conclusion:

Chronic treatment of diabetic rats with naringenin could prevent some abnormal changes in vascular reactivity in diabetic rats through nitric oxide and endothelium integrity is necessary for this beneficial effect.KEY WORDS: Aorta, diabetes mellitus, naringenin, streptozotocin