Trimebutine maleate relaxes the isolated rat thoracic aorta: The role of nitric oxide and L‐type calcium channels

Trimebutine maleate (TMB), a widely prescribed drug for functional gastrointestinal disorders, has been reported to regulate smooth muscle contractility by modulating multiple ion channel activities in the gastrointestinal tract. However, its action on isolated aorta has not yet been reported. The aim of the present study was to evaluate in vitro vasorelaxant properties and the underlying pharmacological mechanisms of TMB in isolated rat thoracic aortic rings. Vascular activity experiments were performed on thoracic aorta isolated from Sprague‐Dawley rats in vitro, including endothelium‐intact and endothelium‐denuded aortic rings. TMB (10^?10‐10^?5 mol/L) induced relaxation in endothelium‐intact aortic rings precontracted by phenylephrine with a potency similar to that of carbachol. TMB‐induced relaxation was not altered by glibenclamide and atropine in endothelium‐intact aortic rings. However, L‐NAME and endothelium denudation significantly reduced but not completely reversed the vasorelaxant effect of TMB. Also, TMB‐induced relaxation wasn't affected by diclofenac in endothelium‐intact aortic rings. TMB at 10^?5 mol/L significantly reduced the CaCl₂‐induced contractions in endothelium‐intact aortic rings stimulated with KCl, but not stimulated with phenylephrine under Ca²⁺free conditions. Moreover, TMB at 10^?5 mol/L effectively attenuated Bay‐K8644‐induced contractions in aortic rings. These results suggest that TMB‐induced relaxation was mediated by both endothelium‐dependent and endothelium‐independent manner in isolated rat thoracic aorta. The mechanism of TMB‐induced relaxation at low concentrations is partially related to NO‐ and endothelium‐dependent but unrelated to prostanoids formation. However, inhibition of Ca²⁺ influx through voltage‐operated calcium channels and L‐type Ca²⁺channel blocking effect appears to be involved in the mechanism of vasorelaxant effect of TMB at high concentrations.     

Vasorelaxant effect of Valeriana edulis ssp. procera (Valerianaceae) and its mode of action as calcium channel blocker

Objectives The aim was to evaluate the relaxant effect of extracts from Valeriana edulis and determine the possible mechanism of action of the hexanic extract as vasorelaxant agent. Methods Extracts from rhizomes obtained by maceration (hexanic (HEVe), dichloromethanic (DEVe), methanolic (MEVe) and hydroalcoholic (HAEVe) (3.03–500 µg/ml)) were evaluated on aortic rat rings with and without endothelium. Key findings Extracts induced a significant concentration‐dependent and endothelium‐independent relaxation on isolated rat aorta pre‐contracted with noradrenaline (0.1 µm ). HEVe, the most potent extract (0.15–50 µg/ml), induced relaxation in aortic rings pre‐contracted with KCl (80 mm ), with an IC50 value of 34.61 ± 1.41 µg/ml and E_max value of 85.0 ± 4.38%. Pretreatment with HEVe (30 µg/ml) also inhibited contractile responses to noradrenaline and CaCl₂. HEVe (9.98 ± 2.0 µg/ml) reduced noradrenaline‐induced transient contraction in Ca²⁺‐free solution, and inhibited contraction induced by KCl (80 mm ). In endothelium‐denuded rings, the vasorelaxant effect of HEVe was not modified by 1‐H‐[1,2,4]‐oxadiazolo‐[4,3a]‐quinoxalin‐1‐one (1 µm ), tetraethylammonium (5 mm ), glibenclamide (10 µm ) or 2‐aminopyridine (100 µm ). Conclusions Our results suggest that HEVe induces relaxation through an endothelium‐independent pathway, involving blockade of Ca²⁺ channels, and this effect could be related to the presence of valepotriates.     

Comparison of cromakalim-induced relaxation of potassium precontracted rabbit,cat, and rat isolated cerebral arteries

Summary The effects of cromakalim were investigated in KCl-precontracted cat, rabbit, and rat isolated cerebral arteries with intact endothelium. Potassium induced contraction of all cerebral arteries studied, but exhibited marked vessel and species variation with no spasm to 20 or 30 mmol/l KCl in the rat basilar artery or 20 mmol/l KCl in the rabbit middle cerebral artery. On sustained tension to 20 mmol/l KCl, cromakalim induced concentration-related relaxation in the rabbit basilar artery and the cat basilar and middle cerebral arteries with Hill coefficients greater than unity. Cromakalim was more potent in the rabbit basilar artery precontracted with 20 or 30 mmol/KCl than in the rabbit middle cerebral artery or the cat basilar or middle cerebral artery. Elevation of the KCl concentration to 50 mmol/l inhibited cromakalim-induced relaxation and produced a decrease in the Hill coefficient. Preincubation of cerebral arteries with glibenclamide (100 nmol/l–1 mol/1) produced concentration-related inhibition of the cromakalim-induced relaxation in the rabbit basilar, cat basilar, and cat middle cerebral arteries precontracted with 20 mmol/l KCl. The degree of rightward shift of concentration-effect curves by glibenclamide was calculated at the EC₂₅, EC₅₀, and EC₇₅ levels. A good correlation was observed between the shifts at the EC₅₀ and EC₅₀ levels. However, the shift in concentration — effect curves for cromakalim produced at the EC₂₅ level was markedly less than the-EC₅₀ or EC₇₅ levels in the presence of 1 mol/1 glibenclamide. The pA ₂ values for glibenclamide calculated at the EC₅₀ level were 6.6 ± 0.09, 7.1 ± 0.1, and 6.5 ± 0.5 in the rabbit basilar, cat basilar, and cat middle cerebral artery, respectively. The slope of the Schild regression for the inhibitory effect of glibenclamide in the rabbit basilar artery was significantly greater than unity but did not differ from unity in cat cerebral arteries. Glibenclamide (1 mol/l) produced a similar degree of inhibition of the cromakalim-induced relaxation in the 30 mmol/l KCl precontracted rabbit middle cerebral artery and in the rabbit basilar artery exposed to 20 mmol/l KCl. In contrast, tolbutamide 10 mol/l was essentially inactive against the cromakalim-induced relaxation in all vessels studied. It is concluded that cromakalim produces concentration-dependent relaxations of rabbit and cat isolated cerebral arteries by a mechanism that is similar to that identified in peripheral vasculature and visceral smooth muscle. In this study we were unable to demonstrate effects of cromakalim on the KCl precontracted rat basilar artery, possibly due to the low sensitivity of this preparation to KCl. Send offprint requests to M. Wahl at the above address     

Vasorelaxant effect of formononetin in the rat thoracic aorta and its mechanisms

The purpose of the present study was to investigate the effect of formononetin and the related mechanisms on isolated rat thoracic aorta. Formononetin concentration dependently relaxed aortic rings precontracted with norepinephrine (NE, 1 μM) or KCl (80 mM). Pretreatment with formononetin noncompetitively inhibited contractile responses of aortas to NE and KCl. The vasorelaxant effect of formononetin partially relied on intact endothelia, which was significantly attenuated by incubation with N^ω-nitro-l-arginine methyl ester (100 μM). In endothelium-denuded rings, glibenclamide (10 μM) and tetraethylammonium (5 mM) showed slight reduction in the vasorelaxant effect of formononetin. Moreover, formononetin reduced NE-induced transient contraction in Ca²⁺-free solution and inhibited the vasocontraction induced by increasing external calcium in medium plus 80 mM KCl. Our results suggested that formononetin induced relaxation in rat aortic rings through an endothelium-dependent manner via nitric oxide synthesis pathway, and also involving an endothelium-independent vasodilatation by the blockade of Ca²⁺ channels. The opening of K⁺ channels might also be one of the mechanisms of formononetin-induced vasorelaxation.     

An evaluation of vardenafil as a calcium channel blocker in pulmonary artery in rats

Objective:

Vardenafil was reported to relax rat pulmonary artery through endothelium-dependent mechanisms. The aim of this in vitro study was to investigate other related mechanisms for this effect.

Materials and Methods:

Endothelium-intact and denuded artery rings were suspended in order to record isometric tension. In the rings with or without endothelium, the concentration-response curves for vardenafil were generated. In the rings without endothelium the contractile response induced by phenylephrine (Phe) or KCl was assessed in the presence or absence of vardenafil. In the last set of experiments, pulmonary artery rings were exposed to calcium-free isotonic depolarizing solution and the contractile response induced by the addition of calcium was evaluated in the presence or absence of vardenafil, nifedipine, verapamil or 1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one (ODQ).

Results:

Vardenafil attenuated pulmonary artery contraction induced by phenylephrine in the presence and absence of endothelium. In addition, vardenafil attenuated both Phe or KCl-induced contraction but, it''s effect on the KCl dose-response curve was more significant. Vardenafil also inhibited the contractile response induced by calcium in a dose-dependent manner. Addition of nifedipine or verapamil did not significantly alter this effect while ODQ incubation significantly inhibited vardenafil-induced relaxation.

Conclusion:

From these findings, it was proposed that vardenafil relaxed rat pulmonary artery through inhibiting calcium influx.KEY WORDS: Calcium, phosphodiesterase type 5 inhibitors, pulmonary artery, vardenafil     

EFFECTS OF NITRO-l-ARGININE ON ENDOTHELIUM-DEPENDENT RELAXATION OF CANINE CEREBRAL ARTERIES

1. The effect of N^G-nitro-l-arginine (NO₂Arg) on the relaxation of canine basilar artery was investigated and compared with those of middle cerebral and femoral arteries. 2. NO₂Arg (10^?7-3 × 10^?5 mol/L) inhibited the substance-P (Sub-P; 10^?12-10^?8 mol/L) induced relaxation in the basilar artery precontracted with prostaglandin F_2α (PGF_2α; 10^?5 mol/L) or KCl (10^?2 mol/L) in a concentration-dependent manner and a ratio of the maximum inhibition by NO₂Arg (3 × 10^?5 mol/L) was more than 90%. 3. The relaxation induced by A23187 (10^?9-3 × 10^?6) was also abolished by NO₂Arg (3 × 10^?5 mol/L), but that by glyceryl trinitrate (GTN; 10^?9-3 × 10^?5 mol/L) was not, in the basilar artery precontracted with PGF_2α (10^?5 mol/L). N^G-nitro-d-arginine (NO₂ArgD; 3 × 10^?5 mol/L) did not affect the relaxation induced by Sub-P (10^?12-10^?8 mol/L). 4. l-arginine (l-Arg; 3 × 10^?5-10^?4 mol/L) did not inhibit Sub-P (10^?12-10^?8 mol/L) induced relaxation in the basilar artery. Pretreatment of l-Arg (10^?4 mol/L) reversed the relaxation inhibited by NO₂Arg (3 × 10^?6 mol/L) in the arteries. 5. NO₂Arg (3 × 10^?5 mol/L) inhibited the Sub-P (10^?12-10^?8 mol/L) induced relaxation in the canine middle cerebral artery as much as in the basilar artery. NO₂Arg (3 × 10^?5 mol/L) also inhibited Sub-P (10^?12-10^?8 mol/L) induced relaxation in the femoral artery, but the degree of the inhibition was less than that in the basilar artery. 6. These results suggest that the endothelium-derived relaxing factor (EDRF) of canine basilar artery is mainly l-Arg derived nitric oxide which may play a more important role in the basilar artery than the femoral artery.     

Differential Vasorelaxant Effects of K+-Channel Openers and Ca2+-Channel Blockers on Canine Isolated Arteries

Abstract— The vasorelaxant effects of the K⁺-channel openers, pinacidil and cromakalim, were compared with those of the Ca²⁺-channel blockers, verapamil and KB-2796 (1-[bis(4-fluorophenyl)methyl]-4-(2,3,4-trimethoxybenzyl)piperazine dihydrochloride), in canine isolated coronary, renal, basilar and mesenteric arteries precontracted with U46619, a thromboxane A₂ mimetic. The relaxation induced by pinacidil and cromakalim was greater in coronary than in other arteries, the magnitude of relaxation being in the order of coronary > renal > basilar > mesenteric arteries. The relaxant responses to both drugs were inhibited by glibenclamide, a blocker of ATP-sensitive K⁺ channels. The relaxation induced by verapamil and KB-2796, in contrast, was greater in basilar than in other arteries, the magnitude of relaxation being in the order of basilar > coronary > renal and mesenteric arteries. In fura-2-loaded, U46619-stimulated arteries, pinacidil and cromakalim produced a greater reduction in intracellular Ca²⁺ concentration and muscle tension in coronary than in mesenteric arteries, while verapamil and KB-2796 reduced these values more potently in basilar than in mesenteric arteries. These results suggest that K⁺-channel openers exhibit a vasorelaxant selectivity for coronary arteries, whereas Ca²⁺-channel blockers exhibit such selectivity for cerebral arteries. The selective vasorelaxant action induced by these drugs appears to correspond, in part, to their effects on the concentration of intracellular Ca²⁺.     

Investigation of Mechanisms Involved in (−)‐Borneol‐Induced Vasorelaxant Response on Rat Thoracic Aorta

Abstract: The monoterpene (?)‐borneol is present in essential oils of several medicinal plants. The aim of this study was to evaluate (?)‐borneol effects on rat thoracic aorta artery rings. The cumulative addition of (?)‐borneol (10^?9–3 × 10^?4 M) on a phenylephrine‐induced pre‐contraction (10^?6 M) promoted a vasorelaxant effect in a concentration‐dependent manner and independent of vascular endothelium. A similar effect was obtained on KCl‐induced pre‐contractions (80 mM). (?)‐Borneol (10^?5–3 × 10^?4 M) inhibited contractions induced by cumulative addition of CaCl₂ (10^?6–3 × 10^?2 M) in depolarizing medium without Ca²⁺ in a concentration‐dependent manner. On S‐(?) Bay K 8644‐induced pre‐contractions (10^?7 M), (?)‐borneol did not induce significant changes compared with KCl‐induced pre‐contractions. In a Ca²⁺‐free medium, (?)‐borneol (10^?5, 10^?4 or 10^?3 M) interfered in calcium mobilization from phenylephrine (10^?6 M)‐ or caffeine (20 mM)‐sensitive intracellular stores. The involvement of K⁺ channels was evaluated by tetraethylammonium (3 mM), 4‐aminopyridine (1 mM) and glibenclamide (10^?5 M) pre‐treatment, and (?)‐borneol‐induced vasorelaxation was markedly attenuated. Thus, this vasorelaxant effect can probably be attributed to calcium influx blockade through voltage‐operated calcium channels (Ca_VL), calcium mobilization from intracellular stores and potassium channels activation.     

Analysis of cromakalim-, pinacidil-, and nicorandil-induced relaxation of the 5-hydroxytryptamine precontracted rat isolated basilar artery

Summary The effects of the K⁺ channel activators cromakalim, pinacidil, and nicorandil were investigated in endothelium intact, 5-hydroxytryptamine (5-HT) precontracted rat isolated basilar artery. Cromakalim, pinacidil, and nicorandil produced concentration-dependent relaxation of rat isolated basilar artery precontracted with 5-HT with a rank order of potency of cromakalim > pinacidil > nicorandil. All compounds produced full or nearly full relaxation. The calculated Hill coefficients for cromakalim-, pinacidil-, and nicorandil-induced relaxation of 5-HT-precontracted rat isolated basilar artery were 2.20 ± 0.36, 1.30 ± 0.07, and 1.00 ± 0.01, respectively. Under conditions of increased tone produced by 50 mmol/1 KCl (which inhibits cromakalim-induced relaxation) pinacidil and nicorandil produced marked reversal of spasm, with pinacidil being more potent than nicorandil. In arteries precontracted with 5-HT, preincubation with glibenclamide (0.1–1 mol/1) produced concentration-related inhibition of relaxation with calculated mean pA ₂ values (and slopes of Schild regression) ± SEM of 6.84 ± 0.20 (1.1 ± 0.20) against cromakalim, 6.60 ± 0.14 (0.95 ± 0.23) against nicorandil,and6.57 ± 0.26(1.04 ± 0.18) against pinacidil. For cromakalim, pinacidil, and nicorandil the slopes of Schild regression were not significantly different from unity. Tolbutamide 10 mol/l was without effect against the cromakalim-, pinacidil-, or nicorandil-induced relaxation. Tetraethylammonium (TEA; 1–10 mmol/l) produced noncompetitive inhibition of the cromakalim-induced relaxation, but appeared to produce competitive inhibition of the pinacidil- and nicorandil-induced relaxations. We conclude that cromakalim, pinacidil, and nicorandil produce relaxation of the 5-HT precontracted rat basilar artery by similar mechanisms to those identified in other peripheral vascular and visceral smooth muscle. Furthermore, pinacidil and nicorandil differ from cromakalim in possessing marked spasmolytic activity in 50 mmol/l KCl precontracted arteries. Send offprint requests to M. Wahl at the above address     

Chemical composition and vasorelaxant effect induced by the essential oil of Lippia alba (Mill.) N.E. Brown. (Verbenaceae) in rat mesenteric artery

Objectives:

To investigate the chemical composition and vasorelaxant effect of the essential oil of Lippia alba (EOLA) in rat mesenteric artery.

Material and Methods:

Chemical composition of EOLA was investigated by gas chromatography-mass spectrometry (GC/MS). Vasorelaxant effect was evaluated in vitro in rat superior mesenteric artery rings.

Results:

GC/MS analysis revealed the presence of 19 compounds, with geranial (48.58%) and neral (35.42%) being the major constituents. In intact rings precontracted with phenylephrine (Phe: 1 μM), EOLA (100-1000 μg/mL) induced relaxation, where the maximal effect (Emax) was 110.8 ± 10.8%. This effect was not modified after endothelium removal (Emax = 134.8 ± 16.5%), after tetraethylammonium (TEA) (Emax = 117.2 ± 4.96%), or in rings precontracted with KCl (80 mM) (Emax = 112.6 ± 6.70%). In addition, EOLA was able to inhibit the contraction caused by CaCl₂ and produced a small but significant (P<0.05) additional effect (from 70.5 ± 3.4 to 105.3 ± 13.5%, n = 5) on the maximal relaxation of nifedipine (NIF: 10 μM).

Conclusions:

The results demonstrated that EOLA induces endothelium-independent vasorelaxation, which appears to be caused, at least in part, by blocking Ca²⁺ influx through voltage-operated Ca²⁺ channels.KEY WORDS: Calcium channel, essential oil, Lippia alba, rat mesenteric artery, vasorelaxant effects     

Vasorelaxant and antihypertensive effects of ZCM298, a dihydropyridine derivative,are through inhibiting extracellular calcium influx

BackgroundZCM298 is a novel 1,4-dihydropyridine derivative. The aim of the study was to investigate its vasodilation and hypotension, and the related mechanisms.MethodsThe isometric tension of artery ring segments was recorded using an in vitro myography system. The blood pressure of spontaneously hypertensive rats (SHRs) was measured in vivo using a non-invasive tail cuff blood pressure system. Changes in the intracellular calcium concentration ([Ca²⁺]_i) in the mesenteric artery were surveyed using real-time confocal microscopy. Regional cerebral blood flow (rCBF) in the pia mater was monitored by laser-Doppler flowmetry (LDP).ResultsZCM298 (10^?9–10^?4 M) relaxed rat mesenteric artery obviously and concentration-dependently, which was not affected by the removal of the endothelium. ZCM298 shifted the concentration-contractile curves of mesenteric arteries in response to phenylephrine, U46619, KCl and CaCl₂ towards the right in a non-parallel manner. The potency of ZCM298 on relaxing basilar artery was much higher than on mesenteric artery. ZCM298 did not depress the phenylephrine-induced vasoconstriction; however, it inhibited the contraction caused by the addition of CaCl₂ in Ca²⁺-free solution. ZCM298 (10^?6 M) inhibited the increase of [Ca²⁺]i induced by KCl in the artery. ZCM298 improved the rCBF in the pia mater of rats at 0.03 and 0.06 mg/kg. ZCM298 depressed the systolic and diastolic blood pressure of SHRs in a dose-dependent manner.ConclusionsZCM298 relaxes arteries probably through inhibiting extracellular calcium influx and decreases the blood pressure of SHRs. ZCM298 is more potent in the basilar artery than in the mesenteric artery and improves rCBF in the pia mater of rats.     

γ-氨基丁酸对离体犬脑血管的作用

用离体犬脑血管(基底动脉)环标本,观察γ-氨基丁酸(GABA)对几种不同激动剂所致最大收缩反应的舒张作用。苯福林(PE)10μmol·L^-1,5-羟色胺(5-HT)10μmol·L^-1以及25mmol·L^-1的KCl均可使脑血管静息张力增加,GABA50μmol·L^-1对以上几种激动剂所致的收缩反应均有舒张作用。对较高浓度的KCI(45mmol·L^-1)所致的收缩无影响,但对由冷刺激(未预热的营养液,28℃)引起的收缩反应有明显的舒张作用。     

Vasorelaxant effects of the extracts and some flavonoids from the buds of Coreopsis tinctoria

Abstract

Context: The buds of Coreopsis tinctoria Nutt (Compositae) are used in the treatment of hypertension in the Uyghur folk medicine in China.

Objective: To investigate vasorelaxant properties of extracts and some flavonoids from C. tinctoria (CT) and their underlying mechanisms in isolated rat thoracic aortic rings.

Materials and methods: Vasorelaxant effects of ethanol extracts of CT (CTA) and its flavonoids as well as water–ethanol eluates from CTA by AB-8 resins (CTAA～CTAF) were evaluated on rat aortic rings pre-contracted with phenylephrine (PE, 1?µM) or high KCl (60?µM). We evaluated the effect of CTA, CTAD and CTAE on PE-induced contraction in a Ca²⁺-free medium and a dose–effect curve of Ca²⁺ in pre-contracted ring with high KCl.

Results: Endothelial removal did not modify the effect of CTAD and CTAE (3.00?g/L) neither on PE-pre-contracted rings (164.78?±?21.44 and 191.47?±?16.75%) nor on KCl-pre-contracted rings (75.68?±?10.76 and 125.14?±?17.41%) compared with intact-endothelium rings pre-contracted with high KCl (100.49?±?17.30 and 110.81?±?16.33%). CTAD and CTAE (3.00?g/L) down-regulated the dose–effect curve of Ca²⁺ in pre-contraction with high KCl, and inhibited the pre-contraction with PE in a Ca²⁺-free medium (p?<?0.05). Seven flavonoids were obtained from CTAD, of which luteolin (5) and quercetin (6) were found to be the most effective relaxation in rings precontracted with PE (EC₅₀: 0.006 and 0.039?g/L, p?<?0.05) or high KCl (EC₅₀: 0.023 and 0.045?g/L, p?<?0.05).

Discussion and conclusion: These data demonstrated the vasorelaxant effect of CT, and its mechanism is likely due to an inhibitory effect on calcium movements through cell membranes.     

Analysis of bradykinin receptor mediating relaxation of cat cerebral arteries in vivo and in vitro

Summary Bradykinin (BK), methionyl-lysyl-BK (M-L-BK) and des-Arg ⁹-BK produced, in decreasing potency, dose-related dilatations of the superficial pial arteries of the cat in vivo. The competitive, specific B₁-receptor antagonist, des-Arg ⁹-Leu⁸-BK was ineffective against BK-induced dilatations in this in vivo model.On the cat middle cerebral artery in vitro (but not the basilar artery), under resting tension and when contracted with 5-hydroxytryptamine (5-HT) or KCl, concentration related relaxations were produced by BK, M-L-BK, and des-Arg ⁹-BK, this being the order of relative potency of the three kinins.There was no increase in the sensitivity of either the middle cerebral or the basilar artery in vitro under resting tension or when contracted with 5-HT or KCl to BK or des-Arg ⁹-BK, concentration effect curves to which were produced at 2 h intervals over an 8 h period. The B₁-receptor antagonist des-Arg ⁹-Leu⁸-BK was ineffective against relaxations to BK or des-Arg ⁹-BK of the middle cerebral artery under resting tension or when contracted with 5-HT.The receptor mediating dilatation of the superficial pial arteries of the cat in vivo and relaxation of the middle cerebral artery in vitro to kinins is of the B₂-type. The cat basilar artery in vitro is relatively insensitive to the action of kinins and this is possibly due to an absence of receptors for kinins on this tissue.     

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.     

Relation between Cyclic GMP Generation and Cerebrovascular Reactivity: Modulation by NPY and α-Trinositol

Abstract It is considered that cyclic guanosine monophosphate (cGMP) plays a pivotal role in mediating the relaxation of vascular and nonvascular smooth muscles. cGMP steady state levels are regulated by guanylyl cyclase, cGMP phosphodiesterases and its flux from cells. The present study examines the possible relation between cerebrovascular vasodilator agents and generation of cGMP in guinea pig cerebral vessels. Acetylcholine, substance P, nitroglycerine and sodium nitroprusside significantly increased the generation of cGMP. The application of acetylcholine, substance P, nitroglycerine and sodium nitroprusside elicited concentration-dependent relaxation of basilar artery segments. Neuropeptide Y increased the generation of cGMP by 2%–46% of control levels (at 10^?7-10^?6 M of neuropeptide Y; *P<0.05). In addition, neuropeptide Y (10^?6 M) induced a transient relaxation of the precontracted guinea pig basilar arteries with endothelium. This transient relaxation was blocked by nitro-L-arginine (10^?4 M). α-Trinositol does not alter the formation of cGMP nor the neuropeptide Y-induced relaxation. In the presence of α-trinositol neuropeptide Y (10^?7-10^?6 M) did not significantly elevate the production of cGMP as compared with controls. The rise in cGMP induced by acetylcholine, substance P and nitroglycerine was slightly increased by the addition of neuropeptide Y (3×10^?7 M). Acetylcholine and substance P induced an endothelium-dependent relaxation of the precontracted guinea pig basilar arteries, while sodium nitroprusside and nitroglycerine induced an endothelium-independent relaxation. Acetylcholine, substance P and nitroglycerine induced concentration-dependent relaxations of basilar artery, respectively. The relaxation elicited by acetylcholine or substance P, but not nitroglycerine, was markedly attenuated by neuropeptide Y (3×10^?7 M). This inhibitory effect of NPY on vasomotor responses was completely reversed by α-trinositol (10^?6 M).     

Analysis of Rabbit Vascular Responses to DBI,an Ingol Derivative Isolated from Euphorbia canariensis

We have analysed the effects of 7,12-O-diacetyl-8-O-benzoil-2,3-diepiingol (DBI), an ingol derivative isolated from E. canariensis, on isometric tension developed by isolated rabbit basilar and carotid arteries. Concentration-response curves to DBI (10^?8 - 3 × 10^?5 m) were obtained cumulatively in both arteries at resting tension and active tone (KC1, 50 mm). At resting tension, DBI induced a concentration-dependent contraction, which was not inhibited in Ca²⁺-free medium. H7 (1-(5-isoquinoline sulphonyl)-2-methylpiperazine dichloride) (10^?4 m) inhibited the DBI-induced contraction both in basilar and in carotid arteries. Calmidazolium (10^?4 m) inhibited the maximum contraction of the carotid artery to DBI, and completely abolished the response in the basilar artery. In pre-contracted basilar arteries DBI induced a concentration-dependent relaxation that was not modified by incubation with N^G-nitro-l-arginine (l-NOARG; 10^?5 m) or indomethacin (10^?5 m). In the carotid artery with active tone DBI induced further contractions, which were not significantly modified by l-NOARG (10^?5 m) and were potentiated by indomethacin (10^?5 m). These results suggest that DBI contracts rabbit basilar and carotid arteries by a mechanism that is independent of extracellular Ca²⁺ and involves the participation both of protein kinase C and of calmodulin. DBI relaxes basilar but not carotid arteries by a mechanism independent of the liberation of nitric oxide and prostacyclin. In the carotid artery prostacyclin but not nitric oxide partially counteracts the contractile action of DBI.     

Influence of depolarization on vasorelaxant potency and efficacy of Ca2+ entry blockers, K+ channel openers, nitrate derivatives, salbutamol and papaverine in rat aortic rings

We examined the effects of various KCl concentrations on the actions of some vasodilators belonging to different pharmacological classes in rat aortic rings. In some experiments, tissues were precontracted with noradrenaline after blocking voltage-dependent channels to assess the effects of depolarisation unaccompanied by the entry of extracellular Ca²⁺ into the cytosol. Concentration/response curves for the vasorelaxant effect of calcium entry blockers (e.g. diltiazem), K⁺ channel openers (e.g. aprikalim), nitrate derivatives (e.g. nitroglycerin), a β₂-adrenergic agonist (salbutamol) and papaverine were obtained by using endothelium-denuded rat aortic rings precontracted with KCl (20–60 mM) to determine the potencies and efficacies of the drugs. The efficacies and potencies of calcium entry inhibitors were virtually independent of the [KCl]. A reduction in the potency (up to 18-fold) of papaverine occurred without changes in efficacy when the [KCl] was raised from 20 to 60 mM. The decline in potency was even greater for nitrate-like compounds. The potency of K⁺ channel openers in aortic rings precontracted with 30 mM KCl decreased by three- to sixfold compared with those precontracted with 20 mM KCl. With the exception of pinacidil, the efficacy of these agents already started to decline in preparations precontracted with 25 mM KCl and was virtually zero in preparations precontracted with 60 mM KCl. In contrast to other K⁺ channel openers, the vasorelaxant action of pinacidil was relatively resistant to glibenclamide. Salbutamol produced only a slight relaxation even in preparations precontracted with 20 mM KCl. In nitrendipine-pretreated, noradrenaline-precontracted aortic rings, the vasorelaxant effects of aprikalim, but not those of linsidomine or papaverine, declined when the [KCl] of the bathing medium was increased. In conclusion, the vasorelaxant potency and efficacy of calcium entry blockers is independent of the [KCl] used to precontract rat aortic rings, and thus, of the degree of membrane depolarisation. In contrast, increasing the [KCl] strongly reduces the potency and the efficacy of K⁺ channel openers not only in this preparation but also in noradrenaline-precontracted rings in which the entry of extracellular Ca²⁺ was prevented with nitrendipine. This indicates that, with the exception of pinacidil, the vasorelaxant activity of K⁺ channel openers depends on the degree of membrane depolarisation. Finally, the vasorelaxant potency and efficacy of nitrate-like compounds and papaverine are independent of depolarisation per se but they are markedly affected by the influx of Ca²⁺ accompanying elevated [KCl]. Thus, the degree of vessel depolarisation should be taken into consideration when attempting to compare potencies and efficacies among vasorelaxant agents. Received: 4 May 1998 / Accepted: 6 July 1998     

Analysis of the mechanism of action of bradykinin on human basilar artery in vitro

Summary Bradykinin (BK) initially produced concentration-related relaxations of human basilar artery in vitro. Concentration-effect curves constructed at 2 h intervals to BK over an 8 h period were reproducible. The rank order of potency of three kinins on the human basilar artery was found to be BK > methionyl-lysyl-BK > des-Arg⁹-BK. The B₂-receptor antagonist Thi^5,8 d-Phe⁷-BK but not the B₁-receptor antagonist des-Arg⁹-Leu⁸-BK selectively blocked BK-induced relaxations of the human basilar artery.The relaxant effects of bradykinin and acetylcholine but not papaverine were attenuated after removal of the endothelium or treating the tissues with BW755C. Indomethacin was without effect. Concentration-effect curves to angiotensin I were markedly attenuated by captopril at a concentration which had no effect on BK, angiotensin II or 5-hydroxytryptamine responses. It is concluded that BK induced relaxations of the human basilar artery are mediated via activation of a B₂ receptor and the response is dependent upon the release of a factor present in the endothelium. Angiotensin converting enzyme is present in the human basilar artery and is important for the conversion of angiotensin I to angiotensin II but apparently not for the degradation of BK. It is likely that other kininases are present and active in the tissue. Send offprint requests to E. T. Whalley at the above address