Differential regulation of Na+ transporters along nephron during ANG II-dependent hypertension: distal stimulation counteracted by proximal inhibition

Delayed Graft Function and the Renin-angiotensin System

Delayed graft function (DGF) is a form of acute kidney injury (AKI) and a common complication following kidney transplantation. It adversely influences patient outcomes increases the financial burden of transplantation, and currently, no specific treatments are available. In developing this form of AKI, activation of the renin-angiotensin system (RAS) has been proposed to play an important role. In this review, we discuss the role of RAS activation and its contribution to the pathophysiology of DGF following the different stages of the transplantation process, from procurement and ischemia to transplantation into the recipient and including data from experimental animal models. Deceased kidney donors, whether during cardiac or brain death, may experience activation of the RAS. That may be continued or further potentiated during procurement and organ preservation. Additional evidence suggests that during implantation of the kidney graft and reperfusion in the recipient, the RAS is activated and may likely remain activated, extrapolating from other forms of AKI where RAS overactivity is well documented. Of particular interest in this setting is the status of angiotensin-converting enzyme 2, a key RAS enzyme essential for the metabolism of angiotensin II and abundantly present in the apical border of the proximal tubules, which is the site of predominant injury in AKI and DGF. Interventions aimed at safely downregulating the RAS using suitable shorter forms of angiotensin-converting enzyme 2 could be a way to offer protection against DGF.

Renal upregulation of NCC counteracts empagliflozin-mediated NHE3 inhibition in normotensive but not in hypertensive male rat

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) reduce blood pressure (BP) in patients with hypertension, yet the precise molecular mechanisms remain elusive. SGLT2i inhibits proximal tubule (PT) NHE3-mediated sodium reabsorption in normotensive rodents, yet no hypotensive effect is observed under this scenario. This study examined the effect of empagliflozin (EMPA) on renal tubular sodium transport in normotensive and spontaneously hypertensive rats (SHRs). It also tested the hypothesis that EMPA-mediated PT NHE3 inhibition in normotensive rats is associated with upregulation of distal nephron apical sodium transporters. EMPA administration for 14 days reduced BP in 12-wk-old SHRs but not in age-matched Wistar rats. PT NHE3 activity was inhibited by EMPA treatment in both Wistar and SHRs. In Wistar rats, EMPA increased NCC activity, mRNA expression, protein abundance, and phosphorylation levels, but not in SHRs. SHRs showed higher NKCC2 activity and an abundance of cleaved ENaC α and γ subunits compared with Wistar rats, none of which were affected by EMPA. Another set of male Wistar rats was treated with EMPA, the NCC inhibitor hydrochlorothiazide (HCTZ), and EMPA combined with HCTZ or vehicle for 14 days. In these rats, BP reduction was observed only with combined EMPA and HCTZ treatment, not with either drug alone. These findings suggest that NCC upregulation counteracts EMPA-mediated inhibition of PT NHE3 in male normotensive rats, maintaining their baseline BP. Moreover, the reduction of NHE3 activity without further upregulation of major apical sodium transporters beyond the PT may contribute to the BP-lowering effect of SGLT2i in experimental models and patients with hypertension.NEW & NOTEWORTHY This study suggests that reduced NHE3-mediated sodium reabsorption in the renal proximal tubule may account, at least in part, for the BP-lowering effect of SGLT2 inhibitors in the setting of hypertension. It also demonstrates that chronic treatment with SGLT2 inhibitors upregulates NCC activity, phosphorylation, and expression in the distal tubule of normotensive but not hypertensive rats. SGLT2 inhibitor-mediated upregulation of NCC seems crucial to counteract proximal tubule natriuresis in subjects with normal BP.

Predicting sex differences in the effects of diuretics in renal epithelial transport during angiotensin II-induced hypertension

Chronic angiotensin II (ANG II) infusion is an experimental model that induces hypertension in rodents. The natriuresis, diuresis, and blood pressure responses differ between males and females. This is perhaps not unexpected, given the rodent kidney, which plays a key role in blood pressure regulation, exhibits marked sex differences. Under normotensive conditions, compared with males, the female rat nephron exhibits lower Na+/H+ exchanger 3 (NHE3) activity along the proximal tubule but higher Na+ transporter activities along the distal segments. ANG II infusion-induced hypertension induces a pressure natriuretic response that reduces NHE3 activity and shifts Na+ transport capacity downstream. The goals of this study were to apply a computational model of epithelial transport along a rat nephron 1) to understand how a 14-day ANG II infusion impacts segmental electrolyte transport in male and female rat nephrons and 2) to identify and explain any sex differences in the effects of loop diuretics, thiazide diuretics, and K+-sparing diuretics. Model simulations suggest that the NHE3 downregulation in the proximal tubule is a major contributor to natriuresis and diuresis in hypertension, with the effects stronger in males. All three diuretics are predicted to induce stronger natriuretic and diuretic effects under hypertension compared with normotension, with relative increases in sodium excretion higher in hypertensive females than in males. The stronger natriuretic responses can be explained by the downstream shift of Na+ transport load in hypertension and by the larger distal transport load in females, both of which limit the ability of the distal segments to further elevate their Na+ transport.NEW & NOTEWORTHY Sex differences in the prevalence of hypertension are found in human and animal models. The kidney, which regulates blood pressure, exhibits sex differences in morphology, hemodynamics, and membrane transporter distributions. This computational modeling study provides insights into how the sexually dimorphic responses to a 14-day angiotensin II infusion differentially impact segmental electrolyte transport in rats. Simulations of diuretic administration explain how the natriuretic and diuretic effects differ between normotension and hypertension and between the sexes.

Sex differences in renal transporters: assessment and functional consequences

Mammalian kidneys are specialized to maintain fluid and electrolyte homeostasis. The epithelial transport processes along the renal tubule that match output to input have long been the subject of experimental and theoretical study. However, emerging data have identified a new dimension of investigation: sex. Like most tissues, the structure and function of the kidney is regulated by sex hormones and chromosomes. Available data demonstrate sex differences in the abundance of kidney solute and electrolyte transporters, establishing that renal tubular organization and operation are distinctly different in females and males. Newer studies have provided insights into the physiological consequences of these sex differences. Computational simulations predict that sex differences in transporter abundance are likely driven to optimize reproduction, enabling adaptive responses to the nutritional requirements of serial pregnancies and lactation - normal life-cycle changes that challenge the ability of renal transporters to maintain fluid and electrolyte homeostasis. Later in life, females may also undergo menopause, which is associated with changes in disease risk. Although numerous knowledge gaps remain, ongoing studies will provide further insights into the sex-specific mechanisms of sodium, potassium, acid-base and volume physiology throughout the life cycle, which may lead to therapeutic opportunities.

Role of Angiotensin II Type 1a Receptor (AT1aR) of Renal Tubules in Regulating Inwardly Rectifying Potassium Channels 4.2 (Kir4.2), Kir4.1, and Epithelial Na+ Channel (ENaC)

BACKGROUND

Kir4.2 and Kir4.1 play a role in regulating membrane transport in the proximal tubule (PT) and in the distal-convoluted-tubule (DCT), respectively.

METHODS

We generated kidney-tubule-specific-AT1aR-knockout (Ks-AT1aR-KO) mice to examine whether renal AT1aR regulates Kir4.2 and Kir4.1.

RESULTS

Ks-AT1aR-KO mice had a lower systolic blood pressure than Agtr1aflox/flox (control) mice. Ks-AT1aR-KO mice had a lower expression of NHE3 (Na+/H+-exchanger 3) and Kir4.2, a major Kir-channel in PT, than Agtr1aflox/flox mice. Whole-cell recording also demonstrated that the membrane potential in PT of Ks-AT1aR-KO mice was lesser negative than Agtr1aflox/flox mice. The expression of Kir4.1 and Kir5.1, Kir4.1/Kir5.1-mediated K+ currents of DCT and DCT membrane potential in Ks-AT1aR-KO mice, were similar to Agtr1aflox/flox mice. However, angiotensin II perfusion for 7 days hyperpolarized the membrane potential in PT and DCT of the control mice but not in Ks-AT1aR-KO mice, while angiotensin II perfusion did not change the expression of Kir4.1, Kir4.2, and Kir5.1. Deletion of AT1aR did not significantly affect the expression of αENaC (epithelial Na+ channel) and βENaC but increased cleaved γENaC expression. Patch-clamp experiments demonstrated that deletion of AT1aR increased amiloride-sensitive Na+-currents in the cortical-collecting duct but not in late-DCT. However, tertiapin-Q sensitive renal outer medullary potassium channel currents were similar in both genotypes.

CONCLUSIONS

AT1aR determines the baseline membrane potential of PT by controlling Kir4.2 expression/activity but AT1aR is not required for determining the baseline membrane potential of the DCT and Kir4.1/Kir5.1 activity/expression. However, AT1aR is required for angiotensin II-induced hyperpolarization of basolateral membrane of PT and DCT. Deletion of AT1aR had no effect on baseline renal outer medullary potassium channel activity but increased ENaC activity in the CCD.

Angiotensin II hypertension along the female rat tubule: predicted impact on coupled transport of Na+ and K. Am J Physiol Renal Physiol 2023;325:F733-F749. [PMID: 37823196 PMCID: PMC10878725 DOI: 10.1152/ajprenal.00232.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Chronic infusion of subpressor level of angiotensin II (ANG II) increases the abundance of Na+ transporters along the distal nephron, balanced by suppression of Na+ transporters along the proximal tubule and medullary thick ascending limb (defined as "proximal nephron"), which impacts K+ handling along the entire renal tubule. The objective of this study was to quantitatively assess the impact of chronic ANG II on the renal handling of Na+ and K+ in female rats, using a computational model of the female rat renal tubule. Our results indicate that the downregulation of proximal nephron Na+ reabsorption (TNa), which occurs in response to ANG II-triggered hypertension, involves changes in both transporter abundance and trafficking. Our model suggests that substantial (∼30%) downregulation of active NHE3 in proximal tubule (PT) microvilli is needed to reestablish the Na+ balance at 2 wk of ANG II infusion. The 35% decrease in SGLT2, a known NHE3 regulator, may contribute to this downregulation. Both depression of proximal nephron TNa and stimulation of distal ENaC raise urinary K+ excretion in ANG II-treated females, while K+ loss is slightly mitigated by cortical NKCC2 and NCC upregulation. Our model predicts that K+ excretion may be more significantly limited during ANG II infusion by ROMK inhibition in the distal nephron and/or KCC3 upregulation in the PT, which remain open questions for experimental validation. In summary, our analysis indicates that ANG II hypertension triggers a series of events from distal TNa stimulation followed by compensatory reduction in proximal nephron TNa and accompanying adjustments to limit excessive K+ secretion.NEW & NOTEWORTHY We used a computational model of the renal tubule to assess the impact of 2-wk angiotensin II (ANG II) infusion on the handling of Na+ and K+ in female rats. ANG II strongly stimulates distal Na+ reabsorption and K+ secretion. Simulations indicate that substantial downregulation of proximal tubule NHE3 is needed to reestablish Na+ balance at 2 wk. Proximal adaptations challenge K+ homeostasis, and regulation of distal NCC and specific K+ channels likely limit urinary K+ losses.

Sex differences in renal electrolyte transport

PURPOSE OF REVIEW

Women experience unique life events, for example, pregnancy and lactation, that challenge renal regulation of electrolyte homeostasis. Recent analyses of nephron organization in female vs. male rodent kidneys, revealed distinct sexual dimorphisms in electrolyte transporter expression, abundance, and activity. This review aims to provide an overview of electrolyte transporters' organization and operation in female compared with the commonly studied male kidney, and the (patho)physiologic consequences of the differences.

RECENT FINDINGS

When electrolyte transporters are assessed in kidney protein homogenates from both sexes, relative transporter abundance ratios in females/males are less than one along proximal tubule and greater than one post macula densa, which is indicative of a 'downstream shift' in fractional reabsorption of electrolytes in females. This arrangement improves the excretion of a sodium load, challenges potassium homeostasis, and is consistent with the lower blood pressure and greater pressure natriuresis observed in premenopausal women.

SUMMARY

We summarize recently reported new knowledge about sex differences in renal transporters: abundance and expression along nephron, implications for regulation by Na + , K + and angiotensin II, and mathematical models of female nephron function.

Collectrin (Tmem27) deficiency in proximal tubules causes hypertension in mice and a TMEM27 variant associates with blood pressure in males in a Latino cohort

Collectrin (Tmem27), an angiotensin-converting enzyme 2 homologue, is a chaperone of amino acid transporters in the kidney and endothelium. Global collectrin knockout (KO) mice have hypertension, endothelial dysfunction, exaggerated salt sensitivity, and diminished renal blood flow. This phenotype is associated with altered nitric oxide and superoxide balance and increased proximal tubule (PT) Na+/H+ exchanger isoform 3 (NHE3) expression. Collectrin is located on the X chromosome where genome-wide association population studies have largely been excluded. In the present study, we generated PT-specific collectrin KO (PT KO) mice to determine the precise contribution of PT collectrin in blood pressure homeostasis. We also examined the association of human TMEM27 single-nucleotide polymorphisms with blood pressure traits in 11,926 Hispanic Community Health Study/Study of Latinos (HCHS/SOL) Hispanic/Latino participants. PT KO mice exhibited hypertension, and this was associated with increased baseline NHE3 expression and diminished lithium excretion. However, PT KO mice did not display exaggerated salt sensitivity or a reduction in renal blood flow compared with control mice. Furthermore, PT KO mice exhibited enhanced endothelium-mediated dilation, suggesting a compensatory response to systemic hypertension induced by deficiency of collectrin in the PT. In HCHS/SOL participants, we observed sex-specific single-nucleotide polymorphism associations with diastolic blood pressure. In conclusion, loss of collectrin in the PT is sufficient to induce hypertension, at least in part, through activation of NHE3. Importantly, our model supports the notion that altered renal blood flow may be a determining factor for salt sensitivity. Further studies are needed to investigate the role of the TMEM27 locus on blood pressure and salt sensitivity in humans.NEW & NOTEWORTHY The findings of our study are significant in several ways: 1) loss of an amino acid chaperone in the proximal tubule is sufficient to cause hypertension, 2) the results in global and proximal tubule-specific collectrin knockout mice support the notion that vascular dysfunction is required for salt sensitivity or that impaired renal tubule function causes hypertension but is not sufficient to cause salt sensitivity, and 3) our study is the first to implicate a role of collectrin in human hypertension.

Relative Contribution of Blood Pressure and Renal Sympathetic Nerve Activity to Proximal Tubular Sodium Reabsorption via NHE3 Activity

We examined the effects of an acute increase in blood pressure (BP) and renal sympathetic nerve activity (rSNA) induced by bicuculline (Bic) injection in the paraventricular nucleus of hypothalamus (PVN) or the effects of a selective increase in rSNA induced by renal nerve stimulation (RNS) on the renal excretion of sodium and water and its effect on sodium-hydrogen exchanger 3 (NHE3) activity. Uninephrectomized anesthetized male Wistar rats were divided into three groups: (1) Sham; (2) Bic PVN: (3) RNS + Bic injection into the PVN. BP and rSNA were recorded, and urine was collected prior and after the interventions in all groups. RNS decreased sodium (58%) and water excretion (53%) independently of BP changes (p < 0.05). However, after Bic injection in the PVN during RNS stimulation, the BP and rSNA increased by 30% and 60% (p < 0.05), respectively, diuresis (5-fold) and natriuresis (2.3-fold) were increased (p < 0.05), and NHE3 activity was significantly reduced, independently of glomerular filtration rate changes. Thus, an acute increase in the BP overcomes RNS, leading to diuresis, natriuresis, and NHE3 activity inhibition.

Cell-Specific Actions of the Prostaglandin E-Prostanoid Receptor 4 Attenuating Hypertension: A Dominant Role for Kidney Epithelial Cells Compared With Macrophages

Background A beneficial role for prostanoids in hypertension is suggested by clinical studies showing nonsteroidal anti-inflammatory drugs, which block the production of all prostanoids, cause sodium retention and exacerbate hypertension. Among prostanoids, prostaglandin E2 and its E-prostanoid receptor 4 receptor (EP4R) have been implicated in blood pressure control. Our previous study found that conditional deletion of EP4R from all tissues in adult mice exacerbates angiotensin II-dependent hypertension, suggesting a powerful effect of EP4R to resist blood pressure elevation. We also found that elimination of EP4R from vascular smooth muscle cells did not affect the severity of hypertension, suggesting nonvascular targets of prostaglandin E mediate this antihypertensive effect. Methods and Results Here we generated mice with cell-specific deletion of EP4R from macrophage-specific EP4 receptor knockouts or kidney epithelial cells (KEKO) to assess the contributions of EP4R in these cells to hypertension pathogenesis. Macrophage-specific EP4 receptor knockouts showed similar blood pressure responses to alterations in dietary sodium or chronic angiotensin II infusion as Controls. By contrast, angiotensin II-dependent hypertension was significantly augmented in KEKOs (mean arterial pressure: 146±3 mm Hg) compared with Controls (137±4 mm Hg; P=0.02), which was accompanied by impaired natriuresis in KEKOs. Because EP4R expression in the kidney is enriched in the collecting duct, we compared responses to amiloride in angiotensin II-infused KEKOs and Controls. Blockade of the epithelial sodium channel with amiloride caused exaggerated natriuresis in KEKOs compared with Controls (0.21±0.01 versus 0.15±0.02 mmol/24 hour per 20 g; P=0.015). Conclusions Our data suggest EP4R in kidney epithelia attenuates hypertension. This antihypertension effect of EP4R may be mediated by reducing the activity of the epithelial sodium channel, thereby promoting natriuresis.

Tubular IL-1β Induces Salt Sensitivity in Diabetes by Activating Renal Macrophages

BACKGROUND

Chronic renal inflammation has been widely recognized as a major promoter of several forms of high blood pressure including salt-sensitive hypertension. In diabetes, IL (interleukin)-6 induces salt sensitivity through a dysregulation of the epithelial sodium channel. However, the origin of this inflammatory process and the molecular events that culminates with an abnormal regulation of epithelial sodium channel and salt sensitivity in diabetes are largely unknown.

METHODS

Both in vitro and in vivo approaches were used to investigate the molecular and cellular contributors to the renal inflammation associated with diabetic kidney disease and how these inflammatory components interact to develop salt sensitivity in db/db mice.

RESULTS

Thirty-four-week-old db/db mice display significantly higher levels of IL-1β in renal tubules compared with nondiabetic db/+ mice. Specific suppression of IL-1β in renal tubules prevented salt sensitivity in db/db mice. A primary culture of renal tubular epithelial cells from wild-type mice releases significant levels of IL-1β when exposed to a high glucose environment. Coculture of tubular epithelial cells and bone marrow-derived macrophages revealed that tubular epithelial cell-derived IL-1β promotes the polarization of macrophages towards a proinflammatory phenotype resulting in IL-6 secretion. To evaluate whether macrophages are the cellular target of IL-1β in vivo, diabetic db/db mice were transplanted with the bone marrow of IL-1R1 (IL-1 receptor type 1) knockout mice. db/db mice harboring an IL-1 receptor type 1 knockout bone marrow remained salt resistant, display lower renal inflammation and lower expression and activity of epithelial sodium channel compared with db/db transplanted with a wild-type bone marrow.

CONCLUSIONS

Renal tubular epithelial cell-derived IL-1β polarizes renal macrophages towards a proinflammatory phenotype that promotes salt sensitivity through the accumulation of renal IL-6. When tubular IL-1β synthesis is suppressed or in db/db mice in which immune cells lack the IL-1R1, macrophage polarization is blunted resulting in no salt-sensitive hypertension.

Dissecting the Effects of Aldosterone and Hypokalemia on the Epithelial Na+ Channel and the NaCl Cotransporter

Primary hyperaldosteronism (PA) is characterized by aldosterone excess and hypertension. This may be linked to increased renal Na⁺ reabsorption via the epithelial Na⁺ channel (ENaC) and the NaCl cotransporter (NCC). The majority of PA patients have normal plasma K⁺ levels, but a subset of cases are associated with hypokalemia. High NCC levels observed in long-term studies with aldosterone-infused rodents have been attributed to direct effects of aldosterone. Aldosterone can also increase active phosphorylated NCC (pT58-NCC) acutely. However, direct effects of aldosterone on NCC have been contested by recent studies indicating that it is rather an indirect effect of hypokalemia. We therefore set out to determine isolated long-term aldosterone and K⁺ effects on ENaC and NCC using various in vivo and ex vivo approaches. In mice, aldosterone-induced hypokalemia was prevented by simultaneous amiloride infusion, coupled to increased cleavage of α- and γENaC but no effect on NCC. Regression analyses of in vivo data showed a positive correlation between aldosterone/K⁺ and αENaC but a negative correlation with NCC and pT58-NCC. Ex vivo, exposure of kidney tubules for 21 h to aldosterone increased cleavage of αENaC and γENaC, but no effects were observed on NCC or pT58-NCC. Exposure of tubules to low K⁺ media reduced αENaC but increased NCC and pT58-NCC. As hypokalemia can enhance cell proliferation markers in the distal convoluted tubule (DCT), we hypothesized that aldosterone infusion would increase proliferating cell nuclear antigen (PCNA) expression. Infusion of aldosterone in mice for 6 days greatly increased PCNA expression in the DCT. Collectively, in vivo and ex vivo data suggest that both aldosterone and K⁺ can increase ENaC directly. In contrast, the observed increase in abundance and phosphorylation of NCC in aldosterone-infused mice is likely an indirect effect of enhanced ENaC-mediated K⁺ secretion and subsequent hypokalemia. Thus, it is possible that NCC may only be increased in PA when the condition is associated with hypokalemia.

PPAR-α knockout leads to elevated blood pressure response to angiotensin II infusion associated with an increase in renal α-1 Na+/K+ ATPase protein expression and activity

Peroxisome proliferator activated receptor alpha (PPAR-α) deletion has been shown to increase blood pressure (BP). We hypothesized that the BP increase in PPAR-α KO mice was mediated by increased expression and activity of basolateral Na+/K+ ATPase (NKA) pump. To address this hypothesis, we treated wild-type (WT) and PPAR-α knockout (KO) mice with a slow-pressor dose of angiotensin II (400 ng/kg·min) for 12 days by osmotic minipump. Radiotelemetry showed no significant differences in baseline mean arterial pressure (MAP) between WT and PPAR-α KO mice; however, by day 12 of infusion, MAP was significantly higher in PPAR-α KO mice (156 ± 16) compared to WT mice (138 ± 11 mmHg). NKA activity and protein expression (α1 subunit) were significantly higher in PPAR-α KO mice compared to WT mice. There was no significant difference in NKA mRNA levels. Angiotensin II further increased the expression and activity of the NKA in both genotypes along with the water channel, aquaporin 1 (Aqp1). In contrast, angiotensin II decreased the expression (64-97% reduction in band density) of sodium‑hydrogen exchanger-3 (NHE3), NHE regulatory factor-1 (NHERF1, Slc9a3r1), sodium‑potassium-2-chloride cotransporter (NKCC2), and epithelial sodium channel (ENaC) β- and γ- subunits in the renal cortex of both WT and PPAR-α KO mice, with no difference between genotypes. The sodium-chloride cotransporter (NCC) was also decreased by angiotensin II, but significantly more in PPAR-α KO (59% WT versus 77% KO reduction from their respective vehicle-treated mice). Our results suggest that PPAR-α attenuates angiotensin II-mediated increased blood pressure potentially via reducing expression and activity of the NKA.

Renal blood flow and oxygenation

Our kidneys receive about one-fifth of the cardiac output at rest and have a low oxygen extraction ratio, but may sustain, under some conditions, hypoxic injuries that might lead to chronic kidney disease. This is due to large regional variations in renal blood flow and oxygenation, which are the prerequisite for some and the consequence of other kidney functions. The concurrent operation of these functions is reliant on a multitude of neuro-hormonal signaling cascades and feedback loops that also include the regulation of renal blood flow and tissue oxygenation. Starting with open questions on regulatory processes and disease mechanisms, we review herein the literature on renal blood flow and oxygenation. We assess the current understanding of renal blood flow regulation, reasons for disparities in oxygen delivery and consumption, and the consequences of disbalance between O₂ delivery, consumption, and removal. We further consider methods for measuring and computing blood velocity, flow rate, oxygen partial pressure, and related parameters and point out how limitations of these methods constitute important hurdles in this area of research. We conclude that to obtain an integrated understanding of the relation between renal function and renal blood flow and oxygenation, combined experimental and computational modeling studies will be needed.

Renal NOXA1/NOX1 Signaling Regulates Epithelial Sodium Channel and Sodium Retention in Angiotensin II-induced Hypertension

Aims: NADPH oxidase (NOX)-derived reactive oxygen species (ROS) are implicated in the pathophysiology of hypertension in chronic kidney disease patients. Genetic deletion of NOX activator 1 (Noxa1) subunit of NOX1 decreases ROS under pathophysiological conditions. Here, we investigated the role of NOXA1-dependent NOX1 activity in the pathogenesis of angiotensin II (Ang II)-induced hypertension (AIH) and possible involvement of abnormal renal function. Results: NOXA1 is present in epithelial cells of Henle's thick ascending limb and distal nephron. Telemetry showed lower basal systolic blood pressure (BP) in Noxa1^-/-versus wild-type mice. Ang II infusion for 1 and 14 days increased NOXA1/NOX1 expression and ROS in kidney of male but not female wild-type mice. Mean BP increased 30 mmHg in wild-type males, with smaller increases in Noxa1-deficient males and wild-type or Noxa1^-/- females. In response to an acute salt load, Na⁺ excretion was similar in wild-type and Noxa1^-/- mice before and 14 days after Ang II infusion. However, Na⁺ excretion was delayed after 1-2 days of Ang II in male wild-type versus Noxa1^-/- mice. Ang II increased epithelial Na⁺ channel (ENaC) levels and activation in the collecting duct principal epithelial cells of wild-type but not Noxa1^-/- mice. Aldosterone induced ROS levels and Noxa1 and Scnn1a expression and ENaC activity in a mouse renal epithelial cell line, responses abolished by Noxa1 small-interfering RNA. Innovation and Conclusion: Ang II activation of renal NOXA1/NOX1-dependent ROS enhances tubular ENaC expression and Na⁺ reabsorption, leading to increased BP. Attenuation of AIH in females is attributed to weaker NOXA1/NOX1-dependent ROS signaling and efficient natriuresis. Antioxid. Redox Signal. 36, 550-566.

Acclimation to a High-Salt Diet Is Sex Dependent

Background Premenopausal women are less likely to develop hypertension and salt-related complications than are men, yet the impact of sex on mechanisms regulating Na+ homeostasis during dietary salt challenges is poorly defined. Here, we determined whether female rats have a more efficient capacity to acclimate to increased dietary salt intake challenge. Methods and Results Age-matched male and female Sprague Dawley rats maintained on a normal-salt (NS) diet (0.49% NaCl) were challenged with a 5-day high-salt diet (4.0% NaCl). We assessed serum, urinary, skin, and muscle electrolytes; total body water; and kidney Na+ transporters during the NS and high-salt diet phases. During the 5-day high-salt challenge, natriuresis increased more rapidly in females, whereas serum Na+ and body water concentration increased only in males. To determine if females are primed to handle changes in dietary salt, we asked the question whether the renal endothelin-1 natriuretic system is more active in female rats, compared with males. During the NS diet, female rats had a higher urinary endothelin-1 excretion rate than males. Moreover, Ingenuity Pathway Analysis of RNA sequencing data identified the enrichment of endothelin signaling pathway transcripts in the inner medulla of kidneys from NS-fed female rats compared with male counterparts. Notably, in human subjects who consumed an Na+-controlled diet (3314-3668 mg/day) for 3 days, women had a higher urinary endothelin-1 excretion rate than men, consistent with our findings in NS-fed rats. Conclusions These results suggest that female sex confers a greater ability to maintain Na+ homeostasis during acclimation to dietary Na+ challenges and indicate that the intrarenal endothelin-1 natriuretic pathway is enhanced in women.

Angiotensin II and AT1a Receptors in the Proximal Tubules of the Kidney: New Roles in Blood Pressure Control and Hypertension

Contrary to public perception, hypertension remains one of the most important public health problems in the United States, affecting 46% of adults with increased risk for heart attack, stroke, and kidney diseases. The mechanisms underlying poorly controlled hypertension remain incompletely understood. Recent development in the Cre/LoxP approach to study gain or loss of function of a particular gene has significantly helped advance our new insights into the role of proximal tubule angiotensin II (Ang II) and its AT₁ (AT_1a) receptors in basal blood pressure control and the development of Ang II-induced hypertension. This novel approach has provided us and others with an important tool to generate novel mouse models with proximal tubule-specific loss (deletion) or gain of the function (overexpression). The objective of this invited review article is to review and discuss recent findings using novel genetically modifying proximal tubule-specific mouse models. These new studies have consistently demonstrated that deletion of AT₁ (AT_1a) receptors or its direct downstream target Na⁺/H⁺ exchanger 3 (NHE3) selectively in the proximal tubules of the kidney lowers basal blood pressure, increases the pressure-natriuresis response, and induces natriuretic responses, whereas overexpression of an intracellular Ang II fusion protein or AT₁ (AT_1a) receptors selectively in the proximal tubules increases proximal tubule Na⁺ reabsorption, impairs the pressure-natriuresis response, and elevates blood pressure. Furthermore, the development of Ang II-induced hypertension by systemic Ang II infusion or by proximal tubule-specific overexpression of an intracellular Ang II fusion protein was attenuated in mutant mice with proximal tubule-specific deletion of AT₁ (AT_1a) receptors or NHE3. Thus, these recent studies provide evidence for and new insights into the important roles of intratubular Ang II via AT₁ (AT_1a) receptors and NHE3 in the proximal tubules in maintaining basal blood pressure homeostasis and the development of Ang II-induced hypertension.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

A five amino acids deletion in NKCC2 of C57BL/6 mice affects analysis of NKCC2 phosphorylation but does not impact kidney function

Aim

The phosphorylation level of the furosemide‐sensitive Na⁺‐K⁺‐2Cl⁻ cotransporter (NKCC2) in the thick ascending limb (TAL) is used as a surrogate marker for NKCC2 activation and TAL function. However, in mice, analyses of NKCC2 phosphorylation with antibodies against phosphorylated threonines 96 and 101 (anti‐pT96/pT101) give inconsistent results. We aimed (a) to elucidate these inconsistencies and (b) to develop a phosphoform‐specific antibody that ensures reliable detection of NKCC2 phosphorylation in mice.

Methods

Genetic information, molecular biology, biochemical techniques and mouse phenotyping was used to study NKCC2 and kidney function in two commonly used mouse strains (ie 129Sv and in C57BL/6 mice). Moreover, a new phosphoform‐specific mouse NKCC2 antibody was developed and characterized.

Results

Amino acids sequence alignment revealed that C57BL/6 mice have a strain‐specific five amino acids deletion (ΔF97‐T101) in NKCC2 that diminishes the detection of NKCC2 phosphorylation with previously developed pT96/pT101 NKCC2 antibodies. Instead, the antibodies cross‐react with the phosphorylated thiazide‐sensitive NaCl cotransporter (NCC), which can obscure interpretation of results. Interestingly, the deletion in NKCC2 does not impact on kidney function and/or expression of renal ion transport proteins as indicated by the analysis of the F2 generation of crossbred 129Sv and C57BL/6 mice. A newly developed pT96 NKCC2 antibody detects pNKCC2 in both mouse strains and shows no cross‐reactivity with phosphorylated NCC.

Conclusion

Our work reveals a hitherto unappreciated, but essential, strain difference in the amino acids sequence of mouse NKCC2 that needs to be considered when analysing NKCC2 phosphorylation in mice. The new pNKCC2 antibody circumvents this technical caveat.

Angiotensin II-induced natriuresis is attenuated in knockout mice lacking the receptors for tumor necrosis factor-α

Intravenous infusion of relatively higher doses of angiotensin II (AngII) elicits natriuresis as opposed to its usual anti-natruretic response. As AngII can induce tumor necrosis factor-α (TNFα) production which elicits natriuresis via its action on TNFα receptor type 1 (TNFR1), we hypothesize that the concomitant release of TNFα contributes to the natriuretic response to AngII. Responses to AngII infusion (1 ng min-1  g-1 for 75 min, iv) were evaluated in anesthetized knockout (KO) mice lacking TNFR1 (n = 6) and TNFR2 (TNFα receptor type 2; n = 6) and compared these responses with those in wild type (WT; n = 6) mice. Arterial pressure (AP) was recorded from a cannula placed in the carotid artery. Renal blood flow (RBF) and glomerular filtration rate (GFR) were measured by PAH and inulin clearances, respectively. Urine was collected from a catheter placed in the bladder. AngII caused similar increases (p < 0.05 vs basal values) in AP (WT, 37 ± 5%; TNFR1KO, 35 ± 4%; TNFR2KO, 30 ± 4%) and decreases (p < 0.05) in RBF (WT, -39 ± 5%; TNFR1KO, -28 ± 6%; TNFR2KO, -31 ± 4%) without significant changes in GFR (WT, -17 ± 7%; TNFR1KO, -18 ± 7%; TNFR2KO, -12 ± 7%). However, despite similar changes in AP and renal hemodynamics, AngII induced increases (p < 0.05) in urinary sodium excretion in WT (3916 ± 942%) were less in the KO strains, more or less in TNFR1KO (473 ± 170%) than in TNFR2KO (1176 ± 168%). These data indicate that TNF-α receptors, particularly TNFR1 are involved in the natriuretic response that occur during acute infusion of AngII and thus, plays a protective role in preventing excessive salt retention at clinical conditions associated with elevated AngII level.

Immune Mechanisms of Dietary Salt-Induced Hypertension and Kidney Disease: Harry Goldblatt Award for Early Career Investigators 2020

Salt sensitivity of blood pressure is an independent risk factor for cardiovascular mortality not only in hypertensive but also in normotensive adults. The diagnosis of salt sensitivity of blood pressure is not feasible in the clinic due to lack of a simple diagnostic test, making it difficult to investigate therapeutic strategies. Most research efforts to understand the mechanisms of salt sensitivity of blood pressure have focused on renal regulation of sodium. However, salt retention or plasma volume expansion is not different between salt-sensitive and salt-resistant individuals. In addition, over 70% of extracellular fluid is interstitial and, therefore, not directly controlled by renal salt and water excretion. We discuss in this review how the seminal work by Harry Goldblatt paved the way for our attempts at understanding the mechanisms that underlie immune activation by salt in hypertension. We describe our findings that sodium, entering antigen-presenting cells via an epithelial sodium channel, triggers a PKC (protein kinase C)- and SGK1 (serum/glucocorticoid kinase 1)-stimulated activation of nicotinamide adenine dinucleotide phosphate oxidase, which, in turn, enhances lipid oxidation with generation of highly reactive isolevuglandins. Isolevuglandins adduct to proteins, with the potential to generate degraded peptide neoantigens. Activated antigen-presenting cells increase production of the TH17 polarizing cytokines, IL (interleukin)-6, IL-1β, and IL-23, which leads to differentiation and proliferation of IL-17A producing T cells. Our laboratory and others have shown that this cytokine contributes to hypertension. We also discuss where this sodium activation of antigen-presenting cells may occur in vivo and describe the multiple experiments, with pharmacological antagonists and knockout mice that we used to unravel this sequence of events in rodents. Finally, we describe experiments in mononuclear cells obtained from normotensive or hypertensive volunteers, which confirm that analogous processes of salt-induced immunity take place in humans.

Landscape of GPCR expression along the mouse nephron

Kidney transport and other renal functions are regulated by multiple G protein-coupled receptors (GPCRs) expressed along the renal tubule. The rapid, recent appearance of comprehensive unbiased gene expression data in the various renal tubule segments, chiefly RNA sequencing and protein mass spectrometry data, has provided a means of identifying patterns of GPCR expression along the renal tubule. To allow for comprehensive mapping, we first curated a comprehensive list of GPCRs in the genomes of mice, rats, and humans (https://hpcwebapps.cit.nih.gov/ESBL/Database/GPCRs/) using multiple online data sources. We used this list to mine segment-specific and cell type-specific expression data from RNA-sequencing studies in microdissected mouse tubule segments to identify GPCRs that are selectively expressed in discrete tubule segments. Comparisons of these mapped mouse GPCRs with other omics datasets as well as functional data from isolated perfused tubule and micropuncture studies confirmed patterns of expression for well-known receptors and identified poorly studied GPCRs that are likely to play roles in the regulation of renal tubule function. Thus, we provide data resources for GPCR expression across the renal tubule, highlighting both well-known GPCRs and understudied receptors to provide guidance for future studies.

New Insights into the Critical Importance of Intratubular Na+/H+ Exchanger 3 and Its Potential Therapeutic Implications in Hypertension

PURPOSE OF REVIEW

The sodium (Na⁺) and hydrogen (H⁺) exchanger 3 (NHE3), known as solute carrier family 9 member 3 (SLC9A3), mediates active transcellular Na⁺ and bicarbonate reabsorption in the small intestine of the gut and proximal tubules of the kidney. The purpose of this article is to review and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure (BP) homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

RECENT FINDINGS

Recently, our and other laboratories have generated or used novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal BP homeostasis and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The new findings demonstrate that NHE3 contributes to about 10 to 15 mmHg to basal blood pressure levels, and that deletion of NHE3 at the whole-kidney or proximal tubule level, or pharmacological inhibition of NHE3 at the kidney level with an orally absorbable NHE3 inhibitor AVE-0657, attenuates ~ 50% of Ang II-induced hypertension in mice. The results support the proof-of-concept hypothesis that NHE3 plays critical roles in physiologically maintaining normal BP and in the development of Ang II-dependent hypertension. Our results also strongly suggest that NHE3 in the proximal tubules of the kidney may be therapeutically targeted to treat poorly controlled hypertension in humans.

Advances in use of mouse models to study the renin-angiotensin system

The renin-angiotensin system (RAS) is a highly complex hormonal cascade that spans multiple organs and cell types to regulate solute and fluid balance along with cardiovascular function. Much of our current understanding of the functions of the RAS has emerged from a series of key studies in genetically-modified animals. Here, we review key findings from ground-breaking transgenic models, spanning decades of research into the RAS, with a focus on their use in studying blood pressure. We review the physiological importance of this regulatory system as evident through the examination of mouse models for several major RAS components: angiotensinogen, renin, ACE, ACE2, and the type 1 A angiotensin receptor. Both whole-animal and cell-specific knockout models have permitted critical RAS functions to be defined and demonstrate how redundancy and multiplicity within the RAS allow for compensatory adjustments to maintain homeostasis. Moreover, these models present exciting opportunities for continued discovery surrounding the role of the RAS in disease pathogenesis and treatment for cardiovascular disease and beyond.

Vascular control of kidney epithelial transporters

A major pathway in hypertension pathogenesis involves direct activation of ANG II type 1 (AT1) receptors in the kidney, stimulating Na+ reabsorption. AT1 receptors in tubular epithelia control expression and stimulation of Na+ transporters and channels. Recently, we found reduced blood pressure and enhanced natriuresis in mice with cell-specific deletion of AT1 receptors in smooth muscle (SMKO mice). Although impaired vasoconstriction and preserved renal blood flow might contribute to exaggerated urinary Na+ excretion in SMKO mice, we considered whether alterations in Na+ transporter expression might also play a role; therefore, we carried out proteomic analysis of key Na+ transporters and associated proteins. Here, we show that levels of Na+-K+-2Cl- cotransporter isoform 2 (NKCC2) and Na+/H+ exchanger isoform 3 (NHE3) are reduced at baseline in SMKO mice, accompanied by attenuated natriuretic and diuretic responses to furosemide. During ANG II hypertension, we found widespread remodeling of transporter expression in wild-type mice with significant increases in the levels of total NaCl cotransporter, phosphorylated NaCl cotransporter (Ser71), and phosphorylated NKCC2, along with the cleaved, activated forms of the α- and γ-epithelial Na+ channel. However, the increases in α- and γ-epithelial Na+ channel with ANG II were substantially attenuated in SMKO mice. This was accompanied by a reduced natriuretic response to amiloride. Thus, enhanced urinary Na+ excretion observed after cell-specific deletion of AT1 receptors from smooth muscle cells is associated with altered Na+ transporter abundance across epithelia in multiple nephron segments. These findings suggest a system of vascular-epithelial in the kidney, modulating the expression of Na+ transporters and contributing to the regulation of pressure natriuresis.NEW & NOTEWORTHY The use of drugs to block the renin-angiotensin system to reduce blood pressure is common. However, the precise mechanism for how these medications control blood pressure is incompletely understood. Here, we show that mice lacking angiotensin receptors specifically in smooth muscle cells lead to alternation in tubular transporter amount and function. Thus, demonstrating the importance of vascular-tubular cross talk in the control of blood pressure.

Local and downstream actions of proximal tubule angiotensin II signaling on Na+ transporters in the mouse nephron

The renal nephron consists of a series of distinct cell types that function in concert to maintain fluid and electrolyte balance and blood pressure. The renin-angiotensin system (RAS) is central to Na⁺ and volume balance. We aimed to determine how loss of angiotensin II signaling in the proximal tubule (PT), which reabsorbs the bulk of filtered Na⁺ and volume, impacts solute transport throughout the nephron. We hypothesized that PT renin-angiotensin system disruption would not only depress PT Na⁺ transporters but also impact downstream Na⁺ transporters. Using a mouse model in which the angiotensin type 1a receptor (AT_1aR) is deleted specifically within the PT (AT_1aR PTKO), we profiled the abundance of Na⁺ transporters, channels, and claudins along the nephron. Absence of PT AT_1aR signaling was associated with lower abundance of PT transporters (Na⁺/H⁺ exchanger isoform 3, electrogenic Na⁺-bicarbonate cotransporter 1, and claudin 2) as well as lower abundance of downstream transporters (total and phosphorylated Na⁺-K⁺-2Cl^- cotransporter, medullary Na⁺-K⁺-ATPase, phosphorylated NaCl cotransporter, and claudin 7) versus controls. However, transport activities of Na⁺-K⁺-2Cl^- cotransporter and NaCl cotransporter (assessed with diuretics) were similar between groups in order to maintain electrolyte balance. Together, these results demonstrate the primary impact of angiotensin II regulation on Na⁺ reabsorption in the PT at baseline and the associated influence on downstream Na⁺ transporters, highlighting the ability of the nephron to integrate Na⁺ transport along the nephron to maintain homeostasis.NEW & NOTEWORTHY Our study defines a novel role for proximal tubule angiotensin receptors in regulating the abundance of Na⁺ transporters throughout the nephron, thereby contributing to the integrated control of fluid balance in vivo.

Abnormal neonatal sodium handling in skin precedes hypertension in the SAME rat

We discovered high Na⁺ and water content in the skin of newborn Sprague–Dawley rats, which reduced ~ 2.5-fold by 7 days of age, indicating rapid changes in extracellular volume (ECV). Equivalent changes in ECV post birth were also observed in C57Bl/6 J mice, with a fourfold reduction over 7 days, to approximately adult levels. This established the generality of increased ECV at birth. We investigated early sodium and water handling in neonates from a second rat strain, Fischer, and an Hsd11b2-knockout rat modelling the syndrome of apparent mineralocorticoid excess (SAME). Despite Hsd11b2^−/− animals exhibiting lower skin Na⁺ and water levels than controls at birth, they retained ~ 30% higher Na⁺ content in their pelts at the expense of K⁺ thereafter. Hsd11b2^−/− neonates exhibited incipient hypokalaemia from 15 days of age and became increasingly polydipsic and polyuric from weaning. As with adults, they excreted a high proportion of ingested Na⁺ through the kidney, (56.15 ± 8.21% versus control 34.15 ± 8.23%; n = 4; P < 0.0001), suggesting that changes in nephron electrolyte transporters identified in adults, by RNA-seq analysis, occur by 4 weeks of age. Our data reveal that Na⁺ imbalance in the Hsd11b2^−/− neonate leads to excess Na⁺ storage in skin and incipient hypokalaemia, which, together with increased, glucocorticoid-induced Na⁺ uptake in the kidney, then contribute to progressive, volume contracted, salt-sensitive hypertension. Skin Na⁺ plays an important role in the development of SAME but, equally, may play a key physiological role at birth, supporting post-natal growth, as an innate barrier to infection or as a rudimentary kidney.

Renal Inflammation Induces Salt Sensitivity in Male db/db Mice through Dysregulation of ENaC

BACKGROUND

Hypertension is considered a major risk factor for the progression of diabetic kidney disease. Type 2 diabetes is associated with increased renal sodium reabsorption and salt-sensitive hypertension. Clinical studies show that men have higher risk than premenopausal women for the development of diabetic kidney disease. However, the renal mechanisms that predispose to salt sensitivity during diabetes and whether sexual dimorphism is associated with these mechanisms remains unknown.

METHODS

Female and male db/db mice exposed to a high-salt diet were used to analyze the progression of diabetic kidney disease and the development of hypertension.

RESULTS

Male, 34-week-old, db/db mice display hypertension when exposed to a 4-week high-salt treatment, whereas equivalently treated female db/db mice remain normotensive. Salt-sensitive hypertension in male mice was associated with no suppression of the epithelial sodium channel (ENaC) in response to a high-salt diet, despite downregulation of several components of the intrarenal renin-angiotensin system. Male db/db mice show higher levels of proinflammatory cytokines and more immune-cell infiltration in the kidney than do female db/db mice. Blocking inflammation, with either mycophenolate mofetil or by reducing IL-6 levels with a neutralizing anti-IL-6 antibody, prevented the development of salt sensitivity in male db/db mice.

CONCLUSIONS

The inflammatory response observed in male, but not in female, db/db mice induces salt-sensitive hypertension by impairing ENaC downregulation in response to high salt. These data provide a mechanistic explanation for the sexual dimorphism associated with the development of diabetic kidney disease and salt sensitivity.

Angiotensin II up-regulates sodium-glucose co-transporter 2 expression and SGLT2 inhibitor attenuates Ang II-induced hypertensive renal injury in mice

Clinical trials indicate that sodium/glucose co-transporter 2 (SGLT2) inhibitors (SGLT2i) improve kidney function, yet, the molecular regulation of SGLT2 expression is incompletely understood. Here, we investigated the role of the intrarenal renin-angiotensin system (RAS) on SGLT2 expression. In adult non-diabetic participants in the Nephrotic Syndrome Study Network (NEPTUNE, n=163), multivariable linear regression analysis showed SGLT2 mRNA was significantly associated with angiotensinogen (AGT), renin, and angiotensin-converting enzyme (ACE) mRNA levels (P<0.001). In vitro, angiotensin II (Ang II) dose-dependently stimulated SGLT2 expression in HK-2, human immortalized renal proximal tubular cells (RPTCs); losartan and antioxidants inhibited it. Sglt2 expression was increased in transgenic (Tg) mice specifically overexpressing Agt in their RPTCs, as well as in WT mice with a single subcutaneous injection of Ang II (1.44 mg/kg). Moreover, Ang II (1000 ng/kg/min) infusion via osmotic mini-pump in WT mice for 4 weeks increased systolic blood pressure (SBP), glomerulosclerosis, tubulointerstitial fibrosis, and albuminuria; canaglifozin (Cana, 15 mg/kg/day) reversed these changes, with the exception of SBP. Fractional glucose excretion (FeGlu) was higher in Ang II+Cana than WT+Cana, whereas Sglt2 expression was similar. Our data demonstrate a link between intrarenal RAS and SGLT2 expression and that SGLT2i ameliorates Ang II-induced renal injury independent of SBP.

Renal Natriuretic Peptide Receptor-C Deficiency Attenuates NaCl Cotransporter Activity in Angiotensin II-Induced Hypertension

Genome-wide association studies have identified that NPR-C (natriuretic peptide receptor-C) variants are associated with elevation of blood pressure. However, the mechanism underlying the relationship between NPR-C and blood pressure regulation remains elusive. Here, we investigate whether NPR-C regulates Ang II (angiotensin II)-induced hypertension through sodium transporters activity. Wild-type mice responded to continuous Ang II infusion with an increased renal NPR-C expression. Global NPR-C deficiency attenuated Ang II-induced increased blood pressure both in male and female mice associated with more diuretic and natriuretic responses to a saline challenge. Interestingly, Ang II increased both total and phosphorylation of NCC (NaCl cotransporter) abundance involving in activation of WNK4 (with-no-lysine kinase 4)/SPAK (Ste20-related proline/alanine-rich kinase) which was blunted by NPR-C deletion. NCC inhibitor, hydrochlorothiazide, failed to induce natriuresis in NPR-C knockout mice. Moreover, low-salt and high-salt diets-induced changes of total and phosphorylation of NCC expression were normalized by NPR-C deletion. Importantly, tubule-specific deletion of NPR-C also attenuated Ang II-induced elevated blood pressure, total and phosphorylation of NCC expression. Mechanistically, in distal convoluted tubule cells, Ang II dose and time-dependently upregulated WNK4/SPAK/NCC kinase pathway and NPR-C/Gi/PLC/PKC signaling pathway mediated NCC activation. These results demonstrate that NPR-C signaling regulates NCC function contributing to sodium retention-mediated elevated blood pressure, which suggests that NPR-C is a promising candidate for the treatment of sodium retention-related hypertension.

Site-1 Protease-Derived Soluble (Pro)Renin Receptor Contributes to Angiotensin II-Induced Hypertension in Mice

Activation of PRR ([pro]renin receptor) contributes to enhancement of intrarenal RAS and renal medullary α-ENaC and thus elevated blood pressure during Ang II (angiotensin II) infusion. The goal of the present study was to test whether such action of PRR was mediated by sPRR (soluble PRR), generated by S1P (site-1 protease), a newly identified PRR cleavage protease. F1 B6129SF1/J mice were infused for 6 days with control or Ang II at 300 ng/kg per day alone or in combination with S1P inhibitor PF-429242 (PF), and blood pressure was monitored by radiotelemetry. S1P inhibition significantly attenuated Ang II-induced hypertension accompanied with suppressed urinary and renal medullary renin levels and expression of renal medullary but not renal cortical α-ENaC expression. The effects of S1P inhibition were all reversed by supplement with histidine-tagged sPRR termed as sPRR-His. Ussing chamber technique was performed to determine amiloride-sensitive short-circuit current, an index of ENaC activity in confluent mouse cortical collecting duct cell line cells exposed for 24 hours to Ang II, Ang II + PF, or Ang II + PF + sPRR-His. Ang II-induced ENaC activity was blocked by PF, which was reversed by sPRR-His. Together, these results support that S1P-derived sPRR mediates Ang II-induced hypertension through enhancement of intrarenal renin level and activation of ENaC.

Role of α2-Adrenoceptors in Hypertension: Focus on Renal Sympathetic Neurotransmitter Release, Inflammation, and Sodium Homeostasis

The kidney is extensively innervated by sympathetic nerves playing an important role in the regulation of blood pressure homeostasis. Sympathetic nerve activity is ultimately controlled by the central nervous system (CNS). Norepinephrine, the main sympathetic neurotransmitter, is released at prejunctional neuroeffector junctions in the kidney and modulates renin release, renal vascular resistance, sodium and water handling, and immune cell response. Under physiological conditions, renal sympathetic nerve activity (RSNA) is modulated by peripheral mechanisms such as the renorenal reflex, a complex interaction between efferent sympathetic nerves, central mechanism, and afferent sensory nerves. RSNA is increased in hypertension and, therefore, critical for the perpetuation of hypertension and the development of hypertensive kidney disease. Renal sympathetic neurotransmission is not only regulated by RSNA but also by prejunctional α2-adrenoceptors. Prejunctional α2-adrenoceptors serve as autoreceptors which, when activated by norepinephrine, inhibit the subsequent release of norepinephrine induced by a sympathetic nerve impulse. Deletion of α2-adrenoceptors aggravates hypertension ultimately by modulating renal pressor response and sodium handling. α2-adrenoceptors are also expressed in the vasculature, renal tubules, and immune cells and exert thereby effects related to vascular tone, sodium excretion, and inflammation. In the present review, we highlight the role of α2-adrenoceptors on renal sympathetic neurotransmission and its impact on hypertension. Moreover, we focus on physiological and pathophysiological functions mediated by non-adrenergic α2-adrenoceptors. In detail, we discuss the effects of sympathetic norepinephrine release and α2-adrenoceptor activation on renal sodium transporters, on renal vascular tone, and on immune cells in the context of hypertension and kidney disease.

Coordinate adaptations of skeletal muscle and kidney to maintain extracellular [K+] during K+-deficient diet

Extracellular fluid (ECF) potassium concentration ([K+]) is maintained by adaptations of kidney and skeletal muscle, responses heretofore studied separately. We aimed to determine how these organ systems work in concert to preserve ECF [K+] in male C57BL/6J mice fed a K+-deficient diet (0K) versus 1% K+ diet (1K) for 10 days (n = 5-6/group). During 0K feeding, plasma [K+] fell from 4.5 to 2 mM; hindlimb muscle (gastrocnemius and soleus) lost 28 mM K+ (from 115 ± 2 to 87 ± 2 mM) and gained 27 mM Na+ (from 27 ± 0.4 to 54 ± 2 mM). Doubling of muscle tissue [Na+] was not associated with inflammation, cytokine production or hypertension as reported by others. Muscle transporter adaptations in 0K- versus 1K-fed mice, assessed by immunoblot, included decreased sodium pump α2-β2 subunits, decreased K+-Cl- cotransporter isoform 3, and increased phosphorylated (p) Na+,K+,2Cl- cotransporter isoform 1 (NKCC1p), Ste20/SPS-1-related proline-alanine rich kinase (SPAKp), and oxidative stress-responsive kinase 1 (OSR1p) consistent with intracellular fluid (ICF) K+ loss and Na+ gain. Renal transporters' adaptations, effecting a 98% reduction in K+ excretion, included two- to threefold increased phosphorylated Na+-Cl- cotransporter (NCCp), SPAKp, and OSR1p abundance, limiting Na+ delivery to epithelial Na+ channels where Na+ reabsorption drives K+ secretion; and renal K sensor Kir 4.1 abundance fell 25%. Mass balance estimations indicate that over 10 days of 0K feeding, mice lose ~48 μmol K+ into the urine and muscle shifts ~47 μmol K+ from ICF to ECF, illustrating the importance of the concerted responses during K+ deficiency.

Sex differences in solute transport along the nephrons: effects of Na+ transport inhibition

Each day, ~1.7 kg of NaCl and 180 liters of water are reabsorbed by nephron segments in humans, with urinary excretion fine tuned to meet homeostatic requirements. These tasks are coordinated by a spectrum of renal Na⁺ transporters and channels. The goal of the present study was to investigate the extent to which inhibitors of transepithelial Na⁺ transport (T_Na) along the nephron alter urinary solute excretion and how those effects may vary between male and female subjects. To accomplish that goal, we developed sex-specific multinephron models that represent detailed transcellular and paracellular transport processes along the nephrons of male and female rat kidneys. We simulated inhibition of Na⁺/H⁺ exchanger 3 (NHE3), bumetanide-sensitive Na⁺-K⁺-2Cl^- cotransporter (NKCC2), Na⁺-Cl^- cotransporter (NCC), and amiloride-sensitive epithelial Na⁺ channel (ENaC). NHE3 inhibition simulations predicted a substantially reduced proximal tubule T_Na, and NKCC2 inhibition substantially reduced thick ascending limb T_Na. Both gave rise to diuresis, natriuresis, and kaliuresis, with those effects stronger in female rats. While NCC inhibition was predicted to have only minor impact on renal T_Na, it nonetheless had a notable effect of enhancing excretion of Na⁺, K⁺, and Cl^-, particularly in female rats. Inhibition of ENaC was predicted to have opposite effects on the excretion of Na⁺ (increased) and K⁺ (decreased) and to have only a minor impact on whole kidney T_Na. Unlike inhibition of other transporters, ENaC inhibition induced stronger natriuresis and diuresis in male rats than female rats. Overall, model predictions agreed well with measured changes in Na⁺ and K⁺ excretion in response to diuretics and Na⁺ transporter mutations.

Aldosterone-dependent and -independent regulation of Na+ and K+ excretion and ENaC in mouse kidneys

We investigated the regulation of Na⁺ and K⁺ excretion and the epithelial Na⁺ channel (ENaC) in mice lacking the gene for aldosterone synthase (AS) using clearance methods to assess excretion and electrophysiology and Western blot analysis to test for ENaC activity and processing. After 1 day of dietary Na⁺ restriction, AS^-/- mice lost more Na⁺ in the urine than AS^+/+ mice did. After 1 wk on this diet, both genotypes strongly reduced urinary Na⁺ excretion, but creatinine clearance decreased only in AS^-/- mice. Only AS^+/+ animals exhibited increased ENaC function, assessed as amiloride-sensitive whole cell currents in collecting ducts or cleavage of αENaC and γENaC in Western blots. To assess the role of aldosterone in the excretion of a K⁺ load, animals were fasted overnight and refed with high-K⁺ or low-K⁺ diets for 5 h. Both AS^+/+ and AS^-/- mice excreted a large amount of K⁺ during this period. In both phenotypes the excretion was benzamil sensitive, indicating increased K⁺ secretion coupled to ENaC-dependent Na⁺ reabsorption. However, the increase in plasma K⁺ under these conditions was much larger in AS^-/- animals than in AS^+/+ animals. In both groups, cleavage of αENaC and γENaC increased. However, Na⁺ current measured ex vivo in connecting tubules was enhanced only in AS^+/+ mice. We conclude that in the absence of aldosterone, mice can conserve Na⁺ without ENaC activation but at the expense of diminished glomerular filtration rate. Excretion of a K⁺ load can be accomplished through aldosterone-independent upregulation of ENaC, but aldosterone is required to excrete the excess K⁺ without hyperkalemia.

Electrolyte and transporter responses to angiotensin II induced hypertension in female and male rats and mice

AIM

Sexual dimorphisms are evident along the nephron: Females (F) exhibit higher ratios of renal distal to proximal Na⁺ transporters' abundance, greater lithium clearance (C_Li ) more rapid natriuresis in response to saline infusion and lower plasma [K⁺ ] vs. males (M). During angiotensin II infusion hypertension (AngII-HTN) M exhibit distal Na⁺ transporter activation, lower proximal and medullary loop transporters, blunted natriuresis in response to saline load, and reduced plasma [K⁺ ]. This study aimed to determine whether responses of F to AngII-HTN mimicked those in M or were impacted by sexual dimorphisms evident at baseline.

METHODS

Sprague Dawley rats and C57BL/6 mice were AngII infused via osmotic minipumps 2 and 3 weeks, respectively, and assessed by metabolic cage collections, tail-cuff sphygmomanometer, semi-quantitative immunoblotting of kidney and patch-clamp electrophysiology.

RESULTS

In F rats, AngII-infusion increased BP to 190 mm Hg, increased phosphorylation of cortical NKCC2, NCC and cleavage of ENaC two to threefold, increased ENaC channel activity threefold and aldosterone 10-fold. K⁺ excretion increased and plasma [K⁺ ] decreased. Evidence of natriuresis in F included increased urine Na⁺ excretion and C_Li , and decreased medullary NHE3, NKCC2 and Na,K-ATPase abundance. In C57BL/6 mice, AngII-HTN increased abundance of distal Na⁺ transporters, suppressed proximal-medullary transporters and reduced plasma [K⁺ ] in both F and M.

CONCLUSION

Despite baseline sexual dimorphisms, AngII-HTN provokes similar increases in BP, aldosterone, distal transporters, ENaC channel activation and K⁺ loss accompanied by similar suppression of proximal and loop Na⁺ transporters, natriuresis and diuresis in females and males.

α2A-Adrenoceptors Modulate Renal Sympathetic Neurotransmission and Protect against Hypertensive Kidney Disease

BACKGROUND

Increased nerve activity causes hypertension and kidney disease. Recent studies suggest that renal denervation reduces BP in patients with hypertension. Renal NE release is regulated by prejunctional α2A-adrenoceptors on sympathetic nerves, and α2A-adrenoceptors act as autoreceptors by binding endogenous NE to inhibit its own release. However, the role of α2A-adrenoceptors in the pathogenesis of hypertensive kidney disease is unknown.

METHODS

We investigated effects of α2A-adrenoceptor-regulated renal NE release on the development of angiotensin II-dependent hypertension and kidney disease. In uninephrectomized wild-type and α2A-adrenoceptor-knockout mice, we induced hypertensive kidney disease by infusing AngII for 28 days.

RESULTS

Urinary NE excretion and BP did not differ between normotensive α2A-adrenoceptor-knockout mice and wild-type mice at baseline. However, NE excretion increased during AngII treatment, with the knockout mice displaying NE levels that were significantly higher than those of wild-type mice. Accordingly, the α2A-adrenoceptor-knockout mice exhibited a systolic BP increase, which was about 40 mm Hg higher than that found in wild-type mice, and more extensive kidney damage. In isolated kidneys, AngII-enhanced renal nerve stimulation induced NE release and pressor responses to a greater extent in kidneys from α2A-adrenoceptor-knockout mice. Activation of specific sodium transporters accompanied the exaggerated hypertensive BP response in α2A-adrenoceptor-deficient kidneys. These effects depend on renal nerves, as demonstrated by reduced severity of AngII-mediated hypertension and improved kidney function observed in α2A-adrenoceptor-knockout mice after renal denervation.

CONCLUSIONS

Our findings reveal a protective role of prejunctional inhibitory α2A-adrenoceptors in pathophysiologic conditions with an activated renin-angiotensin system, such as hypertensive kidney disease, and support the concept of sympatholytic therapy as a treatment.

Angiotensin II-induced superoxide and decreased glutathione in proximal tubules: effect of dietary fructose

Angiotensin II exacerbates oxidative stress in part by increasing superoxide (O2-) production by many renal tissues. However, whether it does so in proximal tubules and the source of O2- in this segment are unknown. Dietary fructose enhances the stimulatory effect of angiotensin II on proximal tubule Na⁺ reabsorption, but whether this is true for oxidative stress is unknown. We hypothesized that angiotensin II causes proximal nephron oxidative stress in part by stimulating NADPH oxidase (NOX)4-dependent O2- production and decreasing the amount of the antioxidant glutathione, and this is exacerbated by dietary fructose. We measured basal and angiotensin II-stimulated O2- production with and without inhibitors, NOX1 and NOX4 expression, and total and reduced glutathione (GSH) in proximal tubules from rats drinking either tap water (control) or 20% fructose. Angiotensin II (10 nM) increased O2- production by 113 ± 42 relative light units·mg protein^-1·s^-1 in controls and 401 ± 74 relative light units·mg protein^-1·s^-1 with 20% fructose (n = 11 for each group, P < 0.05 vs. control). Apocynin and the Nox1/4 inhibitor GKT136901 prevented angiotensin II-induced increases in both groups. NOX4 expression was not different between groups. NOX1 expression was undetectable. Angiotensin II decreased GSH by 1.8 ± 0.8 nmol/mg protein in controls and by 4.2 ± 0.9 nmol/mg protein with 20% fructose (n = 18 for each group, P < 0.047 vs. control). We conclude that 1) angiotensin II causes oxidative stress in proximal tubules by increasing O2- production by NOX4 and decreasing GSH and 2) dietary fructose enhances the ability of angiotensin II to stimulate O2- and diminish GSH, thereby exacerbating oxidative stress in this segment.

Impact of angiotensin II-mediated stimulation of sodium transporters in the nephron assessed by computational modeling

Angiotensin II (ANG II) raises blood pressure partly by stimulating tubular Na⁺ reabsorption. The effects of ANG II on tubular Na⁺ transporters (i.e., channels, pumps, cotransporters, and exchangers) vary between short-term and long-term exposure. To better understand the physiological impact, we used a computational model of transport along the rat nephron to predict the effects of short- and long-term ANG II-induced transporter activation on Na⁺ and K⁺ reabsorption/secretion, and to compare measured and calculated excretion rates. Three days of ANG II infusion at 200 ng·kg^-1·min^-1 is nonpressor, yet stimulates transporter accumulation. The increase in abundance of Na⁺/H⁺ exchanger 3 (NHE3) or activated Na⁺-K⁺-2Cl^- cotransporter-2 (NKCC2-P) predicted significant reductions in urinary Na⁺ excretion, yet there was no observed change in urine Na⁺. The lack of antinatriuresis, despite Na⁺ transporter accumulation, was supported by Li⁺ and creatinine clearance measurements, leading to the conclusion that 3-day nonpressor ANG II increases transporter abundance without proportional activation. Fourteen days of ANG II infusion at 400 ng·kg^-1·min^-1 raises blood pressure and increases Na⁺ transporter abundance along the distal nephron; proximal tubule and medullary loop transporters are decreased and urine Na⁺ and volume output are increased, evidence for pressure natriuresis. Simulations indicate that decreases in NHE3 and NKCC2-P contribute significantly to reducing Na⁺ reabsorption along the nephron and to pressure natriuresis. Our results also suggest that differential regulation of medullary (decrease) and cortical (increase) NKCC2-P is important to preserve K⁺ while minimizing Na⁺ retention during ANG II infusion. Lastly, our model indicates that accumulation of active Na⁺-Cl^- cotransporter counteracts epithelial Na⁺ channel-induced urinary K⁺ loss.

Proximal Tubule-Specific Deletion of the NHE3 (Na+/H+ Exchanger 3) in the Kidney Attenuates Ang II (Angiotensin II)-Induced Hypertension in Mice

The present study directly tested the hypothesis that the NHE3 (Na⁺/H⁺ exchanger 3) in the proximal tubules of the kidney is required for the development of Ang II (angiotensin II)-induced hypertension using PT-Nhe3^-/- (proximal tubule-specific NHE3 knockout) mice. Specifically, PT-Nhe3^-/- mice were generated using the SGLT2-Cre/Nhe3^loxlox approach, whereas Ang II-induced hypertension was studied in 12 groups (n=5-12 per group) of adult male and female wild-type (WT) and PT-Nhe3^-/- mice. Under basal conditions, systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure were significantly lower in male and female PT-Nhe3^-/- than WT mice (P<0.01). A high pressor, 1.5 mg/kg per day, intraperitoneal or a slow pressor dose of Ang II, 0.5 mg/kg per day, intraperitoneal for 2 weeks significantly increased systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure in male and female WT mice (P<0.01), but the hypertensive response to Ang II was markedly attenuated in male and female PT-Nhe3^-/- mice (P<0.01). Ang II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in Ang II-infused PT-Nhe3^-/- mice (P<0.01). AT₁ receptor blocker losartan completely blocked Ang II-induced hypertension in both WT and PT-Nhe3^-/- mice (P<0.01). However, inhibition of nitric oxide synthase with L-N^G-Nitroarginine methyl ester had no effect on Ang II-induced hypertension in WT or PT-Nhe3^-/- mice (not significant). Furthermore, Ang II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney may be a therapeutical target in hypertension induced by Ang II or with increased NHE3 expression in the proximal tubules.

A model of uric acid transport in the rat proximal tubule

The objective of the present study was to theoretically investigate the mechanisms underlying uric acid transport in the proximal tubule (PT) of rat kidneys, and their modulation by factors, including Na+, parathyroid hormone, ANG II, and Na+-glucose cotransporter-2 inhibitors. To that end, we incorporated the transport of uric acid and its conjugate anion urate in our mathematical model of water and solute transport in the rat PT. The model accounts for parallel urate reabsorption and secretion pathways on apical and basolateral membranes and their coupling to lactate and α-ketoglutarate transport. Model results agree with experimental findings at the segment level. Net reabsorption of urate by the rat PT is predicted to be ~70% of the filtered load, with a rate of urate removal from the lumen that is 50% higher than the rate of urate secretion. The model suggests that apical URAT1 deletion significantly reduces net urate reabsorption across the PT, whereas ATP-binding cassette subfamily G member 2 dysfunction affects it only slightly. Inactivation of basolateral glucose transporter-9 raises fractional urate excretion above 100%, as observed in patients with renal familial hypouricemia. Furthermore, our results suggest that reducing Na+ reabsorption across Na+/H+ exchangers or Na+-glucose cotransporters augments net urate reabsorption. The model predicts that parathyroid hormone reduces urate excretion, whereas ANG II increases it. In conclusion, we have developed the first model of uric acid transport in the rat PT; this model provides a framework to gain greater insight into the numerous solutes and coupling mechanisms that affect the renal handing of uric acid.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Angiotensin II-induced hypertension in rats is only transiently accompanied by lower renal oxygenation

Activation of the renin-angiotensin system may initiate chronic kidney disease. We hypothesised that renal hypoxia is a consequence of hemodynamic changes induced by angiotensin II and occurs prior to development of severe renal damage. Male Sprague-Dawley rats were infused continuously with angiotensin II (350 ng/kg/min) for 8 days. Mean arterial pressure (n = 5), cortical (n = 6) and medullary (n = 7) oxygenation (pO₂) were continuously recorded by telemetry and renal tissue injury was scored. Angiotensin II increased arterial pressure gradually to 150 ± 18 mmHg. This was associated with transient reduction of oxygen levels in renal cortex (by 18 ± 2%) and medulla (by 17 ± 6%) at 10 ± 2 and 6 ± 1 hours, respectively after starting infusion. Thereafter oxygen levels normalised to pre-infusion levels and were maintained during the remainder of the infusion period. In rats receiving angiotensin II, adding losartan to drinking water (300 mg/L) only induced transient increase in renal oxygenation, despite normalisation of arterial pressure. In rats, renal hypoxia is only a transient phenomenon during initiation of angiotensin II-induced hypertension.

The Absence of the ACE N-Domain Decreases Renal Inflammation and Facilitates Sodium Excretion during Diabetic Kidney Disease

BACKGROUND

Recent evidence emphasizes the critical role of inflammation in the development of diabetic nephropathy. Angiotensin-converting enzyme (ACE) plays an active role in regulating the renal inflammatory response associated with diabetes. Studies have also shown that ACE has roles in inflammation and the immune response that are independent of angiotensin II. ACE's two catalytically independent domains, the N- and C-domains, can process a variety of substrates other than angiotensin I.

METHODS

To examine the relative contributions of each ACE domain to the sodium retentive state, renal inflammation, and renal injury associated with diabetic kidney disease, we used streptozotocin to induce diabetes in wild-type mice and in genetic mouse models lacking either a functional ACE N-domain (NKO mice) or C-domain (CKO mice).

RESULTS

In response to a saline challenge, diabetic NKO mice excreted 32% more urinary sodium compared with diabetic wild-type or CKO mice. Diabetic NKO mice also exhibited 55% less renal epithelial sodium channel cleavage (a marker of channel activity), 55% less renal IL-1β, 53% less renal TNF-α, and 53% less albuminuria than diabetic wild-type mice. This protective phenotype was not associated with changes in renal angiotensin II levels. Further, we present evidence that the anti-inflammatory tetrapeptide N-acetyl-seryl-asparyl-lysyl-proline (AcSDKP), an ACE N-domain-specific substrate that accumulates in the urine of NKO mice, mediates the beneficial effects observed in the NKO.

CONCLUSIONS

These data indicate that increasing AcSDKP by blocking the ACE N-domain facilitates sodium excretion and ameliorates diabetic kidney disease independent of intrarenal angiotensin II regulation.

Na restriction activates epithelial Na channels in rat kidney through two mechanisms and decreases distal Na+ delivery

KEY POINTS

Dietary Na restriction, through the mineralocorticoid aldosterone, acts on epithelial Na channels via both fast (24 h) and slow (5-7 days) mechanisms in the kidney. The fast effect entails increased proteolytic processing and trafficking of channel protein to the apical membrane. It is rapidly reversible by the mineralocorticoid receptor antagonist eplerenone and is largely lost when tubules are studied ex vivo. The slow effect does not require increased processing or surface expression, is refractory to acute eplerenone treatment, and is preserved ex vivo. Both slow and fast effects contribute to Na retention in vivo. Increased Na⁺ reabsorption in the proximal tubule also promotes Na conservation under conditions of chronic dietary Na restriction, reducing Na⁺ delivery to the distal nephron.

ABSTRACT

Changes in the activity of the epithelial Na channel (ENaC) help to conserve extracellular fluid volume. In rats fed a low-salt diet, proteolytic processing of ENaC increased within 1 day, and was almost maximal after 3 days. The rapid increase in the abundance of cleaved αENaC and γENaC correlated with decreased urinary Na⁺ excretion and with increased ENaC surface expression. By contrast, ENaC activity, measured ex vivo in isolated cortical collecting ducts, increased modestly after 3 days and required 5 days to reach maximal levels. The mineralocorticoid receptor antagonist eplerenone reversed the increase in cleaved γENaC and induced natriuresis after 1 or 3 days but failed to alter either ENaC currents or Na⁺ excretion after 7 days of Na restriction. We conclude that Na depletion, through aldosterone, stimulates ENaC via independent fast and slow mechanisms. In vivo, amiloride-induced natriuresis increased after 1 day of Na depletion. By contrast, hydrochlorothiazide (HCTZ)-induced natriuresis decreased gradually over 7 days, consistent with increased ability of ENaC activity to compensate for decreased Na⁺ reabsorption in the distal convoluted tubule. Administration of amiloride and HCTZ together increased Na⁺ excretion less in Na-depleted compared to control animals, indicating decreased delivery of Na⁺ to the distal nephron when dietary Na is restricted. Measurements of creatinine and Li⁺ clearances indicated that increased Na reabsorption by the proximal tubules is responsible for the decreased delivery. Thus, Na conservation during chronic dietary salt restriction entails enhanced transport by both proximal and distal nephron segments.

Body mass-specific Na+-K+-ATPase activity in the medullary thick ascending limb: implications for species-dependent urine concentrating mechanisms

In general, the mammalian whole body mass-specific metabolic rate correlates positively with maximal urine concentration (U_max) irrespective of whether or not the species have adapted to arid or mesic habitat. Accordingly, we hypothesized that the thick ascending limb (TAL) of a rodent with markedly higher whole body mass-specific metabolism than rat exhibits a substantially higher TAL metabolic rate as estimated by Na⁺-K⁺-ATPase activity and Na⁺-K⁺-ATPase α1-gene and protein expression. The kangaroo rat inner stripe of the outer medulla exhibits significantly higher mean Na⁺-K⁺-ATPase activity (~70%) compared with two rat strains (Sprague-Dawley and Munich-Wistar), extending prior studies showing rat activity exceeds rabbit. Furthermore, higher expression of Na⁺-K⁺-ATPase α1-protein (~4- to 6-fold) and mRNA (~13-fold) and higher TAL mitochondrial volume density (~20%) occur in the kangaroo rat compared with both rat strains. Rat TAL Na⁺-K⁺-ATPase α1-protein expression is relatively unaffected by body hydration status or, shown previously, by dietary Na⁺, arguing against confounding effects from two unavoidably dissimilar diets: grain-based diet without water (kangaroo rat) or grain-based diet with water (rat). We conclude that higher TAL Na⁺-K⁺-ATPase activity contributes to relationships between whole body mass-specific metabolic rate and high U_max. More vigorous TAL Na⁺-K⁺-ATPase activity in kangaroo rat than rat may contribute to its steeper Na⁺ and urea axial concentration gradients, adding support to a revised model of the urine concentrating mechanism, which hypothesizes a leading role for vigorous active transport of NaCl, rather than countercurrent multiplication, in generating the outer medullary axial osmotic gradient.