Differential nNOS gene expression in salt-sensitive and salt-resistant Dahl rats

Oxidative Stress in Kidney Injury and Hypertension

Hypertension (HTN) is a major contributor to kidney damage, leading to conditions such as nephrosclerosis and hypertensive nephropathy, significant causes of chronic kidney disease (CKD) and end-stage renal disease (ESRD). HTN is also a risk factor for stroke and coronary heart disease. Oxidative stress, inflammation, and activation of the renin-angiotensin-aldosterone system (RAAS) play critical roles in causing kidney injury in HTN. Genetic and environmental factors influence the susceptibility to hypertensive renal damage, with African American populations having a higher tendency due to genetic variants. Managing blood pressure (BP) effectively with treatments targeting RAAS activation, oxidative stress, and inflammation is crucial in preventing renal damage and the progression of HTN-related CKD and ESRD. Interactions between genetic and environmental factors impacting kidney function abnormalities are central to HTN development. Animal studies indicate that genetic factors significantly influence BP regulation. Anti-natriuretic mechanisms can reset the pressure-natriuresis relationship, requiring a higher BP to excrete sodium matched to intake. Activation of intrarenal angiotensin II receptors contributes to sodium retention and high BP. In HTN, the gut microbiome can affect BP by influencing energy metabolism and inflammatory pathways. Animal models, such as the spontaneously hypertensive rat and the chronic angiotensin II infusion model, mirror human essential hypertension and highlight the significance of the kidney in HTN pathogenesis. Overproduction of reactive oxygen species (ROS) plays a crucial role in the development and progression of HTN, impacting renal function and BP regulation. Targeting specific NADPH oxidase (NOX) isoforms to inhibit ROS production and enhance antioxidant mechanisms may improve renal structure and function while lowering blood pressure. Therapies like SGLT2 inhibitors and mineralocorticoid receptor antagonists have shown promise in reducing oxidative stress, inflammation, and RAAS activity, offering renal and antihypertensive protection in managing HTN and CKD. This review emphasizes the critical role of NOX in the development and progression of HTN, focusing on its impact on renal function and BP regulation. Effective BP management and targeting oxidative stress, inflammation, and RAAS activation, is crucial in preventing renal damage and the progression of HTN-related CKD and ESRD.

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.

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.

Sodium sensitivity of blood pressure in Chinese populations

Hypertension is an enormous public-health challenge in the world due to its high prevalence and consequent increased cardiovascular disease morbidity and mortality. Observational epidemiologic studies and clinical trials have demonstrated a causal relationship between sodium intake and elevated blood pressure (BP). However, BP changes in response to sodium intervention vary among individuals-a trait called sodium sensitivity. This paper aims to review the recent advances in sodium-sensitivity research in Chinese and other populations. Older age, female gender, and black race are associated with high sodium sensitivity. Both genetic and environmental factors influence BP sodium sensitivity. Physical activity and dietary potassium intake are associated with reduced sodium sensitivity while obesity, metabolic syndrome, and elevated BP are associated with increased sodium sensitivity. Familial studies have documented a moderate heritability of sodium sensitivity. Candidate gene association studies, genome-wide association studies, whole-exome, and whole-genome sequencing studies have been conducted to elucidate the genomic mechanisms of sodium sensitivity. The Genetic Epidemiology Network of Salt Sensitivity (GenSalt) study, the largest family-based feeding study to date, was conducted among 1906 Han Chinese in rural northern China. This study showed that ~32.4% of Chinese adults were sodium sensitive. Additionally, several genetic variants were found to be associated with sodium sensitivity. Findings from the GenSalt Study and others indicate that sodium sensitivity is a reproducible trait and both lifestyle factors and genetic variants play a role in this complex trait. Discovering biomarkers and underlying mechanisms for sodium sensitivity will help to develop individualized intervention strategies for hypertension.

Female sex hormones protect against salt-sensitive hypertension but not essential hypertension

Initial studies found that female Dahl salt-sensitive (DS) rats exhibit greater blood pressure (BP) salt sensitivity than female spontaneously hypertensive rats (SHR). On the basis of the central role played by NO in sodium excretion and BP control, we further tested the hypothesis that blunted increases in BP in female SHR will be accompanied by greater increases in renal inner medullary nitric oxide synthase (NOS) activity and expression in response to a high-salt (HS) diet compared with DS rats. Gonad-intact and ovariectomized (OVX) female SHR and DS rats were placed on normal salt (NS; 0.4% salt) or HS (4% salt) diet for 2 wk. OVX did not alter BP in SHR, and HS diet produced a modest increase in BP. OVX significantly increased BP in DS rats on NS; HS further increased BP in all DS rats, although OVX had a greater increase in BP. Renal inner medullary NOS activity, total NOS3 protein, and NOS3 phosphorylated on serine residue 1177 were not altered by salt or OVX in either strain. NOS1 protein expression, however, significantly increased with HS only in SHR, and this corresponded to an increase in urinary nitrate/nitrite excretion. SHR also exhibit greater NOS1 and NOS3 protein expression than DS rats. These data indicate that female sex hormones offer protection against HS-mediated elevations in BP in DS rats but not SHR. We propose that the relative resistance to HS-mediated increases in BP in SHR is related to greater NOS expression and the ability to increase NOS1 protein expression compared with DS rats.

Genomic epidemiology of blood pressure salt sensitivity

High blood pressure (BP) is a complex trait determined by genetic and environmental factors, as well as their interactions. Over the past few decades, there has been substantial progress elucidating the genetic determinants underlying BP response to sodium intake, or BP salt sensitivity. Research of monogenic BP disorders has highlighted the importance of renal salt handling in BP regulation, implicating genes and biological pathways subsequently identified in candidate gene studies of salt sensitivity. Despite these advancements, certain candidate gene findings await replication evidence, and some biological pathways warrant further investigation. Furthermore, results from genome-wide association studies (GWASs) and sequencing work have yet to be reported. GWAS will be valuable for uncovering novel mechanisms underlying salt sensitivity, whereas future sequencing efforts promise the discovery of functional variants related to this complex trait. Delineating the genetic architecture of salt sensitivity will be critical to understanding how genes and dietary sodium interact to influence BP.

Gene-sodium interaction and blood pressure: findings from genomics research of blood pressure salt sensitivity

High blood pressure (BP) is a complex trait determined by both genetic and environmental factors, as well as the interactions between these factors. Over the past few decades, there has been substantial progress in elucidating the genetic determinants underlying the BP response to sodium intake, or BP salt sensitivity. Research of monogenic BP disorders has highlighted the importance of renal salt handling in BP regulation, implicating genes and biological pathways related to salt sensitivity. Candidate gene studies have contributed important information toward understanding the genomic mechanisms underlying the BP response to salt intake, identifying genes in the renin-angiotensin-aldosterone system, renal sodium channels/transporters, and the endothelial system related to this phenotype. Despite these advancements, genome-wide association studies are still needed to uncover novel mechanisms underlying salt sensitivity, while future sequencing efforts promise the discovery of functional variants related to this complex trait. Delineating the genetic architecture of salt sensitivity will be critical to understanding how genes and dietary sodium interact to influence BP.

Endogenous angiotensin II has fewer effects but neuronal nitric oxide synthase has excitatory effects on renal sympathetic nerve activity in salt-sensitive hypertension-induced heart failure

The effects of endogenous angiotensin II (Ang II) and neuronal nitric oxide synthase (nNOS) on tonic sympathetic activity were studied in salt-sensitive hypertension-induced heart failure. Dahl salt-sensitive rats were fed 8% NaCl diet for 9 weeks to induce chronic heart failure (CHF-DSS). The effects of intravenous administration of a selective nNOS inhibitor, S-methyl-L: -thiocitrulline (SMTC), and an Ang II type 1-receptor blocker, losartan, on renal sympathetic nerve activity (RSNA) were examined in chronically instrumented conscious rats. Baroreceptor (baro)-unloaded RSNA was obtained by decreasing arterial pressure with caval occlusion to determine tonic RSNA. SMTC significantly decreased baro-unloaded RSNA, and subsequent losartan recovered baro-unloaded RSNA to the control level in CHF-DSS rats. To compare the effects of the inhibitors between low- and high-activity states of the renin-angiotensin system (RAS), Sprague-Dawley rats were fed low (0.04%)- or high (8%)-salt diets. A significant difference was found in the effects of SMTC and/or losartan on RSNA between the high- and low-RAS states, which suggested that there is a difference in the effect of endogenous Ang II on RSNA between salt-induced and other-type heart failure. To examine the effects of heart failure on brain-tissue nNOS activity, we measured the activities of the diencephalon in heart-failure rats. Heart failure significantly suppressed diencephalon nNOS activity, which was significantly different from the results in salt-sensitive hypertension without heart failure. These results suggest that endogenous Ang II has fewer effects, but nNOS has excitatory effects on tonic RSNA in salt-sensitive hypertension-induced heart failure.

Nitric oxide in the kidney : its physiological role and pathophysiological implications

Nitric oxide has been implicated in many physiologic processes that influence both acute and long-term control of kidney function. Its net effect in the kidney is to promote natriuresis and diuresis, contributing to adaptation to variations of dietary salt intake and maintenance of normal blood pressure. A pretreatment with nitric oxide donors or L-arginine may prevent the ischemic acute renal injury. In chronic kidney diseases, the systolic blood pressure is correlated with the plasma level of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase. A reduced production and biological action of nitric oxide is associated with an elevation of arterial pressure, and conversely, an exaggerated activity may represent a compensatory mechanism to mitigate the hypertension.

Brain neuronal nitric oxide synthase neuron-mediated sympathoinhibition is enhanced in hypertensive Dahl rats

OBJECTIVE

To elucidate the role of central neurons containing neuronal nitric oxide synthase (nNOS neurons) in the sympathetic nervous system in hypertensive Dahl salt-sensitive (DS) rats.

DESIGN AND METHODS

Dahl rats were fed either a regular-salt (0.4% NaCl) or high-salt (8% NaCl) diet for 4 weeks. The effect of intracerebroventricular administration of S-methyl-L-thiocitrulline, a selective nNOS inhibitor, on renal sympathetic nerve activity was examined in chronically instrumented conscious DS rats. The activity and protein amount of brain nNOS was evaluated by enzyme assay and western blot analysis. The distribution and number of nNOS neurons in the brainstem were examined immunohistochemically in hypertensive and normotensive DS rats.

RESULTS

S-methyl-L-thiocitrulline induced a larger increase in tonic renal sympathetic nerve activity generated before baroreflex-mediated inhibition in hypertensive DS rats than normotensive DS rats. Hypertensive DS rats showed increased nNOS activity in the brainstem, but not in the diencephalon or cerebellum. High nNOS activity was confirmed by an increase in the amount of nNOS protein. nNOS Neurons were localized in several nuclei throughout the brainstem; the dorsolateral periaqueductal gray, pedunculopontine tegmental nucleus, dorsal raphe nucleus, laterodorsal tegmental nucleus, lateral parabrachial nucleus, rostral ventrolateral medulla, nucleus tractus solitarius and raphe magnus. The number of nNOS neurons in these nuclei, except for the two raphes, was significantly greater in hypertensive than in normotensive DS rats.

CONCLUSIONS

These findings suggest that central nNOS-mediated sympathoinhibition may be enhanced in salt-sensitive hypertensive Dahl rats. The upregulated nNOS-mediated inhibition may occur in the central sympathetic control system generated before baroreflex-mediated inhibition.

Salt-induced hypertension in WKY rats: prevention by alpha-lipoic acid supplementation

There is strong evidence that points to excess dietary salt as a major factor contributing to the development of hypertension. Salt sensitivity is associated with glucose intolerance and insulin resistance in both animal models and humans. In insulin resistance, impaired glucose metabolism leads to elevated endogenous aldehydes which bind to vascular calcium channels, increasing cytosolic [Ca2+]i and blood pressure. In an insulin resistant animal model of hypertension, spontaneously hypertensive rats (SHRs), dietary supplementation with lipoic acid lowers tissue aldehydes and plasma insulin levels and normalizes blood pressure. The objective of this study is to examine the effects of a high salt diet on tissue aldehydes, cytosolic [Ca2+]i and blood pressure in WKY rats and to investigate whether dietary supplementation with lipoic acid can prevent a salt induced increase in blood pressure. Starting at 7 weeks of age, WKY rats were divided into three groups of six animals each and treated for 10 weeks with diets as follows: WKY-normal salt (0.7% NaCl); WKY-high salt (8% NaCl); WKY-high salt + lipoic acid (8% NaCl diet + lipoic acid 500 mg/Kg feed). At completion, animals in the high salt group had elevated systolic blood pressure, platelet [Ca2+]i, and tissue aldehyde conjugates compared with the normal salt group and showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary alpha-lipoic acid supplementation in high salt-treated WKY rats normalized systolic blood pressure and cytosolic [Ca2+]i and aldehydes in liver and aorta. Kidney aldehydes and renal vascular changes were attenuated, but not normalized.

Identification of salt-sensitive genes in the kidneys of Dahl rats

OBJECTIVE

Inherited differences in renal function underlie the effect of high salt diets on blood pressure in Dahl rats. We probed the kidneys of inbred Dahl SS/Jr and SR/Jr for anonymous and candidate genes whose expression was regulated by dietary sodium.

METHODS

mRNA quantitation of both candidate genes implicated in sodium excretion and anonymous gene products found by differential hybridization in the kidneys of salt-resistant (SR) and salt sensitive (SS) inbred Dahl rats on high and low salt diets for 21 days.

RESULTS

Differential screening revealed a cDNA clone (H1) that showed increased dietary salt-dependent expression only in SS rats. Sequencing of the H1 cDNA showed it was the Dahl rat homologue to a perchloric acid soluble protein expressed in liver and kidney. Among candidate genes, transcript levels of arginosuccinate synthetase (AS) and arginosuccinate lyase (AL) were higher in SS on low salt diets, and AS mRNA increased in response to a high salt diet in SR. Renal mRNA for the ANP-A and the vasopressin type II receptors did not differ by strain or dietary conditions.

CONCLUSIONS

Three new salt-sensitive genes were detected in the kidneys of inbred Dahl rats. Two genes encode enzymes in the biosynthesis of L-arginine. The upregulation of these genes by dietary salt indicates increased demand and biosynthesis of L-arginine in Dahl SS rats. A third gene encodes a small acid-soluble protein thought to influence the transcription/translation of numerous genes. Further studies will be needed to determine the nature of the association of these genes with salt-sensitivity and blood pressure.

Colocalization of urocortin and neuronal nitric oxide synthase in the hypothalamus and Edinger-Westphal nucleus of the rat

Different lines of studies suggest that both the corticotropin-releasing hormone-related peptide Urocortin I (Ucn) and the neuromodulator nitric oxide (NO) are involved in the regulation of the complex mechanisms controlling feeding and anxiety-related behaviors. The aim of the present study was to investigate the possible interaction between Ucn and NO in the hypothalamic paraventricular nucleus (PVN), an area known to be involved in the modulation of these particular behaviors. Therefore, we mapped local mRNA and peptide/protein presence of both Ucn and the NO producing neuronal NO synthase (nNOS). This investigation was extended to include the hypothalamic supraoptic nucleus (SON) and the Edinger-Westphal nucleus area (EW), the latter being one of the major cellular Ucn-expressing sites. Furthermore, we compared the two predominantly used laboratory rat strains, Wistar and Sprague-Dawley. Ucn mRNA and immunoreactivity were detected in the SON and in the EW. A significant difference between Wistar and Sprague-Dawley rats was found in mRNA levels in the EW. nNOS was detected in all brain areas analyzed, showing a significantly lower immunoreactivity in the PVN and EW of Sprague-Dawley versus Wistar rats. Contrary to some previous reports, no Ucn mRNA and only a very low immunoreactivity were detectable in the PVN of either rat strain. Interestingly, double-labeling immunofluorescence revealed that in the SON approximately 75% of all cells immunoreactive for Ucn were colocalized with nNOS, whereas in the EW only approximately 2% of the Ucn neurons were found to contain nNOS. These findings suggest an interaction between Ucn and NO signaling within the SON, rather than the PVN, that may modulate the regulation of feeding, reproduction, and anxiety-related behaviors.

Increased collecting duct urea transporter expression in Dahl salt-sensitive rats

Because abnormalities of inner medullary function have been proposed in Dahl salt-sensitive (DS) rats vs. salt-resistant (DR) rats, we performed transporter profiling by semiquantitative immunoblotting to determine whether specific solute transporter abundances are altered in inner medullas of DS rats vs. DR rats. Although none of the expressed Na transporters were upregulated in the inner medullas of DS rats compared with DR rats, there were marked increases in the protein abundances of the collecting duct urea transporters UT-A1 (to 212% of DR) and UT-A3 (to 223% of DR). These differences were confirmed by immunocytochemistry. Quantitative real-time RT-PCR showed higher mRNA abundance in DS rats for both UT-A1 (to 256% of DR) and UT-A3 (to 210% of DR). In isolated, perfused inner medullary collecting ducts, urea permeability was significantly greater in DS rats. Because both UT-A1 and UT-A3 are transcriptionally regulated by glucocorticoids, we measured both plasma corticosterone levels and inner medullary 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) abundances. Although the plasma corticosterone concentrations were not different between DS and DR rats, immunoblotting and immunocytochemistry revealed a marked elevation of 11beta-HSD2 abundance in DS rats. Consistent with the view that an elevated 11beta-HSD2 level is responsible for increased urea transporter expression in the inner medullary collecting duct, administration of the 11beta-HSD2 inhibitor carbenoxolone to DS rats decreased the abundances of UT-A1 and UT-A3 to levels similar to those seen in DR rats.

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

Studies were designed to examine the hypothesis that the renal medulla of Dahl salt-sensitive (Dahl S) rats has a reduced capacity to generate nitric oxide (NO), which diminishes the ability to buffer against the chronic hypertensive effects of small elevations of circulating ANG II. NO synthase (NOS) activity in the outer medulla of Dahl S rats (arginine-citrulline conversion assay) was significantly reduced. This decrease in NOS activity was associated with the downregulation of protein expression of NOS I, NOS II, and NOS III isoforms in this region as determined by Western blot analysis. In anesthetized Dahl S rats, we observed that a low subpressor intravenous infusion of ANG II (5 ng. kg(-1). min(-1)) did not increase the concentration of NO in the renal medulla as measured by a microdialysis with oxyhemoglobin trapping technique. In contrast, ANG II produced a 38% increase in the concentration of NO (87 +/- 8 to 117 +/- 8 nmol/l) in the outer medulla of Brown-Norway (BN) rats. The same intravenous dose of ANG II reduced renal medullary blood flow as determined by laser-Doppler flowmetry in Dahl S, but not in BN rats. A 7-day intravenous ANG II infusion at a dose of 3 ng. kg(-1). min(-1) did not change mean arterial pressure (MAP) in the BN rats but increased MAP in Dahl S rats from 120 +/- 2 to 138 +/- 2 mmHg (P < 0.05). ANG II failed to increase MAP after NO substrate was provided by infusion of L-arginine (300 microg. kg(-1). min(-1)) into the renal medulla of Dahl S rats. Intravenous infusion of L-arginine at the same dose had no effect on the ANG II-induced hypertension. These results indicate that an impaired NO counterregulatory system in the outer medulla of Dahl S rats makes them more susceptible to the hypertensive actions of small elevations of ANG II.