Localization of endothelin ETA and ETB receptor-mediated constriction in the renal microcirculation of rats

Mechanism of protective actions of sparsentan in the kidney: lessons from studies in models of chronic kidney disease

Simultaneous inhibition of angiotensin II AT1 and endothelin ETA receptors has emerged as a promising approach for treatment of chronic progressive kidney disease. This therapeutic approach has been advanced by the introduction of sparsentan, the first dual AT1 and ETA receptor antagonist. Sparsentan is a single molecule with high affinity for both receptors. It is US Food and Drug Administration approved for immunoglobulin A nephropathy (IgAN) and is currently being developed as a treatment for rare kidney diseases, such as focal segmental glomerulosclerosis. Clinical studies have demonstrated the efficacy and safety of sparsentan in these conditions. In parallel with clinical development, studies have been conducted to elucidate the mechanisms of action of sparsentan and its position in the context of published evidence characterizing the nephroprotective effects of dual ETA and AT1 receptor inhibition. This review summarizes this evidence, documenting beneficial anti-inflammatory, antifibrotic, and hemodynamic actions of sparsentan in the kidney and protective actions in glomerular endothelial cells, mesangial cells, the tubulointerstitium, and podocytes, thus providing the rationale for the use of sparsentan as therapy for focal segmental glomerulosclerosis and IgAN and suggesting potential benefits in other renal diseases, such as Alport syndrome.

Quantifying the integrated physiological effects of endothelin-1 on cardiovascular and renal function in healthy subjects: a mathematical modeling analysis

Endothelin-1 (ET-1) is a potent vasoconstrictor with strong anti-natriuretic and anti-diuretic effects. While many experimental studies have elucidated the mechanisms of ET-1 through its two receptors, ETA and ETB, the complexity of responses and sometimes conflicting data make it challenging to understand the effects of ET-1, as well as potential therapeutic antagonism of ET-1 receptors, on human physiology. In this study, we aimed to develop an integrated and quantitative description of ET-1 effects on cardiovascular and renal function in healthy humans by coupling existing experimental data with a mathematical model of ET-1 kinetics and an existing mathematical model of cardiorenal function. Using a novel agnostic and iterative approach to incorporating and testing potential mechanisms, we identified a minimal set of physiological actions of endothelin-1 through ETA and ETB receptors by fitting the physiological responses (changes in blood pressure, renal blood flow, glomerular filtration rate (GFR), and sodium/water excretion) to ET-1 infusion, with and without ETA/ETB antagonism. The identified mechanisms align with previous experimental studies on ET-1 and offer novel insights into the relative magnitude and significance of endothelin's effects. This model serves as a foundation for further investigating the mechanisms of ET-1 and its antagonists.

The acute pressure natriuresis response is suppressed by selective ETA receptor blockade

Hypertension is a major risk factor for cardiovascular disease.  In a significant minority of people, it develops when salt intake is increased (salt-sensitivity).  It is not clear whether this represents impaired vascular function or disruption to the relationship between blood pressure (BP) and renal salt-handling (pressure natriuresis, PN).  Endothelin-1 (ET-1) regulates BP via ETA and ETB receptor subtypes.  Blockade of ETA receptors reduces BP, but promotes sodium retention by an unknown mechanism.  ETB blockade increases both BP and sodium retention.  We hypothesised that ETA blockade promotes sodium and water retention by suppressing PN.  We also investigated whether suppression of PN might reflect off-target ETB blockade.  Acute PN was induced by sequential arterial ligation in male Sprague Dawley rats.  Intravenous atrasentan (ETA antagonist, 5mg/kg) halved the normal increase in medullary perfusion and reduced sodium and water excretion by >60%.  This was not due to off-target ETB blockade because intravenous A-192621 (ETB antagonist, 10mg/kg) increased natriuresis by 50% without modifying medullary perfusion.  In a separate experiment in salt-loaded rats monitored by radiotelemetry, oral atrasentan reduced systolic and diastolic BP by ~10mmHg, but additional oral A-192621 reversed these effects.  Endogenous ETA stimulation has natriuretic effects mediated by renal vascular dilation while endogenous ETB stimulation in the kidney has antinatriuretic effects via renal tubular mechanisms.  Pharmacological manipulation of vascular function with ET antagonists modifies the BP set-point, but even highly selective ETA antagonists attenuate PN, which may be associated with salt and water retention.

MicroRNAs as markers to monitor endothelin-1 signalling and potential treatment in renal disease: Carcinoma - proteinuric damage - toxicity

This review highlights new developments in miRNA as diagnostic and surveillance tools in diseases damaging the renal proximal tubule mediated by endothelin in the field of renal carcinoma, proteinuric kidney disease and tubulotoxicity. A new mechanism in the miRNA regulation of proteins leads to the binding of the miRNA directly to the DNA with premature transcriptional termination and hence the formation of truncated protein isoforms (Mxi2, Vim3). These isoforms are mediated through miRNA15a or miRNA 498, respectively. ET-1 can activate a cytoplasmic complex consisting of NF-κB p65, MAPK p38α, and PKCα. Consequently, PKCα does not transmigrate into the nucleus, which leads to the loss of suppression of a primiRNA15a, maturation of this miRNA in the cytoplasm, tubular secretion and detectability in the urine. This mechanism has been shown in renal cell carcinoma and in proteinuric disease as a biomarker for the activation of the signalling pathway. Similarly, ET-1 induced miRNA 498 transmigrates into the nucleus to form the truncated protein Vim3, which is a biomarker for the benign renal cell tumour, oncocytoma. In tubulotoxicity, ET-1 induced miRNa133a down-regulating multiple-drug-resistant related protein-2, relevant for proteinuric and cisplatin/cyclosporine A toxicity. Current advantages and limitations of miRNAs as urinary biomarkers are discussed.

Endothelin contributes to blunted renal autoregulation observed with a high-salt diet

Autoregulation of renal blood flow (RBF) is an essential function of the renal microcirculation that has been previously shown to be blunted by excessive dietary salt. Endogenous endothelin 1 (ET-1) is increased following a high-salt (HS) diet and contributes to the control of RBF but the differential effects of ET-1 on renal microvessel autoregulation in response to HS remain to be established. We hypothesized that a HS diet increases endothelin receptor activation in normal Sprague-Dawley rats and blunts autoregulation of RBF. The role of ET-1 in the blunted autoregulation produced by a HS diet was assessed in vitro and in vivo using the blood-perfused juxtamedullary nephron preparation and anesthetized rats, respectively. Using highly selective antagonists, we observed that blockade of either ETA or ETB receptors was sufficient to restore normal autoregulatory behavior in afferent arterioles from HS-fed rats. Additionally, normal autoregulatory behavior was restored in vivo in HS-fed rats by simultaneous ETA and ETB receptor blockade, whereas blockade of ETB receptors alone showed significant improvement of normal autoregulation of RBF. Consistent with this observation, autoregulation of RBF in ETB receptor-deficient rats fed HS was similar to both ETB-deficient rats and transgenic control rats on normal-salt diets. These data support the hypothesis that endogenous ET-1, working through ETB and possibly ETA receptors, contributes to the blunted renal autoregulatory behavior in rats fed a HS diet.

Bosentan normalizes the GFR response to renal nerve stimulation following reversible unilateral ureteric obstruction in the rat

We investigated the renal response to direct renal nerve stimulation, 2 weeks following reversal of 24-h unilateral (left) ureteric obstruction. Renal nerve stimulation caused a 13-15 % fall in renal blood flow, in 4 groups of anesthetized rats following ureteric obstruction (n=9) or a sham operation (n=7) both with (n=9) and without (n=7) treatment with the mixed ET(A/B) receptor antagonist, bosentan. In the sham-operated rats, renal nerve stimulation did not change glomerular filtration rate but reduced urine flow rate (37+/-3 %, P<0.001), and absolute (38+/-4 %, P<0.001) and fractional (35+/-5 %, P<0.01) sodium excretion. Following unilateral ureteric obstruction, renal nerve stimulation increased glomerular filtration rate by 22+/-3 % (P<0.01), but reduced urine flow rate (14+/-2 %, P<0.001) and fractional sodium excretion (23+/-5 %, P<0.01). Bosentan treatment had no effect on baseline or renal responses to renal nerve stimulation in the sham group but normalized the renal response to renal nerve stimulation in the unilateral ureteric obstruction group. We conclude that 14 days after a 24-h period of unilateral ureteric obstruction there is an increase in GFR in response to direct renal nerve stimulation, which is due, in part, to the actions of endothelin at the time of obstruction.

Endothelin-1, but not angiotensin II, induces afferent arteriolar myosin diphosphorylation as a potential contributor to prolonged vasoconstriction

Bolus administration of endothelin-1 elicits long-lasting renal afferent arteriolar vasoconstriction, in contrast to transient constriction induced by angiotensin II. Vasoconstriction is generally evoked by myosin regulatory light chain (LC20) phosphorylation at Ser19 by myosin light chain kinase (MLCK), which is enhanced by Rho-associated kinase (ROCK)-mediated inhibition of myosin light chain phosphatase (MLCP). LC20 can be diphosphorylated at Ser19 and Thr18, resulting in reduced rates of dephosphorylation and relaxation. Here we tested whether LC20 diphosphorylation contributes to sustained endothelin-1 but not transient angiotensin II-induced vasoconstriction. Endothelin-1 treatment of isolated arterioles elicited a concentration- and time-dependent increase in LC20 diphosphorylation at Thr18 and Ser19. Inhibition of MLCK or ROCK reduced endothelin-1-evoked LC20 mono- and diphosphorylation. Pretreatment with an ETB but not an ETA receptor antagonist abolished LC20 diphosphorylation, and an ETB receptor agonist induced LC20 diphosphorylation. In contrast, angiotensin II caused phosphorylation exclusively at Ser19. Thus, endothelin-1 and angiotensin II induce afferent arteriolar constriction via LC20 phosphorylation at Ser19 due to calcium activation of MLCK and ROCK-mediated inhibition of MLCP. Endothelin-1, but not angiotensin II, induces phosphorylation of LC20 at Thr18. This could contribute to the prolonged vasoconstrictor response to endothelin-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.

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.

ADP-ribosyl cyclase and ryanodine receptors mediate endothelin ETA and ETB receptor-induced renal vasoconstriction in vivo

ADP-ribosyl cyclase (ADPR cyclase) and ryanodine receptors (RyR) participate in calcium transduction in isolated afferent arterioles. We hypothesized that this signaling pathway is activated by ETA and ETB receptors in the renal vasculature to mediate vasoconstriction in vivo. To test this, we measured acute renal blood flow (RBF) responses to ET-1 in anesthetized rats and mice in the presence and absence of functional ADPR cyclase and/or RyR. Inhibitors of ADPR cyclase (nicotinamide) or RyR (ruthenium red) reduced RBF responses to ET-1 by 44% (P < 0.04 for both) in Sprague-Dawley rats. Mice lacking the predominant form of ADPR cyclase (CD38-/-) had RBF responses to ET-1 that were 47% weaker than those seen in wild-type mice (P = 0.01). Selective ETA receptor stimulation (ET-1+BQ788) produced decreases in RBF that were attenuated by 43 and 56% by nicotinamide or ruthenium red, respectively (P < 0.02 for both). ADPR cyclase or RyR inhibition also reduced vasoconstrictor effects of the ETB receptor agonist sarafotoxin 6c (S6c; 77 and 54%, respectively, P < 0.02 for both). ETB receptor stimulation by ET-1 + the ETA receptor antagonist BQ123 elicited responses that were attenuated by 59 and 60% by nicotinamide and ruthenium red, respectively (P < 0.01 for both). Nicotinamide attenuated RBF responses to S6c by 54% during inhibition of nitric oxide synthesis (P = 0.001). We conclude that in the renal microcirculation in vivo 1) ET-1-induced vasoconstriction is mediated by ADPR cyclase and RyR; 2) both ETA and ETB receptors activate this pathway; and 3) ADPR cyclase participates in ETB receptor signaling independently of NO.

Reactive oxygen species participate in acute renal vasoconstrictor responses induced by ETAand ETBreceptors

Reactive oxygen species (ROS) play important roles in renal vasoconstrictor responses to acute and chronic stimulation by angiotensin II and norepinephrine, as well as in long-term effects of endothelin-1 (ET-1). Little is known about participation of ROS in acute vasoconstriction produced by ET-1. We tested the influence of NAD(P)H oxidase inhibition by apocynin [4 mg·kg−1·min−1, infused into the renal artery (ira)] on ETAand ETBreceptor signaling in the renal microcirculation. Both receptors were stimulated by ET-1, ETAreceptors by ET-1 during ETBantagonist BQ-788, and ETBby ETBagonist sarafotoxin 6C. ET-1 (1.5 pmol injected ira) reduced renal blood flow (RBF) 17 ± 4%. Apocynin raised baseline RBF (+10 ± 1%, P < 0.001) and attenuated the ET-1 response to 10 ± 2%, i.e., 35 ± 9% inhibition ( P < 0.05). Apocynin reduced ETA-induced vasoconstriction by 42 ± 12% ( P < 0.05) and that of ETBstimulation by 50 ± 8% ( P < 0.001). During nitric oxide (NO) synthase inhibition ( Nω-nitro-l-arginine methyl ester), apocynin blunted ETA-mediated vasoconstriction by 60 ± 8% ( P < 0.01), whereas its effect on the ETBresponse (by 87 ± 8%, P < 0.001) was even larger without than with NO present ( P < 0.05). The cell-permeable superoxide dismutase mimetic tempol (5 mg·kg−1·min−1ira), which reduces O2−and may elevate H2O2, attenuated ET-1 responses similar to apocynin (by 38 ± 6%, P < 0.01). We conclude that ROS, O2−rather than H2O2, contribute substantially to acute renal vasoconstriction elicited by both ETAand ETBreceptors and to basal renal vasomotor tone in vivo. This physiological constrictor action of ROS does not depend on scavenging of NO. In contrast, scavenging of O2−by NO seems to be more important during ETBstimulation.

In vitro effect of cyclosporine-A on angiotensins secretion by glomerular cells

AIM

Cyclosporine-A (CyA) is used to control transplant rejections and to treat autoimmune diseases. We investigated the possibility that changes induced by CyA on endothelin 1 (ET), angiotensin I (AI) and angiotensin II (AII) concentrations recognize a common pathway through which different mechanisms operate.

METHODS

We measured ET, AI and AII concentrations, before and after either ET or CyA addition to the incubation medium of glomeruli of pig kidneys, isolated in vitro. The measurements were carried out with or without selective (ET(A) and ET(B)) or unselective ET(A)-ET(B) receptor inhibitors.

RESULTS

In the presence of CyA, AI and ET are positively correlated either when ET(B) receptors are blocked, or when both receptors are free, while this correlation becomes negative when ET(A) receptors alone are blocked. Adding ET to the medium, the correlations between AI and ET are negative when either ET(A), or ET(B) or both are blocked. The effects of CyA and ET are significant only during the first 2 h of incubation.

CONCLUSION

Cyclosporine-A recruits angiotensins and ET through ET(A) receptors, a mechanism possibly responsible of glomerular damage. This stimulation is time-dependent. Prevention of the renal damage from CyA should require selective ET(A) receptor blockade.

Endothelin-1 induces NF-kappaB via two independent pathways in human renal tubular epithelial cells

BACKGROUND

Endothelin-1 (ET-1) is a major transcriptional activator of renal proximal tubule cells acting in an autocrine and paracrine manner. In animal studies, ET-1 has been implicated in progressive renal interstitial fibrosis by promoting gene expression, possibly via the inflammatory NF-kappaB signal pathway. While ET-1-dependent mechanisms of signal transduction have been studied mainly in tumor cell lines, we analyzed the mechanism of ET-1-induced, NF-kappaB-mediated target gene activation in proximal tubule cells.

METHODS

Human renal proximal tubule cells were stimulated with ET-1 and gene expression analyzed by protein microarray, Western blot, non-radioactive electromobility shift assay, and quantitative real-time polymerase chain reaction.

RESULTS

Activation of NF-kappaB occurs only via an ET-1-specific type A receptor (not type B as in animals). Induction can be blocked by bosentan, and endothelin-A but not endothelin-B receptor-specific antagonists. Protein microarray screening shows activation of two independent cascades (via the endothelin-A receptor, or via diacylglycerol) leading to NF-kappaB induction. The independent induction is also reflected by target gene expression such as the vascular cell adhesion molecule-1, interleukin-6, and fractalkine at different time points.

CONCLUSION

Thus prohibiting ET-1-mediated gene transcription necessitates blocking of NF-kappaB and diacylglycerol signal transduction in proximal tubule cells.

Distribution of endothelin receptor subtypes ETA and ETB in the rat kidney

The endothelin (ET) receptor system is markedly involved in the regulation of renal function under both physiological and pathophysiological conditions. The present study determined the detailed cellular localization of both ET receptor subtypes, ET(A) and ET(B), in the vascular and tubular system of the rat kidney by immunofluorescence microscopy. In the vascular system we observed both ET(A) and ET(B) receptors in the media of interlobular arteries and afferent and efferent arterioles. In interlobar and arcuate arteries, only ET(A) receptors were present on vascular smooth muscle cells. ET(B) receptor immunoreactivity was sparse on endothelial cells of renal arteries, whereas there was strong labeling of peritubular and glomerular capillaries as well as vasa recta endothelium. ET(A) receptors were evident on glomerular mesangial cells and pericytes of descending vasa recta bundles. In the renal tubular system, ET(B) receptors were located in epithelial cells of proximal tubules and inner medullary collecting ducts, whereas ET(A) receptors were found in distal tubules and cortical collecting ducts. Distribution of ET(A) and ET(B) receptors in the vascular and tubular system of the rat kidney reported in the present study supports the concept that both ET receptor subtypes cooperate in mediating renal cortical vasoconstriction but exert differential and partially antagonistic effects on renal medullary function.

Endothelin-A and -B receptors, superoxide, and Ca2+ signaling in afferent arterioles

It is unknown if endothelin-A and -B receptors (ET(A)R and ET(B)R) activate the production of superoxide via NAD(P)H oxidase and subsequently stimulate the formation of cyclic adenine diphosphate ribose (cADPR) in afferent arterioles. Vessels were isolated from rat kidney and loaded with fura 2. Endothelin-1 (ET-1) rapidly increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) by 303 nM. The superoxide dismutase mimetic tempol, the NAD(P)H oxidase inhibitor apocynin, and nicotinamide, an inhibitor of ADPR cyclase, diminished the response by approximately 60%. The ET(B)R agonist sarafotoxin 6c (S6c) increased peak [Ca(2+)](i) by 117 nM. Subsequent addition of ET-1 in the continued presence of S6c caused an additional [Ca(2+)](i) peak of 225 nM. Neither nicotinamide or 8-bromo- (8-Br) cADPR nor apocynin decreased the [Ca(2+)](i) response to S6c, but inhibited the subsequent [Ca(2+)](i) response to ET-1. The ET(B)R blockers BQ-788 and A-192621 prevented the S6c [Ca(2+)](i) peak and reduced the ET-1 response by more than one-half, suggesting an ET(B)R/ET(A)R interaction. In contrast, the ET(A)R blocker BQ-123 had no effect on the S6c [Ca(2+)](i) peak and obliterated the subsequent ET-1 response. ET-1 immediately stimulated superoxide formation (measured with TEMPO-9-AC, 68 arbitrary units) that was inhibited 95% by apocynin or diphenyl iodonium. S6c or IRL-1620 increased superoxide by 8% of that caused by subsequent ET-1 addition. We conclude that ET(A)R activation of afferent arterioles increases the formation of superoxide that accounts for approximately 60% of subsequent Ca(2+) signaling. ET(B)R activation appears to result in only minor increases in superoxide production. Nicotinamide and 8-Br-cADPR results suggest that ET-1 (and primarily ET(A)R) causes the activation of vascular smooth muscle cell-ADPR cyclase.

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.

L-type calcium channels in the renal microcirculatory response to endothelin

The signaling pathways of endothelin (ET)-1-mediated vasoconstriction in the renal circulation have not been elucidated but appear to be distinct between ETAand ETBreceptors. The purpose of this study was to determine the role of L-type Ca2+channels in the vasoconstrictor response to ET-1 and the ETBreceptor agonist sarafotoxin 6c (S6c) in the rat kidney. Renal blood flow (RBF) was measured with an ultrasonic flow probe in anesthetized rats, and a microcatheter was inserted into the renal artery for drug infusion. All rats were given vehicle (0.9% NaCl) or three successive bolus injections (1, 10, and 100 pmol) of ET-1 or S6c at 30-min intervals ( n = 6 in each group). ET-1 and S6c produced dose-dependent decreases in RBF. The Ca2+channel blocker nifedipine (1.5 μg) significantly attenuated the RBF response only at the highest doses of ET-1 and S6c. In the isolated blood-perfused juxtamedullary nephron preparation, Ca2+channel blockade with diltiazem had a very small inhibitory effect on ET-1-induced decreases in afferent arteriolar diameter only at the lowest concentrations of ET-1. In vascular smooth muscle cells isolated from preglomerular vessels, ET-1 produced a typical biphasic Ca2+response, whereas S6c had no effect on cytosolic Ca2+. Furthermore, Ca2+channel blockade (diltiazem or Ni2+) had no effect on the peak or sustained increase in cytosolic Ca2+produced by ET-1. These results support the hypothesis that L-type Ca2+channels play only a minor role in the constrictor responses to ET-1 in the renal microcirculation.

Muscular ETB receptors develop postnatally and are differentially distributed in specific segments of the rat vasculature

The endothelin/endothelin-receptor system is a key player in the regulation of vascular tone in mammals. We raised and characterized an antiserum against rat ETB receptor and investigated the distribution of ETB receptors in different vascular beds during postnatal development (day 0 through day 28) and in the adult rat. We report the tissue-specific and age-dependent presence of vasoconstrictor ETB receptors. At the time of birth, vascular smooth muscle cells from all tissues examined did not exhibit ETB receptor immunoreactivity. The occurrence of ETB receptor immunoreactivity in the postnatal development was time dependent and started in small coronary and meningeal arteries at day 5, followed by small mesenteric arteries as well as brachial artery and vein at day 14. At day 21, ETB receptors were present in the media of muscular segments of pulmonary artery, large coronary arteries, and intracerebral arterioles. At day 28, ETB receptor immunoreactivity was evident in interlobular renal arteries, vas afferens, and efferens. Large renal arteries, mesenteric artery, and elastic segments of pulmonary arteries, as well as coronary and mesenteric veins, did not exhibit ETB receptor immunoreactivity. These data demonstrate the age-dependent and tissue-specific presence of ETB receptors, mainly on arterial smooth muscle cells in the vascular system of the rat.

Mechanisms underlying the differential control of blood flow in the renal medulla and cortex

There is much evidence that the medullary circulation plays a key role in regulating renal salt and water handling and, accordingly, the long-term level of arterial pressure. It has also recently become clear that various regulatory factors can affect medullary blood flow (MBF) differently from cortical blood flow (CBF). It appears likely that the influence of hormonal and neural factors on the control of arterial pressure is mediated partly through their impact on MBF. In this review, we focus on the mechanisms underlying the differential control of MBF and CBF, particularly the relative insensitivity of MBF to vasoconstrictors such as angiotensin II, endothelin-1 and the sympathetic nerves. The vascular architecture of the kidney appears to be arranged in a way that protects the renal medulla from ischaemic insults, with juxtamedullary arterioles, the source of MBF, having larger calibre than their counterparts in other kidney regions. Indeed, recent studies using vascular casting methodology suggest that juxtamedullary glomerular arterioles are not the chief regulators of MBF, which is consistent with the idea that outer medullary descending vasa recta play a key role in MBF control. Release of vasoactive paracrine factors such as nitric oxide and various eicosanoids from the vascular endothelium, and probably also from the tubular epithelium, appear to differentially modulate responses of MBF and CBF to hormonal and neural factors. The prevailing intrarenal hormonal milieu and existing haemodynamic conditions also appear to strongly modulate these responses, indicating that multiple control systems interact to regulate regional kidney blood flow at an integrative level.

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

1. Medullary blood flow (MBF) is important in the long-term control of arterial pressure. However, it is unclear which vascular elements regulate MBF. 2. Exogenous endothelin (ET)-1 decreases cortical more than medullary blood flow. We hypothesized that ET-1 would therefore constrict afferent (AA) and efferent arterioles (EA) of juxtamedullary glomeruli less than those of cortical glomeruli. 3. Mean arterial pressure, renal blood flow and cortical (CBF) and medullary (MBF) blood flow, via laser-Doppler flowmetry, were measured before and after intrarenal ET-1 (2 ng/kg per min; n = 6) or vehicle (n = 6) in anaesthetized rabbits. Kidneys were perfusion fixed, vascular casts formed, lumen diameters measured via scanning electron microscopy and relative resistance calculated. 4. Mean arterial pressure was not significantly affected by ET-1 infusion. Cortical glomerular arteriole lumen diameters were significantly reduced in the ET-1-infused group (AA approximately 30%, EA approximately 18%; PA < 0.01), compatible with the decrease in CBF (42 +/- 3%; PGT < 0.01). Juxtamedullary arteriole lumen diameters were also significantly reduced in the ET-1-infused group (AA approximately 34%, EA approximately 21%; PA < 0.01); however, MBF did not decrease. 5. In conclusion, our data suggest that juxtamedullary arterioles are not of primary importance in the regulation of MBF because, despite reductions in juxtamedullary arteriole diameters in response to ET-1, MBF was not decreased.

Relaxin-induced changes in renal sodium excretion in the anesthetized male rat

Pregnancy is associated with profound changes in renal hemodynamics and electrolyte handling. Relaxin, a hormone secreted by the corpus luteum, has been shown to induce pregnancy-like increases in renal blood flow and glomerular filtration rate (GFR) and alter osmoregulation in nonpregnant female and male rats. However, its effects on renal electrolyte handling are unknown. Accordingly, the influence of short (2 h)- and long-term (7 day) infusion of relaxin on renal function was determined in the male rat. Short term infusion of recombinant human relaxin (rhRLX) at 4 μg·h−1·100 g body wt−1 induced a significant increase in effective renal blood flow (ERBF) within 45 min, which peaked at 2 h of infusion (vehicle, n = 6, 2.1 ± 0.4 vs. rhRLX, n = 7, 8.1 ± 1.1 ml·min−1·100 g body wt−1, P < 0.01). GFR and urinary excretion of electrolytes were unaffected. After a 7-day infusion of rhRLX at 4 μg/h, ERBF (1.4 ± 0.2 vs. 2.5 ± 0.4 ml·min−1·100 g body wt−1, P < 0.05), urine flow rate (3.1 ± 0.3 vs. 4.3 ± 0.4 μl·min−1·100 g body wt−1, P < 0.05) and urinary sodium excretion (0.8 ± 0.1 vs. 1.2 ± 0.1 μmol·min−1·100 g body wt−1, P < 0.05) were significantly higher; plasma osmolality and sodium concentrations were lower in rhRLX-treated rats. These data show that long-term relaxin infusion induces a natriuresis and diuresis in the male rat. The mechanisms involved are unclear, but they do not involve changes in plasma aldosterone or atrial natriuretic peptide concentrations.

Endothelin B receptor Ca2+ signaling in shark vascular smooth muscle: participation of inositol trisphosphate and ryanodine receptors

In mammals, endothelin receptors are sub-classified into ET(A) receptors (ET(A)R), which are purely constrictive in vascular smooth muscle (VSM), and ET(B)R, which may produce constriction in VSM or dilatation by stimulating the production of nitric oxide (NO) from endothelial cells. In contrast, previous studies suggested that shark VSM is stimulated exclusively by ET(B)R. The Ca(2+) signaling pathways utilized by shark VSM in response to stimulation by endothelin-1 (ET-1) have not previously been investigated. We measured cytosolic Ca(2+) concentration ([Ca(2+)](i)) in fura-2-loaded VSM of anterior mesenteric artery of Squalus acanthias and show that the ET(B)R agonists IRL 1620 and sarafotoxin S6c (SRX) increase [Ca(2+)](i) in VSM to the same extent as ET-1 and ET(B)R appears to be the only ETR subtype in sharks. To investigate the participation of the inositol trisphosphate (IP(3)) receptors (IP(3)R), we utilized two inhibitors of the mammalian IP(3)R, TMB-8 and 2-APB. In Ca(2+)-free Ringer, these agents inhibit the response to ET(B)R agonist stimulation by 71%. The ryanodine-sensitive receptor (RyR) may be activated by low concentrations of ryanodine, by abrupt local increases of [Ca(2+)](i), (calcium-induced calcium release) or by cyclic adeninediphosphate ribose (cADPR). We employed three inhibitors of activation of the RyR, Ruthenium Red, 8-Br cADPR and high concentrations of ryanodine; these agents blocked the [Ca(2+)](i) response to ET(B)R agonist stimulation by a mean of 39%. These data show for the first time that in VSM of the shark, ET(B)R activation stimulates both IP(3)R and RyR, and that cADPR is involved in RyR activation.

Endothelin A and B receptors of preglomerular vascular smooth muscle cells

BACKGROUND

The endothelin (ET) receptors are subclassified into ET(A,) which are purely vasoconstrictive, and ET(B). The ET(B) receptors may cause either vasodilation by stimulating the release of nitric oxide from endothelial cells, or vasoconstriction of vascular smooth muscle cells (VSMC). The relative contribution of ET(A) and ET(B) receptors to calcium signaling and vasoconstriction in the renal microcirculation is not clear. Our goal was to study the cytosolic calcium concentration ([Ca(2+)](i)) responses of fresh rat preglomerular VSMC and afferent arterioles to agonists and antagonists of ET(A) and ET(B) receptors in rats.

METHODS

Fresh VSMC and afferent arterioles were isolated using the magnetized microsphere/sieving technique, followed by gentle collagenase digestion. [Ca(2+)](i) was measured with fura-2 ratiometric fluorescence.

RESULTS

Afferent arterioles and VSMC responded to ET-1 stimulation with a rapid peak increase in [Ca(2+)](i) (Delta= 287 +/- 81 and 342 +/- 55 nmol/L, respectively). The ET(B) receptor agonist IRL 1620 stimulated a rise in [Ca(2+)](i) in afferent arterioles (106 +/- 35 nmol/L); subsequent addition of ET-1 at the IRL 1620 nadir to stimulate ET(A) receptors caused a second peak that was twice as large (213 +/- 44 nmol/L). In VSMC, the ET(B) agonist peak increase was 99 +/- 12 nmol/L; addition of ET-1 then increased [Ca(2+)](i) by 294 +/- 23 nmol/L. The ET(B) inhibitor BQ-788 prevented stimulation of [Ca(2+)](i) by IRL 1620 in afferent arterioles and VSMC; subsequent stimulation of ET(A) receptors with ET-1 caused an increase in [Ca(2+)](i) (239 +/- 17 and 248 +/- 22 nmol/L). Pretreatment with the selective ET(A) inhibitor PD 156707 attenuated but did not abolish the responses to ET-1, suggesting that the residual [Ca(2+)](i) response was caused by ET(B) stimulation.

CONCLUSION

These results indicate that fresh preglomerular VSMC as well as afferent arterioles have both ET(A) and ET(B) receptors, and that the rapid peak [Ca(2+)](i) responses to the ET(B) agonist IRL 1620 are less than half that of subsequent stimulation of ET(A) receptors with ET-1. The similarity of findings in isolated VSMC and afferent arterioles suggests that responses in VSMC in our arteriolar preparation overshadow any potential contribution of endothelial cells when reagents are administered abluminally.

Dual constrictor and dilator actions of ET(B) receptors in the rat renal microcirculation: interactions with ET(A) receptors

The vascular actions of endothelin-1 (ET-1) reflect the combination of vasoconstrictor ET(A) and ET(B) receptors on smooth muscle cells and vasodilator ET(B) receptors on endothelial cells. The present study investigated the contribution of ET receptor subtypes using a comprehensive battery of agonists and antagonists infused directly into the renal artery of anesthetized rats to evaluate the actions of each receptor class alone and their interactions. ET-1 (5 pmol) reduced renal blood flow (RBF) 25+/-1%. ET(A) antagonist BQ-123 attenuated this response to a 15+/-1% decrease in RBF (P < 0.01), indicating net constriction by ET(B) receptors. Combined receptor blockade (BQ-123+BQ-788) resulted in a renal vasoconstriction of 7+/-1% (P = 0.001 vs. BQ-123), supporting a constrictor action of ET(B) receptors. In marked contrast, the ET(B) antagonist BQ-788 enhanced the ET-1 RBF response to 60+/-5% (P < 0.001), suggesting ET(B)-mediated net dilation. Consistent with ET(A) blockade, the ET(B) agonist sarafotoxin 6C (S6C) produced vasoconstriction, reducing RBF by 23+/-5%. Dose-response curves for ET-1 and S6C showed similar degrees of constriction between 0.2 and 100 pmol. Both antagonists (BQ-123, BQ-788) were equally effective at threefold lower than the standard doses, suggesting complete inhibition. We conclude that ET(B) receptors alone exert a net constrictor effect but cause a net dilator influence when costimulated with ET(A) receptors. Such opposing actions indicate more complex than additive interaction between receptor subtypes. Model analysis suggests ET(A)-mediated constriction is appreciably greater without than with costimulation of ET(B) receptors. Possible explanations include ET-1 clearance by ET(B) receptors and/or a dilator ET(B) receptor function that counteracts constriction.

Endothelin receptor antagonist bosentan improves microcirculatory blood flow in splanchnic organs in septic shock

OBJECTIVE

Splanchnic ischemia is believed to play an important role in the development of multiple organ dysfunction in septic shock. The vasoconstrictor peptide endothelin can produce an intense and sustained splanchnic vasoconstriction and is increased in sepsis. The aim of this investigation was to study the effects of an endothelin antagonist on microcirculatory blood flow in multiple abdominal organs during septic shock.

DESIGN

Prospective, controlled animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Fifteen anesthetized and mechanically ventilated pigs.

INTERVENTIONS

Septic shock was induced by fecal peritonitis. After 120 mins of sepsis, eight animals received 10 mg/kg bosentan intravenously followed by an intravenous infusion at 5 mg x kg-1 x hr-1 whereas seven (controls) received isotonic saline. At 240 mins after induction of sepsis both groups received hydroxyethyl starch, 20 mL/kg intravenously, to convert hypodynamic septic shock to hyperdynamic sepsis.

MEASUREMENTS AND MAIN RESULTS

Microcirculatory blood flow was measured simultaneously and continuously in the jejunal muscularis, pancreas, liver, kidney, skeletal muscle, and gastric, jejunal, and colon mucosa by using a multiple-channel laser Doppler flow meter. After 120 mins, all animals had developed signs of hypodynamic sepsis with decreased cardiac index, mean arterial blood pressure, and gastric mucosal pH. Microcirculatory blood flow in the pancreas and liver had decreased by 20% and in the jejunal muscularis by >40% (p <.01) whereas it remained virtually unchanged in the gastric, jejunal, and colonic mucosa. After 240 mins, cardiac index, mean arterial blood pressure, gastric mucosal pH, and microcirculatory blood flow in the gastric mucosa, colon mucosa, jejunal muscularis, and pancreas had all deteriorated in the controls, whereas in the bosentan-treated group, cardiac index and microcirculatory blood flow in the pancreas, gastric, and colon mucosa improved. During hyperdynamic sepsis, cardiac index increased above baseline in both groups but significantly more in the bosentan group. In the control group, microcirculatory flow returned to baseline in most tissues except in skeletal muscle and jejunal muscularis. In the bosentan group, microcirculatory flow returned to or increased above baseline in all tissues except in the muscularis of the jejunum.

CONCLUSIONS

The endothelin receptor antagonist bosentan significantly improved microcirculatory blood flow in many splanchnic organs and in peripheral tissues during septic shock. The results of this study are consistent with the hypothesis that endothelin plays an important role in the regulation of microcirculatory blood flow in splanchnic as well as in peripheral tissues during septic shock.

Rho-kinase inhibition blunts renal vasoconstriction induced by distinct signaling pathways in vivo

In addition to intracellular calcium, which activates myosin light chain (MLC) kinase, MLC phosphorylation and hence contraction is importantly regulated by MLC phosphatase (MLCP). Recent evidence suggests that distinct signaling cascades of vasoactive hormones interact with the Rho/Rho kinase (ROK) pathway, affecting the activity of MLCP. The present study measured the impact of ROK inhibition on vascular F-actin distribution and on vasoconstriction induced by activation/inhibition of distinct signaling pathways in vivo in the microcirculation of the split hydronephrotic rat kidney. Local application of the ROK inhibitors Y-27632 or HA-1077 induced marked dilation of pre- and postglomerular vessels. Activation of phospholipase C with the endothelin ET B agonist IRL 1620, inhibition of soluble guanylyl cyclase with 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), or inhibition of adenylyl cyclase with the adenosine A1 agonist N6-cyclopentyladenosine (CPA) reduced glomerular blood flow (GBF) by about 50% through vasoconstriction at different vascular levels. ROK inhibition with Y-27632 or HA-1077, but not protein kinase C inhibition with Ro 31-8220, blunted ET B-induced vasoconstriction. Furthermore, the reduction of GBF and of vascular diameters in response to ODQ or CPA were abolished by pretreatment with Y-27632. ROK inhibitors prevented constriction of preglomerular vessels and of efferent arterioles with equal effectiveness. Confocal microscopy demonstrated that Y-27632 did not change F-actin content and distribution in renal vessels. The results suggest that ROK inhibition might be considered as a potent treatment of renal vasoconstriction, because it interferes with constriction induced by distinct signaling pathways in renal vessels without affecting F-actin structure.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

Endothelin resets renal blood flow autoregulatory efficiency during acute blockade of NO in the rat

Renal blood flow (RBF) autoregulatory efficiency may be enhanced during NO inhibition in the rat, as recently reported. Under these conditions, endothelin (ET) synthesis and release may be increased. Our purpose was therefore to determine the role of ET in RBF autoregulatory changes induced by NO inhibition. To address this point, ET(A/B) receptors were blocked in anesthetized rats with bosentan, or selectively with BQ-610 or BQ-788. NO synthesis was inhibited with N(G)-nitro-L-arginine methyl ester (L-NAME). Mean arterial pressure (MAP) was decreased after bosentan (-10 mmHg; P < 0.01) or increased after L-NAME (25 mmHg; P < 0.001). RBF measured with an electromagnetic flow probe was reduced by L-NAME (-50%) and by BQ-788 (-24%). The pressure limits of the autoregulatory plateau (P(A) approximately 100 mmHg) and of no RBF autoregulation (P(o) approximately 80 mmHg) were significantly lowered by 15 mmHg after L-NAME but were unchanged after bosentan, BQ-610, or BQ-788. During NO inhibition, autoregulatory resetting was completely hindered by bosentan (P(A) approximately 100 mmHg) and by ET(B) receptor blockade with BQ-788 (P(A) approximately 106 mmHg), but not by ET(A) receptor blockade with BQ-610 (P(A) approximately 85 mmHg). These results suggest that the involvement of ET in the RBF autoregulatory resetting occurs during NO inhibition, possibly by preferential activation of the ET(B) receptor. However, the relative contribution of ET receptor subtypes remains to be further specified.

Effects of ET(A) - and ET(B)-receptor antagonists on regional kidney blood flow, and responses to intravenous endothelin-1, in anaesthetized rabbits

OBJECTIVE

To determine the roles of endothelin (ET)-receptor subtypes in the effects of exogenous and endogenous ETs on regional kidney blood flow in anaesthetized rabbits.

DESIGN AND METHODS

The effects on regional kidney blood flow of the ET(A) antagonist BQ610, and the ET(B) antagonist BQ788, were tested. We also examined the effects of intravenous and renal arterial bolus doses of ET-1, and how these responses are modified by pretreatment with BQ610 and BQ788.

RESULTS

BQ610 reduced mean arterial pressure (MAP, 3%), and increased total renal blood flow (RBF, 10%), cortical perfusion (CBF, 11%) and medullary perfusion (MBF, 16%). BQ788 increased MAP (6%) and reduced RBF (16%) and CBF (13%) but not MBF. The effects of BQ788 were abolished by pretreatment with BQ610. Intravenous ET-1 (300 ng/kg) reduced RBF and CBF, but increased MBF. BQ788 potentiated ET-1 mediated reductions in CBF, and abolished increases in MBF. BQ610 blunted reductions in RBF and CBF produced by ET-1, but did not significantly affect MBF responses. The renal vascular effects of intravenous ET-1 were mimicked by lower doses (1-30 ng/kg) administered into the renal artery.

CONCLUSIONS

Endogenous ETs act at ET(A)-receptors to reduce MBF and CBF, but ET(B)-receptors have little direct role in physiological control of renal haemodynamics. Bolus doses of ET-1 act at ET(B)-receptors in the kidney to increase MBF. The effects of bolus ET-1 on the cortical vasculature appear to result from the competing influences of ET(A)-mediated vasoconstriction and ET(B)-mediated vasodilatation.

Differences in cortical microcirculation in the kidneys of unilaterally congenital hydronephrotic rats

The surgically induced split hydronephrotic kidney has been generally accepted as a valid model for the assessment of renal microcirculation by means of intravital microscopy. Whereas nearly all previous work on this issue has been done with a transillumination technique, we used an epiillumination model that is suitable for investigation of microvascular perfusion in both normal and hydronephrotic kidneys without surgical manipulation of the ureter. By means of the congenital unilaterally hydronephrotic Tauchi rat, microcirculation of the hydronephrotic and that of the nonhydronephrotic kidney were compared. For that purpose both the hydronephrotic and the nonhydronephrotic kidneys of Tauchi rats were exteriorized on a specially designed microscopy stage. After injection of FITC-dextran and rhodamine 6G, microvascular perfusion was assessed in both kidneys. The new model allowed visualization of arterioles, capillaries, and postcapillary venules in both the hydronephrotic and the nonhydronephrotic kidneys. Glomeruli could only be regularly seen in the hydronephrotic kidney, but also in some normal kidneys. Capillary blood cell velocity was significantly higher in the hydronephrotic kidneys (0.67 +/- 0.03 mm/s) compared to the normal kidney (0.32 +/- 0.05 mm/s; P < 0.05), whereas capillary diameters were smaller (4.2 +/- 0.02 microm vs. 5.7 +/- 0.2 microm; P < 0.05). In addition, the hydronephrotic kidney showed a significantly lower density of perfused microvessels compared to the normal controls. Epiillumination intravital microscopy allows assessment of the cortical microcirculation in both the hydronephrotic and the nonhydronephrotic kidneys without surgical induction of hydronephrosis. The hydronephrotic kidney shows significant microcirculatory differences compared to normal kidneys that should be taken into account when using a hydronephrotic model for pharmacological testing.

Electrophysiological effects of endothelin-1 and their relationship to contraction in rat renal arterial smooth muscle

The electophysiological effects of endothelin-1 (ET-1) and their relationship to contraction remain unclear in the renal circulation. Using endotheliumdenuded arteries from the main branch of the renal artery proximal to the kidney of the rat, we have examined its effects on tension and conducted parallel patch-clamp measurements using freshly isolated smooth muscle cells from this tissue. Pharmacological experiments revealed that ET-1 produced constriction of renal arteries dependent on the influx of extracellular Ca(2+), mediated solely through ET(A) receptor stimulation. Current-clamp experiments revealed that renal arterial myocytes had a resting membrane potential of approximately 32 mV, with the majority of cells exhibiting spontaneous transient hyperpolarizations (STHPs). Application of ET-1 produced depolarization and in those cells exhibiting STHPs, either caused their inhibition or made them occur regularly. Under voltage-clamp conditions cells were observed to exhibit spontaneous transient outward currents (STOCs) inhibited by iberiotoxin. Application of voltage-ramps revealed an outward current activated at approximately -30 mV, sensitive to both 4-AP and TEA. Taken together these results suggest that renal arterial myocytes possess both delayed rectifying K(+) (K(V)) and Ca(2+)-activated K(+) (BK(Ca)) channels. Under voltage-clamp, ET-1 attenuated the outward current and reduced the magnitude and incidence of STOCs: effects mediated solely as a consequence of ET(A) receptor stimulation. Thus, in conclusion, activation of ET(A) receptors by ET-1 causes inhibition of K(V) and BK(Ca) channel activity, which could promote and/or maintain membrane depolarization. This effect is likely to favour L-type Ca(2+) channel activity providing an influx pathway for extracellular Ca(2+) essential for contraction.

ET-receptor subtypes: roles in regional renal vascular actions of exogenous and endogenous endothelins in anesthetized rabbits

The roles of endothelin (ET)-receptor subtypes, in the regional renal vascular effects of exogenous and endogenous ETs, were examined in pentobarbitone-anesthetized rabbits. The effects of renal arterial infusion of ET-1 (0.05-12.8 ng/kg/min) and the ET(B)-agonist [Ala1,3,11,15]-ET-1 (12.5-800 ng/kg/min) were compared. We then tested the effects of the ET(A)-antagonist BQ610 and the ET(B)-antagonist BQ788 (both 200 microg/kg plus 100 microg/kg/h, i.v.) on basal hemodynamics and on responses to renal arterial ET-1. Both ET-1 and [Ala1,3,11,15]-ET-1 dose-dependently reduced total renal blood flow (RBF) and cortical blood flow (CBF), but not medullary blood flow (MBF). ET-1 was 34-fold more potent than [Ala1,3,11,15-ET-1. BQ610 reduced mean arterial pressure (MAP; 14%), and increased RBF (21%) and CBF (12%), but not MBF. BQ788 increased MAP (13%), and reduced RBF (29%) and CBF (15%) but not MBF. Coadministration of both agents increased RBF (18%) and CBF (9%), without significantly affecting MAP. Neither antagonist (alone or combined) significantly affected responses to renal arterial ET-1. We conclude that the predominant renal vascular effects of exogenous and endogenous ETs are cortical vasoconstriction, but not at vascular sites controlling MBF. ET(A)-receptors contribute to the renal vasoconstrictor effects of endogenous ETs. ET(B2)-like receptors appear to contribute to the vasoconstrictor effects of [Ala1,3,11,15]-ET-1.

Upregulation of endothelin B receptors in kidneys of DOCA-salt hypertensive rats

Experiments were designed to elucidate the role of endothelin B receptors (ET(B)) on arterial pressure and renal function in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Male Sprague-Dawley rats underwent uninephrectomy and were treated with either DOCA and salt (0.9% NaCl to drink) or placebo. DOCA-salt rats given the ET(B)-selective antagonist, A-192621, for 1 wk (10 mg. kg(-1). day(-1) in the food) had significantly greater systolic arterial pressure compared with untreated DOCA-salt rats (208 +/- 7 vs. 182 +/- 4 mmHg) whereas pressure in placebo rats was unchanged. In DOCA-salt, but not placebo rats, A-192621 significantly decreased sodium and water excretion along with parallel decreases in food and water intake. To determine whether the response in DOCA-salt rats was due to increased expression of ET(B) receptors, endothelin receptor binding was performed by using membranes from renal medulla. Maximum binding (B(max)) of [(125)I]ET-1, [(125)I]ET-3, and [(125)I]IRL-1620 increased from 227 +/- 42, 146 +/- 28, and 21 +/- 1 fmol/mg protein, respectively, in placebo rats to 335 +/- 27, 300 +/- 38, and 61 +/- 6 fmol/mg protein, respectively, in DOCA-salt hypertensive rats. The fraction of receptors that are the ET(B) subtype was significantly increased in DOCA-salt (0.88 +/- 0.07) compared with placebo (0.64 +/- 0.01). The difference between [(125)I]ET-3 and [(125)I]IRL-1620 binding is consistent with possible ET(B) receptor subtypes in the kidney. These results indicate that ET(B) receptors in the renal medulla are up-regulated in the DOCA-salt hypertensive rat and may serve to maintain a lower arterial pressure by promoting salt and water excretion.

Cytochrome P450 and cyclooxygenase metabolites contribute to the endothelin-1 afferent arteriolar vasoconstrictor and calcium responses

Arachidonic acid metabolites contribute to the endothelin-1 (ET-1)-induced decrease in renal blood flow, but the vascular sites of action are unknown. Experiments performed in vitro used the rat juxtamedullary nephron preparation combined with videomicroscopy. The response of afferent arterioles to ET-1 was determined before and after cytochrome P450 (CYP450) or cyclooxygenase (COX) inhibition. Afferent arteriolar diameter averaged 20+/-1 microm (n=17) at a renal perfusion pressure of 100 mm Hg. Superfusion with 0.001 to 10 nmol/L ET-1 caused a graded decrease in diameter of the afferent arteriole. Vessel diameter decreased by 30+/-2% and 41+/-2% in response to 1 and 10 nmol/L ET-1, respectively. The afferent arteriolar response to ET-1 was significantly attenuated during administration of the CYP450 hydroxylase inhibitor N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS), such that afferent arteriolar diameter decreased by 19+/-3% and 22+/-3% in response to 1 and 10 nmol/L ET-1, respectively. COX inhibition also greatly attenuated the vasoconstriction elicited by ET-1, whereas the CYP450 epoxygenase inhibitor N-methylsulfonyl-6-(2-proparglyoxyphenyl) hexanamide enhanced the ET-1-mediated vascular response. Additional studies were performed using freshly isolated smooth muscle cells prepared from preglomerular microvessels. Renal microvascular smooth muscle cells were loaded with the calcium-sensitive dye fura 2 and studied by use of single-cell fluorescence microscopy. Basal renal microvascular smooth muscle cell [Ca(2+)](i) averaged 95+/-3 nmol/L (n=42). ET-1 (10 nmol/L) increased microvascular smooth muscle cell [Ca(2+)](i) to a peak value of 731+/-75 nmol/L before stabilizing at 136+/-8 nmol/L. Administration of DDMS or the COX inhibitor indomethacin significantly attenuated the renal microvascular smooth muscle cell calcium response to ET-1. These data demonstrate that CYP450 hydroxylase and COX arachidonic acid metabolites contribute importantly to the afferent arteriolar diameter and renal microvascular smooth muscle cell calcium responses elicited by ET-1.

Endothelin-mediated calcium signaling in preglomerular smooth muscle cells

This study was performed to test the hypothesis that endothelin peptides differentially influence intracellular calcium concentration ([Ca(2+)](i)) in preglomerular microvascular smooth muscle cells (MVSMC), in part through activation of endothelin (ET)(A) receptors. Experiments were performed in vitro with the use of single MVSMC freshly isolated from rat preglomerular microvessels. The effect of ET-1, ET-2, and ET-3 on [Ca(2+)](i) was measured with the use of the calcium-sensitive dye, fura 2, and standard fluorescence microscopy techniques. Baseline [Ca(2+)](i) averaged 84+/-3 nmol/L (n=141 cells from 23 dispersions). ET-1 concentrations of 1, 10, and 100 nmol/L evoked peak increases in [Ca(2+)](i) of 48+/-16, 930+/-125, and 810+/-130 nmol/L, respectively. The time course of the [Ca(2+)](i) response was biphasic, beginning with a rapid initial increase followed by a sustained plateau phase or a period during which [Ca(2+)](i) oscillated sharply. Similar responses were observed after ET-2 administration. In contrast, ET-3 stimulated monophasic increases in [Ca(2+)](i) of only 14+/-5, 33+/-16, and 44+/-19 nmol/L at peptide concentrations of 1, 10, and 100 nmol/L, respectively. These responses are significantly smaller than responses to ET-1 or ET-2, respectively. The relative contributions of calcium mobilization and calcium influx in the response to ET-1 were also evaluated. Removal of calcium from the bathing medium did not significantly alter the peak response to 10 nmol/L ET-1 but abolished the late phase elevation of [Ca(2+)](i). These data demonstrate that endothelin peptides increase [Ca(2+)](i) in preglomerular MVSMC. The concentration-response profiles are consistent with the response involving activation of ET(A) receptors. Furthermore, these results suggest that ET-1 increases [Ca(2+)](i) by stimulating both the release of intracellular calcium and the influx of calcium from the extracellular medium.

Influence of endothelins and sarafotoxin 6c and L-NAME on renal vasoconstriction in the anaesthetized rat

1. An investigation was performed in pentobarbitone anaesthetized rats to compare the renal vasoconstrictor actions of endothelin-1 (ET-1), endothelin-3 (ET-3) and sarafotoxin 6c and their dependency on NO production. 2. Intra-renal arterial infusion of ET-1 and ET-3, from 1 - 1000 ng had no effect on blood pressure, but reduced renal blood flow maximally by 82 and 81% with EC50 values of 510+/-18 and 1113+/-17 ng, respectively and correspondingly increased renal vascular resistance and decreased conductance. 3. Direct renal arterial administration of sarafotoxin 6c was without effect on blood pressure but caused a maximum reduction in renal blood flow of 56% at 300 ng and had an EC50 of 86+/-4 ng. 4. Administration of the selective ETA receptor antagonist FR139317 at 0.3 and 1.0 mg kg-1 had no effect on basal levels of blood pressure, renal vascular resistance or renal blood flow. The lower dose of FR139317 had no effect on the ET-1 dose-response curve for renal blood flow while at 1.0 mg kg-1, FR139317 reduced the EC50 to 363+/-32 ng (P<0.05). 5. Infusion of L-NAME, 10 microg kg-1 min-1 increased blood pressure by approximately 15%, increased renal vascular resistance and decreased renal blood flow by some 40%. The EC50 values for renal blood flow were reduced to 358+/-68 ng (P<0.05) for ET-1, 638+/-69 ng (P<0.05) for ET-3 and 55+/-10 ng (P<0.01) for sarafotoxin 6c. The maximal reduction in renal blood flow induced by sarafotoxin 6c was raised (P<0.01) from 56% to approximately 100% and renal vascular resistance increased when NO production was blocked. 6. These results showed that the vasoconstrictor actions of ET-1 and ET-3 on resistance vessels controlling renal blood flow are mediated via ETB rather than ETA receptors. Moreover, both ET-1 and ET-3 dependent vasoconstrictions are slightly attenuated by concomitant NO production. By contrast, sarafotoxin 6c appears much more potent at the renal resistance vasculature and is much more powerfully modulated by NO.

Contribution of endothelin to renal vascular tone and autoregulation in the conscious dog

Exogenous endothelin-1 (ET-1) is a strong vasoconstrictor in the canine kidney and causes a decrease in renal blood flow (RBF) by stimulating the ETA receptor subtype. The aim of the present study was to investigate the role of endogenously generated ET-1 in renal hemodynamics under physiological conditions. In six conscious foxhounds, the time course of the effects of the selective ETA receptor antagonist LU-135252 (10 mg/kg iv) on mean arterial blood pressure (MAP), heart rate (HR), RBF, and glomerular filtration rate (GFR), as well as its effects on renal autoregulation, were examined. LU-135252 increased RBF by 20% (from 270 +/- 21 to 323 +/- 41 ml/min, P < 0.05) and HR from 76 +/- 5 to 97 +/- 8 beats/min (P < 0. 05), but did not alter MAP, GFR, or autoregulation of RBF and GFR. Since a number of interactions between ET-1 and the renin-angiotensin system have been reported previously, experiments were repeated during angiotensin converting enzyme (ACE) inhibition by trandolaprilat (2 mg/kg iv). When ETA receptor blockade was combined with ACE inhibition, which by itself had no effects on renal hemodynamics, marked changes were observed: MAP decreased from 91 +/- 4 to 80 +/- 5 mmHg (P < 0.05), HR increased from 85 +/- 5 to 102 +/- 11 beats/min (P < 0.05), and RBF increased from 278 +/- 23 to 412 +/- 45 ml/min (P < 0.05). Despite a pronounced decrease in renal vascular resistance over the entire pressure range investigated (40-100 mmHg), the capacity of the kidneys to autoregulate RBF was not impaired. The GFR remained completely unaffected at all pressure levels. These results demonstrate that endogenously generated ET-1 contributes significantly to renal vascular tone but does not interfere with the mechanisms of renal autoregulation. If ETA receptors are blocked, then the vasoconstrictor effects of ET-1 in the kidney are compensated for to a large extent by an augmented influence of ANG II. Thus ET-1 and ANG II appear to constitute a major interrelated vasoconstrictor system in the control of RBF.

Effects of BQ-123 on systemic and renal hemodynamic responses to endothelin-1 in the rat split hydronephrotic kidney

OBJECTIVE

To assess the site of action of endothelin-1 in vessels of different sizes in the kidney in vivo and investigate the function of endothelin A (ET(A)) receptors in mediating renal and systemic vasoconstriction.

DESIGN

The luminal diameters of different vessels were measured and glomerular blood flow in cortical glomeruli was determined by intravital videomicroscopy in the split hydronephrotic kidney of anesthetized female Wistar rats.

METHODS

The rats were infused with endothelin-1 (40 pmol/kg per min) with or without pretreatment with the selective ET(A)-receptor antagonist BQ-123 (0.5 mg/kg). Aortic clamping was used to control renal blood pressure during the endothelin-1 infusion.

RESULTS

Exogenous endothelin-1 induced a significant rise (30+/-3%) in mean arterial pressure and a marked, long-lasting fall in glomerular blood flow (53+/-3%) related to reduction of the inner diameter of arcuate (-30%), interlobular arteries (-33%) and afferent arterioles (-17%). Aortic clamping to normalize renal blood pressure did not attenuate the vasoconstriction and reduction in glomerular blood flow. Pretreatment with BQ-123 significantly reduced both the endothelin-1-induced rise in mean arterial pressure (12+/-1%) and the fall in glomerular blood flow (-23+/-11%). BQ-123 blunted the response to endothelin-1 in arcuate (-12%), interlobular (-11%) and afferent vessels (-5%). Acetylcholine and nitroprusside completely reversed the vasoconstriction in BQ-123-pretreated animals.

CONCLUSIONS

BQ-123 largely prevented the hemodynamic effects of exogenously administered endothelin-1. Our direct in-vivo techniques showed that ET(A) receptors are, at least in part, involved in endothelin-1 -mediated vasoconstriction in the rat kidney, and support the hypothesis that ET(A) receptors may help to control arterial pressure in anesthetized rats.

Contribution of endothelin receptors in renal microvessels in acute cyclosporine-mediated vasoconstriction in rats

Cyclosporine A (CsA), a widely used immunosuppressive agent, causes renal vasoconstriction and systemic hypertension. Recent data suggest that the renal effect of CsA is possibly mediated by endothelin (ET). We investigated the effects of CsA on renal microvessels and the efficacy of ETA or ETA/ETB receptor antagonists in ameliorating CsA effects in the hydronephrotic rat kidney. Infusion of CsA (30 mg.kg-1) induced a transient increase (20%) in mean arterial pressure (MAP) and a sustained reduction (85%) in glomerular blood flow (GBF) due to preferential constriction of the arcuate artery (39%) and the proximal segment of the interlobular artery (23%). Under basal conditions the ETA receptor antagonist BQ-123 had marginal effects consisting of reduction in MAP, rise in GBF and dilation of preglomerular vessels. The non-selective ETA/ETB receptor antagonist PD 145065 also reduced MAP, but tended to decrease GBF and constrict large preglomerular vessels. The difference in effects of the two antagonists indicated that under basal conditions ETB blockade constricts large preglomerular vessels and reduces GBF. After BQ-123 or PD 145065, the constriction of large preglomerular vessels and reduction in GBF induced by CsA was attenuated by about 50%, but the rise in MAP was not influenced. Our data indicate that a sizable part of renal vasoconstriction due to CsA is mediated via ET production in large preglomerular arteries and can be avoided by the blockade of ETA receptors. Additional blockade of ETB receptors does not attenuate the CsA effects further, possibly because ETB receptors mediate both vasoconstriction and dilation.