Endothelin, the kidney, and hypertension

Sex-Specific Differences in Kidney Function and Blood Pressure Regulation

Premenopausal women generally exhibit lower blood pressure and a lower prevalence of hypertension than men of the same age, but these differences reverse postmenopause due to estrogen withdrawal. Sexual dimorphism has been described in different components of kidney physiology and pathophysiology, including the renin-angiotensin-aldosterone system, endothelin system, and tubular transporters. This review explores the sex-specific differences in kidney function and blood pressure regulation. Understanding these differences provides insights into potential therapeutic targets for managing hypertension and kidney diseases, considering the patient's sex and hormonal status.

Effects of systemic and renal intramedullary endothelin-1 receptor blockade on tissue NO and intrarenal hemodynamics in normotensive and hypertensive rats

Endothelin 1 (ET-1) seems essential in salt-dependent hypertension, and activation of ETA receptors causes renal vasoconstriction. However, the response in the renal medulla and the role of tissue NO availability has never been adequately explored in vivo. We examined effects of ETA and ETB receptor blockade (atrasentan and BQ788) on blood pressure (MAP), medullary blood flow (MBF) and medullary tissue NO. Effects of systemic and intramedullary blocker application were compared in anesthetized normotensive ET-1-pretreated Sprague-Dawley rats (S-D), in salt-dependent hypertension (HS/UNX) and in spontaneously hypertensive rats (SHR). Total renal blood flow (RBF) was measured using a Transonic renal artery probe, MBF as laser-Doppler flux, and tissue NO signal using selective electrodes. In normotensive rats ET-1 significantly increased MAP, decreased RBF (-20%) and renal medullary NO. In HS/UNX rats atrasentan decreased MAP and increased medullary NO, earlier and more profoundly with intravenous infusion. In SHR atrasentan decreased MAP, more effectively with intravenous infusion; the increase in tissue NO (∼10%) was similar with both routes; however, only intramedullary atrasentan increased MBF. No consistent responses to BQ788 were seen. We confirmed dominant role of ETA receptors in regulation of blood pressure and renal hemodynamics in normotensive and hypertensive rats and provided novel evidence for the role of ETA in control of intrarenal NO bioavailability in salt-dependent and spontaneous hypertension. Under conditions of activation of the endothelin system ETB stimulation preserved medullary perfusion.

Brain structure and perfusion in relation to serum renal function indexes in healthy young adults

Prior neuroimaging studies of the relationship between the kidney and the brain have been limited to clinical populations and have largely relied on a single modality. We sought to examine the kidney-brain associations in healthy subjects using a combined analysis of multi-modal imaging data. Structural, diffusion, and perfusion magnetic resonance imaging (MRI) scans were performed to measure cortical thickness, white matter integrity, and cerebral blood flow in 157 healthy young adults. Peripheral venous blood samples were collected to measure serum renal function indexes. Correlation analyses were performed to investigate the relations between brain MRI measures and renal function indexes. Results showed that higher serum uric acid level was associated with increased cortical thickness in the transverse temporal gyrus. We also found that decreased serum creatinine level was linked to lower white matter integrity in the sagittal stratum, anterior corona radiata, superior corona radiata, and external capsule. Furthermore, we observed that increased serum uric acid level was related to hyperperfusion in the opercular and triangular parts of inferior frontal gyrus and supramarginal gyrus, and hypoperfusion in the calcarine sulcus, cuneus and lingual gyrus. More importantly, mediation analysis revealed that the relationship between serum uric acid and working memory performance was mediated by perfusion in the supramarginal gyrus and lingual gyrus. These findings not only may extend current knowledge regarding the relationship between the kidney and the brain, but also may inform real-world clinical practice by identification of potential brain regions vulnerable to renal dysfunction.

Glucose and Blood Pressure-Dependent Pathways-The Progression of Diabetic Kidney Disease

The major clinical associations with the progression of diabetic kidney disease (DKD) are glycemic control and systemic hypertension. Recent studies have continued to emphasize vasoactive hormone pathways including aldosterone and endothelin which suggest a key role for vasoconstrictor pathways in promoting renal damage in diabetes. The role of glucose per se remains difficult to define in DKD but appears to involve key intermediates including reactive oxygen species (ROS) and dicarbonyls such as methylglyoxal which activate intracellular pathways to promote fibrosis and inflammation in the kidney. Recent studies have identified a novel molecular interaction between hemodynamic and metabolic pathways which could lead to new treatments for DKD. This should lead to a further improvement in the outlook of DKD building on positive results from RAAS blockade and more recently newer classes of glucose-lowering agents such as SGLT2 inhibitors and GLP1 receptor agonists.

Preeclampsia: Novel Mechanisms and Potential Therapeutic Approaches

Preeclampsia is a serious complication of pregnancy where it affects 5–8% of all pregnancies. It increases the morbidity and mortality of both the fetus and pregnant woman, especially in developing countries. It deleteriously affects several vital organs, including the kidneys, liver, brain, and lung. Although, the pathogenesis of preeclampsia has not yet been fully understood, growing evidence suggests that aberrations in the angiogenic factors levels and coagulopathy are responsible for the clinical manifestations of the disease. The common nominator of tissue damage of all these target organs is endothelial injury, which impedes their normal function. At the renal level, glomerular endothelial injury leads to the development of maternal proteinuria. Actually, peripheral vasoconstriction secondary to maternal systemic inflammation and endothelial cell activation is sufficient for the development of preeclampsia-induced hypertension. Similarly, preeclampsia can cause hepatic and neurologic dysfunction due to vascular damage and/or hypertension. Obviously, preeclampsia adversely affects various organs, however it is not yet clear whether pre-eclampsia per se adversely affects various organs or whether it exposes underlying genetic predispositions to cardiovascular disease that manifest in later life. The current review summarizes recent development in the pathogenesis of preeclampsia with special focus on novel diagnostic biomarkers and their relevance to potential therapeutic options for this disease state. Specifically, the review highlights the renal manifestations of the disease with emphasis on the involvement of angiogenic factors in vascular injury and on how restoration of the angiogenic balance affects renal and cardiovascular outcome of Preeclamptic women.

Sex-Specific Contributions of Endothelin to Hypertension

PURPOSE OF REVIEW

Men and women differ in the prevalence, pathophysiology and control rate of hypertension in an age-dependent manner. The renal endothelin system plays a central role in sex differences in blood pressure regulation by control of sodium excretion and vascular function. Improving our understanding of the sex differences in the endothelin system, especially in regard to blood pressure regulation and sodium homeostasis, will fill a significant gap in our knowledge and may identify sex-specific therapeutic targets for management of hypertension.

RECENT FINDINGS

The current review will highlight evidence for the potential role for endothelin system in the pathophysiology of hypertension within three female populations: (i) postmenopausal women, (ii) women suffering from preeclampsia, or (iii) pulmonary arterial hypertension. Clinical trials that specifically address cardiovascular and renal diseases in females under different hormonal status are limited. Studies of the modulatory role of gonadal hormones and sex-specific mechanisms on critically important systems involved, such as endothelin, are needed to establish new clinical practice guidelines based on systematic evidence.

All‑trans retinoic acid reduces endothelin‑1 expression and increases endothelial nitric oxide synthase phosphorylation in rabbits with atherosclerosis

All-trans retinoic acid (ATRA) is a natural derivative of vitamin A that ameliorates atherosclerosis (AS) by regulating inflammatory factors. However, studies concerning the role of retinoic acid in artery endothelial function are rare. Therefore, the present study investigated its role in regulating the production of endothelin‑1 (ET‑1) and nitric oxide (NO) in rabbits with AS. The rabbits were randomly divided into 3 groups: The control group was administered an ordinary diet, while the high fat group and the ATRA drug intervention group were administered a high fat diet. After 12 weeks, the blood lipid levels of rabbits, the morphological structure of the arterial wall, the arterial intimal permeability, the activity of blood endothelial nitric oxide synthase (eNOS) and the level of plasma NO were investigated. Western blot analysis was used to detect the levels of ET‑1, eNOS and eNOS phosphorylation at Ser‑1177 (p‑eNOS), and a radioimmunoassay was performed to detect the level of ET‑1 in the plasma. It was identified that plaque formation was alleviated in the ATRA group compared with the high fat group, as revealed by hematoxylin and eosin and oil red O staining, and a similar trend was reflected in the immunofluorescence results for endothelial permeability. Western blotting demonstrated significantly decreased ET‑1 expression levels in the arterial tissue of rabbits in the ATRA group compared with the high fat group, together with increased p‑eNOS level (P<0.05), however, no difference was observed in the expression of eNOS (P>0.05). The trends observed for ET‑1 and the activity of eNOS in plasma were similar to those for arterial tissue. Therefore, the present study demonstrated that ATRA may regulate the grade of AS by the reduction of ET‑1 secretion and increased NO formation via increased phosphorylation of eNOS. ATRA provides a potential novel method for the treatment of atherosclerosis.

Effect of Magnesium on Glomerular Filtration Rate and Recovery of Hypertension in Women with Severe Preeclampsia

INTRODUCTION

Magnesium sulfate is used for preventing seizures in patients with severe preeclampsia. Previous studies have demonstrated that magnesium plays a significant role in the endothelial function and might have clinically beneficial vasodilating properties.

OBJECTIVES

This study is aimed at evaluating the effect of magnesium sulfate on the glomerular filtration rate (GFR) during the first 24 h after delivery and during the duration of recovery from hypertension in preeclampsia.

METHODS

Severe preeclamptic patients who had normal serum creatinine levels (0.4-0.8 mg/dL) were included in the study. Twenty-three women with severe preeclampsia were divided into groups of 9, 8, and 6, and given 1.0, 1.5, and 2.0 g/h of magnesium sulfate, respectively. Magnesium sulfate infusion was used as seizure prophylaxis for 24 h after delivery. The cystatin C-based GFR was monitored for 24 h, and the blood pressure was recorded for 12 weeks postpartum.

RESULTS

Despite the minimal improvement of GFR 24-h after treatment initiation, survival analysis demonstrated a statistically significant relationship (log rank, p = 0.04) between magnesium dosage and recovery period from hypertension. The group receiving 2.0 g/h of magnesium experienced the shortest recovery period from hypertension (6.5 ± 1.8 days). Meanwhile, the other groups required 66.0 ± 26.9 and 48.3 ± 15.6 days to recover after 1.0 and 1.5 g/h of magnesium infusion, respectively.

CONCLUSION

Magnesium sulfate has no impact on GFR improvement during the first 24 h after delivery. However, magnesium maintenance infusion at 2.0 g/h is capable of preventing seizure by optimizing the therapeutic magnesium level (4.8-8.4 mg/dL) and shortening the hypertensive episode in preeclampsia.

The Endothelin Type A Receptor as a Potential Therapeutic Target in Preeclampsia

Preeclampsia (PE) is a disorder of pregnancy typically characterized by new onset hypertension after gestational week 20 and proteinuria. Although PE is one of the leading causes of maternal and perinatal morbidity and death worldwide, the mechanisms of the pathogenesis of the disease remain unclear and treatment options are limited. However, there is increasing evidence to suggest that endothelin-1 (ET-1) plays a critical role in the pathophysiology of PE. Multiple studies report that ET-1 is increased in PE and some studies report a positive correlation between ET-1 and the severity of symptoms. A number of experimental models of PE are also associated with elevated tissue levels of prepro ET-1 mRNA. Moreover, experimental models of PE (placental ischemia, sFlt-1 infusion, Tumor necrosis factor (TNF) -α infusion, and Angiotensin II type 1 receptor autoantibody (AT1-AA) infusion) have proven to be susceptible to Endothelin Type A (ET_A) receptor antagonism. While the results are promising, further work is needed to determine whether ET antagonists could provide an effective therapy for the management of preeclampsia.

Hypertension in an Animal Model of HELLP Syndrome is Associated With Activation of Endothelin 1

Women with hypertensive forms of pregnancy such as hemolysis-elevated liver enzymes-low platelet syndrome have increased circulating endothelin 1; however, the relationship between hypertension and endothelin 1 has not been studied. Using an animal model, we sought to determine whether there was an increased activation/dysfunction of endothelin 1, the effect of endothelin 1 receptor-A blockade on hypertension and other manifestations of hemolysis, elevated liver enzymes, and low platelets syndrome. On gestational day 12, timed-pregnant rats were infused with soluble fms-like tyrosine kinase 1 (sFlt-1) and soluble endoglin (sEndoglin; 4.7 and 7 µg/kg) via mini-osmotic pumps for 8 days. A subset of rats were treated with receptor-A antagonist (ABT-627, 5mg/kg) for 8 days. Rats with hemolysis-elevated liver enzymes-low platelet syndrome had significantly increased hypertension (P = .0001), circulating endothelin 1 (P = .03), and a significant 3.3- and 7.2-fold increase in preproendothelin messenger RNA (mRNA) expression in the placenta and liver (P = .01 and .04). Urinary protein:creatinine ratio was significantly increased in these animals (P = .0007), and circulating factors from these rats stimulated a significant increase in endothelial cell secretion of endothelin 1 (P = .001) in an in vitro assay. Blockade of the endothelin 1 receptor A significantly decreased hypertension (P = .001), circulating endothelin 1, and interleukin 17 (P = .004 and .003), placental preproendothelin mRNA expression (P = .016), and urinary protein:creatinine ratio (P = .007) in rats with hemolysis-elevated liver enzymes-low platelet syndrome. Blockade of the endothelin 1 receptor A significantly decreased hemolysis (P = .009), liver enzymes (P = .011), and significantly increased platelet levels (P = .03) and decreased circulating CD4+ and CD8+ T lymphocytes (P = .0004 and .0001) in rats infused with sFlt-1 and sEndoglin. These data support the hypothesis that endothelin 1 activation has a critical role in pathophysiology of as hemolysis-elevated liver enzymes-low platelet syndrome.

Enodthelin 1 is elevated in plasma and explants from patients having uterine leiomyomas

OBJECTIVE

To determine a role for endothelin (ET) in progression of uterine fibroids.

DESIGN

An in vitro model of fibroid and myometrium cultivation.

PATIENTS

A total of 32 women undergoing hysterectomies for uterine fibroids and 11 women undergoing hysterectomies for abnormal uterine bleeding (control population).

RESULTS

Women with uterine fibroids were hypertensive and displayed significantly greater circulating ET-1 compared to control patients. Secretion of ET-1 was greater from the fibroids compared to myometrium explants. Endothelin 1 secretion was attenuated with blockade of the angiotensin II type 1 or endothelinA receptors. Hypoxia stimulated ET-1 secretion from both myometrium and fibroid explants. Preproendothelin messenger RNA expression increased with hypoxia from fibroid explants compared to normoxic controls.

CONCLUSIONS

These data support the hypothesis that uterine fibroids are associated with hypertension and increased ET-1, which is exacerbated with hypoxia. These data suggest a possible link between mechanisms of blood pressure regulation and development of uterine leiomyoma.

Alagebrium (ALT-711) improves the anti-hypertensive efficacy of nifedipine in diabetic-hypertensive rats

Combining drugs with complementary mechanisms of action may contribute to improved hypertension control in diabetic patients. Advanced glycation end-product (AGE) breakers, a new class of candidate drugs targeting aging-related cardiovascular dysfunction, may be useful as novel adjuvant agents to improve the efficacy of diabetic hypertension (DH) treatment. This study evaluated the effects of alagebrium (ALT-711), an AGE breaker, combined with nifedipine, a Ca(2+) channel blocker, in a rat model of streptozotocin-induced DH. Compared with monotherapy, combination treatment significantly decreased systolic and diastolic blood pressure values, increased the pulse pressure, and decreased the coefficient of variation of the systolic blood pressure. Plasma biochemistry indicated that the concentrations of prostacyclin and nitric oxide were increased. Gene expression analysis showed significantly decreased prepro-endothelin-1expression in the aorta. These results reveal that alagebrium significantly improves the anti-hypertensive actions of nifedipine in a rat model of DH and suggest its potential use in the successful control of clinical DH.

Hypertension: physiology and pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Modulators of the vascular endothelin receptor in blood pressure regulation and hypertension

Endothelin (ET) is one of the most investigated molecules in vascular biology. Since its discovery two decades ago, several ET isoforms, receptors, signaling pathways, agonists and antagonists have been identified. ET functions as a potent endothelium-derived vasoconstrictor, but could also play a role in vascular relaxation. In endothelial cells, preproET and big ET are cleaved by ET converting enzymes into ET-1, -2, -3 and -4. These ET isoforms bind with different affinities to ET(A) and ET(B) receptors in vascular smooth muscle (VSM), and in turn increase [Ca(2+)](i), protein kinase C and mitogen-activated protein kinase and other signaling pathways of VSM contraction and cell proliferation. ET also binds to endothelial ET(B) receptors and stimulates the release of nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. ET, via endothelial ET(B) receptor, could also promote ET re-uptake and clearance. While the effects of ET on vascular reactivity and growth have been thoroughly examined, its role in the regulation of blood pressure and the pathogenesis of hypertension is not clearly established. Elevated plasma and vascular tissue levels of ET have been identified in salt-sensitive hypertension and in moderate to severe hypertension, and ET receptor antagonists have been shown to reduce blood pressure to variable extents in these forms of hypertension. The development of new pharmacological and genetic tools could lead to more effective and specific modulators of the vascular ET system for treatment of hypertension and related cardiovascular disease.

Increased urinary excretion of nephrin, podocalyxin, and βig-h3 in women with preeclampsia

Emerging evidence has shown that podocyte injury and reduced specific podocyte protein expressions contribute to proteinuria in preeclampsia. We collected urine specimens from women with preeclampsia to study whether podocyte-specific protein shedding is associated with renal barrier dysfunction. Urine specimens from women with normal pregnancies and from pregnant women complicated by chronic hypertension were used for comparison. We determined soluble podocyte slit protein nephrin levels in the urine specimens. Podocalyxin, βig-h3, and VEGF concentrations were also measured. We found that nephrin and podocalyxin were barely detectable in the urine specimens from normal pregnant women and from women with chronic hypertension. In preeclampsia, urinary nephrin and podocalyxin concentrations were significantly increased and highly correlated to each other, r(2) = 0.595. Nephrin and podocalyxin were also correlated with urine protein concentrations. βig-h3 was detected in the urine specimens from women with preeclampsia, and it is highly correlated with nephrin and podocalyxin concentrations in preeclampsia. βig-h3 was undetectable in normal pregnancy and pregnancy complicated by chronic hypertension. Elevated VEGF levels were also found in women with preeclampsia compared with those of normal pregnancy and pregnancy complicated by chronic hypertension. These results provide strong evidence that podocyte protein shedding occurs in preeclampsia, and their levels are associated with proteinuria. The finding of urinary βig-h3 excretion in preeclampsia suggests that increased transforming growth factor activity might also be involved in the kidney lesion in this pregnancy disorder.

Regulation of sodium transport in the proximal tubule by endothelin

Human essential hypertension and rodent genetic hypertension are associated with increased sodium transport in the renal proximal tubule and medullary thick ascending limb of Henle. The proximal tubule, which secretes endothelin (ET), expresses the ET(B) receptor. Low (nM) concentrations of ET, via the ET(B) receptor, inhibit sodium and water transport and ATP-driven drug secretion in the proximal tubule. In contrast, very low (pM) and high nM concentrations of ET increase renal proximal sodium transport, but the receptor involved remains to be determined. The natriuretic effect of ET(B) receptor stimulation is impaired in spontaneously hypertensive rats, due in part to a defective interaction with D(3) dopamine and angiotensin II type 1 receptors. Impaired ET(B) receptor function in the renal proximal tubule may be important in the pathogenesis of genetic hypertension.

Autoantibody-mediated angiotensin receptor activation contributes to preeclampsia through tumor necrosis factor-alpha signaling

Preeclampsia is a prevalent life-threatening hypertensive disorder of pregnancy for which the pathophysiology remains largely undefined. Recently, a circulating maternal autoantibody, the angiotensin II type I (AT(1)) receptor agonistic autoantibody (AA), has emerged as a contributor to disease features. Increased circulating maternal tumor necrosis factor alpha (TNF-alpha) is also associated with the disease; however, it is unknown whether this factor directly contributes to preeclamptic symptoms. Here we report that this autoantibody increases the proinflammatory cytokine TNF-alpha in the circulation of AT(1)-AA-injected pregnant mice but not in nonpregnant mice. Coinjection of AT(1)-AA with a TNF-alpha neutralizing antibody reduced cytokine availability in AT(1)-AA-injected pregnant mice. Moreover, TNF-alpha blockade in AT(1)-AA-injected pregnant mice significantly attenuated the key features of preeclampsia. Autoantibody-induced hypertension was reduced from 131+/-4 to 110+/-4 mm Hg, and proteinuria was reduced from 212+/-25 to 155+/-23 microg of albumin per milligram of creatinine (both P<0.05). Injection of AT(1)-AA increased the serum levels of circulating soluble fms-like tyrosine kinase 1 and soluble endoglin (34.1+/-5.1, 2.4+/-0.3 ng/mL, respectively) and coinjection with the TNF-alpha blocker significantly reduced their levels (21.7+/-3.4 and 1.2+/-0.4 ng/mL, respectively). Renal damage and placental abnormalities were also decreased by TNF-alpha blockade. Lastly, the elevated circulating TNF-alpha in preeclamptic patients is significantly correlated with the AT(1)-AA bioactivity in our patient cohort. Similarly, the autoantibody, through AT(1) receptor-mediated TNF-alpha induction, contributed to increased soluble fms-like tyrosine kinase 1, soluble endoglin secretion, and increased apoptosis in cultured human villous explants. Overall, AT(1)-AA is a novel candidate that induces TNF-alpha, a cytokine that may play an important pathogenic role in preeclampsia.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.

Na(+)-ATPase in spontaneous hypertensive rats: possible AT(1) receptor target in the development of hypertension

Clinical and experimental data show an increase in sodium reabsorption on the proximal tubule (PT) in essential hypertension. It is well known that there is a link between essential hypertension and renal angiotensin II (Ang II). The present study was designed to examine ouabain-insensitive Na(+)-ATPase activity and its regulation by Ang II in spontaneously hypertensive rats (SHR). We observed that Na(+)-ATPase activity was enhanced in 14-week-old but not in 6-week-old SHR. The addition of Ang II from 10(-12) to 10(-6) mol/L decreased the enzyme activity in SHR to a level similar to that obtained in WKY. The Ang II inhibitory effect was completely reversed by a specific antagonist of AT(2) receptor, PD123319 (10(-8) mol/L) indicating that a system leading to activation of the enzyme in SHR is inhibited by AT(2)-mediated Ang II. Treatment of SHR with losartan for 10 weeks (weeks 4-14) prevents the increase in Na(+)-ATPase activity observed in 14-week-old SHR. These results indicate a correlation between AT(1) receptor activation in SHR and increased ouabain-insensitive Na(+)-ATPase activity. Our results open new possibilities towards our understanding of the pathophysiological mechanisms involved in the increased sodium reabsorption in PT found in essential hypertension.

Deficiency in Six2 during prenatal development is associated with reduced nephron number, chronic renal failure, and hypertension in Br/+ adult mice

The Br/+ mutant mouse displays decreased embryological expression of the homeobox transcription factor Six2, resulting in hertitable renal hypoplasia. The purpose of this study was to characterize the renal physiological consequences of embryonic haploinsuffiency of Six2 by analyzing renal morphology and function in the adult Br heterozygous mutant. Adult Br/+ kidneys weighed 50% less than those from wild-type mice and displayed glomerulopathy. Stereological analysis of renal glomeruli showed that Br/+ kidneys had an average of 88% fewer glomeruli than +/+ kidneys, whereas individual glomeruli in Br/+ mice maintained an average volume increase of 180% compared with normal nephrons. Immunostaining revealed increased levels of endothelin-1 (ET-1), endothelin receptors A (ET(A)) and B (ET(B)), and Na-K-ATPase were present in the dilated renal tubules of mutant mice. Physiological features of chronic renal failure (CRF) including elevated mean arterial pressure, increased plasma creatinine, and dilute urine excretion were measured in Br/+ mutant mice. Electron microscopy of the Br/+ glomeruli revealed pathological alterations such as hypercellularity, extracellular matrix accumulation, and a thick irregular glomerular basement membrane. These results indicate that adult Br/+ mice suffer from CRF associated with reduced nephron number and renal hypoplasia, as well as glomerulopathy. Defects are associated with embryological deficiencies of Six2, suggesting that proper levels of this protein during nephrogenesis are critical for normal glomerular development and adult renal function.

Endothelin-1 inhibits thick ascending limb transport via Akt-stimulated nitric oxide production

Endothelin-1 inhibits sodium reabsorption in the thick ascending limb (THAL) via stimulation of nitric oxide (NO) production. The mechanism whereby endothelin-1 stimulates THAL NO is unknown. We hypothesized that endothelin-1 stimulates THAL NO production by activating phosphatidylinositol 3-kinase (PI3K), stimulating Akt activity, and phosphorylating NOS3 at Ser1177. This enhances NO production and inhibits sodium transport. We measured 1) NO production by fluorescence microscopy using DAF2-DA, 2) Akt activity using a fluorescence resonance energy transfer-based Akt reporter, 3) phosphorylated NOS3 and Akt by Western blotting, and 4) NKCC2 activity by fluorescence microscopy. In isolated THAL, endothelin-1 (1 nmol/liter) increased NO production from 0.23 +/- 0.24 to 2.81 +/- 0.32 fluorescence units/min (p < 0.001; n = 5) but failed to stimulate NO production in THALs isolated from NOS3-/- mice. Wortmannin (150 nmol/liter), a PI3K inhibitor, reduced endothelin-1-stimulated NO by 83% (0.49 +/- 0.13 versus 3.31 +/- 0.49 fluorescence units/min for endothelin-1 alone; p < 0.006; n = 5). Endothelin-1 stimulated Akt activity by 0.16 +/- 0.02 arbitrary units as measured by fluorescence resonance energy transfer (p < 0.001; n = 5) and increased phosphorylation of Akt at Ser473 by 56 +/- 11% (p < 0.002; n = 7). Dominant-negative Akt blocked endothelin-1-induced NO by 60 +/- 8% (p < 0.001 versus control; n = 6), and an Akt inhibitor had a similar effect. Endothelin-1 increased phosphorylation of NOS3 at Ser1177 by 89 +/- 24% (p < 0.01; n = 7) but had no effect on Ser633. Endothelin-1 inhibited NKCC2 activity, an effect that was blocked by dominant-negative Akt and NOS inhibition. We conclude that endothelin-1 stimulates THAL NO production by activating PI3K, stimulating Akt activity, and phosphorylating NOS3 at Ser1177. This enhances NO production and inhibits sodium transport.

Endothelin, sex and hypertension

The ETs (endothelins) comprise a family of three 21-amino-acid peptides (ET-1, ET-2 and ET-3) and 31-amino-acid ETs (ET-1(1-31), ET-2(1-31) and ET-3(1-31)). ET-1 is synthesized from a biologically inactive precursor, big ET-1, by ECEs (ET-converting enzymes). The actions of ET-1 are mediated through activation of the G-protein-coupled ET(A) and ET(B) receptors, which are found in a variety of cells in the cardiovascular and renal systems. ET-1 has potent vasoconstrictor, mitogenic, pro-inflammatory and antinatriuretic properties, which have been implicated in the pathophysiology of a number of cardiovascular diseases. Overexpression of ET-1 has been consistently described in salt-sensitive models of hypertension and in models of renal failure, and has been associated with disease progression. Sex differences are observed in many aspects of mammalian cardiovascular function and pathology. Hypertension, as well as other cardiovascular diseases, is more common in men than in women of similar age. In experimental models of hypertension, males develop an earlier and more severe form of hypertension than do females. Although the reasons for these differences are not well established, the effects of gonadal hormones on arterial, neural and renal mechanisms that control blood pressure are considered contributing factors. Sex differences in the ET-1 pathway, with males displaying higher ET-1 levels, greater ET-1-mediated vasoconstrictor and enhanced pressor responses in comparison with females, are addressed in the present review. Sex-associated differences in the number and function of ET(B) receptors appear to be particularly important in the specific characteristics of hypertension between females and males. Although the gonadal hormones modulate some of the differences in the ET pathway in the cardiovascular system, a better understanding of the exact mechanisms involved in sex-related differences in this peptidergic system is needed. With further insights into these differences, we may learn that men and women could require different antihypertensive regimens.

Chronic Activation of Endothelin B Receptors

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

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.