Effect of endothelin-1 on regional kidney blood flow and renal arteriole calibre in rabbits

Kidney Microcirculation as a Target for Innovative Therapies in AKI

Acute kidney injury (AKI) is a serious multifactorial conditions accompanied by the loss of function and damage. The renal microcirculation plays a crucial role in maintaining the kidney’s functional and structural integrity for oxygen and nutrient supply and waste product removal. However, alterations in microcirculation and oxygenation due to renal perfusion defects, hypoxia, renal tubular, and endothelial damage can result in AKI and the loss of renal function regardless of systemic hemodynamic changes. The unique structural organization of the renal microvasculature and the presence of autoregulation make it difficult to understand the mechanisms and the occurrence of AKI following disorders such as septic, hemorrhagic, or cardiogenic shock; ischemia/reperfusion; chronic heart failure; cardiorenal syndrome; and hemodilution. In this review, we describe the organization of microcirculation, autoregulation, and pathophysiological alterations leading to AKI. We then suggest innovative therapies focused on the protection of the renal microcirculation and oxygenation to prevent AKI.

Renal biomarkers of acute kidney injury in response to increasing intermittent hypoxia episodes in the neonatal rat

BACKGROUND

We tested the hypotheses that: 1) early exposure to increasing episodes of clinically relevant intermittent hypoxia (IH) is detrimental to the developing kidneys; and 2) there is a critical number of daily IH episodes which will result in irreparable renal damage that may involve angiotensin (Ang) II and endothelin (ET)-1.

METHODS

At birth (P0), neonatal rat pups were exposed to brief IH episodes from the first day of life (P0) to P7 or from P0-P14. Pups were either euthanized immediately or placed in room air (RA) until P21. RA littermates served as controls. Kidneys were harvested at P7, P14, and P21 for histopathology; angiotensin converting enzyme (ACE), ACE-2, ET-1, big ET-1, and malondialdehyde (MDA) levels; immunoreactivity of ACE, ACE-2, ET-1, ET-2, ET receptors (ETAR, ETBR), and hypoxia inducible factor (HIF)1α; and apoptosis (TUNEL stain).

RESULTS

Histopathology showed increased renal damage with 8-12 IH episodes/day, and was associated with Ang II, ACE, HIF1α, and apoptosis. ACE-2 was not expressed at P7, and minimally increased at P14. However, a robust ACE-2 response was seen during recovery with maximum levels noted in the groups recovering from 8 IH episodes/day. ET-1, big ET-1, ETAR, ETBR, and MDA increased with increasing levels of neonatal IH.

CONCLUSIONS

Chronic neonatal IH causes severe damage to the developing kidney with associated elevations in vasoconstrictors, suggesting hypertension, particularly with 8 neonatal IH episodes. ACE-2 is not activated in early postnatal life, and this may contribute to IH-induced vasoconstriction. Therapeutic targeting of ACE and ET-1 may help decrease the risk for kidney injury in the developing neonate to prevent and/or treat neonatal acute kidney injury and/or chronic kidney disease.

The Role of Endothelin and Endothelin Antagonists in Chronic Kidney Disease

Background

Endothelins (ET) are a family of peptides that act as potent vasoconstrictors and pro-fibrotic growth factors. ET-1 is integral to renal and cardiovascular pathophysiology and exerts effects via autocrine, paracrine and endocrine signaling pathways tied to regulation of aldosterone, catecholamines, and angiotensin. In the kidney, ET-1 is critical to maintaining renal perfusion and controls glomerular arteriole tone and hemodynamics. It is hypothesized that ET-1 influences the progression of chronic kidney disease (CKD), and the objective of this review is to discuss the pathophysiology, and role of ET and endothelin receptor antagonists (ERAs) in CKD.

Summary

The use of ERAs in hypertensive nephropathy has the potential to decrease proteinuria, and in diabetic nephropathy has the potential to restore glycocalyx thickness, also decreasing proteinuria. Focal segmental glomerular sclerosis has no specific Food and Drug Administration-approved therapy currently, however, ERAs show promise in decreasing proteinuria and slowing tissue damage. ET-1 is a potential biomarker for autosomal dominant polycystic kidney disease progression and so it is thought that ERAs may be of some therapeutic benefit.

Key Messages

Multiple studies have shown the utility of ERAs in CKD. These agents have shown to reduce blood pressure, proteinuria, and arterial stiffness. However, more clinical trials are needed, and the results of active or recently concluded studies are eagerly awaited.

Therapeutic potential of endothelin receptor antagonism in kidney disease

Our growing understanding of the role of the endothelin (ET) system in renal physiology and pathophysiology is from emerging studies of renal disease in animal models and humans. ET receptor antagonists reduce blood pressure and proteinuria in chronic kidney disease and cause regression of renal injury in animals. However, the therapeutic potential of ET receptor antagonism has not been fully explored and clinical studies have been largely limited to patients with diabetic nephropathy. There remains a need for more work in nondiabetic chronic kidney disease, end-stage renal disease (patients requiring maintenance dialysis and those with a functioning kidney transplant), ischemia reperfusion injury, and sickle cell disease. The current review summarizes the most recent advances in both preclinical and clinical studies of ET receptor antagonists in the field of kidney disease.

Endothelin Blockade in Diabetic Kidney Disease

Diabetic kidney disease (DKD) remains the most common cause of chronic kidney disease and multiple therapeutic agents, primarily targeted at the renin-angiotensin system, have been assessed. Their only partial effectiveness in slowing down progression to end-stage renal disease, points out an evident need for additional effective therapies. In the context of diabetes, endothelin-1 (ET-1) has been implicated in vasoconstriction, renal injury, mesangial proliferation, glomerulosclerosis, fibrosis and inflammation, largely through activation of its endothelin A (ET_A) receptor. Therefore, endothelin receptor antagonists have been proposed as potential drug targets. In experimental models of DKD, endothelin receptor antagonists have been described to improve renal injury and fibrosis, whereas clinical trials in DKD patients have shown an antiproteinuric effect. Currently, its renoprotective effect in a long-time clinical trial is being tested. This review focuses on the localization of endothelin receptors (ET_A and ET_B) within the kidney, as well as the ET-1 functions through them. In addition, we summarize the therapeutic benefit of endothelin receptor antagonists in experimental and human studies and the adverse effects that have been described.

Lipopolysaccharide enhanced renal vascular response to endothelin-1 through ETA overexpression in portal hypertensive rats

BACKGROUND AND AIM

Hypo-perfusion resulting from intense renal vasoconstriction is traditionally contributed to renal dysfunction in advanced liver disease, although cumulative studies demonstrated renal vasodilatation with impaired vascular contractility to endogenous vasoconstrictors in portal hypertension and compensated liver cirrhosis. The pathophysiology of altered renal hemodynamics remains unclear. This study, using a rat model of portal hypertension with superimposed endotoxemia, was designed to delineate the evolution of renal vascular reactivity and vaso-regulatory gene expression during liver disease progression.

METHODS

Rats were randomized into sham surgery (SHAM) or partial portal vein ligation (PVL). Endotoxemia was induced by intraperitoneal injection of lipopolysaccharide (LPS) on the seventh day following surgery. Isolated kidney perfusion was performed at 0.5 h or 5 h after LPS to evaluate renal vascular response to endothelin-1.

RESULTS

In contrast to impaired vascular contractility of SHAM rats, PVL rats displayed enhanced renal vascular reactivity to endothelin-1 at 5 h following endotoxemia. There were extensive upregulations of inducible nitric oxide synthase in kidney tissues of endotoxemic rats. The changes of renal endothelin receptor type A (ETA ) level paralleled with the changes of renal vascular reactivity in LPS-treated rats. Compared with SHAM rats, PVL rats showed increased renal ETA and phosphorylated extracellular-signal-regulated kinases 1/2 (p-ERK1/2) at 5 h after LPS.

CONCLUSION

LPS-induced systemic hypotension induces a paradoxical change of renal vascular response to endothelin-1 between SHAM and PVL rats. LPS-induced renal vascular hyperreactivity in PVL rats was associated with upregulation of renal ETA and subsequent activation of ERK1/2 signaling.

Detection of microvasculature alterations by synchrotron radiation in murine with delayed jellyfish envenomation syndrome

Using the tentacle extract (TE) from the jellyfish Cyanea capillata, we have previously established a delayed jellyfish envenomation syndrome (DJES) model, which is meaningful for clinical interventions against jellyfish stings. However, the mechanism of DJES still remains unclear. Thus, this study aimed to explore its potential mechanism by detecting TE-induced microvasculature alterations in vivo and ex vivo. Using a third-generation synchrotron radiation facility, we, for the first time, directly observed the blood vessel alterations induced by jellyfish venom in vivo and ex vivo. Firstly, microvasculature imaging of whole-body mouse in vivo indicated that the small blood vessel branches in the liver and kidney in the TE-treated group, seemed much thinner than those in the control group. Secondly, 3D imaging of kidney ex vivo showed that the kidneys in the TE-treated group had incomplete vascular trees where distal vessel branches were partly missing and disorderly disturbed. Finally, histopathological analysis found that obvious morphological changes, especially hemorrhagic effects, were also present in the TE-treated kidney. Thus, TE-induced microvasculature changes might be one of the important mechanisms of multiple organ dysfunctions in DJES. In addition, the methods we employed here will probably facilitate further studies on developing effective intervention strategies against DJES.

Endothelin-1 and the kidney--beyond BP

Since its discovery over 20 years ago endothelin-1 (ET-1) has been implicated in a number of physiological and pathophysiological processes. Its role in the development and progression of chronic kidney disease (CKD) is well established and is an area of ongoing intense research. There are now available a number of ET receptor antagonists many of which have been used in trials with CKD patients and shown to reduce BP and proteinuria. However, ET-1 has a number of BP-independent effects. Importantly, and in relation to the kidney, ET-1 has clear roles to play in cell proliferation, podocyte dysfunction, inflammation and fibrosis, and arguably, these actions of ET-1 may be more significant in the progression of CKD than its prohypertensive actions. This review will focus on the potential role of ET-1 in renal disease with an emphasis on its BP-independent actions.

Endothelin and endothelin receptors in the renal and cardiovascular systems

Endothelin-1 (ET-1) is a multifunctional hormone which regulates the physiology of the cardiovascular and renal systems. ET-1 modulates cardiac contractility, systemic and renal vascular resistance, salt and water renal reabsorption, and glomerular function. ET-1 is responsible for a variety of cellular events: contraction, proliferation, apoptosis, etc. These effects take place after the activation of the two endothelin receptors ET(A) and ET(B), which are present - among others - on cardiomyocytes, fibroblasts, smooth muscle and endothelial cells, glomerular and tubular cells of the kidney. The complex and numerous intracellular pathways, which can be contradictory in term of functional response depending on the receptor type, cell type and physiological situation, are described in this review. Many diseases share an enhanced ET-1 expression as part of the pathophysiology. However, the use of endothelin blockers is currently restricted to pulmonary arterial hypertension, and more recently to digital ulcer. The complexity of the endothelin system does not facilitate the translation of the molecular knowledge to clinical applications. Endothelin antagonists can prevent disease development but secondary undesirable effects limit their usage. Nevertheless, the increasing understanding of the effects of ET-1 on the cardiac and renal physiology maintains the endothelin system as a promising therapeutic target.

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.

Endothelin type A and B receptors in the control of afferent and efferent arterioles in mice

BACKGROUND

Endothelin 1 contributes to renal blood flow control and pathogenesis of kidney diseases. The differential effects, however, of endothelin 1 (ET-1) on afferent (AA) and efferent arterioles (EA) remain to be established.

METHODS

We investigated endothelin type A and B receptor (ETA-R, ETB-R) functions in the control of AA and EA. Arterioles of ETB-R deficient, rescued mice [ETB(-/-)] and wild types [ETB(+/+)] were microperfused.

RESULTS

ET-1 constricted AA stronger than EA in ETB(-/-) and ETB(+/+) mice. Results in AA: ET-1 induced similar constrictions in ETB(-/-) and ETB(+/+) mice. BQ-123 (ETA-R antagonist) inhibited this response in both groups. ALA-ET-1 and IRL1620 (ETB-R agonists) had no effect on arteriolar diameter. L-NAME did neither affect basal diameters nor ET-1 responses. Results in EA: ET-1 constricted EA stronger in ETB(+/+) compared to ETB(-/-). BQ-123 inhibited the constriction completely only in ETB(-/-). ALA-ET-1 and IRL1620 constricted only arterioles of ETB(+/+) mice. L-NAME decreased basal diameter in ETB(+/+), but not in ETB(-/-) mice and increased the ET-1 response similarly in both groups. The L-NAME actions indicate a contribution of ETB-R in basal nitric oxide (NO) release in EA and suggest dilatory action of ETA-R in EA.

CONCLUSIONS

ETA-R mediates vasoconstriction in AA and contributes to vasoconstriction in EA in this mouse model. ETB-R has no effect in AA but mediates basal NO release and constriction in EA. The stronger effect of ET-1 on AA supports observations of decreased glomerular filtration rate to ET-1 and indicates a potential contribution of ET-1 to the pathogenesis of kidney diseases.

The effect of hemodilution during normothermic cardiac surgery on renal physiology and function: a review

Although the definitions of renal dysfunction vary, loss of renal function is a common complication following cardiac surgery using cardiopulmonary bypass (CPB). When postoperative dialysis is required, mortality is approximately 50%. CPB-accompanied hemodilution is a major contributing factor to renal damage as it notably reduces oxygen delivery by reducing the oxygen transport capacity of the blood as well as disturbing the microcirculation. To minimize hypoxemic damage during CPB, lowering of body temperature is applied to reduce the patient’s metabolic rate. At present, however, temperature management during elective adult cardiac surgery is shifting from moderate hypothermia to normothermia. To determine whether the currently accepted levels of hemodilution during CPB can suffice the normothermic patient’s high oxygen demand, we focused this study on renal physiology and postoperative renal function. Hemodilution reduces the capillary density through a diminished capillary viscosity, thereby, redistributing blood from the renal medulla to the renal cortex. As the physiology of the renal medulla makes it a hypoxic environment, this part of the kidney appears to be especially at risk for hypoxic damage caused by a hemodilution-induced lowered oxygen transport and oxygen delivery. In addition, hemodilution is also likely to disturb the hormonal systems regulating renal blood distribution. Clinical studies, mostly of retrospective or observational nature, show that perioperative nadir hematocrit levels lower than approximately 24% are associated with an increased risk to develop postoperative renal failure. A better comprehension of the cause-and-effect relation between low perioperative hematocrits and loss of postoperative renal function may enable more effective renal protective strategies.

Contrast angiography of the rat renal microcirculation in vivo using synchrotron radiation

We have developed a new method for contrast microangiography of the rat renal circulation using synchrotron radiation. The method was applied to determine responses of the renal arterial vasculature to angiotensin II and electrical stimulation of the renal nerves (RNS). Iodinated contrast agent was administered directly into the renal artery of pentobarbital-anesthetized rats before and during 1) intravenous infusion of angiotensin II (1.6 μg·kg−1·min−1) or 2) its vehicle, or 3) RNS at 2 Hz. Images were obtained at 30 Hz, before and during these treatments, and vascular caliber was determined by use of a newly developed algorithm described herein. Up to four levels of branching could be observed simultaneously along the arterial tree, comprising vessels with resting diameter of 28–400 μm. Vessel diameter was not significantly altered by vehicle infusion (+3.1 ± 3.5% change) but was significantly reduced by angiotensin II (−24.3 ± 3.4%) and RNS (−17.1 ± 3.8%). Angiotensin II-induced vasoconstriction was independent of vessel size, but RNS-induced vasoconstriction was greatest in vessels with a resting caliber of 100–200 μm and least in vessels with a resting caliber 40–100 μm. In conclusion, the method we describe herein provides a new approach for assessing responses of the renal arterial circulation to vasoactive factors along several orders of branching.

The endothelin receptor antagonist tezosentan improves renal microcirculation in a porcine model of endotoxemic shock

BACKGROUND

Impaired renal microcirculation has been suggested as a factor contributing to the development of renal dysfunction in sepsis. This study was conducted to elucidate the role of endothelin-1 (ET-1)in mediating reductions in renal microcirculatory blood flow during endotoxemic shock.

METHODS

A prospective, randomized, and experimental study was performed with 16 anesthetized and mechanically ventilated pigs. After 2 h of lipopolysaccaride-induced endotoxemia, eight animals received a bolus dose of the dual endothelin receptor antagonist tezosentan (1 mg/kg), followed by a continuous infusion of 1 mg/kg/h throughout the experiment. Eight animals served as the control group. Renal microcirculation, total renal blood flow, plasma creatinine levels, cardiac index, and mean arterial pressure were measured. Plasma samples were collected for the measurement of tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), interleukin-10 (IL-10), ET-1, angiotensin II, and aldosterone.

RESULTS

Endotoxin infusion resulted in a state of circulatory shock with impairment of renal microcirculation. An increase in the plasma levels of TNF-alpha, IL-6, IL-10, ET-1, angiotensin II, and aldosterone was also observed. Tezosentan attenuated the decrease in renal microcirculation and renal blood flow, and attenuated the increase in plasma creatinine. Treatment with tezosentan did not significantly affect the plasma cytokine, angiotensin II, or aldosterone response to endotoxemia.

CONCLUSION

These results indicate that treatment with the dual endothelin receptor tezosentan in endotoxemic shock attenuates the reduction of renal microcirculation and total renal blood flow independently of plasma changes in the renin-angiotensin-aldosterone system or early plasma cytokine response.

The endothelin system and its antagonism in chronic kidney disease

The incidence of chronic kidney disease (CKD) is increasing worldwide. Cardiovascular disease (CVD) is strongly associated with CKD and constitutes one of its major causes of morbidity and mortality. Treatments that slow the progression of CKD and improve the cardiovascular risk profile of patients with CKD are needed. The endothelins (ET) are a family of related peptides, of which ET-1 is the most powerful endogenous vasoconstrictor and the predominant isoform in the cardiovascular and renal systems. The ET system has been widely implicated in both CVD and CKD. ET-1 contributes to the pathogenesis and maintenance of hypertension and arterial stiffness and more novel cardiovascular risk factors such as oxidative stress and inflammation. Through these, ET also contributes to endothelial dysfunction and atherosclerosis. By reversal of these effects, ET antagonists may reduce cardiovascular risk. In particular relation to the kidney, antagonism of the ET system may be of benefit in improving renal hemodynamics and reducing proteinuria. ET likely also is involved in progression of renal disease, and data are emerging to suggest a synergistic role for ET receptor antagonists with angiotensin-converting enzyme inhibitors in slowing CKD progression.

ETA and ETB receptors differentially modulate afferent and efferent arteriolar responses to endothelin

The segment-specific actions of endothelin peptides and agonists have not been thoroughly investigated in the renal microcirculation. The current studies were performed to assess the relative contribution of ET(A) and ET(B) receptors to the renal pre- and postglomerular arteriolar responses to ET-1. Experiments determined the effect of selective ET(A) (A-127722; 30 nM) and ET(B) (A-192621; 30 nM) receptor blockade, on arteriolar responses to ET-1 concentrations of 1 pM to 10 nM in rat kidneys using the isolated juxtamedullary nephron technique. Renal perfusion pressure was set at 110 mmHg. Baseline afferent arteriolar diameter was similar in all groups and averaged 17.8+/-0.6 microm (n=14). In control experiments (n=6), ET-1 produced significant concentration-dependent decreases in arteriolar diameter, with 10 nM ET-1 decreasing diameter by 85+/-1%. Selective blockade of ET(A) receptors (n=6) prevented ET-1-mediated vasoconstriction, except at concentrations of 1 and 10 nM. Similarly, the vasoconstrictor profile was right shifted during selective ET(B) receptor blockade (n=4). Combined ET(A) and ET(B) receptor blockade (n=5) completely abolished afferent arteriolar diameter responses to ET-1. ET(B) selective agonists (S6c and IRL-1620) produced disparate responses. S6c produced a concentration-dependent vasoconstriction of afferent arterioles. In contrast, S6c produced a concentration-dependent dilation of efferent arterioles that could be blocked with an ET(B) receptor antagonist. IRL-1620, another ET(B) agonist, was less effective at altering afferent or efferent diameter and produced a small reduction in pre- and postglomerular arteriolar diameter. These data demonstrate that both ET(A) and ET(B) receptors participate in ET-1-mediated vasoconstriction of afferent arterioles. ET(B) receptor stimulation provides a significant vasodilatory influence on the efferent arteriole. Furthermore, since selective ET(A) and ET(B) receptor antagonists abolished preglomerular vasoconstrictor responses at lower ET-1 concentrations, these data support a possible interaction between ET(A) and ET(B) receptors in the control of afferent arteriolar diameter.

Nitric oxide and superoxide in the renal medulla: a delicate balancing act

PURPOSE OF REVIEW

Endothelial nitric oxide synthase (eNOS) and nicotinamide adenine dinucleotide (phosphate) oxidase [NAD(P)H oxidase] are both expressed in tubular epithelial cells within the renal medulla, particularly the thick ascending limb of the loop of Henle (mTALH). Thick ascending limbs contribute to long-term blood pressure control, both because they reabsorb approximately 30% of filtered sodium, and because they produce paracrine factors like nitric oxide (NO) that control medullary blood flow (MBF), which in turn has a major impact on tubular sodium reabsorption. Herein, we review recent evidence for roles of NO and superoxide (O2*-) in autocrine control of tubular sodium reabsorption, and in paracrine control of MBF.

RECENT FINDINGS

O2*- can have a direct action to reduce MBF, and to enhance sodium reabsorption from mTALH. These actions oppose those of NO produced in mTALH, which inhibits tubular sodium reabsorption (autocrine) and increases MBF (paracrine). NO and O2*- also oppose each other's actions through chemical combination to produce peroxynitrite. Thus, interactions between NO and O2*-, at both the chemical and cellular levels, likely contribute to long-term blood pressure control. This hypothesis is supported by recent data showing that sodium retention and hypertension can develop when the balance of production of these free radicals is tipped towards O2*-, such as in diabetes, atherosclerosis and renin-angiotensin-system activation.

SUMMARY

Interactions between O2*- and NO produced within the mTALH regulate tubular and vascular function in the renal medulla. Dysregulation of these systems in states of oxidative stress likely promotes salt and water retention, and thus hypertension.