Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

Atherosclerosis Development and Aortic Contractility in Hypercholesterolemic Rabbits Supplemented with Two Different Flaxseed Varieties

Alpha-linolenic acid (ALA) is widely regarded as the main beneficial component of flax for the prevention of cardiovascular disease. We evaluated the effect of the transgenic flaxseed W86-which is rich in ALA-on the lipid profile, atherosclerosis progression, and vascular reactivity in hypercholesterolemic rabbits compared to the parental cultivar Linola with a very low ALA content. Rabbits were fed a basal diet (control) or a basal diet supplemented with 1% cholesterol, 1% cholesterol and 10% flaxseed W86, or 1% cholesterol and 10% Linola flaxseed. A high-cholesterol diet resulted in an elevated plasma cholesterol and triglyceride levels compared to the control animals. Aortic sections from rabbits fed Linola had lower deposits of foamy cells than those from rabbits fed W86. A potassium-induced and phenylephrine-induced contractile response was enhanced by a high-cholesterol diet and not influenced by the W86 or Linola flaxseed. Pretreatment of the aortic rings with nitro-L-arginine methyl ester resulted in a concentration-dependent tendency to increase the reaction amplitude in the control and high-cholesterol diet groups but not the flaxseed groups. Linola flaxseed with a low ALA content more effectively reduced the atherosclerosis progression compared with the W86 flaxseed with a high concentration of stable ALA. Aorta contractility studies suggested that flaxseed ameliorated an increased contractility in hypercholesterolemia but had little or no impact on NO synthesis in the vascular wall.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Sulodexide promotes arterial relaxation via endothelium-dependent nitric oxide-mediated pathway

Sulodexide (SDX) is a highly purified glycosaminoglycan with antithrombotic and profibrinolytic properties and reported benefits in thrombotic and atherosclerotic vascular disorders. However, the effects of SDX on vascular function are unclear. We tested whether SDX affects vascular relaxation and examined the potential underlying mechanisms. Isolated segments of male rat abdominal aorta and mesenteric artery were suspended in a tissue bath, and the changes in arterial contraction/relaxation were measured. The α-adrenergic receptor agonist phenylephrine (Phe) (10^-9-10^-5 M) caused concentration-dependent aortic and mesenteric artery contraction that was reduced in tissues pretreated with SDX (1 mg/ml). In aortic and mesenteric arterial segments precontracted with submaximal concentration of Phe (3 × 10^-7-6 × 10^-7 M), SDX (0.001-1 mg/ml) caused concentration-dependent relaxation. To test the role of endothelium, SDX-induced relaxation was compared with that of acetylcholine (ACh), a known activator of endothelium-dependent relaxation. In Phe precontracted aorta, ACh relaxation was abolished and SDX relaxation was significantly inhibited by endothelium removal or the nitric oxide synthase (NOS) inhibitor N_ω-nitro-l-arginine methyl ester (L-NAME), suggesting a role of NO. In mesenteric artery, ACh relaxation was abolished by endothelium removal, partially blocked by L-NAME, and completely blocked by a mixture of indomethacin, a cyclooxygenase inhibitor and blocker of the PGI₂-cAMP pathway, and tetraethylammonium, a blocker of K⁺ channels and EDHF-dependent hyperpolarization pathway. In comparison, SDX relaxation of mesenteric artery was almost completely inhibited by endothelium removal or NOS inhibitor L-NAME. SDX enhanced vascular relaxation and increased nitrate/nitrite production in response to all ACh concentrations in the aorta, but only to low ACh concentrations (<10^-7 M) in mesenteric artery. SDX did not affect aortic or mesenteric artery endothelium-independent relaxation to the NO donor sodium nitroprusside. Thus, SDX promotes arterial relaxation via a mechanism involving endothelium-dependent NO production; an effect that could enhance vasodilation and decrease vasoconstriction in vascular disorders.

Non-adrenergic vasoconstriction and vasodilatation of guinea-pig aorta by β-phenylethylamine and amphetamine - Role of nitric oxide determined with L-NAME and NO scavengers

Sympathomimetic and trace amines, including β-phenylethylamine (PEA) and amphetamine, increase blood pressure and constrict isolated blood vessels. By convention this is regarded as a sympathomimetic response, however, recent studies suggest trace amine-associated receptor (TAAR) involvement. There is also uncertainty whether these amines also release nitric oxide (NO) causing opposing vasodilatation. These questions were addressed in guinea-pig isolated aorta, a species not previously examined. Guinea-pig aortic rings were set up to measure contractile tension. Cumulative concentration-response curves were constructed for the reference α-adrenoceptor agonist, phenylephrine, PEA or d-amphetamine before and in the presence of vehicles, the α₁-adrenoceptor antagonist, prazosin (1µM), the nitric oxide synthase inhibitor, N^ω-nitro-L-arginine (L-NAME), or NO scavengers, curcumin and astaxanthin. Prazosin inhibited phenylephrine contractions with low affinity consistent with α_1L-adrenoceptors. However, PEA and amphetamine were not antagonised, indicating non-adrenergic responses probably via TAARs. L-NAME potentiated contractions to PEA both in the absence and presence of prazosin, indicating that PEA releases NO to cause underlying opposing vasodilatation, independent of α₁-adrenoceptors. L-NAME also potentiated amphetamine and phenylephrine. PEA was potentiated by the NO scavenger astaxanthin but less effectively. Curcumin, an active component of turmeric, however, inhibited PEA. Trace amines therefore constrict blood vessels non-adrenergically with an underlying NO-mediated non-adrenergic vasodilatation. This has implications in the pressor actions of these amines when NO is compromised.

BAY 41-2272 Treatment Improves Acetylcholine-Induced Aortic Relaxation in L-NAME Hypertensive Rats

Hypertension, an emerging problem of recent era, and many pathophysiological factors are participating to produce the disease. Nitric oxide (NO) is an important constituent to ameliorate hypertensive condition. Inhibition of endogenous NO synthase by L-N^G-Nitroarginine methyl ester (L-NAME) was responsible for generating hypertension in rats. BAY 41-2272 (5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridine-3-yl]-pyrimidin-4-ylamine), a soluble guanylyl cyclase activator, restricts rise of blood pressure and shows cardioprotective activity. The aim of the present study was to analyze effect of short-term BAY 41-2272 treatment on blood pressure and vascular function. Male Wistar rats were randomly divided into three groups such as control (group-A), hypertensive (group-B), and BAY 41-2272-treated hypertensive (group-C) rats. Normal saline was administered intramuscularly to control rats for last 3 days (days 40, 41, and 42) of total 42 days treatment, whereas rats of group-B and group-C were treated with L-NAME hydrochloride in drinking water at 50 mg/kg body weight daily for 42 days. Also, normal saline and BAY 41-2272 were administered for last 3 days at two different dosages at 1 and 3 mg/kg body weight/day intramuscularly to group-B and group-C rats, respectively. Administration of BAY 41-2272 for 3 days was not sufficient enough to decrease mean arterial pressure of hypertensive rats significantly. BAY at both the treatment dosages significantly ameliorate acetylcholine-induced maximal aortic relaxation compared with BAY-untreated hypertensive rats. Findings of the present study indicate that even shorter period of BAY 41-2272 treatment (3 days) improves vascular relaxation.

Effects of one resistance exercise session on vascular smooth muscle of hypertensive rats

BACKGROUND

Hypertension is a public health problem and increases the incidence of cardiovascular diseases.

OBJECTIVE

To evaluate the effects of a resistance exercise session on the contractile and relaxing mechanisms of vascular smooth muscle in mesenteric arteries of NG-nitro L-arginine methyl ester (L-NAME)-induced hypertensive rats.

METHODS

Wistar rats were divided into three groups: control (C), hypertensive (H), and exercised hypertensive (EH). Hypertension was induced by administration of 20 mg/kg of L-NAME for 7 days prior to experimental protocols. The resistance exercise protocol consisted of 10 sets of 10 repetitions and intensity of 40% of one repetition maximum. The reactivity of vascular smooth muscle was evaluated by concentration‑response curves to phenylephrine (PHEN), potassium chloride (KCl) and sodium nitroprusside (SNP).

RESULTS

Rats treated with L-NAME showed an increase (p < 0.001) in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) compared to the initial period of induction. No difference in PHEN sensitivity was observed between groups H and EH. Acute resistance exercise reduced (p < 0.001) the contractile response induced by KCl at concentrations of 40 and 60 mM in group EH. Greater (p < 0.01) smooth muscle sensitivity to NPS was observed in group EH as compared to group H.

CONCLUSION

One resistance exercise session reduces the contractile response induced by KCl in addition to increasing the sensitivity of smooth muscle to NO in mesenteric arteries of hypertensive rats.

Increased constrictor tone induced by ouabain treatment in rats

Ouabain (Oua)-induced hypertension in rodents provides a model to study cardiovascular changes associated with human hypertension. We examined vascular function in rats after a long-term treatment with Oua. Systolic blood pressure was measured by tail-cuff plethysmography in male Sprague-Dawley rats treated with Oua (≈ 25 µg/d) or placebo for 8 weeks. Blood pressure increased in Oua-treated animals, reaching 30% above baseline systolic blood pressure after 7 weeks. At the end of treatment, vascular responses were studied in mesenteric resistance arteries (MRAs) by wire myography. Contraction to potassium chloride in intact and denuded arteries showed greater sensitivity in Oua-treated animals. Contraction to phenylephrine and relaxation to acetylcholine were similar between groups with a lower response to sodium nitroprusside in Oua-treated arteries. Sensitivity to endothelin-1 was higher in Oua-treated arteries. Na⁺-K⁺ ATPase activity was decreased in MRAs from Oua-treated animals, whereas protein expression of the Na⁺-K⁺ ATPase α₂ isoform was increased in heart and unchanged in mesenteric artery. Preincubation with indomethacin (10⁻⁵ M) or Nω-nitro-L-arginine methyl ester (10⁻⁴ M) abolished the differences in potassium chloride response and Na⁺-K⁺ ATPase activity. Changes in MRAs are consistent with enhanced vascular smooth muscle cell reactivity, a contributor to the increased vascular tone observed in this model of hypertension.

Effect of age on the contractile response of the rat carotid artery in the presence of sympathetic drugs and L-NAME

For evaluating the age-related change in noradrenaline (NA)-induced contraction of isolated rat carotid artery (CA), the effect of α and β adrenoreceptor (AR) blockers and the role of nitric oxide (NO) were investigated.

METHODS

Concentration-response curves to NA (10-10-10-4 M) and α1 agonist phenylephrine (PE; 10-10-10-5 M) were constructed in isolated CA rings from young and middle-aged rats. The effects of nitric oxide synthase (NOS) inhibitor (L-NAME; 100 μM), α1-AR antagonist (prazosin; 0.1 μM), α2-AR antagonist (yohimbine; 0.1 μM) and β-AR antagonist (propranolol; 1 μM) on NA-induced contraction of isolated CA rings were examined. In CA rings preconstricted with NA, the responses to α2-AR agonist (clonidine; 10-7-10-5 M), β-AR agonist (isoprenaline; 10-8-10-5 M),), sodium nitroprusside (SNP; 10-9-10-5 M) were assessed.

RESULTS

The maximum contractile response of CA to NA and to PE was higher in younger than in middle-aged rats. Prazosin reduced the contractile response to NA in both groups, while propranolol, yohimbine and L-NAME did not affect NA-induced contraction in either of them. Clonidine, isoprenaline and SNP produced a dose-dependent vasorelaxation of CA rings, isoprenaline-induced vasodilatation was lower in middle-aged rats, while there was no difference in clonidine or SNP-induced relaxant effect between the two groups.

CONCLUSIONS

NA-induced contraction of isolated rat CA rings is decreased in old rats, this is related to α1-AR. β-AR mediated dilatation was compromised in middle-aged rats (endothelium-dependent). α2-AR and SNP-mediated dilator effect seems to be unchanged.

Endothelium-dependent vascular relaxation induced by Globularia alypum extract is mediated by EDHF in perfused rat mesenteric arterial bed

The vasodilatory effect of Globularia alypum L. (GA) extract was evaluated in rat mesenteric arterial bed pre-contracted by continuous infusion of phenylephrine (2-4 ng/mL). Bolus injections of GA elicited dose-response vasodilation, which was abolished after endothelium removal. Addition of a nitric oxide synthase inhibitor, N(G)-nitro-l-arginine methyl ester (100 µmol/L), alone or in the presence of a cyclooxygenase inhibitor, indomethacin (10 µmol/L), did not significantly affect the vasodilation of the mesenteric arterial bed in response to GA extract. These results suggest that GA-induced vasodilation is endothelium dependent but nitric oxide and prostacyclin independent. In the presence of high K(+) (60 mmol/L), the GA vasodilatory effect was completely abolished, suggesting that the vasodilation effect is mediated by hyperpolarization of the vascular cells. Also, pre-treatment with atropine (a muscarinic receptors antagonist) antagonized the GA-induced vasodilation, suggesting that the vasodilatory effect is mainly mediated by the endothelium-derived hyperpolarizing factor through activation of endothelial muscarinic receptors.

Pioglitazone treatment improves nitrosative stress in type 2 diabetes

OBJECTIVE

The purpose of this study was to determine the effect of 24 weeks of treatment with 45 mg/day pioglitazone on peripheral skin blood flow (SkBF) and skin nitric oxide (NO) production in vivo in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

This was a randomized, parallel, cross-over, double-blind, within- and between-subject study designed to compare vascular responses before and after treatment. We studied 12 subjects with type 2 diabetes (average age 58.6 +/- 30.8 years, HbA(1c) 7.9 +/- 00.4%, BMI 31.3 +/- 1.2 kg/m(2)). SkBF was measured using laser Doppler techniques in response to ischemia reperfusion and local skin warming, and NO production was assessed in vivo using an amperometric NO meter inserted directly into the skin. These measurements were performed before treatment and at 6 and 24 weeks.

RESULTS

The SkBF response was not significantly improved after 24 weeks in either of the groups. NO production was significantly decreased in the pioglitazone-treated group in the basal condition (area under the curve 6.4 +/- 1.0 vs. 2.8 +/- 0.8, P < 0.01), after local heat stimulation at 40 degrees C (12.9 +/- 2.2 vs. 5.7 +/- 1.7, P < 0.01), and after nociceptor stimulated flow with local heating at 44 degrees C (36.4 +/- 6.3 vs. 16.6 +/- 3.4). Differences were not significant in the placebo-treated group.

CONCLUSIONS

Treatment of patients with type 2 diabetes with pioglitazone for 24 weeks reduced skin NO production, thus probably reducing nitrosative stress without a demonstrable effect on SkBF. Because nitrosative stress is considered to be a factor in the pathogenesis of neurovascular dysfunction, these findings warrant further investigation.

A quantitative description of the contraction of blood vessels following the release of noradrenaline from sympathetic varicosities

A model is presented that highlights the principal factors determining the form and extent of contraction in arteries upon stimulation of their sympathetic nerve supply. This model incorporates a previous quantitative model of the process of noradrenaline (NAd) diffusion into the vascular media and reuptake into sympathetic varicosities during nerve stimulation (J. Theor. Biol. 226 (2004) 359). It is also dependent on a model of how the subsequent activation of metabotropic receptors initiates a G-protein cascade, resulting in the production of inositol trisphosphate (IP3) and an increase in intracellular calcium concentration, [Ca2+]i, in the smooth muscle cells (J. Theor. Biol. 223 (2003) 93). In the present work we couple this rise in [Ca2+]i to the increase in phosphorylated myosin bound to actin in the cells and hence determine the force development in arteries due to nerve stimulation. The model accounts for force development as a function of [Ca2+]i and for the rate of change of force as a function of the rate of change of [Ca2+]i in single smooth muscle cells. It also accounts for the characteristic time course of the force developed by the media of the rat-tail artery upon nerve stimulation. This consists of a rapid rise to a transient peak followed by a sustained plateau of contraction during the stimulation period, after which the contraction slowly decays back to baseline at a rate dependent on the strength of the stimulation. The model indicates that the transient peak is primarily due to the partial block of the IP3 receptor by the rise in [Ca2+]i and that the main determinant of the equilibrium condition indicated by the plateau phase is the rate of pumping of calcium into the sarcoplasmic reticulum. The relatively slow decline of contraction at the end of nerve stimulation is primarily a consequence of the slow rates of removal of NAd from the media by diffusion and reuptake into the sympathetic varicosities. The model thus provides a quantitative account of vascular smooth muscle contraction upon sympathetic nerve stimulation.

Alterations by age of calcium handling in rat resistance arteries

This study investigated the alterations by age of Ca handling in rat small mesenteric arteries. The contractile responses to phenylephrine and caffeine in small mesenteric arteries from young and old rats were studied in Ca -containing and Ca -free medium. In Ca -containing medium the contraction to phenylephrine (100 micro M) but not to caffeine (10 mM) was greater in old than in young rats. Concentration-response curve to phenylephrine was affected to the same extent by nifedipine (1 micro M) in both age groups, whereas ryanodine (20 micro M) decreased the maximal response to phenylephrine only in young rats. These results suggest the participation of intracellular Ca handling on the observed differences by aging. In Ca -free medium, phenylephrine (10 micro M) but not caffeine (10 mM) induced a greater contraction in old than in young animals, corroborating the results obtained in Ca -containing solution. The greater response to phenylephrine observed in old rats cannot be explained by an increase in the inositol 1,4,5-trisphosphate (IP ) formation because the accumulation of inositol phosphates by phenylephrine was not affected by aging. Results obtained in Ca -free medium using caffeine after phenylephrine or vice versa suggest a common intracellular Ca pool. Pretreatment with ryanodine in Ca -free medium almost abolished contractile response to phenylephrine and caffeine in young rats but only partially decreased them in old animals, suggesting an impairment in the Ca -induced Ca release (CICR) mechanism leading to an increase in the stored Ca content. The greater amount of stored Ca could explain the higher contractile response to phenylephrine observed in aged rats. As a consequence of all these changes due to aging, an imbalance between the two Ca release mechanisms from sarcoplasmic reticulum was observed with a major role of Ca induced release by IP at the expense of an impairment of CICR mechanism. This observation will also help explain the results obtained in the presence of extracellular Ca, where phenylephrine induced a greater maximum response in old animals in spite of a decrease in the midrange sensitivity to this agonist.

Influence of the nature of pre-contraction on the responses to commonly employed vasodilator agents in rat-isolated aortic rings

The relaxing properties of vasodilator drugs in vitro may depend on the characteristics of the contractile state of the vessel investigated. Rat-isolated thoracic aortas were exposed to different types of pre-contraction. The following vasoconstrictor agents were used: phenylephrine (PhE), a selective alpha1-adrenoceptor agonist; St 587, a partial alpha1-adrenoceptor stimulant; U46619 (U-46). a thromboxane A2 agonist: and potassium ions causing receptor-independent depolarization of the membrane. After pre-contraction, various differential vasodilator drugs were investigated: methacholine (MCh, endothelium dependent), sodium nitroprusside (SNP, NO donor), forskolin (FSK, adenylyl cyclase stimulant) and nifedipine, a Ca2+-antagonist (selective L-type calcium antagonist). The vasodilator activity of these compounds was quantified by their vasodilator potency value (pD2) and efficacy (Emax) obtained from their concentration-response curves. PhE (0.1, 0.3, 3 microM) caused isometric responses of 4.8 +/- 0.3, 6.5 +/- 0.3 and 7.8 +/- 0.5 mN, respectively. An increase of the PhE concentration from 0.1 to 3 microM did not influence the response to FSK while it reduced the pD2 of SNP (8.6 +/- 0.1 to 7.35 +/- 0.1). Under these conditions, only the Emax of MCh was reduced (96.3 +/- 4.3% to 43.3 +/- 6.9%). U46 (0.18, 0.3, 1 microM) increased the contractile force by 7.4 +/- 0.4, 8.8 +/- 0.3 and 10.4 +/- 0.3 mN, respectively. Increasing the concentration of U-46 from 0.18 to 1 microM affected only the efficacy of SNP (84 +/- 4.4% to 17 +/- 8.8%) and MCh (64.5 +/- 12.3% to 0.0 +/- 9.2%) and reduced the potency of FSK (7.91 +/- 0.26 to 7.15 +/- 0.10). The concentration of K+-ions from 25 to 30 and 40 mM increased the contractile force by 4.0 +/- 0.4, 7.0 +/- 0.5 and 10.8 +/- 0.4 mN, respectively. The increase in [K+] caused a potency decrease of FSK (7.1 +/- 0.0 to 5.8 +/- 0.0) whereas both efficacy and potency were reduced for SNP (95.6 +/- 1.8% to 65.8 +/- 1.9% and 8.7 +/- 0.1 to 7.2 +/- 0.1) and MCh (55.4 +/- 3.5% to 24.5 +/- 0.8% and 7.4 +/- 0.3 to 6.1 +/- 0.4). Inhibiting of the endothelial NO production by L-NAME 100 microM resulted after pre-contraction with PhE and potassium in comparable differences in properties for SNP. Pre-contraction with St 587 1, 3, 10 and 30 microM shows comparable results after nifedepine relaxation. The present experiments clearly demonstrate that the characteristics of the applied pre-contraction strongly, but differentially influence both the potency and efficacy of various vasodilator drugs in vitro. Accordingly, in vitro characterization of vasodilator drugs should be performed under a carefully standardized protocol of pre-contraction.

Increased superoxide and vascular dysfunction in CuZnSOD-deficient mice

Increased superoxide is thought to play a major role in vascular dysfunction in a variety of disease states. Superoxide dismutase (SOD) limits increases in superoxide; however, the functional significance of selected isoforms of SOD within the vessel wall are unknown. We tested the hypothesis that selective loss of CuZnSOD results in increased superoxide and altered vascular responsiveness in CuZnSOD-deficient (CuZnSOD(-/-)) mice compared with wild-type (CuZnSOD(+/+)) littermates. Total SOD activity was reduced (P<0.05) by approximately 60% and CuZnSOD protein was absent in aorta from CuZnSOD(-/-) as compared with wild-type mice. Vascular superoxide levels, measured using lucigenin (5 micro mol/L)-enhanced chemiluminescence and hydroethidine (2 micro mol/L)-based confocal microscopy, were increased (approximately 2-fold; P<0.05) in CuZnSOD(-/-) mice as compared with wild-type mice. Relaxation of the carotid artery in response to acetylcholine and authentic nitric oxide was impaired (P<0.05) in CuZnSOD(-/-) mice. For example, maximal relaxation to acetylcholine (100 micro mol/L) was 50+/-6% and 69+/-5% in CuZnSOD(-/-) and wild-type mice, respectively. Contractile responses of the carotid artery were enhanced (P<0.05) in CuZnSOD(-/-) mice in response to phenylephrine and serotonin, but not to potassium chloride or U46619. In vivo, dilatation of cerebral arterioles (baseline diameter=31+/-1 micro m) to acetylcholine was reduced by approximately 50% in CuZnSOD(-/-) mice as compared with wild-type mice (P<0.05). These findings provide the first direct insight into the functional importance of CuZnSOD in blood vessels and indicate that this specific isoform of SOD limits increases in superoxide under basal conditions. CuZnSOD-deficiency results in altered responsiveness in both large arteries and microvessels.

Ouabain-induced hypertension is accompanied by increases in endothelial vasodilator factors

The involvement of nitric oxide (NO), prostaglandins, and calcium-dependent potassium channel (K(Ca)) activators on the negative modulation of phenylephrine-induced contractions was evaluated on the isolated aorta and caudal (CAU) artery obtained from rats treated with ouabain for 5 wk to induce hypertension. In ouabain-treated rats, the reactivity to phenylephrine was reduced in the endothelium-intact aorta but not the CAU segments. Endothelial modulation of phenylephrine contraction, as demonstrated by endothelium removal, NO synthase (NOS) inhibition with N(omega)-nitro-L-arginine methyl ester and aminoguanidine, as well as K(Ca) inhibition with tetraethylammonium, was more pronounced in segments from ouabain-treated animals, and here greater effects were seen in the aorta than in CAU. An increased expression of endothelial NOS and neuronal NOS was seen in the aorta after ouabain treatment. In CAU, only endothelial NOS was detected and ouabain treatment did not alter its expression. These results suggest that ouabain-induced hypertension is accompanied by increased NO release derived from endothelial NOS and neuronal NOS and increased release of an endothelial hyperpolarizing factor that presumably opens K(Ca), all of which contribute to the increased negative modulation of the phenylephrine contraction.

Thromboxane A2 in vasomotor effects of phenylephrine, acetylcholine, and bradykinin in rat mesenteric bed

In vessels, pharmacological agents displace the balance between relaxing and contracting factors. A cross-talk between those factors has been shown in some vascular beds. To examine whether NO may regulate the vascular tone by modulating prostanoid synthesis or release, we analyzed the response of resistance rat mesenteric arterial bed to vasoactive agents. Phenylephrine, bradykinin (BK), and acetylcholine (ACH) were administered in the absence or in the presence of either NO synthesis inhibition (N(G)-nitro-L-arginine methyl ester [L-NAME]), cyclo-oxygenase inhibition (indomethacin), and/or a thromboxane A2 (TXA2) receptor antagonist (S18886), or a combination thereof. In the presence of L-NAME, the response to phenylephrine was markedly increased. Indomethacin limited these changes, which are attributed to TXA2 release since they were abolished by S18886 and a marked increase in TXB2 release (stable metabolite of TXA2) was found during phenylephrine infusion under NO blockade. Similarly, BK response under NO blockade was markedly attenuated with an improved response with indomethacin and a restoration of vasorelaxation with S18886. In contrast, indomethacin decreased further the response to ACH during L-NAME treatment, and TXA2 inhibition had no effect. Thus, in pathophysiological conditions where an endothelial dysfunction is present, TXA2 stimulation induced by NO release impairment may contribute to an altered response to phenylephrine or BK.

Alterations in phenylephrine-induced contractions and the vascular expression of Na+,K+-ATPase in ouabain-induced hypertension

Hypertension development, phenylephrine-induced contraction and Na(+),K(+)-ATPase functional activity and protein expression in aorta (AO), tail (TA) and superior mesenteric (SMA) arteries from ouabain- (25 microg day(-1), s.c., 5 weeks) and vehicle-treated rats were evaluated. Ouabain treatment increased systolic blood pressure (127+/-1 vs 160+/-2 mmHg, n=24, 35; P<0.001) while the maximum response to phenylephrine was reduced (P<0.01) in AO (102.8+/-3.9 vs 67.1+/-10.1% of KCl response, n=12, 9) and SMA (82.5+/-7.5 vs 52.2+/-5.8%, n=12, 9). Endothelium removal potentiated the phenylephrine response to a greater extent in segments from ouabain-treated rats. Thus, differences of area under the concentration-response curves (dAUC) in endothelium-denuded and intact segments for control and ouabain-treated rats were, respectively: AO, 56.6+/-9.6 vs 198.3+/-18.3 (n=9, 7); SMA, 85.5+/-15.4 vs 165.4+/-24.8 (n=6, 6); TA, 13.0+/-6.1 vs 39.5+/-10.4% of the corresponding control AUC (n=6, 6); P<0.05. The relaxation to KCl (1 - 10 mM) was similar in segments from both groups. Compared to controls, the inhibition of 0.1 mM ouabain on KCl relaxation was greater in AO (dAUC: 64.8+/-4.6 vs 84.0+/-5.1%, n=11, 14; P<0.05), similar in SMA (dAUC: 39.1+/-3.9 vs 43.3+/-7.8%, n=6, 7; P>0.05) and smaller in TA (dAUC: 62.1+/-5.5 vs 41.4+/-8.2%, n=12, 13; P<0.05) in ouabain-treated rats. Protein expression of both alpha(1) and alpha(2) isoforms of Na(+),K(+)-ATPase was augmented in AO, unmodified in SMA and reduced in TA from ouabain-treated rats. These results suggest that chronic administration of ouabain induces hypertension and regional vascular alterations, the latter possibly as a consequence of the hypertension.

The effect of nitric oxide synthesis inhibition on intravenous cocaine self-administration

Adult male rats were implanted with intravenous catheters. After a minimum of 10 days recovery from surgery, rats were trained to intravenously self-administer cocaine (1 mg/kg/infusion) during 3-h test sessions. The nitric oxide synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME) was used to determine the effect of nitric oxide (NO) synthesis inhibition on cocaine self-administration. A 5-day protocol was used and on Days 2 and 5, an intraperitoneal injection of L-NAME (0, 3, 30, 300 mg/kg) was administered 45 to 60 min into a 3-h test session. One to two hours following L-NAME administration, there was a dose-dependent decrease in the amount of self-administered cocaine and an increase in the interresponse time (IRT) between successive cocaine injections. L-NAME appeared to prolong the rewarding effect of cocaine possibly through a pharmacokinetic action.

Role of arterial smooth muscle tone and geometry in the regulation of peripheral conduit artery mechanics by shear stress

1. Although arterial blood flow is recognized as an important modulator of vascular tone and geometry, the effect of acute changes in shear-stress on conduit artery mechanics has not been fully investigated in humans because of technical limitations. 2. To assess, respectively, the effects of decreases and increases in flow and shear stress on radial artery tone and mechanics, arterial pressure (photoplethysmography), total blood viscosity, radial artery internal diameter, wall thickness (echotracking) and blood flow (Doppler) were measured in healthy volunteers (mean (+/-SEM) age 25 +/- 1 years) during a distal flow arrest (n=12) and hand skin heating (n=18). 3. Radial artery flow decreased from 31 +/- 4 to 7 +/- 1 10(-3) L/min during distal flow arrest (P < 0.001) and increased from 10 +/- 2 to 22 +/- 4 and 69 +/- 6 10(-3) L/min during heating (P < 0.001). At mean arterial pressure, these changes in flow were respectively associated with a parallel flow-dependent reduction and increase in diameter and midwall stress. There was no significant modification in mean elastic modulus. Compliance did not change when flow decreased and only increased at the highest level of flow. Finally, the cross-sectional compliance and incremental modulus were fitted as functions of midwall stress. The decrease in flow was associated with an upward shift of the modulus-midwall stress curve and a downward shift of the compliance-midwall stress curve. The increase in flow was associated with a downward shift of the modulus-midwall stress curve and an upward shift of the compliance-midwall stress curve at each level of wall shear stress. 4. By using two different procedures, we obtained similar results concerning the direct effects of increases and decreases in flow on stiffness of the arterial wall and on arterial compliance and demonstrated the presence of a flow-dependent regulation of arterial smooth muscle tone of peripheral conduit arteries in humans.

Enhancement of phenylephrine-induced contraction in the isolated rat aorta with endothelium by H2O-extract from an Oriental medicinal plant Leonuri herba

Leonuri herba (I-mu-ts'ao, the Chinese motherwort) is an ancient Chinese traditional herb. Although the pharmacological effects of extracts of Leonuri herba have been shown in platelets and uteri, the effect on the vascular system has not been determined. In the present study, we investigated the effects of extracts of Leonuri herba on the contraction of the isolated rat aorta. Although the H20-extract (0.3-3 mg/ml) by itself showed a limited effect, the extract enhanced phenylephrine-induced contraction of the aorta with endothelium, but not without endothelium. The H20-extract, like N(G)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase), significantly inhibited the relaxation induced by acetylcholine in the aorta with endothelium. The inhibitory effect of H20-extract on the relaxation decreased by co-addition with 1 mM L-arginine. The vasoconstrictive effect of H20-extract was not due to leonurine, which is a constituent in Leonuri herba and shows uterotonic activity. Intravenous injection of the H20-extract (1.5 mg/kg) to rats caused an increase in blood pressure for 5 min, like L-NAME (1.35 mg/kg). These findings suggest that there is a component(s) in Leonuri herba, which shows a vasoconstrictive activity in rat aorta in vitro and in vivo and has similar pharmacological profile to that of L-NAME.

Alpha-adrenoceptor agonists produce Ca2+ oscillations in isolated rat aorta: role of protein kinase C

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) in responses to norepinephrine (NE) and selective alpha2-adrenoceptor agonist, UK14,304 of the endothelium-denuded rat aorta loaded with fura PE-3. NE (3 x 10(-8) M) evoked a rapid increase in [Ca2+]i followed by slight decreasing to a steady state level and produced a contraction. After the NE-induced increase in [Ca2+]i had reached a maximum, the [Ca2+]i showed persistent oscillations. The Ca2+ oscillations were superimposed on the sustained increase in [Ca2+]i. UK14,304 (3 x 10(-6) M) also evoked an increase in [Ca2+]i and produced a contraction. However, the UK14,304-induced effect on [Ca2+]i was characterized by pronounced oscillations, and the amplitude of the sustained increase in [Ca2+]i was less than that seen with NE. Protein kinase C inhibitor, Ro31-8220 (3 x 10(-6) M) and verapamil (10(-5) M) abolished both NE and UK14,304-evoked Ca2+ oscillations. UK14,304-induced contractions were also strongly inhibited by Ro31-8220 and verapamil. However, NE induced contractions were partly inhibited by these inhibitors. The sustained increases in [Ca2+]i evoked NE and UK14,304 were not significantly inhibited by Ro31-8220 and verapamil. These results suggest that NE and UK14,304 produce Ca2+ oscillations during sustained contractions in rat aorta. The alpha2 adrenoceptor agonist, UK14,304-induced sustained contraction and Ca2+ oscillations may be due to PKC activation and opening of voltage-dependent L type Ca2+ channels.

Antihypertensive properties of a nitric oxide-releasing naproxen derivative in two-kidney, one-clip rats

Nonsteroidal anti-inflammatory drugs have been reported to exacerbate hypertension. In this study, we tested the hypothesis that a nitric oxide-releasing derivative of naproxen would ameliorate hypertension in the rat. Hypertension was induced by partially occluding one renal artery (the "2K,1C" model), and 2 wk later the rats started receiving naproxen, the nitric oxide-releasing derivative HCT-3012, or vehicle each day for 2 wk. Naproxen significantly exacerbated the hypertension. HCT-3012 significantly reduced blood pressure relative to both the naproxen- and vehicle-treated groups. Both naproxen and HCT-3012 markedly suppressed whole blood thromboxane B(2) synthesis. In studies of anesthetized rats, naproxen significantly enhanced the late hypertensive response to endothelin-1 and significantly blunted the early hypotensive response. In contrast, HCT-3102 did not affect either response to endothelin-1. In vitro, HCT-3012 significantly reduced the responsiveness of aortic rings to the contractile effects of phenylephrine. These studies suggest that HCT-3012 reduces blood pressure in hypertensive rats, not simply through the vasodilatory actions of the nitric oxide it releases, but through alterations in the responsiveness of the vasculature to endogenous pressor agents.

Role of nitric oxide in the regulation of regional blood flow and metabolism in anaesthetized pigs

To investigate the contribution of nitric oxide in the regulation of regional blood flow and metabolism in vivo, we administered incremental doses of N omega-L-arginine-methyl ester (L-NAME 1, 3, 10, 30 and 100 mg kg-1, intravenously) in isoflurane anaesthetized pigs. The pulmonary vascular bed exhibited a greater sensitivity to the L-NAME-induced pressor effects compared with the systemic arterial bed as the slope of the dose-response curve was steeper (42.9 +/- 4.3 vs. 24.3 +/- 3.6, P < 0.05) and the dose of L-NAME required to induce a 25% pressure increase was lower (PD25 of 6.2 +/- 2.5 vs. 22.8 +/- 5.2 mg kg-1, P < 0.05). L-NAME infusion produced a dose-dependent reduction in cardiac output that was evenly distributed among the mesenteric, femoral, hepatic and carotid arterial circulation as demonstrated by unchanged regional blood flows-to-cardiac output ratios, except in the kidney where the L-NAME-induced vasoconstriction was most pronounced (renal blood flow/cardiac output decreased from 6.2 +/- 0.6 to 3.7 +/- 0.7% after 100 mg kg-1 of L-NAME, P < 0.05). After the administration of L-NAME 30 mg kg-1, intestinal O2 uptake (Vo2) increased (+39 +/- 3%, P < 0.05) whereas renal Vo2 tended to decrease (-19 +/- 4%, P = 0.07) and whole body Vo2 remained unchanged. Plasma noradrenaline and adrenaline concentrations did not change significantly with L-NAME infusion. These data demonstrate that in anaesthetized pigs, endogenous nitric oxide is most important for the regulation of pulmonary and renal blood flows and in spite of unchanged global metabolic demand, nitric oxide inhibition leads to an increase in intestinal Vo2 associated with enhanced gut motility without rise in circulating lactate levels.

The neuronal NOS inhibitor L-MIN, but not 7-NINA, reduces neurotoxic effects of chronic intrastriatal administration of quinolinic acid

Rat striata were exposed to 15 mM quinolinic acid (QUIN), or QUIN plus the nitric oxide synthase inhibitors S-methyl-L-thiocitrulline dihydrochloride (L-MIN) or 7-nitroindazole monosodium salt (7-NINA) for 21 days. Co-administration of 100 microM or 1 mM L-MIN with QUIN significantly reduced lesion volume compared to QUIN alone. Co-administration of 1 microM or 10 microM L-MIN with QUIN had no significant effect. There was no significant effect of 7-NINA co-administered with QUIN compared to QUIN alone. L-MIN reduction of lesion volume supports the contention that neuronal nitric oxide synthase is a mediator of excitotoxic injury.

Role of nitric oxide and nitric oxide-independent relaxing factor in contraction and relaxation of rabbit blood vessels

It has been shown that spontaneous release of nitric oxide (NO) from the vascular endothelium attenuates contractile responses of vascular smooth muscles to norepinephrine, and that acetylcholine-induced relaxation is mediated by the evoked release of NO and endothelium-derived hyperpolarizing factor. Since the involvement of these substances (or factors) in mechanical responses is heterogeneous among blood vessels, we have investigated the role of these substances in agonist-induced contraction and relaxation in 6 rabbit blood vessels. Vascular reactivity for the contractile response to norepinephrine was potentiated after removal of endothelium and by 100 microM N(G)-nitro-L-arginine (L-NA) but not by 80 nM-0.4 microM clotrimazole. This potentiation was most marked in the mesenteric artery among the blood vessels tested, suggesting that the basal release of NO reduced the contractile response of the vascular smooth muscle to norepinephrine in this artery. Acetylcholine-induced relaxation was abolished by removal of the endothelium and was attenuated by L-NA (1-100 microM) in all blood vessels. The attenuation by 100 microM L-NA was most obvious in aorta and vein and least in mesenteric resistance artery in which the acetylcholine-induced, L-NA-resistant relaxation was inhibited by 80 nM-0.4 microM clotrimazole. These results suggested that there is a regional difference in the degree of involvement of NO in acetylcholine-induced relaxation. In mesenteric resistance artery, the NO-independent, clotrimazole-sensitive factor, possibly hyperpolarizing factor may also contribute to the response to acetylcholine at high concentrations.

Alpha 1-adrenoceptor vasoconstriction in the tail artery during ageing

1. We have studied the alpha 1-adrenoceptor-mediated responses in intact tail artery rings from 3-4 and 20-22 months old Sprague-Dawley rats, focusing on possible endothelial alterations. The influence of nitric oxide released by the endothelium, the number of alpha 1-adrenoceptors and the functional receptor reserve were evaluated to determine their contribution to the contractile response mediated by this receptor. The state of the endothelial layer was assessed by confocal microscopy. 2. Noradrenaline (1 nM-100 microM) induced concentration-dependent vasoconstriction. The maximum contractions to noradrenaline (P < 0.05) and to 75 mM KCl (P < 0.01) were higher in young than in old animals. 3. The density (Bmax) of alpha 1-adrenoceptors and the dissociation constant (KD) obtained in [3H]-prazosin binding experiments were unchanged by age. 4. The apparent affinity (pKA) and the percentage of functional receptors (qx 100) remaining after phenoxybenzamine (0.03 microM) were similar in both age groups. 5. After partial alpha 1-adrenoceptor inactivation with phenoxybenzamine, NG-nitro-L-arginine methylester (30 microM) significantly potentiated the E/[A] curve to noradrenaline in young rats. However, only responses to 0.1 to 1 microM noradrenaline were significantly potentiated in old animals. In addition, 94% of the vessels from young, but only 52% from old rats were relaxed by 80-100% of the noradrenaline (0.03 microM) contraction, with 1 microM acetylcholine. 6. No modifications in the area (micron2) or in the number of endothelial nuclei (per mm2) were observed between age groups. An elongation of the nuclei of endothelial cells was observed in the old animals. 7. These data suggest that the noradrenaline-induced contraction is decreased in old rats probably due to differences in either the contractile machinary or postreceptor mechanisms. These alterations may be accompanied by an impairment of the release or production of NO from endothelial cells.

Effects of L-NG-nitro-arginine on noradrenaline induced contraction in the rat anococcygeus muscle

1. The influence of L-NG-nitro-arginine (L-NOARG, 30 microM) on contractile responses to exogenous noradrenaline was studied in the rat anococcygeus muscle. 2. Noradrenaline (0.1-100 microM) contracted the muscle in a concentration-dependent manner. L-NOARG (30 microM) had no effect on noradrenaline responses. 3. Phenoxybenzamine (Phz 0.1 microM) depressed by 46% (P < 0.001) the maximum response and shifted to the right (P < 0.001) the E/[A] curve to noradrenaline (pEC50 control: 6.92 +/- 0.09; pEC50 Pbz: 5.30 +/- 0.10; n = 20). 4. The nested hyperbolic null method of analysing noradrenaline responses after phenoxybenzamine showed that only 0.61% of the receptors need to be occupied to elicit 50% of the maximum response, indicating a very high functional receptor reserve. 5. Contractile responses to noradrenaline after partial alpha 1-adrenoceptor alkylation with phenoxybenzamine (0.1 microM) were clearly enhanced by L-NOARG. 6. The potentiating effect of L-NOARG on noradrenaline responses after phenoxybenzamine was reversed by (100 microM) L-arginine but not by (100 microM) D-arginine. 7. These results indicate that spontaneous release of NO by nitrergic nerves can influence the alpha -adrenoceptor-mediated response to exogenous noradrenaline.

Endothelial modulation of alpha 1-adrenoceptor contractile responses in the tail artery of spontaneously hypertensive rats

1. Vascular contraction induced by phenylephrine was studied in tail artery rings from spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY) with particular focus on the role of endothelium. The influence of receptor reserve and the density of alpha 1-adrenoceptors on the possible differences observed were also analysed. 2. Phenylephrine (0.01-100 microM) induced concentration-dependent vasoconstrictions. The maximum response (alpha, P < 0.001) was greater but the pEC50 (P < 0.05) smaller in rings from SHR than from WKY rats irrespective of the presence or absence of endothelium. 3. Removal of endothelial cells resulted in a decrease of the maximum contraction with no modification in the pEC50 in arteries from both WKY and SHR. 4. The density of alpha 1-adrenoceptors (Bmax) and the dissociation constant (KD) were found to be the same for preparations from SHR and WKY rats in [3H]-prazosin binding experiments. 5. The apparent affinity (pKA) determined by the nested hyperbolic method and the operational model was similar in tail arteries from the two rat strains, irrespective of the presence or absence of endothelium. However, in endothelium-denuded rings, the pKA value was enhanced when compared with intact rings, in both SHR and WKY rats. 6. In rings from hypertensive rats, the operational parameter maximum possible effect (Em) was greater and the agonist efficacy (tau) was smaller than in rings from normotensive rats. When the endothelium was removed log tau and Em diminished in preparations from both rat strains. 7. In summary, the increased maximum responsiveness to phenylephrine in rings from SHR could be due to enhancement in Em. The log tau values indicate a deterioration in the transduction of the stimulus provided by the agonist in tail arteries from hypertensive animals. This study also suggests that the absence of endothelium modifies the alpha 1-adrenoceptor-mediated vasoconstriction probably by altering the transduction signalling mechanisms. The importance of analysing the degree of endothelium functionality when comparing results from different groups of rats is stated.

Sources of calcium and alpha 1-adrenoceptor-mediated contraction in rat tail artery

1. The relative importance of intracellular and extracellular Ca2+ on alpha 1-adrenoceptor-mediated contraction by noradrenaline and St-587 has been studied and correlated with the binding characteristics in intact tail artery from Sprague-Dawley rats. 2. Noradrenaline and St-587 behaved as full agonists inducing a concentration-dependent vasoconstriction. 3. Nifedipine (1 microM and 10 microM) blocked by 50% (P < 0.001) and 75% (P < 0.001) respectively, the maximum contraction (Emax) induced by St-587. Nevertheless, to reach 40% inhibition of Emax on noradrenaline responses (P < 0.01), 10 microM nifedipine was necessary. 4. Both agonists induced a concentration-dependent accumulation of inositol phosphates. Noradrenaline behaved as a full agonist and St-587 as a partial agonist for this response. 5. [3H]-prazosin binding to intact tail artery rings was saturable and of high affinity (KD = 4.44 +/- 0.46 nM; Bmax = 36.35 +/- 4.22 fmol mg-1 tissue). 6. Competition curves for inhibition of specific [3H]-prazosin binding by WB-4101 suggest that the rat tail artery contains two alpha 1-adrenoceptor subtypes in an approximate ratio of 60:40. 7. After irreversible alkylation of alpha 1B-adrenoceptors with 100 microM chloroethylclonidine (CEC), nifedipine (1 microM) influenced to a greater extent the St-587- than the noradrenaline-induced contraction. 8. Our results indicate that the degree of participation of intracellular and extracellular Ca2+ sources, on the alpha 1-adrenoceptor-mediated contraction, depends on the agonist used. The two alpha 1-adrenoceptor subtypes observed in binding experiments seem to be unrelated to the Ca2+ sources used for contraction.