Salt-dependency of endothelin-induced, chronic hypertension in conscious rats

Renal denervation attenuates hypertension but not salt sensitivity in ETB receptor-deficient rats

Hypertension is a prevalent pathology that increases risk for numerous cardiovascular diseases. Because the etiology of hypertension varies across patients, specific and effective therapeutic approaches are needed. The role of renal sympathetic nerves is established in numerous forms of hypertension, but their contribution to salt sensitivity and interaction with factors such as endothelin-1 are poorly understood. Rats deficient of functional ET_B receptors (ET_B-def) on all tissues except sympathetic nerves are hypertensive and exhibit salt-sensitive increases in blood pressure. We hypothesized that renal sympathetic nerves contribute to hypertension and salt sensitivity in ET_B-def rats. The hypothesis was tested through bilateral renal sympathetic nerve denervation and measuring blood pressure during normal salt (0.49% NaCl) and high-salt (4.0% NaCl) diets. Denervation reduced mean arterial pressure in ET_B-def rats compared with sham-operated controls by 12 ± 3 (SE) mmHg; however, denervation did not affect the increase in blood pressure after 2 wk of high-salt diet (+19 ± 3 vs. +16 ± 3 mmHg relative to normal salt diet; denervated vs. sham, respectively). Denervation reduced cardiac sympathetic-to-parasympathetic tone [low frequency-high frequency (LF/HF)] during normal salt diet and vasomotor LF/HF tone during high-salt diet in ET_B-def rats. We conclude that the renal sympathetic nerves contribute to the hypertension but not to salt sensitivity of ET_B-def rats.

Salt Sensitivity of Angiogenesis Inhibition-Induced Blood Pressure Rise: Role of Interstitial Sodium Accumulation?

In response to salt loading, Na⁺ and Cl^- accumulate in the skin in excess of water, stimulating skin lymphangiogenesis via activation of the mononuclear phagocyte system cell-derived vascular endothelial growth factor-C-vascular endothelial growth factor type 3 receptor signaling pathway. Inhibition of this pathway results in salt-sensitive hypertension. Sunitinib is an antiangiogenic, anticancer agent that blocks all 3 vascular endothelial growth factor receptors and increases blood pressure. We explored the salt dependency of sunitinib-induced hypertension and whether impairment of skin lymphangiogenesis is an underlying mechanism. Normotensive Wistar-Kyoto rats were exposed to a normal or high salt with or without sunitinib administration. Sunitinib induced a 15 mm Hg rise in telemetrically measured blood pressure, which was aggravated by a high-salt diet (HSD), resulting in a decline of the slope of the pressure-natriuresis curve. Without affecting body weight, plasma Na⁺ concentration or renal function, Na⁺ and Cl^- skin content increased by 31% and 32% with the high salt and by 49% and 50% with the HSD plus sunitinib, whereas skin water increased by 17% and 24%, respectively. Skin mononuclear phagocyte system cell density increased both during sunitinib and a HSD, but no further increment was seen when HSD and sunitinib were combined. HSD increased skin lymphangiogenesis, while sunitinib tended to decrease lymphangiogenesis, both during a normal-salt diet and HSD. We conclude that sunitinib induces hypertension that is aggravated by high salt intake and not accompanied by impaired skin lymphangiogenesis.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

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.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Interleukin-1beta, but not interleukin-6, enhances renal and systemic endothelin production in vivo

The inflammatory cytokines IL-1beta and IL-6 have been shown to stimulate production of endothelin-1 (ET-1) by several cell types in vitro, but their effects on renal ET-1 production in vivo are not known. To test whether IL-1beta and IL-6 stimulate renal ET-1 production and release in vivo, urine was collected from male C57BL/6 mice over 24-h periods at baseline and on days 7 and 14 of a 14-day subcutaneous infusion of IL-1beta (10 ng/h), IL-6 (16 ng/h), or vehicle. By day 14, plasma ET-1 was significantly increased by IL-1beta infusion (1.7 +/- 0.1 vs. 0.8 +/- 0.1 pg/ml for vehicle, P < 0.001). Compared with vehicle infusion, IL-1beta infusion induced significant increases in urinary ET-1 excretion rate and urine flow but did not affect conscious mean arterial pressure (telemetry). IL-1beta infusion significantly increased renal cortical and medullary IL-1beta content (ELISA) and prepro-ET-1 mRNA expression (quantitative real-time PCR). In contrast, 14 days of IL-6 infusion had no significant effect on plasma ET-1 or urinary ET-1 excretion rate. To determine whether IL-1beta stimulates ET-1 release via activation of NF-kappaB, inner medullary collecting duct (IMCD-3) cells were incubated for 24 h with IL-1beta, and ET-1 release and NF-kappaB activation were measured (ELISA). IL-1beta activated NF-kappaB and increased ET-1 release in a concentration-dependent manner. The effect of IL-1beta on ET-1 release could be partially inhibited by pretreatment of IMCD-3 cells with an inhibitor of NF-kappaB activation (BAY 11-7082). These results indicate that IL-1beta stimulates renal and systemic ET-1 production in vivo, providing further evidence that ET-1 participates in inflammatory responses.

Chronic Activation of Endothelin B Receptors

Endothelin (ET) exerts powerful pressor actions primarily through activation of the ET(A) receptor subtype. The ET(B) receptor (ET(B)R) subtype, on the other hand, is generally thought to initiate physiological actions that decrease arterial pressure. Such actions include clearing ET from the bloodstream, initiating endothelium-mediated vasodilation, and facilitating renal sodium and water excretion. The effect of long-term activation of the ET(B)R on arterial pressure, however, never has been directly tested. In this study we evaluated cardiovascular responses to chronic (5-day) activation of ET(B)R in male rats using continuous intravenous infusion of the selective agonist sarafotoxin 6c. Surprisingly, we found that sarafotoxin 6c caused a sustained increase in arterial pressure that rapidly reversed on termination of infusion. The hypertension was associated with increased renal excretion of sodium and water and decreased plasma volume. Alterations in daily sodium intake did not affect the magnitude of the hypertension. Hemodynamic studies revealed a decreased cardiac output and increased total peripheral resistance during sarafotoxin 6c infusion. Infusion of sarafotoxin 6c caused a small increase in plasma ET levels. Nevertheless, the hypertension was not affected by coadministration of a selective ET(A) receptor antagonist (atrasentan) but was completely prevented by treatment with a combined ET(A) receptor and ET(B)R antagonist (A186280). These experiments reveal for the first time that chronic activation of ET(B)R in rats causes sustained hypertension.

Endothelin 1, its Endothelin Type A Receptor, Connective Tissue Growth Factor, Platelet-Derived Growth Factor, and Adrenomedullin Expression in Lungs of Pulmonary Hypertensive and Nonhypertensive Chickens

Twenty-four 1-d-old broilers were distributed in 2 groups, pulmonary hypertensive broilers (PHB) and pulmonary nonhypertensive broilers (NPHB), to estimate possible differences between them in the expression of endothelin 1 (ET-1) and its type A receptor, connective tissue growth factor, platelet-derived growth factor, and adrenomedullin expression in the lungs. For this purpose, total RNA extraction and real-time PCR analysis were used. Endothelin 1 mRNA levels in the lungs of PHB were significantly higher than the corresponding level in NPHB (P < 0.001). In contrast, the opposite was true for ET-1 type A receptor mRNA levels (P < 0.001). Connective tissue growth factor mRNA levels in the lungs of PHB were significantly higher than in the lungs of NPHB (P < 0.01). However, no differences were encountered between the 2 groups of broilers in platelet-derived growth factor mRNA expression (P > 0.05). Adrenomedullin mRNA levels in the lungs of PHB were significantly higher than in NPHB (P < 0.01). It has been demonstrated for the first time that ET-1, connective tissue growth factor, and adrenomedullin are upregulated in the lungs of PHB. Furthermore, it is suggested that these peptides may play a major role in pulmonary hypertension pathophysiology. Present data might provide clues for future research directions such as genetic selection and therapeutic intervention to revert the process of pulmonary vasoconstriction and vascular remodeling. Major research goals could be to find endothelium-derived factors that probably trigger endothelial dysfunction, as well as possible interactions with already identified molecules which also intervene in the pulmonary response to hypoxia.

Stimulation of serum- and glucocorticoid-regulated kinase-1 gene expression by endothelin-1

The serum- and glucocorticoid-regulated kinase-1 (SGK1) participates in the regulation of sodium homeostasis and blood pressure by mineralocorticoids. Aldosterone rapidly induces SGK1 transcription, which contributes to the activation of renal epithelial sodium channels. Another important regulator of blood pressure is the vasoactive hormone endothelin-1 (ET-1) that is systemically upregulated in chronic renal failure. In the present study, we investigated whether ET-1 modulates SGK1 expression, and thereby might explain some of its hypertensive effects. As assessed by real-time PCR analysis, ET-1 triggered the rapid increase of SGK1 mRNA levels in A-10 smooth muscle cells and also in intact aortas of adult rats. In A-10 cells transcriptional activation was associated with a more than 6-fold upregulation of SGK1 protein expression and in similar range as found after treatment with aldosterone. A stimulatory effect of ET-1 was not only observed in isolated cells, but also in an animal model. Upon subtotal nephrectomy (SNX) of rats, myocardial ET-1 levels strongly increased, which was followed by a more than 2-fold induction of SGK1 expression in the left ventricle. The myocardial upregulation of SGK1 was completely abrogated by a specific ET(A) receptor antagonist, thereby substantiating the in vivo role of ET-1 in SGK1 expression. Thus, these data demonstrate that ET-1 increases expression of SGK1 in vivo and in vitro, and therefore indicate that SGK1 upregulation might be involved in ET-1-dependent regulation of blood pressure and cardiac modelling during mild renal failure.

Cardiovascular endothelins: essential regulators of cardiovascular homeostasis

The endothelin (ET) system consists of 3 ET isopeptides, several isoforms of activating peptidases, and 2 G-protein-coupled receptors, ETA and ETB, that are linked to multiple signaling pathways. In the cardiovascular system, the components of the ET family are expressed in several tissues, notably the vascular endothelium, smooth muscle cells, and cardiomyocytes. There is general agreement that ETs play important physiological roles in the regulation of normal cardiovascular function, and excessive generation of ET isopeptides has been linked to major cardiovascular pathologies, including hypertension and heart failure. However, several recent clinical trials with ET receptor antagonists were disappointing. In the present review, the authors take the stance that ETs are mainly and foremost essential regulators of cardiovascular function, hence that antagonizing normal ET actions, even in patients, will potentially do more harm than good. To support this notion, we describe the predominant roles of ETs in blood vessels, which are (indirect) vasodilatation and ET clearance from plasma and interstitial spaces, against the background of the subcellular mechanisms mediating these effects. Furthermore, important roles of ETs in regulating and adapting heart functions to different needs are addressed, including recent progress in understanding the effects of ETs on diastolic function, adaptations to changes in preload, and the interactions between endocardial-derived ET-1 and myocardial pump function. Finally, the potential dangers (and gains) resulting from the suppression of excessive generation or activity of ETs occurring in some cardiovascular pathological states, such as hypertension, myocardial ischemia, and heart failure, are discussed.

High salt diet enhances cardiovascular responses from the nucleus tractus solitarius and ventrolateral medulla of Sprague-Dawley rats

High salt intake has been shown to augment the sensitivity of rostral ventrolateral medulla (RVLM) sympathoexcitatory neurons. We examined the effects of 4 weeks of high dietary salt (8%) on the sensitivity of nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM) in controlling RVLM. In chloralose-anesthetized Sprague-Dawley rats, high salt intake did not elevate baseline arterial pressure or heart rate (HR). In high-salt group, NTS, CVLM, and RVLM responses to glutamate were greater. NTS responses to acetylcholine or serotonin, which is independent of baroreflex, also were greater. Phenylephrine or nitroprusside (i.v.) elicited similar changes in arterial pressure and heart rate, the baroreflex sensitivity also was similar in both groups of rats. These results suggest that high salt intake augments the sensitivity of NTS and CVLM sending inhibitory input to RVLM. This presumably may inhibit the RVLM, thereby inhibiting the elevation of arterial pressure.

Endothelium-Restricted Overexpression of Human Endothelin-1 Causes Vascular Remodeling and Endothelial Dysfunction

Background— Endothelin (ET)-1 is a potent vasoconstrictor that contributes to vascular remodeling in hypertension and other cardiovascular diseases. Endogenous ET-1 is produced predominantly by vascular endothelial cells. To directly test the role of endothelium-derived ET-1 in cardiovascular pathophysiology, we specifically targeted expression of the human preproET-1 gene to the endothelium by using the Tie-2 promoter in C57BL/6 mice.

Methods and Results— Ten-week-old male C57BL/6 transgenic (TG) and nontransgenic (wild type; WT) littermates were studied. TG mice exhibited 3-fold higher vascular tissue ET-1 mRNA and 7-fold higher ET-1 plasma levels than did WT mice but no significant elevation in blood pressure. Despite the absence of significant blood pressure elevation, TG mice exhibited marked hypertrophic remodeling and oxidant excess-dependent endothelial dysfunction of resistance vessels, altered ET-1 and ET-3 vascular responses, and significant increases in ET B expression compared with WT littermates. Moreover, TG mice generated significantly higher oxidative stress, possibly through increased activity and expression of vascular NAD(P)H oxidase than did their WT counterparts.

Conclusions— In this new murine model of endothelium-restricted human preproET-1 overexpression, ET-1 caused structural remodeling and endothelial dysfunction of resistance vessels, consistent with a direct nonhemodynamic effect of ET-1 on the vasculature, at least in part through the activation of vascular NAD(P)H oxidase.

Prevention of endothelin-1-induced increases in blood pressure: role of endogenous CGRP

To determine the role of endothelin-1 (ET-1) and its receptors in the regulation of calcitonin gene-related peptide (CGRP) release, male Wistar rats were divided into six groups and subjected to the following treatments for 1 wk with or without ABT-627 (an ET(A) receptor antagonist, 5 mg.kg(-1).day(-1) in drinking water) or A-192621 (an ET(B)-receptor antagonist, 30 mg.kg(-1).day(-1) by oral gavage): control (Con), ET-1 (5 ng.kg(-1).min(-1) iv), Con + ABT-627, Con + A-192621, ET-1 + ABT-627, and ET-1 + A-192621. Baseline mean arterial pressure (MAP, mmHg) was higher (P < 0.05) in Con + A-192621 (122 +/- 4) and ET-1 + A-192621 (119 +/- 4) groups compared with Con (104 +/- 6), ET1 (106 +/- 3), Con + ABT-627 (104 +/- 3), and ET1 + ABT-627 (100 +/- 3) groups. Intravenous administration of CGRP(8-37) (a CGRP receptor antagonist, 1 mg/kg) increased MAP (P < 0.05) in ET-1 (13 +/- 1), Con + A-192621 (12 +/- 1), and ET-1 + A-192621 (15 +/- 3) groups compared with Con (4 +/- 1), Con-ABT-627 (4 +/- 1), and ET-1 + ABT-627 (5 +/- 1) groups. Plasma CGRP levels (in pg/ml) were increased (P < 0.05) in ET-1 (57.5 +/- 6.1), Con + A-192621 (53.9 +/- 3.4), and ET-1 + A-192621 (60.4 +/- 3.0) groups compared with Con (40.4 +/- 1.6), Con + ABT-627 (40.0 +/- 2.9), and ET-1 + ABT-627 (42.6 +/- 1.9) groups. Plasma ET-1 levels (in pg/ml) were higher (P < 0.05) in ET-1 (2.8 +/- 0.2), ET-1 + ABT-627 (3.2 +/- 0.4), Con + A-192621 (3.3 +/- 0.4), and ET-1 + A-192621 (4.6 +/- 0.3) groups compared with Con (1.1 +/- 0.2) and Con-ABT-627 (1.3 +/- 0.2) groups. Therefore, our data show that ET-1 infusion leads to increased CGRP release via activation of the ET(A) receptor, which plays a compensatory role in preventing ET-1-induced elevation in blood pressure.

Endothelin-induced increases in Ca2+ entry mechanisms of vascular contraction are enhanced during high-salt diet

High-salt diet is often associated with increases in arterial pressure, and a role for endothelin (ET)-1 in salt-sensitive hypertension has been suggested; however, the vascular mechanisms involved are unclear. We investigated whether ET increases the sensitivity of the mechanisms of vascular contraction to changes in dietary salt intake. Active stress and 45Ca2+ influx were measured in endothelium-denuded aortic strips of male Sprague-Dawley rats not treated or chronically infused intravenously with ET (5 pmol/kg per minute) and fed either normal-sodium diet (NS, 1%) or high-sodium diet (HS, 8%) for 9 days. Phenylephrine (Phe) caused increases in active stress that were similar in NS and HS, but were greater in NS/ET (maximum, 10.5+/-0.7) than in NS (maximum, 7.4+/-0.9) rats, and further enhanced in HS/ET (maximum, 14.4+/-1.1) compared with HS rats (maximum, 8.0+/-0.8 x 10(4)N/m2). Phe was more potent in causing contraction in NS/ET than in NS rats and in HS/ET than in HS rats. In Ca2+-free (2 mmol/L EGTA) Krebs, stimulation of intracellular Ca2+ release by Phe (10(-5) mol/L) or caffeine (25 mmol/L) caused a transient contraction that was not significantly different in all groups of rats. In contrast, membrane depolarization by high-KCl solution, which stimulates Ca2+ entry from the extracellular space, caused greater contraction in ET-infused rats, particularly those on HS diet. Phe (10(-5) mol/L) caused an increase in 45Ca2+ influx that was greater in NS/ET (27.9+/-1.7) than in NS (20.1+/-1.8) rats and further enhanced in HS/ET (35.2+/-1.8) compared with HS rats (21.8+/-1.9 micromol/kg/min). The Phe-induced 45Ca2+ influx-stress relation was not different between NS and HS rats, but was enhanced in ET-infused rats particularly those on HS. The enhancement of the 45Ca2+ influx-active stress relation in ET-infused rats was not observed in vascular strips treated with the protein kinase C inhibitor GF109203X or calphostin C (10(-6) mol/L). Thus, low-dose infusion of ET, particularly during HS, is associated with increased vascular reactivity that involves Ca2+ entry from the extracellular space, but not Ca2+ release from the intracellular stores. The ET-induced enhancement of the Ca2+ influx-stress relation particularly during HS suggests activation of other mechanisms in addition to Ca2+ entry, possibly involving protein kinase C. The results suggest that ET increases the sensitivity of the mechanisms of vascular smooth muscle contraction to high dietary salt intake and may, in part, explain the possible role of ET in salt-sensitive hypertension.

Role of ET(A) receptors in experimental ANG II-induced hypertension in rats

The objectives were to determine if ANG II-induced hypertension is maintained by activation of endothelin type A (ET(A)) receptors by endogenous ET-1 and if this effect is influenced by salt intake. Male rats were maintained on high sodium intake (HS; 6 meq/day) or on normal sodium intake (NS; 2 meq/day). Hypertension was produced by intravenous infusion of ANG II (5 ng/min) for 15 days. Five-day oral dosing with the selective ET(A)-receptor antagonist ABT-627 (~2 mg. kg(-1). day(-1)) reduced mean arterial pressure (MAP) to baseline levels in rats on HS receiving ANG II infusion, but it did not affect MAP in normotensive HS controls. In rats on NS, ABT-627 only transiently decreased MAP in rats receiving ANG II and slightly reduced MAP in normotensive controls. ABT-627 produced mild retention of sodium and water in NS rats receiving ANG II, but not in any other group. These results indicate that ET-1 plays a role in ANG II-induced hypertension via activation of ET(A) receptors and that this role is more prominent in rats on HS.

Factors affecting endothelin-induced venous tone in conscious rats

There may be a relation between altered venous function, endothelin (ET)-1, and an impairment in the activity of endothelial-derived nitric oxide (NO) and prostanoids in salt-dependent hypertension. The present study examined the effects of salt intake on ET-induced changes in venomotor tone and the effects of blockade of NO synthase with N(G)-nitro-L-arginine methyl ester (L-NAME) and of cyclooxygenase with indomethacin on venomotor tone caused by the ET(B) selective agonist sarafotoxin 6c (S6c) in awake rats. Rats were anesthetized for permanent placement of catheters for measurements of arterial and venous pressures. A silicone balloon catheter was also fixed in the right atrium to produce brief circulatory arrest. Venomotor tone was estimated from measurements of mean circulatory filling pressure (MCFP) in conscious rats. There were no differences in mean arterial pressure, heart rate, or MCFP responses to short-term administration of ET-1 or S6c at different levels of salt intake. L-NAME or indomethacin did not change MCFP or the response of MCFP to short-term injection of S6c. In conclusion, neither basal MCFP nor integrated venomotor responses to short-term injection of ET-1 or S6c were altered by short-term changes in salt intake, blockade of NO synthase or cyclooxygenase. These data do not support the hypothesis that increased salt intake alters reactivity of veins to ET-1, NO, or prostanoids.

Role of endothelin in intermittent hypoxia-induced hypertension

Clinical studies suggest that sleep apnea causes systemic hypertension. In addition, patients with sleep apnea have elevated plasma levels of endothelin-1 (ET-1). We hypothesized that the intermittent hypoxia/hypercapnia (IH) associated with sleep apnea causes hypertension by increasing ET-1 production. To test this hypothesis, rats with arterial and venous catheters were placed in Plexiglas chambers. IH rat chambers were flushed with an N(2)-CO(2) mixture for 90 seconds to achieve hypoxia/hypercapnia (5% O(2)-5% CO(2)) followed by 90 seconds of compressed air to achieve normoxia (21% O(2)-0% CO(2)). Control rat chambers were flushed with 90 seconds of air-air cycles. Cycles for both groups were repeated 8 hours per day for 11 days. Resting mean arterial pressure (MAP) and heart rate were recorded daily before the start of exposure. After 11 days, MAP was significantly elevated in IH rats compared with initial MAP (109+/-5 mm Hg initial, 139+/-11 mm Hg day 11) and compared with air-air rats (110+/-4 mm Hg). On day 11, cumulative doses of PD145065 (a nonselective ET-receptor antagonist) were administered intravenously to the rats breathing room air. PD145065 caused a dose-dependent decrease in MAP in IH rats but did not alter MAP in air-air rats. Plasma ET-1 measured by radioimmunoassay was significantly increased on days 5 and 11 in the IH rats compared with day 1 and compared with air-air rats. There was no significant change in plasma ET-1 over time in air-air rats. We conclude that IH exposure increases both MAP and plasma ET-1 and that the increased ET-1 may contribute to the hypertension.

Acute hypertension inhibits thirst stimulated by ANG II, hyperosmolality, or hypovolemia in rats

The present study sought to determine whether increases in arterial blood pressure inhibited drinking behavior evoked by ANG II, hyperosmolality, or hypovolemia in rats. Cumulative water intakes in 60- or 90-min tests and latency to the first lick were recorded as indexes of thirst. During intravenous infusions of 100 ng. kg(-1). min(-1) ANG II, attenuation of the induced increases in arterial pressure with the arteriolar vasodilator diazoxide resulted in greater water intakes and shorter latencies to drink. Drinking behavior stimulated by intravenous infusion of hypertonic saline was significantly inhibited by increases in arterial pressure caused by intravenous infusion of phenylephrine or endothelin-1, and this inhibition of drinking was proportional to the induced increase in pressure. Upon termination of the phenylephrine infusion, mean arterial pressure returned to basal values, and drinking was restored. Phenylephrine-induced increases in arterial pressure also inhibited drinking behavior in response to hypovolemia that could not be explained by differences in plasma renin activity, plasma protein concentration, or plasma osmolality. Thus increases in arterial pressure inhibit water drinking behavior in response to each of these three thirst stimuli in rats.

Increased renal medullary endothelin-1 synthesis in prehypertensive DOCA- and salt-treated rats

To investigate the role of renal endothelin-1 (ET-1) synthesis in water-sodium homeostasis, we measured mRNA expressions, protein levels, enzyme activity, and receptor binding of the renal ET-1 system in a DOCA- and salt-treated rat model. Male Wistar rats were divided into control and DOCA- and salt-treated (DOCA-Salt) groups. The DOCA-Salt group received 25 mg/kg body wt DOCA and was maintained on 1% NaCl drinking water. Rats were killed on days 1, 2, 4, and 10 of the experiment. Urinary ET-1-like immunoreactivity significantly increased from the second day in the DOCA-Salt group and correlated well with the urinary sodium excretion rate (r = 0.81, P < 0.001). Renal endothelin-converting enzyme (ECE) activity, ET-1, and ECE-1 mRNA expressions were significantly increased in the renal medullary area of DOCA-Salt rats. In situ hybridization and immunohistochemical studies showed that the increase in ET-1 synthesis was mainly localized in the inner medullary collecting ducts. The maximum binding of endothelin B receptor also increased from the second day in the renal medulla of the DOCA-Salt group. Our results suggest that renal medullary synthesized ET-1 may be a natriuretic factor and may participate in the intrarenal regulation of water and salt homeostasis in prehypertensive DOCA-and salt-treated rats.

Mechanisms of increased venous smooth muscle tone in desoxycorticosterone acetate-salt hypertension

The purpose of the present study was to identify mechanisms that contribute to increased venous smooth muscle tone in desoxycorticosterone acetate (DOCA)-salt hypertension in rats. Male Sprague-Dawley rats were uninephrectomized, received subcutaneous implants of DOCA, and drank 1% sodium chloride/0.2% potassium chloride solutions. Sham-operated rats received only uninephrectomy and drank tap water. Three to 4 weeks later, arterial and venous catheters were implanted for measurements of arterial and central venous pressures, respectively, and a silicone balloon catheter was permanently fixed in the right atrium to produce brief circulatory arrest. Venous smooth muscle activity was estimated on the basis of repeated measurements of mean circulatory filling pressure in conscious rats resting in their home cages. DOCA-salt-treated rats were hypertensive and had elevated mean circulatory filling pressure compared with normotensive sham-operated rats. Blockade of the endothelin subtype A receptor with 1 mg/kg ABT-627 IV decreased arterial blood pressure and mean circulatory filling pressure significantly more in hypertensive rats than in normotensive rats. Ganglionic blockade with 30 mg/kg hexamethonium IV also decreased arterial blood pressure and mean circulatory filling pressure more in hypertensive than in normotensive rats. Pretreatment with ABT-627 did not affect subsequent hemodynamic responses to ganglionic blockade. We conclude that venous smooth muscle tone is increased in DOCA-salt hypertension through the independent actions of both endogenous endothelin-1 acting on subtype A receptors and sympathetically mediated venoconstrictor activity.

Endothelin and the central and peripheral nervous systems: a decade of endothelin research

1. During the past decade, extensive investigation of the endothelin (ET) system, primarily characterized by its potent vasoactive peptide element ET-1, has suggested a prominent role for this humoral agent and its isopeptides in cardiovascular and neural regulation. 2. Major elements of this system, including its peptide isoforms, converting enzymes involved in their formation and metabolism, as well as multiple receptor subtypes, have been localized within various components of the cardiovascular system and the central and peripheral nervous systems. 3. An understanding of the possible roles for the ET system in neural regulation and development has progressed over the past several years; most notable is the influence of ET on the central control of cardiovascular function and sympathetic tone. 4. The present concentrated review of ET and the central and peripheral nervous systems is presented to introduce the other papers within this symposium by briefly describing the presence and influence of components of this unique peptide system within pertinent physiological structures involved in cardiovascular, adrenal, sympathetic and enteric developmental function.

Pathophysiological roles of endothelin-1 in Dahl salt-sensitive hypertension

The purpose of the present experiment was to study the pathophysiological roles of endothelin-1 (ET-1) in salt-sensitive hypertension with the use of Dahl salt-sensitive (DS) and salt-resistant (DR) rats. PreproET-1 mRNA expression was determined by reverse transcription-polymerase chain reaction. In the kidney, expression of preproET-1 mRNA was greater in DS rats on a normal salt diet compared with DR rats of the same age. In DS rats, the level of preproET-1 mRNA expression in kidney had a significant correlation with systolic blood pressure. The expression of preproET-1 mRNA in aorta and kidney was increased by 3-week high salt intake in DS rats but not in DR rats. Expression of preproET-1 mRNA and ET-1 levels in left ventricle was exaggerated by high salt intake in DS rats. However, there was no significant difference in plasma ET-1 levels between DS and DR rats regardless of salt intake. Pressor response curves for ET-1 in DS rats with or without high salt intake were significantly shifted to the left compared with those in DR rats. A single oral dose (3 to 10 mg/kg) of J-104132 (L-753 037), a potent, orally active mixed endothelin A and B (ET(A)/ET(B)) receptor antagonist, reduced blood pressure to normotensive levels in DS rats with high salt intake, and its action was maintained for >/=24 hours. In DS rats with normal salt intake, J-104132 (10 mg/kg) slightly but significantly decreased blood pressure. DR rats did not show obvious depressor responses to J-104132 (10 mg/kg) regardless of salt intake. These results suggest that ET-1 acts as one of the pathophysiological factors in the development and maintenance of salt-sensitive hypertension, and a mixed ET(A)/ET(B) receptor antagonist could be useful in the treatment for salt-sensitive hypertension.

Dietary salt intake alters cardiovascular responses evoked from the rostral ventrolateral medulla

The present experiments examined whether in rats consuming diets with either high NaCl content (8%) or low Na+ content (0.01%) for 2 wk excitatory inputs to the rostral ventrolateral medulla (RVLM) would be altered. In chloralose-anesthetized rats, injection of glutamate into the RVLM elicited a pressor response that, compared with rats fed a control diet, was 50% larger in rats fed a diet containing 8% NaCl and was 25% smaller in rats fed a diet containing 0.01% Na+. Pressor responses produced by electrical stimulation of sciatic nerve afferents, as well as by microinjections into the RVLM of L-dihydroxyphenylalanine or carbachol, were all potentiated by high dietary salt intake and reduced by low dietary salt intake. Dietary salt intake had no effect on pressor responses produced by intravenous injection of phenylephrine, indicating that salt-related alterations in cardiovascular responses produced by central activation could not be accounted for by changes in peripheral vascular reactivity. The decrease in arterial pressure produced by injection of glutamate into the nucleus of the solitary tract was also potentiated by the high salt diet, suggesting that the sensitivity of central baroreceptor reflex pathways may be altered by dietary NaCl. These results indicate that the amount of NaCl consumed in the diet can change the sensitivity of RVLM sympathoexcitatory neurons, and this change in sensitivity is not restricted to any particular class of cell surface receptors.

Renal haemodynamic effects of endothelin-1 and the ETA/ETB antagonist TAK-044 in anaesthetized rabbits

OBJECTIVE

The aim of this study was to test the effects of exogenous endothelin-1 (ET-1) on regional kidney blood flow and renal function, and the renal haemodynamic effects of endogenous ET, in anaesthetized rabbits.

METHODS

ET-1 was infused into the left renal artery at 2 ng/kg/min for 30 min, then at 1 ng/kg/min. Cumulative doses of TAK-044 (0.1-3 mg/kg, i.v.) or its vehicle were given at 30-min intervals. In other rabbits, an extracorporeal circuit was established to adjust renal arterial pressure (RAP) independently of systemic arterial pressure (MAP). RAP was set at 65 mmHg, and either TAK-044 (3 mg/kg, i.v.) or its vehicle was administered.

RESULTS

In the infused kidney ET-1 (2 ng/kg/min) reduced renal blood flow (RBFprobe; 52+/-8%), cortical perfusion (37+/-7%), glomerular filtration rate (GFR; 49+/-8%), urine flow (47+/-14%) and sodium excretion (49+/-13%), but not medullary perfusion (5+/-6%). No effects of ET-1 on MAP or on the contralateral kidney were observed. TAK-044 dose-dependently reversed the effects of ET-1 on RBFprobe and cortical perfusion. TAK-044 also reduced MAP (by up to 11+/-3%) and increased effective renal blood flow in the contralateral kidney (by up to 46+/-27%). In the extracorporeal circuit model, TAK-044 decreased MAP by 12+/-2% and RAP by 10+/-3%, and increased RBF by 9+/-3%.

CONCLUSION

Exogenous ET-1 reduces cortical more than medullary perfusion, and reduces GFR without affecting net tubular sodium and fluid reabsorption. TAK-044 antagonizes local renal vascular responses to ET-1. Endogenous ETs appear to contribute markedly to resting renal vasomotor tone and MAP.

Role of endothelin in hypertension of experimental chronic renal failure

Surgical ablation of renal mass leads to a reduction in kidney function and commonly to the development of hypertension and chronic renal failure (CRF) in rats. The objective of this study was to determine whether endothelin (ET)-1 is involved in the maintenance of the hypertension that accompanies loss of renal mass. First, we demonstrated the antihypertensive efficacy of PD 155080, a selective, orally active ET(A) receptor antagonist, in a group of rats made hypertensive by continuous intravenous infusion of ET-1 (2.5 pmol x kg(-1) x min[-1]) for 7 days. ET-1 produced a sustained hypertension and PD 155080 (56.4 micromol/kg [25mg/kg] BID PO) normalized blood pressure (BP) during the 5 days of drug administration. In a second experiment, Sprague-Dawley rats underwent a 5/6 reduction in renal mass (RRM); 4 weeks later, PD 155080 administered for 7 days resulted in a sustained reduction in BP. Sham-operated rats also showed a slight hypotensive response to PD 155080 administration. Plasma urea nitrogen, plasma creatinine, urinary protein excretion, and creatinine clearance were not altered by PD 155080 administration in RRM or sham rats. In a third experiment, we investigated the contribution of the renin-angiotensin system to BP control in RRM rats given PD 155080. In these rats, PD 155080 reduced BP during 5 treatment days, and this antihypertensive effect was not altered by coadministration of the angiotensin-converting enzyme inhibitor enalapril in the drinking water (508 micromol/L [250 mg/L]). These results demonstrate that (1) ET-1 plays a role in established RRM hypertension through activation of the ET(A) receptor subtype, (2) lowering blood pressure with PD 155080 in RRM rats does not adversely affect renal function, and 3) the antihypertensive effect of ET(A) receptor antagonism is not opposed by the renin-angiotensin system.

Endothelin-1 upregulation in the kidney of uninephrectomized spontaneously hypertensive rats and its modification by the angiotensin-converting enzyme inhibitor quinapril

Endothelin (ET-1) is a potent vasoconstrictor that plays an important role in the control of renal circulation and tubular function. The contribution of this peptide to the pathogenesis of systemic hypertension and renal failure remains largely undefined. In spontaneously hypertensive rats (SHR) uninephrectomized at 20 weeks of age (UNX-SHR) and followed until 45 weeks of age, we determined ET-1 gene expression in renal tissue by reverse transcription-polymerase chain reaction and its localization by in situ hybridization in paraffin-embedded kidney sections. Age-matched SHR and normotensive Wistar-Kyoto (WKY) rats were chosen as controls. At the end of the follow-up, UNX-SHR had high systolic blood pressure, intense proteinuria, mesangial expansion, focal and segmental glomerular sclerosis, and tubulointerstitial lesions. In relation to WKY and SHR, UNX-SHR exhibited an increase in ET-1 gene expression in renal cortex and medulla. By in situ hybridization and immunoperoxidase staining, an overexpression of ET-1 gene and protein were seen in mesangial and glomerular epithelial cells and in some proximal tubules and vessels. Angiotensin-converting enzyme (ACE) activity was significantly increased in the renal brush border. Since in mesangial cells, angiotensin II induces ET-1 synthesis, a group of UNX-SHR received the ACE inhibitor quinapril from the time of UNX. These animals had a decrease in blood pressure, proteinuria, and serum and brush border ACE activity and in the expression and synthesis of ET-1 in all renal areas. On the whole, these data show that UNX-SHR have an upregulation of ET-1 gene and protein in several structures of the kidney compared with SHR and WKY rats. Quinapril diminished ACE activity and ET-1 expression and synthesis coincidentally with an improvement in proteinuria and morphological lesions. The beneficial effects of ACE inhibitors may be due to the diminution of both angiotensin II and ET-1 generation.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Endothelin: potential role in hypertension and vascular hypertrophy

The endothelins are a family of 21-amino acid peptides that are powerful vasoconstrictors. They may also induce vascular hypertrophy. These peptides may participate through these two mechanisms in the pathogenesis of the elevation of blood pressure and/or in the maintenance of hypertension in both experimental animal models and human essential hypertension. This review presents evidence both in favor and against the involvement of endothelins in hypertension. Plasma levels of endothelin-1 are either normal or slightly elevated in experimental and human essential hypertension. Responses of blood vessels to endothelin-1 may be normal or depressed in many models of experimental hypertension and also in essential hypertension in humans. It has recently been demonstrated that endothelin content and mRNA are increased in blood vessels of deoxycorticosterone acetate-salt hypertensive rats. When endothelin receptor antagonists are administered chronically, elevation of blood pressure and development of vascular hypertrophy are blunted in this experimental model of hypertension. In contrast, spontaneously hypertensive rats do not exhibit any increase in either endothelin-1 mRNA or immunoreactive endothelin in blood vessels and fail to respond with lowering of blood pressure to longterm treatment with endothelin receptor antagonists. Blood pressure development in young spontaneously hypertensive rats is also unaffected by long-term administration of endothelin antagonists. Molecular genetic studies appear to support a genetic role of components of the endothelin system in Dahl salt-sensitive rats. In human essential hypertension, there is some evidence of activation of the endothelin system despite depressed responses of small arteries to endothelin-1 and normal circulating levels of endothelin-1 in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased dietary salt sensitizes vasomotor neurons of the rostral ventrolateral medulla

Excess dietary sodium is a major contributing factor to the incidence and severity of hypertension. However, the precise mechanism or mechanisms by which salt contributes to the severity of hypertension are unknown. The region of the rostral ventrolateral medulla (RVLM) is a principal brain stem locus critical for the regulation of arterial blood pressure by the sympathetic nervous system. The purpose of this study was to determine if excess dietary sodium chloride might alter the function or responsiveness of neurons in the RVLM. Male Sprague-Dawley rats were given either tap water or 0.9% sodium chloride solution to drink for 10 to 14 days. Excess sodium chloride did not affect baseline blood pressure. However, when neurons of the RVLM were stimulated by microinjections of L-glutamate, evoked increases in arterial pressure were potentiated in rats given sodium chloride. Augmented pressor responses could not be accounted for by increased vascular reactivity because both groups responded similarly to intravenously administered phenylephrine and norepinephrine. Additionally, electrical stimulation of descending spinal sympathoexcitatory axons produced identical pressor responses in both groups, indicating that altered synaptic transmission at central or peripheral neuroeffector junctions distal to the RVLM could not explain enhanced pressor responses produced by direct stimulation of RVLM cell somata. Finally, impaired arterial baroreceptor reflexes could not account for augmented RVLM pressor responses, as depressor and bradycardic responses produced by electrical stimulation of aortic baroreceptor afferents were not reduced in rats given excess dietary sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)