Arginine vasopressin modulates expression of neuronal NOS in rat renal medulla

Animal models of hypertension: The status of nitric oxide and oxidative stress and the role of the renal medulla

Hypertension significantly contributes to overall morbidity and mortality worldwide, and animal models of hypertension provide important tools to verify the physiological and molecular mechanisms underlying the development of the disease. A review of the most important models available would provide an insight into the appropriate targets to be addressed in the treatment of different forms of human hypertension. In the animal models discussed a special attention is given to the status and pathophysiological role of nitric oxide and its interaction with reactive oxygen species and oxidative stress. Another focus of the review are the processes running in the renal medulla which are still insufficiently explored. Deficient nitric oxide synthesis and its reduced bioavailability are important determinants of hypertension since NO is recognized as a major control factor of vascular tone homeostasis. For decades perfusion of the renal medulla has also been regarded as one of the blood pressure control factors and, noteworthily, the renal medulla belongs to the tissues with the highest NO content. The list of most often applied animal hypertension models reviewed here includes variants of salt-induced hypertension, the models with genetic background: such as spontaneously hypertensive rats (SHR) and Dahl salt sensitive (SS/SR) rats, Goldblatt 2K-1C hypertensive rats, and also the pharmacologically-plus-dietary salt-induced model known as DOCA-salt hypertension.

Tetramethylpyrazine alleviates diabetic nephropathy through the activation of Akt signalling pathway in rats

In the whole world, the principal cause of end-stage renal disease is diabetic nephropathy (DN), which is one of the most relentless complications of diabetes. However, there is a shortfall of compelling DN treatments and the mechanism potentially able to alleviate renal injury remains ambiguous. In this experiment, we estimated the preventive actions of tetramethylpyrazine (TMP) on DN in rats and further investigated the underlying mechanism. The different doses of TMP (100 mg/kg, 150 mg/kg and 200 mg/kg) were orally given each day for 8 weeks in streptozotocin (STZ) - nicotinamide (NCT) - induced type-2 diabetic (T2D) rats. The metabolic parameters of diabetes, blood urea nitrogen (BUN), serum creatinine (SCR), urinary protein and oxidative stress parameters were assessed. Microstructural changes in kidney were observed, and the expression of Akt signalling pathway proteins was measured by western blotting. TMP administration in T2D rats improved diabetic condition, as demonstrated by significant (P < 0.05) increase of body weight and fasting serum insulin (FSI) level, reduction of fasting blood glucose (FBG) and glycosylated haemoglobin (HbA1c) level and regulation of lipid profile and oral glucose tolerance in a dose-dependent manner. TMP treatment also reduced BUN, SCR, urinary protein and oxidative stress and prevented renal injury in diabetic rats. TMP activated Akt signalling pathway, increased the levels of p-Akt and Bcl-2, and diminished the expressions of p-GSK-3β, Bax and cleaved caspase-3. In conclusion, TMP ameliorates diabetic nephropathy in T2D rats by initiating the Akt signalling, improving the metabolic markers of diabetes and suppressing oxidative stress.

Collecting duct-specific knockout of nitric oxide synthase 3 impairs water excretion in a sex-dependent manner

Nitric oxide (NO) inhibits collecting duct (CD) Na⁺ and water reabsorption. Mice with CD-specific knockout (KO) of NO synthase 1 (NOS1) have salt-sensitive hypertension. In contrast, the role of NOS3 in CD salt and water reabsorption is unknown. Mice with CD NOS3 KO were generated with loxP-flanked exons 9-12 (encodes the calmodulin binding site) of the NOS3 gene and the aquaporin-2 promoter-Cre transgene. There were no differences between control and CD NOS3 KO mice, irrespective of sex, in food intake, water intake, urine volume, urinary Na⁺ or K⁺ excretion, plasma renin concentration, blood pressure, or pulse during 7 days of normal (0.3%), high (3.17%), or low (0.03%) Na⁺ intake. Blood pressure was similar between genotypes during DOCA-high salt. CD NOS3 KO did not alter urine volume or urine osmolality after water deprivation. In contrast, CD NOS3 KO male, but not female, mice had lower urine volume and higher urine osmolality over the course of 7 days of water loading compared with control mice. Male, but not female, CD NOS3 KO mice had reduced urinary nitrite+nitrate excretion compared with controls after 7 days of water loading. Urine AVP and AVP-stimulated cAMP accumulation in isolated inner medullary CD were similar between genotypes. Western analysis did not reveal a significant effect of CD NOS3 KO on renal aquaporin expression. In summary, these data suggest that CD NOS3 may be involved in the diuretic response to a water load in a sex-specific manner; the mechanism of this effect remains to be determined.

Altered Nitric Oxide System in Cardiovascular and Renal Diseases

Nitric oxide (NO) is synthesized by a family of NO synthases (NOS), including neuronal, inducible, and endothelial NOS (n/i/eNOS). NO-mediated effects can be beneficial or harmful depending on the specific risk factors affecting the disease. In hypertension, the vascular relaxation response to acetylcholine is blunted, and that to direct NO donors is maintained. A reduction in the activity of eNOS is mainly responsible for the elevation of blood pressure, and an abnormal expression of iNOS is likely to be related to the progression of vascular dysfunction. While eNOS/nNOS-derived NO is protective against the development of atherosclerosis, iNOS-derived NO may be proatherogenic. eNOS-derived NO may prevent the progression of myocardial infarction. Myocardial ischemia/reperfusion injury is significantly enhanced in eNOS-deficient animals. An important component of heart failure is the loss of coronary vascular eNOS activity. A pressure-overload may cause severer left ventricular hypertrophy and dysfunction in eNOS null mice than in wild-type mice. iNOS-derived NO has detrimental effects on the myocardium. NO plays an important role in regulating the angiogenesis and slowing the interstitial fibrosis of the obstructed kidney. In unilateral ureteral obstruction, the expression of eNOS was decreased in the affected kidney. In triply n/i/eNOS null mice, nephrogenic diabetes insipidus developed along with reduced aquaporin-2 abundance. In chronic kidney disease model of subtotal-nephrectomized rats, treatment with NOS inhibitors decreased systemic NO production and induced left ventricular systolic dysfunction (renocardiac syndrome).

The role of cullin 5-containing ubiquitin ligases

The suppressor of cytokine signaling (SOCS) box consists of the BC box and the cullin 5 (Cul5) box, which interact with Elongin BC and Cul5, respectively. SOCS box-containing proteins have ubiquitin ligase activity mediated by the formation of a complex with the scaffold protein Cul5 and the RING domain protein Rbx2, and are thereby members of the cullin RING ligase superfamily. Cul5-type ubiquitin ligases have a variety of substrates that are targeted for polyubiquitination and proteasomal degradation. Here, we review the current knowledge on the identification of Cul5 and the regulation of its expression, as well as the signaling pathways regulated by Cul5 and how viruses highjack the Cul5 system to overcome antiviral responses.

Increase in the expression of inducible nitric oxide synthase on exposure to bisphenol A: a possible cause for decline in steroidogenesis in male mice

Bisphenol A (BPA) is a well-known plasticizer and xenoestrogen that is responsible for many acquired reproductive difficulties, especially in men. Despite the prevalence of BPA in society, the mechanism behind reproductive deficits remains elusive. The present study investigates the mode of BPA's action by evaluating its effect on the expression of inducible nitric oxide synthase (iNOS) and steriodogenic acute regulatoryprotein (StAR) in male mice testis. Swiss albino mice were treated with a range BPA concentrations of 0.5, 50 and 100μg/kg body weight/day intraperitoneally for 60 days. Several markers of oxidative stress and male fertility were investigated. Nitrite levels, malondialdehyde levels and testicular injury scores were elevated whereas the sperm count, serum testosterone levels and catalase activity were reduced in the BPA groups. Mechanistically, an increase in iNOS expression was observed in the testis whereas the expression of the StAR was down regulated in the BPA treated mouse. These results suggest that BPA induces oxidative stress by altering the expression of iNOS, which consequently leads to the down regulation of StAR expression in the testis of male mouse.

Withania somnifera alleviates parkinsonian phenotypes by inhibiting apoptotic pathways in dopaminergic neurons

Maneb (MB) and paraquat (PQ) are environmental toxins that have been experimentally used to induce selective damage of dopaminergic neurons leading to the development of Parkinson's disease (PD). Although the mechanism of this selective neuronal toxicity in not fully understood, oxidative stress has been linked to the pathogenesis of PD. The present study investigates the mechanisms of neuroprotection elicited by Withania somnifera (Ws), a herb traditionally recognized by the Indian system of medicine, Ayurveda. An ethanolic root extract of Ws was co-treated with the MB-PQ induced mouse model of PD and was shown to significantly rescue canonical indicators of PD including compromised locomotor activity, reduced dopamine in the substantia nigra and various aspects of oxidative damage. In particular, Ws reduced the expression of iNOS, a measure of oxidative stress. Ws also significantly improved the MB + PQ mediated induction of a pro-apoptotic state by reducing Bax and inducing Bcl-2 protein expression, respectively. Finally, Ws reduced expression of the pro-inflammatory marker of astrocyte activation, GFAP. Altogether, the present study suggests that Ws treatment provides nigrostriatal dopaminergic neuroprotection against MB-PQ induced Parkinsonism by the modulation of oxidative stress and apoptotic machinery possibly accounting for the behavioural effects.

Renal response to L-arginine in diabetic rats. A possible link between nitric oxide system and aquaporin-2

The aim of this study was to evaluate whether L-Arginine (L-Arg) supplementation modifies nitric oxide (NO) system and consequently aquaporin-2 (AQP2) expression in the renal outer medulla of streptozotocin-diabetic rats at an early time point after induction of diabetes. Male Wistar rats were divided in four groups: Control, Diabetic, Diabetic treated with L-Arginine and Control treated with L-Arginine. Nitric oxide synthase (NOS) activity was estimated by [¹⁴C] L-citrulline production in homogenates of the renal outer medulla and by NADPH-diaphorase staining in renal outer medullary tubules. Western blot was used to detect the expression of AQP2 and NOS types I and III; real time PCR was used to quantify AQP2 mRNA. The expression of both NOS isoforms, NOS I and NOS III, was decreased in the renal outer medulla of diabetic rats and L-Arg failed to prevent these decreases. However, L-Arg improved NO production, NADPH-diaphorase activity in collecting ducts and other tubular structures, and NOS activity in renal homogenates from diabetic rats. AQP2 protein and mRNA were decreased in the renal outer medulla of diabetic rats and L-Arg administration prevented these decreases. These results suggest that the decreased NOS activity in collecting ducts of the renal outer medulla may cause, at least in part, the decreased expression of AQP2 in this model of diabetes and constitute additional evidence supporting a role for NO in contributing to renal water reabsorption through the modulation of AQP2 expression in this pathological condition. However, we cannot discard that another pathway different from NOS also exists that links L-Arg to AQP2 expression.

Chronic activation of vasopressin V2 receptor signalling lowers renal medullary oxygen levels in rats

AIM

In the present study, we aimed to elucidate the effects of chronic vasopressin administration on renal medullary oxygen levels.

METHODS

Adult Sprague Dawley or vasopressin-deficient Brattleboro rats were treated with the vasopressin V2 receptor agonist, desmopressin (5 ng/h; 3d), or its vehicle via osmotic minipumps. Immunostaining for pimonidazole and the transcription factor HIF-1α (hypoxia-inducible factor-1α) were used to identify hypoxic areas. Activation of HIF-target gene expression following desmopressin treatment was studied by microarray analysis.

RESULTS

Pimonidazole staining was detected in the outer and inner medulla of desmopressin-treated rats, whereas staining in control animals was weak or absent. HIF-1α immunostaining demonstrated nuclear accumulation in the papilla of desmopressin-treated animals, whereas no staining was observed in the controls. Gene expression analysis revealed significant enrichment of HIF-target genes in the group of desmopressin-regulated gene products (P = 2.6*10(-21) ). Regulated products included insulin-like growth factor binding proteins 1 and 3, angiopoietin 2, fibronectin, cathepsin D, hexokinase 2 and cyclooxygenase 2.

CONCLUSION

Our results demonstrate that an activation of the renal urine concentrating mechanism by desmopressin causes renal medullary hypoxia and an upregulation of hypoxia-inducible gene expression.

mPGES-1-derived PGE2 mediates dehydration natriuresis

PGE(2) is a natriuretic factor whose production is elevated after water deprivation (WD) but its role in dehydration natriuresis is not well-defined. The goal of the present study was to investigate the role of microsomal prostaglandin E synthase-1 (mPGES-1) in dehydration natriuresis. After 24-h WD, wild-type (WT) mice exhibited a significant increase in 24-h urinary Na(+) excretion accompanied with normal plasma Na(+) concentration and osmolality. In contrast, WD-induced elevation of urinary Na(+) excretion was completely abolished in mPGES-1 knockout (KO) mice in parallel with increased plasma Na(+) concentration and a trend increase in plasma osmolality. WD induced a 1.8-fold increase in urinary PGE(2) output and a 1.6-fold increase in PGE(2) content in the renal medulla of WT mice, both of which were completely abolished by mPGES-1 deletion. Similar patterns of changes were observed for urinary nitrate/nitrite and cGMP. The natriuresis in dehydrated WT mice was associated with a significant downregulation of renal medullary epithelial Na channel-α mRNA and protein, contrasting to unaltered expressions in dehydrated KO mice. By quantitative RT-PCR, WD increased the endothelial nitric oxide synthase (eNOS), inducible NOS, and neuronal NOS expressions in the renal medulla of WT mice by 3.9-, 1.48-, and 2.6-fold, respectively, all of which were significantly blocked in mPGES-1 KO mice. The regulation of eNOS expression was further confirmed by immunoblotting. Taken together, our results suggest that mPGES-1-derived PGE(2) contributes to dehydration natriuresis likely via NO/cGMP.

Interaction of nitric oxide and the cytochrome P-450 system on blood pressure and renal function in the rat: dependence on sodium intake

AIM

Interaction was examined of nitric oxide (NO) and cytochrome P-450 (CYP-450)-dependent arachidonic acid derivatives, 20-HETE and EETs, in control of arterial pressure (MABP) and renal function. Modification of this interaction by changing sodium intake was also studied.

METHODS

On low, standard or high Na diet (LS, STD and HS rats respectively) effects of sequential blockade of NO synthases (NOS) and CYP-450 enzyme activity on MABP, renal blood flow (RBF, Transonic probe), renal medullary perfusion (MBF, laser-Doppler technique), medullary tissue NO (selective electrode) and renal excretion were examined in anaesthetized rats. All NOS were blocked with N(ϖ) -nitro-l-arginine methyl ester (l-NAME), the neuronal NOS with S-methyl-l-thiocitrulline (SMTC), and CYP-450 with 1-aminobenzotriazole (ABT).

RESULTS

In each diet group the baseline MABP was highest in rats pre-treated with l-NAME. CYP-450 inhibition significantly decreased MABP only in LS (-9%) and HS rats (-22%) pre-treated with l-NAME. This MABP decrease correlated directly with the dietary sodium content (r = 0.644, P < 0.001). CYP-450 inhibition decreased RBF in LS and HS rats (not in HS pre-treated with l-NAME). Acute exclusion of CYP-450 significantly increased MBF only in STD, SMTC pre-treated rats; in HS group it significantly increased medullary tissue NO by about 1.0 nA. The post-ABT changes in renal excretion occurred in LS and HS rats, irrespective of the status of NO synthesis.

CONCLUSIONS

Both NO- and CYP-450-dependent agents contribute to blood pressure and kidney function control, however, the role of 20-HETE and EETs becomes crucial only under conditions of high sodium intake or after NOS inhibition.

Involvement of nitric oxide in maneb- and paraquat-induced Parkinson's disease phenotype in mouse: is there any link with lipid peroxidation?

The study aimed to investigate the involvement of nitric oxide (NO) in maneb (MB)- and paraquat (PQ)-induced Parkinson's disease (PD) phenotype in mouse and its subsequent contribution to lipid peroxidation. Animals were treated intraperitoneally with or without MB and PQ, twice a week for 3, 6 and 9 weeks. In some sets of experiments (9 weeks treated groups), the animals were treated intraperitoneally with or without inducible nitric oxide synthase (iNOS) inhibitor-aminoguanidine, tyrosine kinase inhibitor-genistein, nuclear factor-kappa B (NF-kB) inhibitor-pyrrolidine dithiocarbamate (PDTC) or p38 mitogen activated protein kinase (MAPK) inhibitor-SB202190. Nitrite content and lipid peroxidation were measured in all treated groups along with respective controls. RNA was isolated from the striatum of control and treated mice and reverse transcribed into cDNA. RT-PCR was performed to amplify iNOS mRNA and western blot analysis was done to check its protein level. MB- and PQ-treatment induced nitrite content, expressions of iNOS mRNA and protein and lipid peroxidation as compared with respective controls. Aminoguanidine resulted in a significant attenuation of iNOS mRNA expression, nitrite content and lipid peroxidation demonstrating the involvement of nitric oxide in MB- and PQ-induced lipid peroxidation. Genistein, SB202190 and PDTC reduced the expression of iNOS mRNA, nitrite content and lipid peroxidation in MB- and PQ-treated mouse striatum. The results obtained demonstrate that nitric oxide contributes to an increase of MB- and PQ-induced lipid peroxidation in mouse striatum and tyrosine kinase, p38 MAPK and NF-kB regulate iNOS expression.

Altered regulation of renal nitric oxide, atrial natriuretic peptide and cyclooxygenase systems in aldosterone escape in rats

The present study was aimed to determine whether there is an altered role of local nitric oxide (NO), atrial natriuretic peptide (ANP) and cyclooxygenase (COX) systems in the kidney in association with the aldosterone escape. Male Sprague-Dawley rats were used. Aldosterone (200 microg/day) was infused through entire time course. The control group was kept on a low sodium diet (0.02 mEq/day), and the experimental group was supplied with a higher sodium diet (2. /day). Four days after beginning the regimen, the kidneys were taken. The protein expression of NO synthase (NOS) and COX isoforms was determined by semiquantitative immunoblotting. The mRNA expression of components of ANP system was determined by real-time polymerase chain reaction. The activities of soluble and particulate guanylyl cyclases were determined by the amount of cGMP generated in responses to sodium nitroprusside and ANP, respectively. There developed aldosterone escape in the experimental group. Accordingly, the renal content and the urinary excretion of NO increased. The expression of nNOS was increased in the inner medulla. Neither the expression of eNOS nor that of iNOS was changed. The expression and the catalytic activity of soluble guanylyl cyclase remained unaltered. The mRNA expression of ANP was increased. Neither the expression of NPR-A or NPR-C nor the activity of particulate guanylyl cyclase was altered in the papilla. The protein expression of COX-2 was increased in the inner medulla, while that of COX-1 remained unchanged. In conclusion, the upregulation of nNOS, ANP, and COX-2 may be causally related with the aldosterone escape.

Bypassing vasopressin receptor signaling pathways in nephrogenic diabetes insipidus

Water reabsorption in the kidney represents a critical physiological event in the maintenance of body water homeostasis. This highly regulated process relies largely on vasopressin (VP) action and on the VP-sensitive water channel (AQP2) that is expressed in principal cells of the kidney collecting duct. Defects in the VP signaling pathway and/or in AQP2 cell surface expression can lead to an inappropriate reduction in renal water reabsorption and the development of nephrogenic diabetes insipidus, a disease characterized by polyuria and polydipsia. This review focuses on the major regulatory steps that are involved in AQP2 trafficking and function. Specifically, we begin with a discussion on VP-receptor-independent mechanisms of AQP2 trafficking, with special emphasis on the nitric oxide-cyclic guanosine monophosphate signaling pathway, followed by a review of the mechanisms that govern AQP2 endocytosis and exocytosis. We then discuss emerging data illustrating roles played by the actin cytoskeleton on AQP2 trafficking, and lastly we consider elements that affect AQP2 protein expression in cells. Recent advances in each topic are summarized and are presented in the context of their potential to serve as a basis for the development of novel therapies that may ultimately improve life quality of nephrogenic diabetes insipidus patients.

Vasopressin regulates the renin-angiotensin-aldosterone system via V1a receptors in macula densa cells

The neuropeptide hormone arginine-vasopressin (AVP) is well known to exert its antidiuretic effect via the vasopressin V2 receptor (V2R), whereas the role of the vasopressin V1a receptor (V1aR) in the kidney remains to be clarified. Previously, we reported decreased plasma volume and blood pressure in V1a receptor-deficient (V1aR-/-) mice (Koshimizu T, Nasa Y, Tanoue A, Oikawa R, Kawahara Y, Kiyono Y, Adachi T, Tanaka T, Kuwaki T, Mori T. Proc Natl Acad Sci USA 103: 7807-7812, 2006). In this study, we investigated the role of V1aR in urine concentration, renal function, and the renin-angiotensin system (RAS) using V1aR-/- mice. Urine volume of V1aR-/- mice was greater than that of wild-type mice, particularly when water was loaded, while the glomerular filtration rate (GFR), urinary NaCl excretion, AVP-dependent cAMP generation, V2R, and aquaporin 2 (AQP2) expression in the kidney were lower, indicating that the diminished GFR and V2R-AQP2 system led to impaired urinary concentration in V1aR-/- mice. Since the GFR and V2R-AQP2 system are regulated by RAS, we analyzed renin and angiotensin II in V1aR-/- mice and found that the plasma renin and angiotensin II were decreased. The expression of renin in granule cells was decreased in V1aR-/- mice, which led to a decreased level of plasma renin. In addition, the expression of renin stimulators such as neuronal nitric oxide synthase and cyclooxygenase-2 in macula densa (MD) cells, where V1aR was specifically expressed, was decreased in V1aR-/- mice. These data indicate that AVP regulates body fluid homeostasis and GFR via the V1aR in MD cells by activating RAS and subsequently the V2R-AQP2 system.

Prostaglandin E2 inhibits vasotocin-induced osmotic water permeability in the frog urinary bladder by EP1-receptor-mediated activation of NO/cGMP pathway

PGE(2) is a well-known inhibitor of the antidiuretic hormone-induced increase of osmotic water permeability (OWP) in different osmoregulatory epithelia; however, the mechanisms underlying this effect of PGE(2) are not completely understood. Here, we report that, in the frog Rana temporaria urinary bladder, EP(1)-receptor-mediated inhibition of arginine-vasotocin (AVT)-induced OWP by PGE(2) is attributed to increased generation of nitric oxide (NO) in epithelial cells. It was shown that the inhibitory effect of 17-phenyl-trinor-PGE(2) (17-ph-PGE(2)), an EP(1) agonist, on AVT-induced OWP was significantly reduced in the presence of 7-nitroindazole (7-NI), a neuronal NO synthase (nNOS) inhibitor. NO synthase (NOS) activity in both lysed and intact epithelial cells measured as a rate of conversion of l-[(3)H]arginine to l-[(3)H]citrulline was Ca(2+) dependent and inhibited by 7-NI. PGE(2) and 17-ph-PGE(2), but not M&B-28767 (EP(3) agonist) or butaprost (EP(2) agonist), stimulated NOS activity in epithelial cells. The above effect of PGE(2) was abolished in the presence of SC-19220, an EP(1) antagonist. 7-NI reduced the stimulatory effect of 17-ph-PGE(2) on NOS activity. 17-ph-PGE(2) increased intracellular Ca(2+) concentration and cGMP in epithelial cells. Western blot analysis revealed an nNOS expression in epithelial cells. These results show that the inhibitory effect of PGE(2) on AVT-induced OWP in the frog urinary bladder is based at least partly on EP(1)-receptor-mediated activation of the NO/cGMP pathway, suggesting a novel cross talk between AVT, PGE(2), and nNOS that may be important in the regulation of water transport.

Nitric oxide in the kidney: functions and regulation of synthesis

In the kidney nitric oxide (NO) has numerous important functions including the regulation of renal haemodynamics, maintenance of medullary perfusion, mediation of pressure-natriuresis, blunting of tubuloglomerular feedback, inhibition of tubular sodium reabsorption and modulation of renal sympathetic neural activity. The net effect of NO in the kidney is to promote natriuresis and diuresis. Significantly, deficient renal NO synthesis has been implicated in the pathogenesis of hypertension. All three isoforms of nitric oxide synthase (NOS), namely neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2) and endothelial NOS (eNOS or NOS3) are reported to contribute to NO synthesis in the kidney. The regulation of NO synthesis in the kidney by NOSs is complex and incompletely understood. Historically, many studies of NOS regulation in the kidney have emphasized the role of variations in gene transcription and translation. It is increasingly appreciated, however, that the constitutive NOS isoforms (nNOS and eNOS) are also subject to rapid regulation by post-translational mechanisms such as Ca(2+) flux, serine/threonine phosphorylation and protein-protein interactions. Recent studies have emphasized the role of post-translational regulation of nNOS and eNOS in the regulation of NO synthesis in the kidney. In particular, a role for phosphorylation of nNOS and eNOS at both activating and inhibitory sites is emerging in the regulation of NO synthesis in the kidney. This review summarizes the roles of NO in renal physiology and discusses recent advances in the regulation of eNOS and nNOS in the kidney by post-translational mechanisms such as serine/threonine phosphorylation.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

Vasopressin and nitric oxide synthesis after three days of water or food deprivation

Nitric oxide has been suggested to be involved in the regulation of fluid and nutrient homeostasis. In the present investigation, vasopressin and nitric oxide metabolite (nitrite and nitrate) levels were determined in plasma of male Wistar rats submitted to water or food deprivation for three days. Hematocrit and plasma sodium showed marked increase in dehydrated and starved rats. Potassium levels and plasma volume decreased in both treated groups. Plasma osmolality and vasopressin levels were significantly elevated in water deprived (362.8 +/- 7.1 mOsm/kg H2O, 17.3 +/- 2.7 pg/ml, respectively, p < 0.001) rats, but not in food deprived (339.9 +/- 5.0, 1.34 +/- 0.28) rats, compared to the controls (326.1 +/- 4.1, 1.47 +/- 0.32). The alterations observed in plasma vasopressin levels were related to plasma osmolality rather than plasma volume. Plasma levels of nitrite and nitrate were markedly increased in both water and food deprived rats (respectively, 2.19 +/- 0.29 mg/l and 2.22 +/- 0.17 mg/l versus 1.33 +/- 0.19 mg/l, both p < 0.01). There was a significant negative correlation between plasma nitrite and nitrate concentration and plasma volume. These results suggest that both dehydration and starvation increase plasma nitric oxide, probably by activation of nitric oxide synthases. The release of nitric oxide may participate in the regulation of the alteration in blood flow, fluid and nutrient metabolism caused by water deprivation or starvation.

Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra)

Vasopressin-stimulated insertion of the aquaporin 2 (AQP2) water channel into the plasma membrane of kidney collecting duct principal cells is a key event in the urinary concentrating mechanism. The paradigm for vasopressin-receptor signaling involves cAMP-mediated protein kinase A activation, which results in the functionally critical phosphorylation of AQP2 on amino acid serine 256. We previously showed that a parallel cGMP-mediated signaling pathway also leads to AQP2 membrane insertion in AQP2-transfected LLC-PK1 (LLC-AQP2) cells and in outer medullary collecting duct principal cells in situ (Bouley R, Breton S, Sun T, McLaughlin M, Nsumu NN, Lin HY, Ausiello DA, and Brown D. J Clin Invest 106: 1115-1126, 2000). In the present report, we show by immunofluorescence microscopy, and Western blotting of plasma membrane fractions, that 45-min exposure of LLC-AQP2 cells to the cGMP phosphodiesterase type 5 (PDE5) inhibitors sildenafil citrate (Viagra) or 4-{[3',4'-methylene-dioxybenzyl]amino}-6-methoxyquinazoline elevates intracellular cGMP levels and results in the plasma membrane accumulation of AQP2; i.e., they mimic the vasopressin effect. Importantly, our data also show that acute exposure to PDE5 inhibitors for 60 min induces apical accumulation of AQP2 in kidney medullary collecting duct principal cells both in tissue slices incubated in vitro as well as in vivo after intravenous injection of Viagra into rats. These data suggest that AQP2 membrane insertion can be induced independently of vasopressin-receptor activation by activating a parallel cGMP-mediated signal transduction pathway with cGMP PDE inhibitors. These results provide proof-of-principle that pharmacological activation of vasopressin-independent, cGMP signaling pathways could aid in the treatment of those forms of nephrogenic diabetes insipidus that are due to vasopressin-2 receptor dysfunction.

C-peptide increases the expression of vasopressin-activated calcium-mobilizing receptor gene through a G protein-dependent pathway

OBJECTIVE

Although an increasing number of reports suggest that physiological concentrations of C-peptide protect against the development of diabetic nephropathy, possibly through the modulation of Na-K pump activity, the intracellular pathways controlled by C-peptide are still unrecognized. C-peptide and vasopressin share similar intracellular effects including the activation of calcium influx and endothelial nitric oxide synthase. Both hormones stimulate also the activity of Na-K pump activity. Whether the activity of C-peptide is mediated by the recently identified vasopressin-activated calcium-mobilizing receptor (VACM-1) has never been previously investigated.

DESIGN AND METHODS

To clarify this issue, we evaluated the effect of C-peptide on VACM-1 RNA (measured by semiquantitative RT-PCR) and protein expression (measured by immunoblotting) in human skin fibroblasts (where a specific binding of C-peptide was demonstrated) and in human mesangial cells, the cellular target of diabetic nephropathy.

RESULTS

C-peptide-induced activation of VACM-1 was demonstrated in fibroblasts from six healthy individuals (0.51+/-0.1 vs 1.48+/-0.4, arbitrary units+/-s.e., P = 0.025). This finding was paralleled by an increased VACM-1 protein expression (5.64+/-1.0 vs 8.47+/-1.2, arbitrary units+/-s.e., P= 0.043). Similar results were confirmed in three independent cultures of human mesangial cells. VACM-1 activation in fibroblasts was insensitive to phosphatidylinositol-3-kinase inhibitor LY294002, but was inhibited by pertussis toxin, suggesting that activation of VACM-1 could be mediated by a G protein-coupled receptor.

CONCLUSIONS

This study demonstrates for the first time that C-peptide activates VACM-1, possibly through a G protein-coupled receptor. Further studies are needed to clarify whether VACM-1 is involved in the protective effect of C-peptide against the development of diabetic nephropathy.

NaCl transport across the opercular epithelium ofFundulus heteroclitusis inhibited by an endothelin to NO, superoxide, and prostanoid signaling axis

Recent evidence suggests that paracrine signaling agents, such as endothelin (ET), nitric oxide (NO), superoxide (O2-), and prostanoids can modulate mammalian renal function by affecting both hemodynamic and epithelial ionic transport pathways. Since these signaling pathways have been described in fish blood vessels, we hypothesized that they may control salt transport across the gill epithelium—the primary site of ion excretion in marine teleost fishes. We found that ET, the NO donors sodium nitroprusside and spermine NONOate, and the prostanoid PGE2each can produce a concentration-dependent reduction in the short circuit current ( Isc) across the isolated opercular epithelium of the killifish ( Fundulus heteroclitus), the generally accepted model for the marine teleost gill epithelium. Sarafotoxin S6c was equipotent to ET-1, suggesting that ETBreceptors are involved. Incubation with NG-nitro-l-arginine methyl ester (l-NAME) or indomethacin reduced the effect of subsequent addition of SRXS6c by 17 and 89%, respectively, suggesting the presence of an ET to NO and PGE axis. The effects of l-NAME and indomethacin were not additive, but the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL) reduced the effect of SRXS6c by 34% and preincubation with l-NAME, indomethacin, and TEMPOL reduced the SRXS6c response to zero. This suggests a direct role for O2-in this axis. COX-2 appears to be the major enzyme involved in this axis because the specific COX-2 inhibitor NS-398 was twice as effective as the COX-1 inhibitor SC560 in inhibiting the SRXS6c effect. The Iscwas stimulated by the EP2agonist butaprost and inhibited by the EP1,3agonist sulprostone, suggesting both stimulatory and inhibitory PGE receptors in this tissue. Carbaprostacyclin (PGI2analog), thromboxane A2, PGF2α, and PGD2did not affect the Isc. Our data are the first to suggest the importance of an ET-stimulated and NO-, O2--, and PGE2-mediated signaling axis that can modify active extrusion of NaCl across the killifish opercular epithelium and, by inference, the marine teleost gill epithelium.

Effet d'une déshydratation sur le monoxyde d'azote, l'axe vasopressinergique et l'axe corticotrope chez le rat

Effect of dehydration on nitric oxide, corticotropic and vasopressinergic axis in rat. The purpose of our work is to study, in the male 'Wistar' rat, the effects of a chronic dehydration, by deprivation of water for three days out of four, during four repeated cycles, on the evolution of certain blood variables, on the activities of both corticotropic and vasopressinergic axis and on the synthesis of nitric oxide. The chronic dehydration causes a considerable reduction of the body weight, an activation of the vasopressinergic axis and an increase in the circulating rates of the nitrates/nitrites, which represent the final metabolites of the reaction of oxidation of nitric oxide. The pituitary-adrenal axis is not statistically affected by the chronic dehydration. This seems to be in favour of a possible adaptation of corticotropic axis to chronic water deprivation. The activation of synthesis of nitric oxide shows its implication in the regulation of the water balance and its buffer effect on vasoconstriction and hypertension induced by water stress.

Tetrandrine ameliorates cirrhosis and portal hypertension by inhibiting nitric oxide in cirrhotic rats

To examine the role and effect of nitric oxide synthase type II (NOS II) in cirrhotic rats, expression of NOS II mRNA was detected by real time RT-PCR. The enzymatic activity of nitric oxide synthase and the circulating levels of NO, systemic and portal hemodynamics and quantification of cirrhosis were measured. Chinese traditional medicine was used to treat cirrhotic rats and the effect of NO was evaluated. Double-blind method was used in experiment. Our results showed the concentration of NO and the enzymatic activity of NOS increased markedly at all stages of cirrhosis and iNOSmRNA was strongly expressed. Meanwhile, the portal-venous-pressure (PVP) and portal-venous-flow (PVF) were significantly increased. NO, NOS and iNOSmRNA were positively correlated to the degree of hepatic fibrosis. Tetrandrine significantly inhibited NO production and the expression of iNOSmRNA. Our results suggested that increased hepatic expression of NOS II is one of the important factors causing cirrhosis and portal hypertension. Tetrandrine can significantly ameliorate cirrhosis and portal hypertension.

High-salt diet increases sensitivity to NO and eNOS expression but not NO production in THALs

L-Arginine inhibits thick ascending limb (THAL) NaCl absorption by activating endothelial NO synthase (eNOS) and increasing NO production. Inhibition of renal NO production combined with a high-salt diet produces hypertension, and the THAL has been implicated in salt-sensitive hypertension. We hypothesized that a high-salt diet enhances the inhibitory action of L-arginine on NaCl absorption by THALs because of increased eNOS expression and NO production. To test this, we used isolated THALs from rats on a normal-salt (NS) or high-salt diet (HS) for 7 to 10 days. L-Arginine (1 mmol/L) decreased chloride absorption by 56+/-10% in THALs from rats on a HS diet, but only 29+/-3% in THALs from rats on a NS diet. eNOS expression in isolated THALs from rats on a HS diet was increased by 3.9-fold compared with NS (P<0.03). However, L-arginine increased NO levels to the same extent in THALs from both groups, as measured with DAF-2 DA or a NO-sensitive electrode. To determine whether a HS diet increases the sensitivity of the THAL to NO, we tested the effects of the NO donor spermine NONOate on chloride absorption. In THALs from rats on a HS diet, 1 and 5 micromol/L spermine NONOate reduced chloride absorption by 35+/-5% and 58+/-6%, respectively. In contrast, these same concentrations of spermine NONOate reduced chloride absorption by 4+/-4% (P<0.03 versus HS diet) and 43+/-9% in THALs from rats on a NS diet. We conclude that a HS diet enhances the effect of NO in the THAL. L-Arginine-stimulated NO production was not enhanced by a HS diet, despite increased eNOS protein.