Blockade of distal nephron sodium transport attenuates pressure natriuresis in dogs

High blood pressure induced by vitamin D deficiency is associated with renal overexpression and hyperphosphorylation of Na+-K+-2Cl- cotransporter type 2

OBJECTIVES

Clinical and epidemiological studies have suggested a correlation between vitamin D deficiency (VDD) and high blood pressure (BP). This study aimed to test the hypothesis that high BP induced by VDD is associated with altered expression and covalent modification of apical sodium transporters along the nephron. The contributions of the intrarenal renin-angiotensin system (RAS) and oxidative stress were also investigated.

METHODS

Male Wistar rats were fed a vitamin D-free (n = 26) or standard diet (n = 25) for 30 days. BP was recorded using noninvasive and invasive procedures. The expression levels of total and phosphorylated apical sodium transporters in rat renal cortex and medulla were evaluated by immunoblotting. Intrarenal RAS components were assessed by immunoblotting and ELISA. Renal oxidative stress was analyzed by measuring the concentrations of thiobarbituric acid reactive substances and reduced glutathione.

RESULTS

Higher BP levels in VDD rats than controls were accompanied by overexpression and hyperphosphorylation of renal cortical and medullary Na+-K+-2Cl- cotransporter type 2, enhanced levels of phosphorylated Na+/H+ exchanger type 3, and reduced expression levels of total and phosphorylated Na+/Cl- cotransporter. Changes in intrarenal RAS induced by VDD vs. controls included the marked elevation of medullary renin expression, higher expression of cortical angiotensinogen, higher urinary angiotensinogen excretion, and higher cortical and medullary angiotensin II content. VDD rats displayed higher thiobarbituric acid reactive substances/glutathione ratios in the renal cortex and medulla than controls.

CONCLUSION

These results suggest that the molecular mechanisms underlying the effects of VDD on BP may include the upregulation of Na+-K+-2Cl- cotransporter type 2 and activation of intrarenal RAS and oxidative stress.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

AT2 receptor non-peptide agonist C21 promotes natriuresis in obese Zucker rats

Previously, we demonstrated that angiotensin II type 2 (AT(2)) receptors have a role in natriuresis in obese Zucker rats (OZR). In the present study, we investigated the role of a novel, non-peptide agonist, C21, in natriuresis via AT(2) receptor activation in OZR. Infusion of C21 (1 and 5 μg kg(-1) min(-1)) into rats under anesthesia caused a dose-dependent increase in urine flow (UF) and urinary Na volume (U(Na)V). These effects of C21 were blocked by pre-infusion of the AT(2) receptor antagonist, PD123319, (50 μg kg(-1) min(-1)), suggesting involvement of the AT(2) receptor. Infusion of C21 (5 μg kg(-1) min(-1)) significantly increased the fractional excretion of sodium without changing the glomerular filtration rate or blood pressure, suggesting a tubular effect. Similarly, C21 infusion increased the fractional excretion of lithium, suggesting a proximal tubular effect. Furthermore, we tested the effect of C21 on natriuresis after blocking two main, distal-nephron Na transporters, the epithelial Na channels (ENaC), with amiloride (AM, 3 mg kg(-1) body wt), and the NaCl cotransporters (NCC), with bendroflumethiazide (BFTZ, 7 mg kg(-1) body wt). Infusion of AM + BFTZ caused significant increases in both diuresis and natriuresis, which were further increased by infusion of C21 (5 μg kg(-1) min(-1)). Natriuresis in response to C21 was associated with increases in urinary NO and cGMP levels. The data indicate that the AT(2) receptor agonist, C21, promotes natriuresis via AT(2) receptor activation and that this effect is potentially based in the proximal tubules and linked to the nitric oxide/cyclic guanosine monophosphate pathway. The natriuretic response to C21 may have therapeutic significance by improving kidney function in obesity.

Inhibition of soluble epoxide hydrolase improves the impaired pressure-natriuresis relationship and attenuates the development of hypertension and hypertension-associated end-organ damage in Cyp1a1-Ren-2 transgenic rats

OBJECTIVE

In the present study, we compared the effects of treatment with the novel soluble epoxide hydrolase (sEH) inhibitor (c-AUCB) with those of the AT1 receptor antagonist losartan on blood pressure (BP), autoregulation of renal blood flow (RBF) and on glomerular filtration rate (GFR) and the pressure-natriuresis relationship in response to stepwise reduction in renal arterial pressure (RAP) in Cyp1a1-Ren-2 transgenic rats.

METHODS

Hypertension was induced in Cyp1a1-Ren-2 rats through dietary administration for 11 days of the natural xenobiotic indole-3-carbinol (I3C) which activates the renin gene. Treatment with c-AUCB and losartan was started 48 h before initiating administration of the diet containing I3C. Rats were prepared for renal functional studies to evaluate in-vivo renal autoregulatory efficiency when RAP was gradually decreased by an aortic clamp.

RESULTS

I3C administration resulted in the development of severe hypertension which was associated with markedly lower basal RBF and GFR and substantially impaired autoregulatory efficiency as well as a suppression of the pressure-natriuresis relationship when compared with noninduced rats. Treatment with c-AUCB significantly decreased BP, improved autoregulatory efficiency of RBF and GFR and the slope of pressure-natriuresis relationship. Treatment with losartan completely prevented the impaired autoregulation and pressure-natriuresis relationship as well as the development of hypertension in I3C-induced rats.

CONCLUSION

Our present findings indicate that chronic treatment with the sEH inhibitor c-AUCB substantially attenuates the development of malignant hypertension in I3C-induced rats likely via improvement of the renal autoregulatory efficiency and the pressure-natriuresis relationship.

Role of cytochrome P-450 metabolites in the regulation of renal function and blood pressure in 2-kidney 1-clip hypertensive rats

Alterations in renal function contribute to Goldblatt two-kidney, one-clip (2K1C) hypertension. A previous study indicated that bioavailability of cytochrome P-450 metabolites epoxyeicosatrienoic acids (EETs) is decreased while that of 20-hydroxyeicosatetraenoic acids (20-HETE) is increased in this model. We utilized the inhibitor of soluble epoxide hydrolase cis-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB) and HET-0016, the inhibitor of 20-HETE production, to study the role of EETs and 20-HETE in the regulation of renal function. Chronic c-AUCB treatment significantly decreased systolic blood pressure (SBP) (133 ± 1 vs. 163 ± 3 mmHg) and increased sodium excretion (1.23 ± 0.10 vs. 0.59 ± 0.03 mmol/day) in 2K1C rats. HET-0016 did not affect SBP and sodium excretion. In acute experiments, renal blood flow (RBF) was decreased in 2K1C rats (5.0 ± 0.2 vs. 6.9 ± 0.2 ml·min(-1)·g(-1)). c-AUCB normalized RBF in 2K1C rats (6.5 ± 0.6 ml·min(-1)·g(-1)). HET-0016 also increased RBF in 2K1C rats (5.8 ± 0.2 ml·min(-1)·g(-1)). Although RBF and glomerular filtration rate (GFR) remained stable in normotensive rats during renal arterial pressure (RAP) reductions, both were significantly reduced at 100 mmHg RAP in 2K1C rats. c-AUCB did not improve autoregulation but increased RBF at all RAPs and shifted the pressure-natriuresis curve to the left. HET-0016-treated 2K1C rats exhibited impaired autoregulation of RBF and GFR. Our data indicate that c-AUCB displays antihypertensive properties in 2K1C hypertension that are mediated by an improvement of RBF and pressure natriuresis. While HET-0016 enhanced RBF, its anti-natriuretic effect likely prevented it from producing a blood pressure-lowering effect in the 2K1C model.

ANP-mediated inhibition of distal nephron fractional sodium reabsorption in wild-type and mice overexpressing natriuretic peptide receptor

Atrial natriuretic peptide (ANP) elicits natriuresis; however, the relative contributions of proximal and distal nephron segments to the overall ANP-induced natriuresis have remained uncertain. This study was performed to characterize the effects of ANP on distal nephron sodium reabsorption determined after blockade of the two major distal nephron sodium transporters with amiloride (5 microg/g body wt) plus bendroflumethiazide (12 microg/g body wt) in male anesthetized C57/BL6 and natriuretic peptide receptor-A gene (Npr1) targeted four-copy mice. The lower dose of ANP (0.1 ng x g body wt(-1) x min(-1), n = 6) increased distal sodium delivery (DSD, 2.4 +/- 0.4 vs. 1.6 +/- 0.2 mueq/min, P < 0.05) but did not change fractional reabsorption of DSD compared with control (86.3 +/- 2.0 vs. 83.9 +/- 3.6%, P > 0.05), thus limiting the magnitude of the natriuresis. In contrast, the higher dose (0.2 ng x g body wt(-1) x min(-1), n = 6) increased DSD (2.8 +/- 0.3 mueq/min, P < 0.01) and also decreased fractional reabsorption of DSD (67.4 +/- 4.5%, P < 0.01), which markedly augmented the natriuresis. In Npr1 gene-duplicated four-copy mice (n = 6), the lower dose of ANP increased urinary sodium excretion (0.6 +/- 0.1 vs. 0.3 +/- 0.1 mueq/min, P < 0.05) and decreased fractional reabsorption of DSD compared with control (72.2 +/- 3.4%, P < 0.05) at similar mean arterial pressures (91 +/- 6 vs. 92 +/- 3 mmHg, P > 0.05). These results provide in vivo evidence that ANP-mediated increases in DSD alone exert modest effects on sodium excretion and that inhibition of fractional reabsorption of distal sodium delivery is requisite for the augmented natriuresis in response to the higher dose of ANP or in Npr1 gene-duplicated mice.

Cholesterol induces renal vasoconstriction and anti-natriuresis by inhibiting nitric oxide production in anesthetized rats

Although hypercholesterolemia is implicated in the pathophysiology of many renal disorders as well as hypertension, its direct actions in the kidney are not yet clearly understood. In the present study, we evaluated renal responses to administration of cholesterol (8 microg x min(-1).100 g body wt(-1); bound by polyethylene glycol) into the renal artery of anesthetized male Sprague-Dawley rats. Total renal blood flow (RBF) was measured by a Transonic flow probe, and glomerular filtration rate (GFR) was determined by Inulin clearance. In control rats (n = 8), cholesterol induced reductions of 10 +/- 2% in RBF [baseline (b) 7.6 +/- 0.3 microg x min(-1).100 g(-1)], 17 +/- 3% in urine flow (b, 10.6 +/- 0.9 microg x min(-1).100 g(-1)), 29 +/- 3% in sodium excretion (b, 0.96 +/- 0.05 mumol.min(-1).100 g(-1)) and 24 +/- 2% in nitrite/nitrate excretion (b, 0.22 +/- 0.01 nmol.min(-1).100 g(-1)) without an appreciable change in GFR (b, 0.87 +/- 0.03 ml.min(-1).100 g(-1)). These renal vasoconstrictor and anti-natriuretic responses to cholesterol were absent in rats pretreated with nitric oxide (NO) synthase inhibitor, nitro-l-arginine methylester (0.5 microg x min(-1).100 g(-1); n = 6). In rats pretreated with superoxide (O(2)(-)) scavenger tempol (50 microg x min(-1).100 g(-1); n = 6), the cholesterol-induced renal responses remained mostly unchanged, although there was a slight attenuation in anti-natriuretic response. This anti-natriuretic response to cholesterol was abolished in furosemide-pretreated rats (0.3 microg x min(-1).100 g(-1); n = 6) but remained unchanged in amiloride-pretreated rats (0.2 microg x min(-1).100 g(-1); n = 5), indicating that Na(+)/K(+)/2Cl(-) cotransport is the dominant mediator of this effect. These data demonstrate that cholesterol-induced acute renal vasoconstrictor and antinatriuretic responses are mediated by a decrease in NO production. These data also indicate that tubular effect of cholesterol on sodium reabsorption is mediated by the furosemide sensitive Na(+)/K(+)/2Cl(-) cotransporter.

Enhanced distal nephron sodium reabsorption in chronic angiotensin II-infused mice

Chronic angiotensin II (Ang II) infusions enhance urinary excretion of angiotensinogen, suggesting augmentation of distal nephron sodium reabsorption. To assess whether chronic Ang II infusions (15 ng/min for 2 weeks) enhance distal nephron sodium reabsorption, we compared sodium excretion before and after blockade of the 2 main distal nephron sodium transporters by IV amiloride (5 mg/kg of body weight) plus bendroflumethiazide (12 mg/kg of body weight) in male C57/BL6 anesthetized control mice (n=10) and in chronic Ang II-infused mice (n=8). Chronic Ang II infusions increased systolic blood pressure to 141+/-6 mm Hg compared with 106+/-4 mm Hg in control mice. After anesthesia, mean arterial pressure averaged 97+/-4 mm Hg in chronic Ang II-infused mice compared with 94+/-3 mm Hg in control mice, allowing comparison of renal function at similar arterial pressures. Ang II-infused mice had lower urinary sodium excretion (0.16+/-0.04 versus 0.30+/-0.05 microEq/min; P<0.05), higher distal sodium reabsorption (1.74+/-0.18 versus 1.12+/-0.18 microEq/min; P<0.05), and higher fractional reabsorption of distal sodium delivery (91.1+/-1.8% versus 77.9+/-4.3%; P<0.05) than control mice. Urinary Ang II concentrations, measured during distal blockade, were greater in Ang II-infused mice (1235.0+/-277.2 versus 468.9+/-146.9 fmol/mL; P<0.05). In chronic Ang II-infused mice treated with spironolactone (n=5), fractional reabsorption of distal sodium delivery was similarly augmented as in chronic Ang II-infused mice (94.6+/-1.7%; P<0.01). These data provide in vivo evidence that there is enhanced distal sodium reabsorption dependent on sodium channel and Na(+)-Cl(-) cotransporter activity and increased urinary Ang II concentrations in mice infused chronically with Ang II.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Acute angiotensin II infusions elicit pressure natriuresis in mice and reduce distal fractional sodium reabsorption

Acute angiotensin II (Ang II) infusions into mice increase arterial pressure (AP) and elicit pressure natriuresis. We used this model of pressure natriuresis to delineate the distal nephron responses to AP-mediated increases in distal sodium delivery. In the first group, we measured changes in urinary sodium excretion (U(Na)V) in male C57/BL6 anesthetized mice (n=9) before and during acute Ang II infusions (5 ng/g of body weight per minute). Acute Ang II infusions increased AP (98+/-3 to 126+/-5 mm Hg; P<0.001), urine flow (2.7+/-0.5 to 6.0+/-0.8 microL/min; P<0.01), and U(Na)V (0.6+/-0.2 to 1.3+/-0.2 microEq/min; P<0.05). There were significant relationships between U(Na)V and urine flow (y=0.207x+0.030; P<0.0001) and between U(Na)V and AP (y=0.027x-2.100). In a separate series, distal sodium delivery and fractional reabsorption of distal sodium delivery were determined in control (n=12) and Ang II-infused mice (n=8) by comparing U(Na)V before and after blockade of the 2 major distal nephron sodium transporters with amiloride (5 mg/kg of body weight) plus bendroflumethiazide (12 mg/kg of body weight). A positive relationship was found between U(Na)V (y=0.015x-1.100; P<0.0001) or distal sodium delivery (y=0.027x-0.900; P<0.0001) and AP. An inverse relationship was found between fractional reabsorption of distal sodium delivery and AP (y=-0.511x+128.300; P<0.01). These data indicate that Ang II-mediated pressure natriuresis involves an increase in distal sodium delivery combined with a reduced distal nephron fractional sodium reabsorption, suggesting that increased AP prevents the distal nephron transport mechanisms from accommodating the increased distal delivery.

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.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Blood pressure maintenance in NHE3-deficient mice with transgenic expression of NHE3 in small intestine

NHE3 Na+/H+ exchanger knockout ( Nhe3−/−) mice have severe absorptive deficits in the kidney proximal tubule and intestinal tract. The resulting hypovolemia has confounded efforts to carefully evaluate the specific effects of NHE3 deficiency on kidney function. Development of mice with transgenic expression of NHE3 in the small intestine (tg Nhe3−/−) has allowed us to analyze the role of renal NHE3 in overall maintenance of blood pressure, pressure natriuresis, and autoregulation of both glomerular filtration rate (GFR) and renal blood flow (RBF). Ambulatory blood pressure, measured by telemetry, was lower in tg Nhe3−/− mice than in wild-type controls (tg Nhe3+/+) when the mice were maintained on a normal NaCl diet but was normalized when they were provided with a high NaCl intake. Furthermore, administration of the AT1-receptor blocker losartan showed that circulating ANG II plays a major role in maintaining blood pressure in tg Nhe3−/− mice fed normal NaCl but not in those receiving high NaCl. Clearance studies revealed a blunted pressure-natriuresis response in tg Nhe3−/− mice at lower blood pressures but a robust response at higher blood pressures. Autoregulation of GFR and RBF was normal in tg Nhe3−/− mice. These results show that dietary NaCl loading normalizes blood pressure in awake tg Nhe3−/− mice and that alterations in NHE3 activity are not essential for normal autoregulation of GFR and RBF. Furthermore, the data strongly support the hypothesis that NHE3 plays an important role in the diuretic and natriuretic responses to increases in blood pressure but also show that mechanisms not involving NHE3 mediate pressure natriuresis in the higher range of blood pressures studied.

Role of WNK kinases in regulating tubular salt and potassium transport and in the development of hypertension

A recently discovered family of protein kinases is responsible for an autosomal-dominant disease known as Gordon's syndrome or pseudohypoaldosteronism type II (PHA-II) that features hyperkalemia and hyperchloremic metabolic acidosis, accompanied by hypertension and hypercalciuria. Four genes have been described in this kinase family, which has been named WNK, due to the absence of a key lysine in kinase subdomain II (with no K kinases). Two of these genes, WNK1 and WNK4 located in human chromosomes 12 and 17, respectively, are responsible for PHA-II. Immunohystochemical analysis revealed that WNK1 and WNK4 are predominantly expressed in the distal convoluted tubule and collecting duct. The physiological studies have shown that WNK4 downregulates the activity of ion transport pathways expressed in these nephron segments, such as the apical thiazide-sensitive Na+-Cl−cotransporter and apical secretory K+channel ROMK, as well as upregulates paracellular chloride transport and phosphorylation of tight junction proteins such as claudins. In addition, WNK4 downregulates other Cl−influx pathways such as the basolateral Na+-K+-2Cl−cotransporter and Cl−/HCO3−exchanger. WNK4 mutations behave as a loss of function for the Na+-Cl−cotransporter and a gain of function when it comes to ROMK and claudins. These dual effects of WNK4 mutations fit with proposed mechanisms for developing electrolyte abnormalities and hypertension in PHA-II and point to WNK4 as a multifunctional regulator of diverse ion transporters.

Altered expression of renal apical plasma membrane Na+ transporters in the early phase of genetic hypertension

The present study explores whether the development of hypertension in the Milan strain of rats (MHS) rats is preceded or paralleled by alterations of mRNA and/or protein levels of the major luminal Na+ transporters. MHS rats were studied at 23-25 days after birth; age-matched Milan normotensive (MNS) rats were used as controls. The glomerular filtration rate (GFR), measured by inulin clearance, was higher in MHS than in MNS rats, while the mean blood pressure was not different in the two strains of animals indicating that the MHS rats were still in the prehypertensive state. Type 3 sodium/hydrogen exchanger (NHE3), bumetanide-sensitive sodium-potassium-2 chloride cotransporter (NKCC2), sodium-chloride cotransporter (NCC) and alpha-ENaC mRNA abundances were quantified by competitive PCR. In MHS compared with MNS, mRNA abundance was unchanged for NHE3 in proximal tubules, higher for NKCC2 in medullary thick ascending limbs of Henle's loops (TAL) and lower for NCC in distal convoluted tubules (DCT) and for alpha-ENaC along collecting ducts (CD). Western blot experiments revealed 1) unchanged NHE3; 2) a significant increase in NKCC2 in the outer medulla; 3) a significant decrease in NCC in the renal cortex and of alpha-ENaC in both the renal cortex and outer medulla, whereas beta- and gamma-ENaC remained unchanged. These data indicate that, in MHS rats, there is a strong upregulation of NKCC2 along the TAL associated with increased GFR, robust inhibition of NCC cotransporter along the DCT and modest downregulation of alpha-ENaC along the CD. The interplay of the various Na+ transporters may well explain why, at this age, the rats are still in the prehypertensive state.

Chronic renal injury-induced hypertension alters renal NHE3 distribution and abundance

Renal cortical phenol injection provokes acute sympathetic nervous system-dependent hypertension and a shift of proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3) and Na(+)-P(i) cotransporter type 2 (NaPi2) to apical microvilli. This study aimed to determine whether proximal tubule (PT) Na(+) transporter redistribution persists chronically and whether the pool sizes of renal Na(+) transporters are altered. At 5 wk after a 50-microl 10% phenol injection, blood pressure is elevated: 154 +/- 8 vs. 113 +/- 11 mmHg after saline injection. Cortical membranes were fractionated into three "windows" enriched in apical brush border (WI), mixed apical and intermicrovillar cleft (WII), and intracellular membranes (WIII). NHE3 relative distribution in these windows, assessed by immunoblots and expressed as %total, remained shifted to apical from intracellular membranes (WI: 25.3 +/- 3 in phenol vs.12.7 +/- 3% in saline and WIII: 9.1 +/- 1.3 in phenol vs. 18.9 +/- 3% in saline). NaPi2 and dipeptidyl-peptidase IV also remained shifted to WI, and alkaline phosphatase activity increased 100.9 +/- 29.7 (WI) and 51.4 +/- 17.5% (WII) in phenol-injected membranes. Na(+) transporter total abundance [NHE3, NaPi2, thiazide-sensitive Na-Cl cotransporter, bumetanide-sensitive Na-K-2Cl cotransporter, Na-K-ATPase alpha(1)- and beta(1)-subunits, and epithelial Na(+) channel (ENaC) alpha- and beta-subunits] was profiled by immunoblotting. Only cortical NHE3 abundance was altered, decreasing to 0.56 +/- 0.06. The results demonstrate that phenol injury provokes a persistant shift of PT NHE3 and NaPi2 to the apical microvilli, along with a 44% decrease in total NHE3, evidence for an escape mechanism that would counteract the redistribution of a larger fraction of NHE3 to the apical surface by normalizing the total amount of NHE3 in apical membranes.

The renal thiazide-sensitive Na-Cl cotransporter as mediator of the aldosterone-escape phenomenon

The kidneys "escape" from the Na-retaining effects of aldosterone when circulating levels of aldosterone are inappropriately elevated in the setting of normal or expanded extracellular fluid volume, e.g., in primary aldosteronism. Using a targeted proteomics approach, we screened renal protein extracts with rabbit polyclonal antibodies directed to each of the major Na transporters expressed along the nephron to determine whether escape from aldosterone-mediated Na retention is associated with decreased abundance of one or more of renal Na transporters. The analysis revealed that the renal abundance of the thiazide-sensitive Na-Cl cotransporter (NCC) was profoundly and selectively decreased. None of the other apical solute-coupled Na transporters displayed decreases in abundance, nor were the total abundances of the three ENaC subunits significantly altered. Immunocytochemistry showed a strong decrease in NCC labeling in distal convoluted tubules of aldosterone-escape rats with no change in the cellular distribution of NCC. Ribonuclease protection assays (RPAs) revealed that the decrease in NCC protein abundance was not associated with altered NCC mRNA abundance. Thus, the thiazide-sensitive Na-Cl cotransporter of the distal convoluted tubule appears to be the chief molecular target for regulatory processes responsible for mineralocorticoid escape, decreasing in abundance via a posttranscriptional mechanism.

Use of positron emission tomography to study AT1 receptor regulation in vivo

Increased sodium intake and enhanced sodium sensitivity are implicated in the pathogenesis of hypertension and in the control of a major regulator of BP, the type 1 angiotensin receptor (AT(1) receptor). An in vivo technique to study changes of renal AT(1) receptors by dietary sodium was developed that uses positron emission tomography (PET). PET revealed that renal cortical AT(1) receptor binding was increased in sodium-loaded compared with sodium-deprived dogs, which correlated with ex vivo estimations of AT(1) receptor numbers. Plasma renin activity, angiotensin II, and aldosterone were inversely related to changes in AT(1) receptor binding. These results demonstrate, for the first time in vivo, that the renal AT(1) receptor is inversely related to the activity of the renin angiotensin system, which may provide a compensatory mechanism to prevent inappropriate fluctuations in arterial BP. The ability to measure AT(1) receptor binding in vivo has potential significance for clinical studies of AT(1) receptors, because PET is a noninvasive imaging technique that is readily applicable in humans.

Nitric oxide dependency of arterial pressure-induced changes in renal interstitial hydrostatic pressure in dogs

A direct relationship between renal arterial pressure (RAP) and renal interstitial hydrostatic pressure (RIHP) has been shown under conditions of efficient renal blood flow autoregulation. Experiments were performed in six anesthetized dogs to evaluate whether these RIHP responses to changes in RAP were modified during nitric oxide (NO) inhibition with nitro-L-arginine (NLA) or after administration of NO donor agents. A microtip catheter transducer was placed underneath the renal capsule to measure RIHP. Stepwise reductions in RAP (140 to 80 mm Hg) during control conditions resulted in decreases in RIHP from its basal value of 4.7+/-1.1 mm Hg with a slope of 0.04+/-0.026 mm Hg. mm Hg(-)(1) along with decreases in urinary nitrate/nitrite excretion rate (U(NOx)V). Renal cortical and medullary blood flows, measured by laser-Doppler flowmetry, exhibited high autoregulatory efficiency over this RAP range. The changes in RIHP during alterations in RAP were positively correlated (r=0.743; P:<0.001) with the changes in U(NOx)V but not with cortical or medullary blood flow. NLA infusion decreased RIHP to 1.9+/-0.5 mm Hg and also reduced U(NOx)V from 1.8+/-0.2 to 0.9+/-0.01 nmol. min(-)(1). g(-)(1). Infusion of NO donors restored RIHP (4.3+/-0.9 mm Hg) and U(NOx)V (1.5+/-0.2 nmol. min(-)(1). g(-)(1)). During NLA infusion, the RIHP responses to reductions in RAP were markedly attenuated and were not restored even during constant-rate infusion of NO donors. The results suggest that changes in RIHP in response to alterations in RAP are associated with changes in intrarenal NO, suggesting a direct effect of NO to regulate RIHP.

Influence of nitric oxide derived from neuronal nitric oxide synthase on glomerular filtration

The neuronal isoform of nitric oxide synthase (nNOS) has been localized to specific regions of the kidney, including the thick ascending limb of the loop of Henle and the macula densa. Because of this discrete localization in the renal cortex, nitric oxide (NO) produced by nNOS has been suggested to play an important role in the regulation of macula densa-mediated arteriole tone and therefore could play an important role in the regulation of whole-kidney glomerular filtration rate (GFR). We hypothesized that selective blockade of nNOS would decrease GFR. Renal hemodynamics were measured before and after acute selective blockade of nNOS by 50 mg/kg 7-nitroindazole (7-NI) in anesthetized rats. Administration of 7-NI had no significant effect on basal blood pressure (from 105 +/- 3 to 101 +/- 2 mm Hg), renal blood flow [from 6.08 +/- 0.39 to 6.31 +/- 0.33 ml/min/gram of kidney weight (gkw)], or total renal vascular resistance (from 18.1 +/- 1.6 to 16.4 +/- 1.0 mm Hg/ml/min/gkw) but decreased GFR by 26% (from 1.36 +/- 0.15 to 1.00 +/- 0.13 ml/min/gkw; p < 0.02), urinary flow rate by 28% (from 24.7 +/- 1.8 to 17.8 +/- 2.2 microl/min; p < 0.05), and sodium excretion by 22% (from 5.55 +/- 0.53 to 4.30 +/- 0.52 microEq/min; p < 0.05). However, fractional sodium excretion was not changed by nNOS inhibition. There were no such changes in vehicle-treated time controls. We conclude that, in the renal cortex, NO produced by nNOS plays an important role in the regulation of whole-kidney GFR and excretion in normal, sodium-replete rats.

Effect of interactions between nitric oxide and angiotensin II on pressure diuresis and natriuresis

The present study examined the effect of an angiotensin II AT1 or AT2 receptor antagonist on the impairment of the pressure diuresis and natriuresis response produced by nitric oxide (NO) synthesis blockade. N omega-nitro-L-arginine methyl ester (L-NAME, 37 nmol.kg-1.min-1) lowered renal blood flow and reduced the slopes of the pressure diuresis and natriuresis responses by 44 and 40%, respectively. Blockade of AT1 receptors with valsartan increased slightly sodium and water excretion at low renal perfusion pressure (RPP). Blockade of AT2 receptors with PD-123319 had no effect on renal function. The administration of valsartan or PD-123319 to rats given L-NAME had no effect on the renal vasoconstriction induced by NO synthesis blockade. In addition, in rats given L-NAME, valsartan elevated baseline excretory values at all RPP studied, but it had no effect on the sensitivity of the pressure diuresis and natriuresis response. However, the administration of PD-123319 to L-NAME-pretreated rats shifted the slopes of the pressure diuresis and natriuresis responses toward control values, indicating that the impairment produced by NO synthesis blockade on pressure diuresis is dependent on the activation of AT2 angiotensin receptors.

The kidney in blood pressure regulation and development of hypertension

Systemic arterial pressure is a dynamic and responsive physiologic parameter that can be influenced by many different factors. In particular, short-term changes in arterial pressure are caused by a myriad of mechanisms that affect cardiac output, total peripheral resistance, and cardiovascular capacitance. In the long run, however, most of these actions can be buffered or compensated by appropriate renal adjustments of sodium balance, ECFV, and blood volume. As long as the mechanisms regulating sodium excretion can maintain sodium balance by appropriately modulating the sensitivity of the pressure-natriuresis relationship, normal arterial pressure can be sustained. Derangements that compromise the ability of the kidneys to maintain sodium balance, however, can result in the kidney's need for an elevated arterial pressure to reestablish net salt and water balance.

Nitric oxide in the mediation of pressure natriuresis

1. Recent studies have indicated that nitric oxide (NO) production in the kidney contributes to the regulation of renal haemodynamics and excretory function. Inhibition of nitric oxide synthase (NOS) reduces renal blood flow by approximately 25% and markedly reduces sodium excretion without reductions in filtered load. In particular, inhibition of NO synthesis markedly suppresses the slope of the arterial pressure-mediated response in sodium excretion. 2. Further studies have shown that constant intrarenal infusion of a NO donor in dogs treated with a NOS inhibitor produced diuretic and natriuretic responses but failed to restore the slope of the pressure-induced natriuretic response. These data indicate that an alteration in intrarenal NO activity, rather than the simple presence of NO during changes in arterial pressure is required for full expression of pressure natriuretic responses. 3. In support of the hypothesis that NO is involved in the mediation of pressure natriuresis, we also recently demonstrated a direct relationship between changes in arterial pressure and urinary excretion rate of sodium as well as nitrate and nitrite (a marker for endogenous NO activity) in the presence of efficient autoregulation of cortical and medullary blood flow. 4. The direct inhibitory actions of NO on tubular sodium reabsorption have also been observed in cultured tubular cells as well as isolated, perfused cortical collecting duct segments. 5. Thus, the collective data suggest that acute changes in arterial pressure induce changes in intrarenal NO production, which may directly alter tubular reabsorptive function to manifest the phenomenon of pressure natriuresis.

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

An abnormal L-arginine-nitric oxide axis has been suggested to be relevant to the genesis of salt-sensitive hypertension. In the present study we investigated the activities of three isoforms of nitric oxide synthase (NOS) in the kidney of Dahl salt-sensitive and salt-resistant rats. Five-week-old Dahl Iwai salt-sensitive (n = 9) and salt-resistant (n = 10) rats were maintained on a high salt diet (4% sodium chloride) for 4 weeks. We measured calcium-dependent and calcium-independent NOS activities in each particulate and soluble fraction of kidney by conversion of L-[3H]arginine to L-[3H]citrulline. Systolic blood pressure was elevated significantly (P < .001) in salt-sensitive but not salt-resistant rats. Calcium-dependent NOS activity in the soluble fraction was significantly lower in salt-sensitive rats than in salt-resistant rats (25.8 +/- 9.0 versus 48.2 +/- 19.2 disintegrations per microgram protein, respectively; P < .01). There were no differences in calcium-dependent NOS activity in the particulate fraction and calcium-independent NOS activity in the soluble fraction between groups. Renal norepinephrine content was lower in salt-sensitive rats than in salt-resistant rats (P < .05) and was positively correlated with calcium-dependent NOS activity in the soluble fraction (P < .01). Although no differences in endothelial and inducible-type NOS activity were observed a significant reduction in calcium-dependent NOS activity in the soluble fraction of the kidney of salt-sensitive rats suggests that the decreased neural-type NOS activity may in part be involved in the mechanism of salt-sensitive hypertension, possibly through alterations in renal sympathetic nervous activity and sodium handling.

Relation between pressure natriuresis and urinary excretion of nitrate/nitrite in anesthetized dogs

Alterations in intrarenal nitric oxide (NO) formation during changes in renal arterial pressure (RAP) have been suggested as a mechanism mediating pressure natriuresis. To test this hypothesis further, we examined the relation between RAP and the urinary excretion rate of nitrate/nitrite (NO3-/NO2-; NO metabolites) in anesthetized sodium-replete dogs before (n = 9) and during (n = 6) intrarenal infusion of the NO synthesis inhibitor nitro-L-arginine (NLA; 50 micrograms.kg-1.min-1). Urinary NO3-/NO2- concentrations were measured with the Griess reaction and spectrophotometry methods after enzymatic reduction of NO3- to NO2- in the samples. During control conditions, there were decreases in the urinary NO3-/NO2- excretion rate in response to reductions in RAP (150 to 75 mm Hg; slope, 0.04 +/- 0.01 nmol.min-1.g-1.mm Hg-1) in association with decreases in urinary sodium excretion (UNaV). There was a positive correlation between changes in NO3-/NO2- excretion rate and changes in RAP (r = .48; P < .005) or UNaV (r = .59; P < .001). NLA infusion resulted in decreases in NO3-/NO2- excretion rate (4.8 +/- 1.4 to 1.0 +/- 0.3 nmol.min-1.g-1) in association with reductions in UNaV (4.3 +/- 0.3 to 0.7 +/- 0.2 microL.min-1.g-1), fractional excretion of sodium (2.9 +/- 0.2% to 0.5 +/- 0.1%), and renal blood flow (4.8 +/- 0.3 to 3.3 +/- 0.2 mL.min-1.g-1), without changes in glomerular filtration rate. Furthermore, there was a marked attenuation of the NO3-/NO2- and sodium excretory responses to alterations in RAP during NO synthesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)