Role of nitric oxide in the regulation of nephron transport

Role of atrial natriuretic peptide in mediating the blood pressure-independent natriuresis elicited by systemic inhibition of nitric oxide

While it is clearly recognized that increased intrarenal nitric oxide (NO) levels elicit natriuresis, confounding data showing that systemic nitric oxide synthase inhibition (NOSi) also increases sodium excretion (U_NaV) poses a conundrum. This response has been attributed to the associated increases in arterial pressure (AP); however, the increases in AP and in U_NaV are temporally dissociated. The changes in regional renal haemodynamics induced by NOSi could also contribute to the alterations of U_NaV. To evaluate the roles of AP and non-AP mechanisms mediating the natriuresis, N_ω-nitro-l-arginine methyl ester hydrochloride (L-NAME) was infused i.v. at doses ranging from 5 to 50 μg/kg/min in anaesthetized rats. U_NaV, perfusion of the cortex (cortical blood flow, CBF) and medulla (medullary blood flow, MBF) with laser-Doppler flowmetry and glomerular filtration rate (GFR) were measured. U_NaV increased from 0.6 ± 0.2 to 1.6 ± 0.1 μmol/kg/min (P < 0.05) with the lower nonpressor doses. With the higher doses, AP increased from 116 ± 4 to 122 ± 4 mmHg and U_NaV increased from 1.1 ± 0.3 to 3.3 ± 0.7 μmol/min/g (P < 0.002). U_NaV increased similarly in a group where renal AP was maintained at baseline levels. The associated reductions in CBF (17 ± 5 and 38 ± 5 %) and MBF (27 ± 6 and 52 ± 6 %) would be expected to attenuate rather than contribute to the natriuresis. Plasma atrial natriuretic peptide (ANP) concentrations increased significantly following NOSi. Anantin, a natriuretic peptide receptor-A blocker, prevented or reversed the L-NAME-induced natriuresis without altering the L-NAME-induced changes in AP or CBF. The results indicate that increased ANP and related natriuretic peptides mediate the AP-independent natriuresis, at least partly, elicited by systemic L-NAME infusion and help resolve the conundrum of natriuresis during systemic NOSi.

Angiotensin II dose-dependently stimulates human renal proximal tubule transport by the nitric oxide/guanosine 3',5'-cyclic monophosphate pathway

Stimulation of renal proximal tubule (PT) transport by angiotensin II (Ang II) is critical for regulation of BP. Notably, in rats, mice, and rabbits, the regulation of PT sodium transport by Ang II is biphasic: transport is stimulated by picomolar to nanomolar concentrations of Ang II but inhibited by nanomolar to micromolar concentrations of Ang II. However, little is known about the effects of Ang II on human PT transport. By functional analysis with isolated PTs obtained from nephrectomy surgery, we found that Ang II induces a dose-dependent profound stimulation of human PT transport by type 1 Ang II receptor (AT1)-dependent phosphorylation of extracellular signal-regulated kinase (ERK). In PTs of wild-type mice, the nitric oxide (NO) /cGMP/cGMP-dependent kinase II (cGKII) pathway mediated the inhibitory effect of Ang II. In PTs of cGKII-deficient mice, the inhibitory effect of Ang II was lost, but activation of the NO/cGMP pathway failed to phosphorylate ERK. Conversely, in human PTs, the NO/cGMP pathway mediated the stimulatory effect of Ang II by phosphorylating ERK independently of cGKII. These contrasting responses to the NO/cGMP pathway may largely explain the different modes of PT transport regulation by Ang II, and the unopposed marked stimulation of PT transport by high intrarenal concentrations of Ang II may be an important factor in the pathogenesis of human hypertension. Additionally, the previously unrecognized stimulatory effect of the NO/cGMP pathway on PT transport may represent a human-specific therapeutic target in hypertension.

Nitric oxide regulation of Na, K-ATPase activity in ocular ciliary epithelium involves Src family kinase

The nitric oxide (NO) donor sodium nitroprusside (SNP) is known to reduce aqueous humor (AH) secretion in the isolated porcine eye. Previously, SNP was found to inhibit Na,K-ATPase activity in nonpigmented ciliary epithelium (NPE), AH-secreting cells, through a cGMP/protein kinase G (PKG)-mediated pathway. Here we show Src family kinase (SFK) activation in the Na,K-ATPase activity response to SNP. Ouabain-sensitive (86) Rb uptake was reduced by >35% in cultured NPE cells exposed to SNP (100 µM) or exogenously added cGMP (8-Br-cGMP) (100 µM) and the SFK inhibitor PP2 (10 µM) prevented the response. Ouabain-sensitive ATP hydrolysis was reduced by ~40% in samples detected in material obtained from SNP- and 8-Br-cGMP-treated cells following homogenization, pointing to an intrinsic change of Na,K-ATPase activity. Tyrosine-10 phosphorylation of Na,K-ATPase α1 subunit was detected in SNP and L-arginine-treated cells and the response prevented by PP2. SNP elicited an increase in cell cGMP. Cells exposed to 8-Br-cGMP displayed SFK activation (phosphorylation) and inhibition of both ouabain-sensitive (86) Rb uptake and Na,K-ATPase activity that was prevented by PP2. SFK activation, which also occurred in SNP-treated cells, was suppressed by inhibitors of soluble guanylate cyclase (ODQ; 10 µM) and PKG (KT5823; 1 µM). SNP and 8-Br-cGMP also increased phosphorylation of ERK1/2 and p38 MAPK and the response prevented by PP2. However, U0126 did not prevent SNP or 8-Br-cGMP-induced inhibition of Na,K-ATPase activity. Taken together, the results suggest that NO activates guanylate cyclase to cause a rise in cGMP and subsequent PKG-dependent SFK activation. Inhibition of Na,K-ATPase activity depends on SFK activation.

Oxidative stress in hypertension: role of the kidney

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES AND FUTURE DIRECTIONS

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

Hypertension: physiology and pathophysiology

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

Pre- or post-treatment with aminoguanidine attenuates a renal distal acidification defect induced by acute ureteral obstruction in rats

Acute unilateral ureteral obstruction (UUO) impairs distal nephron acid secretion and stimulates expression of inducible nitric oxide synthase (iNOS) in post-obstructed kidney (POK). This study investigated the influence of pre- or post-treatment with aminoguanidine as a selective iNOS inhibitor on UUO-induced renal functional disturbances. To induce acute UUO, the left ureter in rats was ligated and released after 24 h. Then, a 3 h clearance period followed by bicarbonate loading and thereafter a 30 min clearance period were allocated. Aminoguanidine was administered either prior to the UUO induction or after release of the obstruction in the different rat groups, while untreated and sham groups received normal saline. During the first clearance period, fractional bicarbonate excretion and urinary pH increased markedly in the POK of the untreated group compared with the left kidney of sham group, and a large drop in the difference between urine and blood pCO2 (U–B pCO2) was observed after bicarbonate loading; all of these parameters were ameliorated in the pre-treated and post-treated groups. However, the UUO-induced decreases in creatinine clearance, sodium reabsorption, urine osmolality, and free-water reabsorption in the POK were attenuated only in the post-treated group. Therefore, the in vivo application of a selective iNOS inhibitor partially improved the acute UUO-induced distal nephron acidification defect, while post-treatment but not pre-treatment with aminoguanidine ameliorated decrements of glomerular filtration, sodium reabsorption, and urine-concentrating ability.

Adipokines as a link between obesity and chronic kidney disease

Adipocytes secrete a number of bioactive adipokines that activate a variety of cell signaling pathways in central and peripheral tissues. Obesity is associated with the altered production of many adipokines and is linked to a number of pathologies. As an increase in body weight is directly associated with an increased risk for developing chronic kidney disease (CKD), there is significant interest in the link between obesity and renal dysfunction. Altered levels of the adipokines leptin, adiponectin, resistin, and visfatin can decrease the glomerular filtration rate and increase albuminuria, which are pathophysiological changes typical of CKD. Specifically, exposure of the glomerulus to altered adipokine levels can increase its permeability, fuse the podocytes, and cause mesangial cell hypertrophy, all of which alter the glomerular filtration rate. In addition, the adipokines leptin and adiponectin can act on tubular networks. Thus, adipokines can act on multiple cell types in the development of renal pathophysiology. Importantly, most studies have been performed using in vitro models, with future studies in vivo required to further elucidate the specific roles that adipokines play in the development and progression of CKD.

Role of collecting duct endothelin in control of renal function and blood pressure

Over 26,000 manuscripts have been published dealing with endothelins since their discovery 25 years ago. These peptides, and particularly endothelin-1 (ET-1), are expressed by, bind to, and act on virtually every cell type in the body, influencing multiple biological functions. Among these actions, the effects of ET-1 on arterial pressure and volume homeostasis have been most extensively studied. While ET-1 modulates arterial pressure through regulation of multiple organ systems, the peptide's actions in the kidney in general, and the collecting duct in particular, are of unique importance. The collecting duct produces large amounts of ET-1 that bind in an autocrine manner to endothelin A and B receptors, causing inhibition of Na(+) and water reabsorption; absence of collecting duct ET-1 or its receptors is associated with marked salt-sensitive hypertension. Collecting duct ET-1 production is stimulated by Na(+) and water loading through local mechanisms that include sensing of salt and other solute delivery as well as shear stress. Thus the collecting duct ET-1 system exists, at least in part, to detect alterations in, and maintain homeostasis for, extracellular fluid volume. Derangements in collecting duct ET-1 production may contribute to the pathogenesis of genetic hypertension. Blockade of endothelin receptors causes fluid retention due, in large part, to inhibition of the action of ET-1 in the collecting duct; this side effect has substantially limited the clinical utility of this class of drugs. Herein, the biology of the collecting duct ET-1 system is reviewed, with particular emphasis on key issues and questions that need addressing.

Anandamide inhibits transport-related oxygen consumption in the loop of Henle by activating CB1 receptors

The energy required for active Na chloride reabsorption in the thick ascending limb (TAL) depends on oxygen consumption and oxidative phosphorylation (OXP). In other cells, Na transport is inhibited by the endogenous cannabinoid anandamide through the activation of the cannabinoid receptors (CB) type 1 and 2. However, it is unclear whether anandamide alters TAL transport and the mechanisms that could be involved. We hypothesized that anandamide inhibits TAL transport via activation of CB1 receptors and NO. For this, we measured oxygen consumption (Q(O(2))) in TAL suspensions to monitor the anandamide effects on transport and OXP. Anandamide reduced Q(O(2)) in a concentration-dependent manner. During Na-K-2Cl cotransport and Na/H exchange inhibition, anandamide did not inhibit TAL Q(O(2)). To test the role of the cannabinoid receptors, we used specific agonists and antagonists of CB1 and CB2 receptors. The CB1-selective agonist WIN55212-2 reduced Q(O(2)) in a concentration-dependent manner. Also, the CB1 receptor antagonist rimonabant blocked the effect of anandamide on Q(O(2)). In contrast, the CB2-selective agonist JHW-133 had no effect on Q(O(2)), while the CB2 receptor antagonist AM-630 failed to block the anandamide effects on Q(O(2)). To confirm these results, we measured CB1 and CB2 receptor expression and only CB1 expression was detected. Because CB1 receptors are strong nitric oxide synthase (NOS) stimulators and NO inhibits transport in TALs, we evaluated the role of NO. Anandamide stimulated NO production and the NOS inhibitor N(G)-nitro-L-arginine methyl ester blocked the anandamide effects on Q(O(2)). We conclude that anandamide inhibits TAL Na transport-related Q(O(2)) via activation of CB1 receptor and NOS.

Nitric oxide activates IL-6 production and expression in human renal epithelial cells

BACKGROUND/AIMS

Increased nitric oxide (NO) production or inducible form of NO synthase activity have been documented in patients suffering from urinary tract infection (UTI), but the role of NO in this infection is unclear. We investigated whether NO can affect the host response in human renal epithelial cells by modulating IL-6 production and mRNA expression.

METHODS

The human renal epithelial cell line A498 was infected with a uropathogenic Escherichia coli (UPEC) strain and/or the NO donor DETA/NO. The IL-6 production and mRNA expression were evaluated by ELISA and real-time RT-PCR. IL-6 mRNA stability was evaluated by analyzing mRNA degradation by real-time RT-PCR.

RESULTS

DETA/NO caused a significant (p < 0.05) increase in IL-6 production. Inhibitors of p38 MAPK and ERK1/2 signaling, but not JNK, were shown to significantly suppress DETA/NO-induced IL-6 production. UPEC-induced IL-6 production was further increased (by 73 ± 23%, p < 0.05) in the presence of DETA/NO. The IL-6 mRNA expression increased 2.1 ± 0.17-fold in response to DETA/NO, while the UPEC-evoked increase was pronounced (20 ± 4.5-fold). A synergistic effect of DETA/NO on UPEC-induced IL-6 expression was found (33 ± 7.2-fold increase). The IL-6 mRNA stability studies showed that DETA/NO partially attenuated UPEC-induced degradation of IL-6 mRNA.

CONCLUSIONS

NO was found to stimulate IL-6 in renal epithelial cells through p38 MAPK and ERK1/2 signaling pathways and also to increase IL-6 mRNA stability in UPEC-infected cells. This study proposes a new role for NO in the host response during UTI by modulating the transcription and production of the cytokine IL-6.

Salt sensitivity of nitric oxide generation and blood pressure in mice with targeted knockout of the insulin receptor from the renal tubule

To elucidate the role of the insulin receptor (IR) on kidney nitric oxide generation and blood pressure (BP) control, we generated mice with targeted deletion of renal tubule IR using loxP recombination driven by a Ksp-cadherin promoter. Male knockout (KO) and wild-type (WT) littermates (~4 mo old) were transitioned through three 1-wk treatments: 1) low-NaCl diet (0.085%); 2) high-NaCl diet (HS; 5%); and 3) HS diet plus 3 mM tempol, a superoxide dismutase mimetic, in the drinking water. Mice were then switched to medium-NaCl (0.5%) diet for 5 days and kidneys harvested under pentobarbital anesthesia. Twenty-four-hour urinary nitrates plus nitrites were significantly higher in the WT mice under HS (2,067 ± 280 vs. 1,550 ± 230 nmol/day in WT and KO, respectively, P < 0.05). Tempol attenuated genotype differences in urinary nitrates plus nitrites. A rise in BP with HS was observed only in KO mice and not affected by tempol (mean arterial pressure, dark period, HS, 106 ± 5 vs. 119 ± 4 mmHg, for WT and KO, respectively, P < 0.05). Renal outer medullary protein levels of nitric oxide synthase (NOS) isoforms by Western blot (NOS1-3 and phosphorylated-S1177-NOS3) revealed significantly lower band density for NOS1 (130-kDa isoform) in the KO mice. A second study, when mice were euthanized under HS conditions, confirmed significantly lower NOS1 (130 kDa) in the KO, with an even more substantial (>50%) reduction of the 160-kDa NOS1 isoform. These studies suggest that the loss of renal IR signaling impairs renal nitric oxide production. This may be important in BP control, especially in insulin-resistant states, such as the metabolic syndrome.

Effects of atorvastatin on systemic and renal nitric oxide in healthy man

Statin treatment improves endothelial function but the effects of statins on renal nitric oxide have not been clarified. In this crossover study, 26 healthy subjects received atorvastatin 80 mg per day or placebo for 5 days. After 5 days of treatment, L-N(G)-monomethyl arginine caused a similar increase in blood pressure and decrease in urine output and glomerular filtration rate. The decrease in fractional excretion of sodium to L-N(G)-monomethyl arginine was more pronounced after atorvastatin treatment. Atorvastatin did not change the response to several vasoactive hormones. The results indicate that atorvastatin increase renal nitric oxide, which may explain a part of the pleiotropic effects of statins.

Glycerol-induced renal damage improved by 7-O-galloyl-D-sedoheptulose treatment through attenuating oxidative stress

The protective effect of 7-O-galloyl-D-sedoheptulose (GS), isolated from Corni Fructus as an active component, against acute renal failure (ARF) induced by glycerol was investigated. The administration of GS led to a decline in the levels of blood urea nitrogen and creatinine; on the other hand, it did not have a significant effect on creatinine clearance. Furthermore, GS also significantly decreased the urine volume and fractional excretion of sodium, but it increased the urine osmolarity, suggesting the protective role of GS against renal dysfunction. Oxidative stress under ARF was attenuated by GS through the inhibition of lipid peroxidation, scavenging of reactive oxygen species (ROS), and elevation of the antioxidative status. Renal oxidative stress is related to the overproduction of ROS by nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase; therefore, in the present study, the protein expression of p22(phox) and NAD(P)H oxidase-4 (Nox-4) was investigated. GS down-regulated the protein expression of p22(phox); on the other hand, it did not significantly affect the expression of Nox-4. This indicates that GS inhibits the production of superoxide by regulating a component of NAD(P)H oxidase, p22(phox). Furthermore, GS down-regulated the expressions of nuclear factor-κB (NF-κΒ) and inducible nitric oxide (NO) synthase (iNOS), suggesting that GS protects against NO-induced inflammatory pathological conditions under ARF through the regulation of NF-κB and iNOS expressions. The present study indicates that GS exerts a protective effect against ARF through the recovery of renal dysfunction and attenuation of renal oxidative stress by regulating related protein expression.

Effect of nitric oxide inhibition on blood pressure and renal sodium handling: a dose-response study in healthy man

Nitric oxide (NO) is a ubiquitous vasodilator and an important regulator of renal sodium excretion. To further investigate the role of NO in renal sodium handling, we studied the effects of the NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA), in a crossover dose-response study. During NO inhibition mean arterial pressure increased dose-dependently and reached a plateau after 20 minutes of infusion. On the contrary, the fractional excretion of sodium was reduced equally in all three L-NMMA doses. This indicates that sodium excretion is highly sensitive to even small changes in renal NO bioavailability in healthy human.

Effect of aminoguanidine on ischemia/reperfusion injury in rat small intestine

Ischemia/reperfusion (I/R) injury is induced by reactive oxygen species (ROS). During intestinal I/R, the amount of nitric oxide (NO), which is a ROS, is increased. In this study, we examined the protection against I/R injury by inhibition of NO generation. Wistar/ST rats were exposed to 1 h of ischemia, followed by reperfusion for 4 h. The rats were intravenously injected with 100 mg/kg aminoguanidine (AG), which is a selective inducible NO synthase (iNOS) inhibitor, for 5 min before ischemia. The increase in NO(2)(-) by intestinal I/R was significantly inhibited by AG 1 h after reperfusion. Moreover, the increase in area under curve of 0 to 1 h after reperfusion (AUC(0-1)) of paracellular marker was inhibited. However, 3 h after reperfusion, the survival ratio of rats was significantly decreased in the intestinal I/R condition with AG. The amount of NO(2)(-) and AUC of 3 to 4 h after reperfusion (AUC(3-4)) of paracellular marker in intestinal I/R groups were increased by AG compared with those in the I/R condition without AG 3 h after reperfusion. These data indicated that AG, which was given by single pre-administration, can clearly inhibit intestinal I/R injury 1 h after reperfusion. However, the injury occurs again 3 h after reperfusion and grows worse.

Protein kinase G inhibits flow-induced Ca2+ entry into collecting duct cells

The renal cortical collecting duct (CCD) contributes to the maintenance of K(+) homeostasis by modulating renal K(+) secretion. Cytosolic Ca(2+) ([Ca(2+)](i)) mediates flow-induced K(+) secretion in the CCD, but the mechanisms regulating flow-induced Ca(2+) entry into renal epithelial cells are not well understood. Here, we found that atrial natriuretic peptide, nitric oxide, and cyclic guanosine monophosphate (cGMP) act through protein kinase G (PKG) to inhibit flow-induced increases in [Ca(2+)](i) in M1-CCD cells. Coimmunoprecipitation, double immunostaining, and functional studies identified heteromeric TRPV4-P2 channels as the mediators of flow-induced Ca(2+) entry into M1-CCD cells and HEK293 cells that were coexpressed with both TRPV4 and TRPP2. In these HEK293 cells, introducing point mutations at two putative PKG phosphorylation sites on TRPP2 abolished the ability of cGMP to inhibit flow-induced Ca(2+) entry. In addition, treating M1-CCD cells with fusion peptides that compete with the endogenous PKG phosphorylation sites on TRPP2 also abolished the cGMP-mediated inhibition of the flow-induced Ca(2+) entry. Taken together, these data suggest that heteromeric TRPV4-P2 channels mediate the flow-induced entry of Ca(2+) into collecting duct cells. Furthermore, substances such as atrial natriuretic peptide and nitric oxide, which increase cGMP, abrogate flow-induced Ca(2+) entry through PKG-mediated inhibition of these channels.

Gasotransmitters: novel regulators of epithelial na(+) transport?

The vectorial transport of Na(+) across epithelia is crucial for the maintenance of Na(+) and water homeostasis in organs such as the kidneys, lung, or intestine. Dysregulated Na(+) transport processes are associated with various human diseases such as hypertension, the salt-wasting syndrome pseudohypoaldosteronism type 1, pulmonary edema, cystic fibrosis, or intestinal disorders, which indicate that a precise regulation of epithelial Na(+) transport is essential. Novel regulatory signaling molecules are gasotransmitters. There are currently three known gasotransmitters: nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S). These molecules are endogenously produced in mammalian cells by specific enzymes and have been shown to regulate various physiological processes. There is a growing body of evidence which indicates that gasotransmitters may also regulate Na(+) transport across epithelia. This review will summarize the available data concerning NO, CO, and H(2)S dependent regulation of epithelial Na(+) transport processes and will discuss whether or not these mediators can be considered as true physiological regulators of epithelial Na(+) transport biology.

Basolateral P2X receptors mediate inhibition of NaCl transport in mouse medullary thick ascending limb (mTAL)

Extracellular nucleotides regulate epithelial transport via luminal and basolateral P2 receptors. Renal epithelia express multiple P2 receptors, which mediate significant inhibition of solute absorption. Recently, we identified several P2 receptors in the medullary thick ascending limb (mTAL) including luminal and basolateral P2Y2 receptors (Jensen ME, Odgaard E, Christensen MH, Praetorius HA, Leipziger J. J Am Soc Nephrol 18: 2062–2070, 2007). In addition, we found evidence for a basolateral P2X receptor. Here, we investigate the effect of basolateral ATP on NaCl absorption in isolated, perfused mouse mTALs using the electrical measurement of equivalent short-circuit current ( I′sc). Nonstimulated mTALs transported at a rate of 1,197 ± 104 μA/cm2 ( n = 10), which was completely blockable with luminal furosemide (100 μM). Basolateral ATP (100 μM) acutely (1 min) and reversibly reduced the absorptive I′sc. After 2 min, the reduction amounted to 24.4 ± 4.0% ( n = 10). The nonselective P2 receptor antagonist suramin blocked the effect. P2Y receptors were found not to be involved in this effect. The P2X receptor agonist 2-methylthio ATP mimicked the ATP effect, and the P2X receptor antagonist periodate-oxidized ATP blocked it. In P2X7−/− mice, the ATP effect remained unaltered. In contrast, in P2X4−/− mice the ATP-induced inhibition of transport was reduced. A comprehensive molecular search identified P2X4, P2X5, and P2X1 receptor subunit mRNA in isolated mouse mTALs. These data define that basolateral ATP exerts a significant inhibition of Na+ absorption in mouse mTAL. Pharmacological, molecular, and knockout mouse data identify a role for the P2X4 receptor. We suggest that other P2X subunits like P2X5 are part of the P2X receptor complex. These data provide the novel perspective that an ionotropic receptor and thus a nonselective cation channel causes transport inhibition in an intact renal epithelium.

Aquaporin-2 promoter is synergistically regulated by nitric oxide and nuclear factor of activated T cells

Background/Aims

We have previously shown that aquaporin-2 (AQP2) is down-regulated in the renal medulla of rats made hypertensive by chronic inhibition of nitric oxide synthase. It has been shown that AQP2 expression is regulated by the calcineurin/nuclear factor of activated T cells (NFATc). Nitric oxide (NO) regulates the activity of NFATc via c-Jun-N-terminal kinase 2 (JNK2). Therefore, we hypothesized that increases in NO enhance NFATc-mediated up-regulation of AQP2 promoter activity.

Methods

AQP2 mRNA and protein expression were detected in mouse renal papilla. AQP2 promoter luciferase reporter- and NFAT luciferase reporter-transfected MDCK cells were used to determine AQP2 promoter activity and NFATc activity, respectively. Cells were incubated with classic activators and inhibitors of NFATc and the NO pathway.

Results

Our results demonstrate that both Ca²⁺ and NO have a synergistic effect resulting in an increase in AQP2 mRNA and protein in mouse papilla and activation of the AQP2 promoter in kidney-derived cells. In addition, NO enhances Ca²⁺-induced NFATc activation. The underlying mechanism involves increased NFATc nuclear import and decreased export via protein kinase G-mediated inhibition of JNK1/2.

Conclusions

This is the first study defining novel regulatory roles for NO and NFATc in the control of AQP2, which is an important renal protein.

The effect of cadmium toxicity on renal nitric oxide synthase isoenzymes

The aim of this study was to assess the cadmium (Cd) toxicity on renal nitric oxide synthase (NOS) isoenzymes. The study was carried out on 18 inbred male (Cd group: 10 and control group: 8) Wistar rats. Cd group received drinking water containing 15 mg/L Cd for 30 days; and at the end of the 30 days, plasma Cd was analysed. One kidney was snap frozen to assess the endothelial NOS (eNOS), inducible NOS (iNOS) and neuronal NOS (nNOS) expressions by Western blot analyses, and the other kidney was preserved for histopathological examination. Plasma Cd levels were significantly elevated in the Cd group. The Western blot analyses found higher levels of eNOS, iNOS and nNOS in the Cd group but only eNOS and nNOS levels were statistically significant. There was no difference in pathological assessment of the renal tissues. Cd toxicity increases NOS isoenzyme levels and may affect renal physiology.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

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.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Gender Differences in Pressure-Natriuresis and Renal Autoregulation

Sexual dimorphism in arterial pressure regulation has been observed in humans and animal models. The mechanisms underlying this gender difference are not fully known. Previous studies in rats have shown that females excrete more salt than males at a similar arterial pressure. The renin-angiotensin system is a powerful regulator of arterial pressure and body fluid volume. This study examined the role of the angiotensin type 2 receptor (AT 2 R) in pressure-natriuresis in male and female rats because AT 2 R expression has been reported to be enhanced in females. Renal function was examined at renal perfusion pressures of 120, 100, and 80 mm Hg in vehicle-treated and AT 2 R antagonist-treated (PD123319; 1 mg/kg/h) groups. The pressure-natriuresis relationship was gender-dependent such that it was shifted upward in female vs male rats ( P <0.001). AT 2 R blockade modulated the pressure-natriuresis relationship, shifting the curve downward in male ( P <0.01) and female ( P <0.01) rats to a similar extent. In females, AT 2 R blockade also reduced the lower end of the autoregulatory range of renal blood flow ( P <0.05) and glomerular filtration rate ( P <0.01). Subsequently, the renal blood flow response to graded angiotensin II infusion was also measured with and without AT 2 R blockade. We found that AT 2 R blockade enhanced the renal vasoconstrictor response to angiotensin II in females but not in males ( P <0.05). In conclusion, the AT 2 R modulates pressure-natriuresis, allowing the same level of sodium to be excreted at a lower pressure in both genders. However, a gender-specific role for the AT 2 R in renal autoregulation was evident in females, which may be a direct vascular AT 2 R effect.

Renal function and vasomotor activity in mice lacking the Cyp4a14 gene

The production of 20-hydroxyeicosatetraenoic acid (20-HETE) in the kidney is thought to be involved in the control of renal vascular tone and tubular sodium and chloride reabsorption. Cytochrome (Cyp) P-450 enzymes of the Cyp4a family in the mouse, namely 4a10, -12 and 14, are involved in 20-HETE synthesis. Recent advances in the molecular genetics of the mouse have produced mice in which Cyp4a isoforms have been disrupted and the consequence of such an approach is examined. This study evaluated the effect of deletion of the Cyp4a14 gene on blood pressure, renal vascular responses and tubular function. When compared with the wild-type (WT) litter mates, systolic blood pressure was greater in Cyp4a14 null (KO) mice as were renal vascular responses to angiotensin II or phenyephrine, G protein-coupled receptor (GPCR) agonists, but not KCl, a non-GPCR agonist. Renal vascular responses to guanosine 5'-O-(gamma-thio)triphosphate, a non-hydrolyzable GTP analog, or NaF(4), an activator of G-proteins, were also enhanced. However, vasodilation to bradykinin or apocynin but not sodium nitroprusside was blunted in Cyp4a14 null (KO) kidneys. These changes in KO mice were accompanied by increased 20-HETE synthesis, reduced renal production of nitric oxide (NO), increased lipid hydroperoxides and increased apocynin-inhibitable vascular NADPH oxidase activity that was prevented by administration of NO synthase (NOS) inhibitor, suggesting endothelial nitric oxide synthase (eNOS) uncoupling. Cyp4a14 KO mice also exhibited a diminished capacity to excrete an acute sodium load (0.9% NaCl, 2.5 mL/kg). These data suggest that deletion of the Cyp4a gene conferred a prohypertensive status via mechanisms involving increased 20-HETE synthesis and eNOS uncoupling leading to increased oxidative stress, enhanced vasoconstriction but diminished vasodilation as well as a defect in the renal excretory capacity in Cyp4a14 KO mice. These mechanisms suggest that the Cyp4a14-deficient mouse may be a useful model for evaluation of NO/20-HETE interactions.

Shear stress increases nitric oxide production in thick ascending limbs

We showed that luminal flow stimulates nitric oxide (NO) production in thick ascending limbs. Ion delivery, stretch, pressure, and shear stress all increase when flow is enhanced. We hypothesized that shear stress stimulates NO in thick ascending limbs, whereas stretch, pressure, and ion delivery do not. We measured NO in isolated, perfused rat thick ascending limbs using the NO-sensitive dye DAF FM-DA. NO production rose from 21 ± 7 to 58 ± 12 AU/min (P < 0.02; n = 7) when we increased luminal flow from 0 to 20 nl/min, but dropped to 16 ± 8 AU/min (P < 0.02; n = 7) 10 min after flow was stopped. Flow did not increase NO in tubules from mice lacking NO synthase 3 (NOS 3). Flow stimulated NO production by the same extent in tubules perfused with ion-free solution and physiological saline (20 ± 7 vs. 24 ± 6 AU/min; n = 7). Increasing stretch while reducing shear stress and pressure lowered NO generation from 42 ± 9 to 17 ± 6 AU/min (P < 0.03; n = 6). In the absence of shear stress, increasing pressure and stretch had no effect on NO production (2 ± 8 vs. 8 ± 8 AU/min; n = 6). Similar results were obtained in the presence of tempol (100 μmol/l), a O(2)(-) scavenger. Primary cultures of thick ascending limb cells subjected to shear stresses of 0.02 and 0.55 dyne/cm(2) produced NO at rates of 55 ± 10 and 315 ± 93 AU/s, respectively (P < 0.002; n = 7). Pretreatment with the NOS inhibitor l-NAME (5 mmol/l) blocked the shear stress-induced increase in NO production. We concluded that shear stress rather than pressure, stretch, or ion delivery mediates flow-induced stimulation of NO by NOS 3 in thick ascending limbs.

Intrarenal purinergic signaling in the control of renal tubular transport

Renal tubular epithelial cells receive hormonal input that regulates volume and electrolyte homeostasis. In addition, numerous intrarenal, local signaling agonists have appeared on the stage of renal physiology. One such system is that of intrarenal purinergic signaling. This system involves all the elements necessary for agonist-mediated intercellular communication. ATP is released from epithelial cells, which activates P2 receptors in the apical and basolateral membrane and thereby modulates tubular transport. Termination of the signal is conducted via the breakdown of ATP to adenosine. Recent far-reaching advances indicate that ATP is often used as a local transmitter for classical sensory transduction. This transmission apparently also applies to sensory functions in the kidney. Locally released ATP is involved in sensing of renal tubular flow or in detecting the distal tubular load of NaCl at the macula densa. This review describes the relevant aspects of local, intrarenal purinergic signaling and outlines its integrative concepts.

Interaction of oxidative stress, nitric oxide and peroxisome proliferator activated receptor gamma in acute renal failure

Oxidative stress has been reported to play a critical role in the pathology of acute renal failure (ARF). An interaction between different reactive species and/or their sources have been the focus of extensive studies. The exact sources of reactive species generated in biological systems under different disease states are always elusive because they are also a part of physiological processes. Exaggerated involvement of different oxidation pathways including NAD(P)H oxidase has been proposed in different models of ARF. An interaction between oxygen species and nitrogen species has drawn extensive attention because of the deleterious effects of peroxynitrite and their possible effects on antioxidant systems. Recent advances in molecular biology have allowed us to understand glomerular function more precisely, especially the organization and importance of the slit diaphragm. Identification of slit diaphragm proteins came as a breakthrough and a possibility of therapeutic manipulation in ARF is encouraging. Transcriptional regulation of the expression of slit diaphragm protein is of particular importance because their presence is crucial in the maintenance of glomerular function. This review highlights the involvement of oxidative stress in ARF, sources of these reactive species, a possible interaction between different reactive species, and involvement of PPARgamma, a nuclear transcription factor in this process.

Nitric oxide produced by endothelial nitric oxide synthase promotes diuresis

Extracellular fluid volume is highly regulated, at least in part, by peripheral resistance and renal function. Nitric oxide (NO) produced by NO synthase type 3 (NOS 3) in the nonrenal vasculature may promote fluid retention by reducing systemic vascular resistance and arterial pressure. In contrast, NO produced by renal NOS 3 promotes water excretion by reducing renal vascular resistance, increasing glomerular filtration, and inhibiting reabsorption along the nephron. Thus, the net effect of NO from NOS 3 on urinary volume (UV) is unclear. We hypothesized that NO produced by NOS 3 promotes water excretion primarily due to renal tubular effects. We gave conscious wild-type and NOS 3 -/- mice an acute volume load and measured UV, blood pressure, plasma renin concentration (PRC), Na(+), vasopressin, and urinary Na(+) and creatinine concentrations. To give the acute volume load, we trained mice to drink a large volume of water while in metabolic cages. On the day of the experiment, water was replaced with 1% sucrose, and mice had access to it for 1 h. Volume intake was similar in both groups. Over 3 h, wild-type mice excreted 62 +/- 10% of the volume load, but NOS 3 -/- excreted only 42 +/- 5% (P < 0.05). Blood pressure in NOS 3 -/- was 118 +/- 3 compared with 110 +/- 2 mmHg in wild-type mice (P < 0.05), but it did not change following volume load in either strain. PRC, vasopressin, and glomerular filtration rate were similar between groups. Urinary Na(+) excretion was 49.3 +/- 7.0 in wild-type vs. 37.8 +/- 6.4 mumol/3 h in NOS 3 -/- mice (P < 0.05). Bumetanide administration eliminated the difference in volume excretion between wild-type and NOS 3 -/- mice. We conclude that 1) NO produced by NOS 3 promotes water and Na(+) excretion and 2) the renal epithelial actions of NO produced by NOS 3 supersede the systemic and renal vascular actions.

Nitric oxide originating from NOS1 controls oxygen utilization and electrolyte transport efficiency in the diabetic kidney

Nitric oxide (NO) is a potent regulator of both vascular tone and cellular oxygen consumption (Q(O(2)). Diabetic kidneys have reduced NO availability and increased Q(O(2)). However, the exact nitric oxide synthase (NOS) isoform regulating Q(O(2)), hemodynamics, and excretory function in the diabetic kidney remains unclear. We therefore investigated the effects of both selective neuronal NOS (NOS1) inhibition and nonselective NOS inhibition. Oxygen utilization, electrolyte transport efficiency [tubular Na(+) transport (T(Na))/Q(O(2))], renal blood flow (RBF), glomerular filtration rate (GFR), and mean arterial pressure (MAP) were measured in vivo in control and streptozotocin-diabetic rats before and after administration of the selective NOS1 inhibitor S-methyl-L-thiocitrulline (SMTC) or the nonselective NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Diabetic rats had higher baseline Q(O(2)) and GFR than control rats, although RBF was similar in the groups. SMTC and L-NAME increased Q(O(2)) and reduced T(Na)/Q(O(2)) only in the diabetic animals, whereas both inhibitors increased MAP and reduced RBF in both groups. GFR was reduced by L-NAME, but SMTC had no effect in either group. Carbachol increased RBF and decreased MAP in SMTC-treated rats, whereas it had no effect in L-NAME-treated rats, indicating that SMTC selectively inhibited NOS1. In conclusion, NO regulates RBF and GFR similarly in both control and diabetic rats. However, selective NOS1 inhibition increased Qo(2) and reduced T(Na)/Q(O(2)) in the diabetic rat kidney, indicating a pivotal role of NO produced by NOS1 in maintaining control of Q(O(2)) and tissue oxygenation in these kidneys.

Role for endothelial nitric oxide synthase in nitrite-induced protection against renal ischemia-reperfusion injury in mice

Nitrite is protective against renal ischemia/reperfusion injury (IRI); an effect due to its reduction to nitric oxide (NO). In addition to other reductase pathways, endothelial NO synthase (eNOS) may also facilitate nitrite reduction in ischemic environments. We investigated the role of eNOS in sodium nitrite (60 microM, 10 ml/kg applied topically 1 min before reperfusion)-induced protection against renal IRI in C57/BL6 wild-type (WT) and eNOS knockout (eNOS KO) mice subjected to bilateral renal ischemia (30 min) and reperfusion (24h). Markers of renal dysfunction (plasma [creatinine] and [urea]), damage (tubular histology) and inflammation (cell recruitment) were elevated following IRI in WT mice; effects significantly reduced following nitrite treatment. Chemiluminescence analysis of cortical and medullary sections of the kidney demonstrated rapid (within 1 min) distribution of nitrite following application. Whilst IRI caused a significant (albeit substantially reduced compared to WT mice) elevation of markers of renal dysfunction and damage in eNOS KO mice, the beneficial effects of nitrite were absent or reduced, respectively. Moreover, nitrite treatment enhanced renal dysfunction in the form of increased plasma [creatinine] in eNOS KO mice. Confirmation of nitrite reductase activity of eNOS was provided by demonstration of nitrite (100 microM)-derived NO production by kidney homogenates of WT mice, that was significantly reduced by L-NMMA. L-NMMA was without effect using kidney homogenates of eNOS KO mice. These results support a role for eNOS in the pathways activated during renal IRI and also identify eNOS as a nitrite reductase in ischemic conditions; activity which in part underlies the protective effects of nitrite.

Responses of sodium-hydrogen exchange to nitric oxide in porcine cultured nonpigmented ciliary epithelium

PURPOSE

To better understand how nitric oxide (NO) alters the function of the nonpigmented ciliary epithelium (NPE), studies were performed to determine the influence of NO on sodium-hydrogen exchanger (NHE) activity.

METHODS

Cytoplasmic pH (pH(i)) was measured in cultured porcine NPE loaded with BCECF (2',7'-bis(2-carboxyl)-5(6)-carboxyfluorescein-acetoxyethyl ester). Na-H exchanger (NHE) was examined by immunolocalization.

RESULTS

In cells acidified by 5 minutes of exposure to 20 mM ammonium chloride, pH(i) recovery was partially inhibited by sodium nitroprusside (SNP), an NO donor, and l-arginine, the endogenous substrate for NO synthase. SNP and dimethyl amiloride (DMA), an NHE inhibitor, inhibited pH(i) recovery to a similar degree. In bicarbonate-free buffer SNP+DMA elicited no additional change in pH(i) recovery beyond that elicited by DMA alone. This suggests that SNP causes NHE inhibition. the SNP's effect on pH(i) recovery was mimicked by 8-pCPT-cGMP but suppressed by ODQ and H-8. Ouabain alone reduced pH(i) recovery, but SNP+ouabain caused significant further reduction. Immunolocalization studies revealed NHE1 and -4 in native and cultured NPE.

CONCLUSIONS

NHE1 and -4 are expressed at the NPE basolateral margin. The findings suggest the NHE is inhibited by NO which acts via a cGMP and protein kinase G signaling pathway. The NHE response does not appear to be the consequence of NO-induced Na,K-ATPase inhibition. Because NO synthases are expressed in porcine NPE, NO could act as an autocrine regulator of NHE activity. Although NHE inhibitors are known to lower intraocular pressure (IOP), further studies are needed to understand whether changes in NHE activity contribute to the IOP-lowering effect of NO donors.

Inhibition of natriuretic factors increases blood pressure in rats

Renal dopamine and nitric oxide contribute to natriuresis during high-salt intake which maintains sodium and blood pressure homeostasis. We wanted to determine whether concurrent inhibition of these natriuretic factors increases blood pressure during high-sodium intake. Male Sprague-Dawley rats were divided into the following groups: 1) vehicle (V)-tap water, 2) NaCl-1% NaCl drinking water, 3) 30 mM l-buthionine sulfoximine (BSO), an oxidant, 4) BSO plus NaCl, and 5) BSO plus NaCl with 1 mM tempol (antioxidant). Compared with V, NaCl intake for 10 days doubled sodium intake and increased urinary dopamine level but reduced urinary nitric oxide content. NaCl intake also reduced basal renal proximal tubular Na-K-ATPase activity with no effect on blood pressure. However, NaCl intake in BSO-treated rats failed to reduce basal Na-K-ATPase activity despite higher urinary dopamine levels. Also, dopamine failed to inhibit proximal tubular Na-K-ATPase activity and these rats exhibited reduced urinary nitric oxide levels and high blood pressure. Tempol supplementation in NaCl plus BSO-treated rats reduced blood pressure. BSO treatment alone did not affect the urinary nitric oxide and dopamine levels or blood pressure. However, dopamine failed to inhibit proximal tubular Na-K-ATPase activity in BSO-treated rats. BSO treatment also increased basal protein kinase C activity, D1 receptor serine phosphorylation, and oxidative markers like malondialdehyde and 8-isoprostane. We suggest that NaCl-mediated reduction in nitric oxide does not increase blood pressure due to activation of D1 receptor signaling. Conversely, oxidative stress-provoked inhibition of D1 receptor signaling fails to elevate blood pressure due to presence of normal nitric oxide. However, simultaneously decreasing nitric oxide levels with NaCl and inhibiting D1 receptor signaling with BSO elevated blood pressure.

Production and actions of hydrogen sulfide, a novel gaseous bioactive substance, in the kidneys

Hydrogen sulfide (H(2)S), a novel endogenous gaseous bioactive substance, has recently been implicated in the regulation of cardiovascular and neuronal functions. However, its role in the control of renal function is unknown. In the present study, incubation of renal tissue homogenates with L-cysteine (L-Cys) (as a substrate) produced H(2)S in a concentration-dependent manner. This H(2)S production was completely abolished by inhibition of both cystathionine beta-synthetase (CBS) and cystathionine gamma-lyase (CGL), two major enzymes for the production of H(2)S, using amino-oxyacetic acid (AOAA), an inhibitor of CBS, and propargylglycine (PPG), an inhibitor of CGL. However, inhibition of CBS or CGL alone induced a small decrease in H(2)S production. In anesthetized Sprague-Dawley rats, intrarenal arterial infusion of an H(2)S donor (NaHS) increased renal blood flow, glomerular filtration rate (GFR), urinary sodium (U(Na) x V), and potassium (U(K) x V) excretion. Consistently, infusion of both AOAA and PPG to inhibit the endogenous H(2)S production decreased GFR, U(Na) x V, and U(K) x V, and either one of these inhibitors alone had no significant effect on renal functions. Infusion of L-Cys into renal artery to increase the endogenous H(2)S production also increased GFR, U(Na) x V, and U(K) x V, which was blocked by AOAA plus PPG. It was shown that H(2)S had both vascular and tubular effects and that the tubular effect of H(2)S might be through inhibition of Na(+)/K(+)/2Cl(-) cotransporter and Na(+)/K(+)/ATPase activity. These results suggest that H(2)S participates in the control of renal function and increases urinary sodium excretion via both vascular and tubular actions in the kidney.

Abundance of the Na-K-2Cl cotransporter NKCC2 is increased by high-fat feeding in Fischer 344 X Brown Norway (F1) rats

Insulin resistance is associated with hypertension by mechanisms likely involving the kidney. To determine how the major apical sodium transporter of the thick ascending limb, the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) is regulated by high-fat feeding, we treated young male, Fischer 344 X Brown Norway (F344BN) rats for 8 wk with diets containing either normal (NF, 4%) or high (HF, 36%) fat, by weight, primarily as lard. HF-fed rats had impaired glucose tolerance, increased urine excretion of 8-isoprostane (a marker of oxidative stress), increased protein levels for NKCC2 (50-125%) and the renal outer medullary potassium channel (106%), as well as increased natriuretic response to furosemide (20-40%). To test the role of oxidative stress in this response, in study 2, rats were fed the NF or HF diet plus plain drinking water, or water containing N(G)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor (100 mg/l), or tempol, a superoxide dismutase mimetic (1 mmol/l). The combination of tempol with HF nullified the increase in medullary NKCC2, while l-NAME with HF led to the highest expression of medullary NKCC2 (to 498% of NF mean). However, neither of these drugs dramatically affected the elevated natriuretic response to furosemide with HF. Finally, l-NAME led to a marked increase in blood pressure (measured by radiotelemetry), which was significantly enhanced with HF. Mean arterial blood pressure at 7 wk was as follows (mmHg): NF, 100 +/- 2; NF plus l-NAME, 122 +/- 3; and HF plus l-NAME, 131 +/- 2. Overall, HF feeding increased the abundance of NKCC2. Inappropriately high sodium reabsorption in the thick ascending limb via NKCC2 may contribute to hypertension with insulin resistance.

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.

Leptin: linking obesity, the metabolic syndrome, and cardiovascular disease

The incidence and prevalence of obesity and the metabolic syndrome have risen markedly in the past decade, representing a serious cardiovascular health hazard with significant morbidity and mortality. The etiology of the metabolic syndrome and its various pathogenic mechanisms are incompletely defined and under intense investigation. Contemporary research suggests that the adipocyte-derived hormone leptin may be an important factor linking obesity, the metabolic syndrome, and cardiovascular disorders. Although recent evidence indicates that under normal conditions leptin may be an important factor in regulating pressure and volume, during situations of chronic hyperleptinemia and leptin resistance, this hormone may function pathophysiologically for the development of hypertension and cardiac and renal diseases. Future research will determine if reduction of circulating leptin and/or blockade of its peripheral actions can confer cardiovascular and renal protection in hyperleptinemic patients with obesity and the metabolic syndrome.

Nitric oxide decreases expression of osmoprotective genes via direct inhibition of TonEBP transcriptional activity

During antidiuresis, renal medullary cells adapt to the hyperosmotic interstitial environment by increased expression of osmoprotective genes, which is driven by a common transcriptional activator, tonicity-responsive enhancer binding protein (TonEBP). Because nitric oxide (NO) is abundantly produced in the renal medulla, the present studies addressed the effect of NO on expression of osmoprotective genes and TonEBP activation in MDCK cells. Several structurally unrelated NO donors blunted tonicity-induced up-regulation of TonEBP target genes involved in intracellular accumulation of organic osmolytes. These effects were mediated by reduced transcriptional activity of TonEBP, as assessed by tonicity-responsive elements- and aldose reductase promoter-driven reporter constructs. Neither total TonEBP abundance nor nuclear translocation of TonEBP was affected by NO. Furthermore, 8-bromo-cGMP and peroxynitrite failed to reproduce the inhibitory effect of NO, indicating that NO acts directly on TonEBP rather than through classical NO signaling pathways. In support of this notion, electrophoretic mobility shift assays showed reduced binding of TonEBP to its target sequence in nuclear extracts prepared from MDCK cells treated with NO in vivo and in nuclear extracts exposed to NO in vitro. Furthermore, immunoprecipitation of S-nitrosylated proteins and the biotin-switch method identified TonEBP as a target for S-nitrosylation, which correlates with reduced DNA binding and transcriptional activity. These observations disclose a novel direct inhibitory effect of NO on TonEBP, a phenomenon that may be relevant for regulation of osmoprotective genes in the renal medulla.

Intrarenal oxygenation: unique challenges and the biophysical basis of homeostasis

The kidney is faced with unique challenges for oxygen regulation, both because its function requires that perfusion greatly exceeds that required to meet metabolic demand and because vascular control in the kidney is dominated by mechanisms that regulate glomerular filtration and tubular reabsorption. Because tubular sodium reabsorption accounts for most oxygen consumption (Vo2) in the kidney, renal Vo2 varies with glomerular filtration rate. This provides an intrinsic mechanism to match changes in oxygen delivery due to changes in renal blood flow (RBF) with changes in oxygen demand. Renal Vo2 is low relative to supply of oxygen, but diffusional arterial-to-venous (AV) oxygen shunting provides a mechanism by which oxygen superfluous to metabolic demand can bypass the renal microcirculation. This mechanism prevents development of tissue hyperoxia and subsequent tissue oxidation that would otherwise result from the mismatch between renal Vo2 and RBF. Recent evidence suggests that RBF-dependent changes in AV oxygen shunting may also help maintain stable tissue oxygen tension when RBF changes within the physiological range. However, AV oxygen shunting also renders the kidney susceptible to hypoxia. Given that tissue hypoxia is a hallmark of both acute renal injury and chronic renal disease, understanding the causes of tissue hypoxia is of great clinical importance. The simplistic paradigm of oxygenation depending only on the balance between local perfusion and Vo2 is inadequate to achieve this goal. To fully understand the control of renal oxygenation, we must consider a triad of factors that regulate intrarenal oxygenation: local perfusion, local Vo2, and AV oxygen shunting.

Hemodynamic parameters during normal and hypertensive pregnancy in rats: evaluation of renal salt and water transporters

OBJECTIVE

To determine whether alterations in extracellular volume expansion observed during normal and hypertensive pregnancy run in parallel to changes in the mRNA expression of renal transporters.

METHODS

Wistar rats were divided into four groups: control (C, n = 5); pregnancy (P, n = 5); N(omega)-nitro-l-arginine methyl ester (L-NAME; 50 mg/kg/d)-treated control (H, n = 6); and pregnant rats (HP, n = 6). Hemodynamic studies were performed on day 14 of pregnancy, at which time we also analyzed of the sodium transporters (NHE3, Na/K/2Cl and Na/Cl), potassium channel (ROMK2) and water channel (AQP2).

RESULTS

As expected, P rats presented high cardiac output (CO) and normal blood pressure (BP), whereas H rats presented lower CO and elevated BP. A significant (threefold) increase in total vascular resistance and a decrease in stroke volume were observed in the HP group. Hypertension resulting from nitric oxide (NO) synthesis inhibition blunted systemic hemodynamic adaptations during pregnancy. Compared with C rats, mRNA expression of ROMK2 in P rats was lower, whereas that of AQP2 was higher. Expression of AQP2 was significantly higher in H than in C or HP groups. Expression of BSC and NHE3 was lower in the HP than in the P group. The NO inhibition also provoked renal transporter alterations in HP.

CONCLUSIONS

Our results suggest that tubule transporter variants may mediate the hemodynamic adaptations seen during pregnancy, although we cannot rule out the hypothesis that other factors are also mediating hemodynamic changes.