Blood pressure and vascular effects of endothelin blockade in chronic nitric oxide-deficient hypertension

Endothelium-specific amyloid precursor protein deficiency causes endothelial dysfunction in cerebral arteries

The exact physiological function of amyloid-β precursor protein (APP) in endothelial cells is unknown. Endothelium-specific APP-deficient (eAPP^-/-) mice were created to gain new insights into the role of APP in the control of vascular endothelial function. Endothelium-dependent relaxations to acetylcholine were significantly impaired in basilar arteries of global APP knockout (APP^-/-) and eAPP^-/- mice ( P < 0.05). In contrast, endothelium-independent relaxations to nitric oxide (NO)-donor diethylamine-NONOate were unchanged. Western blot analysis revealed that protein expression of endothelial nitric oxide synthase (eNOS) was significantly downregulated in large cerebral arteries of APP^-/- mice and eAPP^-/- mice as compared to respective wild-type littermates ( P < 0.05). Furthermore, basal levels of cyclic guanosine monophosphate (cGMP) were also significantly reduced in large cerebral arteries of APP-deficient mice ( P < 0.05). In contrast, protein expression of prostacyclin synthase as well as levels of cyclic adenosine monophosphate (cAMP) was not affected by genetic inactivation of APP in endothelial cells. By using siRNA to knockdown APP in cultured human brain microvascular endothelial cells we also found a significant downregulation of eNOS mRNA and protein expressions in APP-deficient endothelium ( P < 0.05). These findings indicate that under physiological conditions, expression of APP in cerebral vascular endothelium plays an important protective function by maintaining constitutive expression of eNOS .

The trilateral link between anaesthesia, perioperative visual loss and Flammer syndrome

Background

A variety of factors have been linked to perioperative visual loss during or directly after nonocular and ocular surgeries. Prolonged immobilization, biochemical factors and hemodynamic instability have been discussed as factors in the pathogenesis of this devastating complication. Perioperative visual loss in four consecutive patients, all featuring Flammer syndrome, is reported herein. To our knowledge, we present the first case series, which associates perioperative visual loss with Flammer syndrome. We assume that a low perfusion pressure, disturbed autoregulation of the ocular blood flow and altered drug sensitivity in such subjects, play significant role in the pathogenesis of this dreaded complication.

Cases presentation

We analysed the medical records of four consecutive patients with permanent perioperative visual loss and complemented our findings with additional history taking and clinical examinations. A variety of tests was performed, including colour Doppler ultrasonography of the retroocular vessels, static and dynamic retinal vessel analysis. The visual loss was unilateral in three patients and bilateral in one. An extensive review of published perioperative vision loss cases was conducted.

All four patients were male Caucasians, and exhibited prominent signs and symptoms of Flammer syndrome. The visual loss originated from a propensity for unstable ocular blood flow, combined with hyperreactivity toward pharmacological stimuli, leading together to disturbed autoregulation of the blood supply, and subsequently - to ocular hypoxia. An identified intrinsic hypoperfusion diathesis was a crucial pathophysiologic link in all of the patients. Other, yet unknown systemic or local factors may also be involved in this process.

Conclusions

A review of numerous publications of perioperative visual loss and our data, support our hypothesis for a novel pathophysiologic model and incorporate Flammer syndrome as a distinct risk factor for paradoxical visual loss, during nonocular and ocular surgeries, or invasive procedures. To prevent the complications produced by disturbed blood flow autoregulation in such patients, guidelines for screening and tailored preoperative approach are given.

End o' the line revisited: moving on from nitric oxide to CGRP

When endothelin-1(ET-1) was discovered it was hailed as the prototypical endothelium-derived contracting factor (EDCF). However, over the years little evidence emerged convincingly demonstrating that the peptide actually contributes to moment-to-moment changes in vascular tone elicited by endothelial cells. This has been attributed to the profound inhibitory effect of nitric oxide (NO) on both the production (by the endothelium) and the action (on vascular smooth muscle) of ET-1. Hence, the peptide is likely to initiate acute changes in vascular diameter only under extreme conditions of endothelial dysfunction when the NO bioavailability is considerably reduced if not absent. The present essay discusses whether or not this concept should be revised, in particular in view of the potent inhibitory effect exerted by calcitonin gene related peptide (CGRP) released from sensorimotor nerves on vasoconstrictor responses to ET-1.

Endothelium and its alterations in cardiovascular diseases: life style intervention

The endothelium, which forms the inner cellular lining of blood vessels and lymphatics, is a highly metabolically active organ that is involved in many physiopathological processes, including the control of vasomotor tone, barrier function, leukocyte adhesion, and trafficking and inflammation. In this review, we summarized and described the following: (i) endothelial cell function in physiological conditions and (ii) endothelial cell activation and dysfunction in the main cardiovascular diseases (such as atherosclerosis, and hypertension) and to diabetes, cigarette smoking, and aging physiological process. Finally, we presented the currently available evidence that supports the beneficial effects of physical activity and various dietary compounds on endothelial functions.

Signal transduction and modulating pathways in tryptamine-evoked vasopressor responses of the rat isolated perfused mesenteric bed

Tryptamine is an endogenous and dietary indoleamine-based trace amine implicated in cardiovascular pathologies, including hypertension, migraine and myocardial infarction. This study aimed at identifying the signalling pathways for the vasoconstrictor response to tryptamine in rat isolated perfused mesenteric arterial beds and co-released vasodilator modulators of tryptamine-mediated vasoconstriction. Tryptamine caused concentration-dependent vasoconstriction of the mesenteric bed, measured as increases in perfusion pressure. These were inhibited by the 5-HT_2A receptor antagonist, ritanserin, indicating mediation via 5-HT_2A receptors. The response was inhibited by the phospholipase C (PLC) and phospholipase A₂ (iPLA₂) inhibitors, U-73122 and PACOCF₃, suggesting involvement of phospholipase pathways. Activation of these pathways by tryptamine releases cyclooxygenase (COX) products since indomethacin (non-selective inhibitor of COX-1/2) and nimesulide (selective COX-2 inhibitor) reduced the vasoconstriction. The most likely COX vasoconstrictor product was prostaglandin PGE₂ since the responses to tryptamine were reduced by AH-6809, a non-selective EP₁ receptor antagonist. Involvement of the Rho-kinase pathway in the tryptamine-evoked vasoconstriction was also indicated by its reduction by the Rho-kinase inhibitors, Y-27,632 and fasudil. The tryptamine vasoconstriction is modulated by the co-released endothelial vasodilator, nitric oxide. Thus, circulating tryptamine can regulate mesenteric blood flow through a cascade of signalling pathways secondary to stimulation of 5-HT_2A receptors.

The thromboxane/endoperoxide receptor (TP): the common villain

The stimulation of thromboxane/endoperoxide receptors (TP) elicits diverse physiological/pathophysiological reactions, including platelet aggregation and contraction of vascular smooth muscle. Furthermore, the activation of endothelial TP promotes the expression of adhesion molecules and favors adhesion and infiltration of monocytes/macrophages. In various cardiovascular diseases, endothelial dysfunction is predominantly the result of the release of endothelium-derived contracting factors that counteract the vasodilator effect of nitric oxide produced by the endothelial nitric oxide synthase. Endothelium-dependent contractions involve the activation of cyclooxygenases, the production of reactive oxygen species along with that of endothelium-derived contracting factors, which diffuse toward the vascular smooth muscle cells and activate their TP. TP antagonists curtail the endothelial dysfunction in diseases such as hypertension and diabetes, are potent antithrombotic agents, and reduce vascular inflammation. Therefore, TP antagonists, because of this triple activity, may have a unique potential for the treatment of cardiovascular disorders.

The endocrine system in chronic nitric oxide deficiency

The experimental model of chronic inhibition of nitric oxide (NO) production has proven to be a useful tool to study cardiovascular and renal lesions produced by this type of hypertension, which are similar to those found in human hypertension. It also offers a unique opportunity to study the interaction of NO with the humoral systems, known to have a role in the normal physiology of vascular tone and renal function. This review provides a thorough and updated analysis of the interactions of NO with the endocrine system. There is special focus on the main vasoactive factors, including the renin-angiotensin-aldosterone system, catecholamines, vasopressin, and endothelin among others. Recent discoveries of crosstalk between the endocrine system and NO are also reported. Study of these humoral interactions indicates that NO is a molecule with ubiquitous function and that its inhibition alters virtually to all other known regulatory systems. Thus, hypothyroidism attenuates the pressor effect of NO inhibitor N-nitro-L-arginine methyl ester, whereas hyperthyroidism aggravates the effects of NO synthesis inhibition; the sex hormone environment determines the blood pressure response to NO blockade; NO may play a homeostatic role against the prohypertensive effects of mineralocorticoids, thyroid hormones and insulin; and finally, NO deficiency affects not only blood pressure but also glucose and lipid homeostasis, mimicking the human metabolic syndrome X, suggesting that NO deficiency may be a link between metabolic and cardiovascular disease.

The potential of endothelin antagonism as a therapeutic approach

Endothelin (ET) is a pivotal physiological regulator of blood pressure through its effects on blood vessels, heart, lung and kidneys, and the ET system can be overactive in disorders such as pulmonary hypertension, heart failure and renal disease. Such observations stimulated interest among scientists and pharmaceutical companies that have set up high-throughput screens to search for antagonists of ET receptors. The emerging compounds have been tested in animals with exciting results, leading to great hope that such inhibitors could be translated into human drugs with desirable therapeutic activities and few side effects. This review will describe the most relevant results obtained in experimental animals in a wide variety of disease conditions and focus on the data of selected compounds that have been employed in clinical trials.

Nitric oxide synthase-inhibition hypertension is associated with altered endothelial cyclooxygenase function

We reported previously that endothelium-intact superior mesenteric arteries (SMA) from Nω-nitro-l-arginine (l-NNA)-treated hypertensive rats (LHR) contract more to norepinephrine (NE) than SMA from control rats. Others have shown that nitric oxide (NO) synthase (NOS) inhibition increases cyclooxygenase (COX) function and expression. We hypothesized that augmented vascular sensitivity to NE in LHR arteries is caused by decreased NOS-induced dilation and increased COX product-induced constriction. We observed that the EC50 for NE is lower in LHR SMA compared with control SMA (control −6.37 ± 0.04, LHR −7.89 ± 0.09 log mol/l; P < 0.05). Endothelium removal lowered the EC50 (control −7.95 ± 0.11, LHR −8.44 ± 0.13 log mol/l; P < 0.05) and increased maximum tension in control (control 1,036 ± 38 vs. 893 ± 21 mg; P < 0.05) but not LHR (928 ± 30 vs. 1,066 ± 31 mg) SMA. Thus augmented NE sensitivity in LHR SMA depends largely on decreased endothelial dilation. NOS inhibition (l-NNA, 10−4 mol/l) increased maximum tension and EC50 in control arteries but not in LHR arteries. In contrast, COX inhibition decreased maximum tension in control arteries, suggesting that COX products augment contraction. Indomethacin did not affect NE-induced contraction in l-NNA-treated or denuded arteries. In control SMA loaded with the fluorescent NO indicator 4-amino-5-methylamino-2′,7′-difluorofluorescein diacetate, indomethacin increased and l-NNA decreased NO release. Therefore, COX products appear to inhibit NO production to augment NE-induced contraction. With chronic NOS inhibition, this modulating influence is greatly diminished. Thus, in NOS-inhibition hypertension, decreased activity of both COX and NOS pathways profoundly disrupts endothelial modulation of contraction.

Nitric oxide inhibition accelerates hypertension and induces perivascular inflammation in rats

1. Inflammatory changes in peripheral arteries have been reported in animal models of hypertension. Whether they occur in cerebral arteries (CA) with hypertension induced by deprivation of endogenous nitric oxide (NO) remains unknown. 2. In the present study, we compared the arteriolar injury score (AIS) and perivascular inflammation in CA between hypertensive and normotensive rats following NO deprivation with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Five-week-old male spontaneously hypertensive rats (SHR) and Wistar -Kyoto (WKY) rats were fed with L-NAME (1 mg/mL) for 4 weeks. 3. Nitric oxide deprivation resulted in time-dependent elevations in tail-cuff pressure (representing systolic blood pressure (SBP)) in both SHR and WKY rats. The magnitude of increase in SBP was larger in SHR (+81.0 +/- 3.2 vs+25.0 +/- 2.2 mmHg; P < 0.01). Arteriolar hyalinosis and AIS in various segments of the CA were assessed with periodic acid-Schiff staining and inflammatory cells were immunostained with the antibody against macrophage/monocyte marker (ED1). The ED1+ cells appeared in the middle CA of L-NAME-treated SHR as early as 2 weeks after treatment. These cells were not observed in L-NAME-treated WKY rats and untreated SHR. More ED1+ cells were found in L-NAME-treated SHR than L-NAME-treated WKY rats after 4 weeks treatment. 4. The AIS and number of ED1+ cells around the perivascular area of the internal carotid artery were significantly higher in L-NAME-treated compared with untreated rats (AIS: 137 +/- 28 vs 46 +/- 10 for WKY rats, respectively; 169 +/- 18 vs 53 +/- 6 for SHR, respectively (P < 0.01); ED1+ cells: 7.9 +/- 0.6 vs 1.3 +/- 0.9 for WKY rats, respectively; 13.6 +/- 2.7 vs 2.1 +/- 0.9 for SHR, respectively (P < 0.01)), although SBP was higher in untreated SHR than in L-NAME-treated WKY rats (170 +/- 4 vs 137 +/- 4 mmHg, respectively; P < 0.05). 5. These findings suggest that ED1+ cells appeared in the middle CA of L-NAME-SHR as early as 2 weeks after treatment. Chronic inhibition of NO accelerates hypertension and induces perivascular inflammation.

Endothelin receptor antagonists in heart failure: current status and future directions

Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of heart failure. Plasma endothelin (ET)-1 levels are increased in patients with heart failure, independent of the aetiology, and correlate with the severity of the disease. Furthermore, tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several experimental models of left ventricular remodelling and/or heart failure, treatment with nonselective ET-A and -B as well as selective ET-A antagonists exerted beneficial cardiovascular effects. In patients with heart failure, short-term studies of treatment with endothelin antagonists demonstrated an improvement of haemodynamic parameters; however, long-term treatment with these drugs did not significantly improve combined morbidity/mortality endpoints. Furthermore, in the recently completed Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) trial in patients with chronic heart failure, the selective ET-A receptor antagonist darusentan did not significantly affect left ventricular remodelling as assessed by cardiac magnetic resonance imaging. Potential reasons for the lack of beneficial effects of long-term treatment with ET antagonists in patients with heart failure include the following. Firstly, adverse effects on left ventricular healing have been observed when endothelin antagonist therapy was introduced early after myocardial infarction in rats. Secondly, the role of the ET-B receptor in the pathophysiology of heart failure and remodelling processes has not been clearly defined. Finally, for the detection of improvement in left ventricular remodelling, a study needs to be conducted in patients with recent myocardial infarction and signs of heart failure.

Time-dependent reduction of acetylcholine-induced relaxation in corpus cavernosum of cholestatic rats: role of nitric oxide and cyclooxygenase pathway

The endothelium-dependent relaxation of corpus cavernosum smooth muscle and the roles of nitric oxide (NO) and arachidonic acid products of cyclooxygenase were investigated in non-operated, SHAM-operated, and bile duct-ligated rats. We further investigated the time-dependent alterations of corpus cavernosum relaxation in 2-, 7-, and 14-day bile duct-ligated animals. Acetylcholine produced concentration-dependent relaxation in phenylephrine-precontracted strips of corpus cavernosum. A significant reduction in the acetylcholine-induced relaxation was observed 2 days after bile duct ligation, and a greater reduction was observed on subsequent days. Incubation with 20 microM indomethacin reduced the acetylcholine-induced relaxation of the corpus cavernosum of unoperated rats while it had no effect in the corpus cavernosum of bile duct-ligated rats. Chronic treatment with Nomega-Nitro-L-Arginine Methyl Ester (L-NAME, 3 mg/kg/day, intraperitoneally) reduced the relaxation responses in the unoperated group while it had no effect in the bile duct-ligated group. These results show that acetylcholine-induced corporal relaxation is impaired in cholestatic rats, and this may be related to deficient nitric oxide production by the endothelium. The involvement of prostaglandins in this impairment seems unlikely.

The Distinct Alterations Produced in Cardiovascular Functions by Prednisolone and Nitro-prednisolone (NCX-1015) in the Rat Highlight a Causal Role for Endothelin-1

Daily administration of prednisolone, but not the derivative NCX-1015 (or prednisolone 21-[4'-nitrooxymethyl]benzoate), to rats resulted in a time- and dose-dependent increase in mean arterial blood pressure (MABP), significant after 1 week for the dose of 6.9 micromol/kg i.p. (n = 10; P < 0.05), and 3 weeks for the lower dose of 1.38 micromol/kg. A similar dichotomy of behavior was observed with respect to myocardial contractility and renal vascular resistance, in either case augmented by 3-week treatment with prednisolone but not NCX-1015. In contrast, both NCX-1015 and prednisolone reduced plasma levels of corticosterone in a dose- (dose range of 0.69-6.9 micromol/kg i.p.) and time-dependent (1-3 weeks) manner. Similar profiles were obtained for plasma nitrate values, although they were increased selectively after NCX-1015 administration. In contrast, prednisolone, but not NCX-1015, augmented plasma endothelin 1 (ET-1) with a profile that mirrored the changes observed in MABP and renal blood flow. Supply in the drinking water of the ET-1 receptor type A (ETA) antagonist FR139317 [(R-2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]-carbonyl]amino-4-methylpentanoyl]-amino-3-(2-pyridil)propionic] or mixed ETA/B, but not of selective ETB, antagonists prevented the changes produced by a 21-day treatment with prednisolone. In conclusion, this study indicates 1) a lack of occurrence of cardiovascular alterations by nitro-releasing derivative of prednisolone (NCX-1015), and 2) a functional link between prednisolone effects and the endogenous endothelin-1 system.

Cyclooxygenase pathway is involved in the vascular reactivity and inhibition of the Na+, K+-ATPase activity in the tail artery from L-NAME-treated rats

L-NAME (LN) induces hypertension by blocking nitric oxide (NO) synthesis. It produces vascular hyperreactivity to phenylephrine (PHE) associated with a reduced vascular Na+, K+-ATPase activity. The aim of this work was to investigate whether products of the cyclooxygenase pathway are involved in alterations of vascular reactivity and Na+-pump activity in the tail artery from LN-induced hypertension rats. Four groups of rats were used: Control (CT, normotensive), LN (50 mg/kg/day, hypertensive), indomethacin (Indo-4 mg/kg/day, normotensive), and LN plus Indo (LN + Indo, partially prevented hypertension). All drugs were administered in drinking water during 7 days. In isolated rat tail vascular beds; the reactivity to PHE, acetylcholine (ACh), sodium nitroprusside (SNP), the functional activity of the Na+, K+-ATPase (K+-induced relaxation) and the modulation of PHE-induced vasoconstriction by constitutively available NO were evaluated. LN increased vascular sensitivity (pD2) and reactivity (Emax) to PHE and Indo blocked the effect of LN on Emax without changing pD2. Emax and pD2 values for ACh were reduced by LN and partially reverted by Indo. SNP-induced vasodilatation was similar in all groups. LN reduced the activity of Na+, K+-ATPase and Indo prevented LN effects. LN also abolished NO ability to modulate PHE-induced contractions. This effect was partially prevented by Indo suggesting that products from the cyclooxygenase pathway might reduce NO actions. Indo itself did not affect vascular reactivity to PHE, ACh or SNP or the Na+,K+-ATPase activity. Results suggested that products from cyclooxygenase pathway are involved in the genesis or maintenance of LN-induced hypertension, playing a role in the increased vascular reactivity, in the reduction of the endothelium-dependent relaxation and in the inhibition of the functional activity of the Na+, K+-ATPase.

The role of hydrogen sulfide generation in the pathogenesis of hypertension in rats induced by inhibition of nitric oxide synthase

OBJECTIVE

The present study intended to investigate whether the impaired H2S synthase/H2S pathway is associated with hypertension.

METHODS

Hypertension in Wistar rats was induced by the oral administration of the l-arginine analog, NG-nitro-l-arginine methyl ester (l-NAME) in their drinking water for a period of 6 weeks. The control rats were given plain tap water only. Sodium hydrosulfide (NaHS) was given by intraperitoneal injection to both the control group and the l-NAME-treated group. The systolic BP (blood pressure) was measured by a tail-cuff method using a pulse transducer. Plasma hydrogen sulfide (H2S), and H2S generation by thoracic aorta and superior mesenteric artery, were determined. In addition, the activity of cystathionine-gamma-lyase (CSE) in thoracic aorta and superior mesenteric artery, most responsible for H2S production, was also measured. Competitive reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine CSE mRNA in thoracic aorta.

RESULTS

l-NAME caused a time-dependent elevation of systolic BP. The heart-to-body weight ratio of l-NAME-treated rats was 27% higher than that of controls. The systolic BP in the NaHS-treated l-NAME group was significantly decreased, by 19% (P < 0.01), in comparison with the l-NAME group. The heart-to-body weight ratio decreased significantly by 12%. l-NAME inhibited CSE gene expression significantly. The inhibition of H2S generation and CSE activity by l-NAME was greatly attenuated in the NaHS-treated l-NAME group. However, there was no significant difference in nitric oxide (NO) generation between the l-NAME group and the NaHS-treated l-NAME group.

CONCLUSION

In summary, dysfunction of the vascular H2S synthase/H2S pathway was found in l-NAME-induced hypertensive rats. Exogenous H2S effectively prevented the development of hypertension induced by l-NAME. These findings suggest that the H2S synthase/H2S pathway participates in hypertension.

ERK1/2-mediated vasoconstriction normalizes wall stress in small mesenteric arteries during NOS inhibition in vivo

As in essential hypertension, chronic nitric-oxide synthase (NOS) inhibition leads to hypertrophic remodeling in conduit and muscular arteries and inward eutrophic remodeling in small resistance arteries with activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in both vessel types. The authors tested the hypothesis that this remodeling heterogeneity could be related to distinct vasoreactivity patterns in small and larger arteries, with a vessel-specific function of ERK1/2 signaling. Using intravital microscopy in rats we have demonstrated that acute NOS inhibition (l-NA injection, 100 mg/kg) produced vasoconstriction of small mesenteric arteries. Consequently, the calculated in vivo wall stress was not significantly modified, despite the local rise in pressure. This could explain the lack of vascular protein synthesis elevation in vivo, an early index of hypertrophy. Inhibition of ERK1/2 activation with PD98059 blunted mesenteric artery contractions. Femoral arteries did not contract and were thus submitted to an enhanced wall stress and underwent hypertrophic remodeling in chronic conditions. In conclusion, the heterogeneous vascular remodeling in the l-NAME model is associated with a heterogeneous vasoconstriction response to acute NOS inhibition. Indeed, in contrast to larger arteries, l-NA-induced vasoconstriction in small arteries normalized wall stress and prevented early signs of hypertrophy. The results also suggest that ERK1/2 is a signaling element in NOS inhibition-induced vasoconstriction of small arteries in vivo.

Role of endothelins in animal models of hypertension: focus on cardiovascular protection

Investigation of the regulation of vascular function by endothelium-derived factors has been a prominent topic of research in the field of hypertension during the last decade. Of the different endothelial factors, endothelins, which play an important role in vasodilatation-vasoconstriction balance, have been the subject of great interest and an impressive number of publications. This peptide, a very potent vasoconstrictor, triggers as well events involved in growth, proliferation, matrix production and local inflammation. In parallel, its role in hypertension has evolved from a simple vasoconstrictor to a central local regulator of vascular homeostasis contributing not only to the elevation of blood pressure, but also to the complications of hypertension. This review summarizes research on endothelins and its receptor antagonists in experimental hypertension, with special emphasis on vascular remodeling and target-organ protection.

Reduced Endothelial Vasomotor Function and Enhanced Neointimal Formation after Vascular Injury in a Rat Model of Blood Pressure Lability

Increased short-term blood pressure variability is known to be associated with hypertensive target organ damage. Sinoaortic denervation (SAD) induces a marked increase in blood pressure lability without affecting the average blood pressure level. The aim of this study was to investigate the effects of blood pressure lability on endothelial vasomotor function and neointimal formation after balloon injury in SAD rats. Direct longterm measurement of mean arterial pressure showed no significant difference in the average of mean arterial pressure between the SAD group and sham-operated control group. In contrast, the standard deviation of mean arterial pressure, as an index of blood pressure lability, was 3-fold greater in SAD rats. To study endothelial function, isometric tension of aortic rings was measured 4 weeks after SAD or sham operation. Endothelium-dependent vasorelaxation induced by acetylcholine was significantly reduced in the SAD group (20% reduction at maximum relaxation). Endothelium-independent vasorelaxation induced by sodium nitroprusside was similar in each group. Acetylcholine-induced NO release from aortic rings was significantly reduced in the SAD group. Next, we examined neointimal formation in carotid arteries in SAD and sham-operated rats at 2 weeks after balloon injury. The neointimal-to-medial area ratio in the SAD group was 50% higher than that in the sham-operated group. The percentage of proliferating cell nuclear antigen-positive cells in the intima was significantly higher in the SAD group. These findings suggest that increased blood pressure lability, independently of average blood pressure level, impairs endothelial function by inhibiting NO production, enhances neointimal formation after balloon injury, and may thereby contribute to atherogenesis.

Protective effects of the flavonoid quercetin in chronic nitric oxide deficient rats

OBJECTIVES

The present study analysed, for the first time, the effects of the flavonoid quercetin in rats after chronic inhibition of nitric oxide (NO) synthesis with Nomega-nitro-L-arginine methyl ester (L-NAME).

DESIGN

Rats were divided randomly into five different treatment groups for 6 weeks: (1) vehicle (control, 1 ml of 1% methylcellulose once daily); (2) vehicle plus L-NAME (75 mg/100 ml in drinking water); (3) quercetin (10 mg/kg p.o. once daily); (4) quercetin (5 mg/kg p.o.) plus L-NAME; and (5) quercetin (10 mg/kg p.o.) plus L-NAME.

METHODS

The evolution of systolic blood pressure, morphological variables, proteinuria, plasma malondialdehyde and nitrite and nitrate concentrations, hepatic glutathione and malondialdehyde content, glutathione enzymes activity and vascular reactivity at the end of the experiment were analysed.

RESULTS

Quercetin markedly inhibited the development of L-NAME-induced hypertension. This effect was accompanied by a partial or full prevention of most of the effects induced by L-NAME, such as: (1) increases in the left ventricular and kidney weight indices; (2) proteinuria; (3) renal histological lesions, including hyaline arteriopathy and thickening of the vascular wall with moderate decrease of the lumen; (4) increased endothelium-dependent contraction; (5) increased vascular thromboxane B2 (TXB2) synthesis; (6) reduced plasma concentrations of nitrites plus nitrates (NOx); (7) increased plasma and hepatic malondialdehyde (MDA) concentrations; and (8) reduced glutathione peroxidase activity. In most cases these effects were dose dependent, but none of them were observed in normotensive animals.

CONCLUSIONS

This study confirms and extends the previous evidence about the antihypertensive effects and end-organ protection of the flavonoid quercetin in animal models of hypertension.

Therapeutic role of bosentan in hypertension: lessons from the model of perinephritic hypertension

Since its discovery in 1988, there has been increasing evidence that endothelin-1 (ET-1) plays an important role in the pathophysiology of hypertension and its related end-organ damages. First studies, using ET-1 administration in animals or in humans suspected this role by demonstrating the hypertensive properties of ET-1. The latter, due to stimulation of ET(A) receptors inducing sustained vasoconstriction have been reported to follow transient vasodilation linked with activation of an endothelial ET(B) receptor releasing nitric oxide (NO). In certain instances, ET(B) smooth-muscle receptors might also induce contraction. Cloning of these receptors helped to develop ET-1 receptor antagonists. As soon as one of them became available, bosentan, a dual (ET(A) and ET(B)) ET-1 receptor antagonist, we tested its effects in the canine model of perinephritic hypertension. Bosentan was found to exert striking hypotensive effects, due to peripheral vasodilation but without affecting cardiac function. In further experiments, we observed that effects of bosentan were additional to those of ACE inhibitors or angiotensin II antagonists. This opened new therapeutic perspectives and also suggested a proper role of ET-1 in hypertension, independent of the renin-angiotensin system. To explain this role, we demonstrated a real imbalance characterized by an impairment of the NO system in favor of the ET-1 pathway. Recent studies suggest that such an imbalance may also occur in human hypertension. Furthermore, the contribution of ET-1 to human hypertension appears more convincing since bosentan was shown to decrease blood pressure in hypertensive subjects. Finally, ET-1 receptor antagonists might be of therapeutic interest to prevent hypertension induced end-organ damages. Whether or not these compounds are able to prevent or to reverse target organ injuries in man remains to be investigated.

Crosstalk between endothelin and nitric oxide in the control of vascular tone

Several lines of evidence indicate that nitric oxide (NO) impairs endothelin (ET) production/action in vitro. Acute pressor responses caused by the blockade of NO formation with arginine analogues in vivo are blunted by selective ET(A) or dual ET(A)/ET(B) receptor blockade whereas blockade of NO formation magnifies ET-induced constriction of various vascular territories. Given that ET receptor blockade has normally limited effects on mean arterial pressure, the reversal of pressor responses caused by the blockade of NO formation with ET receptor blockade most likely reflects a significant crosstalk between NO and ET. Suppression of NO formation also leads to significant increases in ET production caused by agents targeting the endothelium, such as acetylcholine and thrombin. In addition, the inhibitory effect of shear stress on endothelial cells ET production also involves NO as an intermediate.Paradoxically, chronic exposure to organic nitrates which causes nitrate tolerance leads to an augmented vascular ET content. An increased angiotensin II (AII) production is apparently pivotal in this process. This article reviews observations pointing to the importance of NO/ET interactions as a fundamental and common regulatory mechanism shared across species. As a consequence of this crosstalk between NO and ET, experimental strategies designed to assess endothelial NO-dependent activity by the blockade of NO formation may be mitigated by magnified ET-dependent influences.

Endothelin A receptor antagonists in congestive heart failure: blocking the beast while leaving the beauty untouched?

Congestive heart failure (CHF) is a disease process characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3, and ET-4, which cause vasoconstriction, cell proliferation, and myocardial effects through activation of ET(A) receptors. In contrast, endothelial ET(B) receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ET(A) receptor antagonist into the brachial artery in healthy humans leads to vasodilation whereas infusion of an ET(B) receptor antagonist causes vasoconstriction. ET-1 plasma levels are elevated in CHF and correlate both with the hemodynamic severity and with symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death in patients after myocardial infarction and with CHF. ET-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia, and renal impairment in CHF. Selective ET(A) as well as combined ET(A/B) receptor antagonists have been studied in patients with CHF showing impressive hemodynamic improvements (i.e. reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET receptor antagonists indeed have a potential to improve hemodynamics, symptoms, and potentially prognosis of CHF which still carries a high mortality.

An update on the status of endothelin receptor antagonists for hypertension

Endothelin receptor antagonists (ETRA) are actively developed by the pharmaceutical industry for several cardiovascular indications. In the context of hypertension, preclinical studies are increasingly focused on prevention or regression of end-organ damage and drug combination than on control of arterial pressure in monotherapy, as most experimental models have already been studied. In general, the antihypertensive effect of ETRA is limited but the overwhelming efficacy of this class of drugs to prevent several end-organ damages warrants judicious combination. However, the few studies looking at regression of hypertension-induced cardiovascular alterations proved less successful, suggesting that ETRA should be used early in the treatment of hypertension to obtain full benefit. Judging from the progression of ongoing trials and the development of new trials patients suffering from pulmonary hypertension and heart failure may be the first to benefit from this new class of drugs. However, it is expected that once on the market, responsive subsets of hypertensive patients will be identified and will benefit from end-organ protection.

Current strategies and perspectives for correcting endothelial dysfunction in atherosclerosis

The vascular endothelium synthesizes and releases a spectrum of vasoactive substances such as nitric oxide and endothelin. In atherosclerosis, the delicate balance between endothelium-derived factors is disturbed. Endothelin acts as the natural counterpart to endothelium-derived nitric oxide, which exerts vasodilating, antithrombotic and antiproliferative effects, and inhibits leukocyte adhesion to the vascular wall. Besides its blood pressure increasing effect in man, endothelin also induces vascular and myocardial hypertrophy, which are independent risk factors for cardiovascular morbidity and mortality. The derangement of endothelial function in atherosclerosis is likely to be caused in part by genetic factors, but is also due to cardiovascular risk factors. Endothelial dysfunction in atherosclerosis is crucial for the development of the disease process in the vasculature and is therefore an important therapeutic target. However, the efficacy of pharmacotherapy aimed at an improvement in endothelial function depends on the individual risk factor profile of the patient.

Endothelin receptor antagonists and cardiovascular diseases of aging

Our understanding of the role of the endothelin system in human cardiovascular physiology and pathophysiology has evolved very rapidly since the initial description of its constituent parts in 1988. Endothelin-1 (ET-1) is the predominant endothelin isoform in the human cardiovascular system and has potent vasoconstrictor, mitogenic and antinatriuretic properties which have implicated it in the pathophysiology of a number of cardiovascular diseases. The effects of ET-1 have been shown to be mediated by 2 principal endothelin receptor subtypes: ET(A) and ET(B). The development of a range of peptidic and nonpeptidic endothelin receptor antagonists represents an exciting breakthrough in human cardiovascular therapeutics. Two main classes of endothelin receptor antagonist have been developed for possible human therapeutic use: ET(A)-selective and nonselective antagonists. Extensive laboratory and clinical research with these agents has highlighted their promise in various cardiovascular diseases. Randomised, placebo-controlled clinical trials have yielded very encouraging results in patients with hypertension and chronic heart failure with more preliminary data suggesting a possible role in the treatment and prevention of atherosclerosis and stroke. Much more research is needed, however, before endothelin receptor antagonists can be considered for clinical use.

Regression by ACE inhibition of arteriosclerotic changes induced by chronic blockade of NO synthesis in rats

We previously reported that chronic inhibition of nitric oxide (NO) synthesis with N(omega)-nitro-L-arginine methyl ester (L-NAME) induces vascular inflammation at week 1 and produces subsequent arteriosclerosis at week 4 and that cotreatment with an angiotensin-converting enzyme (ACE) inhibitor prevents such changes. In the present study, we tested the hypothesis that treatment with an ACE inhibitor after development of vascular inflammation could inhibit arteriosclerosis in rats. Wistar-Kyoto rats were randomized to four groups: the control group received no drugs, the 4wL-NAME group received L-NAME (100 mg x kg(-1) x day(-1)) for 4 wk, the 1wL + 3wNT group received L-NAME for 1 wk and no treatment for the subsequent 3 wk, and the 1wL + 3wACEI group received L-NAME for 1 wk and the ACE inhibitor imidapril (20 mg x kg(-1) x day(-1)) for the subsequent 3 wk. After 4 wk, we observed significant arteriosclerosis of the coronary artery (medial thickening and fibrosis) and increased cardiac ACE activity in the 1wL + 3wNT group as well as in the 4wL-NAME group, but not in the 1wL + 3wACEI group. In a separate study, we examined apoptosis formation and found that posttreatment with imidapril (20 mg x kg(-1) x day(-1)) or an ANG II AT1-receptor antagonist, CS-866 (5 mg x kg(-1) x day(-1)), induced apoptosis (TdT-mediated nick end-labeling) in monocytes and myofibroblasts appearing in the inflammatory lesions associated with a clear degradation in the heart (DNA electrophoresis). In conclusion, treatment with the ACE inhibitor after 1 wk of L-NAME administration inhibited arteriosclerosis by inducing apoptosis in the cells with inflammatory lesions in this study, suggesting that increased ANG II activity inhibited apoptosis of the cells with inflammatory lesions and thus contributed to the development of arteriosclerosis.

Endothelin receptor antagonists in congestive heart failure: a new therapeutic principle for the future?

Congestive heart failure (CHF) is characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3 and ET-4, which cause vasoconstriction, cell proliferation and myocardial effects through activation of ETA receptors. In contrast, endothelial ETB receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ETB receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ETA-receptor antagonist into the brachial artery in healthy humans leads to vasodilation, whereas infusion of an ETB-receptor antagonist causes vasoconstriction. Endothelin-1 plasma levels are elevated in CHF and correlate both with hemodynamic severity and symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death after myocardial infarction as well as in CHF. Endothelin-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia and renal impairment in CHF. Selective ETA, as well as combined ETA/B-receptor antagonists, have been studied in patients with CHF, and their use has shown impressive hemodynamic improvement (i.e., reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET-receptor antagonists, indeed, have a potential to improve hemodynamics, symptoms and, potentially, prognosis in patients with CHF, which still carries a high mortality.

Effects of blood pressure and glucose on endothelial function

Hypertension and diabetes mellitus are associated with accelerated atherosclerosis and an increased prevalence of cardiovascular disease. Loss of the modulatory role of the endothelium can be considered the link between these conditions and cardiovascular disease. Substantial evidence suggests that vasodilation mediated by endothelium-derived nitric oxide (NO) is impaired in animal models and in patients with hypertension and diabetes mellitus. NO is a principal factor involved in the anti-atherosclerotic properties of the endothelium. Therefore, the pathogenesis of hypertensive and diabetic vascular disease may involve a reduced bioavailability of endothelium-derived NO. Inactivation of NO by reactive oxygen species is an important common mechanism by which endothelial dysfunction may occur. This review summarizes experimental and clinical evidence for impaired NO-mediated vasodilation in the presence of high blood pressure and hyperglycemia. A better understanding of the mechanisms leading to endothelial dysfunction may unmask new preventive strategies to reduce cardiovascular morbidity and mortality in these conditions.

Therapeutic potential for endothelin receptor antagonists in cardiovascular disorders

The endothelins are synthesized in vascular endothelial and smooth muscle cells, as well as in neural, renal, pulmonal, and inflammatory cells. These peptides are converted by endothelin-converting enzymes (ECE-1 and -2) from 'big endothelins' originating from large preproendothelin peptides cleaved by endopeptidases. Endothelin (ET)-1 has major influence on the function and structure of the vasculature as it favors vasoconstriction and cell proliferation through activation of specific ET(A) and ET(B) receptors on vascular smooth muscle cells. In contrast, ET(B )receptors on endothelial cells cause vasodilation via release of nitric oxide (NO) and prostacyclin. Additionally, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Indeed, ET-1 contributes to the pathogenesis of important disorders as arterial hypertension, atherosclerosis, and heart failure. In patients with atherosclerotic vascular disease (as well as in many other disease states), ET-1 levels are elevated and correlate with the number of involved sites. In patients with acute myocardial infarction, they correlate with 1-year prognosis. ET receptor antagonists have been widely studied in experimental models of cardiovascular disease. In arterial hypertension, they prevent vascular and myocardial hypertrophy. Experimentally, ET receptor blockade also prevents endothelial dysfunction and structural vascular changes in atherosclerosis due to hypercholesterolemia. In experimental myocardial ischemia, treatment with an ET receptor antagonist reduced infarct size and prevented left ventricular remodeling after myocardial infarction. Most impressively, treatment with the selective ET(A) receptor antagonist BQ123 significantly improved survival in an experimental model of heart failure. In many clinical conditions, such as congestive heart failure, both mixed ET(A/B )as well as selective ET(A) receptor antagonism ameliorates the clinical status of patients, i.e. symptoms and hemodynamics. A randomized clinical trial showed that a mixed ET(A/B) receptor antagonist effectively lowered arterial blood pressure in patients with arterial hypertension. In patients with primary pulmonary hypertension or pulmonary hypertension related to scleroderma, treatment with a mixed ET(A/B) receptor antagonist resulted in an improvement in exercise capacity. ET receptor blockers thus hold the potential to improve the outcome in patients with various cardiovascular disorders. Randomized clinical trials are under way to evaluate the effects of ET receptor antagonism on morbidity and mortality.

Nitric oxide synthase inhibition promotes carcinogen-induced preneoplastic changes in the colon of rats

l-Arginine is metabolized either to polyamines through arginase and ornithine decarboxylase (ODC) activities or to citrulline and nitric oxide (NO, nitrogen monoxide) through the NO synthase (NOS) pathway. Polyamine levels and ODC activity are high in tumor cells. The aim of this study was to test whether N(G)-nitro-l-arginine methyl ester (l-NAME), an inhibitor of NOS, modulates colon carcinogenesis. Adult male Wistar rats were treated with azoxymethane (AOM, 15 mg/kg ip), a chemical carcinogen, once a week for 2 weeks. One week after the second injection the rats were randomly divided into two groups. One group (n = 8) received l-NAME (10 mg/kg body wt/day) in drinking water. The control group (n = 8) received tap water. After 5 weeks, the rats receiving l-NAME showed enhanced mean basal arterial blood pressure, decreased heart rate, and a significant decrease of the cGMP content in the colonic mucosa. In both groups, AOM induced the formation of colonic aberrant crypt foci (ACF). In l-NAME-treated rats, the number of ACF was higher than in controls by 47%. ODC activity was enhanced by 11-fold. S-Adenosyl-methionine-decarboxylase activity and putrescine concentration were significantly increased in the colonic mucosa of l-NAME-treated rats. The data suggest that l-NAME promotes carcinogen-induced preneoplastic changes in the colon by inhibiting NOS activity and by stimulating polyamine biosynthesis.

Endothelin and blood pressure regulation in the female rat: studies in normal pregnancy and with nitric oxide synthase inhibition-induced hypertension

OBJECTIVE

To evaluate the role of endothelin (ET) in blood pressure regulation in normal pregnant and nonpregnant rats and with nitric oxide synthase (NOS) inhibition.

METHODS

Pregnant and nonpregnant Sprague-Dawley rats were treated for 7 days with an ET(A)-selective (A-127722 or FR-139317), ET(B)-selective (A-192621), or ET(A)/ET(B) nonselective (A-182086) endothelin receptor antagonist, and/or with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 2. 5 mg/kg/h). In pregnant rats, the ET antagonists and L-NAME were administered from gestational day 14 through day 21 (term = 22 days). All rats received indwelling arterial catheters for blood pressure measurement. Mean arterial pressures were recorded on infusion days 1, 4, and 7 and these data were compared by analysis of variance among experimental groups with p < 0.05 considered significant.

RESULTS

The ET(A) receptor antagonism lowered blood pressure in both pregnant and nonpregnant rats (p < 0.05), whereas ET(B) antagonism resulted in hypertension (p < 0.001). ET(B) antagonism-induced hypertension was attenuated by pregnancy (p < 0. 001). Hypertension was induced in all rats treated with L-NAME (p < 0.001). Endothelin receptor antagonism, regardless of specificity, did not ameliorate L-NAME-induced hypertension in pregnant or nonpregnant female rats. The only observed effect of ET(A) antagonism on NOS inhibition-induced hypertension was the prevention of a continued rise at infusion day 7 in nonpregnant rats.

CONCLUSIONS

Endothelin, acting via both the ET(A) and ET(B) receptors, contributes to blood pressure homeostasis in pregnant and nonpregnant rats. Endothelin receptor antagonism does not ameliorate NOS-inhibition-induced hypertension in pregnant rats.

Blood pressure changes after intravenous administration of cell-free hemoglobin A and hemoglobin H in the rat

Hemoglobin H (HbH) is a tetramer of four beta chains (present in erythrocytes of alpha thalassemia), whereas hemoglobin A is a tetramer of two alpha and two beta chains. Since HbH is known to bind four times more nitric oxide (a vasodilator) at its sulfhydryls compared to HbA, the present studies were conducted to see the effect of HbH and HbA on rat blood pressure. The acute administration (20-2000 nmol/kg) of both HbH and HbA produced a dose-dependent effect on blood pressure. The net change in mean arterial pressure was significantly higher with HbH compared to HbA. Partially nitrosylated (in which SH groups are occupied with NO) HbH retained the property of raising blood pressure to some extent while HbA lost this property. Completely nitrosylated (in which both heme and SH groups are occupied with NO) derivatives of both HbH and HbA reduced the blood pressure to the same extent. The preliminary studies with chronic administration of HbA and HbH resulted in nonsignificant increase in blood pressure. It is concluded that these findings may explain the earlier observations of increased risk of hypertension in individuals with alpha thalassemia.

Cyclooxygenase inhibition reduces blood pressure elevation and vascular reactivity dysfunction caused by inhibition of nitric oxide synthase in rats

In the present study we investigated the role of cyclooxygenase (COX)-dependent vasoconstrictors in the hypertension and altered vascular reactivity following prolonged nitric oxide (NO) synthase inhibition. Male Wistar rats (250-270 g) were divided into four groups and treated for 7 days with Placebo (control), L-NAME (48 mg/kg/day), indomethacin (4 mg/kg/day) and L-NAME in combination with indomethacin. L-NAME treatment induced arterial hypertension, in vitro aortic hyperresponsiveness to phenylephrine, impaired vasodilatory response to acetylcholine and no significant change in response to sodium nitroprusside. Indomethacin co-treatment partially prevented blood pressure elevation, restored responsiveness to phenylephrine and improved sensitivity to acetylcholine. Indomethacin treatment alone did not modify blood pressure and aortic vascular reactivity. Both enhanced phenylphrine-induced contraction and impaired acetylcholine-evoked vasodilation induced by acute NO synthase inhibition with L-NAME (10(-4) M) in normal rat aortas were not modified by indomethacin (10(-5) M). These results are consistent with the hypothesis that constricting factors, which arise from the COX pathway, contribute to hypertension and altered vascular reactivity following continued inhibition of NO synthase.

Endothelin receptor antagonists: novel agents for the treatment of hypertension?

Excitement always greets the development of a new class of therapeutic drug, representing as it does the combined efforts of the pharmaceutical industry, research laboratories and clinicians. Endothelin (ET)-receptor antagonists are being actively developed as new therapeutic agents for cardiovascular diseases, and may also be of use in other pathological conditions. Based on early and indirect evidence, ET has been implicated in the pathophysiology of hypertension; the receptor antagonists have been studied quite extensively in this setting at the preclinical level. We now possess direct evidence that such drugs are effective as antihypertensives in some experimental models of hypertension. Furthermore, the ability of ET-receptor antagonists to prevent hypertension-induced end-organ damage is also well documented. Their capacity to reverse already established target organ alterations remains poorly defined. Based on our current preclinical and clinical knowledge, this review presents the anticipated clinical usefulness of these new drugs, both in terms of blood pressure reduction and the protection of target organs.

Evidence for a common defect associated with pressure in the aorta of two hypertensive rat strains

1. Thoracic aortas of normotensive (Wistar-Kyoto (WKY) and Lyon normotensive (LN)) and hypertensive (spontaneously hypertensive rats (SHR) and Lyon hypertensive (LH)) rats from two groups (Japanese (WKY rats and SHR) and Lyon (LN and LH rats)) were compared using organ chambers. Changes in endothelium and smooth muscle reactivity to noradrenaline (NA), carbamylcholine and N omega-nitro-L-arginine (L-NNA) were analysed to distinguish between changes in reactivity that are associated with the presence of hypertension and those that are dependent on group (Japanese vs Lyon). 2. Aortas of hypertensive rats had lower pD2 values for NA than aortas from normotensive rats. These differences were associated with hypertension (P < 0.005 and P < 0.01) and group (P < 0.005 and P < 0.005) in presence or absence of endothelium, respectively, whereas no difference was seen in the maximal developed tension in response to NA. 3. Aortas also differed by a reduced ability to relax in response to carbamylcholine in hypertensive rats; this effect is hypertension (P < 0.05) and group (P < 0.005) dependent, without any change in carbamylcholine pD2 values. 4. Changes in maximum developed tension in the presence of L-NNA were found to be endothelium dependent and pressure and group independent. Furthermore, the change in tension induced by L-NNA appears significantly more pronounced in SHR than in LH rats (P < 0.05). 5. These results indicate that the common defect associated with hypertension appears to be linked to the endothelium through alpha-adrenoceptors and muscarinic receptors in both the Japanese and Lyon groups. However, SHR differs markedly from LH rats by having a higher developed tension in response to NA, this increased tension being counterbalanced by the release of nitric oxide, as observed in the presence of L-NNA.

Differences in aortic response to vasoactive stimuli in Japanese and Lyon rats. The role of hypertension

OBJECTIVE

We have previously shown that conduit arteries of normotensive (WKY) and hypertensive (SHR) Japanese rats differ from normotensive (LN) and hypertensive (LH) Lyon rats in terms of lower aortic thickness and higher collagen III content, whereas differences in vasoactive properties are unknown.

METHODS

Aortic rings with (E+) and without (E-) endothelium were studied under resting and noradrenaline-stimulated conditions in the presence of N(omega)-nitro-L-arginine (L-NNA) alone or in association with indomethacin, bosentan and/or BQ123.

RESULTS

Under resting conditions, aortas of normotensive and hypertensive Japanese rats differed from Lyon rats by higher developed tension in the presence of L-NNA and endothelium. In the absence of endothelium, normotensives differed from hypertensives in terms of stronger developed tensions in the presence of L-NNA in the two strains. Addition of indomethacin to L-NNA induced relaxation in E+ SHR and E- WKY and contraction in E-LH. By contrast, tensions were unchanged after addition of bosentan and BQ123. Under stimulated conditions, tensions were equally increased by L-NNA in E+ and unchanged in E- both in Japanese and Lyon rats whether they were normotensive or hypertensive, and indomethacin (but not bosentan) elicited higher response in Lyon than in Japanese rats in E+ and E- aorta.

CONCLUSION

Under NO synthase inhibition, the vasoactive properties of Japanese and Lyon aorta differ in the presence of a cyclo-oxygenase blocker but not endothelin blockers. These results indicate that the aorta vasorelaxant tone is associated to prostanoid regulation in Lyon but not in Japanese rats. This observation appears dependent on the genetic and/or environmental background linked to the origin and not the presence of hypertension.

Endothelin antagonists for hypertension and renal disease

The endothelin system has been implicated in the pathogenesis of arterial hypertension and renal disorders. Endothelin-1, the predominant isoform of the endothelin peptide family, regulates vasoconstriction and cell proliferation in tissues both within and outside the cardiovascular system through activation of Gi-protein-coupled ET(A) and ET(B) receptors. Endothelin synthesis is regulated through autocrine mechanisms by endothelin converting enzymes, chymases, and non-endothelin converting enzyme metalloproteases. In-vitro experiments have demonstrated that endothelin-1 stimulates growth in vascular smooth muscle and in the kidney. Recent studies indicate that endothelin mRNA and protein are also increased in vivo in the kidney and vasculature in hypertension and renal disease. Studies using molecular or pharmacological inhibition of the endothelin system demonstrate that endothelin-1 contributes to the functional and structural changes associated with arterial hypertension and glomerulosclerosis, and that these effects are only in part dependent on blood pressure. These experimental studies and first clinical trials suggest that endothelin antagonists may offer therapeutic potential to reduce end-organ damage in diseases associated with vascular remodeling and renal injury.

Angiotensin converting enzyme inhibitors and vascular protection in hypertension

Strategically located between the circulating blood and the vascular smooth muscle, endothelial cells release numerous vasoactive substances regulating the function of vascular smooth muscle and circulating blood cells. Endothelium-derived vasodilators include prostacyclin, nitric oxide and endothelium-derived hyperpolarizing factor. In particular, nitric oxide inhibits cellular growth and migration. In concert with prostacyclin, nitric oxide exerts potent antiatherogenic and thromboresistant properties by preventing platelet aggregation and cell adhesion. These effects are counterbalanced by vasoconstrictors, such as angiotensin II and endothelin (ET)-1, both of which exert prothrombotic and growth-promoting properties. In hypertension, elevated blood pressure causes vascular disease by inducing endothelial dysfunction. Hence, modern therapeutic strategies in human hypertension focus on preserving or restoring endothelial integrity. Beyond inhibiting the renin-angiotensin system, angiotensin-converting enzyme (ACE) inhibitors diminish the inactivation of bradykinin, thus leading to an augmentation of nitric oxide release. In addition, the compounds stabilize the B2-receptor, and reduce oxidative stress and tissue ET-1 levels. In patients with coronary artery disease, chronic ACE inhibition improves endothelial function. Further clinical studies are already under way which will prove whether these beneficial vascular effects of ACE inhibitors on endothelial dysfunction result in a clinical benefit for patients with hypertension.

Chronic studies on the interaction between nitric oxide and endothelin in cardiovascular and renal function

1. Chronic inhibition of nitric oxide synthase (NOS) results in a persistent hypertension, while chronic blockade of endothelin ETA receptors has little effect on arterial pressure. These findings indicate that nitric oxide (NO) plays a more significant role than ET-1 in the long-term maintenance of arterial pressure. 2. Although endothelin (ET) appears to contribute to the hypertension in the early stages of NOS inhibition, blockade of either ETA or both ETA and ETB receptors has only a minor effect on the hypertension beyond the initial 2 weeks of NOS inhibition. 3. Endothelin may play a role in vascular lesion development associated with NOS inhibition, at least within the kidney, which may be related to angiotensin II activity. 4. The processes involved in the hypertension associated with chronic NOS inhibition appear to be dynamic and may include an evolution of ET-1 action. Variability in results from different laboratories may be related to genetic factors and choice of pharmacological agents.

Endothelin and its antagonists in hypertension: can we foresee the future?

Endothelin-receptor antagonists may soon become a new therapeutic class of agents used to treat cardiovascular diseases. Although the first clinical trials are anxiously awaited to position this new family of compounds in the treatment of essential or secondary forms of hypertension, we dispose of an impressive amount of studies in which plasma endothelin levels have been measured, in addition to chronic preclinical studies that provide a general picture of what we can expect from these drugs. The experimental models that do respond to endothelin- receptor antagonists share vascular overexpression of endothelin, which seems directly linked with vascular hypertrophy of resistance arteries. In addition, salt sensitivity may represent an unbalanced condition between relaxing and constrictive properties of the renal endothelium that can respond favorably to endothelin blockade. Thus, African-American hypertensives may well be a responsive target population for the new drugs. In addition to blood pressure control, endothelin may also be involved in the evolution of end-organ damage by its potent vasoactive and vasoproliferative properties. The kidney, heart, large arteries and brain may therefore benefit from these drugs, but it is still unclear if this benefit goes beyond what can be attributed to the reduction of arterial pressure. Moreover, most studies performed so far have looked at prevention of end-organ damage, while very few have addressed the clinically relevant question of regression of lesions already installed by the disease process.

Effect of endothelin blockade on pressure natriuresis in nitric oxide-deficient hypertensive rats

OBJECTIVE

Chronic inhibition of nitric oxide synthesis has been shown to cause arterial hypertension and an important blunting of the pressure diuresis and natriuresis response. The mechanisms mediating these abnormalities are not completely established. We therefore studied the effects of endothelin on these alterations.

MATERIALS AND METHODS

Pressure diuretic and natriuretic relationships were evaluated in rats treated chronically (3 weeks) with the nitric oxide synthesis inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 40 mg/kg per day), alone or in combination with bosentan sodium salt (acute treatment: 10 mg/kg, intravenously; chronic treatment: 10 mg/kg per day).

RESULTS

Chronic treatment with L-NAME significantly elevated mean arterial pressure (143.7 +/- 2.8 mmHg versus 102.8 +/- 1.6 in controls), reduced the glomerular filtration rate and renal blood flow and shifted the pressure diuretic and natriuretic responses to the right. Treatment with bosentan, either acute or chronically, did not attenuate the arterial hypertension of the L-NAME-treated rats but normalized the glomerular filtration rate and renal blood flow. In spite of the normalization of renal hemodynamics, the pressure diuretic and natriuretic responses of the bosentan-treated groups were not normalized, although chronic bosentan significantly improved the pressure natriuretic response.

CONCLUSIONS

These results indicate that endothelin participates in the renal hemodynamic and excretory alterations that follow chronic inhibition of nitric oxide synthesis. However, the arterial hypertension is not mediated by endothelin activation.

Functional response of the rat kidney to inhibition of nitric oxide synthesis: role of cytochrome p450-derived arachidonate metabolites

1. We tested the hypothesis that nitric oxide (NO) exerts a tonic inhibitory influence on cytochrome P450 (CYP450)-dependent metabolism of arachidonic acid (AA). 2. N(omega)-nitro-L-Arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase (NOS), increased mean blood pressure (MBP), from 91+/-6 to 137+/-5 mmHg, renal vascular resistance (RVR), from 9.9+/-0.6 to 27.4+/-2.5 mmHg ml(-1) min(-1), and reduced renal blood flow (RBF), from 9.8+/-0.7 to 6.5+/-0.6 ml min(-1)) and GFR from 1.2+/-0.2 to 0.6+/-0.2 ml 100 g(-1) min(-1)) accompanied by diuresis (UV, 1.7+/-0.3 to 4.3+/-0.8 microl 100 g(-1) min (-1)), and natriuresis (U(Na)V, 0.36+/-0.04 to 1.25+/-0.032 micromol 100 g(-1) min(-1)). 3. 12, 12 dibromododec-enoic acid (DBDD), an inhibitor of omega hydroxylase, blunted L-NAME-induced changes in MBP, RVR, UV and U(Na)V by 63+/-8, 70+/-5, 45+/-8 and 42+/-9%, respectively, and fully reversed the reduction in GFR by L-NAME. Clotrimazole, an inhibitor of the epoxygenase pathway of CYP450-dependent AA metabolism, was without effect. 4. BMS182874 (5-dimethylamino)-N-(3,4-dimethyl-5-isoxazolyl)-1-naphthalenesulfo namide), an endothelin (ET)A receptor antagonist, also blunted the increases in MBP and RVR and the diuresis/natriuresis elicited by L-NAME without affecting GFR. 5. Indomethacin blunted L-NAME-induced increases in RVR, UV and U(Na)V. BMS180291 (1S-(1alpha,2alpha,3alpha,4alpha)]-2-[[3-[4-[(++ +pentylamino)carbonyl]-2-oxazolyl]-7-oxabicyclo[2.2.1]hept-2-yl ]methyl]benzenepropanoic acid), an endoperoxide receptor antagonist, attenuated the pressor and renal haemodynamic but not the renal tubular effects of L-NAME. 6. In conclusion, the renal functional effects of the CYP450-derived mediator(s) expressed after inhibition of NOS with L-NAME were prevented by inhibiting either CYP450 omega hydroxylase or cyclooxygenase or by antagonizing either ET(A) or endoperoxide receptors. 20-hydroxyeicosatetraenoic acid (20-HETE) fulfils the salient properties of this mediator.

Chronic blockade of endothelin ETA receptors improves flow dependent dilation in resistance arteries of hypertensive rats

OBJECTIVE

Flow (shear stress)-induced dilation (FD) is attenuated in hypertension. Flow triggers the release by endothelial cells of dilators, such as NO or cyclo-oxygenase (COX) derivatives and constrictor factors such as endothelin-1 (ET-1) which might be involved in several cardiovascular diseases. We hypothesized that ET-1 might play a functional role in FD and participate in the endothelial dysfunction in hypertension.

METHODS

We investigated the effect of chronic treatment with the ETA receptor blocker LU135252 (50 mg/kg/day) for 2 weeks on the dilator response to flow in normotensive (Wistar-Kyoto; WKY) or hypertensive (SHR, n = 7 or 8 per group) rats.

RESULTS

Systolic arterial pressure was not significantly affected by chronic ETA receptor blockade in both strains. In mesenteric resistance arteries (diameter: approximately 100 microns), isolated in vitro, FD was lower and myogenic tone higher in SHR than in WKY rats. Chronic ETA receptor blockade increased FD by 73% (7.5 +/- 1.5 to 13.0 +/- 2.7 microns dilation with a flow-rate of 150 microliters/min) in SHR (no effect in WKY). The participation of NO to FD was increased in SHR and the participation of dilator COX product(s) (blocked by indomethacin 10 mumol/l) to FD was significantly increased in SHR and in WKY. In control rats FD was improved by acute ETA receptor blockade in WKY rats (18.5 +/- 2.0 to 23.2 +/- 1.8 microns dilation to flow-rate of 150 microliters/min) and significantly more in SHR (6.0 +/- 1.8 to 15.1 +/- 1.6 microns). Acetylcholine-induced dilation was also improved by chronic ETA receptor blockade (no effect of an acute blockade). Myogenic and phenylephrine-induced tone were not affected by chronic or acute ETA receptor blockade. The improvement of endothelium-dependent dilation was not related to a change in blood pressure.

CONCLUSION

Chronic ETA receptor blockade increased flow-induced dilation in SHR possibly by suppressing flow-induced ETA stimulation and by improving the release of dilator products by the endothelium.

Different contributions of the endothelin ET(A) receptor to hypertension induced by acute or chronic inhibition of nitric oxide synthesis

The effects of FR139317((R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carb onyl] amino-4-methyl-pentanoyl] amino-3-[3-(1-methyl-1H-indoyl)]propionyl]-amino-3-(2-pyridyl)prop ionic acid), an endothelin ET(A) receptor antagonist, on systemic and renal haemodynamic responses and excretory responses to chronic or acute nitric oxide (NO) synthase inhibition with NG-nitro-L-arginine (NOARG) have been examined. An intravenous bolus injection of FR139317 (10 mg kg(-1)) to chronic NO-deficient hypertensive rats (2.74 mM NOARG in drinking water for 4 weeks) elicited only a slight decrease in mean arterial pressure (MAP), to the same extent as seen in normotensive control rats. Injection of this drug induced no alteration of the renal haemodynamics of this chronic hypertensive model. Urine formation in control rats was significantly reduced by administration of FR139317. No significant decrease in urine formation was observed in the chronic NO-deficient rats. Acute intravenous injection of NOARG (5 mg kg(-1)) induced a gradual and significant increase in MAP, with a significant decrease in renal blood flow. A slight but insignificant diuretic effect was observed. In animals pretreated with FR139317 (10 mg kg(-1) i.v.) NOARG induced a significantly less potent increase in MAP, whereas similar renal haemodynamic responses to NOARG were observed. In contrast to the FR139317-untreated group, urine formation tended to decrease after administration of NOARG. These results suggest that endothelin, via the ET(A) receptor, contributes to the systemic pressor response to acute NO synthase inhibition, although renal vasoconstriction and functional changes induced by acute NO synthase inhibition are independent of ET(A) receptor-related effects. These results imply that action of endothelin via the ET(A) receptor is not involved in the maintenance of sustained hypertension induced by chronic NO synthase inhibition.