Studies of the cellular mechanisms underlying the vasorelaxant effects of rutaecarpine, a bioactive component extracted from an herbal drug

Decreased Human Platelet Activation and Mouse Pulmonary Thrombosis by Rutaecarpine and Comparison of the Relative Effectiveness with BAY11-7082: Crucial Signals of p38-NF-κB

Platelets play a critical role in arterial thrombosis. Rutaecarpine (RUT) was purified from Tetradium ruticarpum, a well-known Chinese medicine. This study examined the relative activity of RUT with NF-κB inhibitors in human platelets. BAY11-7082 (an inhibitor of IκB kinase [IKK]), Ro106-9920 (an inhibitor of proteasomes), and RUT concentration-dependently (1-6 μM) inhibited platelet aggregation and P-selectin expression. RUT was found to have a similar effect to that of BAY11-7082; however, it exhibits more effectiveness than Ro106-9920. RUT suppresses the NF-κB pathway as it inhibits IKK, IκBα, and p65 phosphorylation and reverses IκBα degradation in activated platelets. This study also investigated the role of p38 and NF-κB in cell signaling events and found that SB203580 (an inhibitor of p38) markedly reduced p38, IKK, and p65 phosphorylation and reversed IκBα degradation as well as p65 activation in a confocal microscope, whereas BAY11-7082 had no effects in p38 phosphorylation. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay shows that RUT and BAY11-7082 did not exhibit free radical scavenging activity. In the in vivo study, compared with BAY11-7082, RUT more effectively reduced mortality in adenosine diphosphate (ADP)-induced acute pulmonary thromboembolism without affecting the bleeding time. In conclusion, a distinctive pathway of p38-mediated NF-κB activation may involve RUT-mediated antiplatelet activation, and RUT could act as a strong prophylactic or therapeutic drug for cardiovascular diseases.

Rutaecarpine, an Alkaloid from Evodia rutaecarpa, Can Prevent Platelet Activation in Humans and Reduce Microvascular Thrombosis in Mice: Crucial Role of the PI3K/Akt/GSK3β Signal Axis through a Cyclic Nucleotides/VASP-Independent Mechanism

The role of activated platelets in acute and chronic cardiovascular diseases (CVDs) is well established. Therefore, antiplatelet drugs significantly reduce the risk of severe CVDs. Evodia rutaecarpa (Wu-Chu-Yu) is a well-known Chinese medicine, and rutaecarpine (Rut) is a main bioactive component with substantial beneficial properties including vasodilation. To address a research gap, we investigated the inhibitory mechanisms of Rut in washed human platelets and experimental mice. At low concentrations (1–5 μM), Rut strongly inhibited collagen-induced platelet aggregation, whereas it exerted only a slight or no effect on platelets stimulated with other agonists (e.g., thrombin). Rut markedly inhibited P-selectin expression; adenosine triphosphate release; [Ca²⁺]i mobilization; hydroxyl radical formation; and phospholipase C (PLC)γ2/protein kinase C (PKC), mitogen-activated protein kinase, and phosphoinositide 3-kinase (PI3K)/Akt/glycogen synthase kinase-3β (GSK3β) phosphorylation stimulated by collagen. SQ22536 (an adenylate cyclase inhibitor) or ODQ (a guanylate cyclase inhibitor) did not reverse Rut-mediated antiplatelet aggregation. Rut was not directly responding to vasodilator-stimulated phosphoprotein phosphorylation. Rut significantly increased the occlusion time of fluorescence irradiated thrombotic platelet plug formation. The findings demonstrated that Rut exerts a strong effect against platelet activation through the PLCγ2/PKC and PI3K/Akt/GSK3β pathways. Thus, Rut can be a potential therapeutic agent for thromboembolic disorders.

Synthetic Fluororutaecarpine Inhibits Inflammatory Stimuli and Activates Endothelial Transient Receptor Potential Vanilloid-Type 1

The natural product, rutaecarpine (RUT), is the main effective component of Evodia rutaecarpa which is a widely used traditional Chinese medicine. It has vasodilation, anticoagulation, and anti-inflammatory activities. However, further therapeutic applications are limited by its cytotoxicity. Thus, a derivative of RUT, 10-fluoro-2-methoxyrutaecarpine (F-RUT), was designed and synthesized that showed no cytotoxicity toward RAW264.7 macrophages at 20 μM. In an anti-inflammation experiment, it inhibited the production of nitric oxide (NO) and tumor necrosis factor (TNF)-α in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophages; cyclooxygenase (COX)-2 and inducible NO synthase (iNOS) induced by LPS were also downregulated. After 24 h of treatment, F-RUT significantly inhibited cell migration and invasion of ovarian A2780 cells. Furthermore, F-RUT promoted expressions of transient receptor potential vanilloid type 1 (TRPV1) and endothelial (e)NOS in human aortic endothelial cells, and predominantly reduced the inflammation in ovalbumin/alum-challenged mice. These results suggest that the novel synthetic F-RUT exerts activities against inflammation and vasodilation, while displaying less toxicity than its lead compound.

Traditional herbs: a remedy for cardiovascular disorders

BACKGROUND

Medicinal plants have been used in patients with congestive heart failure, systolic hypertension, angina pectoris, atherosclerosis, cerebral insufficiency, venous insufficiency and arrhythmia since centuries. A recent increase in the popularity of alternative medicine and natural products has revived interest in traditional remedies that have been used for the treatment of cardiovascular diseases.

AIM

The purpose of this review is to provide updated, comprehensive and categorized information on the history and traditional uses of some herbal medicines that affect the cardiovascular system in order to explore their therapeutic potential and evaluate future research opportunities.

METHODS

Systematic literature searches were carried out and the available information on various medicinal plants traditionally used for cardiovascular disorders was collected via electronic search (using Pubmed, SciFinder, Scirus, GoogleScholar, JCCC@INSTIRC and Web of Science) and a library search for articles published in peer-reviewed journals. No restrictions regarding the language of publication were imposed.

RESULTS

This article highlights the cardiovascular effects of four potent traditional botanicals viz. Garlic (Allium sativum), Guggul (Commiphora wightii), Hawthorn (Crataegus oxyacantha) and Arjuna (Terminalia arjuna). Although these plants have been used in the treatment of heart disease for hundreds of years, current research methods show us they can be utilized effectively in the treatment of cardiovascular diseases including ischemic heart disease, congestive heart failure, arrhythmias and hypertension.

CONCLUSION

Although the mechanisms of action are not very clear, there is enough evidence of their efficacy in various cardiovascular disorders. However, for bringing more objectivity and also to confirm traditional claims, more systematic, well-designed animal and randomized clinical studies with sufficient sample sizes are necessary. Multidisciplinary research is still required to exploit the vast potential of these plants. Potential synergistic and adverse side effects of herb-drug interactions also need to be studied. These approaches will help in establishing them as remedies for cardiovascular diseases and including them in the mainstream of healthcare system.

Low-cytotoxic synthetic bromorutaecarpine exhibits anti-inflammation and activation of transient receptor potential vanilloid type 1 activities

Rutaecarpine (RUT), the major bioactive ingredient isolated from the Chinese herb Evodia rutaecarpa, possesses a wide spectrum of biological activities, including anti-inflammation and preventing cardiovascular diseases. However, its high cytotoxicity hampers pharmaceutical development. We designed and synthesized a derivative of RUT, bromo-dimethoxyrutaecarpine (Br-RUT), which showed no cytotoxicity at 20 μM. Br-RUT suppressed nitric oxide (NO) production and tumor necrosis factor-α release in concentration-dependent (0~20 μM) manners in lipopolysaccharide (LPS)-treated RAW 264.7 macrophages; protein levels of inducible NO synthase (iNOS) and cyclooxygenase-2 induced by LPS were downregulated. Br-RUT inhibited cell migration and invasion of ovarian carcinoma A2780 cells with 0~48 h of treatment. Furthermore, Br-RUT enhanced the expression of transient receptor potential vanilloid type 1 and activated endothelial NOS in human aortic endothelial cells. These results suggest that the synthetic Br-RUT possesses very low cytotoxicity but retains its activities against inflammation and vasodilation that could be beneficial for cardiovascular disease therapeutics.

Phenols, Alkaloids and Terpenes from Medicinal Plants with Antihypertensive and Vasorelaxant Activities. A Review of Natural Products as Leads to Potential Therapeutic Agents

Numerous studies support the cardiovascular effects of medicinal plants. This review examines plants whose antihypertensive and vasorelaxant effects have been scientifically validated. Our study selected only chemically characterized plants whose mode of action had already been investigated. The aim of the paper is to provide a quick way to identify medicinal plants and their constituents with antihypertensive and vasorelaxant activities.

Vasorelaxant activity of indole alkaloids from Tabernaemontana dichotoma

The aim of this study was to search for bioactive natural products from medicinal plants targeting vasorelaxant activity and we found the methanol extract from bark of Tabernaemontana dichotoma showed vasorelaxant activity on rat aorta. We isolated eight indole alkaloids including 10-methoxyalstonerine (1), a new macroline type indole alkaloid, from bark of T. dichotoma. These were respectively identified as 10-methoxyaffinisine (2), lochnerine (3), cathafoline (4), (-)-alstonerine (5), 19,20-dehydro-10-methoxytalcarpine (6), alstonisine (7), and alstonal (8) based on spectroscopic analysis. Among them, sarpagine type (2 and 3), akuammiline type (4), and macroline oxindole type (7 and 8) showed potent vasorelaxant activity. Mechanism of action on vasorelaxant activity of 10-methoxyaffinisine (2), cathafoline (4), and alstonisine (7) was clarified. Effects of 10-methoxyaffinisine (2), cathafoline (4), and alstonisine (7) were partially mediated the NO release from endothelial cells. Furthermore, 10-methoxyaffinisine (2) and alstonisine (7) attribute to the inhibitory effect of VDC and ROC, and cathafoline (4) have inhibitory effect on Ca(2+) influx via ROC. In addition, 10-methoxyaffinisine (2) as a major compound from bark of T. dichotoma showed hypotensive effect on normotensive rats in vivo.

Anti-inflammatory and anti-infectious effects of Evodia rutaecarpa (Wuzhuyu) and its major bioactive components

This article reviews the anti-inflammatory relative and anti-infectious effects of Evodia rutaecarpa and its major bioactive components and the involvement of the nitric oxide synthases, cyclooxygenase, NADPH oxidase, nuclear factor kappa B, hypoxia-inducible factor 1 alpha, reactive oxygen species, prostaglandins, tumor necrosis factor, LIGHT, amyloid protein and orexigenic neuropeptides. Their potential applications for the treatment of endotoxaemia, obesity, diabetes, Alzheimer's disease and their uses as cardiovascular and gastrointestinal protective agents, analgesics, anti-oxidant, anti-atherosclerosis agents, dermatological agents and anti-infectious agents are highlighted. Stimulation of calcitonin gene-related peptide release may partially explain the analgesic, cardiovascular and gastrointestinal protective, anti-obese activities of Evodia rutaecarpa and its major bioactive components.

Pharmacological effects of rutaecarpine as a cardiovascular protective agent

Many studies indicate that traditional Chinese herbs are beneficial in the prevention and treatment of cardiovascular diseases. Evodia rutaecarpa (‘Wu-Chu-Yu’) remains the most popular and multi-purpose herb traditionally used in China for treatment of headache, abdominal pain, postpartum hemorrhage, dysentery and amenorrhea. Rutaecarpine is one of the intriguing indolopyridoquinazoline alkaloids isolated from ‘Wu-Chu-Yu’. Rutaecarpine has been shown to have cardiovascular biological effects such as inotropic and chronotropic, vasorelaxant, anti-platelet aggregation and anti-inflammatory effects. Furthermore, it has been reported that rutaecarpine has beneficial effects on some cardiovascular diseases. This review summarizes data on the cardiovascular pharmacological actions of rutaecarpine the published over the recent years, aiming to provide more evidence supporting its use in the treatment of cardiovascular diseases.

Impairment of endothelium-dependent ACh-induced relaxation in aorta of diabetic db/db mice--possible dysfunction of receptor and/or receptor-G protein coupling

Diabetes is a risk factor of ischemic heart disease, cerebral ischemia, and atherosclerosis, in which endothelial dysfunction plays a role in the pathogenesis. We examined vascular responses in the aorta of pre-diabetic db/db mice with normoglycemia, hyperlipidemia, and hyperinsulinemia (6 weeks old), and diabetic db/db mice with hyperglycemia, hyperlipidemia, and hyperinsulinemia (11 weeks old) in comparison with age-matched non-diabetic db/+ mice. Prostaglandin F2alpha (PGF2alpha)-induced contraction was significantly enhanced in the aorta of diabetic but not pre-diabetic db/db mice compared to age-matched non-diabetic db/+ mice. Acetylcholine (ACh), adenosine-5'-diphosphate (ADP), NaF, a G protein activator and A-23187, a Ca-ionophore, caused endothelium-dependent and nitric oxide (NO)-mediated relaxation, and sodium nitroprusside (SNP), an NO donor, caused endothelium-independent relaxation in the pre-contracted aorta of db/db mice. Maximal endothelium-dependent ACh-induced relaxation was reduced in diabetic but not pre-diabetic db/db mice compared to age-matched db/+ mice, while maximal SNP-induced relaxation was not different between diabetic and non-diabetic mice. ACh-induced relaxation in diabetic db/db mice was not affected by ozagrel, a thromboxane A2 (TXA2) synthetase inhibitor, or acetylsalicylic acid (aspirin), a cyclooxygenase inhibitor, suggesting no involvement of endogenous TXA2 or prostanoids in the reduction of relaxation. Maximal endothelium-dependent ADP-, A-23187-, and NaF-induced relaxation was not reduced in diabetic db/db mice. EC50 values for ACh- and SNP-induced relaxation were increased in diabetic but not pre-diabetic db/db mice, suggesting decreases in sensitivity to NO in diabetic mice. Two-week treatment with KV-5070, a PPARgamma agonist, lowered plasma glucose, triglyceride (TG), and insulin but not cholesterol, and reversed the reduced ACh-induced relaxation. In conclusion, ACh-induced endothelium-dependent relaxation is impaired in diabetic db/db mice, probably due to the dysfunction of ACh receptors and/or receptor-G protein coupling. Endothelial dysfunction was not genetic and was considered to be initiated primarily by hyperglycemia, and was improved by anti-diabetic treatment with a PPARgamma agonist.

A systematic review of mechanisms by which natural products of plant origin evoke vasodilatation

This article reviews the body of work aimed at elucidating the mechanisms of action by which natural products of plant origin exert a vasodilatory effect at the level of the vasculature. The search was restricted to 4 mechanisms: the nitric oxide system and (or) reactive oxygen species, the eicosanoid system, potassium channel function, and calcium channel function. The National Library of Medicine database was searched using "PubMed" without restriction to language. The search generated 266 references on 15 November 2005. Most studies were in vitro in nature and of these, most involved studies in the rat aorta. Many of the natural products evoked vasodilatation through an endothelium-dependent mechanism. The vasodilatation was attenuated or abolished by a nitric oxide synthase inhibitor and, in some of these studies, by an inhibitor of guanylate cyclase. A few studies reported a cyclooxygenase component, but most found no effect of the cyclooxygenase inhibitor, indomethacin. The vasorelaxation evoked by several natural products was attenuated by various potassium channel blocking agents, suggesting that some natural products exerted their effect either directly or indirectly through activation of potassium channels. Finally, a significant number of natural products evoked vasodilatation either through blockade of calcium channels or by inhibiting the release of calcium from intracellular stores. Many natural products evoked vasodilatation through multiple mechanisms. The information in this review on mechanisms of action should facilitate good clinical practice by increasing the predictive capabilities of the practitioner, notably the ability to predict adverse effects and interactions among medications. The knowledge should also help to provide leads to the ultimate goal of developing new therapeutic medications.

In Vitro and in Vivo Antiallergic Effect of the Fructus of Evodia rutaecarpa and Its Constituents

The unripe fruit of Evodia rutaecarpa (JUSS) BENTH (ER, Family Rutaceae) has been used frequently as a traditional medicine against inflammatory diseases in Korea, China and Japan. To evaluate antiallergic effect of ER, we isolated its main constituents, evodiamine and rutaecarpine, and evaluated in vivo their inhibitory effects against passive cutaneous anaphylaxis (PCA) reaction induced by IgE-antigen complex and scratching behaviors by compound 48/80. ER and its constituents, evodiamine and rutaecarpine, potently inhibited PCA reaction and scratching behaviors in mice, although ER weakly inhibited scratching behaviors. Evodiamine and rutaecarpine inhibited TNF-alpha and IL-4 protein expression in RBL-2H3 cells induced by IgE-antigen complex, although these did not inhibit degranulation of RBL-2H3 cells induced by IgE-antigen complex and rat peritoneal mast cells induced by compound 48/80. These findings suggest that ER and its constituents, evodiamine and rutaecarpine, may be effective for IgE-induced allergic diseases such as atopic dermatitis and rhinitis.

Pertussis toxin-induced histamine sensitisation: an aspecific phenomenon independent from the nitric oxide system?

Mechanisms were studied initially to develop an in vitro safety test for detecting pertussis toxin toxicity in acellular pertussis vaccines based on the histamine sensitisation test. Maximal contractions and sensitivities to different agonists and adrenoceptor-induced contractions in Ca2+-free medium of isolated rat small mesenteric resistance arteries were significantly reduced by in vivo [30 microg/kg, intravenously (i.v.), day 5] or in vitro (10 microg/ml, 2 h) pertussis toxin pretreatment. Pertussis toxin-induced decrease in sensitivity of small mesenteric resistance arteries to noradrenaline was endothelium-dependent. Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM, 20 min) did not reestablish the sensitivity to noradrenaline. In vivo L-NAME treatment (0, 1, 10 or 30 mg/kg) of pertussis toxin-pretreated (15 microg/kg) rats did not reduce pertussis toxin-induced enhancement of the histamine-induced decrease in blood pressure and histamine (10, 30, 100 or 300 mg/kg) induced mortality. Finally, in vivo pertussis toxin pretreatment sensitises rats for sodium nitroprusside (50 microg/kg/min). We conclude that pertussis toxin-induced histamine sensitisation is caused by an interference of pertussis toxin with the contractile mechanisms of vascular smooth muscle of resistance arteries which indicates only an indirect role for histamine in the histamine sensitisation test.

Nitric oxide, human diseases and the herbal products that affect the nitric oxide signalling pathway

1. Nitric oxide (NO) is formed enzymatically from l-arginine in the presence of nitric oxide synthase (NOS). Nitric oxide is generated constitutively in endothelial cells via sheer stress and blood-borne substances. Nitric oxide is also generated constitutively in neuronal cells and serves as a neurotransmitter and neuromodulator in non-adrenergic, non-cholinergic nerve endings. Furthermore, NO can also be formed via enzyme induction in many tissues in the presence of cytokines. 2. The ubiquitous presence of NO in the living body suggests that NO plays an important role in the maintenance of health. Being a free radical with vasodilatory properties, NO exerts dual effects on tissues and cells in various biological systems. At low concentrations, NO can dilate the blood vessels and improve the circulation, but at high concentrations it can cause circulatory shock and induce cell death. Thus, diseases can arise in the presence of the extreme ends of the physiological concentrations of NO. 3. The NO signalling pathway has, in recent years, become a target for new drug development. The high level of flavonoids, catechins, tannins and other polyphenolic compounds present in vegetables, fruits, soy, tea and even red wine (from grapes) is believed to contribute to their beneficial health effects. Some of these compounds induce NO formation from the endothelial cells to improve circulation and some suppress the induction of inducible NOS in inflammation and infection. 4. Many botanical medicinal herbs and drugs derived from these herbs have been shown to have effects on the NO signalling pathway. For example, the saponins from ginseng, ginsenosides, have been shown to relax blood vessels (probably contributing to the antifatigue and blood pressure-lowering effects of ginseng) and corpus cavernosum (thus, for the treatment of men suffering from erectile dysfunction; however, the legendary aphrodisiac effect of ginseng may be an overstatement). Many plant extracts or purified drugs derived from Chinese medicinal herbs with proposed actions on NO pathways are also reviewed.

Rutaecarpine-induced block of delayed rectifier K+ current in NG108-15 neuronal cells

The effects of rutaecarpine on ionic currents of NG108-15 neuronal cells were investigated in this study. Rutaecarpine (2-100 microM) suppressed the amplitude of delayed rectifier K+ current (I(K(DR))) in a concentration-dependent manner. The IC50 value for rutaecarpine-induced inhibition of I(K(DR)) was 11 microM. I(K(DR)) present in these cells is sensitive to the inhibition by quinidine and dendrotoxin, yet not by E-4031. The presence of rutaecarpine enhanced the rate and extent of I(K(DR)) inactivation, although it had no effect on the initial activation phase of I(K(DR)). Recovery from block by rutaecarpine (5 microM) was fitted by a single exponential with a value of 2.87 s. Crossover of tail currents in the presence of rutaecarpine was also observed. Cell-attached single-channel recordings revealed that rutaecarpine decreased channel activity, but it did not alter single-channel amplitude. With the aid of the binding scheme, a quantitative description of the rutaecarpine actions on I(K(DR)) was provided. However, rutaecarpine (20 microM) had no effect on L-type Ca2+ current. Under current-clamp configuration, rutaecarpine prolonged action potential duration in NG108-15 cells. These results show that rutaecarpine is a blocker of the K(DR) channel. The increase in action potential duration induced by rutaecarpine can be explained mainly by its blocking actions on I(K(DR)).

In vitro relaxation of rabbit and human internal anal sphincter by rutaecarpine, an alkaloid isolated from Evodia rutaecarpa

Rutaecarpine, a compound extracted from the Chinese medicinal herb Evodia rutaecarpa, has been shown to possess relaxing action on vascular smooth muscle from rat thoracic aorta. The internal anal sphincter is a specialized smooth muscle regulating important anorectal physiology. To investigate the effect and underlying mechanisms of rutaecarpine on internal anal sphincter, muscle strips from rabbit internal anal sphincter were used. The results showed that rutaecarpine (1 x 10(-10) M to 1 x 10(-4) M) produced a concentration-dependent muscular relaxation effect in our preparations, which were precontracted with acetylcholine. This muscular relaxation effect was not affected by treatment with L-N(G)-nitro-arginine methyl ester (a nitric oxide synthase inhibitor), methylene blue (a guanylate cyclase inhibitor), N-ethylmaleimide (an adenylate cyclase inhibitor), or by removal of the mucosa and submucosa tissue. Pretreatment with nifedipine (a calcium channel blocker) or extracellular Ca+2 removal by ethylenediaminetetraacetic acid (EDTA) greatly attenuated the relaxation effect, suggesting that calcium ion might be involved. In experiments using strips from human internal anal sphincter, an even more prominent relaxation effect was shown. It is thus concluded that rutaecarpine caused relaxation on internal anal sphincter from rabbits and human subjects. The relaxation action was not related to NO-cGMP pathway, instead calcium ion might play an important role and shed insight into clinical implications for those anorectal disorders with hyperactive anal tone.

Mechanisms involved in the vasorelaxant effect of (-)-stepholidine in rat mesenteric small arteries

The purpose of the present investigation was to clarify whether the hypotensive action of the protoberberine alkaloid, and dopamine receptor antagonist, (-)-stepholidine, can be ascribed to an effect on peripheral small arteries. For this purpose isolated mesenteric small arteries were suspended in microvascular myographs for isometric tension recording. Relaxations mediated by dopamine D1 receptors were antagonized by (-)-stepholidine. (-)-Stepholidine inhibited in a concentration-dependent manner the contractile responses evoked by noradrenaline (10(-6) M), but not the contractile responses evoked by depolarizing solution (KCl, 60 mM) or 9,11-dideoxy-11alpha,9alpha-epoxymethano prostaglandin F2alpha (U46619, 10(-7) M). Mechanical endothelial cell removal, blockade of K+ channels, muscarinic receptors or adrenoceptors did not influence the inhibitory effect of (-)-stepholidine on the contractile response evoked with noradrenaline in the segments. (-)-Stepholidine caused rightward shifts of the concentration-response curves for noradrenaline and phenylephrine. The pA2 values for (-)-stepholidine were 6.05 and 5.94 against noradrenaline and phenylephrine, respectively. Electrical field stimulation induced prazosin-sensitive frequency-dependent contractions in mesenteric small arteries. These contractions were significantly inhibited by 10(-6) and 10(-5) M (-)-stepholidine. In membranes from the rat cerebral cortex labelled with [3H]prazosin, (-)-stepholidine (10(-7)-10(-4) M) completely inhibited the specific binding of the ligand with a pKi of 5.6. The present investigation suggests the inhibitory effect of (-)-stepholidine on the alpha1-adrenoceptor-mediated contractions induced by exogenously added and nerve-released noradrenaline in peripheral small arteries might contribute to a hypotensive effect of the drug.