Nitric oxide, endothelin and nephron transport: potential interactions

Amikacin induced renal damage and the role of the antioxidants on neonatal rats

Amikacin (AK) is frequently used on the treatment of Gram-negative infections on neonates, but its usage is restricted because of nephrotoxicity. In this study, on neonatal rats, we aimed to investigate the effects of erythropoietin and vitamin E on AK induced nephrotoxicity. A total of 35 newborn Wistar Albino rats were divided into four groups: (1) injected with saline (serum physiological was administered to placebo controls), (2) injected with AK (1200 mg/kg), (3) injected with AK + vitamin E (150 mg/kg), (4) injected with AK + erythropoietin (EPO) (300 IU/kg/day). In renal tissue, AK levels were significantly high in all groups except the control. Tissue malondialdehyde (MDA) and nitric oxide (NO) levels were statistically higher in AK -treated group than the control. MDA and NO levels were significantly decreased with the administration of vitamin E and EPO. Glutathione peroxidase (GPX) levels were statistically low in AK group compared with the controls. The levels of GPX, in vitamin E group, were increased significantly. However, superoxide dismutase and catalase levels were not significantly different in none of the groups. Insulin-like growth factor-1 values in AK, EPO and vitamin E groups were significantly higher than the control group. Histomorphological changes such as tubular epithelial necrosis were seen in AK treated group. Histopathological improvements observed with EPO and vitamin E administration. AK nephrotoxicity is related to oxidative stress and is supported with biochemical and histopathological findings. Vitamin E and EPO, as antioxidants, can be useful renoprotective agents for ameliorating AK induced nephrotoxicity in neonates.

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.

Renal pericytes: regulators of medullary blood flow

Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla.

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.

Flow regulation of collecting duct endothelin-1 production

Collecting duct (CD) endothelin-1 (ET-1) is an important autocrine inhibitor of CD Na(+) reabsorption. Salt loading is thought to increase CD ET-1 production; however, definitive evidence of this, as well as understanding of the mechanisms transducing this effect, is lacking. Tubule fluid flow increases in response to Na(+) loading; hence, we studied flow modulation of CD ET-1 production. Three days of a high-salt diet increased mouse and rat inner medullary CD (IMCD) ET-1 mRNA expression. Acute furosemide infusion increased urinary ET-1 excretion in anesthetized rats. Primary cultures of mouse or rat IMCD detached in response to flow using a closed perfusion chamber, consequently a CD cell line (mpkCCDcl4) was examined. Flow increased ET-1 mRNA at shear stress rates exceeding 1 dyne/cm(2), with the maximal effect seen between 2 and 10 dyne/cm(2). Induction of ET-1 mRNA was first evident after 1 h, and most apparent after 2 h, of flow. Inhibition of calmodulin or dihydropyridine-sensitive Ca(2+) channels did not alter the flow response; however, chelation of intracellular Ca(2+) or removal of extracellular Ca(2+) largely prevented flow-stimulated ET-1 mRNA accumulation. Downregulation of protein kinase C (PKC) using phorbol 12-myristate 13-acetate, or PKC inhibition with calphostin C, markedly reduced flow-stimulated ET-1 mRNA levels. Flow-stimulated ET-1 mRNA accumulation was abolished by inhibition of phospholipase C (PLC). Taken together, these data indicate that flow increases CD ET-1 production and this is dependent on extracellular and intracellular Ca(2+), PKC, and PLC. These studies suggest a novel pathway for coupling alterations in extracellular fluid volume to CD ET-1 production and ultimately control of CD Na(+) reabsorption.

Changes in endothelin receptor type B and neuronal nitric oxide synthase in puromycin aminonucleoside-induced nephrotic syndrome

The collecting duct endothelin (ET) system, which involves ET-1 and its two receptors, may play a role in the regulation of renal sodium in association with the nitric oxide synthase (NOS) system. We determined whether sodium retention is associated with changes in the endothelin and NOS systems at different stages (i.e., a sodium retaining stage and a compensatory stage) of nephrotic syndromes. On day 7 after puromycin aminonucleoside (PAN) injection, urinary sodium excretion was decreased, ascites had developed, and there was a positive sodium balance. ET-1 mRNA expression was increased in the inner medulla of the kidney, whereas protein expression of ET receptor type B (ET(B)R) was unchanged. The expression of neuronal NOS (nNOS) was decreased in the inner medulla. On day 14, urinary sodium excretion was unchanged compared with controls. The expression of ET(B)R increased, while nNOS expression in the inner medulla was comparable to controls. These findings suggest that decreased nNOS plays a role in the development of sodium retention in the nephrotic syndrome. Recovery of nNOS and increased renal ET(B)R synthesis may promote sodium excretion in later stages of the nephrotic syndrome (on day 14).

Darusentan, a selective endothelin A receptor antagonist, for the oral treatment of resistant hypertension

Resistant hypertension is defined as failure to lower blood pressure to target when a patient adheres to the maximum tolerated doses of three antihypertensive drugs including a diuretic. Notwithstanding the wide availability of several antihypertensive agents and the continued recommendation of dietary and lifestyle modifications, the prevalence of resistant hypertension remains high and is expected to increase thus underscoring the need for potential new treatment modalities in resistant hypertension. Endothelin-1 is a long-lasting potent vasoconstrictor and plays a key role in cardiovascular haemostasis. Endothelin mediates its biological activity in humans through the endothelin A and B receptors. The clinical experience and the evidence for therapy with darusentan in resistant systemic hypertension are reviewed. The leading journals that publish basic science and clinical research in the area of cardiovascular diseases and PubMed were scanned. While results from early clinical studies suggested that darusentan might emerge as new treatment option in patients with resistant hypertension, results from recent studies suggests that darusentan appears unlikely to find its way in the armamentarium for treatment of resistant hypertension.

Efficacy and safety of atrasentan in patients with cardiovascular risk and early atherosclerosis

Endothelin plays an important role in the pathogenesis of atherosclerosis. The aim of the study was to evaluate the safety and hemodynamic and metabolic responses to 6 months treatment with atrasentan, the selective endothelin-A receptor antagonist. Seventy-two patients with multiple cardiovascular risk factors and nonobstructive coronary artery disease on coronary angiogram were randomly assigned in a double-blind manner to atrasentan or placebo. Mean aortic blood pressure decreased from 92+/-10 to 80+/-10 mm Hg (P<0.001) in the atrasentan group and did not change in the placebo group (93+/-10 and 92+/-11 mm Hg; P=0.84). The difference between the groups was significant (P<0.001). No effect on heart rate was observed. In a subgroup of patients not treated with angiotensin-converting enzyme inhibitor, creatinine level decreased in the atrasentan versus the placebo group (P=0.011). Fasting glucose (P=0.026), glycosylated hemoglobin level (P=0.041), triglyceride l (P=0.013), lipoprotein-A (P=0.046), and uric acid levels (P=0.048) decreased significantly in the atrasentan group compared with the placebo group. No progression of angiographic coronary disease was observed. The most common adverse effects with atrasentan were nasal stuffiness, headache, and edema. In conclusion, 6 months of treatment with atrasentan results in a reduction of blood pressure and improvement in glucose and lipid metabolism. These findings suggest the beneficial role of atrasentan in the treatment of hypertension and metabolic syndrome.

Teleost fish osmoregulation: what have we learned since August Krogh, Homer Smith, and Ancel Keys

In the 1930s, August Krogh, Homer Smith, and Ancel Keys knew that teleost fishes were hyperosmotic to fresh water and hyposmotic to seawater, and, therefore, they were potentially salt depleted and dehydrated, respectively. Their seminal studies demonstrated that freshwater teleosts extract NaCl from the environment, while marine teleosts ingest seawater, absorb intestinal water by absorbing NaCl, and excrete the excess salt via gill transport mechanisms. During the past 70 years, their research descendents have used chemical, radioisotopic, pharmacological, cellular, and molecular techniques to further characterize the gill transport mechanisms and begin to study the signaling molecules that modulate these processes. The cellular site for these transport pathways was first described by Keys and is now known as the mitochondrion-rich cell (MRC). The model for NaCl secretion by the marine MRC is well supported, but the model for NaCl uptake by freshwater MRC is more unsettled. Importantly, these ionic uptake mechanisms also appear to be expressed in the marine gill MRC, for acid-base regulation. A large suite of potential endocrine control mechanisms have been identified, and recent evidence suggests that paracrines such as endothelin, nitric oxide, and prostaglandins might also control MRC function.

Endothelin role in kidney acidification

Endothelin is a potent vasoconstrictor that recent studies show modulates transport in kidney tubules, including that related to acidification. The data support a physiologic role for endothelin in mediating enhanced kidney tubule acidification in response to an acid challenge to systemic acid-base balance status. The data to date do not support an endothelin role in maintaining kidney tubule acidification in control, nonacid-challenged states. Endothelin also contributes to the enhanced acidification of some pathophysiologic states and might have a role in some of the untoward outcomes associated with these conditions. This reviews supports continuation of studies into the physiologic and possibly pathophysiologic role of endothelin in settings of increased tubule acidification.

Regulation of kidney acid excretion by endothelins

Endothelin (ET) is a potent vasoconstrictor that is now known to modulate kidney tubule transport, including kidney tubule acidification. Animals undergoing an acid challenge to systemic acid-base status and with some models of chronic metabolic acidosis have increased kidney ET production. Increased ET production/activity contributes to enhanced kidney tubule acidification that facilitates kidney acid excretion in response to an acid challenge to systemic acid-base status. The data to date support a physiologic role for ET in mediating enhanced kidney acidification in response to acid challenges, but do not support an ET role in maintaining kidney tubule acidification in control, non-acid-challenged states. ET increases acidification in both the proximal and distal nephron and appears to exert its effects both directly and indirectly, the latter through modulating the levels and/or activity or other mediators of kidney tubule acidification. ET also contributes to enhanced kidney acidification in some pathophysiologic states and might contribute to some untoward outcomes associated with these conditions. Whether ET should be a therapeutic target in treating and/or preventing some of these untoward outcomes remains an open question. This review supports continued research into the physiologic and possibly pathophysiologic role of ET in settings of increased kidney tubule acidification.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

Endothelin-1 stimulates NO production and inhibits cAMP accumulation in rat inner medullary collecting duct through independent pathways

Endothelin-1 (ET-1) inhibition of vasopressin (AVP)-stimulated cAMP accumulation in the collecting duct has been hypothesized to be mediated, at least in part, by nitric oxide (NO). To examine this, the effect of ET-1 on NO production by acutely isolated rat inner medullary collecting duct (IMCD) cell suspensions and the role of NO in mediating ET-1 effects on AVP-stimulated cAMP accumulation were studied. ET-1 dose dependently (first evident at 100 pM ET-1) increased IMCD NO production as determined by DAF-FM fluorescence. ET(B) receptor (BQ-788), but not ET(A) receptor (BQ-123), antagonism blocked this effect. Nonspecific NO synthase (NOS) inhibitors [N(G)-nitro-L-arginine methyl ester (L-NAME) or N(G)-monomethyl-L-arginine] or NOS-1 inhibitors (SMTC or VNIO) inhibited the ET-1 response, whereas NOS-2 or NOS-3 inhibitors (L-NAA or 1400W) were ineffective. ET-1 also increased cGMP accumulation. ET-1 caused a 35% reduction in AVP-stimulated cAMP levels; however, this response was not affected by L-NAME or SMTC. The addition of L-arginine, NADPH, tetrahydrobiopterin, or tempol (to reduce superoxide-dependent conversion of NO to peroxynitrate) did not affect the response. NO donors (SNAP or spermine NONOate), at concentrations that stimulated DAF-FM fluorescence and increased cGMP levels, did not alter AVP-stimulated cAMP accumulation in the IMCD cell suspensions. In conclusion, ET-1 stimulates IMCD NO production through activation of the ET(B) receptor and NOS-1. However, neither ET-1-mediated NO production nor NO donors inhibit AVP-stimulated cAMP accumulation, indicating that NO does not mediate ET-1 inhibition of cAMP production by the IMCD.

The potential of endothelin antagonism as a therapeutic approach

Endothelin (ET) is a pivotal physiological regulator of blood pressure through its effects on blood vessels, heart, lung and kidneys, and the ET system can be overactive in disorders such as pulmonary hypertension, heart failure and renal disease. Such observations stimulated interest among scientists and pharmaceutical companies that have set up high-throughput screens to search for antagonists of ET receptors. The emerging compounds have been tested in animals with exciting results, leading to great hope that such inhibitors could be translated into human drugs with desirable therapeutic activities and few side effects. This review will describe the most relevant results obtained in experimental animals in a wide variety of disease conditions and focus on the data of selected compounds that have been employed in clinical trials.

Atrial natriuretic peptide and endothelin-3 target renal sodium-glucose cotransporter

Atrial natriuretic peptide (ANP) and endothelin (ET) are endogenous vasoactive factors that exert potent diuretic and natriuretic actions. We have previously shown that ANP and ET-3 act through an NO pathway to inhibit the sodium-glucose cotransporter (SGLT) in the intestine [Gonzalez Bosc LV, Elustondo PA, Ortiz MC, Vidal NA. Effect of atrial natriuretic peptide on sodium-glucose cotransport in the rat small intestine. Peptides 1997; 18: 1491-5; Gonzalez Bosc LV, Majowicz MP, Ortiz MC, Vidal NA. Effects of endothelin-3 on intestinal ion transport. Peptides 2001; 22: 2069-75.]. Here we address the role of ANP and ET-3 on SGLT activity in renal proximal tubules. In rat renal cortical brush border membranes (BBV), fluorescein isothiocianate (FITC) labeling revealed a specific 72-kD peptide that exhibits increased FITC labeling in the presence of Na+ and D-glucose. Using alpha-14C-methylglucose active uptake, rat BBV were shown to possess SGLT activity with an affinity constant (K(0.5) approximately 2.4 mM) that is consistent with the expression of the low-affinity, high-capacity SGLT2 isoform. SGLT2 activity in these preparations is dramatically inhibited by ANP and ET-3. This inhibition is independent of changes in membrane lipids and is mimicked by the cGMP analogue, 8-Br-cGMP, suggesting the involvement of cGMP/PKG pathways. These results are the first demonstration that both ANP and ET-3 inhibit rat cortical renal SGLT2 activity, and suggest a novel mechanism by which these vasoactive substances modulate hydro-saline balance at the proximal tubular nephron level.

NO inhibits Na+-K+-2Cl- cotransport via a cytochrome P-450-dependent pathway in renal epithelial cells (MMDD1)

Nitric oxide (NO) exerts direct effects on nephron transport. We determined the effect of NO on Na(+)-K(+)-2Cl(-) cotransport in a cell line (MMDD1) with properties of macula densa. Na(+)-K(+)-2Cl(-) cotransport was measured as bumetanide-sensitive (86)Rb(+) uptake in the presence of ouabain. MMDD1 cells expressed mRNA for the neuronal isoform of nitric oxide synthase, as well as NKCC1 and NKCC2(B) isoforms of the Na(+)-K(+)-2Cl(-) cotransporter. Preincubation of cells with the NO donors sodium nitroprusside (SNP) or S-nitroso-N-acetylpenicillamine (SNAP) caused concentration-dependent inhibition of Na(+)-K(+)-2Cl(-) cotransport. Both apical and basolateral Na(+)-K(+)-2Cl(-) cotransport was inhibited by NO donors. SNP or SNAP had no significant effect on cellular levels of cGMP, cAMP, cytosolic calcium, or phosphorylation of ERK1 and ERK2. In contrast, the inhibitors of cytochrome P-450, 1-aminobenzotriazole (ABT; 10(-3) M) or ketoconazole (1.5 x 10(-5) M), completely reversed the inhibitory effect of SNAP on apical or basolateral Na(+)-K(+)-2Cl(-) cotransport [apical: control 1.18 +/- 0.15 vs. SNAP (10(-4) M) 0.41 +/- 0.05 pmol x mg(-1) x 5 min(-1); P < 0.001; SNAP (10(-4) M) + ABT 1.32 +/- 0.10 pmol x mg(-1) x 5 min(-1); P = not significant vs. control; n = 5]. The cytochrome P-450 epoxyeicosatrienoic acid (EET) metabolite 14,15-EET (5 x 10(-7) M) inhibited both apical and basolateral cotransport, whereas 8,9-EET and 11,12-EET had no significant effect. Although 20-hydroxyeicosatetraenoic acid inhibited apical cotransport, the inhibitor of omega-hydroxylase activity HET0016 did not reverse SNAP-mediated inhibition of apical cotransport. These data indicate that NO inhibits apical and basolateral Na(+)-K(+)-2Cl(-) cotransport in MMDD1 cells. The results suggest that the inhibitory pathway is independent of cGMP and might involve stimulation of a cytochrome P-450-dependent pathway.

Role of NO in endothelin-regulated drug transport in the renal proximal tubule

We previously demonstrated in intact killifish renal proximal tubules that endothelin (ET), acting through an ET(B) receptor and protein kinase C (PKC), reduced transport mediated by multidrug resistance-associated protein 2 (Mrp2), i.e., luminal accumulation of fluorescein methotrexate (FL-MTX) (Masereeuw R, Terlouw SA, Van Aubel RAMH, Russel FGM, and Miller DS. Mol Pharmacol 57: 59-67, 2000). In the present study, we used confocal microscopy and quantitative image analysis to measure Mrp2-mediated transport of FL-MTX in killifish tubules as an indicator of the status of this ET-fired, intracellular signaling pathway. Exposing tubules to sodium nitroprusside (SNP), a nitric oxide (NO) donor, signaled a reduction in luminal accumulation of FL-MTX, which suggested pathway activation. N(G)-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor, blocked the action of ET-1 on transport. Because SNP effects on transport were blocked by bisindoylmaleide, a PKC-selective inhibitor, but not by RES-701-1, an ET(B)-receptor antagonist, generation of NO occurred after ET(B) receptor signaling but before PKC activation. NO generation was implicated in the actions of several nephrotoxicants, i.e., diatrizoate, gentamicin, amikacin, HgCl(2), and CdCl(2), each of which decreased Mrp2-mediated transport by activating ET signaling. For each nephrotoxicant, decreased FL-MTX transport was prevented when tubules were exposed to L-NMMA. ET-1 and each nephrotoxicant stimulated NO production by the tubules, as determined by a fluorescence-based assay. Together, the data show that NO generation follows ET binding to the basolateral ET(B) receptor and that, in activating the ET-signaling pathway, nephrotoxicants produce NO, a molecule that could contribute to subsequent toxic effects.

Role of cytochrome P-450 arachidonate metabolites in endothelin signaling in rat proximal tubule

We examined the rat proximal tubule (PT) response to endothelin-1 (ET-1) in terms of 20-hydroxyeicosatetraenoic acid (HETE) dependency. Arachidonic acid (AA) (1 microM) decreased ouabain-sensitive (86)Rb uptake from 2.1 +/- 0.1 to 0.3 +/- 0.08 ng Rb. 10 microg protein(-1). 2 min(-1) (P < 0.05); 20-HETE (1 microM) had similar effects. Dibromododecenoic acid (DBDD) (2 microM), an inhibitor of omega-hydroxylase, abolished the inhibitory action of AA on (86)Rb uptake whereas the PT response to 20-HETE was unaffected. ET-1 at 0.1, 1, 10, and 100 nM reduced (86)Rb uptake from 2.8 +/- 0.3 in control PTs to 2.4 +/- 0.2, 1.7 +/- 0.1, 0.67 +/- 0.08, and 0.1 +/- 0.03 ng Rb. 10 microg protein(-1). 2 min(-1), respectively. DBDD (2 microM) abolished the inhibitory effect of ET-1 on (86)Rb uptake as did BMS182874 (1 microM), an ET(A)-selective receptor antagonist. ET-1 (100 nM) significantly increased PT 20-HETE release by approximately 50%, an effect prevented by DBDD. N(omega)-nitro-L-arginine-methyl ester (L-NAME), given for 4 days to inhibit nitric oxide synthase (NOS), increased arterial pressure from 92 +/- 12 to 140 +/- 8 mmHg and increased endogenous release of 20-HETE from isolated PTs (measured by gas chromatography/mass spectrometry). In L-NAME-treated PTs, but not in control PTs, 0.1 microM AA inhibited ouabain-sensitive (86)Rb uptake by > 40%; the response to AA was attenuated by DBDD. We conclude that, in the PTs, 1) 20-HETE is a second messenger for ET-1 and 2) conversion of AA to 20-HETE is augmented when NOS is inhibited.

Endothelins: vasoactive modulators of renal function in health and disease

Vasoactive autocoids with directly opposing actions on the renal vasculature, glomerular function, and in salt and water homeostasis have been demonstrated in the kidney. In the renal cortex, endothelin (ET)-1 and angiotensin-II cause vasoconstriction, decreasing renal blood flow, and glomerular filtration rate, whereas bradykinin and atrial natriuretic peptide cause vasodilation and increase glomerular capillary permeability. ET-1 causes vasoconstriction of the afferent and efferent arteries and outer medullary descending vasa recta, thereby decreasing vasa recta and papillary blood flow, while bradykinin has the opposite effect. ET-1 stimulates cell proliferation, increasing the expression of several genes, including collagenase, prostaglandin endoperoxidase synthase, and platelet-derived growth factor. ET-1 promotes natriuresis via the ET-B receptor, causing down-regulation of the epithelial Na(+) channel in the renal tubule. Thus, ETs affect three major aspects of renal physiology: vascular and mesangial tone, Na(+) and water excretion, and cell proliferation and matrix formation.

Renal tubular effects of endothelin-B receptor signaling: its role in cardiovascular homeostasis and extracellular volume regulation

The role of the endothelin-B receptor in vascular homeostasis is controversial because the receptor has both pressor and depressor effects in vivo. One potential depressor mechanism of endothelin-B activation is through the promotion of natriuresis and diuresis in the renal tubule. Recent studies demonstrate that rodents genetically deficient for the endothelin-B exhibit sodium-dependent hypertension due to an absence of tonic inhibition of the epithelial sodium channel in the distal nephron. These studies suggest that the predominant role of endothelin-B receptors in the basal physiologic state may be to regulate renal sodium excretion relative to the level of oral salt intake.

Renal endothelin in hypertension

Due to the potent vasoconstrictor action of endothelin-1 and its synthesis throughout the vasculature and other tissues, most investigators believe that it is an active participant in the pathogenesis of hypertension. However, the autocrine and paracrine nature of the endothelin system has made its role difficult to define. In recent years, it has become apparent that endothelin-1 contributes to the regulation of renal salt and water excretion and that it is a major contributor to the hypertension associated with salt-dependency. Evidence suggests that endothelin-1 within the renal medulla is activated in conditions of salt loading and inhibits reabsorption of sodium in a nitric oxide-dependent manner. Blockade of endothelin A receptors lowers arterial pressure in animal models of salt-dependent hypertension. Furthermore, circulating levels of endothelin-1 are generally higher in African-Americans compared to white Americans as is the prevalence of salt-dependent hypertension. Therefore, it would appear that use of endothelin A-selective receptor antagonists should be targeted to those individuals at risk for salt-dependent hypertension. Blockade of endothelin B receptors would not be desirable because of their important role in eliminating a salt load.