Endogenous endothelin modulates blood pressure, plasma volume, and albumin escape after systemic nitric oxide blockade

Endothelin-1 in Health and Disease

Discovered almost 40 years ago, the potent vasoconstrictor peptide endothelin-1 (ET-1) has a wide range of roles both physiologically and pathologically. In recent years, there has been a focus on the contribution of ET-1 to disease. This has led to the development of various ET receptor antagonists, some of which are approved for the treatment of pulmonary arterial hypertension, while clinical trials for other diseases have been numerous yet, for the most part, unsuccessful. However, given the vast physiological impact of ET-1, it is both surprising and disappointing that therapeutics targeting the ET-1 pathway remain limited. Strategies aimed at the pathways influencing the synthesis and release of ET-1 could provide new therapeutic avenues, yet research using cultured cells in vitro has had little follow up in intact ex vivo and in vivo preparations. This article summarises what is currently known about the synthesis, storage and release of ET-1 as well as the role of ET-1 in several diseases including cardiovascular diseases, COVID-19 and chronic pain. Unravelling the ET-1 pathway and identifying therapeutic targets has the potential to treat many diseases whether through disease prevention, slowing disease progression or reversing pathology.

Chronic hypoxia changes gene expression profile of primary rat carotid body cells: consequences on the expression of NOS isoforms and ET-1 receptors

Sustained chronic hypoxia (CH) produces morphological and functional changes in the carotid body (CB). Nitric oxide (NO) and endothelin-1 (ET-1) play a major role as modulators of the CB oxygen chemosensory process. To characterize the effects of CH related to normoxia (Nx) on gene expression, particularly on ET-1 and NO pathways, primary cultures of rat CB cells were exposed to 7 days of CH. Total RNA was extracted, and cDNA-32P was synthesized and hybridized with 1,185 genes printed on a nylon membrane Atlas cDNA Expression Array. Out of 324 differentially expressed genes, 184 genes were upregulated, while 140 genes were downregulated. The cluster annotation and protein network analyses showed that both NO and ET-1 signaling pathways were significantly enriched and key elements of each pathway were differentially expressed. Thus, we assessed the effect of CH at the protein level of nitric oxide synthase (NOS) isoforms and ET-1 receptors. CH induced an increase in the expression of endothelial NOS, inducible NOS, and ETB. During CH, the administration of SNAP, a NO donor, upregulated ETB. Treatment with Tezosentan (ET-1 receptor blocker) during CH upregulated all three NOS isoforms, while the NOS blocker L-NAME induced upregulation of iNOS and ETB and downregulated the protein levels of ETA. These results show that CH for 7 days changed the cultured cell CB gene expression profile, the NO and ET-1 signaling pathways were highly enriched, and these two signaling pathways interfered with the protein expression of each other.

Vascular toxicities with VEGF inhibitor therapies-focus on hypertension and arterial thrombotic events

The vascular endothelial growth factor (VEGF) signaling pathway (VSP) fulfills a cardinal role in endothelial cells and its inhibition has profound cardiovascular impact. This is true not only for the normal vasculature but also for the tumor vasculature when VSP inhibitors are used as anti-angiogenic therapies. Generalized endothelial dysfunction predisposes to vasoconstriction, atherosclerosis, platelet activation, and thrombosis (arterial more than venous). All of these have been reported with VSP inhibitors and collectively give rise to vascular toxicities, the most concerning of which are arterial thromboembolic events (ATE). VSP inhibitors include antibodies, acting extracelluarly on VEGF, such as bevacizumab and tyrosine kinases inhibitors, acting intracellularly on the kinase domain of VEGF receptors, such as sunintib and sorafenib. The addition of bevacizumab and VSP tyrosine kinase inhibitor therapy to the cancer treatment regimen is associated with a 1.5-2.5-fold and 2.3-4.6-fold increase risk of ATEs, respectively. Risk factors for ATEs while on VSP inhibitor therapy include age older than 65 years, previous thromboembolic events, history of atherosclerotic disease, and duration of VSP inhibitor therapy. In clinical practice, hypertension remains the most commonly noted vascular manifestation of VSP inhibition. Optimal blood pressure goals and preferred therapeutic strategies toward reaching these goals are not defined at present. This review summarizes current data on this topic and proposes a more intensive management approach to patients undergoing VSP inhibitor therapy including Systolic Blood PRessure Intervention Trial (SPRINT) blood pressure goals, pleiotropic vasoprotective agents such as angiotensin converting enzyme inhibitors, amlodipine, and carvedilol, high-dose statin therapy, and aspirin.

Is Change in Hemoglobin Level a Predictive Biomarker of Tyrosine Kinase Efficacy in Metastatic Renal Cell Carcinoma? A Turkish Oncology Group Study

BACKGROUND

There are insufficient predictive markers for renal cell carcinoma (RCC).

METHODS

A total of 308 metastatic RCC patients were analyzed retrospectively.

RESULTS

The increased hemoglobin (Hb) group had significantly higher progression-free survival and overall survival (OS) compared with the decreased Hb group at 11.5 versus 6.35 months (p < .001) and 21.0 versus 11.36 months (p < .001) respectively. The 1- and 3-year OS rates were higher in the Hb increased group, i.e., 84% versus 64% and 52% versus 35% respectively.

CONCLUSIONS

The present study showed that increased Hb levels after tyrosine kinase inhibitor therapy could be a predictive marker of RCC.

Natriuretic Peptides as Cardiovascular Safety Biomarkers in Rats: Comparison With Blood Pressure, Heart Rate, and Heart Weight

Cardiovascular (CV) toxicity is an important cause of failure during drug development. Blood-based biomarkers can be used to detect CV toxicity during preclinical development and prioritize compounds at lower risk of causing such toxicities. Evidence of myocardial degeneration can be detected by measuring concentrations of biomarkers such as cardiac troponin I and creatine kinase in blood; however, detection of functional changes in the CV system, such as blood pressure, generally requires studies in animals with surgically implanted pressure transducers. This is a significant limitation because sustained changes in blood pressure are often accompanied by changes in heart rate and together can lead to cardiac hypertrophy and myocardial degeneration in animals, and major adverse cardiovascular events (MACE) in humans. Increased concentrations of NPs in blood correlate with higher risk of cardiac mortality, all-cause mortality, and MACE in humans. Their utility as biomarkers of CV function and toxicity in rodents was investigated by exploring the relationships between plasma concentrations of NTproANP and NTproBNP, blood pressure, heart rate, and heart weight in Sprague Dawley rats administered compounds that caused hypotension or hypertension, including nifedipine, fluprostenol, minoxidil, L-NAME, L-thyroxine, or sunitinib for 1-2 weeks. Changes in NTproANP and/or NTproBNP concentrations were inversely correlated with changes in blood pressure. NTproANP and NTproBNP concentrations were inconsistently correlated with relative heart weights. In addition, increased heart rate was associated with increased heart weights. These studies support the use of natriuretic peptides and heart rate to detect changes in blood pressure and cardiac hypertrophy in short-duration rat studies.

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.

Acute nitric oxide synthase inhibition and endothelin-1-dependent arterial pressure elevation

Key evidence that endogenous nitric oxide (NO) inhibits the continuous, endothelin (ET)-1-mediated drive to elevate arterial pressure includes demonstrations that ET-1 mediates a significant component of the pressure elevated by acute exposure to NO synthase (NOS) inhibitors. This review examines the characteristics of this pressure elevation in order to elucidate potential mechanisms associated with the negative regulation of ET-1 by NO and, thereby, provide potential insight into the vascular pathophysiology underlying NO dysregulation. We surmise that the magnitude of the ET-1-dependent component of the NOS inhibitor-elevated pressure is (1) independent of underlying arterial pressure and other pressor pathways activated by the NOS inhibitors and (2) dependent on relatively higher NOS inhibitor dose, release of stored and de novo synthesized ET-1, and ET_A receptor-mediated increased vascular resistance. Major implications of these conclusions include: (1) the marked variation of the ET-1-dependent component, i.e., from 0 to 100% of the pressure elevation, reflects the NO-ET-1 regulatory pathway. Thus, NOS inhibitor-mediated, ET-1-dependent pressure elevation in vascular pathophysiologies is an indicator of the level of compromised/enhanced function of this pathway; (2) NO is a more potent inhibitor of ET-1-mediated elevated arterial pressure than other pressor pathways, due in part to inhibition of intravascular pressure-independent release of ET-1. Thus, the ET-1-dependent component of pressure elevation in vascular pathophysiologies associated with NO dysregulation is of greater magnitude at higher levels of compromised NO.

Role of endothelin-1 in the hyper-responsiveness to nitrovasodilators following acute NOS inhibition

BACKGROUND AND PURPOSE

Acute NOS inhibition in humans and animals is associated with hypersensitivity to NO donors. The mechanisms underlying this phenomenon have not been fully elucidated. The purpose of the present study was to assess whether hypersensitivity to NOS-blockade is linked to endothelin-1 (ET-1) signalling.

EXPERIMENTAL APPROACH

Sprague Dawley rats were instrumented with indwelling arterial and venous catheters for continuous assessments of haemodynamic parameters and drug delivery, respectively. Mesenteric arteries were isolated and tested for reactivity by wire myography.

KEY RESULTS

NOS blockade with L-N(G)-nitroarginine methyl ester (L-NAME) caused a pronounced increase in arterial blood pressure (BP) (∼40 mmHg). In L-NAME-treated animals, the dose of sodium nitroprusside (SNP) required to cause a significant reduction in arterial BP was lower than in vehicle-treated rats (P < 0.001), and the magnitude of the reduction in BP was greater. Similar results were obtained with other NO mimetics, but not isoprenaline; moreover, decreasing the BP back to baseline levels with prazosin after L-NAME treatment did not attenuate the hyper-responsiveness to NO donors. The increased responsiveness to NO donors was abolished by pretreatment with the ET(A/B) receptor antagonist, PD145065, or the ET(A) receptor-specific antagonist ABT627. Ex vivo, L-NAME treatment potentiated the constriction induced by big endothelin-1 (bET-1), the precursor to active ET-1, but had no effect on the ET-1-mediated constriction.

CONCLUSIONS AND IMPLICATIONS

These data suggest that the increased sensitivity to NO donors is mediated, at least in part, by ET-1 in vivo, and the mechanism may involve the conversion of bET-1 to ET-1.

The interaction between endothelin-1 and nitric oxide in the vasculature: new perspectives

Nitric oxide (NO) and endothelin-1 (ET-1) are natural counterparts in vascular function, and it is becoming increasingly clear that an imbalance between these two mediators is a characteristic of endothelial dysfunction and is important in the progression of vascular disease. Here, we review classical and more recent data that suggest that ET-1 should be regarded as an essential component of NO signaling. In particular, we review evidence of the role of ET-1 in models of acute and chronic NO synthase blockade. Furthermore, we discuss the possible mechanisms by which NO modulates ET-1 activity. On the basis of these studies, we suggest that NO tonically inhibits ET-1 function, and in conditions of diminished NO bioavailability, the deleterious effects of unmitigated ET-1 actions result in vasoconstriction and eventually lead to vascular remodeling and dysfunction.

Modelling pulse wave propagation in the rabbit systemic circulation to assess the effects of altered nitric oxide synthesis

Pulse wave propagation in the mature rabbit systemic circulation was simulated using the one-dimensional equations of blood flow in compliant vessels. A corrosion cast of the rabbit circulation was manufactured to obtain arterial lengths and diameters. Pulse wave speeds and inflow and outflow boundary conditions were derived from in vivo data. Numerical results captured the main features of in vivo pressure and velocity pulse waveforms in the aorta, brachiocephalic artery and central ear artery. This model was used to elucidate haemodynamic mechanisms underlying changes in peripheral pulse waveforms observed in vivo after administering drugs that alter nitric oxide synthesis in the endothelial cells lining blood vessels. According to our model, these changes can be explained by single or combined alterations of blood viscosity, peripheral resistance and compliance, and the elasticity of conduit arteries.

Regulation of glial inflammatory mediators synthesis: possible role of endothelins

Endothelins are well known as modulators of inflammation in the periphery, but little is known about their possible role in brain inflammation. Stimulation of astrocyte prostaglandin, an inflammatory mediator, synthesis was shown so far only by endothelin 3 (ET-3). By contrast, several studies showed no change or slight decrease of basal nitric oxide synthesis after treatment of astrocytes with endothelin 1 (ET-1) and ET-3. However, a significant increase in astrocytic and microglial nitric oxide synthase (NOS) was observed after exposure to ET-1 and ET-3 in a model of forebrain ischaemia. Here we demonstrate that all three endothelins (ET-1, ET-2, ET-3) significantly enhanced the synthesis of prostaglandin E(2) and nitric oxide in glial cells. Each of the selective antagonists for ETA and ETB receptors (BQ123 and BQ788 respectively), significantly inhibited endothelins-induced production of both nitric oxide and prostaglandin E(2). These results suggest a regulatory mechanism of endothelins, interacting with both endothelin receptors, on glial inflammation. Therefore, inhibition of endothelin receptors may have a therapeutic potential in pathological conditions of the brain, when an uncontrolled inflammatory response is involved.

Gender differences in vascular reactivity to endothelin-1 (1-31) in mesenteric arteries from diabetic mice

Endothelin-1 (1-31) [ET-1 (1-31)], a novel member of the ET family, comprises 31 amino acids and is derived from the selective hydrolysis of big ET-1 by chymase. Although ET-1 (1-31) reportedly exerts biological effects by direct or indirect [via its conversion to ET-1 (1-21)] mechanisms, it is unclear whether in diabetes the vascular effects of ET-1 (1-31) display gender differences. We investigated this question by exposing mesenteric artery rings to ET-1 (1-31), using arteries from mice in the early or chronic phase of diabetes. In the early stage of diabetes, the ET-1 (1-31)-induced contraction was similar between age- and sex-matched control and streptozotocin (STZ)-induced diabetic mice. In the chronic stage of diabetes, the ET-1 (1-31)-induced contraction was enhanced in diabetic female mice, but not in diabetic male mice (vs. both age-matched control and early-stage diabetic mice). This enhancement was largely prevented by Y27632 (Rho kinase inhibitor), PD98059 [inhibitor of extracellular signal related kinases 1 and 2 (ERK1/2)], or SP600125 [C-jun terminal kinase (JNK) inhibitor]. These data indicate that the ET-1 (1-31)-induced vasoconstriction in the mesenteric artery may be specifically enhanced in established diabetic female mice, and that this enhancement may be due to alterations in the activities of Rho/Rho kinase or mitogen-activated protein kinase.

Attenuation of capillary leakage by hydroxyethyl starch (130/0.42) in a porcine model of septic shock

OBJECTIVES

Capillary leakage, a frequent complication in septic shock, is characterized by loss of intravasal fluid resulting in generalized edema and hemodynamic instability despite massive fluid therapy. We have shown that administration of an established colloid 200/0.5 hydroxyethyl starch (HES) stabilized plasma volume in a porcine septic shock model. Recently, a new HES with a low molecular weight (130 kD) and lower molar substitution (0.42) has been developed. In this study, we compared effects of HES 130/0.42 and HES 200/0.5 on capillary leakage in porcine septic shock.

DESIGN

Prospective randomized, controlled animal study.

SETTING

University department of anesthesiology.

SUBJECTS

Fourteen pigs (22.9 +/- 2.8 kg).

INTERVENTIONS

Anesthetized and mechanically ventilated pigs were observed over 6 hrs.

MEASUREMENT AND MAIN RESULTS

Septic shock was induced with fecal peritonitis (0.75 g.kg of body weight autologous feces). Animals were allocated to volume-replacement therapy with either HES 130/0.42 (n = 5) or HES 200/0.5 (n = 5) and compared with nonseptic controls receiving HES 130/0.42 (n = 4). Infusion rate was titrated to maintain a central venous pressure of 12 mm Hg. Albumin escape rate was calculated using iodine 125-labeled albumin. Plasma volume was determined using chromium-51-tagged erythrocytes. Albumin escape rate increased significantly in both groups in comparison to controls (HES 200/0.5, 45% +/- 3; HES 130/0.42, 38% +/- 5), but this increase was significantly smaller with HES 130/0.42. Both HES 200/0.5 (-14%, not significant) and HES 130/0.42 (-1%, not significant) stabilized plasma volume compared with controls. Systemic oxygenation was not significantly altered in either group.

CONCLUSIONS

In this porcine septic shock model, HES 130/0.42 attenuated capillary leakage significantly more effectively than HES 200/0.5.

Resuscitation from septic shock with capillary leakage: hydroxyethyl starch (130 kd), but not Ringer's solution maintains plasma volume and systemic oxygenation

There is evidence suggesting that early fluid resuscitation is beneficial in the treatment of septic shock. The question as to which solution should be used remains controversial. Using a porcine septic shock model, we tested the effects of a new synthetic colloid hydroxyethyl starch (HES 130 kD) and a crystalloid regimen with Ringer's solution (RS) on plasma volume (PV) maintenance as well as on systemic and regional hemodynamics. Fourteen anaesthetized mechanically ventilated pigs received 0.75 g kg body weight of feces into the abdominal cavity to induce sepsis. They were randomly allocated to receive 6% HES 130 kD (n = 5) or RS (n = 5) and were compared with nonseptic controls receiving 6% HES 130 kD (n = 4). The infusion rate was titrated to maintain a central venous pressure of 12 mmHg. PV was determined by chromium-51-tagged erythrocytes and hematocrit. Albumin escape rate (AER) was calculated using iodine-125-labeled albumin. Arterio-intramucosal pCO2 gap, systemic hemodynamics, and oxygenation were obtained before and 6 h after induction of sepsis. AER increased in the HES (+38%) and RS groups (+38%) compared with control. PV was reduced in the RS group (-39%), but was maintained in the HES group (-1%). After 6 h of sepsis, HES 130 kD-treated animals had a significantly higher cardiac output (166 +/- 28 mL min kg vs. 90 +/- 18 mL min kg, P < 0.05), and a significantly higher mixed-venous oxygen saturation (65% +/- 8% vs. 40% +/- 14%, P < 0.05) than RS animals. In this porcine septic shock model with concomitant capillary leakage syndrome, resuscitation with HES 130 kD but not RS could maintain PV and preserve systemic hemodynamics and oxygenation.

Terminal complement complex in septic shock with capillary leakage: marker of complement activation?

BACKGROUND AND OBJECTIVE

The aim of this study was to evaluate the value of terminal complement complex (C5b-9) plasma levels as a marker for complement activation in septic shock with concomitant capillary leak syndrome.

METHODS

In a prospective animal study 10 fasted, anaesthetized, mechanically ventilated and multi-catheterized pigs (20.6 +/- 1.3 kg) were investigated over a period of 8 h. Sepsis was induced by faecal peritonitis (1 g kg(-1) body weight faeces, n = 5) and compared to controls (n = 5). The animals received 6% hydroxyethyl starch 200/0.5 to maintain a central venous pressure of 12 mmHg. To quantify capillary leak syndrome, albumin escape rate was measured using 99mTc-labelled human serum albumin. Plasma levels of terminal complement complex were measured in a double antibody immunoassay (neoepitope-specific MoAb aE 11 as catching antibody). Immunohistological studies of renal specimens were performed to detect terminal complement complex deposition.

RESULTS

Albumen escape rate increased in septic animals (+ 52%) compared to controls (+ 3%, P < 0.05). Plasma levels of terminal complement complex decreased during the study period in both groups. In septic animals this finding was accompanied by a significant deposition of terminal complement complex in renal specimens (P < 0.05).

CONCLUSION

We found an activation of the complement system proven by marked deposition of terminal complement complex in renal specimen, while its plasma levels decreased during the study period in septic and control animals. These results suggest that in septic shock with capillary leak syndrome plasma level of terminal complement complex may not be a reliable marker of complement activation.

Anti-inflammatory strategies in hypertension: focus on COX-1 and COX-2

An increasing body of evidence suggests that elevated levels of blood pressure may induce a proinflammatory response. Indeed, both C-reactive protein and blood pressure are independent determinants of cardiovascular risk, and, in combination, each parameter has additional predictive value. Hence, strategies targeted to lower blood pressure and reduce vascular inflammation may potentially provide clinical benefit. In this review, we discuss the role of chronic low-grade inflammation in the context of cardiovascular disease with a focus on roles of cyclooxygenase-1 and -2 in potential anti-inflammatory treatment strategies.

Mechanisms underlying enhanced contractile response to endothelin-1 in diabetic rat basilar artery

We investigated the influence of streptozotocin-induced diabetes on the responsiveness of the rat basilar artery to endothelin-1 (ET-1) and nitric oxide (NO), which is known to counteract ET-1. In basilar arteries isolated from diabetic rats: (a) the ET-1-induced contraction was enhanced, (b) the contraction induced by N(G)-nitro-l-arginine [a nitric oxide synthase (NOS) inhibitor] was weaker, and (c) the levels of the mRNAs for ET(A)/ET(B) receptors and prepro-ET-1, but not for NOS, were significantly elevated (all versus age-matched controls). These data indicate that ET-1-induced vasoconstriction may be increased in the diabetic rat basilar artery, and that this hyper-reactivity to ET-1 may be due to an overproduction of ET-1, an up-regulation of ET(A)/ET(B) receptors, and a defect in the bioavailability of NO.

Fluid resuscitation with colloids of different molecular weight in septic shock

OBJECTIVE

The aim of this study was to investigate the short-term effect of fluid resuscitation with 4% modified fluid gelatine (GEL) versus 6% hydroxyethyl starch (HES) on haemodynamics and oxygenation in patients with septic shock and acute lung injury (ALI).

DESIGN

Prospective randomised clinical trial.

SETTING

Twenty-bed intensive care unit in a university hospital.

PATIENTS

Thirty hypovolemic patients (intrathoracic blood volume index, ITBVI <850 ml/m(2)) in septic shock with ALI were randomised into HES (mean molecular weight: 200,000 Dalton, degree of substitution 0.6) and GEL (mean molecular weight: 30,000 Dalton) groups (15 patients each).

INTERVENTIONS

For fluid resuscitation 250 ml/15 min boluses (max. 1,000 ml) were given until the end point of ITBVI >900 ml/m(2) was reached. Repeated haemodynamic measurements were done at baseline (t(b)), at the end point (t(ep)) then at 30 min and 60 min after the end point was reached (t(30), t(60)). Cardiac output, stroke volume, extravascular lung water (EVLW), and oxygen delivery was determined at each assessment point. For statistical analysis two-way ANOVA was used.

MEASUREMENTS AND RESULTS

ITBVI, cardiac index, and oxygen delivery index increased significantly at t(ep) and remained elevated for t(30) and t(60), but there was no significant difference between the two groups. The increase in the ITBVI by 100 ml of infusion was similar in both groups (HES: 26+/-19 ml/m(2) vs GEL: 30+/-19 ml/m(2)). EVLW, remained unchanged, and there was no significant difference between the groups (HES, t(b): 8+/-6, t(60): 8+/-6; GEL, t(b): 8+/-3, t(60): 8+/-3 ml/kg). The PaO(2)/FiO(2) did not change significantly over time or between groups (HES, t(b): 207+/-114, t(60): 189+/-78; GEL, t(b): 182+/-85, t(60): 182+/-85 mmHg).

CONCLUSION

The results of this study indicate that both HES and GEL infusions caused similar short-term change in ITBVI in septic shock, without increasing EVLW or worsening oxygenation.

Role of endothelins in septic, cardiogenic, and hemorrhagic shock

Shock is a condition where blood flow is inadequate for tissue needs. In all forms of shock, the concentrations of endothelins (ETs) are elevated, and they are especially high in septic shock. The rise in ETs plasma levels may initially have some positive homeostatic effects, for ETs can help restore normal vascular tone. However, high levels of ETs compromise the appropriate matching of flow to tissue needs and contribute to the pathophysiology of shock. Attempts at regulating the effects of ETs by the use of pharmacological blockers is made complicated by important interactions between the ETA and ETB receptors and potentially different effects on different tissues. We conclude that antagonism of ET receptors is unlikely to be helpful for cardiogenic or hemorrhagic shock. Furthermore, selective blockade is unlikely to be helpful. However, moderate doses of a mixed ET receptor antagonist may be of use for the management of septic patients.

Fluid therapy in sepsis with capillary leakage

Sepsis is associated with a profound intravascular fluid deficit due to vasodilatation, venous pooling and capillary leakage. Fluid therapy is aimed at restoration of intravascular volume status, haemodynamic stability and organ perfusion. Circulatory stability following fluid resuscitation is usually achieved in the septic patient at the expense of tissue oedema formation that may significantly influence vital organ function. The type of fluid therapy, crystalloid or colloid, in sepsis with capillary leakage remains an area of intensive and controversial discussion. The current understanding of the physiology of increased microvascular permeability in health and sepsis is incomplete. Furthermore, there is a lack of appropriate clinical study end-points for fluid resuscitation. This review considers critically the clinical and experimental data analysing the assessment of capillary leakage in sepsis and investigating the effects of different fluid types on increased microvascular permeability in sepsis.

Effects of bosentan on cellular processes involved in pulmonary arterial hypertension: do they explain the long-term benefit?

Pulmonary arterial hypertension is a rapidly progressing disease characterized by an over- expression of endothelin. In addition to its potent pulmonary vasoconstrictor effects, endothelin has been shown to produce many of the aberrant changes, such as hypertrophy, fibrosis, inflammation, and neurohormonal activation that underlie the shortened life span in pulmonary arterial hypertensive patients. The fact that endothelin expression correlates significantly with disease severity and outcome in these patients suggests that endothelin, through binding to both ETA and ETB receptor subtypes, is a key causative agent in the pathophysiology of pulmonary arterial hypertension. The orally active dual endothelin receptor antagonist bosentan competitively antagonizes the binding of endothelin to both endothelin receptor subtypes with high affinity and specificity. In animal models relevant for the pathophysiology of pulmonary hypertension, bosentan not only causes selective pulmonary vasodilation, but also prevents vascular hypertrophy and cardiac remodeling, attenuates pulmonary fibrosis, decreases vascular inflammation, and blunts neurohormonal activation. These experimental data may explain the effects on disease progression and the long-term benefit observed with bosentan in pulmonary arterial hypertension.

Effect of in vivo nitrate tolerance on hypersensitivity to NO donors after NO-synthase blockade

Animals treated with nitric oxide synthase (NOS) inhibitors exhibit marked hypersensitivity to the blood pressure lowering effects of exogenous nitric oxide (NO) donors. We used this model as a sensitive index to evaluate the relative importance of reduced biotransformation of glyceryl trinitrate (GTN) to NO in the development of nitrate tolerance. NOS-blockade hypertension using N(G)-nitro-L-arginine methyl ester (L-NAME) caused a marked enhancement of the mean arterial pressure (MAP) decrease mediated by GTN in nontolerant rats. However, even large doses of GTN were unable to change the MAP in GTN-tolerant, NOS-blockade hypertensive animals. In contrast, the MAP responses to the spontaneous NO donor sodium nitroprusside (SNP) were completely unaltered in either tolerant rats or tolerant NOS-blockade hypertensive animals, indicating that NO-dependent vasodilatory mechanisms remain intact despite the development of GTN tolerance. The MAP-lowering effects of GTN in NOS-blockade hypertensive animals were restored 48 h after cessation of chronic GTN exposure. These alterations in the pharmacodynamic response to GTN during tolerance development and reversal were associated with parallel changes in the pattern of GTN metabolite formation, suggesting that the activity of one or more enzymes involved in nitrate metabolism was altered as a consequence of chronic GTN exposure. These findings suggest that the vasodilation resulting from the vascular biotransformation of GTN to NO (or a closely related species) is severely compromised in nitrate-tolerant animals, and that although other mechanisms may contribute to the vascular changes observed following the development of GTN tolerance, decreased GTN bioactivation is likely the most important.

Vascular control in larval Xenopus laevis: the role of endothelial-derived factors

We used novel digital contrasting methods to measure the effects of endothelin-1 (ET-1, 10–6 to 10–8 mol l–1), the nitric-oxide-releasing agent sodium nitroprusside (SNP, 10–2 mol l–1) and Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME, 10–4 mol l–1), a non-selective inhibitor of nitric oxide synthase, on the diameter of tiny arteries and veins of the head in anaesthetized Xenopus laevis tadpoles (stage NF 50–53). Perfusion of the main artery and vein supplying the head with ET-1 caused an immediate, significant and dose-dependent vasoconstriction. While preincubation with l-NAME caused no significant change in vessel diameter, the constriction provoked by subsequent endothelin administration was significantly enhanced. Application of SNP caused a vasodilation that was significant only after preconstriction with ET-1 (10–6 mol l–1). Our results provide strong evidence that the vasculature of developing Xenopus laevis tadpoles (NF stage 50–53) is influenced by endogenously released nitric oxide and endothelin. Vasoactive mediators released from vascular endothelial cells could be particularly important in vascular control in early embryos when the autonomic innervation is undifferentiated, poorly developed or even absent.

Short-term endothelin receptor blockade with tezosentan has both immediate and long-term beneficial effects in rats with myocardial infarction

OBJECTIVES

We investigated the effects of short-term tezosentan treatment on cardiac function, pulmonary edema and long-term evolution of heart failure (HF) in a rat model of myocardial infarction (MI).

BACKGROUND

Endothelin (ET) may play a major role in the progression from MI to HF. Tezosentan is a new dual ET(A)/ET(B) receptor antagonist.

METHODS

Rats were subjected to coronary artery ligation and were treated with either vehicle or tezosentan (10 mg/kg IV bolus) at 1 h and 24 h after MI. Cardiac hemodynamics and lung weight were measured at 48 h after MI. Survival was assessed over a five-month period.

RESULTS

At 48 h after ligation, vehicle-treated rats developed HF, as evidenced by a marked increase in left ventricular end-diastolic pressure (LVEDP), reduction in dP/dt(max) and mean arterial pressure (MAP), and development of pulmonary edema. Tezosentan treatment attenuated the increase in LVEDP and in lung weight and slightly reduced MAP without affecting dP/dt(max). Infarct size was not modified by tezosentan. Despite the fact that treatment with tezosentan was stopped after 24 h, the initial tezosentan administration significantly reduced cardiac hypertrophy (22%) and decreased mortality by 51% at five months (50% survival vs. 19% survival in vehicle-treated rats, p < 0.001).

CONCLUSIONS

Tezosentan administered during the first day after MI in rats, in addition to improving acutely hemodynamic conditions, markedly increases long-term survival. This increase is associated with a decrease of pulmonary edema and prevention of cardiac hypertrophy. Tezosentan could be a safe and useful therapeutic agent in the prevention and treatment of ischemic HF.

Vasopeptidase inhibition and endothelial function in hypertension

Vasopeptidase inhibitors are a new class of drugs capable of inhibiting both angiotensin-converting enzyme and neutral endopeptidase 24.11. This involves simultaneous inhibition with a single molecule of two key enzymes, ACE and NEP, which are both involved in the regulation of cardiovascular homeostasis in many ways. This includes metabolism of several vasoactive peptides and their clearance from the circulation, therefore contributing to neurohumoral modulation, which might have therapeutic advantages in the prevention of endothelial dysfunction in hypertension.

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.

Basic and therapeutic relevance of endothelin-mediated regulation

Three endothelin family peptides (endothelin-1, -2 and -3) exert an extremely potent and long-lasting vasoconstrictor action as well as other various actions through stimulating two subtypes of receptor (ETA and ETB). Vascular endothelial cells produce only endothelin-1. Although the pharmacological actions of exogenous endothelin-1 have been extensively analyzed, the physiological roles of endogenous endothelin-1 have long been obscure. Using potent and selective receptor antagonists, endothelin-1 has been demonstrated to contribute slightly to the maintenance of regional vascular tone. In gene-targeted mice, endothelin family peptides and their receptors have been shown to play an important role in the embryonic development of neural crest-derived tissues. In addition to its potent vasoconstrictor action, endothelin-1 has direct mitogenic actions on cardiovascular tissues, as well as co-mitogenic actions with a wide variety of growth factors and vasoactive substances. Endothelin-1 also promotes the synthesis and secretion of various substances including extracellular constituents. These effects of endogenous endothelin-1 would appear to be naturally concerned with the development and/or aggravation of chronic cardiovascular diseases, e.g. hypertension, pulmonary hypertension, vascular remodeling (restenosis, atherosclerosis), renal failure, and heart failure. A great many non-peptide and orally active endothelin receptor antagonists have been developed, and shown to exert excellent therapeutic effects in animal models as well as human patients with these diseases.

Is nitric oxide overproduction the target of choice for the management of septic shock?

Sepsis is a heterogeneous class of syndromes caused by a systemic inflammatory response to infection. Septic shock, a severe form of sepsis, is associated with the development of progressive damage in multiple organs, and is a leading cause of patient mortality in intensive care units. Despite important advances in understanding its pathophysiology, therapy remains largely symptomatic and supportive. A decade ago, the overproduction of nitric oxide (NO) had been discovered as a potentially important event in this condition. As a result, great hopes arose that the pharmacological inhibition of NO synthesis could be developed into an efficient, mechanism-based therapeutic approach. Since then, an extraordinary effort by the scientific community has brought a deeper insight regarding the feasibility of this goal. Here we present in summary form the present state of knowledge of the biological chemistry and physiology of NO. We then proceed to a systematic review of experimental and clinical data, indicating an up-regulation of NO production in septic shock; information on the role of NO in septic shock, as provided by experiments in transgenic mice that lack the ability to up-regulate NO production; effects of pharmacological inhibitors of NO production in various experimental models of septic shock; and relevant clinical experience. The accrued evidence suggests that the contribution of NO to the pathophysiology of septic shock is highly heterogeneous and, therefore, difficult to target therapeutically without appropriate monitoring tools, which do not exist at present.

Improvement of respiratory function by bosentan during endotoxic shock in the pig

We evaluated the role of endothelin-1 (ET-1) and the involvement of nitric oxide in cardiovascular and respiratory dysfunction, during endotoxic shock, in 18 anaesthetised, mechanically ventilated pigs, divided into three groups. Group 1 was i.v. infused with LPS (20 microg/Kg/h for 240 min). Group 2 was pre-treated with bosentan, a dual inhibitor of ET-1 receptors, and at 180 min of endotoxic shock, L-NAME (N(G)-nitro-L-arginine methyl ester, 10 mg/Kg), a non-selective inhibitor of NO synthases, was i.v. administered. Group 3 was infused with LPS and L-NAME was administered similarly to group 2. Results show that LPS caused systemic hypotension, pulmonary biphasic hypertension, decrease in compliance (C(rs)) and increase in resistance (R(max,rs)) of respiratory system. Bosentan completely abolished the pulmonary hypertension and the changes in C(rs)and R(max,rs). L-NAME does not affect the LPS-dependent changes in respiratory mechanics, but it worsens the cardiovascular effects, causing death of pigs. Pre-treatment with bosentan prevents this deleterious effect. Our study demonstrates that the LPS-dependent respiratory effects are mediated by ET-1, which, probably causing pulmonary oedema, is responsible for the decrease in C(rs)and the increase of R(max,rs).

L-canavanine, a selective inhibitor of inducible NO synthase, increases plasma endothelin-1 concentrations in dogs with endotoxic shock

PURPOSE

The purpose of this article is to elucidate the effect of L-canavanine, a selective inhibitor of inducible NO synthase (iNOS), on hemodynamics, blood gas parameters, and plasma concentrations of lactate and endothelin-1 (ET-1) during endotoxic shock.

MATERIALS AND METHODS

Eleven mongrel dogs under pentobarbital anesthesia were divided into two groups: (1) bacterial lipopolysaccharide (LPS) plus vehicle group (n = 5) receiving infusion of LPS (3 mg/kg/h for 1 h) followed by vehicle (2 mL/h for 5 hours); (2) LPS plus L-canavanine group (n = 6) receiving infusion of LPS (3 mg/kg/h for 1 hour) followed by L-canavanine (10 mg/kg/h for 5 hours).

RESULTS

LPS caused a significant (P < .05) decrease in mean arterial pressure (MAP) at 1 hour, but there was no significant difference in MAP during 6-hour period between the two groups. LPS alone did not cause significant changes in other hemodynamics, whereas L-canavanine caused a significant (P < .05) increase in pulmonary vascular resistance index (PVRI) and a decrease in oxygen delivery at 6 hours. The LPS-induced lactic acidosis and hypersecretion of ET-1 were aggravated after L-canavanine infusion. Plasma ET-1 showed positive correlations to lactate levels and PVRI, and negative correlations to cardiac output and oxygen delivery only in the LPS plus L-canavanine group, but not in the LPS plus vehicle group.

CONCLUSIONS

This study suggests that L-canavanine induces tissue hypoperfusion and ischemia with concomitant hypersecretion of ET-1 in dogs with endotoxic shock.

Hemodynamic and renal effects of acute and progressive nitric oxide synthesis inhibition in anesthetized dogs

This study evaluated the effects of progressive nitric oxide (NO) inhibition in the regulation of systemic and regional hemodynamics and renal function in anesthetized dogs. The N(G)-nitro-L-arginine methyl ester group (n = 9) received progressive doses of 0.1, 1, 10, and 50 microg. kg(-1). min(-1). Renal (RBF), mesenteric (MBF), iliac (IBF) blood flows, mean arterial pressure (MAP), pulmonary pressures, cardiac output (CO), and systemic and pulmonary vascular resistances were measured. During N(G)-nitro-L-arginine methyl ester infusion, MAP and systemic vascular resistances increased in a dose-dependent manner. Mean pulmonary pressure and pulmonary vascular resistances increased in both the N(G)-nitro-L-arginine methyl ester and the control group, but the increase was more marked in the N(G)-nitro-L-arginine methyl ester group during the last two infusion periods. CO decreased progressively, before any significant change in blood pressure was noticeable in the N(G)-nitro-L-arginine methyl ester group. IBF decreased significantly from the first N(G)-nitro-L-arginine methyl ester dose, whereas RBF and MBF only decreased significantly during the highest N(G)-nitro-L-arginine methyl ester dose. Urinary volume and sodium excretion only increased significantly in the time control group during the two last time periods. The pulmonary vasculature was more sensitive than the systemic vasculature, whereas skeletal muscle and renal vasculatures showed a greater sensitivity to the inhibition of NO production than the mesenteric vasculature. NO synthesis inhibition induces a progressive antidiuretic and antinatriuretic effect, which is partially offset by the increase in blood pressure.

Calcium handling and role of endothelin-1 in monocrotaline right ventricular hypertrophy of the rat

We investigated the role of endothelin-1 (ET-1) in right ventricular function and intracellular Ca(2+)(Ca(2+)(i)) handling of isolated perfused rat hearts with right ventricular hypertrophy induced by monocrotaline (50 mg/kg). Nine weeks after monocrotaline (n=9) or saline (control n=9) treatment, hearts were perfused isovolumically at 37 degrees C and right ventricular function (fluid-filled balloon), right ventricular intracellular Ca(2+) transients (aequorin bioluminescence method) and the effects of ET-1 were determined. Monocrotaline-treated rats developed considerable right ventricular hypertrophy (right ventricular weight:body weight ratio: 1.07+/-0.13 v. 0.60+/-0.03 in controls P<0.05) and these hearts generated higher right ventricular systolic and diastolic pressure, but similar systolic and diastolic wall stress, indicating a compensated functional state. Hypertrophied hearts demonstrated a prolonged duration of isovolumic contraction (time to 90% decline from peak: 105+/-1 v 89+/-4 ms at 3 m M extracellular Ca(2+) P<0.05), but neither the time to peak pressure (71+/-3 ms) nor time to peak light (25+/-3 ms) were different from controls. The increased duration of contraction correlated with a similar prolongation of the Ca(2+)transient (time to 90% decline from peak: 72+/-4 v 50+/-3 ms P<0.05), indicating a reduced rate of Ca(2+)sequestration in hypertrophic right ventricles. Peak systolic intracellular Ca(2+)was similar in control and hypertrophied hearts (1.04+/-0.02 and 0.99+/-0.02 microM, P>0.05, n=6). ET-1 (1-300 p M) affected neither the time course of right ventricular contraction nor that of the Ca(2+)transient or peak systolic Ca(2+)concentrations. These data are the first measurements of right ventricular Ca(2+)transients in beating normal and hypertrophic hearts. We conclude that ET-1 plays no role in compensated hypertrophy because it affected neither right ventricular function nor intracellular Ca(2+)handling in this model.

Say NO to ET

The endothelial cells release both relaxing [nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), prostacyclin] and contracting factors [endoperoxides, thromboxane A(2), superoxide anions, endothelin-1 (ET)]. The production of ET is inhibited by NO. The latter also strongly opposes the direct effects of the former on vascular smooth muscle. With aging and vascular disease, the production of enothelial NO declines, and thus ET can be released, act and contribute to the symptoms.

Effects of estrogen on venous function in rats with chronic heart failure

The effect of 17beta-estradiol on venous function was investigated in ovariectomized rats with heart failure. Rats (50-60 days old) were ovariectomized and implanted with 60-day-release pellets that contain 17beta-estradiol (1.5 mg) or vehicle. The left coronary artery was ligated 7 days later. Another group of ovariectomized rats was given vehicle pellets and then a sham operation was performed. The rats were studied while under pentobarbital anesthesia at 7 wk after ligation. Ligated rats, relative to sham groups, had lower mean arterial pressure (MAP, -34 mmHg) and cardiac output (CO, -38%); higher arterial resistance (R(A), +12%) and venous resistance (R(V), +116%); mean circulatory filling pressure (MCFP, +40%) and left ventricular end-diastolic pressure (LVEDP, +11 mmHg); and similar cardiovascular responses to norepinephrine (NE). Treatment of ligated rats with 17beta-estradiol increased CO (+16%); reduced R(A) (-16%), R(V) (-35%), MCFP (-23%), and LVEDP (-3 mmHg); and augmented MAP, R(V,) and MCFP responses to NE. Therefore, 17beta-estradiol reduced MCFP, and this reduced preload (LVEDP). 17beta-Estradiol decreased R(V), which, along with decreased R(A) (afterload), led to an increase in CO. 17beta-Estradiol likely augmented vasoconstriction to NE through an improvement on the cardiovascular status.

Role for endogenous endothelin in the regulation of plasma volume and albumin escape during endotoxin shock in conscious rats

To explore the role of endogenous endothelin (ET) in the regulation of vascular functions, we studied the effects endothelin receptor blockade on blood pressure, plasma volume and albumin escape during endotoxin shock in conscious, chronically catheterized rats. Red blood cell volume and plasma volume were determined by using chromium-51-tagged erythrocytes and iodine-125-labelled albumin, respectively. Intravenous injection of lipopolysaccharide (LPS, 10 mg kg(-1)) resulted in hypotension, haemoconcentration, and increased total-body albumin escape, which is reflected by a 30% reduction in plasma volume. Plasma ET-1 concentrations increased 2.1 fold and 5.4 fold at 30 and 120 min post-LPS, respectively. LPS-induced losses in plasma volume and albumin escape were significantly attenuated by pretreatment of animals with the dual ET(A)/ET(B) receptor antagonist bosentan (17.4 micromol kg(-1), i.v. 15 min prior to LPS) or the ET(A) receptor antagonist FR 139317 (3.8 micromol kg(-1), i.v.) during both the immediate and delayed phases of endotoxin shock. The inhibitory actions of bosentan and FR 139317 were similar. Both antagonists augmented the hypotensive action of LPS. Administration of bosentan or FR 139317 70 min after injection of LPS also attenuated further losses in plasma volume and increases in total body and organ albumin escape rates with the exception of the lung and kidney. These results indicate a role for endogenous endothelin in mediating losses in plasma volume and albumin escape elicited by LPS predominantly through activation of ET(A) receptors, and suggest that by attenuating these events, ET(A) or dual ET(A)/ET(B) receptor blockers may be useful agents in the therapy of septic shock.

Endothelins and the lung

Since endothelins were discovered by Yanasigawa in 1988 it has been recognised that they may have an important role in lung pathophysiology. Despite their biological importance as vasoconstrictors the physiological role of endothelin has not yet been defined within the lungs. This review explores their role in acute and chronic disease. During acute inflammation and ischaemia-reperfusion injury cytokines may induce release of endothelin. This is important in the realm of acute lung injury and during surgical procedures such as cardiopulmonary operations including lung resections and transplantation. Complications of surgery including primary organ failure resulting in poor gas exchange as well as increased pulmonary vascular resistance have been linked to the presence of excessive endothelin. Endothelin may have an important role in transplantation biology. The complex process leading to successful lung transplantation includes optimising the donor with brain death, harvesting the lungs, managing acute and chronic rejection, and protecting the vital organs from toxic effects of immunosuppressants. During chronic disease processes, the mitotic action of endothelin may be important in vascular and airway remodelling by means of smooth muscle cell proliferation. We also explore recent advances in drug development, animal models and future directions for research.

Coronary endothelial function is preserved with chronic endothelin receptor antagonism in experimental hypercholesterolemia in vitro

Hypercholesterolemia is associated with increased circulating and tissue endothelin-1 immunoreactivity, decreased nitric oxide (NO) activity, and altered endothelial function. We tested the hypothesis that chronic endothelin receptor antagonism preserves endothelial function and increases NO in experimental porcine hypercholesterolemia. Pigs were randomized to 3 groups: Group 1, a 2% high-cholesterol (HC) diet alone (n=7); group 2, RO-48-5695, a combined endothelin receptor antagonist, and an HC diet (n=8); and group 3, ABT-627, a selective endothelin-A receptor antagonist, and an HC diet (n=8). Coronary epicardial and arteriolar endothelial function was determined by a dose-response relaxation to bradykinin (10(-11) to 10(-6) mol/L), in all groups and in pigs maintained on a normal diet. Plasma total oxidized products of NO (NO(x)) were determined by chemiluminescence at baseline and after 12 weeks. Bradykinin-stimulated coronary epicardial and arteriolar relaxation in group 1 was attenuated compared with normal-diet controls. This relaxation was normalized with endothelin receptor antagonism. Plasma NO(x) decreased after 12 weeks in group 1 (-74.8+/-5.5%). This decrease was attenuated in the endothelin receptor antagonist groups (group 2, -28.2+/-15.0%; group 3, -38.9+/-20.6%). Chronic endothelin receptor antagonism preserves coronary endothelial function and increases NO in hypercholesterolemia. This study supports a role of endothelin-1 in the regulation of NO activity and suggests a possible therapeutic role for endothelin receptor antagonists in hypercholesterolemia.

The effect of an endothelin antagonist on blood pressure in a rat model of preeclampsia

OBJECTIVE

We attempted to determine the role of endothelin in a previously characterized animal model of preeclampsia by studying the effect of a specific endothelin antagonist, BQ123, on blood pressure.

STUDY DESIGN

A preeclampsia-like condition was induced by infusing pregnant rats with the nitric oxide synthase inhibitor N(G)-nitro-L -arginine methyl ester. Osmotic minipumps were inserted subcutaneously into timed pregnant Harlan-Sprague-Dawley rats on day 17 of pregnancy (term, 22 days). The pumps were loaded to continuously deliver either vehicle (control group) or N(G)-nitro-L -arginine methyl ester 50 mg/d, either alone or with BQ123 at 0.5 mg/d. In a similar but separate experiment, the dose of BQ123 was increased to 1 mg/d. Blood pressure was measured with the tail-cuff method before pump insertion and then daily until postpartum day 2.

RESULTS

Except for a decrease on the day after pump insertion, BQ123 0.5 mg/d had no significant effect on the hypertension induced by N(G)-nitro-L -arginine methyl ester. At the higher dose, however, BQ123 significantly attenuated the increase in blood pressure induced by N(G)-nitro-L -arginine methyl ester during most of the study period.

CONCLUSION

The effect of nitric oxide inhibition can be successfully attenuated by the use of an endothelin antagonist, thereby supporting the role of endothelin in the hypertension described with the preeclampsialike condition seen in pregnant rats.

Contribution of endogenous endothelin to large epicardial coronary artery tone in dogs and humans

Nitric oxide (NO) may normally impair endothelin (ET) activity in epicardial coronary arteries. Lifting this inhibitory feedback could reveal ET-dependent effects involving ET(A)- and/or ET(B)-receptor activation. In conscious dogs, the blockade of ET(A) receptors (intracoronary Ro-61-1790) increased external circumflex coronary artery diameter (CD) (sonomicrometry) by 0.10 +/- 0.01 from 3.04 +/- 0.12 mm (P < 0.01) without altering coronary blood flow (Doppler). Similarly, CD increased (0.09 +/- 0.01 from 2.91 +/- 0.14 mm; P < 0. 01) when Ro-61-1790 was given after blockade of NO formation with intracoronary N(omega)-nitro-L-arginine methyl ester (L-NAME). In contrast, ET(B)-receptor blockade (intracoronary Ro-46-8443) did not influence baseline CD with and without L-NAME. In vitro, increases in tension caused by N(omega)-nitro-L-arginine (L-NNA) or PGF(2alpha) in arterial rings were reduced by ET(A)- but not ET(B)-receptor blockade. ET(A)-receptor blockade also reduced the increase in tension caused by L-NNA in human coronary arterial rings. Thus ET(A) receptors, but not ET(B) receptors, account for ET-dependent constriction in canine epicardial coronary arteries in vivo. ET-dependent effects were independent of the level of NO formation in vitro and in vivo. In human epicardial coronary arterial rings, ET(A)-receptor blockade also caused significant relaxation.

Role of angiotensin II in modulating the hemodynamic effects of nitric oxide synthesis inhibition

This study examined the role of ANG II in modulating the increase of hematocrit and vascular permeability that follows nitric oxide (NO) synthesis blockade, that are contributing to the decrease in cardiac index (CI) in conscious, chronically catheterized rats. Pretreatment with losartan attenuated the N(omega)-nitro-L-arginine methyl ester (L-NAME)-induced increase in total peripheral resistance by 26% and also blunted the fall in CI (28%) and stroke volume. L-NAME produced an increase in hematocrit (4.5%) and in (125)I-labeled albumin content in the heart and small intestine in untreated rats, but the increase was prevented in rats pretreated with losartan. Furthermore, L-NAME induced a transient increase of plasma protein concentration and tissue intestinal blood flow, which was abolished in rats given losartan. The results of the present study indicate that the systemic hemodynamic responses, the fall in plasma volume, and the increase in albumin escape observed after inhibition of NO synthesis are in part the consequence of unmasking the actions of endogenous ANG II. These data suggest a physiological role for NO by restraint of the vascular actions of the renin-angiotensin system.

Acute interactions between endothelin and nitric oxide in the control of renal haemodynamics

1. Endogenous endothelin (ET) does contribute to control of renal vascular tone via nitric oxide (NO)-dependent vasodilation in the rat. 2. Endothelin mediates some of the renal vascular responses to acute nitric oxide synthase (NOS) inhibition, being particularly important when a rise in renal perfusion pressure occurs. 3. Tonically produced NO blunts the renal vasoconstrictor responses to acutely administered ET. 4. The similarity between the renal vascular responses to ET administration and NOS inhibition is not fortuitous but, in part, reflects important interactions between these vasoactive agents.

Human renal and systemic hemodynamic, natriuretic, and neurohumoral responses to different doses of L-NAME

Experimental evidence indicates that the renal circulation is more sensitive to the effects of nitric oxide (NO) synthesis inhibition than other vascular beds. To explore whether in men the NO-mediated vasodilator tone is greater in the renal than in the systemic circulation, the effects of three different intravenous infusions of NG-nitro-L-arginine methyl ester (L-NAME; 1, 5, and 25 microg. kg-1. min-1 for 30 min) or placebo on mean arterial pressure (MAP), systemic vascular resistance (SVR), renal blood flow (RBF), renal vascular resistance (RVR), glomerular filtration rate (GFR), and fractional sodium and lithium excretion (FENa and FELi) were studied in 12 healthy subjects, each receiving randomly two of the four treatments on two different occasions. MAP was measured continuously by means of the Finapres device, and stroke volume was calculated by a model flow method. GFR and RBF were estimated from the clearances of radiolabeled thalamate and hippuran. Systemic and renal hemodynamics were followed for 2 h after start of infusions. During placebo, renal and systemic hemodynamics and FENa and FELi remained stable. With the low and intermediate L-NAME doses, maximal increments in SVR and RVR were similar: 20.4 +/- 19.6 and 23.5 +/- 16.0%, respectively, with the low dose and 31.4 +/- 26.7 and 31.2 +/- 14.4%, respectively, with the intermediate dose (means +/- SD). With the high L-NAME dose, the increment in RVR was greater than the increment in SVR. Despite a decrease in RBF, FENa and FELi did not change with the low L-NAME dose, but they decreased by 31.2 +/- 11.0 and 20.2 +/- 6.3%, respectively, with the intermediate dose and by 70.8 +/- 8.1 and 31.5 +/- 15.9% with the high L-NAME dose, respectively. It is concluded that in men the renal circulation is not more sensitive to the effects of NO synthesis inhibition than the systemic circulation and that the threshold for NO synthesis inhibition to produce antinatriuresis is higher than the threshold level to cause renal vasoconstriction.

Estrogen attenuates endothelin-1 production by bovine endothelial cells via estrogen receptor

To investigate the mechanism underlying the antiatherogenic effect of estrogen, we examined the role of estrogen in the regulation of endothelin-1 (ET-1) production by cultured bovine carotid arterial endothelial cells. Treatment of the cells with 17beta-estradiol dose dependently inhibited gene expression and peptide secretion of ET-1. This inhibitory effect of 17beta-estradiol was more marked when ET-1 production was stimulated by transforming growth factor-beta1. Simultaneous addition of ICI 182,780, a pure estrogen receptor antagonist, completely blocked the effect of 17beta-estradiol, indicating that 17beta-estradiol acted via estrogen receptor. These results suggest that reduced ET-1 production by endothelial cells may be involved in the vasoprotective effect of estrogen.

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.