Effect of selective blockade of endothelin ETB receptors on the liver dysfunction and injury caused by endotoxaemia in the rat

Stellate cell in hepatic inflammation and acute injury

The perisinusoidal hepatic stellate cells (HSCs) have been investigated extensively for their role as the major fibrogenic cells during chronic liver injury. HSCs also produce numerous cytokines, chemokines, and growth mediators, and express cell adhesion molecules constitutively and in response to stimulants such as endotoxin (lipopolysaccharide). With this property and by interacting with resident and recruited immune and inflammatory cells, HSCs regulate hepatic immune homeostasis, inflammation, and acute injury. Indeed, experiments with HSC-depleted animal models and cocultures have provided evidence for the prominent role of HSCs in the initiation and progression of inflammation and acute liver damage due to various toxic agents. Thus HSCs and/or mediators derived thereof during acute liver damage may be considered as potential therapeutic targets.

Inhibition of COX-1 attenuates the formation of thromboxane A2 and ameliorates the acute decrease in glomerular filtration rate in endotoxemic mice

Thromboxane (Tx) A2 has been suggested to be involved in the development of sepsis-induced acute kidney injury (AKI). Therefore, we investigated the impact of cyclooxygenase (COX)-1 and COX-2 activity on lipopolysaccharide (LPS)-induced renal TxA2 formation, and on endotoxemia-induced AKI in mice. Injection of LPS (3 mg/kg ip) decreased glomerular filtration rate (GFR) and the amount of thrombocytes to ∼50% of basal values after 4 h. Plasma and renocortical tissue levels of TxB2 were increased ∼10- and 1.7-fold in response to LPS, respectively. The COX-1 inhibitor SC-560 attenuated the LPS-induced fall in GFR and in platelet count to ∼75% of basal levels. Furthermore, SC-560 abolished the increase in plasma and renocortical tissue levels of TxB2 in response to LPS. The COX-2 inhibitor SC-236 further enhanced the LPS-induced decrease in GFR to ∼40% of basal values. SC-236 did not alter thrombocyte levels nor the LPS-induced increase in plasma and renocortical tissue levels of TxB2. Pretreatment with clopidogrel inhibited the LPS-induced drop in thrombocyte count, but did not attenuate the LPS-induced decrease in GFR and the increase in plasma TxB2 levels. This study demonstrates that endotoxemia-induced TxA2 formation depends on the activity of COX-1. Our study further indicates that the COX-1 inhibitor SC-560 has a protective effect on the decrease in renal function in response to endotoxin. Therefore, our data support a role for TxA2 in the development of AKI in response to LPS.

Current trends in inflammatory and immunomodulatory mediators in sepsis

Sepsis refers to severe systemic inflammation in response to invading pathogens. An overwhelming immune response, as mediated by the release of various inflammatory mediators, can lead to shock, multiple organ damage, and even death. Cytokines, proteases, lipid mediators, gaseous substances, vasoactive peptides, and cell stress markers play key roles in sepsis pathophysiology. Various adhesion molecules and chemokines sequester and activate neutrophils into the target organs, further augmenting inflammation and tissue damage. Although the anti-inflammatory substances counterbalance proinflammatory mediators, prolonged immune modulation may cause host susceptibility to concurrent infections, thus reflecting enormous challenge toward developing effective clinical therapy against sepsis. To understand the complex interplay between pro- and anti-inflammatory phenomenon in sepsis, there is still an unmet need to study newly characterized mediators. In addition, revealing the current trends of novel mediators will upgrade our understanding on their signal transduction, cross-talk, and synergistic and immunomodulating roles during sepsis. This review highlights the latest discoveries of the mediators in sepsis linking to innate and adaptive immune systems, which may lead to resolution of many unexplored queries.

Endothelin-mediated gut microcirculatory dysfunction during porcine endotoxaemia

BACKGROUND

The potent vasoconstrictor endothelin-1 has been implicated in the pathogenesis of the microcirculatory dysfunction seen in sepsis. The mixed endothelin receptor antagonist tezosentan and the selective endothelin A-receptor antagonist TBC3711 were used to investigate the importance of the different endothelin receptors in modulating splanchnic regional blood flow and microvascular blood flow in endotoxaemia.

METHODS

Eighteen anaesthetized pigs were i.v. infused with endotoxin (Escherichia coli lipopolysaccharide, serotype 0111:b4) for 300 min. After 120 min, six animals received tezosentan and six animals received TBC3711. Six animals served as endotoxin-treated controls. Laser Doppler flowmetry was used to measure microcirculatory blood flow in the liver and ileum. Superior mesenteric artery flow (SMA(FI)) and portal vein flow (PV(FI)) were measured with ultrasonic flow probes, and air tonometry was used to measure Pco₂ in the ileal mucosa.

RESULTS

TBC3711 did not improve splanchnic regional blood flow or splanchnic microvascular blood flow compared with endotoxin-treated controls. Tezosentan increased PV(FI) (P<0.05), but SMA(FI) was not improved compared with the other groups. In the tezosentan group, microvascular blood flow in the ileal mucosa (MCQ(muc)) improved and mucosal-arterial Pco₂ gap decreased (P<0.05 for both) compared with endotoxin-treated controls and the TBC3711 group.

CONCLUSIONS

Tezosentan improved MCQ(muc) without any concomitant increase in SMA(FI), implying a direct positive effect on the microcirculation. TBC3711 was not effective in improving regional splanchnic blood flow or splanchnic microvascular blood flow. Dual endothelin receptor antagonism was necessary to improve MCQ(muc), indicating a role for the endothelin B-receptor in mediating the microcirculatory failure in the ileal mucosa.

The role of endogenous and exogenous ligands for the peroxisome proliferator-activated receptor alpha (PPAR-alpha) in the regulation of inflammation in macrophages

The aim of the present study was to evaluate the role of endogenous and exogenous peroxisome proliferator-activated receptor alpha (PPAR-alpha), a nuclear receptor, on the regulation of inflammation in macrophages. To address this question, we have stimulated peritoneal macrophages from PPAR-alpha wild-type mice and PPAR-alpha knockout mice (PPAR-alpha) with 10 microg/mL LPS and 100 U/mL IFN-gamma. We report here that the absence of a functional PPAR-alpha gene in PPAR-alpha knockout mice resulted in a significant augmentation of various inflammatory parameters in peritoneal macrophages. In particular, we have clearly demonstrated that PPAR-alpha gene deletion increases (1) the mitogen-activated protein kinase phosphorylation (extracellular signal-regulated kinase, c-Jun NH2-terminal kinase, and p38), (2) nuclear factor-kappaB activation, (3) IkappaB-alpha degradation, (4) iNOS expression and NO formation, and (5) cyclooxygenase 2 expression and prostaglandin E2 formation caused by LPS/IFN-gamma stimulation. On the contrary, the incubation of peritoneal macrophages from PPAR-alpha wild type with clofibrate (2 mM) at 2 h before the LPS and IFN-gamma stimulation significantly reduced the expression and the release of the proinflammatory mediators. To elucidate whether the protective effects of clofibrate is related to activation of the PPAR-alpha receptor, we also investigated the effect of clofibrate treatment on PPAR-alpha-deficient mice. The absence of the PPAR-alpha receptor significantly abolished the protective effect of the PPAR-alpha agonist against LPS/IFN-gamma-induced macrophage inflammation. In conclusion, our study demonstrates that the endogenous and exogenous PPAR-alpha ligands reduce the degree of macrophage inflammation caused by LPS/IFN-gamma stimulation.

Tezosentan normalizes hepatomesenteric perfusion in a porcine model of cardiac tamponade

BACKGROUND

To investigate endothelin-1 (ET-1)-dependent hepatic and mesenteric vasoconstriction, and oxygen and lactate fluxes in an acute, fixed low cardiac output (CO) state.

METHODS

Sixteen anesthetized, mechanically ventilated pigs were studied. Cardiac tamponade was established to reduce portal venous blood flow (Q(PV)) to 2/3 of the baseline value. CO, hepatic artery blood flow (Q(HA)), Q(PV), hepatic laser-Doppler flow (LDF), hepatic venous and portal pressure, and hepatic and mesenteric oxygen and lactate fluxes were measured. Hepatic arterial (R(HA)), portal (R(HP)) and mesenteric (R(mes)) vascular resistances were calculated. The combined ET(A)-ET(B) receptor antagonist tezosentan (RO 61-0612) or normal saline vehicle was infused in the low CO state. Measurements were made at baseline, after 30, 60, 90 min of tamponade, and 30, 60, 90 min following the infusion of tesozentan at 1 mg/kg/h.

RESULTS

Tamponade decreased CO, Q(PV), Q(HA), LDF, hepatic and mesenteric oxygen delivery, while hepatic and mesenteric oxygen extraction and lactate release increased. R(HA), R(HP) and R(mes) all increased. Ninety minutes after tesozentan, Q(PV), LDF and hepatic and mesenteric oxygen delivery and extraction increased approaching baseline values, but no effect was seen on CO or Q(HA). Hepatic and mesenteric handling of lactate converted to extraction. R(HA), R(HP) and R(mes) returned to baseline values. No changes were observed in these variables among control animals not receiving tesozentan.

CONCLUSION

In a porcine model of acute splanchnic hypoperfusion, unselective ET-1 blockade restored hepatomesenteric perfusion and reversed lactate metabolism. These observations might be relevant when considering liver protection in low CO states.

Endotoxin causes pulmonary hypertension by upregulating smooth muscle endothelin type-B receptors: role of aldose reductase

Endothelin-1 (ET-1), a potent vasoconstrictor and mitogen, is upregulated in pulmonary tissue during endotoxemia and contributes markedly to endotoxin-induced pulmonary hypertension. It is, however, unknown whether the ET receptors, ET(A) and ET(B), are differentially regulated in endotoxemic pulmonary vasculature and how this may impact on pulmonary vascular tone. To investigate this topic, we used isolated perfused lungs, pulmonary endothelial cells (ECs), and pulmonary vascular smooth muscle cells (SMCs) of the rat. During a 6-h endotoxin exposure, isolated perfused lungs developed significant pulmonary hypertension that was markedly attenuated by antagonizing ET(A) or ET(B) receptors using subtype-selective or a mixed ET(A/B) receptor antagonist. Peptide levels of big ET-1 and ET-1 and gene expression of prepro-ET-1 were increased after endotoxin challenge in all tissues. In endotoxemic isolated perfused lungs and ECs, the significant rise of mature ET-1 seen in controls after ET(B) receptor or mixed antagonism disappeared completely. However, this effect was preserved in endotoxemic SMCs. In ECs, endotoxin markedly downregulated maximum ET(B) receptor sites and ET(B) mRNA levels, whereas in SMCs, it generated substantial ET(B) receptor upregulation and moderate ET(A) receptor downregulation. The aldose reductase inhibitors sorbinil and zopolrestat mitigated endotoxin-induced pulmonary hypertension, ET-1 stimulation, and differential ET(B) receptor regulation. We conclude that endotoxin-induced pulmonary hypertension in the rat results from a loss of endothelial and concomitant gain of vascular smooth muscle ET(B) receptors. These changes are at least partly mediated by aldose reductase.

Tezosentan attenuates organ injury and mesenteric blood flow decrease in endotoxemia and cecal ligation and puncture

BACKGROUND

Decreased mesenteric blood flow and multiple organ injury due to free radicals are the consequences of septic shock. Since the blockade of endothelin receptors was reported to exert beneficial effects, we investigated the effects of tezosentan, a novel dual endothelin receptor antagonist, in two different experimental models of septic shock induced either by the injection of Escherichia coli endotoxin (ETX, 20 mg/kg, i.p.) or by cecal ligation and puncture (CLP).

STUDY DESIGN

Swiss albino mice received tezosentan (10 mg/kg, i.p.) or its solvent saline (0.9% NaCl, w/v) twice at 2 and 22 h after ETX or CLP. At 24 h, the animals were anesthetized and the mesenteric blood flow was monitored for 15 min by using perivascular ultrasonic Doppler flowmeter. Then the animals were exsanguinated, and spleen, liver, and kidneys were isolated accordingly for histopathological examination. Thiobarbituric acid reacting substances and glutathione and myeloperoxides activities were also determined in the liver.

RESULTS

In both ETX and CLP models, there was a decrease in mesenteric blood flow which was blocked by tezosentan. Similarly, tezosentan significantly attenuated the histopathological injury inflicted by both models. Although the glutathione levels were decreased and thiobarbituric acid reacting substances and myeloperoxidase activity were increased by ETX and CLP, tezosentan has failed to block these alterations in a consistent manner. However, a significant interaction between CLP and tezosentan with regard to myeloperoxidase activity and glutathione should be taken as partial evidence to explain the underlying mechanism of protection offered by tezosentan against liver injury.

CONCLUSIONS

Therefore, we concluded that tezosentan, by working via mechanisms mostly other than the blockade of free radical induced damage, is a useful treatment option for combating the deleterious effects of septic shock such as mesenteric ischemia as well as liver, spleen, and kidney injury.

Endothelin-1 is secreted after total knee arthroplasty regardless of the use of a tourniquet

Early infections and wound healing disorders after implantation of a total knee replacement occur regardless of the intraoperative use of a tourniquet. The biochemical regulatory processes responsible for the disturbances in microcirculation and thus the potential therapeutic options have yet to be elucidated. The hypothesis of the present paper was that endothelin-1 (ET-1), a mediator of microcirculation disturbances in parenchymatous organs, also is released after major operations on peripheral joints. The concentration of ET-1 in the plasma was determined preoperatively and at 10 postoperative time points (5 min-48 h) with (group A, n=10) and without the use of a tourniquet (group B, n=10). The ET-1 concentration achieved its maximum 6h after opening the tourniquet, which corresponded to 3.3 times the preoperative value. Without a tourniquet, the concentration maximum (2.9 times the baseline value) was achieved already 1.5 h after the end of the operation. However, the total amount of ET-1 secreted over 24 h was identical in both groups (p>0.5). We conclude that the tissue hypoxia resulting from the use of a tourniquet modulates ET-1 secretion, but that traumatization during the operation has a much stronger influence on the total amount secreted. ET-1 antagonists thus should be discussed for the drug prophylaxis of wound healing disorders, regardless of the use of a tourniquet.

Regulatory and functional interaction of vasoactive factors in the kidney and extracellular pH

A growing body of evidence suggests that vasoactive factors produced in the kidney such as nitric oxide, endothelins, angiotensin, and prostaglandins participate actively in the regulation of acid-base homeostasis under physiologic conditions. In addition, recent reports indicate that alterations in the systemic acid-base status may also influence the generation of vasoactive cytokines in the kidney, which in turn may mediate the renal effector processes that tend to restore normality under such conditions. Metabolic acidosis, which so frequently accompanies many forms of chronic renal failure (CRF), may contribute to down-regulation of intrarenal nitric oxide production that characterizes CRF. Reduced extracellular pH inhibits inducible nitric oxide production in mesangial cells by altering the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, an important posttranslational mechanism in the inducible nitric oxide synthase (iNOS) activation. The underlying defects resulting in the uncoupling of NADPH oxidation in acidemic microenvironment are discussed. Acidosis stimulates renal production of endothelins, which mediate proximal tubular acidification by enhancing sodium-hydrogen exchanger-3 (NHE-3) activity. Renal endothelins mediate enhanced urinary acid excretion following dietary acid ingestion, an effect that is effectively blocked by endothelin receptor blockers. Reduced extracellular pH stimulates endothelin secretion from renal microvascular endothelial cells, which may promote enhanced acid excretion from the distal tubule under conditions of acidosis. These phenomena as well as the role of angiotensin and renal prostaglandins in mediating renal acidification in normal and acidotic conditions are discussed in this review, which describe the regulatory interaction between extracellular pH and renal vasoactive factors.

Role of thromboxane in producing portal hypertension following trauma-hemorrhage

Thromboxane A2 (TXA2) and endothelin-1 (ET-1) have been proposed as the important vasoconstrictors that increase portal venous resistance in paracrine or autocrine fashion. We hypothesized that the hepatic damage following trauma-hemorrhage (T-H) is induced by the impaired hepatic circulation due to the increased production of vasoconstrictors such as ET-1 and TXA2 by the liver. To test this, male Sprague-Dawley rats (n = 6/group) were subjected to trauma (i.e., midline laparotomy) and hemorrhage (35-40 mmHg for 90 min followed by fluid resuscitation) or sham operation. At 2 or 5 h after the end of resuscitation, the liver was isolated and perfused and portal inflow pressure, bile flow, and release of ET-1 and thromboxane B2 (TXB2; a stable metabolite of TXA2) into the perfusate were measured. The level of portal pressure was higher at 5 h following T-H compared with 2 h after T-H and sham. The portal pressure was inversely correlated to the amount of bile production. Furthermore, the bile flow was significantly correlated to the hepatic damage as evidenced by release of lactate dehydrogenase into the perfusate. The level of ET-1 at 5 h following T-H in the perfusate after 30 min of recirculation did not show any difference from sham. However, the levels of TXB2 in the T-H group were significantly higher than those in sham at that interval. These results indicate that the increased release of TXA2 but not ET-1 following T-H might be responsible for producing the increased portal resistance, decreased bile production, and hepatic damage.

Role of endothelin ETA receptors in sepsis-induced mortality, vascular leakage, and tissue injury in rats

The role of endothelin ETA receptors in sepsis-induced mortality and edema formation was evaluated with a selective antagonist ABT-627 [2-(4-methoxyphenyl)-4-(1,3-benzodioxol-5-yl)-1-(N,N-di(n-butyl)amino carbonylmethyl)-pyrrolidine-3-carboxylic acid]. Sprague-Dawley rats received saline (control group), Escherichia coli endotoxin (10 mg/kg, sepsis group) or infusion of ABT-627 prior and immediately after saline and endotoxin injection. Mortality, edema formation (wet/dry ratios), and multiple tissue injury (indicated by serum concentrations of creatinine, urea, bilirubin, creatine kinase, lactate dehydrogenase, and aspartate aminotransferase) were monitored within 5 h. Endotoxin injection elicited 64% mortality, significantly augmented edema formation in liver, heart, lung, and kidney, and raised serum levels of tissue injury markers. Pretreatment with ABT-627 completely reversed endotoxin-induced mortality, significantly attenuated wet/dry ratios of the heart, liver, and kidney, but not lungs, and reduced serum levels of creatine kinase, creatinine, aspartate aminotransferase, and lactate dehydrogenase, but not that of urea and bilirubin. These results suggest that endothelin ETA receptors play a significant role in promoting mortality, edema formation (except in the lungs), and tissue injury in animals with severe sepsis.

BQ-788, a selective endothelin ET(B) receptor antagonist

We describe characteristics of a selective endothelin (ET) ET(B) receptor antagonist, BQ-788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine], which is widely used to demonstrate the role of endogenous or exogenous ETs in vitro and in vivo. In vitro, BQ-788 potently and competitively inhibited (125)I-labeled ET-1 binding to ET(B) receptors in human Girrardi heart cells (hGH) with an IC(50) of 1.2 nM, but only poorly inhibited the binding to ET A receptors in human neuroblastoma cell line SK-N-MC cells (IC(50), 1300 nM). In isolated rabbit pulmonary arteries, BQ-788 showed no agonistic activity up to 10 microM and competitively inhibited the vasoconstriction induced by an ET(B)-selective agonist (pA(2), 8.4). BQ-788 also inhibited several bioactivities of ET-1, such as bronchoconstriction, cell proliferation, and clearance of perfused ET-1. Thus, it is confirmed that BQ-788 is a potent, selective ET(B) receptor antagonist. In vivo, in conscious rats, BQ-788, 3 mg/kg/h, i.v., completely inhibited a pharmacological dose of ET-1- or sarafotoxin6c (S6c) (0.5 nmol/kg, i.v.)-induced ET(B) receptor-mediated depressor, but not pressor responses. Furthermore, BQ-788 markedly increased the plasma concentration of ET-1, which is considered an index of potential ET(B) receptor blockade in vivo. In Dahl salt-sensitive hypertensive (DS) rats, BQ-788, 3 mg/kg/h, i.v., increased blood pressure by about 20 mm Hg. It is reported that BQ-788 also inhibited ET-1-induced bronchoconstriction, tumor growth and lipopolysaccharide-induced organ failure. These data suggest that BQ-788 is a good tool for demonstrating the role of ET-1 and ET(B) receptor subtypes in physiological and/or pathophysiological conditions.

Carboxy-PTIO, a scavenger of nitric oxide, selectively inhibits the increase in medullary perfusion and improves renal function in endotoxemia

The acute renal failure associated with septic shock is associated with a high mortality despite dialytic therapies. Endotoxemia leads to marked changes in the distribution of intrarenal perfusion that may be independent of alterations in total renal blood flow or systemic hemodynamics. Modulation of this intrarenal redistribution may protect against acute renal failure. This study examines the effect of carboxy-2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO), a scavenger of nitric oxide (NO), on systemic and intrarenal hemodynamics measured by laser Doppler flowmetry following the induction of endotoxemia in the anesthetized rat. Infusion of lipopolysaccharide (LPS) led to a prompt reduction in inulin clearance at 60 min, which remained reduced for 6 h in saline-treated rats. Administration of carboxy-PTIO led to a sustained increase in inulin clearance over 360 min post-LPS. During endotoxemia, cortical perfusion fell acutely by 29 +/- 8%, whereas medullary perfusion increased by 71 +/- 11%. The increase in medullary perfusion was potently and selectively inhibited by carboxy-PTIO. We propose that inhibition of medullary hyperemia maintains glomerular hydrostatic pressure, thus leading to the improved renal function during endotoxemia and that scavenging of NO may prove to be a useful therapeutic option in the acute renal failure associated with septic shock.

Endothelin receptor blockers reduce I/R-induced intestinal mucosal injury: role of blood flow

The aim of the present study was to assess the role of endothelin (ET) in ischemia-reperfusion (I/R)-induced mucosal injury. Mucosal permeability ((51)Cr-EDTA clearance) and tissue myeloperoxidase (MPO) activity were significantly increased after 30 min of ischemia followed by 30 min of reperfusion. The I/R-induced increases in mucosal permeability and polymorphonuclear leukocyte (PMN) infiltration were significantly attenuated by pretreatments with ET(A) (BQ-485) and/or ET(B) (BQ-788) receptor antagonists. Monoclonal antibody (MAb) directed against intercellular adhesion molecule-1 (ICAM-1; MAb 1A29) and superoxide dismutase (SOD) pretreatments significantly attenuated the increased mucosal permeability and PMN infiltration in a similar manner as with ET receptor antagonists. Superior mesenteric artery blood flow was significantly reduced during the reperfusion period. Both ET receptor antagonists caused a significant rise in blood flow compared with an untreated I/R group. In conclusion, our data suggest that ET(A) and/or ET(B) receptors, ICAM-1, and superoxide play an important role in I/R-induced mucosal dysfunction and PMN infiltration. Furthermore, ET is involved in the pathogenesis of post-reperfusion-induced damage and beneficial effects of ET receptor antagonism are related to an improvement of disturbed blood flow during the reperfusion period.

The timing of endothelin and nitric oxide inhibition affects survival in a mice model of septic shock

The effect of endothelin and nitric oxide (NO) inhibition on survival from septic shock was investigated in male Swiss albino mice (20-40 g), with particular emphasis on the timing of the administration of their blockers after Escherichia coli endotoxin (lipopolysaccharide, O55:B5, 60 mg kg(-1), i.p.) challenge. Mice were injected with the endothelin receptor antagonist bosentan (30 mg kg(-1), i.p., either 2 or 12 h after endotoxin) alone or in addition to the NO synthase blockers L-canavanine (100 mg kg(-1), i.p.), N(G)-nitro-L-arginine methyl ester (L-NAME, 3 mg kg(-1), i.p.) or aminoguanidine (15 mg kg(-1), i.p.), which were also given twice at 2 and 6 h after endotoxin. Control animals received saline, and survival rates in each group (n=10) were recorded over 24 h at 6-h intervals. At 24 h, the survival rate was 10% in controls, but 30% (n.s.) and 70% (P<0.05) in animals that received only bosentan at 2 and 12 h, respectively, indicating a relatively late involvement of endothelin in comparison to NO. In contrast, these figures were 70% (P<0.05) and 80% (P<0.05) at 12 h for L-NAME and L-canavanine, respectively, and 10% (n.s.) at 24 h, implying a relatively early involvement of NO compared to endothelin. Interestingly, survival in the aminoguanidine group (75% at 24 h, P<0.05 vs. controls) was markedly higher than that in the L-NAME and L-canavanine groups, an effect that was attributed to mechanisms other than NO inhibition. Survival was better (60%, P<0.05 vs. endotoxin alone) when bosentan was given at 2 h in combination with L-NAME, but the best outcome (90% survival, P<0.05) was observed in animals when bosentan was given at 12 h and L-NAME was injected twice at 2 and 6 h. However, the statistical analysis revealed no significant additional beneficial effect of L-NAME coadministered with bosentan. Therefore, we conclude that NO is involved during the earlier phases of septic shock in comparison to a relatively late involvement of endothelin peptides, and that bosentan alone appears to be beneficial when administered at least 12 h after the endotoxin challenge in our mice model of septic shock.

Endotoxin treatment causes an upregulation of the endothelin system in the liver: amelioration of increased portal resistance by endothelin receptor antagonism

BACKGROUND

Mechanisms underlying hepatic microcirculatory failure during endotoxemia are incompletely understood. Because endothelin-1 (ET-1) has been implicated in endotoxin-induced liver injury, we investigated the hepatic ET-1 system in endotoxin-treated rats.

METHODS

Rats were treated with endotoxin (Escherichia coli lipopolysaccharide; 3 mg/kg, i.p.), and various determinations were made 24 h later.

RESULTS

Endotoxin treatment caused 11.2 +/- 1.6% weight loss, a decrease in mean arterial pressure (MAP; 96 +/- 5 mmHg vs 108 +/- 3 mmHg; P < 0.05) and an increase in portal pressure (11.6 +/- 1.3 mmHg vs 7.4 +/- 1 mmHg; P < 0.02). No significant changes in the serum levels of liver enzymes or hepatocellular necrosis were observed. Endotoxin caused increases in hepatic ET-1 (from 345 +/- 31 to 565 +/- 38 pg/g; P < 0.01), ET-1 receptor density (from 179 +/- 16 to 340 +/- 26 fmol/mg; P < 0.02), and mRNA expression of preproendothelin-1, and ET(A) and ET(B) receptors. While the serum nitric oxide (nitrite +/- nitrate) concentration was increased in endotoxin-treated rats, that of ET-1 remained unchanged. A mixed ET(A)/ET(B) receptor antagonist, TAK-044 (10 mg/kg, i.v.), reduced the weight loss from 11.2 +/- 1.6% to 5.9 +/- 2.9% (P < 0.05) and the portal pressure from 11.6 +/- 1.3 mmHg to 8.6 +/- 0.7 mmHg (P < 0.05) in endotoxin-treated rats. The mixed ET(A)/ET(B) receptor antagonist also caused an increase in serum ET-1 concentration, but did not affect serum nitric oxide and MAP in endotoxin-treated rats.

CONCLUSIONS

These results suggest that the upregulated hepatic ET-1 system is an important mechanism of increased portal resistance and related complications of endotoxemia.

Levels of circulating endothelin-1 and nitrates/nitrites in patients with virus-related hepatocellular carcinoma

A balance between endothelins (ET) and nitric oxide (NO) might interfere with liver haemodynamics and disease progression in various liver diseases. Increased levels of endothelin 1 (ET-1) and nitrites and nitrates (NOx, the end products of NO metabolism) have been reported in hepatocellular carcinoma (HCC), but the balance has not been studied. The purpose of this study was to assess the ratio of NOx to ET-1 in patients with virus-related hepatocellular carcinoma and to investigate its correlation with the extent of the disease. Eighteen patients with virus-related HCC (six Okuda stage I, six Okuda stage II and six Okuda stage III) were included in the study and were compared with 22 patients with viral cirrhosis (14 decompensated, eight compensated) and seven normal controls. ET-1 was measured with an ELISA assay and NOx with a modification of the Griess reaction. Patients with virus-related HCC had the highest levels of circulating ET-1 and NOx (13.24 +/- 0.82 pg/ml and 112.28 +/- 18.56 micromol/l) compared to compensated cirrhosis (9.47 +/- 0.50 pg/ml, P < 0.004 and 54.47 +/- 2.36 micromol/l, P < 0.01), decompensated cirrhosis (9.57 +/- 0.32 pg/ml, P < 0.001 and 90.20 +/- 11.23 micromol/l, NS) and normal controls (8.84 +/- 0.61 pg/ml, P < 0.001 and 51.17 +/- 6.18 micromol/l, P < 0.01). There was a significant increase of ET-1 and NOx at HCC stage III compared to HCC stages I and II, cirhotics and controls. HCC stage III patients also had a NOx/ET-1 ratio that was higher than HCC stages I and II patients, normal controls and patients with compensated cirrhosis. Virus-related HCC patients have high levels of circulating ET-1, compared to compensated or decompensated cirrhosis. Highest levels of ET-1 are produced in Okuda III tumours. NOx are also increased but only in Okuda stage III tumours. The NOx/ET-1 ratio is increased in virus-related HCC and DC. This increase may account for the known increase in tumour blood flow.

The endothelin system in septic and endotoxin shock

The view of the endothelium as a passive barrier has gradually changed as a number of endothelium-derived substances have been discovered. Substances like nitric oxide, prostaglandins and endothelins have potent and important properties, involving not only the circulation as such but also the response to stimuli like inflammation and trauma. The endothelin system, discovered in 1988, has not only strong vasoconstrictor properties, but also immunomodulating, endocrinological and neurological effects exerted through at least two types of receptors. Septic shock, a condition with high mortality, is associated with vast cardiovascular changes, organ dysfunction with microcirculatory disturbances and dysoxia. In the experimental setting, endotoxaemia resembles these changes and is, as well as septic shock, accompanied by a pronounced increase in plasma endothelin levels. The pathophysiology in septic and endotoxin shock remains to be fully elucidated, but several studies indicate that endothelial dysfunction is one contributing mechanism. Activation of the endothelin system is associated with several pathological conditions complicating septic shock, such as acute respiratory distress syndrome, cardiac dysfunction, splanchnic hypoperfusion and disseminated intravascular coagulation. Through the development of both selective and nonselective endothelin receptor antagonists, the endothelin system has been the object of a large number of studies during the last decade. This review highlights systematically the findings of previous studies in the area. It provides strong indications that the endothelin system, apart from being a marker of vascular injury, is directly involved in the pathophysiology of septic and endotoxin shock. Interventions with endothelin receptor antagonists during septic and endotoxin shock have so far only been done in animal studies but the results are interesting and promising.

Basal release of endothelin-1 and the influence of the ETB receptor on single vessel hydraulic permeability

BACKGROUND

Endothelin-1 (ET-1) has a direct permeability decreasing effect on the microvasculature. The present study was designed to test the hypothesis that this effect is mediated via the endothelin B (ETB) receptor located on the microvascular endothelium and to determine whether basal microvascular permeability is dependent on constitutive release of ET-1. To isolate the direct effect of ET-1, experiments were conducted under conditions in which hydraulic and oncotic pressures were controlled.

METHODS

Postcapillary venules in the rat mesentery were perfused in situ, and paired measurements of hydraulic permeability (Lp) were obtained using the modified Landis micro-occlusion method. Lp measured after a 15-minute perfusion with the ETB receptor blocker BQ-788 (1 micromol/L) was compared with measures of Lp obtained after perfusion with a combined mixture of BQ-788 and ET-1 (80 pmol/L) (n = 6). In addition, the effect of basal endogenous ET-1 was tested by measuring the effects of BQ-788 perfusion on Lp (n = 6).

RESULTS

Units for Lp are mean +/- SE x 10(-8) cm x s(-1) cm H2O(-1). ETB receptor blockade prevented any decrease in Lp induced by ET-1 (BQ-788 alone = 7.9 +/- 0.7; BQ-788 + ET-1 = 8.2 +/- 0.8;p = 0.5). Under basal conditions and in the absence of exogenous ET-1, ETB receptor blockade led to a significant increase in Lp from 6.8 +/- 0.9 to 9.7 +/- 1.2 (p = 0.001).

CONCLUSION

Decreases in microvascular permeability in single postcapillary venules after the administration of ET-1 are mediated via the ETB receptor. Constitutive release of ET-1 from the microvascular endothelium also plays a role in maintaining basal levels of permeability. These findings suggest important roles for ET-1 in maintaining and modulating microvascular permeability.

Endothelin receptor blockade attenuates lipopolysaccharide-induced pulmonary nitric oxide production

Increased nitric oxide (NO) synthesis by the inducible nitric oxide synthase (iNOS) has been shown to contribute to the development of acute lung injury and delayed hypotension in animals injected with bacterial lipopolysaccharides (LPS). Recent evidence indicates that endothelin-1 (ET-1) is also elevated in septic humans and in animals. To assess the contribution of ETs to LPS-induced pulmonary NO production and iNOS expression, we used P1/fl, a 22 amino acid peptide, to selectively antagonize endothelin-A receptors. Anesthetized, mechanically ventilated rats were injected with either saline or LPS (E. coli endotoxin, 20 mg/kg) and studied for 5 h. Two other groups of rats were pretreated 15 min earlier with P1/fl peptide (20 microg/kg). Unlike saline-treated rats, rats injected with LPS showed a progressive decline in arterial pressure and a significant rise in plasma ET concentration and serum nitrite-nitrate level. In the lungs, LPS injection elicited a several-fold rise in lung iNOS activity and exhaled NO concentration and increased lung wet/dry ratio significantly. Pretreatment with P1/fl peptide eliminated the decline in arterial pressure, the rise in lung wet/dry ratio, lung NOS activity, and iNOS protein expression and significantly attenuated the increase in pulmonary exhaled NO production but had no effect on plasma ET concentration. We conclude that activation of ET-A receptors by rising ET-1 concentration enhances NO production and iNOS expression in the respiratory and vascular systems and contributes to both LPS-induced hypotension and acute lung injury.

Receptor subtype mediating the action of circulating endothelin on glucose metabolism and hemodynamics in perfused rat liver

The subtype of endothelin receptor that mediates metabolic and hemodynamic effects of circulating endothelin was explored using perfused rat liver. Infusion of endothelin (ET)-1 or ET-3 into the portal vein at a concentration of 0.3 nM increased glucose and lactate output and decreased perfusion flow, although ET-3 was less effective than ET-1. The metabolic effects of ET-1 were observed even under costant-flow perfusion. Infusion of either sarafotoxin S6b or S6c, an ET(A)- or ET(B)-receptor agonist, mimicked the actions of ET-1 to an equal extent. The flow reduction and glucose production induced by ET-1 were partly attenuated by the ET(A)-receptor antagonist BQ485. By contrast, ET(B)-receptor antagonist BQ788 enhanced glucose production caused by ET-1 and ET-3 without affecting the hemodynamic change. The effects of ET-1 and ET-3 were almost totally inhibited by the combination of BQ485 and BQ788. These results suggest that both ET(A) and ET(B) receptors are involved in the metabolic and hemodynamic effects of circulating endothelin in rat liver, while the ET(A)-receptor-mediated action appears to be dominant.

The effects of bosentan, aminoguanidine and L-canavanine on mesenteric blood flow, spleen and liver in endotoxaemic mice

The modulatory effects of a non-selective endothelin receptor antagonist, bosentan, were investigated together with those of relatively selective inducible nitric oxide synthase inhibitors, aminoguanidine and L-canavanine, on mesenteric blood flow decrease, liver and spleen injury elicited by endotoxaemia. Swiss albino mice (20-40 g) were administered intraperitoneally bosentan (3, 10 or 30 mg kg(-1)), aminoguanidine (15 mg kg(-1)) or L-canavanine (20 or 100 mg kg(-1)) 10 min before they received saline or Escherichia coli endotoxin (10 mg kg(-1)). After 4 h, the mice were anaesthetized, mesenteric blood flow values were measured, spleen and liver weight/body weight ratios were determined and the organs were examined histopathologically. Endotoxin decreased mesenteric blood flow (ml min(-1), saline: 3.0 +/- 0.2; endotoxin: 2.2 +/- 0.2: n = 10, P < 0.05), increased the weight of liver (g per kg body weight, saline: 47.5 +/- 2.0; endotoxin: 60.8 +/- 1.9: n = 10, P < 0.05) and spleen (g per kg body weight, saline: 3.9 +/- 0.5; endotoxin: 8.6 +/- 0.9; n = 10, P < 0.01) while it inflicted significant histopathological injury to both organs. Bosentan was ineffective at 3 mg kg(-1) but at 10 and 30 mg kg(-1) doses, it abolished all the deleterious effects of endotoxin without exception. Aminoguanidine blocked most of the effects of endotoxin except those on spleen. In contrast, L-canavanine blocked only the endotoxin-induced increase in liver weight but itself increased spleen weight and failed to block any other effects of endotoxin. Thus, it can be speculated that the beneficial effects of aminoguanidine are produced largely by mechanisms other than selective inducible nitric oxide synthase inhibition since L-canavanine was not fully effective. The beneficial effects of endothelin inhibition by using bosentan in endotoxaemia can be further exploited for the understanding and the therapy of sepsis-related syndromes.

Differentiated effects on splanchnic homeostasis by selective and non-selective endothelin receptor antagonism in porcine endotoxaemia

1. The non-selective endothelin (ET) receptor antagonist bosentan has been shown to restore systemic and gut oxygen delivery and reverse intestinal mucosal acidosis in porcine endotoxin shock. 2. To further elucidate the specific role of the ETA as opposed to the ETB receptor and their effects in the splanchnic region a non-selective (ET(MIX)ra) A-182086 and selective ETA (ET(A)ra) PD155080 and ETB (ET(B)ra) A-192621 receptor antagonists were administered, separately or simultaneously (ET(A+B)ra) 2 h after onset of endotoxin shock. These four groups were compared to a control group receiving only endotoxin and vehicle. 3. Thirty-nine pigs were anaesthetized and catheterized for measurement of central and regional haemodynamics. A tonometer in the distal ileum was used for measurement of mucosal PCO2. Blood gases and plasma ET-1-LI levels as well as histological samples from the gut were assessed. Intervention was started 2 h after onset of endotoxemia and the experiments were terminated after 5 h. 4. Endotoxin-induced changes in systemic, gut oxygen delivery and portal hepatic vascular resistance and systemic acidosis were effectively counteracted by both ET(A+B)ra an ET(MIX)ra. ET(A)ra administration was not effective while ET(B)ra proved to be fatal as all animals in this group died prior to full time of the experiment. While both ET(A+B)ra and ET(MIX)ra improved gut oxygen delivery only the latter attenuated the profound endotoxin-induced ileal mucosal acidosis. 5. The lethal effect seen from selective ETB receptor antagonism in the current study may be due to increased ETA receptor activity as plasma levels of ET-1 is increased several fold by blocking the ETB receptor and thereby the plasma-ET-1-clearing function. Furthermore, a loss of endothelial ETB receptor vasodilating properties may also have contributed to the lethal course in the ET(B)ra group. 6. The findings in this study suggest that ET is involved in the profound endotoxin-induced disturbances in splanchnic homeostasis in porcine endotoxaemia. Furthermore, antagonism of both ETA and ETB receptors is necessary to effectively counteract these changes.

Hypoxia inhibits increased ETB receptor-mediated NO synthesis in hypertensive rat lungs

Although hypertensive lungs of chronically hypoxic rats express increased levels of nitric oxide (NO) synthases (NOSs) and produce increased amounts of NO-containing compounds (NOx) during normoxic ventilation, the level of NO production during hypoxic exposure is unclear. Because hypoxia inhibits NO synthesis in normotensive lungs, we investigated whether hypoxic ventilation inhibited NO synthesis in isolated hypertensive lungs and chronically hypoxic rats. Measurement of perfusate NOx concentration in hypertensive lungs from male rats exposed to 4 wk of hypobaric hypoxia showed that basal NOx production was reduced during hypoxic (0% O2) vs. normoxic (21% O2) ventilation. Similarly, plasma NOx concentration was lower in chronically hypoxic rats breathing 10% O2 than in those breathing 21% O2. Hypoxic inhibition of lung NOx production was not prevented by supplementary L-arginine or tetrahydrobiopterin and was not mimicked by inhibition of Ca2+ influx. However, it was mimicked by inhibition of constitutive NOS with NG-monomethyl-L-arginine and chelation of intracellular Ca2+. The endothelin type B-receptor antagonist BQ-788 prevented the increases in NOx production associated with normoxic ventilation in both isolated hypertensive lungs and intact chronically hypoxic rats. These results suggest that a reduced supply of the cosubstrate molecular O2 to NOS counteracts an endothelin type B receptor-mediated stimulation of NO synthesis in hypertensive rat lungs. Thus, despite increased NOS protein in the lungs and pulmonary arteries of chronically hypoxic rats, direct hypoxic inhibition of NO production may contribute to the development of pulmonary hypertension.

Endothelin A-receptor blockade worsens endotoxin-induced hepatic microcirculatory changes and necrosis

BACKGROUND & AIMS

Endothelin 1 is considered to be an important regulator of sinusoidal blood flow and increases during endotoxemia. The purpose of this study was to investigate the role of endothelin 1 in hepatic microcirculation, oxygen transport, and liver injury during endotoxemia.

METHODS

Male Sprague-Dawley rats were continuously infused with 2.5 mL/h of saline, 0.8 mg . kg-1 . h-1 of lipopolysaccharide (LPS), 3 mg . kg-1 . h-1 of BQ-485, an endothelin A-receptor antagonist, or LPS plus BQ-485 for 7 hours.

RESULTS

BQ-485 infusion had no significant effect on hepatic microcirculation and liver injury. LPS increased the plasma levels of aspartate aminotransferase (AST) and total bilirubin and decreased the hepatic adenosine triphosphate (ATP) level and bile flow rate. LPS + BQ-485 infusion further increased the plasma levels of AST and total bilirubin and decreased the bile flow rate and the hepatic ATP level. Dual-spot microspectroscopy revealed mild decreases in sinusoidal erythrocyte velocity and oxygen transport in the LPS group and profound decreases in these parameters in the LPS + BQ-485 group. Histological examinations revealed massive necrotic changes in the pericentral regions of the LPS + BQ-485 group.

CONCLUSIONS

These results suggest that blockade of endothelin A receptors disturbs hepatic microcirculation and oxygen transport and aggravates the necrotic injury induced by endotoxin.

Endothelin association with cultured rat hepatic endothelial cells: functional characterization

Endothelin is a potent vasoactive peptide whose concentration increases in a number of pathophysiological states. In the intact animal, the liver is known to sequester approximately 12% of an injected bolus of [125I]endothelin-1 ([125I]ET-1). Endothelial cells (ECs) isolated from rat liver were maintained in culture in order to examine their role in ET sequestration. LECs were shown to express predominantly ET(B) receptors both by association assays and by Northern blot analysis. In these cells the reaction between [125I]ET-1 and its receptor was essentially irreversible. Ligand binding experiments performed at 4 degrees C showed that LECs in early culture (approximately 3 h) had 4.3 +/- 0.8 fmol of ET receptors per 10(6) cells; this number fell progressively to < or = 1 fmol/10(6) cells during 24 h of culture. The decrease in receptor numbers could be blocked by maintaining the cells at 4 degrees C. Northern blot analysis showed that relative to freshly isolated cells, mRNA for the ET(B) receptor decreased 4-fold in early culture, and recovered somewhat at 24 h. At 37 degrees C [125I]ET-1 bound by the cells was rapidly internalized, with concomitant down-regulation of ET receptors. Recovery of down-regulated ET receptors was sensitive to cycloheximide, making short-term receptor recycling unlikely. Metabolism of [125I]ET-1 was low at short (< 4 h) exposure times, and at 24 h showed a concentration dependence similar to that of ligand association, suggesting that ET-1 metabolism primarily was intracellular. ET stimulation of Kupffer cells and other hepatic cell types is known to activate phosphoinositide signaling, but no such activation was seen in LECs. Moreover, ET did not appear to stimulate protein tyrosine kinase activity in LECs. While hepatic LECs may lack some of the ET-dependent responses seen in other cell types, they likely contribute substantially to the liver's previously reported ability to sequester systemically administered ET.