The potentiation of nitroglycerin-induced relaxation by PKG inhibition in rat aortic rings

• 1.1. In rat aortic rings precontracted by phenylephrine, H7 (10⁻⁵M) and staurosporine (10⁻⁷M), which inhibit PKA, PKG and PKC, and H-89 (10⁻⁶M), which inhibits PKA and PKG, potentiated relaxations induced by nitroglycerin. Forskolin-induced relaxations were not affected by H7 (10⁻⁵M).
• 2.2. Nitroglycerin-induced relaxations were not affected by calphostin-C (10⁻⁷M), which inhibits PKC, H-89 (10⁻⁷M), which inhibits PKA, and staurosporine (2 × 10⁻⁹M), which inhibits PKC.
• 3.3. Iberiotoxin (3 × 10⁻⁸M), an inhibitor of large conductance Kca channels, partly inhibited the relaxation induced by nitroglycerin and completely inhibited the potentiating effect of H7 on nitroglyc. erin-induced relaxations.
• 4.4. The potentiating effect of zaprinast (10⁻⁵M), an inhibitor of cGMP-phosphodiesterase, on nitroglycerin-induced relaxation was not affected by iberiotoxin. In the presence of methylene blue (10⁻⁵M), an inhibitor of guanylate cyclase, the residual relaxing response to nitroglycerin was not affected by H7, but it was inhibited by iberiotoxin.
• 5.5. These results suggest that the potentiation of nitroglycerin-induced relaxation by H7, staurosporine and H-89 may be due to inhibition of PKG.

     

The potentiating effect of genistein on the relaxation induced by isoproterenol in rat aortic rings.

In rat aortic rings, the mechanism of potentiating effect of genistein, a tyrosine kinase inhibitor, on the relaxation induced by isoproterenol was examined. Pretreatment of the aortic rings by genistein, but not by daidzein, an inactive analogue of genistein, potentiated the relaxation induced by isoproterenol. Genistein also potentiated the relaxation induced by forskolin, an activator of guanylyl cyclase, and dibutyryl cyclic AMP. In addition, theophylline, an inhibitor of phosphodiesterase, potentiated the relaxation induced by isoproterenol and forskolin. Theophylline partly inhibited the potentiation of isoproterenol-induced relaxation by genistein while it completely inhibited the potentiation of forskolin-induced relaxation by genistein. Iberiotoxin, an inhibitor of Ca-activated K (KCa) channels, partly inhibited the isoproterenol-induced relaxation and the potentiating effect of genistein on the relaxation induced by isoproterenol. Quinacrine (an inhibitor of phospholipase A2), alpha-naphthoflavone (an inhibitor of cytochrome P-450 enzymes), and 8-methoxypsoralen (an inhibitor of cytochrome P-450 enzymes), partly inhibited the potentiating effect of genistein on the isoproterenol-induced relaxation, but metyrapone (an inhibitor of cytochrome P-450 enzymes), indomethacin (an inhibitor of cyclooxygenase), and AA861 (an inhibitor of 5-lipoxygenase) did not. These results suggest that the potentiation of isoproterenol-induced relaxation by genistein may be related to the activities of phosphodiesterase, KCa channels, and cytochrome P-450 enzymes.     

Endothelium-dependent and -independent relaxation induced by pinocembrin in rat aortic rings

The aim of present study was to evaluate the vasorelaxant effects of the flavonone pinocembrin and its possible mechanisms in isolated rat aortic rings. Pinocembrin (5 approximately 100 microM) induced relaxation in aortic rings pre-contracted with norepinephrine (NE, 1 microM) or KCl (60 mM), with pEC(50) value 4.37+/-0.02 and 4.52+/-0.04. Pretreatment with pinocembrin (30 or 50 microM) also inhibited contractile responses to NE and KCl. The vasorelaxant effect of pinocembrin relied on intact endothelium partially, and incubation with n(omega)-nitro-l-arginine methyl ester (l-NAME, 100 microM) or methylene blue (10 microM) significantly inhibited the effect, however indomethacin (5 microM) had no influence on the action. In endothelium-denuded rings, the vasorelaxant effect of pinocembrin was reduced by glibenclamide (10 microM), tetraethylammonium (5 mM) and 4-aminopyridine (100 microM). Pinocembrin also reduced NE-induced transient contraction in Ca(2+)-free solution and inhibited contraction induced by increasing external calcium in Ca(2+)-free medium plus 60 mM KCl. Our results suggest that pinocembrin induces relaxation in rat aortic rings through an endothelium-dependent pathway, involving NO-cGMP, and also through an endothelium-independent pathway, opening K(+) channels and blockade of Ca(2+) channels.     

L-citrulline mediated relaxation in the control and lipopolysaccharide-treated rat aortic rings.

The present study was undertaken to investigate relaxant effect of L-citrulline in phenylephrine precontracted endothelium intact thoracic aortic rings obtained from control or lipopolysaccharide (1 mg/kg)-treated rats. L-citrulline produced 40+/-3% (n=36) and 60+/-5% (n=24) relaxations in control and lipopolysaccharide-treated rings, respectively. Nitric oxide (NO) release and cyclic guanosine-3',5'-monophosphate levels from the rings were also increased following treatment with L-citrulline. Inhibition of guanylate cyclase, L-citrulline recycling to L-arginine or denudation of the endothelium, significantly reduced L-citrulline-induced relaxations both in control and lipopolysaccharide-treated rings. Treatment of rings with protein synthesis inhibitors prevented relaxations to L-citrulline. Inhibitor of Ca2+-activated K+ channels, tetrabutylammonium or precontraction of the rings with KCl (80 mM), significantly attenuated L-citrulline mediated relaxations in control and lipopolysaccharide-treated rings. Thus, L-citrulline seems to exert significant relaxation by supplementing the release of NO due to its recycling to L-arginine, which gets further augmented after lipopolysaccharide treatment.     

Ambient particulate matter induces relaxation of rat aortic rings in vitro

Epidemiological studies have shown an association between ambient levels of particulate matter (PM) and increased mortality from cardiovascular diseases. However, the underlying mechanisms are still not clear. We hypothesised that PM, when translocated after inhalation, could affect vascular smooth muscle function. Therefore, total suspended particulate matter (TSP) was sampled and investigated for its ability to affect aortic muscle contraction. Both TSP and TSP supernatant (TSP-sup) induced a concentration-dependent relaxation of phenylephrine (PE)-precontracted aortic rings. Relaxation induced by 100 microg/ml TSP was 51.5 +/- 3.1% of total contraction. At 60 and 100 microg/ml, relaxation induced by TSP was significantly higher compared to TSP-sup. Ultrafine TiO2, used as a model to investigate the role of ultrafine particles, did not show an effect. Soluble iron, present in TSP suspensions, seems not to be involved, as chelating with deferoxamine did not affect TSP-induced relaxation. However, TSP effects were inhibited by Trolox, suggesting a role of oxidants. Nudation of aortic rings showed that effects of TSP were only partly endothelium-dependent, while preincubation with L-NAME increased TSP-induced relaxation. From these data, we conclude that both the particle core and soluble components of TSP can affect the smooth muscle function, leading to changes in the vascular contractile response.     

Phorbol esters impair endothelium-dependent and independent relaxation in rat aortic rings

• 1.1. This study examined the ability of various nitro-vasodilators, 8-bromo cyclic guanosine 3′:5′ monophosphate (8-BrcGMP) and forskolin to relax rings of rat thoracic aorta pre-contracted with either noradrenaline (0.1 μM) or the protein kinase C activators, phorbol 12,13-dibutyrate (PDB, 0.1 μM) or phorbol 12-myristate 13-acetate (PMA, 0.5 μM).
• 2.2. In noradrenaline pre-contracted rings, acetylcholine (10 nM−10 μM), sodium nitroprusside (1 nM−0.5 μM), the calcium ionophore A23187 (10 nM−10 μM) and 8-BrcGMP (10 mM) totally reversed the smooth muscle contraction. In PDB-contracted aortic rings acetylcholine, sodium nitroprusside and 8-BrcGMP-induced relaxation was reduced compared to that in noradrenaline-contracted aortic rings, but A23187 and forskolin-induced relaxations were unaffected. Both acetylcholine and A23187-induced relaxations in PDB-contracted rings were abolished in the presence of the nitric oxide synthesis inhibitor Nω-nitro-l-arginine (NOLA, 100 μM).
• 3.3. Acetylcholine and sodium nitroprusside were even less potent in their ability to relax PMA-contracted aortic rings compared with noradrenaline and PDB-contracted rings. A23187-induced relaxation was also inhibited in PMA-contracted rings.
• 4.4. These results show that protein kinase C activation reduces the ability of agents which liberate nitric oxide to induce smooth muscle relaxation, and also inhibits the biochemical pathways which are subsequently activated by nitric oxide and lead to vascular smooth muscle relaxation.

     

Peroxynitrite-induced relaxation in isolated rat aortic rings and mechanisms of action

The present study was designed to evaluate the effects of peroxynitrite (ONOO-), the product of superoxide and nitric oxide, on isolated segments of rat aorta. In the absence of any vasoactive agent, ONOO- (from 10(-8) to 10(-4) M) failed to alter the basal tension. In phenylephrine (PE; 5 x 10(-7) M)-precontracted rat aortic rings (RAR), ONOO- elicited concentration-dependent relaxation at concentrations of from 10(-8) to 10(-4) M. The effective concentrations producing approximately 50% of maximal relaxation (ED50) to ONOO- were 1.84 x 10(-5) M and 1.96 x 10(-5) M in intact and denuded RAR, respectively (P > 0.05). No significant differences in the relaxation responses were found between RAR with or without endothelium (P > 0.05). The presence of either 5 microM methylene blue (MB) or 5 microM 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one (ODQ) significantly inhibited the relaxations induced by ONOO-. Sildenafil (10(-7) M), on the other hand, significantly potentiated the ONOO--induced relaxations. Tetraethylammonium chloride (T-2265) significantly decreased the ONOO--induced relaxations in a concentration-dependent manner. However, ONOO- had no effect on RAR precontracted by high KCL (40 mM, n = 6, P > 0.05). Addition of calyculin A also significantly decreased the ONOO--induced relaxation in a dose-dependent manner. Furthermore, ONOO- significantly inhibited calcium-induced contractions of K+-depolarized aortic rings in a concentration-related manner. Lastly, a variety of other pharmacological agents and antagonists including L-NMMA, L-arginine, indomethacin, atropine, naloxone, diphenhydramine, cimetine, glibenclamide, haloperidol, superoxide dismutase (SOD), and catalase did not influence the relaxant effects of ONOO- on RAR. Our new results suggest that ONOO--triggered relaxation on rat aortic rings is mediated by elevation of cGMP levels, membrane hyperpolarization via K+-channel activation, activation of myosin phosphatase activity, and interference with calcium movement and cellular membrane Ca2+ entry.     

黄芪甲苷对正常大鼠离体血管功能的影响

目的观察黄芪甲苷对血管功能的影响并探讨其作用机制。方法采用大鼠离体主动脉环灌流模型,观察黄芪甲苷对血管环收缩和舒张功能的影响。结果黄芪甲苷能够浓度依赖性舒张血管。一氧化氮合酶抑制剂、鸟苷酸环化酶及环加氧酶抑制剂可抑制黄芪甲苷诱导的血管舒张作用。黄芪甲苷能够抑制苯肾上腺、KC l和CaC l2引起的血管收缩。结论黄芪甲苷具有内皮依赖性的舒张血管作用,此作用主要通过NO-cGMP途径发挥作用。黄芪甲苷抑制血管收缩主要通过拮抗外钙内流实现。     

Endophytic fungal metabolite fumigaclavine C causes relaxation of isolated rat aortic rings

Two indole alkaloids were isolated from the culture of Aspergillus fumigatus (strain No. CY018), an endophytic fungus harboring inside the elder leaf of Cynodon dactylon. These two chemicals were identified as fumigaclavine C and fumitremorgin C. In screening the bioactivity of these two indole alkaloids, their vasorelaxant effects on isolated rat thoracic aortic rings were observed. The results showed that fumigaclavine C exhibited potent concentration-dependent vasorelaxant actions in isolated rat aortic rings pre-contracted by high K+ or phenylephrine (with EC50 values of 5.62 micromol/L and 1.58 micromol/L, respectively) whereas fumitremorgin C displayed a weaker vasorelaxation. A detailed investigation was therefore performed with fumigaclavine C. The vasorelaxing action of fumigaclavine C is independent of the presence of endothelium, suggesting its effect of vasorelaxation was not related to endothelial mediators. Blockage of L-type voltage-dependent calcium channels, activation of ATP-sensitive potassium channels and inhibition of Ca2+ release from intracellular Ca2+ stores may be involved in fumigaclavine C induced relaxation of rat isolated aortic rings. These results demonstrate that fumigaclavine C from the endophytic fungus has a potential capacity in vascular protection and thus may have therapeutic use in protection against cardiovascular disease.     

Captopril restores endothelium-dependent relaxation of rat aortic rings after exposure to homocysteine

This study was designed to investigate the effects of captopril, an angiotensin-converting enzyme inhibitor, on inhibition of endothelium-dependent relaxation induced by homocysteine in isolated rat aorta. Isometric tension recordings were used to assess inhibitory effects of homocysteine and protective effects of captopril on endothelium-dependent relaxation of aortic rings. Exposure of aortic rings to homocysteine (0.3 approximately 3 mmol/L) for 30 min induced a significant concentration-dependent inhibition of endothelium-dependent relaxation response to acetylcholine (ACh), but did not affect endothelium-independent relaxation response to sodium nitroprusside. Pre-incubation of aortic rings with captopril (3 approximately 30 micromol/L) for 15 min and co-incubation of aortic rings with homocysteine (1 mmol/L) for another 30 min attenuated the inhibition of homocysteine in a dose-dependent manner. Moreover, superoxide dismutase (SOD, 200 U/mL), a scavenger of superoxide anions, reduced homocysteine-induced inhibition. L-Arginine (3 mmol/L), a precursor of nitric oxide (NO), also attenuated the impairment of vasorelaxation induced by homocysteine. However, in the combined presence of SOD and L-arginine, the inhibitory effect of homocysteine was reversed, which was very similar to the effect of 30 micromol/L captopril. These results suggest that captopril can prevent the inhibition of endothelium-dependent relaxation induced by homocysteine in isolated rat aorta, which may be related to scavenging oxygen free radicals and enhancing NO production.     

Histamine relaxation of aortic rings from diabetic rats

The effect of histamine-induced relaxation on thoracic aortic rings from rats 5, 12, 24 and 52 weeks following streptozotocin-induced diabetes was determined. Preliminary studies confirmed the dependence of histamine-induced relaxation and the independence of nitroglycerin-relaxation (GTN) on the presence of endothelium (EDRF). Diabetes was confirmed by blood glucose levels exceeding 300 mg/dl. Rings with endothelium were depolarized several times with 50 mM KCL and then contracted with phenylephrine (10(-6)). Dose-response curves were plotted from data obtained following exposure to histamine (10(-7)-10(-3)) and GTN (10(-9)-10(-7)) and compared to responses from age-matched untreated controls, diabetic and diabetic rats treated with insulin (2 U/day). The relaxation produced by histamine on phenylephrine pre-contracted rings was similar in all three groups from 5-week age matched rats. However, histamine-induced relaxation from untreated diabetic rats was significantly depressed at 12, 24 and 52 weeks (p less than 0.001). Conversely there was no difference in the relaxation elicited by GTN on rings obtained from the three groups at any age. Pretreatment with diphenhydramine (5 x 10(-7)) on aortic rings from 12 and 52-week age matched rats resulted in qualitatively similar histamine dose-response curves that were displaced about two orders of magnitude to the right, indicative of H1 receptor competitive antagonism. These results demonstrate that the duration of diabetes alters the responsiveness of rat thoracic aortic rings to histamine but not to GTN and suggests that the responses elicited by certain agonists on target tissues may be significantly altered depending on the duration of diabetes.     

Genistein potentiates the relaxation induced by beta1- and beta2-adrenoceptor activation in rat aortic rings

In rat aortic rings, genistein, an inhibitor of tyrosine kinase, but not daidzein, an inactive analogue of genistein, potentiated the relaxation induced by isoproterenol. Atenolol, a beta1-adrenoceptor antagonist, or ICI-118,551, a beta2-adrenoceptor antagonist, inhibited the relaxation induced by isoproterenol. The potentiating effect of genistein on the relaxation induced by isoproterenol in the presence of ICI-118,551 was apparently greater than that in the presence of atenolol. In the presence of ICI-118,551, theophylline, an inhibitor of cyclic adenosine monophosphate (cAMP)-dependent phosphodiesterase (cAMP-PDE), markedly inhibited the potentiating effect of genistein on the isoproterenol-induced relaxation, whereas in the presence of atenolol, theophylline only partly inhibited the potentiating effect of genistein. The relaxation induced by forskolin, an activator of adenylyl cyclase, was potentiated by genistein or theophylline. In the presence of theophylline, the relaxation induced by forskolin was not further affected by genistein. Genistein also inhibited the activities of cAMP-PDE. In the presence of atenolol, but not ICI-118,551, iberiotoxin, an inhibitor of Ca-activated K channels, inhibited the relaxation induced by isoproterenol and the potentiating effect of genistein. In the presence of atenolol, quinacrine, an inhibitor of phospholipase A2, and metyrapone, an inhibitor of P-450 enzymes, but not alpha-naphthoflavone, an inhibitor of P-450 enzymes, indomethacin, a cyclooxygenase inhibitor, or AA861, a 5-lipoxygenase inhibitor, inhibited the potentiating effect of genistein. These results suggest that the potentiation of the beta1-adrenoceptor-induced relaxation by activation of genistein may mostly be due to inhibition of cAMP-PDE activities. In addition, the potentiation of the relaxation induced by activation of beta2-adrenoceptors by genistein may be related to the inhibition of arachidonic acid metabolism and cAMP-PDE activities.     

Actions of amiloride analogues on prostacyclin synthesis by rat aortic rings.

1 Fresh rat aortic rings were incubated in HEPES-buffered salt solutions (pH 8.0) in the presence or absence of amiloride analogues. The effect of these drugs on prostacyclin (PGI2) synthesis was determined by radioimmunoassay of the stable hydrolysis product 6-oxo-prostaglandin (PG)F1 alpha. 2 Amiloride and phenamil (potent inhibitors of epithelial Na+ transport) had no significant effect on basal or Ca2+-stimulated PGI2 synthesis. 3 Several analogues previously reported to inhibit Na+/Ca2+ exchange caused a dose-related increase in 6-oxo-PGF1 alpha production in media containing NaCl 120 mM and CaCl2 2.5 mM. 2',3'-Benzobenzamil was the most potent analogue with a maximum stimulation of 4.51 +/- 0.89 fold, and an EC50 of 3 x 10(-5) M. 4 Amiloride analogues bearing substituents on the 5-amino group of the pyrazine ring have been reported to inhibit Na+/H+ exchange more potently than Na+/Ca2+ exchange. Three of these compounds inhibited Ca2+-stimulated 6-oxo-PGF1 alpha production at concentrations that did not significantly influence basal 6-oxo-PGF1 alpha production.     

Endothelium-independent relaxation of aortic rings by the nitric oxide synthase inhibitor diphenyleneiodonium

1. The flavoprotein binder diphenyleneiodonium (DPI) is a potent, irreversible inhibitor of nitric oxide synthase (NOS), but produces only a transient pressor response following systemic administration to animals, despite evidence of persistent NOS inhibition. To characterize further the effects of DPI on vascular tone, isometric tension was recorded from rat isolated aortic rings mounted between steel wires in an organ bath.
2. The NOS inhibitor N^G-nitro-L-arginine methyl ester (L-NAME, 1 mM) initiated an additional contraction of prostaglandin F_2α-preconstricted rings with endothelium which was sustained throughout the period of L-NAME exposure (+234±39% at 15 min). In contrast, addition of DPI (5 μM) to rings with endothelium produced a transient initial contraction (+111±27% at 2 min) followed by a more sustained relaxation (−27±19% at 15 min, P<0.001 vs L-NAME).
3. The contraction to DPI was also observed in rings without endothelium, was abolished by L-NAME pretreatment, and was unaffected by the α-adrenoreceptor inhibitor prazosin. Relaxation in response to DPI was not inhibited by endothelium removal or by pretreatment with either L-NAME or with the ATP-sensitive potassium channel blocker glibenclamide.
4. The endothelium-independent relaxation to DPI was inhibited at 23°C and its time course was delayed by pretreatment with the guanylate cyclase inhibitor methylene blue.
5. Thus, in addition to a transient initial contraction due to NOS inhibition, DPI produces an endothelium-independent, temperature-dependent relaxation which appears in part due to activation of guanylate cyclase. This relaxant effect of DPI may explain the transient nature of its pressor effect in vivo despite sustained NOS inhibition.

     

Kallikrein rK10-induced kinin-independent, direct activation of NO-formation and relaxation of rat isolated aortic rings.

1. rK10, a weak T-kininogenase isolated from the rat submandibular gland, is a protein belonging to the rat kallikrein family. In the present work, we have studied the biological effects of rK10 with respect to its ability to alter vascular resistance, either directly like rK9, i.e. another kallikrein-like protein, trypsin and thrombin, or through the release of kinins like tissue kallikrein (rK1). The direct effect was studied by its vasomotor activity on rat isolated aortic rings since this preparation was insensitive to the action of kinins. Its ability to induce altered vascular resistance through kinin-generation was investigated by blood pressure studies in whole animals. The studies were performed in comparison to rK1. 2. Unlike rK1, which induces hypotension when administered intravenously to rats (delta BP = -56 +/- 5 mmHg, 5 micrograms kg-1), rK10 did not have any effect on systemic blood pressure (delta BP = -3 +/- 1, 5 micrograms kg-1, i.v.). 3. rK10 was without effect on uncontracted aortic rings, but showed a concentration-dependent (10(-8)-10(-6) M) relaxant effect on tissue precontracted with phenylephrine (10(-6) M). After removal of endothelial cells, no relaxation was observed. The relaxant response to rK10 was transient. rK1 (with and without endothelium), bradykinin and T-kinin (with endothelium) had no effect on contracted or uncontracted aortic rings. 4. The relaxant effect of rK10 was dependent on its enzymatic activity since preincubation with aprotinin (1.02 mM) significantly reduced vasorelaxation from 74 +/- 4% to 24 +/- 3%.(ABSTRACT TRUNCATED AT 250 WORDS)     

吗啉环和哌嗪环类新衍生物对血管舒张功能的影响

目的　利用ETA与M受体的相似性和差异性 ,设计合成吗啉环和哌嗪环类新结构化合物 ,分析其对血管舒张功能的影响。方法　采用离体大鼠胸主动脉环 ,观察化合物对血管张力的影响 ;并观察L NAME ,吲哚美辛和阿托品对血管环最大舒张率的影响。结果　 81个化合物中有 57个能舒张血管 ,其中 8个最大舒张率在 50 %～ 85 % ;活性化合物按母核可分为 8类 ;DMHPPP和PPVP诱发的内皮依赖性舒血管反应可被L NAME和吲哚美辛所拮抗 ,但不被阿托品拮抗 ;DMHPPP和PPVP还能显著增强乙酰胆碱 (ACh)诱发的内皮依赖性舒血管反应的最大舒张率。结论　具有舒血管作用的活性化合物通过促进内皮细胞释放一氧化氮(NO)和前列环素 (PGI2 )共同实现舒张效应 ;并可调节血管内皮细胞乙酰胆碱靶标的功能     

Inhibition of nitric oxide/cyclic GMP-mediated relaxation by purified flavonoids, baicalin and baicalein, in rat aortic rings

The dried roots of Scutellaria baicalensis Georgi (Huangqin) are widely used in traditional Chinese medicine. We purified two flavonoids, baicalin and baicalein from S. baicalensis Georgi and examined their effects on isolated rat aortic rings. Baicalin (3-50 microM) inhibited endothelium/nitric oxide (NO)-dependent relaxation induced by acetylcholine (Ach) or cyclopiazonic acid (CPA). Baicalein at 50 microM abolished Ach-induced relaxation and markedly reduced CPA-induced relaxation. Treatment with 1mM L-arginine partially but significantly reversed the effects of baicalin (50 microM) or baicalein (50 microM) on Ach-induced relaxation. In endothelium-denuded rings, treatment with baicalin, baicalein or methylene blue partially inhibited relaxations induced by the NO donors, sodium nitroprusside (SNP) and hydroxylamine. Both flavonoids markedly reduced the increase in cyclic GMP levels stimulated by Ach in endothelium-intact rings and by SNP in endothelium-denuded rings. In contrast, exposure of endothelium-denuded rings to baicalin or baicalein did not affect relaxations induced by pinacidil or NS 1619, putative K+ channel activators. Neither flavonoids affected agonist-induced increase in the endothelial [Ca2+]i. Our results indicate that baicalin and baicalein attenuated NO-mediated aortic relaxation and cyclic GMP increases, likely through inhibition of NO-dependent guanylate cyclase activity.     

Time-dependent reduction of acetylcholine-induced relaxation in aortic rings of cholestatic rats

Changes in vascular responsiveness are the basis for some of the cardiovascular complications in cholestasis. Since the duration of cholestasis is important in determining the degree of the complications, we investigated the time-course dependent evolution of vascular relaxation responsiveness in the aortic rings of cholestatic rats. Acetylcholine-induced endothelium-dependent relaxation was investigated in the isolated aortic rings of unoperated, sham-operated and two-, five-, seven- and fourteen-day bile-duct ligated rats. There was a significant reduction in acetylcholine-induced relaxation of the aortic rings by the second day after the bile-duct ligation operation, compared to those of unoperated and sham-operated groups, but more reduction still occurs in 5- and 7-day bile-duct ligated groups, reaching a plateau by the seventh day. The relaxation response to sodium nitroprusside in the aortic rings of the unoperated and the 7-day bile-duct ligated rats did not differ, implying the intact smooth muscle component of the relaxation pathway. L-NAME ( N(omega)-nitro-L-arginine methyl ester), a nitric oxide (NO) synthase inhibitor, attenuated the acetylcholine-induced relaxation in both groups (unoperated and bile-duct ligated), while L-arginine prevents this inhibitory effect. Indomethacin potentiated the acetylcholine-induced relaxation in the aortic rings of the bile-duct ligated rats while it has no effect on unoperated controls, providing evidence for the possible role of vasoconstrictor prostanoids in cholestasis-induced reduction in acetylcholine-induced relaxation. These results state that the reduced acetylcholine-induced relaxation in the cholestatic aortic rings during the first week, when no portal hypertension was reported to be present, may be due to the decreased acetylcholine-induced NO release from endothelium or increased NO inactivation.     

The ginsenoside Rg3 evokes endothelium-independent relaxation in rat aortic rings: role of K+ channels

The purpose of the present study was to characterize the mechanism underlying the direct relaxing activity of ginsenosides on vascular smooth muscle. The total ginsenoside mixture, ginsenosides from either the protopanaxadiol group or the protopanaxatriol group, and the ginsenoside Rg3 from the protopanaxatriol group caused a concentration-dependent relaxation of rat aortic rings without endothelium contracted with 25 x 10(-3) M KCl but affected only minimally those contracted with 60 x 10(-3) M KCl. Ginsenoside Rg3 was the most potent relaxing agonist. Relaxations elicited by ginsenoside Rg3 were markedly reduced by tetraethylammonium, a blocker of non-selective K+ channels, but not by glibenclamide, a blocker of ATP-sensitive K+ channels. Ginsenoside Rg3 significantly inhibited Ca2+-induced concentration-contraction curves and the 45Ca2+ influx in aortic rings incubated with 25 x 10(-3) M KCl whereas these responses were not affected in rings incubated with 60 x 10(-3) M KCl. Ginsenoside Rg3 caused a time- and concentration-dependent efflux of 86Rb from aortic rings that was inhibited by tetraethylammonium but not by glibenclamide. These findings indicate that ginsenoside Rg3 is a potent inhibitor of vascular smooth muscle tone and that this effect seems to be due to an inhibition of Ca2+ influx and stimulation of K+ efflux, possibly via activation of tetraethylammonium-sensitive K+ channels.     

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.