An unexpected role for angiotensin II in the link between dietary salt and proximal reabsorption

Dietary sodium and health: How much is too much for those with orthostatic disorders?

High dietary salt (NaCl) increases blood pressure (BP) and can adversely impact multiple target organs including the vasculature, heart, kidneys, brain, autonomic nervous system, skin, eyes, and bone. However, patients with orthostatic disorders are told to increase their NaCl intake to help alleviate symptoms. While there is evidence to support the short-term benefits of increasing NaCl intake in these patients, there are few studies assessing the benefits and side effects of long-term high dietary NaCl. The evidence reviewed suggests that high NaCl can adversely impact multiple target organs, often independent of BP. However, few of these studies have been performed in patients with orthostatic disorders. We conclude that the recommendation to increase dietary NaCl in patients with orthostatic disorders should be done with care, keeping in mind the adverse impact on dietary NaCl in people without orthostatic disorders. Modest, rather than robust, increases in NaCl intake may be sufficient to alleviate symptoms but also minimize any long-term negative effects.

Silencing of Central (Pro)renin Receptor Ameliorates Salt-Induced Renal Injury in Chronic Kidney Disease

Aims: A high-salt diet can aggravate oxidative stress, and renal fibrosis via the brain and renal renin-angiotensin system (RAS) axis in chronic kidney disease (CKD) rats. (Pro)renin receptor (PRR) plays a role in regulating RAS and oxidative stress locally. However, whether central PRR regulates salt-induced renal injury in CKD remains undefined. Here, we hypothesized that the reduction of central PRR expression could ameliorate central lesions and thereby ameliorate renal injury in high-salt-load CKD rats. Results: We investigated RAS, sympathetic nerve activity, oxidative stress, inflammation, and tissue injury in subfornical organs and kidneys in high-salt-load 5/6 nephrectomy CKD rats after the silencing of central PRR expression by intracerebroventricular lentivirus-RNAi. We found that the sympathetic nerve activity was reduced, and the levels of inflammation and oxidative stress were decreased in both brain and kidney. Renal injury and fibrosis were ameliorated. To explore the mechanism by which central inhibition of PRR expression ameliorates kidney damage, we blocked central MAPK/ERK1/2 and PI3K/Akt signaling pathways as well as angiotensin converting enzyme 1-angiotensin II-angiotensin type 1 receptors (ACE1-Ang II-AT1R) axis. Salt-induced overexpression of renal RAS, inflammation, oxidative stress, and fibrosis in CKD rats were prevented by central blockade of the pathways. Innovation: This study provides new insights into the mechanisms underlying salt-induced kidney damage. Targeting central PRR or PRR-mediated signaling pathway may be a novel strategy for the treatment of CKD. Conclusions: These results suggested that the silencing of central PRR expression ameliorates salt-induced renal injury in CKD through Ang II-dependent and -independent pathways.

Angiotensin receptors in the kidney and vasculature in hypertension and kidney disease

Kidney disease, blood pressure determination, hypertension pathogenesis, and the renin-angiotensin system (RAS) are inextricably linked. Hence, understanding the RAS is pivotal to unraveling the pathophysiology of hypertension and the determinants to maintaining normal blood pressure. The RAS has been the subject of intense investigation for over a century. Moreover, medications that block the RAS are mainstay therapies in clinical medicine and have been shown to reduce morbidity and mortality in patients with diabetes, cardiovascular, and kidney diseases. The main effector peptide of the RAS is the interaction of the octapeptide- Ang II with its receptor. The type 1 angiotensin receptor (AT₁R) is the effector receptor for Ang II. These G protein-coupled receptors (GPCRs) are ubiquitously expressed in a variety of cell lineages and tissues relevant to cardiovascular disease throughout the body. The advent of cell specific deletion of genes using Cre LoxP technology in mice has allowed for the identification of discreet actions of AT₁Rs in blood pressure control and kidney disease. The kidney is one of the major targets of the RAS, which is responsible in maintaining fluid, electrolyte balance, and blood pressure. In this review we will discuss the role of AT₁Rs in the kidney, vasculature, and immune cells and address their effects on hypertension and kidney disease.

Assessment of nephron number and single-nephron glomerular filtration rate in a clinical setting

Total nephron counts vary widely between individuals and may affect susceptibility to certain diseases, including hypertension and chronic kidney disease. Detailed analyses of whole kidneys collected from autopsy patients remain the only method for accurately counting nephrons in humans, with no equivalent option in living subjects. Current technological advances have enabled estimations of nephron numbers in vivo, particularly the use of total nephron number and whole-kidney glomerular filtration rate to estimate the mean single-nephron glomerular filtration rate. The use of this method would allow physicians to detect dynamic changes in filtration function at the single-nephron level rather than to simply count the number of nephrons that appear to be functioning. Currently available methods for estimating total nephron number in clinical practice have the potential to overcome limitations associated with autopsy analyses and may therefore pave the way for new therapeutic interventions and improved clinical outcomes.

Dietary salt modifies the blood pressure response to renin-angiotensin inhibition in experimental chronic kidney disease

Chronic kidney disease contributes to hypertension, but the mechanisms are incompletely understood. To address this, we applied the 5/6th nephrectomy rat model to characterize hypertension and the response to dietary salt and renin-angiotensin inhibition. 5/6th nephrectomy caused low-renin, salt-sensitive hypertension with hyperkalemia and unsuppressed aldosterone. Compared with sham rats, 5/6th nephrectomized rats had lower Na⁺/H⁺ exchanger isoform 3, Na⁺-K⁺-2Cl^- cotransporter, Na⁺-Cl^- cotransporter, α-epithelial Na⁺ channel (ENaC), and Kir4.1 levels but higher serum and glucocorticoid-regulated kinase 1, prostasin, γ-ENaC, and Kir5.1 levels. These differences correlated with plasma renin, aldosterone, and/or K⁺. On a normal-salt diet, adrenalectomy (0 ± 9 mmHg) and spironolactone (-11 ± 10 mmHg) prevented a progressive rise in blood pressure (10 ± 8 mmHg), and this was enhanced in combination with losartan (-41 ± 12 and -43 ± 9 mmHg). A high-salt diet caused skin Na⁺ and water accumulation and aggravated hypertension that could only be attenuated by spironolactone (-16 ± 7 mmHg) and in which the additive effect of losartan was lost. Spironolactone also increased natriuresis, reduced skin water accumulation, and restored vasorelaxation. In summary, in the 5/6th nephrectomy rat chronic kidney disease model, salt-sensitive hypertension develops with a selective increase in γ-ENaC and despite appropriate transporter adaptations to low renin and hyperkalemia. With a normal-salt diet, hypertension in 5/6th nephrectomy depends on angiotensin II and aldosterone, whereas a high-salt diet causes more severe hypertension mediated through the mineralocorticoid receptor.NEW & NOTEWORTHY Chronic kidney disease (CKD) causes salt-sensitive hypertension, but the interactions between dietary salt and the renin-angiotensin system are incompletely understood. In rats with CKD on a normal-salt diet targeting aldosterone, the mineralocorticoid receptor (MR) and especially angiotensin II reduced blood pressure. On a high-salt diet, however, only MR blockade attenuated hypertension. These results reiterate the importance of dietary salt restriction to maintain renin-angiotensin system inhibitor efficacy and specify the MR as a target in CKD.

Cardiac and renal protective effects of 2,5-dimethylcelecoxib in angiotensin II and high-salt-induced hypertension model mice

BACKGROUND

We reported that 2,5-dimethylcelecoxib (DM-celecoxib), a celecoxib derivative that is unable to inhibit cyclooxygenase-2, prevented cardiac remodeling induced by sarcomeric gene mutation, left ventricular pressure overload, or β-adrenergic receptor stimulation. This effect seemed to be mediated by the inhibition of the canonical Wnt/β-catenin signaling pathway, which has been suggested to play a key role in the development of chronic kidney disease and chronic heart failure.

METHOD

We investigated the effect of DM-celecoxib on cardiac remodeling and kidney injury in hypertension model mice induced by angiotensin II infusion in the absence or presence of high-salt load.

RESULTS

DM-celecoxib prevented cardiac remodeling and markedly reduced urinary albumin excretion without altering blood pressure in those mice. Moreover, DM-celecoxib prevented podocyte injury, glomerulosclerosis, and interstitial fibrosis in the kidney of mice loaded with angiotensin II and high-salt load. DM-celecoxib reduced the phosphorylation level of Akt and activated glycogen synthase kinase-3, which led to the suppression of the Wnt/β-catenin signal in the heart and kidney. DM-celecoxib also reduced the expression level of snail, a key transcription factor for the epithelial-mesenchymal transition and of which gene is a target of the Wnt/β-catenin signal.

CONCLUSION

Results of the current study suggested that DM-celecoxib could be beneficial for patients with hypertensive heart and kidney diseases.

Salt-dependent hypertension and inflammation: targeting the gut-brain axis and the immune system with Brazilian green propolis

Systemic arterial hypertension (SAH) is a major health problem around the world and its development has been associated with exceeding salt consumption by the modern society. The mechanisms by which salt consumption increase blood pressure (BP) involve several homeostatic systems but many details have not yet been fully elucidated. Evidences accumulated over the last 60 decades raised the involvement of the immune system in the hypertension development and opened a range of possibilities for new therapeutic targets. Green propolis is a promising natural product with potent anti-inflammatory properties acting on specific targets, most of them participating in the gut-brain axis of the sodium-dependent hypertension. New anti-hypertensive products reinforce the therapeutic arsenal improving the corollary of choices, especially in those cases where patients are resistant or refractory to conventional therapy. This review sought to bring the newest advances in the field articulating evidences that show a cross-talking between inflammation and the central mechanisms involved with the sodium-dependent hypertension as well as the stablished actions of green propolis and some of its biologically active compounds on the immune cells and cytokines that would be involved with its anti-hypertensive properties.

Blood pressure regulation by the angiotensin type 1 receptor in the proximal tubule

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) plays a critical role in the pathogenesis of hypertension. Homeostatic actions of the RAS, such as increasing blood pressure (BP) and vasoconstriction, are mediated via type 1 (AT1) receptors for angiotensin II. All components of the RAS are present in the renal proximal tubule, which reabsorbs the bulk of the glomerular filtrate, making this segment of the nephron a location of great interest for solute handling under RAS influence. This review highlights recent studies that illustrate the key role of renal proximal tubule AT1 receptors in BP regulation.

RECENT FINDINGS

A variety of investigative approaches have demonstrated that angiotensin II signaling via AT1a receptors, specifically in the renal proximal tubule, is a major regulator of BP and sodium homeostasis. Reduction of proximal tubule AT1a receptors led to lower BPs, whereas overexpression generally caused increased BPs.

SUMMARY

AT1a receptors in the proximal tubule are critical to the regulation of BP by the kidney and the RAS. The pattern of BP modulation is associated with alterations in sodium transporters. As a key site for sodium homeostasis, the renal proximal tubule could hence be a potential target in the treatment of hypertension.

Gq activity- and β-arrestin-1 scaffolding-mediated ADGRG2/CFTR coupling are required for male fertility

Luminal fluid reabsorption plays a fundamental role in male fertility. We demonstrated that the ubiquitous GPCR signaling proteins Gq and β-arrestin-1 are essential for fluid reabsorption because they mediate coupling between an orphan receptor ADGRG2 (GPR64) and the ion channel CFTR. A reduction in protein level or deficiency of ADGRG2, Gq or β-arrestin-1 in a mouse model led to an imbalance in pH homeostasis in the efferent ductules due to decreased constitutive CFTR currents. Efferent ductule dysfunction was rescued by the specific activation of another GPCR, AGTR2. Further mechanistic analysis revealed that β-arrestin-1 acts as a scaffold for ADGRG2/CFTR complex formation in apical membranes, whereas specific residues of ADGRG2 confer coupling specificity for different G protein subtypes, this specificity is critical for male fertility. Therefore, manipulation of the signaling components of the ADGRG2-Gq/β-arrestin-1/CFTR complex by small molecules may be an effective therapeutic strategy for male infertility.

Dietary sodium induces a redistribution of the tubular metabolic workload

Key points

Body Na⁺ content is tightly controlled by regulated urinary Na⁺ excretion.

The intrarenal mechanisms mediating adaptation to variations in dietary Na⁺ intake are incompletely characterized.

We confirmed and expanded observations in mice that variations in dietary Na⁺ intake do not alter the glomerular filtration rate but alter the total and cell‐surface expression of major Na⁺ transporters all along the kidney tubule.

Low dietary Na⁺ intake increased Na⁺ reabsorption in the proximal tubule and decreased it in more distal kidney tubule segments.

High dietary Na⁺ intake decreased Na⁺ reabsorption in the proximal tubule and increased it in distal segments with lower energetic efficiency.

The abundance of apical transporters and Na⁺ delivery are the main determinants of Na⁺ reabsorption along the kidney tubule.

Tubular O₂ consumption and the efficiency of sodium reabsorption are dependent on sodium diet.

Abstract

Na⁺ excretion by the kidney varies according to dietary Na⁺ intake. We undertook a systematic study of the effects of dietary salt intake on glomerular filtration rate (GFR) and tubular Na⁺ reabsorption. We examined the renal adaptive response in mice subjected to 7 days of a low sodium diet (LSD) containing 0.01% Na⁺, a normal sodium diet (NSD) containing 0.18% Na⁺ and a moderately high sodium diet (HSD) containing 1.25% Na⁺. As expected, LSD did not alter measured GFR and increased the abundance of total and cell‐surface NHE3, NKCC2, NCC, α‐ENaC and cleaved γ‐ENaC compared to NSD. Mathematical modelling predicted that tubular Na⁺ reabsorption increased in the proximal tubule but decreased in the distal nephron because of diminished Na⁺ delivery. This prediction was confirmed by the natriuretic response to diuretics targeting the thick ascending limb, the distal convoluted tubule or the collecting system. On the other hand, HSD did not alter measured GFR but decreased the abundance of the aforementioned transporters compared to NSD. Mathematical modelling predicted that tubular Na⁺ reabsorption decreased in the proximal tubule but increased in distal segments with lower transport efficiency with respect to O₂ consumption. This prediction was confirmed by the natriuretic response to diuretics. The activity of the metabolic sensor adenosine monophosphate‐activated protein kinase (AMPK) was related to the changes in tubular Na⁺ reabsorption. Our data show that fractional Na⁺ reabsorption is distributed differently according to dietary Na⁺ intake and induces changes in tubular O₂ consumption and sodium transport efficiency.

Body Na⁺ content is tightly controlled by regulated urinary Na⁺ excretion.

The intrarenal mechanisms mediating adaptation to variations in dietary Na⁺ intake are incompletely characterized.

We confirmed and expanded observations in mice that variations in dietary Na⁺ intake do not alter the glomerular filtration rate but alter the total and cell‐surface expression of major Na⁺ transporters all along the kidney tubule.

Low dietary Na⁺ intake increased Na⁺ reabsorption in the proximal tubule and decreased it in more distal kidney tubule segments.

High dietary Na⁺ intake decreased Na⁺ reabsorption in the proximal tubule and increased it in distal segments with lower energetic efficiency.

The abundance of apical transporters and Na⁺ delivery are the main determinants of Na⁺ reabsorption along the kidney tubule.

Tubular O₂ consumption and the efficiency of sodium reabsorption are dependent on sodium diet.

Biomarkers for Chronic Kidney Disease Associated with High Salt Intake

High salt intake has been related to the development to chronic kidney disease (CKD) as well as hypertension. In its early stages, symptoms of CKD are usually not apparent, especially those that are induced in a “silent” manner in normotensive individuals, thereby providing a need for some kind of urinary biomarker to detect injury at an early stage. Because traditional renal biomarkers such as serum creatinine are insensitive, it is difficult to detect kidney injury induced by a high-salt diet, especially in normotensive individuals. Recently, several new biomarkers for damage of renal tubular epithelia such as neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (Kim-1) have been identified. Previously, we found a novel renal biomarker, urinary vanin-1, in several animal models with renal tubular injury. However, there are few studies about early biomarkers of the progression to CKD associated with a high-salt diet. This review presents some new insights about these novel biomarkers for CKD in normotensives and hypertensives under a high salt intake. Interestingly, our recent reports using spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) fed a high-salt diet revealed that urinary vanin-1 and NGAL are earlier biomarkers of renal tubular damage in SHR and WKY, whereas urinary Kim-1 is only useful as a biomarker of salt-induced renal injury in SHR. Clinical studies will be needed to clarify these findings.

Immunohistochemical expression of intrarenal renin angiotensin system components in response to tempol in rats fed a high salt diet

AIM

To determine the effect of tempol in normal rats fed high salt on arterial pressure and the balance between antagonist components of the renal renin-angiotensin system.

METHODS

Sprague-Dawley rats were fed with 8% NaCl high-salt (HS) or 0.4% NaCl (normal-salt, NS) diet for 3 wk, with or without tempol (T) (1 mmol/L, administered in drinking water). Mean arterial pressure (MAP), glomerular filtration rate (GFR), and urinary sodium excretion (UVNa) were measured. We evaluated angiotensin II (Ang II), angiotensin 1-7 (Ang 1-7), angiotensin converting enzyme 2 (ACE2), mas receptor (MasR), angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptor (AT2R) in renal tissues by immunohistochemistry.

RESULTS

The intake of high sodium produced a slight but significant increase in MAP and differentially regulated components of the renal renin-angiotensin system (RAS). This included an increase in Ang II and AT1R, and decrease in ACE-2 staining intensity using immunohistochemistry. Antioxidant supplementation with tempol increased natriuresis and GFR, prevented changes in blood pressure and reversed the imbalance of renal RAS components. This includes a decrease in Ang II and AT1R, as increase in AT2, ACE2, Ang (1-7) and MasR staining intensity using immunohistochemistry. In addition, the natriuretic effects of tempol were observed in NS-T group, which showed an increased staining intensity of AT2, ACE2, Ang (1-7) and MasR.

CONCLUSION

These findings suggest that a high salt diet leads to changes in the homeostasis and balance between opposing components of the renal RAS in hypertension to favour an increase in Ang II. Chronic antioxidant supplementation can modulate the balance between the natriuretic and antinatriuretic components of the renal RAS.

Crosstalk between the renal sympathetic nerve and intrarenal angiotensin II modulates proximal tubular sodium reabsorption

NEW FINDINGS

What is the topic of this review? The sympathetic control of renal sodium tubular reabsorption is dependent on activation of the intrarenal renin-angiotensin system and activation of the angiotensin II type 1 (AT1 ) receptor by angiotensin II. What advances does it highlight? Despite the fact that the interaction between the sympathetic nervous system and angiotensin II regarding salt reabsorption is a well-known classical mechanism for the maintenance of extracellular volume homeostasis, the underlying molecular signalling is not clearly understood. It has been shown recently that renal nerve stimulation increases intrarenal angiotensin II and activates the AT1 receptor, triggering a signalling cascade that leads to elevations of Na(+) -H(+) exchanger isoform 3-mediated tubular transport. In this short review, the crosstalk between intrarenal angiotensin II and renal nerve activity and its effect on sodium reabsorption is addressed. In this review, we address the importance of the interaction between the sympathetic nervous system and intrarenal renin-angiotensin system in modulating renal tubular handling of sodium and water. We have recently shown that increased Na(+) -H(+) exchanger isoform 3 (NHE3) activity induced by renal nerve stimulation (RNS) depends on the activation of the angiotensin II type 1 (AT1 ) receptor by angiotensin II (Ang II). Low-frequency RNS resulted in higher levels of intrarenal angiotensinogen and Ang II independent of changes in blood pressure, the glomerular filtration rate and systemic angiotensinogen. Angiotensin II, via the AT1 receptor, triggered an intracellular pathway activating NHE3 in the renal cortex, leading to antinatriuresis and antidiuresis. Pharmacological blockade of the AT1 receptor with losartan prior to RNS abolished both the functional and the molecular responses, suggesting that intrarenal Ang II acting via the AT1 receptor is a major factor for NHE3-mediated sodium and water reabsorption induced by RNS.

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

Angiotensin II type 2 receptor regulates ROMK-like K⁺ channel activity in the renal cortical collecting duct during high dietary K⁺ adaptation

The kidney adjusts K⁺ excretion to match intake in part by regulation of the activity of apical K⁺ secretory channels, including renal outer medullary K⁺ (ROMK)-like K⁺ channels, in the cortical collecting duct (CCD). ANG II inhibits ROMK channels via the ANG II type 1 receptor (AT1R) during dietary K⁺ restriction. Because AT1Rs and ANG II type 2 receptors (AT2Rs) generally function in an antagonistic manner, we sought to characterize the regulation of ROMK channels by the AT2R. Patch-clamp experiments revealed that ANG II increased ROMK channel activity in CCDs isolated from high-K⁺ (HK)-fed but not normal K⁺ (NK)-fed rats. This response was blocked by PD-123319, an AT2R antagonist, but not by losartan, an AT1R antagonist, and was mimicked by the AT2R agonist CGP-42112. Nitric oxide (NO) synthase is present in CCD cells that express ROMK channels. Blockade of NO synthase with N-nitro-l-arginine methyl ester and free NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt completely abolished ANG II-stimulated ROMK channel activity. NO enhances the synthesis of cGMP, which inhibits phosphodiesterases (PDEs) that normally degrade cAMP; cAMP increases ROMK channel activity. Pretreatment of CCDs with IBMX, a broad-spectrum PDE inhibitor, or cilostamide, a PDE3 inhibitor, abolished the stimulatory effect of ANG II on ROMK channels. Furthermore, PKA inhibitor peptide, but not an activator of the exchange protein directly activated by cAMP (Epac), also prevented the stimulatory effect of ANG II. We conclude that ANG II acts at the AT2R to stimulate ROMK channel activity in CCDs from HK-fed rats, a response opposite to that mediated by the AT1R in dietary K⁺-restricted animals, via a NO/cGMP pathway linked to a cAMP-PKA pathway.

Intrarenal renin-angiotensin system in regulation of glomerular function

PURPOSE OF REVIEW

The purpose of this review is to provide an update on the current knowledge regarding the role of the intrarenal rennin-angiotensin system (RAS) in the regulation of glomerular function including glomerular dynamics and filtration rate, glomerular permeability and structural alterations during chronic increases in intrarenal angiotensin (Ang) II.

RECENT FINDINGS

Recent studies have continued to delineate the complex interactions among the various RAS components that participate in regulating glomerular function. Although Ang II acting on AT1 receptors remains as the predominant influence on glomerular dynamics, some of these effects are indirectly mediated by Ang II modulating the sensitivity of the macula densa tubuloglomerular feedback mechanism as well as the more recently described feedback mechanism from the connecting tubule. Interestingly, the actions of Ang II on these systems cause opposite effects on glomerular function demonstrating the complexities associated with the influences of Ang II on glomerular function. When chronically elevated, Ang II also stimulates and/or interacts with other factors, including reactive oxygen species, cytokines and growth factors and other hormones or paracrine agents, to elicit structural alterations.

SUMMARY

Recent studies have provided further evidence for the presence of many components of the RAS in glomerular structures, which supports the importance of locally produced angiotensin peptides to regulate glomerular haemodynamics, filtration rate and macromolecular permeability and contribute to fibrosis and glomerular injury when inappropriately augmented.

Renal neurohormonal regulation in heart failure decompensation

Decompensation in heart failure occurs when the heart fails to balance venous return with cardiac output, leading to fluid congestion and contributing to mortality. Decompensated heart failure can cause acute kidney injury (AKI), which further increases mortality. Heart failure activates signaling systems that are deleterious to kidneys such as renal sympathetic nerve activity (RSNA), renin-angiotensin-aldosterone system, and vasopressin secretion. All three reduce renal blood flow (RBF) and increase tubular sodium reabsorption, which may increase renal oxygen consumption causing AKI through renal tissue hypoxia. Vasopressin contributes to venous congestion through aquaporin-mediated water retention. Additional water retention may be mediated through vasopressin-induced medullary urea transport and hyaluronan but needs further study. In addition, there are several systems that could protect the kidneys and reduce fluid retention such as natriuretic peptides, prostaglandins, and nitric oxide. However, the effect of natriuretic peptides and nitric oxide are blunted in decompensation, partly due to oxidative stress. This review considers how neurohormonal signaling in heart failure drives fluid retention by the kidneys and thus exacerbates decompensation. It further identifies areas where there is limited data, such as signaling systems 20-HETE, purines, endothelin, the role of renal water retention mechanisms for congestion, and renal hypoxia in AKI during heart failure.

Glomerular and tubular function in the diabetic kidney

Diabetes mellitus with its attendant complications is a significant cause of morbidity and mortality with diabetic nephropathy being the leading cause of end stage renal disease in the Western world. Characteristic structural and functional changes in the kidney early in the course of diabetes have been shown to have enduring effects on the progression of disease. A better understanding of the mechanisms underlying these changes is imperative to the development of new therapeutic strategies. Renal hypertrophy and hyperfiltration along with proximal tubular hyperreabsorption are among the distinctive features of early diabetic nephropathy. Additionally, there are particular alterations in the sensitivity of the glomerular and tubular function to dietary salt intake in early diabetes. Herein, we focus on these early physiologic changes and discuss some of the primary and secondary mechanisms discovered in recent years which lead to these alterations in kidney function.

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.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Salt sensitivity of tubuloglomerular feedback in the early remnant kidney

We previously reported internephron heterogeneity in the tubuloglomerular feedback (TGF) response 1 wk after subtotal nephrectomy (STN), with 50% of STN nephrons exhibiting anomalous TGF (Singh P, Deng A, Blantz RC, Thomson SC. Am J Physiol Renal Physiol 296: F1158-F1165, 2009). Presently, we tested the theory that anomalous TGF is an adaptation of the STN kidney to facilitate increased distal delivery when NaCl balance forces the per-nephron NaCl excretion to high levels. To this end, the effect of dietary NaCl on the TGF response was tested by micropuncture in STN and sham-operated Wistar rats. An NaCl-deficient (LS) or high-salt NaCl diet (HS; 1% NaCl in drinking water) was started on day 0 after STN or sham surgery. Micropuncture followed 8 days later with measurements of single-nephron GFR (SNGFR), proximal reabsorption, and tubular stop-flow pressure (PSF) obtained at both extremes of TGF activation, while TGF was manipulated by microperfusing Henle's loop (LOH) from the late proximal tubule. Activating TGF caused SNGFR to decline by similar amounts in Sham-LS, Sham-HS and STN-LS [ΔSNGFR (nl/min) = -16 ± 2, -11 ± 3, -11 ± 2; P = not significant by Tukey]. Activating TGF in STN-HS actually increased SNGFR by 5 ± 2 nl/min (P < 0.0005 vs. each other group by Tukey). HS had no effect on the PSF response to LOH perfusion in sham [ΔPSF (mmHg) = -9.6 ± 1.1 vs. -9.8 ± 1.0] but eliminated the PSF response in STN (+0.3 ± 0.9 vs. -5.7 ± 1.0, P = 0.0002). An HS diet leads to anomalous TGF in the early remnant kidney, which facilitates NaCl and fluid delivery to the distal nephron.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

K+-induced natriuresis is preserved during Na+ depletion and accompanied by inhibition of the Na+-Cl- cotransporter

During hypovolemia and hyperkalemia, the kidneys defend homeostasis by Na(+) retention and K(+) secretion, respectively. Aldosterone mediates both effects, but it is unclear how the same hormone can evoke such different responses. To address this, we mimicked hypovolemia and hyperkalemia in four groups of rats with a control diet, low-Na(+) diet, high-K(+) diet, or combined diet. The low-Na(+) and combined diets increased plasma and kidney ANG II. The low-Na(+) and high-K(+) diets increased plasma aldosterone to a similar degree (3-fold), whereas the combined diet increased aldosterone to a greater extent (10-fold). Despite similar Na(+) intake and higher aldosterone, the high-K(+) and combined diets caused a greater natriuresis than the control and low-Na(+) diets, respectively (P < 0.001 for both). This K(+)-induced natriuresis was accompanied by a decreased abundance but not phosphorylation of the Na(+)-Cl(-) cotransporter (NCC). In contrast, the epithelial Na(+) channel (ENaC) increased in parallel with aldosterone, showing the highest expression with the combined diet. The high-K(+) and combined diets also increased WNK4 but decreased Nedd4-2 in the kidney. Total and phosphorylated Ste-20-related kinase were also increased but were retained in the cytoplasm of distal convoluted tubule cells. In summary, high dietary K(+) overrides the effects of ANG II and aldosterone on NCC to deliver sufficient Na(+) to ENaC for K(+) secretion. K(+) may inhibit NCC through WNK4 and help activate ENaC through Nedd4-2.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

High-salt diets during pregnancy affected fetal and offspring renal renin-angiotensin system

Intrauterine environments are related to fetal renal development and postnatal health. Influence of salty diets during pregnancy on renal functions and renin-angiotensin system (RAS) was determined in the ovine fetuses and offspring. Pregnant ewes were fed high-salt diet (HSD) or normal-salt diet (NSD) for 2 months during middle-to-late gestation. Fetal renal functions, plasma hormones, and mRNA and protein expressions of the key elements of renal RAS were measured in the fetuses and offspring. Fetal renal excretion of sodium was increased while urine volume decreased in the HSD group. Fetal blood urea nitrogen was increased, while kidney weight:body weight ratio decreased in the HSD group. The altered ratio was also observed in the offspring aged 15 and 90 days. Maternal and fetal plasma antidiuretic hormone was elevated without changes in plasma renin activity and Ang I levels, while plasma Ang II was decreased. The key elements of local renal RAS, including angiotensinogen, angiotensin converting enzyme (ACE), ACE2, AT1, and AT2 receptor expression in both mRNA and protein, except renin, were altered following maternal high salt intake. The results suggest that high intake of salt during pregnancy affected fetal renal development associated with an altered expression of the renal key elements of RAS, some alterations of fetal origins remained after birth as possible risks in developing renal or cardiovascular diseases.

Proximal tubule angiotensinogen modulation of arterial pressure

PURPOSE OF REVIEW

Although the existence of a complete intrarenal renin-angiotensin system is now well established, its role in modulating tubule sodium transport and blood pressure is incompletely understood. Several recent studies have shed light on one component of the system, proximal tubule-derived angiotensinogen (AGT). This review discusses the synthesis, regulation and function of AGT in the proximal tubule.

RECENT FINDINGS

Under normal sodium intake, AGT within the S1 and S2 segments of the proximal tubule may derive from the systemic circulation, whereas the S3 segment synthesizes AGT. Urinary AGT likely primarily reflects proximal tubule-derived AGT. Proximal tubule AGT synthesis is regulated by high Na intake, angiotensin-II and inflammatory cytokines. Transgenic expression of mouse AGT in the proximal tubule causes hypertension. Overexpression of rat AGT in the proximal tubule leads to hypertension, enhanced reactive oxygen species generation via NADPH oxidase, tubular apoptosis and tubulointerstitial fibrosis; these effects can be mitigated by catalase overexpression.

SUMMARY

Proximal tubule-derived AGT has the potential to modulate blood pressure and sodium balance, and promote renal injury. Interactions with the systemic renin-angiotensin system may influence the role of proximal tubule-derived AGT in the kidney.

A possible interaction between systemic and renal angiotensinogen in the control of blood pressure

BACKGROUND

Angiotensinogen (AGT) is synthesized in the liver and proximal tubule. AGT overexpression at either site might increase blood pressure (BP). We used transgenic mice with AGT overexpression in proximal tubule (K), liver (L), or both sites (KL) to determine the relative contributions of hepatic- and proximal tubule-derived AGT in modulating BP.

METHODS

Hepatic AGT overexpression was obtained using the albumin enhancer promoter; the kidney androgen protein gene was used for proximal tubule AGT overexpression. BP and renin angiotensin system parameters were examined in male KL, K, L, and wild-type mice on normal and high-sodium diets.

RESULTS

Compared with wild-type mice, K and KL mice had higher BP on normal and high-sodium diets. L mice had similar BP to wild-type mice on a normal-sodium diet, but high sodium intake caused hypertension. There were no differences in plasma AGT, plasma renin concentration, urine volume, or urine sodium excretion between the groups. Urine AGT and angiotensin II (Ang II) excretion were higher in KL and K mice than in L or wild-type mice on a normal-sodium diet and increased with high sodium intake. During high sodium intake, urine AGT and Ang II were higher in all transgenic mice vs wild-type mice.

CONCLUSIONS

Mice with liver AGT overexpression manifest salt-sensitive hypertension, whereas mice with renal AGT overexpression are hypertensive regardless of salt intake. Systemic AGT may stimulate endogenous renal AGT synthesis during high sodium intake, leading to hypertension in L mice. This suggests that systemic and renal AGT may interact to modulate BP.

Perinatal Na+ overload programs raised renal proximal Na+ transport and enalapril-sensitive alterations of Ang II signaling pathways during adulthood

BACKGROUND

High Na(+) intake is a reality in nowadays and is frequently accompanied by renal and cardiovascular alterations. In this study, renal mechanisms underlying perinatal Na(+) overload-programmed alterations in Na(+) transporters and the renin/angiotensin system (RAS) were investigated, together with effects of short-term treatment with enalapril in terms of reprogramming molecular alterations in kidney.

METHODOLOGY/PRINCIPAL FINDINGS

Male adult Wistar rats were obtained from dams maintained throughout pregnancy and lactation on a standard diet and drinking water (control) or 0.17 M NaCl (saline group). Enalapril (100 mg/l), an angiotensin converting enzyme inhibitor, was administered for three weeks after weaning. Ninety day old offspring from dams that drank saline presented with proximal tubules exhibiting increased (Na(+)+K(+))ATPase expression and activity. Ouabain-insensitive Na(+)-ATPase activity remained unchanged but its response to angiotensin II (Ang II) was lost. PKC, PKA, renal thiobarbituric acid reactive substances (TBARS), macrophage infiltration and collagen deposition markedly increased, and AT(2) receptor expression decreased while AT(1) expression was unaltered. Early treatment with enalapril reduced expression and activity of (Na(+)+K(+))ATPase, partially recovered the response of Na(+)-ATPase to Ang II, and reduced PKC and PKA activities independently of whether offspring were exposed to high perinatal Na(+) or not. In addition, treatment with enalapril per se reduced AT(2) receptor expression, and increased TBARS, macrophage infiltration and collagen deposition. The perinatally Na(+)-overloaded offspring presented high numbers of Ang II-positive cortical cells, and significantly lower circulating Ang I, indicating that programming/reprogramming impacted systemic and local RAS.

CONCLUSIONS/SIGNIFICANCE

Maternal Na(+) overload programmed alterations in renal Na(+) transporters and in its regulation, as well as severe structural lesions in adult offspring. Enalapril was beneficial predominantly through its influence on Na(+) pumping activities in adult offspring. However, side effects including down-regulation of PKA, PKC and AT(2) receptors and increased TBARS could impair renal function in later life.

Renal function in diabetic disease models: the tubular system in the pathophysiology of the diabetic kidney

Diabetes mellitus affects the kidney in stages. At the onset of diabetes mellitus, in a subset of diabetic patients the kidneys grow large, and glomerular filtration rate (GFR) becomes supranormal, which are risk factors for developing diabetic nephropathy later in life. This review outlines a pathophysiological concept that focuses on the tubular system to explain these changes. The concept includes the tubular hypothesis of glomerular filtration, which states that early tubular growth and sodium-glucose cotransport enhance proximal tubule reabsorption and make the GFR supranormal through the physiology of tubuloglomerular feedback. The diabetic milieu triggers early tubular cell proliferation, but the induction of TGF-β and cyclin-dependent kinase inhibitors causes a cell cycle arrest and a switch to tubular hypertrophy and a senescence-like phenotype. Although this growth phenotype explains unusual responses like the salt paradox of the early diabetic kidney, the activated molecular pathways may set the stage for tubulointerstitial injury and diabetic nephropathy.

Acute saline expansion increases nephron filtration and distal flow rate but maintains tubuloglomerular feedback responsiveness: role of adenosine A(1) receptors

Temporal adaptation of tubuloglomerular feedback (TGF) permits readjustment of the relationship of nephron filtration rate [single nephron glomerular filtration rate (SNGFR)] and early distal tubular flow rate (V(ED)) while maintaining TGF responsiveness. We used closed-loop assessment of TGF in hydropenia and after acute saline volume expansion (SE; 10% body wt over 1 h) to determine whether 1) temporal adaptation of TGF occurs, 2) adenosine A(1) receptors (A(1)R) mediate TGF responsiveness, and 3) inhibition of TGF affects SNGFR, V(ED), or urinary excretion under these conditions. SNGFR was evaluated in Fromter-Wistar rats by micropuncture in 1) early distal tubules (ambient flow at macula densa), 2) recollected from early distal tubules while 12 nl/min isotonic fluid was added to late proximal tubule (increased flow to macula densa), and 3) from proximal tubules of same nephrons (zero flow to macula densa). SE increased both ambient SNGFR and V(ED) compared with hydropenia, whereas TGF responsiveness (proximal-distal difference in SNGFR, distal SNGFR response to adding fluid to proximal tubule) was maintained, demonstrating TGF adaptation. A(1)R blockade completely inhibited TGF responsiveness during SE and made V(ED) more susceptible to perturbation in proximal tubular flow, but did not alter ambient SNGFR or V(ED). Greater urinary excretion of fluid and Na(+) with A(1)R blockade may reflect additional effects on the distal nephron in hydropenia and SE. In conclusion, A(1)R-independent mechanisms adjust SNGFR and V(ED) to higher values after SE, which facilitates fluid and Na(+) excretion. Concurrently, TGF adapts and stabilizes early distal delivery at the new setpoint in an A(1)R-dependent mechanism.

AT1A angiotensin receptors in the renal proximal tubule regulate blood pressure

Hypertension affects more than 1.5 billion people worldwide but the precise cause of elevated blood pressure (BP) cannot be determined in most affected individuals. Nonetheless, blockade of the renin-angiotensin system (RAS) lowers BP in the majority of patients with hypertension. Despite its apparent role in hypertension pathogenesis, the key cellular targets of the RAS that control BP have not been clearly identified. Here we demonstrate that RAS actions in the epithelium of the proximal tubule have a critical and nonredundant role in determining the level of BP. Abrogation of AT(1) angiotensin receptor signaling in the proximal tubule alone is sufficient to lower BP, despite intact vascular responses. Elimination of this pathway reduces proximal fluid reabsorption and alters expression of key sodium transporters, modifying pressure-natriuresis and providing substantial protection against hypertension. Thus, effectively targeting epithelial functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.

The proximal tubule in the pathophysiology of the diabetic kidney

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Salt and hypertension

Current salt intake is too high. Current evidence documents that salt is crucial to the genesis of hypertension.

High-salt diet reveals the hypertensive and renal effects of reduced nephron endowment

The extent to which a reduced nephron endowment contributes to hypertension and renal disease is confounded in models created by intrauterine insults that also demonstrate other phenotypes. Furthermore, recent data suggest that a reduced nephron endowment provides the "first hit" and simply increases the susceptibility to injurious stimuli. Thus we examined nephron number, glomerular volume, conscious mean arterial pressure (MAP), and renal function in a genetic model of reduced nephron endowment before and after a high-salt (5%) diet. One-yr-old glial cell line-derived neurotrophic factor wild-type (WT) mice, heterozygous (HET) mice born with two kidneys (HET2K), and HET mice born with one kidney (HET1K) were used. Nephron number was 25% lower in HET2K and 65% lower in HET1K than WT mice. Glomeruli hypertrophied in both HET groups by 33%, resulting in total glomerular volumes that were similar between HET2K and WT mice but remained 50% lower in HET1K mice. On a normal-salt diet, 24-h MAP was not different between WT, HET2K, and HET1K mice (102 +/- 1, 103 +/- 1, and 102 +/- 2 mmHg). On a high-salt diet, MAP increased 9.1 +/- 1.9 mmHg in HET1K mice (P < 0.05) and 5.4 +/- 0.9 mmHg in HET2K mice (P < 0.05) and did not change significantly in WT mice. Creatinine clearance was 60% higher in WT mice but 30% lower in HET2K and HET1K mice fed a high-salt diet than in controls maintained on a normal-salt diet. Thus a reduction in nephron number (or total glomerular volume) alone does not lead to hypertension or kidney disease in aged mice, but exposure to high salt uncovers a hypertensive and renal phenotype.

Angiotensin II stimulates trafficking of NHE3, NaPi2, and associated proteins into the proximal tubule microvilli

Angiotensin II (ANG II) stimulates proximal tubule (PT) sodium and water reabsorption. We showed that treating rats acutely with the angiotensin-converting enzyme inhibitor captopril decreases PT salt and water reabsorption and provokes rapid redistribution of the Na(+)/H(+) exchanger isoform 3 (NHE3), Na(+)/Pi cotransporter 2 (NaPi2), and associated proteins out of the microvilli. The aim of the present study was to determine whether acute ANG II infusion increases the abundance of PT NHE3, NaPi2, and associated proteins in the microvilli available for reabsorbing NaCl. Male Sprague-Dawley rats were infused with a dose of captopril (12 microg/min for 20 min) that increased PT flow rate approximately 20% with no change in blood pressure (BP) or glomerular filtration rate (GFR). When ANG II (20 ng x kg(-1) x min(-1) for 20 min) was added to the captopril infusate, PT volume flow rate returned to baseline without changing BP or GFR. After captopril, NHE3 was localized to the base of the microvilli and NaPi2 to subapical cytoplasmic vesicles; after 20 min ANG II, both NHE3 and NaPi2 redistributed into the microvilli, assayed by confocal microscopy and density gradient fractionation. Additional PT proteins that redistributed into low-density microvilli-enriched membranes in response to ANG II included myosin VI, DPPIV, NHERF-1, ezrin, megalin, vacuolar H(+)-ATPase, aminopeptidase N, and clathrin. In summary, in response to 20 min ANG II in the absence of a change in BP or GFR, multiple proteins traffic into the PT brush-border microvilli where they likely contribute to the rapid increase in PT salt and water reabsorption.

Renal NMDA receptors independently stimulate proximal reabsorption and glomerular filtration

N-methyl-D-aspartate receptors (NMDA) are expressed in the kidney, where little is known of their functional role. Several series of micropuncture experiments were performed in hydropenic rats using the NMDA channel blocker, MK801, and the NMDA coagonist, L-glycine, to probe NMDA for effects on single-nephron glomerular filtration rate (SNGFR) and proximal reabsorption (J(prox)). During intravenous infusion of MK801 or L-glycine, Henle's loop was perfused to manipulate SNGFR via tubuloglomerular feedback (TGF), thereby facilitating analysis of glomerulotubular balance. To confirm local actions on the kidney, MK801 was delivered to the glomerulus by microperfusion past the macula densa and to the proximal tubule by microperfusion into the early S1 segment. By all measures, MK801 acted on the glomerulus to reduce SNGFR, and acted on the proximal tubule to suppress J(prox), while having no effect on the responsiveness of TGF. L-Glycine raised SNGFR, dampened the TGF response, and could not be proved to independently stimulate proximal reabsorption. NMDA exerts a tonic vasodilatory influence on the glomerulus and a proreabsorptive effect on the proximal tubule. These combined effects allow NMDA to modulate SNGFR with minimal impact on late proximal flow. The full effects of L-glycine infusion on proximal tubule and TGF response do not extrapolate from the response to NMDA blockade.

Adenosine A(1) receptors determine glomerular hyperfiltration and the salt paradox in early streptozotocin diabetes mellitus

BACKGROUND

In early type 1 diabetes mellitus, changes in proximal reabsorption influence glomerular filtration rate (GFR) through tubuloglomerular feedback (TGF). Due to TGF, a primary increase in proximal reabsorption causes early diabetic hyperfiltration, while a heightened sensitivity of the proximal tubule to dietary salt leads to the so-called salt paradox, where a change in dietary salt causes a reciprocal change in GFR ('tubulocentric principle'). Here, experiments were performed in adenosine A(1) receptor knockout mice (A(1)R-/-), which lack an immediate TGF response, to determine whether A(1)Rs are essential for early diabetic hyperfiltration and the salt paradox.

METHODS

GFR was measured by inulin disappearance in conscious A(1)R-/- and wild-type (WT) mice after 4 weeks of streptozotocin diabetes on a control NaCl diet (1%), and measurements were repeated after 6 days of equilibration on a low-NaCl (0.1%) or a high-NaCl (4%) diet.

RESULTS

A(1)R-/- and WT were similar with respect to blood glucose, dietary intakes and body weight changes on a given diet. Diabetic hyperfiltration occurred in WT, but was blunted in A(1)R-/-. A reciprocal relationship between GFR and dietary salt was found in WT diabetics, but not A(1)R-/- diabetics or nondiabetics of either strain.

CONCLUSION

A(1)Rs determine glomerular hyperfiltration and the salt paradox in early diabetes, which is consistent with the tubulocentric principle.

Unexpected effect of angiotensin AT1 receptor blockade on tubuloglomerular feedback in early subtotal nephrectomy

After subtotal nephrectomy (STN), the remaining nephrons engage in hyperfiltration, which may be facilitated by a reduced sensitivity of the tubuloglomerular feedback (TGF) response to increased distal delivery. However, a muted TGF response would contradict the notion of remnant kidney as a prototype of angiotensin II (ANG II) excess, since ANG II normally sensitizes the TGF response and stimulates proximal reabsorption. We examined the role of ANG II as a modulator of TGF and proximal reabsorption in 7 days after STN in male rats. Single-nephron glomerular filtration rate (SNGFR) and proximal reabsorption (J(prox)) were measured in late proximal collections while perfusing Henle's loop for minimal and maximal TGF stimulation in rats treated with the angiotensin type 1 (AT(1)) receptor blocker losartan or placebo in drinking water for 7 days. Perfusion of Henle's loop yielded a robust TGF response in sham-operated rats. In STN, the feedback responses were highly variable and nil, on average. Paradoxical TGF responses to augmented late proximal flow were confirmed in SNGFR measurements from the early distal nephron. Chronic losartan treatment normalized the average TGF response without reducing the variability. J(prox) was subtly affected by chronic losartan in sham surgery or STN, after controlling for differences in SNGFR. However, when administered acutely into the early S1 segment, losartan potently suppressed J(prox) in STN and sham-operated rats alike. Chronic losartan stabilizes the TGF system in remnant kidneys. This cannot be explained by currently known actions of AT(1) receptors but is commensurate with a salutary effect of an intact TGF system on dynamic autoregulation of intraglomerular flow and pressure.

Micropuncturing the nephron

To achieve the role of the kidney in maintaining body homeostasis, the renal vasculature, the glomeruli, and the various segments of the nephron and the collecting duct system have to fulfill very diverse and specific functions. These functions are dependent on a complex renal architecture and are regulated by systemic hemodynamics, hormones, and nerves. As a consequence, to better understand the physiology of the kidney, methods are necessary that allow insights on the function of these diverse structures in the physiological context of the intact kidney. The renal micropuncture technique allows direct access to study superficial nephrons in vivo. In this review, the application of micropuncture techniques on the single nephron level is outlined as an approach to better understand aspects of glomerular filtration, tubular transport, and tubulo-glomerular communication. Studies from the author's lab, including experiments in gene-targeted mice, are briefly presented to illustrate some of the approaches and show how they can further advance our understanding of the molecular mechanisms involved in the regulation of kidney function.

Effects of dietary salt on renal Na+ transporter subcellular distribution, abundance, and phosphorylation status

During high-salt (HS) diet the kidney increases urinary Na+ and volume excretion to match intake. We recently reported that HS provokes a redistribution of distal convoluted tubule Na+-Cl- cotransporter (NCC) from apical to subapical vesicles and decreases NCC abundance. This study aimed to test the hypothesis that the other renal Na+ transporters' abundance and or subcellular distribution is decreased by HS diet. Six-week-old Sprague-Dawley rats were fed a normal (NS) 0.4% NaCl diet or a HS 4% NaCl diet for 3 wk or overnight. Kidneys excised from anesthetized rats were fractionated on density gradients or analyzed by microscopy; transporters and associated regulators were detected with specific antibodies. Three-week HS doubled Na+/H+ exchanger (NHE)3 phosphorylation at serine 552 and provoked a redistribution of NHE3, dipeptidyl peptidase IV (DPPIV), myosin VI, Na+-Pi cotransporter (NaPi)-2, ANG II type 2 receptor (AT2R), aminopeptidase N (APN), Na+-K+-2Cl- cotransporter (NKCC2), epithelial Na+ channel (ENaC) beta-subunit, and Na+-K+-ATPase (NKA) alpha1- and beta1-subunits from low-density plasma membrane-enriched fractions to higher-density intracellular membrane-enriched fractions. NHE3, myosin VI, and AT2R retraction to the base of the microvilli (MV) during HS was evident by confocal microscopy. HS did not change abundance of NHE3, NKCC, or NKA alpha1- or beta1-subunits but increased ENaC-beta in high-density intracellular enriched membranes. Responses to HS were fully apparent after just 18 h. We propose that retraction of NHE3 to the base of the MV, driven by myosin VI and NHE3 phosphorylation and accompanied by redistribution of the NHE3 regulator DPPIV, contributes to a decrease in proximal tubule Na+ reabsorption during HS and that redistribution of transporters out of low-density plasma membrane-enriched fractions in the thick ascending limb of the loop of Henle and distal nephron may also contribute to the homeostatic natriuretic response to HS diet.

Ornithine decarboxylase inhibitor eliminates hyperresponsiveness of the early diabetic proximal tubule to dietary salt

Heightened sensitivity of the diabetic proximal tubule to dietary salt leads to a paradoxical effect of salt on glomerular filtration rate (GFR) via tubuloglomerular feedback. Diabetic hyperfiltration is a feedback response to growth and hyperreabsorption by the proximal tubule. The present studies were performed to determine whether growth and hyperfunction of the proximal tubule are essential for its hyperresponsiveness to dietary salt and, hence, to the paradoxical effect of dietary salt on GFR. Micropuncture was performed in four groups of inactin-anesthetized Wistar rats after 10 days of streptozotocin diabetes drinking tap water or 1% NaCl. Kidney growth was suppressed with ornithine decarboxylase (ODC) inhibitor, DFMO (200 mg.kg(-1).day(-1)), or placebo. Single nephron GFR (SNGFR) was manipulated by perfusing Henle's loop so that proximal reabsorption (Jprox) could be expressed as a function of SNGFR in each nephron, dissociating primary effects on the tubule from the effects of glomerulotubular balance. Alone, DFMO or high salt reduced SNGFR and suppressed Jprox independent of SNGFR. Suppression of Jprox was eliminated and SNGFR increased when high salt was given to rats receiving DFMO. ODC is necessary for hyperresponsiveness of the proximal tubule to dietary salt and for the paradoxical effect of dietary salt on GFR in early diabetes. This coupling of effects adds to the body of evidence that feedback from the proximal tubule is the principal governor of glomerular filtration in early diabetes.

Roles of ERK and cPLA2 in the angiotensin II-mediated biphasic regulation of Na+-HCO3(-) transport

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A2 (PLA2) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT1A)-deficient, and group IVA cytosolic phospholipase A2 (cPLA2alpha)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na+-HCO3(-) cotransporter activity are both AT1-mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT1A-deficient mice, suggesting that this occurs through AT1B. In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA2alpha activation because high-concentration Ang II stimulated Na+-HCO3(-) cotransporter activity when cPLA2alpha activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA2alpha/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA2alpha in a concentration-dependent manner via AT1, and that the balance between ERK and cPLA2alpha activities determines the ultimate response to Ang II in intact proximal tubules.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

Changes in salt intake are associated in general with corresponding changes in arterial blood pressure. An exaggerated increment in blood pressure driven by a salt load is characteristic of salt-sensitive hypertension, a condition affecting more than two thirds of individuals with essential hypertension who are older than 60 years. In the last decade, significant insight was gained about the role of the kidney in the increment in blood pressure induced by sodium retention. The present review focuses on the pathophysiological characteristics of the blood pressure increase driven by expansion of extracellular fluid and the increment in plasma sodium concentration. In addition, we discuss systemic and renal conditions that result in decreased urinary sodium excretion and were implicated in the development of salt-sensitive hypertension.