Renal medullary nitric oxide deficit of Dahl S rats enhances hypertensive actions of angiotensin II

Renal Medulla in Hypertension

Studies have found that blood flow to the renal medulla is an important determinant of pressure-natriuresis and the long-term regulation of arterial pressure. First, a brief review of methods developed enabling the study of the medullary circulation is presented. Second, studies performed in rats are presented showing medullary blood flow plays a vital role in the pressure-natriuresis relationship and thereby in hypertension. Third, it is shown that chronic reduction of medullary blood flow results in hypertension and that enhancement of medullary blood flow reduces hypertension hereditary models of both salt-sensitive rats and salt-resistant forms of hypertension. The key role that medullary nitric oxide production plays in protecting this region from ischemic injury associated with circulating vasoconstrictor agents and reactive oxygen species is presented. The studies cited are largely the work of my students, research fellows, and colleagues with whom I have performed these studies dating from the late 1980s to more recent years.

Renal Epithelial Mitochondria: Implications for Hypertensive Kidney Disease

According to the Centers for Disease Control and Prevention, 1 in 2 U.S. adults have hypertension, and more than 1 in 7 chronic kidney disease. In fact, hypertension is the second leading cause of kidney failure in the United States; it is a complex disease characterized by, leading to, and caused by renal dysfunction. It is well-established that hypertensive renal damage is accompanied by mitochondrial damage and oxidative stress, which are differentially regulated and manifested along the nephron due to the diverse structure and functions of renal cells. This article provides a summary of the relevant knowledge of mitochondrial bioenergetics and metabolism, focuses on renal mitochondrial function, and discusses the evidence that has been accumulated regarding the role of epithelial mitochondrial bioenergetics in the development of renal tissue dysfunction in hypertension. © 2024 American Physiological Society. Compr Physiol 14:5225-5242, 2024.

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

Effects of Exercise Training on the Renin-Angiotensin System in the Kidneys of Dahl Salt-Sensitive Rats

PURPOSE

Exercise training (Ex) has antihypertensive and renal protective effects; however, the precise mechanisms remain unclear. The renal renin-angiotensin system (RAS) plays a vital role in renal function and pathology. Therefore, we investigated the effects of Ex on the renal RAS components in Dahl salt-sensitive (Dahl-S) rats.

METHODS

Male Dahl-S rats were divided into four groups: normal salt diet + sedentary, normal salt diet + Ex, high-salt diet (HS, 8% NaCl) + sedentary, and HS + Ex. Treadmill running was performed for 8 wk in the Ex groups.

RESULTS

Ex attenuated the HS-induced renal dysfunction and glomerular injury without causing blood pressure alterations. HS increased urinary excretion of both total and intact angiotensinogen. Ex decreased the HS-induced increased urinary excretion of total angiotensinogen. However, it did not change the HS-induced urinary excretion of intact angiotensinogen, indicating reduced intact angiotensinogen cleaving. Ex restored the HS-induced increased angiotensinogen and angiotensin II type 1 receptor expressions in the outer medulla and the HS-induced increased angiotensin-converting enzyme expression in the cortex. Ex restored the HS-induced decreased renin expression in the cortex and outer medulla, and the HS-induced decreased angiotensin-converting enzyme 2, angiotensin II type 2 receptor, and Mas receptor expressions in the outer medulla.

CONCLUSIONS

Ex attenuates HS-induced renal dysfunction, glomerular injury, and renal RAS dysregulation in Dahl-S rats.

Overexpression of MicroRNA-429 Transgene Into the Renal Medulla Attenuated Salt-Sensitive Hypertension in Dahl S Rats

BACKGROUND

We have previously shown that high salt stimulates the expression of miR-429 in the renal medulla, which induces mRNA decay of HIF prolyl-hydroxylase 2 (PHD2), an enzyme to promote the degradation of hypoxia-inducible factor (HIF)-1α, and increases the HIF-1α-mediated activation of antihypertensive genes in the renal medulla, consequently promoting extra sodium excretion. Our preliminary results showed that high salt-induced increase of miR-429 was not observed in Dahl S rats. This present study determined whether correction of this impairment in miR-429 would reduce PHD2 levels, increase antihypertensive gene expression in the renal medulla and attenuate salt-sensitive hypertension in Dahl S rats.

METHODS

Lentiviruses encoding rat miR-429 were transfected into the renal medulla in uninephrectomized Dahl S rats. Sodium excretion and blood pressure were then measured.

RESULTS

Transduction of lentiviruses expressing miR-429 into the renal medulla increased miR-429 levels, decreased PHD2 levels, and upregulated HIF-1α target gene NOS-2, which restored the adaptive mechanism to increase the antihypertensive gene after high-salt intake in Dahl S rats. Functionally, overexpression of miR-429 transgene in the renal medulla significantly improved pressure natriuretic response, enhanced urinary sodium excretion, and reduced sodium retention upon extra sodium loading, and consequently, attenuated the salt-sensitive hypertension in Dahl S rats.

CONCLUSIONS

Our results suggest that the impaired miR-429-mediated PHD2 inhibition in response to high salt in the renal medulla may represent a novel mechanism for salt-sensitive hypertension in Dahl S rats and that correction of this impairment in miR-429 pathway could be a therapeutic approach for salt-sensitive hypertension.

Mechanisms of decreased tubular flow-induced nitric oxide in Dahl salt-sensitive rat thick ascending limbs

Dahl salt-sensitive (SS) rat kidneys produce less nitric oxide (NO) than those of salt-resistant (SR) rats. Thick ascending limb (TAL) NO synthase 3 (NOS3) is a major source of renal NO, and luminal flow enhances its activity. We hypothesized that flow-induced NO is reduced in TALs from SS rats primarily due to NOS uncoupling and diminished NOS3 expression rather than scavenging. Rats were fed normal-salt (NS) or high-salt (HS) diets. We measured flow-induced NO and superoxide in perfused TALs and performed Western blots of renal outer medullas. For rats on NS, flow-induced NO was 35 ± 6 arbitrary units (AU)/min in TALs from SR rats but only 11 ± 2 AU/min in TALs from SS (P < 0.008). The superoxide scavenger tempol decreased the difference in flow-induced NO between strains by about 36% (P < 0.020). The NOS inhibitor N-nitro-l-arginine methyl ester (l-NAME) decreased flow-induced superoxide by 36 ± 8% in TALs from SS rats (P < 0.02) but had no effect in TALs from SR rats. NOS3 expression was not different between strains on NS. For rats on HS, the difference in flow-induced NO between strains was enhanced (SR rats: 44 ± 10 vs. SS: 9 ± 2 AU/min, P < 0.005). Tempol decreased the difference in flow-induced NO between strains by about 37% (P < 0.012). l-NAME did not significantly reduce flow-induced superoxide in either strain. HS increased NOS3 expression in TALs from SR rats but not in TALs from SS rats (P < 0.003). We conclude that 1) on NS, flow-induced NO is diminished in TALs from SS rats mainly due to NOS3 uncoupling such that it produces superoxide and 2) on HS, the difference is enhanced due to failure of TALs from SS rats to increase NOS3 expression.NEW & NOTEWORTHY The Dahl rat has been used extensively to study the causes and effects of salt-sensitive hypertension. Our study suggests that more complex processes other than simple scavenging of nitric oxide (NO) by superoxide lead to less NO production in thick ascending limbs of the Dahl salt-sensitive rat. The predominant mechanism involved depends on dietary salt. Impaired flow-induced NO production in thick ascending limbs most likely contributes to the Na+ retention associated with salt-sensitive hypertension.

L-phenylalanine attenuates high salt-induced hypertension in Dahl SS rats through activation of GCH1-BH4

Amino acid metabolism plays an important role in controlling blood pressure by regulating the production of NO and ROS. The present study examined amino acid levels in the serum of Dahl SS rats and SS.13BN rats fed a low or high salt diet. We observed that 8 of 27 amino acids responded to a high salt diet in SS rats. Thus, we hypothesized that a defect in amino acids may contribute to the development of salt-induced hypertension. L-phenylalanine was used to treat SS rats with a low or high salt diet. The results demonstrated that L-phenylalanine supplementation significantly enhanced the serum nitrite levels and attenuated the high salt-induced hypertension in SS rats. Low levels of BH4 and nitrite and the impaired vascular response to acetylcholine were rescued by L-phenylalanine supplementation. Moreover, increased GTP cyclohydrolase (GCH1) mRNA, levels of BH4 and nitrite, and reduced superoxide production were observed in the kidneys of hypertensive SS rats with L-phenylalanine. The antihypertensive effects of L-phenylalanine might be mediated by enhancing BH4 biosynthesis and decreasing superoxide production from NO synthase, thereby protecting vascular and kidney function with reduced ROS and elevated NO levels. The present study demonstrated that L-phenylalanine supplementation restored vascular function, suggesting L-phenylalanine represented a potential target to attenuate high salt-sensitive hypertension through GCH1-BH4.

Renal metabolism and hypertension

Hypertension is a leading risk factor for disease burden worldwide. The kidneys, which have a high specific metabolic rate, play an essential role in the long-term regulation of arterial blood pressure. In this review, we discuss the emerging role of renal metabolism in the development of hypertension. Renal energy and substrate metabolism is characterized by several important and, in some cases, unique features. Recent advances suggest that alterations of renal metabolism may result from genetic abnormalities or serve initially as a physiological response to environmental stressors to support tubular transport, which may ultimately affect regulatory pathways and lead to unfavorable cellular and pathophysiological consequences that contribute to the development of hypertension.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

High salt intake shifts the mechanisms of flow-induced dilation in the middle cerebral arteries of Sprague-Dawley rats

The goal of the present study was to examine the effect of 1 wk of high salt (HS) intake and the role of oxidative stress in changing the mechanisms of flow-induced dilation (FID) in isolated pressurized middle cerebral arteries of male Sprague-Dawley rats ( n = 15–16 rats/group). Reduced FID in the HS group was restored by intake of the superoxide scavenger tempol (HS + tempol in vivo group). The nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine methyl ester, cyclooxygenase inhibitor indomethacin, and selective inhibitor of microsomal cytochrome P-450 epoxidase activity N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide significantly reduced FID in the low salt diet-fed group, whereas FID in the HS group was mediated by NO only. Cyclooxygenase-2 mRNA (but not protein) expression was decreased in the HS and HS + tempol in vivo groups. Hypoxia-inducible factor-1α and VEGF protein levels were increased in the HS group but decreased in the HS + tempol in vivo group. Assessment by direct fluorescence of middle cerebral arteries under flow revealed significantly reduced vascular NO levels and increased superoxide/reactive oxygen species levels in the HS group. These results suggest that HS intake impairs FID and changes FID mechanisms to entirely NO dependent, in contrast to the low-salt diet-fed group, where FID is NO, prostanoid, and epoxyeicosatrienoic acid dependent. These changes were accompanied by increased lipid peroxidation products in the plasma of HS diet-fed rats, increased vascular superoxide/reactive oxygen species levels, and decreased NO levels, together with increased expression of hypoxia-inducible factor-1α and VEGF.

NEW & NOTEWORTHY High-salt (HS) diet changes the mechanisms of flow-induced dilation in rat middle cerebral arteries from a combination of nitric oxide-, prostanoid-, and epoxyeicosatrienoic acid-dependent mechanisms to, albeit reduced, a solely nitric oxide-dependent dilation. In vivo reactive oxygen species scavenging restores flow-induced dilation in HS diet-fed rats and ameliorates HS-induced increases in the transcription factor hypoxia-inducible factor-1α and expression of its downstream target genes.

Role of immune factors in angiotensin II-induced hypertension and renal damage in Dahl salt-sensitive rats

The present study assessed the importance of immunity in angiotensin (ANG) II (5 ng·kg^-1·min^-1 iv)-mediated hypertension in Dahl salt-sensitive (SS) rats and SS rats deficient in T and B lymphocytes (SS^Rag1-/-) fed a 0.4% NaCl diet. Baseline mean arterial blood pressure (MAP) was not different between groups. ANG II infusion significantly increased MAP in both groups, although MAP increased more rapidly in SS rats, and the maximal MAP achieved was significantly greater in SS than SS^Rag1-/- rats (190 ± 3 vs. 177 ± 3 mmHg) after 12 days. Renal damage, as assessed by albumin excretion rate, was significantly increased after 12 days of ANG lI infusion in SS (from 32 ± 4 to 81 ± 9 mg/day) and SS^Rag1-/- (from 12 ± 2 to 51 ± 8 mg/day) rats; albumin excretion rate was significantly different between SS and SS^Rag1-/- rats at all points measured. After 9 days of recovery from ANG II, MAP was decreased to a greater extent in SS^Rag1-/- than SS rats (143 ± 5 vs. 157 ± 8 mmHg) compared with the peak MAP during ANG II infusion. At this same time point, albumin excretion rate was significantly lower in SS^Rag1-/- than SS rats (42 ± 8 vs. 66 ± 7 mg/day). Further studies demonstrated an increase in CD45⁺ total leukocytes, CD11b/c⁺ macrophages/monocytes, and CD3⁺ T cells in kidneys of ANG II- compared with vehicle-treated SS rats. The present data suggest that infiltrating T cells in the kidney exacerbate renal damage in ANG II-induced hypertension in SS rats maintained on a 0.4% NaCl diet, similar to results observed with a salt stimulus in SS rats.

Diuretic usage for protection against end-organ damage in liver cirrhosis and heart failure

Volume overload is common in liver cirrhosis, heart failure, and chronic kidney disease, being an independent risk factor for mortality. Loop diuretics have been widely used for treating volume overload in these patients. However, there is a tendency to increase the dose of loop diuretics partly because of diuresis resistance. Neurohormonal factors are also enhanced in these patients, which play a role in volume overload and organ ischemia. Loop diuretics cannot improve neurohormonal factors and could result in end-organ damage. The water diuretic tolvaptan has been approved for use for volume overload in heart failure and liver cirrhosis. Despite causing similar increases in urine volume, its characteristics differ from those of loop diuretics. Renal blood flow is maintained with tolvaptan but decreased with furosemide in heart failure patients. Neurohormonal factors and blood pressure are not markedly altered by tolvaptan administration. It is expected that these mechanisms of tolvaptan can protect against worsening renal function by volume overload diseases compared with loop diuretics. It has also been reported that some patients do not respond well to tolvaptan. Loop diuretics and tolvaptan share the same mechanism with regard to decreasing renal interstitial osmolality, which plays a fundamental role in water diuresis. Thus, a high dose of loop diuretics could result in resistance to tolvaptan, so tolvaptan should be administered before increasing the loop diuretic dose. Therefore, volume control without enhancing end-organ damage can be achieved by adding tolvaptan to a tolerable dose of Na-sparing diuretics.

Nitric oxide, prostaglandins and angiotensin II in the regulation of renal medullary blood flow during volume expansion

Regulation of medullary blood flow (MBF) is essential in maintaining renal function and blood pressure. However, it is unknown whether outer MBF (OMBF) and papillary blood flow (PBF) are regulated independently when extracellular volume (ECV) is enhanced. The aim of this study was to determine whether OMBF and PBF are differently regulated and whether there is an interaction between nitric oxide (NO), prostaglandins (PGs) and angiotensin II (Ang II) in regulating OMBF and PBF when ECV is enhanced. To achieve these goals, OMBF and PBF were measured by laser-Doppler in volume-expanded rats treated with a cyclooxygenase inhibitor (meclofenamate, 3 mg/kg) and/or a NO synthesis inhibitor (L-nitro-arginine methyl ester (L-NAME), 3 μg/kg/min) and/or Ang II (10 ng/kg/min). OMBF was unchanged by NO or PGs synthesis inhibition but decreased by 36 % (P < 0.05) when L-NAME and meclofenamate were infused simultaneously. PBF was similarly reduced by L-NAME (12 %), meclofenamate (17 %) or L-NAME + meclofenamate (19 %). Ang II did not modify OMBF, but it led to a similar decrease (P < 0.05) in OMBF when it was administered to rats with reduced NO (32 %), PGs (36 %) or NO and PGs (37 %) synthesis. In contrast, the fall in PBF induced by Ang II (12 %) was enhanced (P < 0.05) by the simultaneous PGs (30 %) or PGs and NO (31 %) synthesis inhibition but not in L-NAME-treated rats (20 %). This study presents novel findings suggesting that blood flows to the outer medulla and renal papilla are differently regulated and showing that there is a complex interaction between NO, PGs and Ang II in regulating OMBF and PBF when ECV is enhanced.

The Dahl salt-sensitive rat is a spontaneous model of superimposed preeclampsia

The mechanisms of the pathogenesis of preeclampsia, a leading cause of maternal morbidity and death worldwide, are poorly understood in part due to a lack of spontaneous animal models of the disease. We hypothesized that the Dahl salt-sensitive (S) rat, a genetic model of hypertension and kidney disease, is a spontaneous model of superimposed preeclampsia. The Dahl S was compared with the Sprague-Dawley (SD) rat, a strain with a well-characterized normal pregnancy, and the spontaneously hypertensive rat (SHR), a genetic model of hypertension that does not experience a preeclamptic phenotype despite preexisting hypertension. Mean arterial pressure (MAP, measured via telemetry) was elevated in the Dahl S and SHR before pregnancy, but hypertension was exacerbated during pregnancy only in Dahl S. In contrast, SD and SHR exhibited significant reductions in MAP consistent with normal pregnancy. Dahl S rats exhibited a severe increase in urinary protein excretion, glomerulomegaly, increased placental hypoxia, increased plasma soluble fms-like tyrosine kinase-1 (sFlt-1), and increased placental production of tumor necrosis factor-α (TNF-α). The Dahl S did not exhibit the expected decrease in uterine artery resistance during late pregnancy in contrast to the SD and SHR. Dahl S pups and litter sizes were smaller than in the SD. The Dahl S phenotype is consistent with many of the characteristics observed in human superimposed preeclampsia, and we propose that the Dahl S should be considered further as a spontaneous model to improve our understanding of the pathogenesis of superimposed preeclampsia and to identify and test new therapeutic targets for its treatment.

Angiotensin II-induced hypertension blunts thick ascending limb NO production by reducing NO synthase 3 expression and enhancing threonine 495 phosphorylation

Thick ascending limbs reabsorb 30% of the filtered NaCl load. Nitric oxide (NO) produced by NO synthase 3 (NOS3) inhibits NaCl transport by this segment. In contrast, chronic angiotensin II (ANG II) infusion increases net thick ascending limb transport. NOS3 activity is regulated by changes in expression and phosphorylation at threonine 495 (T495) and serine 1177 (S1177), inhibitory and stimulatory sites, respectively. We hypothesized that NO production by thick ascending limbs is impaired by chronic ANG II infusion, due to reduced NOS3 expression, increased phosphorylation of T495, and decreased phosphorylation of S1177. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 1 and 5 days. ANG II infusion for 5 days decreased NOS3 expression by 40 ± 12% (P < 0.007; n = 6) and increased T495 phosphorylation by 147 ± 26% (P < 0.008; n = 6). One-day ANG II infusion had no significant effect. NO production in response to endothelin-1 was blunted in thick ascending limbs from ANG II-infused animals [ANG II -0.01 ± 0.06 arbitrary fluorescence units (AFU)/min vs. 0.17 ± 0.02 AFU/min in controls; P < 0.01]. This was not due to reduced endothelin-1 receptor expression. Phosphatidylinositol 3,4,5-triphosphate (PIP3)-induced NO production was also reduced in ANG II-infused rats (ANG II -0.07 ± 0.06 vs. 0.13 ± 0.04 AFU/min in controls; P < 0.03), and this correlated with an impaired ability of PIP3 to increase S1177 phosphorylation. We conclude that in ANG II-induced hypertension NO production by thick ascending limbs is impaired due to decreased NOS3 expression and altered phosphorylation.

Reactive oxygen species as important determinants of medullary flow, sodium excretion, and hypertension

The physiological evidence linking the production of superoxide, hydrogen peroxide, and nitric oxide in the renal medullary thick ascending limb of Henle (mTAL) to regulation of medullary blood flow, sodium homeostasis, and long-term control of blood pressure is summarized in this review. Data obtained largely from rats indicate that experimentally induced elevations of either superoxide or hydrogen peroxide in the renal medulla result in reduction of medullary blood flow, enhanced Na(+) reabsorption, and hypertension. A shift in the redox balance between nitric oxide and reactive oxygen species (ROS) is found to occur naturally in the Dahl salt-sensitive (SS) rat model, where selective reduction of ROS production in the renal medulla reduces salt-induced hypertension. Excess medullary production of ROS in SS rats emanates from the medullary thick ascending limbs of Henle [from both the mitochondria and membrane NAD(P)H oxidases] in response to increased delivery and reabsorption of excess sodium and water. There is evidence that ROS and perhaps other mediators such as ATP diffuse from the mTAL to surrounding vasa recta capillaries, resulting in medullary ischemia, which thereby contributes to hypertension.

Serelaxin reduces oxidative stress and asymmetric dimethylarginine in angiotensin II-induced hypertension

Recent findings suggest the therapeutic action of relaxin during hypertension is dependent on nitric oxide synthase (NOS) activation; however, the mechanisms underlying the beneficial effects of relaxin on the NOS system have not been fully elucidated. We hypothesized that the protective effects of relaxin include reducing both oxidative stress and the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA). We examined the effect of Serelaxin [human recombinant relaxin-2 (RLX)] in male Sprague-Dawley rats given high-dose angiotensin (ANG) II (400 ng·kg(-1)·min(-1) sc) for 6 wk or shams. RLX was administered (4 μg/h sc) to half of the rats in each group after 2 wk of ANG II for the remaining 4 wk. ANG II induced hypertension and proteinuria, reduced NO oxidation products (NOx), and increased oxidative stress (NADPH oxidase activity, thiobarbituric acid-reactive substances, and 8-isoprostane excretion) and plasma ADMA. While RLX had no effect on sham rats, RLX attenuated the ANG II-dependent hypertension (165 ± 5 vs. 135 ± 13 mmHg, P < 0.05) and proteinuria at 6 wk (62 ± 6 vs. 41 ± 4 mg·day(-1)·100 g(-1), P < 0.05) and normalized oxidative stress and circulating ADMA, in association with restored NOx excretion and kidney cortex NOx. We found that RLX had no impact on the ADMA-regulatory enzymes protein arginine methyltransferase and dimethylarginine-dimethylaminohydrolase (DDAH). Furthermore, RLX treatment did not increase DDAH activity in kidney cortex or liver. These data suggest that benefits of RLX treatment include reduced ADMA levels and increased NO bioavailability, possibly due to its antioxidant effects.

Silencing of HIF prolyl-hydroxylase 2 gene in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats

BACKGROUND

In response to high salt intake, transcription factor hypoxia-inducible factor (HIF) 1α activates many antihypertensive genes, such as heme oxygenase 1 (HO-1) 1 and cyclooxygenase 2 (COX-2) in the renal medulla, which is an important molecular adaptation to promote extra sodium excretion. We recently showed that high salt inhibited the expression of HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, thereby upregulating HIF-1α, and that high salt-induced inhibition in PHD2 and subsequent activation of HIF-1α in the renal medulla was blunted in Dahl salt-sensitive hypertensive rats. This study tested the hypothesis that silencing the PHD2 gene to increase HIF-1α levels in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats.

METHODS

PHD2 short hairpin RNA (shRNA) plasmids were transfected into the renal medulla in uninephrectomized Dahl S rats. Renal function and blood pressure were then measured.

RESULTS

PHD2 shRNA reduced PHD2 levels by >60% and significantly increased HIF-1α protein levels and the expression of HIF-1α target genes HO-1 and COX-2 by >3-fold in the renal medulla. Functionally, pressure natriuresis was remarkably enhanced, urinary sodium excretion was doubled after acute intravenous sodium loading, and chronic high salt-induced sodium retention was remarkably decreased, and as a result, salt-sensitive hypertension was significantly attenuated in PHD2 shRNA rats compared with control rats.

CONCLUSIONS

Impaired PHD2 response to high salt intake in the renal medulla may represent a novel mechanism for hypertension in Dahl S rats, and inhibition of PHD2 in the renal medulla could be a therapeutic approach for salt-sensitive hypertension.

Oxidative stress in hypertension: role of the kidney

SIGNIFICANCE

Renal oxidative stress can be a cause, a consequence, or more often a potentiating factor for hypertension. Increased reactive oxygen species (ROS) in the kidney have been reported in multiple models of hypertension and related to renal vasoconstriction and alterations of renal function. Nicotinamide adenine dinucleotide phosphate oxidase is the central source of ROS in the hypertensive kidney, but a defective antioxidant system also can contribute.

RECENT ADVANCES

Superoxide has been identified as the principal ROS implicated for vascular and tubular dysfunction, but hydrogen peroxide (H2O2) has been implicated in diminishing preglomerular vascular reactivity, and promoting medullary blood flow and pressure natriuresis in hypertensive animals.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Increased renal ROS have been implicated in renal vasoconstriction, renin release, activation of renal afferent nerves, augmented contraction, and myogenic responses of afferent arterioles, enhanced tubuloglomerular feedback, dysfunction of glomerular cells, and proteinuria. Inhibition of ROS with antioxidants, superoxide dismutase mimetics, or blockers of the renin-angiotensin-aldosterone system or genetic deletion of one of the components of the signaling cascade often attenuates or delays the onset of hypertension and preserves the renal structure and function. Novel approaches are required to dampen the renal oxidative stress pathways to reduced O2(-•) rather than H2O2 selectivity and/or to enhance the endogenous antioxidant pathways to susceptible subjects to prevent the development and renal-damaging effects of hypertension.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Overexpression of HIF prolyl-hydoxylase-2 transgene in the renal medulla induced a salt sensitive hypertension

Renal medullary hypoxia-inducible factor (HIF)-1α and its target genes, such as haem oxygenase and nitric oxide synthase, have been indicated to play an important role in the regulation of sodium excretion and blood pressure. HIF prolyl hydroxylase domain-containing proteins (PHDs) are major enzymes to promote the degradation of HIF-1α. We recently reported that high salt intake suppressed the renal medullary PHD2 expression and thereby activated HIF-1α-mediated gene regulation in the renal medulla in response to high salt. To further define the functional role of renal medullary PHD2 in the regulation of renal adaptation to high salt intake and the longer term control of blood pressure, we transfected PHD2 expression plasmids into the renal medulla in uninephrectomized rats and determined its effects on pressure natriuresis, sodium excretion after salt overloading and the long-term control of arterial pressure after high salt challenge. It was shown that overexpression of PHD2 transgene increased PHD2 levels and decreased HIF-1α levels in the renal medulla, which blunted pressure natriuresis, attenuated sodium excretion, promoted sodium retention and produced salt sensitive hypertension after high salt challenge compared with rats treated with control plasmids. There was no blood pressure change in PHD2-treated rats that were maintained in low salt diet. These results suggested that renal medullary PHD2 is an important regulator in renal adaptation to high salt intake and a deficiency in PHD2-mediated molecular adaptation in response to high salt intake in the renal medulla may represent a pathogenic mechanism producing salt sensitive hypertension.

Medullary thick ascending limb buffer vasoconstriction of renal outer-medullary vasa recta in salt-resistant but not salt-sensitive rats

We have demonstrated previously that paracrine signaling occurs between medullary thick ascending limb (mTAL) and the contractile pericytes of outer-medullary vasa recta (VR), termed "tubular-vascular cross-talk." The aim of the current study was to determine whether tubular-vascular cross-talk has a functional effect on vasoconstrictor responses to angiotensin II and to determine whether this is altered in the Dahl salt-sensitive (SS) rat. Studies were performed on salt-resistant consomic SS.13 Brown Norway (BN) and SS rats using a novel outer medullary tissue strip preparation in which freshly isolated VRs within VR bundles were perfused either alone or in combination with nearby mTAL. In VRs from SS.13BN rats, angiotensin II (1 µmol/L) increased VR bundle intracellular Ca2+ concentration 19±9 nmol/L (n=8) and reduced focal diameter in perfused VRs by -20±7% (n=5). In the presence of nearby mTAL, however, VR bundle intracellular Ca2+ concentration (-9±8 nmol/L; n=8) and VR diameter (-1±4%, n=7) in SS.13BN rats were unchanged by angiotensin II. In contrast, in Dahl SS rats, angiotensin II resulted in rapid and sustained increase in VR bundle intracellular Ca2+ concentration (89±48 nmol/L, n=7; 50±24%, n=8) and a reduction in VR diameter of (-17±7%, n=7; -11±4%, n=5) in both isolated VRs and VRs with nearby mTAL, respectively. In VRs with mTAL from SS13BN rats, inhibiton of purinergic receptors resulted in an increase in VR bundle intracellular Ca2+ concentration, indicating that purinergic signaling buffers vasoconstriction. Importantly, our in vitro data were able to predict medullary blood flow responses to angiotensin II in SS and SS.13BN rats in vivo. We conclude that paracrine signaling from mTAL buffers angiotensin II vasoconstriction in Dahl salt-resistant SS.13BN rats but not SS rats.

Role of renal medullary oxidative and/or carbonyl stress in salt-sensitive hypertension and diabetes

1. Salt-sensitive hypertension is commonly associated with diabetes, obesity and chronic kidney disease. The present review focuses on renal mechanisms involved in the development of this type of hypertension. 2. The renal medullary circulation plays an important role in the development of salt-sensitive hypertension. In vivo animal studies have demonstrated that the balance between nitric oxide (NO) and reactive oxygen species (ROS) in the renal medulla is an important element of salt-sensitive hypertension. The medullary thick ascending limb (mTAL) in the outer medulla is an important source of NO and ROS production and we have explored the mechanisms that stimulate their production, as well as the effects of NO superoxide and hydrogen peroxide on mTAL tubular sodium reabsorption and the regulation of medullary blood flow. 3. Angiotensin II-stimulated NO produced in the mTAL is able to diffuse from the renal mTAL to the surrounding vasa recta capillaries, providing a mechanism by which to increase medullary blood flow and counteract the direct vasoconstrictor effects of angiotensin II. Enhanced oxidative stress attenuates NO diffusion in this region. 4. Carbonyl stress, like oxidative stress, can also play an important role in the pathogenesis of chronic kidney disease, such as insulin resistance, salt-sensitive hypertension and renal vascular complications. 5. Despite the large number of studies undertaken in this area, there is as yet no drug available that directly targets renal ROS. Oxidative and/or carbonyl stress may be the next target of drug discovery to protect against salt-sensitive hypertension and associated end-organ damage.

Overexpression of HIF-1α transgene in the renal medulla attenuated salt sensitive hypertension in Dahl S rats

Hypoxia inducible factor (HIF)-1α-mediated gene activation in the renal medulla in response to high salt intake plays an important role in the control of salt sensitivity of blood pressure. High salt-induced activation of HIF-1α in the renal medulla is blunted in Dahl S rats. The present study determined whether the impairment of the renal medullary HIF-1α pathway was responsible for salt sensitive hypertension in Dahl S rats. Renal medullary HIF-1α levels were induced by either transfection of HIF-1α expression plasmid or chronic infusion of CoCl₂ into the renal medulla, which was accompanied by increased expressions of anti-hypertensive genes, cyclooxygenase-2 and heme oxygenase-1. Overexpression of HIF-1α transgenes in the renal medulla enhanced the pressure natriuresis, promoted the sodium excretion and reduced sodium retention after salt overload. As a result, hypertension induced by 2-week high salt was significantly attenuated in rats treated with HIF-1α plasmid or CoCl₂. These results suggest that an abnormal HIF-1α in the renal medulla may represent a novel mechanism mediating salt-sensitive hypertension in Dahl S rats and that induction of HIF-1α levels in the renal medulla could be a therapeutic approach for the treatment of salt-sensitive hypertension.

Role of L-arginine uptake mechanisms in renal blood flow responses to angiotensin II in rats

AIM

To examine whether reduced renal arginine transport increases the responsiveness of the renal circulation to angiotensin II in salt sensitivity, renal perfusion responses to angiotensin II were examined in the presence of L-arginine transport inhibitor, L-lysine and subsequent L-arginine in Sprague Dawley (SD) and Dahl salt-sensitive (Dahl S) rats.

METHODS

Laser Doppler probes and a transonic flow probe were used to measure regional renal perfusion and total renal perfusion respectively. Renal perfusion responses to intravenous (i.v.) angiotensin II were sequentially examined under control conditions and during i.v. infusion of L-lysine, L-arginine or nitric oxide synthase inhibitor, N(G)-nitro-L-arginine.

RESULTS

Angiotensin II (10 and 100 ng kg(-1) min(-1) , i.v.) reduced total renal (-10 ± 3 and -36 ± 5%) and cortical (-10 ± 2 and -28 ± 4%) but not medullary perfusion in SD rats. In these rats L-lysine enhanced the renal perfusion response (P = 0.003), whereas subsequent L-arginine reversed this effect (P = 0.04). Angiotensin II reduced total renal, cortical and medullary perfusion in Dahl S rats. In Dahl S rats fed high salt, L-lysine did not affect renal perfusion responses to angiotensin II, but subsequent L-arginine blunted the renal blood flow response (P = 0.01) and increased the medullary perfusion during angiotensin II infusion (P = 0.006).

CONCLUSION

Intact renal L-arginine transport attenuates the vasoconstrictor effects of circulating angiotensin II in the renal cortex in SD rats. L-arginine also plays an important role in protecting the renal medullary circulation from the ischemic effects of angiotensin II in Dahl S rats.

Modulation of pressure-natriuresis by renal medullary reactive oxygen species and nitric oxide

The renal pressure-natriuresis mechanism is the dominant controller of body fluid balance and long-term arterial pressure. In recent years, it has become clear that the balance of reactive oxygen and nitrogen species within the renal medullary region is a key determinant of the set point of the renal pressure-natriuresis curve. The development of renal medullary oxidative stress causes dysfunction of the pressure-natriuresis mechanism and contributes to the development of hypertension in numerous disease models. The purpose of this review is to point out the known mechanisms within the renal medulla through which reactive oxygen and nitrogen species modulate the pressure-natriuresis response and to update the reader on recent advances in this field.

Hypoxia-inducible factor prolyl-hydroxylase 2 senses high-salt intake to increase hypoxia inducible factor 1alpha levels in the renal medulla

High salt induces the expression of transcription factor hypoxia-inducible factor (HIF) 1alpha and its target genes in the renal medulla, which is an important renal adaptive mechanism to high-salt intake. HIF prolyl-hydroxylase domain-containing proteins (PHDs) have been identified as major enzymes to promote the degradation of HIF-1alpha. PHD2 is the predominant isoform of PHDs in the kidney and is primarily expressed in the renal medulla. The present study tested the hypothesis that PHD2 responds to high salt and mediates high-salt-induced increase in HIF-1alpha levels in the renal medulla. In normotensive rats, high-salt intake (4% NaCl, 10 days) significantly inhibited PHD2 expressions and enzyme activities in the renal medulla. Renal medullary overexpression of the PHD2 transgene significantly decreased HIF-1alpha levels. PHD2 transgene also blocked high-salt-induced activation of HIF-1alpha target genes heme oxygenase 1 and NO synthase 2 in the renal medulla. In Dahl salt-sensitive hypertensive rats, however, high-salt intake did not inhibit the expression and activities of PHD2 in the renal medulla. Correspondingly, renal medullary HIF-1alpha levels were not upregulated by high-salt intake in these rats. After transfection of PHD2 small hairpin RNA, HIF-1alpha and its target genes were significantly upregulated by high-salt intake in Dahl salt-sensitive rats. Overexpression of PHD2 transgene in the renal medulla impaired renal sodium excretion after salt loading. These data suggest that high-salt intake inhibits PHD2 in the renal medulla, thereby upregulating the HIF-1alpha expression. The lack of PHD-mediated response to high salt may represent a pathogenic mechanism producing salt-sensitive hypertension.

Angiotensin II stimulates thick ascending limb NO production via AT(2) receptors and Akt1-dependent nitric-oxide synthase 3 (NOS3) activation

Angiotensin II (Ang II) acutely stimulates thick ascending limb (TAL) NO via an unknown mechanism. In endothelial cells, activation of Ang II type 2 receptor (AT(2)) stimulates NO. Akt1 activates NOS3 by direct phosphorylation. We hypothesized that Ang II stimulates TAL NO production via AT(2)-mediated Akt1 activation, which phosphorylates NOS3 at serine 1177. We measured NO production by fluorescence microscopy. In isolated TALs, Ang II (100 nm) increased NO production by 1.1 +/- 0.2 fluorescence units/min (p < 0.01). Ang II increased cGMP accumulation by 4.9 +/- 1.3 fmol/microg (p < 0.01). Upon adding the AT(2) antagonist PD123319 (1 microm), Ang II failed to stimulate NO (0.1 +/- 0.1 fluorescence units/min; p < 0.001 versus Ang II); adding the AT(1) antagonist losartan (1 microm) resulted in Ang II stimulating NO by 0.9 +/- 0.1 fluorescence units/min. Akt inhibitor (5 microm) blocked Ang II-stimulated NO (-0.1 +/- 0.2 fluorescence units/min versus inhibitor alone). Phospho-Akt1 increased by 72% after 5 min (p < 0.006), returning to basal after 10 min. Phospho-Akt2 did not change after 5 min but increased by 115 and 163% after 10 and 15 min (p < 0.02). Phospho-Akt3 did not change. An AT(2) agonist increased pAkt1 by 78% (p < 0.02), PI3K inhibition blocked this effect. In TALs transduced with dominant negative Akt1, Ang II failed to stimulate NO (0.1 +/- 0.2 fluorescence units/min versus 1.2 +/- 0.2 for controls; p < 0.001). Ang II increased phospho-NOS3 at serine 1177 by 130% (p < 0.01) and 150% after 5 and 10 min (p < 0.02). Ang II increased phosphoNOS3 at serine 633 by 50% after 5 min (p < 0.01). Akt inhibition prevented NOS3 phosphorylation. We concluded that Ang II enhances TAL NO production via activation of AT(2) and Akt1-dependent phosphorylation of NOS3 at serines 1177 and 633.

Early life stress sensitizes rats to angiotensin II-induced hypertension and vascular inflammation in adult life

Maternal separation during early life is an established chronic behavioral model of early life stress in rats. It is known that perinatal adverse environments increase activity of the renin-angiotensin (Ang) system, specifically Ang II, in adulthood. The aim of this study was to investigate whether the effects of early life stress augment the sensitivity of the Ang II pathway. Using Wistar Kyoto rats, the maternal separation (MS) protocol was performed by separating approximately half of the male pups from their mother 3 h/d from days 2 to 14 of life. Pups remaining with the mother at all times were used as controls. Maternal separation did not influence the plasma basal parameters, such as blood glucose, insulin, Ang II, Ang 1-7 and plasma renin activity. Furthermore, body weight, blood pressure, and heart rate were similar in MS and control rats. The acute pressor response to Ang II was not different in anesthetized MS and control rats. However, the chronic infusion of Ang II (65 ng/min SC) elicited an exaggerated hypertensive response in MS compared with control rats (P<0.05). Surprisingly, HR was dramatically increased during the second week of Ang II infusion in MS compared with control rats (P<0.05). This enhanced Ang II sensitivity was accompanied by a greater vascular inflammatory response in MS versus control rats. Chronic Ang II infusion increased vascular wall structure in both groups similarly. These data indicate that early life stress sensitizes rats to an increased hemodynamic and inflammatory response during Ang II-induced hypertension.

Silent Partner in Blood Vessel Homeostasis? Pervasive Role of Nitric Oxide in Vascular Disease

The endothelium generates powerful mediators that regulate blood flow, temper inflammation and maintain a homeostatic environment to prevent both the initiation and progression of vascular disease. Nitric oxide (NO) is arguably the single most influential molecule in terms of dictating blood vessel homeostasis. In addition to direct effects associated with altered NO production (e.g. vasoconstriction, excessive inflammation, endothelial dysfunction), NO is a critical modulator of vaso-relevant pathways including cyclooxygenase (COX)-derived prostaglandin production and angiotensin II generation by the renin-angiotensin system. Furthermore, NO may influence the selectivity of COX-2 inhibitors and ultimately contribute to controversies associated with the use of these drugs. Consistent with a central role for NO in vascular disease, disruptions in the production and bioavailability of NO have been linked to hypertension, diabetes, hypercholesterolemia, obesity, aging, and smoking. The ability of the vessel wall to control disease-associated oxidative stress may be the most critical determinant in maintaining homeostatic levels of NO and subsequently the prospect of stroke, myocardial infarction and other CV abnormalities. To this end, investigation of mechanisms that alter the balance of protective mediators, including pathways that are indirectly modified by NO, is critical to the development of effective therapy in the treatment of CV disease.

Exogenous L-arginine ameliorates angiotensin II-induced hypertension and renal damage in rats

Experiments were performed to determine whether exogenous L-arginine could ameliorate angiotensin II-induced hypertension and renal damage. Rats were instrumented with chronic indwelling femoral venous and arterial catheters for infusions of drugs and measurement of conscious arterial pressure. Arterial blood pressure significantly increased from 124+/-1 to 199+/-4 mm Hg, after 9 days of continuous infusion of angiotensin II (20 ng/kg per minute; IV; n=6 to 9). In contrast, the increase in arterial pressure after 9 days of angiotensin II infusion was significantly blunted by 45% (P=0.0003) in rats coadministered L-arginine (300 microg/kg per minute; IV; n=7 to 9). The glomerular injury index was significantly greater in rats administered angiotensin II in comparison with rats administered saline vehicle (P<0.001). Coinfusion of L-arginine significantly increased plasma nitrate/nitrite concentrations (P<0.001) and completely prevented angiotensin II-induced glomerular damage (P<0.001). Angiotensin II infusion alone and combined angiotensin II plus L-arginine infusion significantly increased urinary albumin excretion. Albuminuria in rats administered angiotensin II plus L-arginine is likely to be because of increased intraglomerular pressure. Our experiments demonstrate that L-arginine can blunt angiotensin II-induced hypertension and associated renal damage. This latter observation is most exciting because it indicates that increasing NO bioavailability, in addition to lowering arterial pressure, can greatly reduce hypertension-induced renal damage.

Salt-sensitive hypertension induced by decoy of transcription factor hypoxia-inducible factor-1alpha in the renal medulla

Hypoxia inducible factor (HIF)-1alpha, a transcription factor, is abundantly expressed in the renal medulla and regulates many oxygen-sensitive genes such as nitric oxide synthase, cyclooxygenase-2, and heme oxygenase-1. Given the important roles of these genes in the control of arterial pressure, the present study was to test the hypothesis that HIF-1alpha-mediated gene activation serves as an antihypertensive pathway by regulating renal medullary function and sodium excretion. HIF-1alpha decoy oligodeoxynucleotides (ODNs) or scrambled ODNs were transfected into the renal medulla in uninephrectomized Sprague-Dawley rats. Two weeks after ODN transfection, the HIF-1alpha binding activities were significantly inhibited by 45%, and high salt-induced increases of nitric oxide synthase-2 and heme oxygenase-1 transcriptions were also inhibited by 70% and 61% in the renal medulla from decoy rats. The natriuretic responses and increases of renal medullary blood flow responding to the elevations of renal perfusion pressure were significantly blunted by 50% and 37% in decoy rats. Intravenously acute sodium loading increased medullary blood flow and urinary sodium excretion, which was remarkably attenuated in decoy rats. In decoy rats, high salt intake caused a greater positive sodium balance. Consequently, arterial pressure was remarkably increased (from 118+/-1.9 to 154+/-6.3 mm Hg) in decoy rats but not in control rats when the rats were challenged with a high salt diet. There was no blood pressure change in decoy rats that were maintained in normal salt diet. In conclusion, HIF-1alpha-mediated gene activation importantly participates in the regulation of renal medullary function and long-term arterial blood pressure.

Role of nitric oxide deficiency in the development of hypertension in hydronephrotic animals

Hydronephrotic animals develop renal injury and hypertension, which is associated with an abnormal tubuloglomerular feedback (TGF). The TGF sensitivity is coupled to nitric oxide (NO) in the macula densa. The involvement of reduced NO availability in the development of hypertension in hydronephrosis was investigated. Hydronephrosis was induced by ureteral obstruction in young rats. Blood pressure and renal excretion were measured in adulthood, under different sodium conditions, and before and after chronic administration of either NG-nitro-l-arginine methyl ester (l-NAME) or l-arginine. Blood samples for ADMA, SDMA, and l-arginine analysis were taken and the renal tissue was used for histology and determination of NO synthase (NOS) proteins. TGF characteristics were determined by stop-flow pressure technique before and after administration of 7-nitroindazole (7-NI) or l-arginine. Hydronephrotic animals developed salt-sensitive hypertension, which was associated with pressure natriuresis and diuresis. The blood pressure response to l-NAME was attenuated and l-arginine supplementation decreased blood pressure in hydronephrotic animals, but not in the controls. Under control conditions, reactivity and sensitivity of the TGF response were greater in the hydronephrotic group. 7-NI administration increased TGF reactivity and sensitivity in control animals, whereas, in hydronephrotic animals, neuronal NOS (nNOS) inhibition had no effect. l-Arginine attenuated TGF response more in hydronephrotic kidneys than in controls. The hydronephrotic animals displayed various degrees of histopathological changes. ADMA and SDMA levels were higher and the renal expressions of nNOS and endothelial NOS proteins were lower in animals with hydronephrosis. Reduced NO availability in the diseased kidney in hydronephrosis, and subsequent resetting of the TGF mechanism, plays an important role in the development of hypertension.

Vascular oxidative stress is associated with insulin resistance in hyper-reninemic nonmodulating essential hypertension

BACKGROUND

Nonmodulating hypertension (NMHT) is a high-renin subtype of salt-sensitive hypertension due to renal hemodynamic alterations.

AIMS

To evaluate, in NMHT, whether the increased oxidative stress, which interferes with endothelial function, could be the consequence of an elevated renin-angiotensin activity and insulin resistance.

METHODS

Fourteen patients with NMHT and 12 with modulating hypertension (MHT) were included. Plasma renin activity (PRA) and glucose/insulin tolerance test were performed and homeostasis model assessment (HOMA) index and areas under the curves (AUC) calculated. Urinary nitrites and nitrates (NOx), urinary cyclic guanosine monophosphate (cGMP) activity, urinary isoprostanes and plasma nitrotyrosine levels were also measured.

RESULTS

PRA was higher in NMHT than MHT. In addition, L-arginine infusion increased effective renal plasma flow in MHT but not in NMHT. Insulin levels were higher in NMHT both at fasting and at 120 min, as were HOMA and AUC values. In MHT, NOx and cGMP significantly increased when moving from low to high Na+ intake, while nitrotyrosine mass and isoprostanes failed to show any change. On the contrary, in NMHT under low Na+ intake, urinary NOx levels were significantly higher than MHT under high Na+ intake, and failed to show any change under high Na intake; cGMP also failed to show any change when patients moved from low to high Na+ intake. Nitrotyrosine mass and isoprostanes, like to NOx, were significantly higher in NMHT under both low and high Na+ intake.

CONCLUSIONS

It is suggested that, in NMHT, a possible association between higher renin-angiotensin system activity, insulin resistance and endothelial dysfunction, showed for the first time in the same subjects, might result in systemic vascular and renal endothelial dysfunction, salt-sensitive hypertension and high cardiovascular risk.

Enhanced Superoxide Production in Renal Outer Medulla of Dahl Salt-Sensitive Rats Reduces Nitric Oxide Tubular-Vascular Cross-Talk

Studies were conducted to determine whether the diffusion of NO from the renal medullary thick ascending limb (mTAL) to the contractile pericytes of surrounding vasa recta was reduced and, conversely, whether diffusion of oxygen free radicals was enhanced in the salt-sensitive Dahl S rat (SS/Mcwi). Angiotensin II ([Ang II] 1 μmol/L)–stimulated NO and superoxide (O 2 ·− ) production were imaged by fluorescence microscopy in thin tissue strips from the inner stripe of the outer medulla. In prehypertensive SS/Mcwi rats and a genetically designed salt-resistant control strain (consomic SS-13 BN ), Ang II failed to increase either NO or O 2 ·− in pericytes of isolated vasa recta. Ang II stimulation resulted in production of NO in epithelial cells of the mTAL that diffused to vasa recta pericytes of SS-13 BN rats but not in SS/Mcwi rats except when tissues were preincubated with the superoxide scavenger TIRON (1 mmol/L). Ang II resulted in a greater increase of O 2 ·− in the mTAL of SS/Mcwi compared with SS.13 BN mTAL. The O 2 ·− diffused to adjoining pericytes in tissue strips only in SS/Mcwi rats but not in control SS-13 BN rats. Diffusion of Ang II-stimulated O 2 ·− from mTAL to vasa recta pericytes was absent when tissue strips from SS/Mcwi rats were treated with the NO donor DETA-NONOate (20 μmol/L). We conclude that the SS/Mcwi rat exhibits increased production of O 2 ·− in mTAL that diffuses to surrounding vasa recta and attenuates NO cross-talk. Diffusion of O 2 ·− from mTAL to surrounding tissue could contribute to reduced bioavailability of NO, reductions of medullary blood flow, and interstitial fibrosis in the outer medulla of SS/Mcwi rats.

Vitamin E reduces glomerulosclerosis, restores renal neuronal NOS, and suppresses oxidative stress in the 5/6 nephrectomized rat

Chronic kidney disease is accompanied by nitric oxide (NO) deficiency and oxidative stress, which contribute to progression. We investigated whether the antioxidant vitamin E could preserve renal function and NO bioavailability and reduce oxidative stress in the 5/6th nephrectomy (NX) rat model. We studied the following three groups of male Sprague-Dawley rats: sham (n = 6), 5/6 NX control (n = 6), and 5/6 NX treated with vitamin E (5,000 IU/kg chow; n = 5). The 5/6 NX group showed increased severity of glomerulosclerosis vs. sham, and this was ameliorated by vitamin E therapy. Both 5/6 NX groups showed similar elevations in plasma creatinine and proteinuria and decreased 24-h creatinine clearance compared with sham. There was increased NADPH-dependent superoxide production in 5/6 NX rats vs. sham that was prevented by vitamin E. Total NO production was similarly reduced in both 5/6 NX groups. There was unchanged abundance of endothelial nitric oxide synthesis (NOS) in renal cortex and medulla and neuronal (n) NOS in medulla. However, in kidney cortex, 5/6 NX rats had lower nNOS abundance than sham, which was restored by vitamin E. An increased plasma asymmetric dimethylarginine occurred with 5/6 NX associated with decreased renal dimethylarginine dimethylaminohydrolase activity and increased type 1 protein arginine methyltransferase expression.

Role of Renal Medullary Heme Oxygenase in the Regulation of Pressure Natriuresis and Arterial Blood Pressure

Recent studies have demonstrated that inhibition of renal medullary heme oxygenase (HO) activity and carbon monoxide (CO) significantly decreases renal medullary blood flow and sodium excretion. Given the crucial role of renal medullary blood flow in the control of pressure natriuresis, the present study was designed to determine whether renal medullary HO activity and resulting CO production participate in the regulation of pressure natriuresis and thereby the long-term control of arterial blood pressure. In anesthetized Sprague-Dawley rats, increases in renal perfusion pressure induced significant elevations of CO concentrations in the renal medulla. Renal medullary infusion of chromium mesoporphyrin (CrMP), an inhibitor of HO activity, remarkably inhibited HO activity and the renal perfusion pressure-dependent increases in CO levels in the renal medulla and significantly blunted pressure natriuresis. In conscious Sprague-Dawley rats, continuous infusion of CrMP into the renal medulla significantly increased mean arterial pressure (129±2.5 mm Hg in CrMP group versus 118±1.6 mm Hg in vehicle group) when animals were fed a normal salt diet (1% NaCl). After rats were switched to a high-salt diet (8% NaCl) for 10 days, CrMP-treated animals exhibited further increases in mean arterial pressure compared with CrMP-treated animals that were kept on normal salt diet (152±4.1 versus 130±4.2 mm Hg). These results suggest that renal medullary HO activity plays a crucial role in the control of pressure natriuresis and arterial blood pressure and that impairment of this HO/CO-mediated antihypertensive mechanism in the renal medulla may result in the development of hypertension.

Salt sensitivity and hypertension after menopause: role of nitric oxide and angiotensin II

Hypertension is a major risk factor for cardiovascular disease and renal disease. After menopause, the incidence of hypertension increases in women to levels that equal or exceed that in men, suggesting a protective role of female sex hormones. Salt sensitivity of blood pressure is associated with an increased risk for development of hypertension and cardiovascular disease. We and others have demonstrated that after menopause, the prevalence of salt sensitivity increases, suggesting that female sex hormones influence renal sodium handling and blood pressure regulation. A homeostatic balance between the counteracting effects of nitric oxide (NO) and angiotensin (Ang) II on pressure natriuresis, renal hemodynamics, tubular sodium reabsorption, and oxidative stress plays an important role in modulating salt sensitivity as well as hypertensive end-organ injury. Estrogens modulate the activity and expression of NO and Ang II. We infer that after menopause, estrogen deficiency promotes an unbalance between NO and Ang II, resulting in disturbed renal sodium handling, oxidative stress, and hypertension, particularly in genetically prone women. A better understanding of the mechanisms underlying the development of postmenopausal hypertension and associated cardiovascular and renal diseases should provide insights into preventive and therapeutic strategies.

Molecular mechanisms and therapeutic strategies of chronic renal injury: physiological role of angiotensin II-induced oxidative stress in renal medulla

Renal medullary circulation has now been found to play a fundamental role in regulating long-term blood pressure control and fluid balance. Elevation of superoxide or reduction of nitric oxide (NO) in renal medulla decreases medullary blood flow and Na excretion, resulting in sustained hypertension. Angiotensin II (Ang II)-induced interaction of superoxide and NO was determined in thin tissue strips isolated from the renal outer medullary region of Sprague-Dawley rats using fluorescent microscopy techniques. Ang II can induce diffusion of NO, but not superoxide, from the medullary thick ascending limb (mTAL) to the surrounded vasa recta. However, when NO is reduced by the NO scavenger carboxy-PTIO, Ang II can induce superoxide diffusion from mTAL to vasa recta pericytes. Therefore, the physiological action of oxidative stress in renal medullary region is demonstrated as balance of superoxide and NO diffusion ("tubulo-vascular cross-talk"). These results explain how chronically hypoxic medulla can maintain blood flow. In other studies using chronically instrumented rats, we found that nearly 70% of Ang II-induced medullary renal injury was dependent on pressure determined by servo-control of renal perfusion pressure, whereas 30% of the injury was non-hemodynamic. We conclude that oxidative stress within the renal medulla can induce hypertension and also make the kidney functionally more vulnerable to the effects of Ang II.

Effect of renal medullary H2O2 on salt-induced hypertension and renal injury

Dahl salt-sensitive (SS) and consomic, salt-resistant SS-13(BN) rats possess substantial differences in blood pressure salt-sensitivity even with highly similar genetic backgrounds. The present study examined whether increased oxidative stress, particularly H2O2, in the renal medulla of SS rats contributes to these differences. Blood pressure was measured using femoral arterial catheters in three groups of rats: 1) 12-wk-old SS and consomic SS-13(BN) rats fed a 0.4% NaCl diet, 2) SS rats fed a 4% NaCl diet and chronically infused with saline or catalase (6.9 microg x kg(-1) x min(-1)) directly into the renal medulla, and 3) SS-13(BN) fed high salt (4%) and infused with saline or H2O2 (347 nmol x kg(-1) x min(-1)) into the renal medullary interstitium. After chronic blood pressure measurements, renal medullary interstitial H2O2 concentration ([H2O2]) was collected by microdialysis and analyzed with Amplex red. Blood pressure and [H2O2] were both significantly higher in SS (126 +/- 3 mmHg and 145 +/- 17 nM, respectively) vs. SS-13(BN) rats (116 +/- 2 mmHg and 56 +/- 14 nM) fed a 0.4% diet. Renal interstitial catalase infusion significantly decreased [H2O2] (96 +/- 41 vs. 297 +/- 52 nM) and attenuated the hypertension (146 +/- 2 mmHg catalase vs. 163 +/- 4 mmHg saline) in SS rats after 5 days of high salt (4%). H2O2 infused into the renal medulla of consomic SS-13(BN) fed high salt (4%) for 7 days accentuated the salt sensitivity (145 +/- 2 mmHg H2O2 vs. 134 +/- 1 mmHg saline). [H2O2] was also increased in the treated group (83 +/- 1 nM H2O2 vs. 44 +/- 9 nM saline). These data show that medullary production of H2O2 may contribute to salt-induced hypertension in SS rats and that chromosome 13 of the Brown Norway contains gene(s) that protect against renal medullary oxidant stress.

Alterations in aldosterone and angiotensin II levels in salt-induced hypertension

Several studies have demonstrated that plasma renin-angiotensin activity is reduced in rats administered a high salt diet. We evaluated changes in plasma and tissue levels of aldosterone (ALDO) and angiotensin II (A-II), as well as the reduced-to-oxidized glutathione ratio. Male Dahl salt-sensitive (SS) rats were placed on either a high-salt (8% NaCl; HS) or a normal-salt (0.3% NaCl; NS) diet for 3 weeks. Prior to and weekly on the diets, systolic blood pressure was measured by tail cuff plethysmography. Levels of A-II and ALDO in plasma, heart, and kidney were analyzed by enzyme immunoassay. Reduced and oxidized gluthatione were simultaneously measured by HPLC fluorescence detection. Heart and kidney tissues were prepared for histological analysis. Systolic blood pressure in animals on a HS diet was significantly elevated above that of those on a NS diet. High salt caused a reduction in both plasma A-II and ALDO levels; while their levels in the heart and kidney were increased. Exposure to a high-salt diet led to the enlargement of both heart and kidney. The reduced-to-oxidized glutathione ratio in plasma, heart and kidney was lowered by exposure to a HS diet. Kidneys from animals on a high-salt diet showed fibroid necrosis associated with wrinkling and thickening of the glomerular capillary wall, while hearts were hypertrophic. Taken together, high dietary salt induces inappropriate activation of the local renin-angiotensin-aldosterone systems. Tissue levels of angiotensin II and aldosterone may be more reflective of the severity of vascular maladaptations than are plasma levels, and may play a greater role in the maintenance of hypertension.

Recent advances in the regulation of nitric oxide in the kidney

Nitric oxide (NO) plays important roles in the regulation of renal function and the long-term control of blood pressure. New roles of NO have been proposed recently in diabetes, nephrotoxicity, and pregnancy. NO derived from all 3 NOS isoforms contributes to the overall regulation of kidney function, and recent advances in our understanding of their regulation have been made lately. In this regard, substrate and cofactor availability play important roles in regulating nitric oxide synthase (NOS) activity not only by limiting enzyme activity but also by influencing the coupling of NOS with its cofactors, tetrahydrobiopterin and NADPH. Protein-protein interactions are now recognized to be important negative and positive regulators of NOS. Phosphorylation is another component of the mechanism whereby NOS is activated or deactivated. Increased NOS expression can also influence enzyme activity; however, the degree of expression does not always correlate with enzyme activity because increased NO levels can result in inhibition of NOS. Finally, other potential regulators of NOS such as endogenous L-arginine analogs may also be important. In this article, we summarize recent advances in the regulation of activity and expression of the NOS isoforms within the kidney.

Atorvastatin Prevents End-Organ Injury in Salt-Sensitive Hypertension

Statins, inhibitors of cholesterol biosynthesis, are endowed with pleiotropic effects that may contribute to their favorable clinical results. Hypertensive Dahl salt-sensitive (DS) rats have endothelial dysfunction and cardiorenal injury associated with decreased NO bioavailability and increased superoxide (O2-) production linked to a functional upregulation of angiotensin II. We investigated whether atorvastatin (30 mg/kg per day; by gavage) would prevent endothelial nitric oxide (eNOS) downregulation and the increase in O2- in DS rats, thereby reducing end-organ injury. DS rats given a high-salt diet (4% NaCl) for 10 weeks developed hypertension (systolic blood pressure [SBP] 200+/-8 versus 150+/-2 mm Hg in DS rats fed 0.5% NaCl diet [NS]; P<0.05), impaired endothelium-dependent relaxation, functional upregulation of endothelin-1, left ventricular hypertrophy (LVH; 30%), and proteinuria (167%), accompanied by downregulation of aortic eNOS activity (0.7+/-0.2 versus 1.8+/-0.3 nmol/min per gram protein in NS; P<0.05) and increased aortic O2- (2632+/-316 versus 1176+/-112 counts/min per milligram in NS; P<0.05) and plasma 8-F2alpha isoprostanes. Atorvastatin prevented the decrease in eNOS activity (1.5+/-0.3 nmol/min per gram protein) as well as the increase in O2- (1192+/-243 counts/min per milligram) and plasma 8-F2alpha isoprostanes, reduced LVH and proteinuria, and normalized endothelial function and vascular response to endothelin-1, although reduction in SBP was modest (174+/-8 mm Hg). Atorvastatin combined with removal of high salt normalized aortic eNOS activity, SBP, LVH, and proteinuria. These findings strongly suggest that concomitant prevention of vascular eNOS downregulation and inhibition of oxidative stress may contribute to the protection against end-organ injury afforded by this statin in salt-sensitive hypertension.