Nitric oxide synthase isoform activities in kidney of Dahl salt-sensitive 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.

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.

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.

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.

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.

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.

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.

Renal medullary endothelin-1 is decreased in Dahl salt-sensitive rats

Although it is well established that the renal endothelin (ET-1) system plays an important role in regulating sodium excretion and blood pressure through activation of renal medullary ET(B) receptors, the role of this system in Dahl salt-sensitive (DS) hypertension is unclear. The purpose of this study was to determine whether the DS rat has abnormalities in the renal medullary endothelin system when maintained on a high sodium intake. The data indicate that Dahl salt-resistant rats (DR) on a high-salt diet had a six-fold higher urinary endothelin excretion than in the DR rats with low Na(+) intake (17.8 ± 4 pg/day vs. 112 ± 44 pg/day). In sharp contrast, urinary endothelin levels increased only twofold in DS rats in response to a high Na(+) intake (13 ± 2 pg/day vs. 29.8 ± 5.5 pg/day). Medullary endothelin concentration in DS rats on a high-Na(+) diet was also significantly lower than DR rats on a high-Na(+) diet (31 ± 2.8 pg/mg vs. 70.9 ± 5 pg/mg). Furthermore, DS rats had a significant reduction in medullary ET(B) receptor expression compared with DR rats while on a high-Na(+) diet. Finally, chronic infusion of ET-1 directly into the renal medulla blunted Dahl salt-sensitive hypertension. These data indicate that a decrease in medullary production of ET-1 in the DS rat could play an important role in the development of salt-sensitive hypertension observed in the DS rat.

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.

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.

Salt-induced cardiac hypertrophy is independent of blood pressure and endothelin in obese, heart failure-prone SHHF rats

The interaction of salt sensitivity and obesity in development of cardiac hypertrophy is incompletely understood. The SHHF/Mcc-fa(cp) (SHHF) rat model was used to examine the effect of high salt on cardiac hypertrophy and expression of endothelin (ET) and nitric oxide synthase (NOS) isoforms. Homozygous lean (+/+) and obese (fa(cp)/fa(cp)) SHHF were fed a low-salt diet (0.3% NaCl) for seven days followed by a high-salt diet (8.0% NaCl) for seven days. To assess the role of ET in mediating cardiac hypertrophy and gene expression with high salt, additional groups were treated with an ET(A)/ET(B) receptor antagonist (bosentan) while on high salt. Obese SHHF showed an increase in systolic blood pressure and cardiac hypertrophy in response to the high-salt diet. High salt resulted in decreased expression of preproET as well as all three NOS isoforms in the Obese, while cytokine induced NOS (iNOS) and neuronal NOS (nNOS) increased in Leans. Though the salt-sensitive component of the hypertension observed in the Obese was prevented by bosentan, cardiac hypertrophy still occurred and expression of all NOS isoforms remained lower in Obese compared to Lean. Endothelial NOS (eNOS) expression increased in the Lean with bosentan. These studies suggest that cardiac hypertrophy is independent of the level of hypertension and may be mediated by altered production of NOS isoforms in salt-sensitive, obese SHHF.

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.

The vascular endothelium in hypertension

The vascular endothelium plays a fundamental role in the basal and dynamic regulation of the circulation. Thus, it has a crucial role in the pathogenesis of hypertension. A spectrum of vasoactive substances is synthesised in the endothelium; of these, nitric oxide (NO), prostacyclin (PGI2) and endothelin (ET)-1 are the most important. There is a continuous basal release of NO determining the tone of peripheral blood vessels. Systemic inhibition of NO synthesis or scavenging of NO through oxidative stress causes an increase in arterial blood pressure. Also, the renin-angiotensin-aldosterone system has a major role in hypertension as it has a direct vasoconstrictor effect and important interactions with oxygen free radicals and NO. Prostacyclin, in contrast to NO, does not contribute to the maintenance of basal vascular tone of conduit arteries, but its effect on platelets is most important. ET acts as the natural counterpart to endothelium-derived NO and has an arterial blood pressure-raising effect in man. Anti-hypertensive therapy lowers blood pressure and may influence these different mediators, thus influencing endothelial function. In summary, due to its position between the blood pressure and smooth muscle cells responsible for peripheral resistance, the endothelium is thought to be both victim and offender in arterial hypertension. The delicate balance of endothelium-derived factors is disturbed in hypertension. Specific anti-hypertensive and anti-oxidant treatment is able to restore this balance.

Chronic administration of adrenomedullin attenuates the hypertension and increases renal nitric oxide synthase in Dahl salt-sensitive rats

Adrenomedullin reduces systemic blood pressure and increases urinary sodium excretion partly through the release of nitric oxide. We hypothesized that chronic adrenomedullin infusion ameliorates salt-sensitive hypertension and increases the expression of renal nitric oxide synthase (NOS) in Dahl salt-sensitive (DS) rats, because the reduced renal NOS expression promotes salt sensitivity. DS rats and Dahl salt-resistant (DR) rats were fed a high sodium diet (8.0% NaCl) for 3 weeks. The high sodium diet resulted in an increase in blood pressure and a reduction of urinary sodium excretion in association with increased renal adrenomedullin concentrations and decreased expression of renal neuronal NOS (nNOS) and renal medullary endothelial NOS (eNOS) in DS rats compared with DR rats. Chronic adrenomedullin infusion partly inhibited the increase of blood pressure and proteinuria in association with a restoration of renal nNOS and medullary eNOS expression in DS rats under the high sodium diet. The immunohistochemical analysis revealed that the restored renal nNOS expression induced by chronic adrenomedullin infusion may reflect the restoration of nNOS expression in the macula densa and inner medullary collecting duct. These results suggest that adrenomedullin infusion has beneficial effects on this hypertension probably in part through restored renal NOS expression in DS rats.

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.

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.

Mechanisms of salt-sensitive hypertension: role of renal medullary inducible nitric oxide synthase

The goal of this study was to determine the role of renal medullary inducible nitric oxide synthase (iNOS) in the arterial pressure, renal hemodynamic, and renal excretory changes that occur in Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats during high Na intake. Forty R and S rats, equipped with indwelling arterial, venous, and renal medullary catheters, were subjected to high (8%) Na intake, and selective iNOS inhibition was achieved with continuous intravenous or renal medullary interstitial infusion of aminoguanidine (AG; 3.075 mg. kg(-1). h(-1)). After 5 days of AG, mean arterial pressure increased to 132 +/- 2% control in the S rats with high Na intake and intramedullary AG compared with 121 +/- 4% control (P < 0.05) in the S rats with high Na intake alone and 121 +/- 2% control (P < 0.05) in the S rats with high Na intake and intravenous AG. AG did not change arterial pressure in R rats. AG also caused little change in renal hemodynamics, urinary Na, or H(2)O excretion or ACh-induced aortic vasorelaxation in R or S rats. The data suggest that during high Na intake, nitric oxide produced by renal medullary iNOS helps to prevent excessive increases in arterial pressure in the Dahl S rat but not the R rat.

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.

Magnesium influx enhanced by nitric oxide in hypertensive rat proximal tubule cells

An abnormal handling of renal magnesium has been suggested to cause salt-sensitive hypertension. The filtered magnesium is first reabsorbed in the proximal tubule. Amiloride has been shown to enhance renal magnesium conservation, but the regulatory mechanisms are unknown yet. High-salt (8% NaCl) diet decreased serum magnesium concentration, while increased urinary magnesium in Dahl salt-sensitive (DS) rat. Furthermore, the expression of nitric oxide synthase type 3 and nitric oxide (NO) content were decreased in high-salt loaded DS rat. In isolated proximal tubule cells, amiloride (0.1 mM) increased intracellular free magnesium concentration ([Mg(2+)](i)). However, the net [Mg(2+)](i) increase in the high-salt loaded DS rat was smaller than other groups. NOR1 (0.1 mM), a NO donor, restored the increase of [Mg(2+)](i) to the same level of other groups. On the contrary, L-NMMA (0.1 mM), an inhibitor of NO production, inhibited the increase of [Mg(2+)](i) in all groups. These results suggest that intracellular NO has an important role to up-regulate amiloride-elicited magnesium influx.

Nitric-oxide-mediated relaxations in salt-induced hypertension: effect of chronic beta1 -selective receptor blockade

BACKGROUND

Nebivolol is a new beta1-selective adrenergic receptor antagonist with a direct vasorelaxant effect that involves activation of the l-arginine-nitric oxide (NO) pathway. Therefore, treatment with nebivolol may protect against endothelial injury in hypertension.

OBJECTIVE

To investigate whether chronic selective beta1-blockade with nebivolol could prevent endothelial dysfunction in salt-induced hypertension, and to compare it with atenolol.

METHODS

Dahl salt-sensitive rats were treated for 8 weeks with standard chow or chow containing 4% NaCl alone or in combination with nebivolol (10 mg/kg per day) or atenolol (100 mg/kg per day). Isometric tension was continuously recorded in isolated aorta and small mesenteric arteries. Constitutive NO synthase (cNOS) activity was determined by [3H]citrulline assay.

RESULTS

Chronic salt administration increased systolic blood pressure by 38 +/- 5 mmHg in salt-treated rats as compared with that in control rats. Both nebivolol and atenolol prevented a salt-induced increase in pressure. cNOS activity was significantly decreased by a high-salt diet. The impairment of endothelium-dependent relaxations in response to acetylcholine in salt-treated rats was prevented only by nebivolol, in both large and small arteries. In contrast, the reduced endothelium-independent relaxations and contractions in response to sodium nitroprusside and endothelin-1, respectively, were restored by both drugs. Nebivolol, but not atenolol, restored cNOS activity.

CONCLUSIONS

Despite nebivolol and atenolol having the same blood-pressure-decreasing effect, only nebivolol was able to prevent endothelial dysfunction. This study demonstrates for the first time that the acute NO-mediated vasodilatory action of nebivolol is also present during chronic treatment. Hence, nebivolol might become a new therapeutic tool with which to exert vascular protective effects against end-organ damage in conditions associated with NO deficiency.

Inhibition of inducible nitric oxide synthase during high dietary salt intake

BACKGROUND

Previous studies indicate that nitric oxide (NO) is involved in the regulation of blood pressure (BP) and natriuresis in response to high sodium intake. We investigated the role of inducible NO synthase (iNOS) in response to an increased salt intake.

METHODS

Conscious, chronically catheterized rats were exposed to a high-salt (6%) diet for 14 days while receiving vehicle or aminoguanidine ([AG]; 250 mg/kg/24 h), which selectively inhibits iNOS. A group of rats on normal salt intake + AG were also studied.

RESULTS

Aminoguanidine had no impact on BP (120 +/- 2 v 116 +/- 1 mm Hg, control v day 14) or 24-h urinary nitrite and nitrate excretion (UNOxV), in rats on normal salt but prevented lipopolysaccharide-induced hypotension. High salt alone had no impact on BP (120 +/- 1 v 121 +/- 1 mm Hg), whereas UNaV (1.3 +/- 0.2 v 3.5 +/- 0.6 microeq/min, P < .001) and UNOxV increased with high salt intake. The natriuretic response persisted (1.5 +/- 0.2 v 4.3 +/- 0.8 microeq/min, P < .005), but the increase in UNOXV was prevented with chronic AG although BP fell slightly (121 +/- 1 v 115 +/- 1 mm Hg, P < .05). There was no change in plasma volume with high salt, and 24-h UNaV increased appropriately in the presence of AG. The in vitro NOS activity was not increased in kidney homogenates by high salt diet, nor was it affected by chronic AG treatment.

CONCLUSION

We conclude that NO from an iNOS source is not essential for the regulation of sodium excretion and BP in the presence of a high-salt diet in a normal rat.

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

BACKGROUND

Several indications exist to suggest that an impaired production of nitric oxide might have a role in the development of salt-sensitive hypertension.

OBJECTIVE

To examine whether the gene expression of the nitric oxide synthases (NOS) is altered in the salt-sensitive Dahl rat compared with that in the salt-resistant Dahl rat.

DESIGN AND METHODS

The abundance of NOS mRNA was measured by RNase protection assay in different organs of salt-resistant and salt-sensitive Dahl rats. In addition, the zonal expression of NOS genes in the kidney under salt load and salt restriction was determined.

RESULTS

The abundance of endothelial NOS mRNA was similar between the salt-resistant and salt-sensitive Dahl rat strains in all organs. Inducible NOS mRNA was not detectable by RNase protection assay in any organ. Neuronal NOS (nNOS) mRNA expression, however, was about 50% lower in brain and kidney of salt-sensitive Dahl rats than in salt-resistant Dahl rats. Within the kidney, nNOS mRNA levels were significantly decreased in salt-sensitive Dahl rats compared with those in salt-resistant Dahl rats, in cortex, outer and inner medulla (50, 40 and 30%, respectively) under all dietary conditions. A comparison of renal nNOS gene expression in Dahl rats with that in salt-insensitive Sprague- Dawley rats revealed that the abundance of renal nNOS was similar in salt-sensitive Dahl and Sprague-Dawley rats, but was increased in salt-resistant Dahl rats relative to that in Sprague-Dawley rats.

CONCLUSION

These data suggest that nNOS gene expression is increased in salt-resistant Dahl rats compared with that in salt-sensitive Dahl rats. This increased nNOS expression of the salt-resistant Dahl strain might play a part in compensating for a defect of renal salt excretion in the Dahl strains.

Decreased renal expression of nitric oxide synthase isoforms in adrenocorticotropin-induced and corticosterone-induced hypertension

Administration of adrenocorticotropic hormone (ACTH) leads to the development of hypertension. Because glucocorticoids can affect the nitric oxide system at several sites, the present study tested the hypothesis that nitric oxide synthase (NOS) expression may be altered in ACTH-induced and corticosterone-induced hypertension in the rat. This was addressed by measuring Nos1, Nos2, and Nos3 mRNA in the kidney, adrenal gland, heart, and hypothalamus of 16 ACTH-treated and 16 vehicle-treated rats as well as in 10 corticosterone-treated and 10 control rats. In addition, in situ hybridization and immunohistochemistry were used to confirm changes by detection of Nos in RNA and NOS protein in tissues. Systolic blood pressure of ACTH and corticosterone rats was elevated (165+/-6 and 162+/-11 mm Hg; P<0.001 versus control). Each Nos isoform mRNA was measured by reverse transcriptase-polymerase chain reaction technique. In ACTH rats, mRNA for Nos2 was reduced in renal cortex by 58+/-5% and in medulla by 68+/-7%; for Nos3, mRNA reductions of 59+/-6% and 51+/-11% were seen (P<0.001 after Hochberg correction for multiple comparisons). In corticosterone rats, Nos2 mRNA decreased in cortex by 68+/-5% and in medulla by 62+/-6%; Nos3 mRNA by 50+/-8% in cortex, and Nos1 by 29+/-7% in medulla (all P<0.001 after Hochberg correction). Reductions seen in kidney were supported by in situ hybridization and immunohistochemistry. Apart from a 62+/-2% decrease in Nos2 mRNA in adrenal of ACTH rats (corrected P<0.05), no significant changes were seen in the other nonrenal tissues for any isoform. In conclusion, we have shown for the first time that the physiological components of glucocorticoid action (ACTH and corticosterone) when given chronically in vivo reduce Nos2 and Nos3 expression in the kidney. Such changes are consistent with a role in hypertension for ACTH and corticosterone.

Mechanisms of salt-sensitive hypertension: role of inducible nitric oxide synthase

The goal of this study was to determine the role of inducible nitric oxide synthase (iNOS) in the arterial pressure, renal hemodynamic, renal excretory, and hormonal changes that occur in Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats during changes in Na intake. Thirty-two R and S rats, equipped with indwelling arterial and venous catheters, were subjected to low (0.87 mmol/day) or high (20.6 mmol/day) Na intake, and selective iNOS inhibition was achieved with intravenous aminoguanidine (AG, 12.3 mg. kg(-1). h(-1)). After 5 days of AG, mean arterial pressure increased to 121 +/- 3% control in the R-high Na AG rats compared with 98 +/- 1% control (P < 0.05) in the R-high Na alone rats, and S-high Na rats increased their arterial pressure to 123 +/- 3% control compared with 110 +/- 2% control (P < 0.05) in S-high Na alone rats. AG caused no significant changes in renal hemodynamics, urinary Na or H(2)O excretion, plasma renin activity, or cerebellar Ca-dependent NOS activity. The data suggest that nitric oxide produced by iNOS normally helps to prevent salt-sensitive hypertension in the Dahl R rat and decreases salt sensitivity in the Dahl S rat.

Tubulointerstitial injury and loss of nitric oxide synthases parallel the development of hypertension in the Dahl-SS rat

OBJECTIVE

Alterations in renal nitric oxide (NO) are involved in the hypertension of the Dahl salt-sensitive (Dahl-SS) rat We sought to identify the kinetics and sites of expression of the major NO synthase (NOS) isoforms.

DESIGN

The renal expression of the major NOS were examined in Dahl-SS and salt-resistant rats (Dahl-SR) while on a low salt (0.1% NaCl) diet at 3 and 9 weeks of age.

METHODS

Renal biopsies from Dahl-SS and Dahl-SR rats were compared for evidence of renal injury and for alterations in expression of the NOS enzymes by quantitative immunohistochemistry.

RESULTS

At 3 weeks of age Dahl-SS and Dahl-SR rats have normal renal histology and similar immunohistochemical expression of NOS1, -2, and -3. At 9 weeks Dahl-SS rats had significantly higher blood pressure than Dahl-SR rats (P< 0.005 ), and lower macula densa NOS1 (P< 0.05) and cortical and medullary NOS3 (P< 0.05). NOS2 was reduced in cortical tubules in biopsies showing severe tubulointerstitial damage, but was not significantly different between Dahl-SS and Dahl-SR groups as a whole. Dahl-SS rats also manifested glomerular and tubulointerstitial injury. Tubular expression of osteopontin (OPN), which is an inhibitor of NOS2, correlated with the systolic BP in individual Dahl-SS rats (r2 = 0.80, P < 0.0001 ).

CONCLUSION

Tubulointerstitial injury and the loss of NOS occur after birth and parallel the development of hypertension. We suggest that the structural and functional changes that occur with renal injury in the Dahl-SS rat may contribute to the development of hypertension.

Renal cyclic 3',5'-guanosine monophosphate and sodium excretion in Dahl salt-resistant and Dahl salt-sensitive rats: comparison of the roles of bradykinin and nitric oxide

OBJECTIVE

The purpose of this study was to determine the relative importance of bradykinin and nitric oxide (NO) in mediating renal responses to altered sodium intake in Dahl salt-resistant (Dahl-SR) and salt-sensitive (Dahl-SS) rats.

DESIGN AND METHODS

Dahl-SR and Dahl-SS rats consumed a diet containing 0.15% (low) or 4.0% (high) sodium chloride for 10 days. A microdialysis technique was then used to measure renal cortical interstitial fluid (RIF) cyclic 3',5'-guanosine monophosphate (cGMP) production in anesthetized rats, under baseline conditions and during acute cortical infusion of either the bradykinin B2 receptor antagonist icatibant or the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME). Urine sodium excretion was monitored simultaneously by ureter cannulation. Results Baseline sodium excretion was similar in the two types of rats, but RIF cGMP was significantly elevated in Dahl-SR compared to Dahl-SS rats on both low and high sodium diets. Icatibant infusion significantly reduced both RIF cGMP and sodium excretion in Dahl-SR rats during low sodium intake, but had no effect in Dahl-SS rats on either diet L-NAME infusion significantly reduced sodium excretion in Dahl-SR and Dahl-SS rats, during both low and high sodium intake. L-NAME infusion caused a significant reduction in RIF cGMP in Dahl-SR and Dahl-SS rats on low sodium diet, but reduced RIF cGMP only in Dahl-SR rats on high sodium diet. Conclusion These data suggest a potential role for cortical bradykinin, but not NO, in mediating the differences in the renal response to low sodium intake between Dahl-SR and Dahl-SS rats.

Overexpression of chloride channel CLC-K2 mRNA in the renal medulla of Dahl salt-sensitive rats

OBJECTIVE

The present study aimed to characterize the influence of salt intake on the gene expression of the kidney specific chloride channels CLC-K1 and CLC-K2 in the kidneys of salt-resistant and salt-sensitive Dahl rats.

DESIGN

For this purpose Dahl salt-resistant (Dahl-R) and Dahl salt-sensitive rats (Dahl-S) were fed a low (0.02%), normal (0.6%) or high (4%) salt diet for 19 days and CLC-K1 and -K2 mRNA expression was semiquantitated in cortex, outer and inner medulla.

METHODS

Kidneys were macroscopically dissected, total RNA was isolated according to the guanidinium-thiocyanate-phenol-chloroform method and messenger RNAs for the kidney specific chloride channels CLC-K1 and CLC-K2 were measured by ribonuclease protection assay.

RESULTS

Systolic blood pressure in high salt-treated Dahl-S rats increased to 204 +/- 5 mmHg versus 150 +/- 7 mmHg in Dahl-S controls. Dahl R and low salt Dahl-S rats showed no increase in blood pressure. For CLC-K1 mRNA we found an order of abundance inner medulla >> outer medulla >> cortex. There was no difference in mRNA abundance between Dahl-R and -S, nor any effect of the rate of salt intake on CLC-K1 mRNA abundance in the different kidney zones. CLC-K2 mRNA expression in cortex and outer medulla was similar between Dahl-R and -S rats. In the inner medulla, however, CLC-K2 mRNA was 1.7-fold higher in Dahl-S than in Dahl-R rats. In the cortex we found no influence of salt intake on CLC-K2 mRNA. In outer and inner medulla of Dahl-R rats and Dahl-S rats high salt diet led to a marked downregulation of CLC-K2 mRNA expression. Consequently, CLC-K2 gene expression in the inner medulla was 2.2-fold higher in Dahl-S than in Dahl-R rats in states of high salt diet.

CONCLUSION

Given that the CLC-K2 chloride channel in the outer and inner medulla contributes to salt reabsorption, our findings would suggest that Dahl-S rats have an increased medullary salt reabsorption. This may contribute to the inability of these animals to excrete an increased salt load at a normal renal perfusion pressure leading to the development of hypertension.

Dysfunctional renal nitric oxide synthase as a determinant of salt-sensitive hypertension: mechanisms of renal artery endothelial dysfunction and role of endothelin for vascular hypertrophy and Glomerulosclerosis

This study investigated the role of renal nitric oxide synthase (NOS), endothelin, and possible mechanisms of renovascular dysfunction in salt-sensitive hypertension. Salt-sensitive (DS) and salt-resistant (DR) Dahl rats were treated for 8 wk with high salt diet (4% NaCl) alone or in combination with the ET(A) receptor antagonist LU135252 (60 mg/kg per d). Salt loading markedly increased NOS activity (pmol citrulline/mg protein per min) in renal cortex and medulla in DR but not in DS rats by 270 and 246%, respectively. Hypertension in DS rats was associated with renal artery hypertrophy, increased vascular and renal endothelin-1 (ET-1) protein content, and glomerulosclerosis. In the renal artery but not in the aorta of hypertensive DS rats, endothelium-dependent relaxation to acetylcholine was unchanged; however, endothelial dysfunction due to enhanced prostanoid-mediated, endothelium-dependent contractions and attenuation of basal nitric oxide release was present. Treatment with LU135252 reduced hypertension in part, but completely prevented activation of tissue ET-1 without affecting ET-3 levels. This was associated with a slight increase of renal NOS activity, normalization of endothelial dysfunction and renal artery hypertrophy, and marked attenuation of glomerulosclerosis. Thus, DS rats fail to increase NOS activity in response to salt loading. This abnormality may predispose to activation of the tissue ET-1 system, abnormal renal vasoconstriction, and renal injury. Chronic ET(A) receptor blockade normalized salt-induced changes in the renal artery and reduced glomerular injury, suggesting therapeutic potential for ET antagonists in salt-sensitive forms of hypertension.

Potassium supplementation increases sodium excretion and nitric oxide production in hypertensive Dahl rats

The present study was designed to investigate whether antihypertensive and natriuretic effects of K were achieved by elevation of nitric oxide (NO) production in Dahl salt-sensitive (DS) rats. The rats were placed in individual metabolic cage and fed a high sodium diet with or without K supplementation for 4 weeks. K supplementation counteracted the blood-pressure raising effect of NaCl. K supplementation significantly enhanced sodium excretion and reduced sodium retention, increased the urinary nitrite plus nitrate excretion and kidney constitutive NO synthase activity in salt-loaded DS rats. These effect did not occur in the rats fed a low sodium diet with K supplementation. These results suggest that K supplementation attenuates development of hypertension with reduction of sodium retention in salt-loaded DS rats, which is mediated by the recovery of salt-induced NO production mechanism.

Inhibitory effect of nitric oxide on the renin-angiotensin system in Dahl salt-sensitive rats

1. To explore the role of nitric oxide (NO) in the regulation of the renin-angiotensin system (RAS) in Dahl salt-sensitive (DS) rats, the effects of NG-nitro-L-arginine methyl ester (L-NAME) on plasma renin activity (PRA), and concentrations of angiotensin (Ang)I and AngII in the plasma, aorta and kidney were investigated in DS and Dahl salt-resistant (DR) rats. 2. NG-Nitro-L-arginine methyl ester (12-18 mg/kg per day) administration for 1 week increased mean arterial pressure (MAP) in DS and DR rats fed a 0.3% NaCl diet and in DR rats fed an 8% NaCl diet compared with corresponding vehicle (water)-treated groups. However, L-NAME administration did not change MAP in DS rats fed an 8% NaCl diet. 3. NG-Nitro-L-arginine methyl ester administration increased PRA in DS rats fed an 8% NaCl diet, but not in DR rats fed an 8% NaCl diet. NG-Nitro-L-arginine methyl ester administration increased AngI and AngII concentrations in plasma, aorta and kidney only in DS rats fed an 8% NaCl diet. The ratio of AngI to AngII did not change following L-NAME administration in any rats. 4. These results suggest that NO has an inhibitory role on renin release in DS rats fed a high-salt diet.

Role of nitric oxide in the control of renal function and salt sensitivity

Nitric oxide (NO) plays critical roles in the control of renal and glomerular hemodynamics, tubuloglomerular feedback response, release of renin and sympathetic transmitters, tubular ion transport, and renal water and sodium excretion. This paper explores the importance of NO in the control of renal water and sodium excretion and in the long-term control of arterial blood pressure. Synthesis of NO, characteristics of NO tissue redox forms, NO synthase activity, and NO synthase isoforms in the kidney are reviewed. To define the role of NO as a natriuretic and antihypertensive factor, the most supportive evidence is summarized, and some contradictory results are also noted. Given the evidence that high salt intake results in high NO concentrations and great NO synthase expression and activity selectively in the renal medulla of the kidney, as well as evidence of a deficiency of the NO synthase activity in Dahl salt-sensitive rats confined in the renal medulla, this report emphasizes the mechanisms by which the renal medullary l-arginine/NO system controls sodium excretion and arterial blood pressure. Other mechanisms for the action of NO on sodium homeostasis such as the action on glomerular filtration rate and the direct effect on tubules are also discussed. We conclude that there is strong evidence that under physiologic conditions, NO plays an important role in the regulation of renal blood flow to the renal medulla and in the tubular regulation of sodium excretion. It is thereby involved in the long-term control of arterial blood pressure, and inhibition or deficiency of NO synthase may result in a sustained hypertension.

Isoform-specific regulation of nitric oxide synthase mRNA in the kidney by sodium and blood pressure

BACKGROUND

The roles of nitric oxide synthases (NOS) in kidney function are still controversial, principally due to the lack of isoform-specific inhibitors of NOS.

OBJECTIVE

To investigate the relative roles of each isoform of NOS in regulation of sodium and volume homeostasis.

DESIGN

We studied the effects of long-term modifications of sodium diet and blood pressure on expression of NOS mRNA in the renal cortex, where the three isoforms of NOS are present.

METHODS

We used quantitative reverse-transcription-polymerase chain reaction assays specific to each isoform of NOS to determine amounts of their respective mRNA in control rats, deoxycorticosterone acetate (DOCA)-salt hypertensive rats, rats fed a high-salt diet, and furosemide-treated rats fed a low-sodium diet. Nicotinamide adenine nucleotide phosphate H (NADPH) diaphorase staining was performed on DOCA-salt and control rat kidneys.

RESULTS

Levels of NOS I mRNA in DOCA-salt rats were decreased by treatment, those in low-salt-diet rats remained unaffected and those in high-salt diet rats tended to be intermediate between those of the other rat groups. Expression of NOS III mRNA was not significantly modified by either treatment Levels of NOS II mRNA in DOCA-salt rats were increased, those in high-salt-diet rats remained unaffected, and those in low-salt-diet were decreased by treatment, but these levels are more than 100-fold lower than those observed for the other isoforms of NOS. NADPH diaphorase staining in macula densa of DOCA-salt rats was markedly decreased compared with that in macula densa of control rats but staining in renal inflammatory and fibrous lesions became detectable, and staining in the vessels did not differ from that for control rats.

CONCLUSIONS

Our results show that intake of sodium and extracellular fluid volume regulate levels of mRNA of the three NOS isoforms in the renal cortex differently, suggesting that each of them plays a specific role.

Is the nitric oxide system involved in genetic hypertension in Dahl rats?

To address this issue, a series of genetic tests were carried out. Linkage studies showed that the inducible nitric oxide synthase (Nos2) locus cosegregated with blood pressure in three F2 populations originated from crosses of Dahl salt-sensitive (S) rats with rats of various normotensive strains. However, the brain nitric oxidase synthase (Nos1) and endothelial nitric oxide synthase (Nos3) loci did not cosegregate with blood pressure in five F2 populations. Thus, only Nos2, but not Nos1 and Nos3, was considered as a candidate gene for being a quantitative trait locus (QTL) for blood pressure in the S rat. To further test this hypothesis, congenic strains were constructed by substituting regions on Chromosome 10 of the S rat with the homologous regions of the Milan normotensive (MNS) rat. Results showed that the chromosome region including Nos2 did not contain a blood pressure QTL. In consequence, Nos2 per se is not supported as a candidate QTL capable of causing a blood pressure difference between the S and MNS rats. Nevertheless, the nitric oxide system appears to be involved secondarily in the regulation of blood pressure in the S rat, as evidenced by physiological data.

The role of nitric oxide in angiotensin II-induced renal vasoconstriction in renovascular hypertension

OBJECTIVE

To evaluate the contribution of nitric oxide to the regulation of angiotensin II-induced renal vasoconstriction in normotensive rats and in rats with aortic coarctation-induced hypertension.

METHODS

We evaluated the renal vascular reactivity of nonischemic kidney to angiotensin II with and without nitric oxide synthesis inhibitor (NG-nitro-L-arginine methyl ester) in the isolated perfused kidney. The nitrite concentration in renal perfusate of nonischemic kidney was measured as an index of nitric oxide released and the activity of nitric oxide synthase in renal tissue was determined by production of [3H]-L-citrulline.

RESULTS

The perfusion of NG-nitro-L-arginine methyl ester potentiated angiotensin II-induced renal vasoconstriction in normotensive rats but had no effect on hypertensive rats. The release of nitrites in kidneys from hypertensive rats was lower than that in kidneys from normotensive rats. The activity of renal nitric oxide synthase was less in the hypertensive rats than it was in the normotensive rats.

CONCLUSIONS

Nitric oxide counteracts the vasoconstrictor effect of angiotensin II in normotensive rats, whereas this protective mechanism is impaired in hypertensive rats. This impairment potentiates effect of angiotensin II on vascular resistance, thereby contributing to the development of high blood pressure.

Nitric oxide synthase and renin-angiotensin system gene expression in salt-sensitive and salt-resistant Sabra rats

The molecular mechanisms of salt sensitivity and the contribution of the kidney to salt-induced hypertension in Sabra rats are imperfectly defined. We investigated the expression of the nitric oxide (NO) system (endothelial, inducible, and neural NO synthases) and renin-angiotensin system (renin, angiotensinogen, and angiotensin II type 1A receptor) gene components in the kidneys of SBN/y (salt-resistant) and SBH/y (salt-sensitive) Sabra rat substrains, with and without deoxycorticosterone acetate (DOCA)-salt treatment. We also looked for immunocytochemical evidence of angiotensin II, the effector peptide of the renin-angiotensin system. Inducible and neural NO synthase gene expression values were lower in SBH/y than in SBN/y before and after DOCA-salt treatment. The gene expression level of endothelial NO synthase was not different in SBH/y and SBN/y, either with or without DOCA salt. Renin gene expression was significantly higher in kidneys of SBN/y than in kidneys of SBH/y rats, whereas angiotensinogen gene expression was significantly lower in SBN/y. After DOCA-salt treatment, renin gene expression was strongly suppressed in both strains but more so in SBH/y. Angiotensinogen gene expression, on the other hand, was increased by DOCA salt in SBN/y rats so that the two strains were no longer different. Angiotensin II immunoreactivity was significantly higher in SBN/y than in SBH/y; however, after DOCA salt, immunoreactivity in both strains was no longer detectable. Angiotensin II type 1A receptor gene expression was not different between the two strains, either before or after DOCA-salt administration. We conclude that DOCA salt induced a decrease in the activity of the renin-angiotensin system but did not change NO synthase gene expression in SBH/y and SBN/y. Inducible and neural NO synthase gene expression values were less in SBH/y than in SBN/y, independent of DOCA-salt administration. Thus, the NO system could explain, at least in part, the salt resistance of SBN/y.

Nitric oxide in renal health and disease

Nitric oxide (NO) is a labile radical gas that is widely acclaimed as one of the most important molecules in biology. Through covalent modifications of target proteins and redox reactions with oxygen and superoxide radical and transition metal prosthetic groups, NO plays a critical role in many vital biological processes, including the control of vascular tone, neurotransmission, ventilation, hormone secretion, inflammation, and immunity. Moreover, NO has been shown to influence a host of fundamental cellular functions, such as RNA synthesis, mitochondrial respiration, glycolysis, and iron metabolism. NO is formed from L-arginine by NO synthases (NOSs), a family of related enzymes encoded by separate unlinked genes. The different NOS isozymes exhibit disparate tissue and intrarenal distributions and are governed by unique regulatory mechanisms. In the kidney, NO participates in several vital processes, including the regulation of glomerular and medullary hemodynamics, the tubuloglomerular feedback response, renin release, and the extracellular fluid volume. While NO serves beneficial roles as a messenger and host defense molecule, excessive NO production can be cytotoxic, the result of NO's reaction with reactive oxygen and nitrogen species, leading to peroxynitrite anion formation, protein tyrosine nitration, and hydroxyl radical production. Indeed, NO may contribute to the evolution of several commonly encountered renal diseases, including immune-mediated glomerulonephritis, postischemic renal failure, radiocontrast nephropathy, obstructive nephropathy, and acute and chronic renal allograft rejection. Moreover, impaired NO production has been implicated in the pathogenesis of volume-dependent hypertension. This duality of NO's beneficial and detrimental effects has created extraordinary interest in this molecule and the need for a detailed understanding of NO biosynthesis.

Absence of linkage for "endothelial" nitric oxide synthase locus to blood pressure in Dahl rats

Nitric oxide is thought to be involved in blood pressure regulation. Nitric oxide synthase (NOS) genes are logical candidates for genetic hypertension. Of the three known forms of NOS, the "neuronal" and "inducible" Nos genes have been tested as candidate genes for causing inherited hypertension in Dahl salt-sensitive rats. In the present work, we analyzed the endothelial Nos gene, designated Nos3, directly and indirectly for cosegregation with blood pressure in six F2 populations independently generated from crosses of Dahl salt-sensitive rats with rats of various other strains. The Nos3 alleles did not cosegregate with blood pressure in these populations. Therefore, Nos3 is an improbable, if not impossible, candidate gene for causing hypertension in the Dahl salt-sensitive rat.