Renal ENaC subunit, Na-K-2Cl and Na-Cl cotransporter abundances in aged, water-restricted F344 x Brown Norway rats

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

AUP1 Regulates the Endoplasmic Reticulum-Associated Degradation and Polyubiquitination of NKCC2

Inactivating mutations of kidney Na-K-2Cl cotransporter NKCC2 lead to antenatal Bartter syndrome (BS) type 1, a life-threatening salt-losing tubulopathy. We previously reported that this serious inherited renal disease is linked to the endoplasmic reticulum-associated degradation (ERAD) pathway. The purpose of this work is to characterize further the ERAD machinery of NKCC2. Here, we report the identification of ancient ubiquitous protein 1 (AUP1) as a novel interactor of NKCC2 ER-resident form in renal cells. AUP1 is also an interactor of the ER lectin OS9, a key player in the ERAD of NKCC2. Similar to OS9, AUP1 co-expression decreased the amount of total NKCC2 protein by enhancing the ER retention and associated protein degradation of the cotransporter. Blocking the ERAD pathway with the proteasome inhibitor MG132 or the α-mannosidase inhibitor kifunensine fully abolished the AUP1 effect on NKCC2. Importantly, AUP1 knock-down or inhibition by overexpressing its dominant negative form strikingly decreased NKCC2 polyubiquitination and increased the protein level of the cotransporter. Interestingly, AUP1 co-expression produced a more profound impact on NKCC2 folding mutants. Moreover, AUP1 also interacted with the related kidney cotransporter NCC and downregulated its expression, strongly indicating that AUP1 is a common regulator of sodium-dependent chloride cotransporters. In conclusion, our data reveal the presence of an AUP1-mediated pathway enhancing the polyubiquitination and ERAD of NKCC2. The characterization and selective regulation of specific ERAD constituents of NKCC2 and its pathogenic mutants could open new avenues in the therapeutic strategies for type 1 BS treatment.

Alterations in Kidney Structures Caused by Age Vary According to Sex and Dehydration Condition

Aging is a complex biological process, resulting in gradual and progressive decline in structure and function in many organ systems. Our objective is to determine if structural changes produced by aging vary with sex in a stressful situation such as dehydration. The expression of Slc12a3 mRNA in the renal cortex, α-smooth muscle actin (α-SMA), and fibronectin was evaluated in male and female rats, aged 3 and 18 months, submitted and not submitted to water deprivation (WD) for 48 h, respectively. When comparing ages, 18-month-old males showed a lower expression of Slc12a3 mRNA than 3-month-old males, and control and WD 18-month-old male and female rats exhibited a higher expression of α-SMA than the respective 3-month-old rats. Fibronectin was higher in both control and WD 18-month-old males than the respective 3-month-old males. In females, only the control 18-month-old rats showed higher fibronectin than the control 3-month-old rats. When we compared sex, control and WD 3-month-old female rats had a lower expression of Slc12a3 mRNA than the respective males. The WD 18-month-old male rats presented a higher expression of fibronectin and α-SMA than the WD 18-month-old female rats. When we compared hydric conditions, the WD 18-month-old males displayed a lower relative expression of Slc12a3 mRNA and higher α-SMA expression than the control 18-month-old males. Aging, sex, and dehydration lead to alterations in kidney structure.

Incidence and risk factors for hyperkalaemia in patients treated for COVID-19 with nafamostat mesylate

What is known and objective

Nafamostat mesylate (NM) is used clinically in combination with antiviral drugs to treat coronavirus disease (COVID‐19). One of the adverse events of NM is hyperkalaemia due to inhibition of the amiloride‐sensitive sodium channels (ENaC). The incidence and risk factors for hyperkalaemia due to NM have been studied in patients with pancreatitis but not in COVID‐19. COVID‐19 can be associated with hypokalaemia or hyperkalaemia, and SARS‐CoV‐2 is thought to inhibit ENaC. Therefore, frequency and risk factors for hyperkalaemia due to NM may differ between COVID‐19 and pancreatitis. Hyperkalaemia may worsen the respiratory condition of patients. The objective of this study was to determine the incidence and risk factors for hyperkalaemia in COVID‐19 patients treated with favipiravir, dexamethasone and NM.

Methods

This retrospective study reviewed the records of hospitalized COVID‐19 patients treated with favipiravir and dexamethasone, with or without NM, between March 2020 and January 2021. Multivariable logistic regression analysis was performed to identify the risk factors for hyperkalaemia.

Results and Discussion

Of 45 patients who received favipiravir and dexamethasone with NM for the treatment of COVID‐19, 21 (47%) experienced hyperkalaemia. The duration of NM administration was a significant predictor of hyperkalaemia (odds ratio: 1.55, 95% confidence interval: 1.04–2.31, p = 0.031). The receiver‐operating characteristic curve analysis determined that the cut‐off value for predicting the number of days until the onset of hyperkalaemia was 6 days and the area under the curve was 0.707.

What is new and conclusion

This study revealed that the incidence of hyperkalaemia is high in patients treated for COVID‐19 with NM, and that the duration of NM administration is a key risk factor. When NM is administered for the treatment of COVID‐19, it should be discontinued within 6 days to minimize the risk of hyperkalaemia.

Pressure-Natriuresis Response Is Diminished in Old Age

BACKGROUND

Age-related alterations in renal sodium handling affect blood pressure (BP). We aimed to clarify whether the pressure-natriuresis response changes with age, leading to BP elevation.

METHODS

A total of 4,859 participants with normal renal function from the Korean Genome and Epidemiology Study (KoGES) and 235 patients with non-diabetic chronic kidney disease (CKD) from the ESPECIAL trial were included and divided into the younger and older groups. In ESPECIAL, participants took olmesartan from weeks 0 to 16 and were educated about a low-salt diet (LSD) from weeks 8 to 16.

RESULTS

In both studies, older participants showed lower estimated glomerular filtration rate (eGFR) and urine concentration index and higher albuminuria. In KoGES, BP was higher and urine sodium was lower in older participants. In ESPECIAL, diastolic BP at 0 week was lower in older participants. Olmesartan reduced BP in both groups, whereas LSD decreased systolic BP only in older participants. Urine sodium increased in younger participants but decreased in older participants after olmesartan use. In KoGES, urine sodium was correlated with BP in both groups after adjusting for age, sex, and eGFR; however, the correlation coefficient was lower in older participants. In ESPECIAL, only younger participants showed a significant positive association between systolic BP and urine sodium in multiple regression analysis.

CONCLUSIONS

The pressure-natriuresis response was diminished in older participants with or without CKD.

Golgi Alpha1,2-Mannosidase IA Promotes Efficient Endoplasmic Reticulum-Associated Degradation of NKCC2

Mutations in the apically located kidney Na-K-2Cl cotransporter NKCC2 cause type I Bartter syndrome, a life-threatening kidney disorder. We previously showed that transport from the ER represents the limiting phase in NKCC2 journey to the cell surface. Yet very little is known about the ER quality control components specific to NKCC2 and its disease-causing mutants. Here, we report the identification of Golgi alpha1, 2-mannosidase IA (ManIA) as a novel binding partner of the immature form of NKCC2. ManIA interaction with NKCC2 takes place mainly at the cis-Golgi network. ManIA coexpression decreased total NKCC2 protein abundance whereas ManIA knock-down produced the opposite effect. Importantly, ManIA coexpression had a more profound effect on NKCC2 folding mutants. Cycloheximide chase assay showed that in cells overexpressing ManIA, NKCC2 stability and maturation are heavily hampered. Deleting the cytoplasmic region of ManIA attenuated its interaction with NKCC2 and inhibited its effect on the maturation of the cotransporter. ManIA-induced reductions in NKCC2 expression were offset by the proteasome inhibitor MG132. Likewise, kifunensine treatment greatly reduced ManIA effect, strongly suggesting that mannose trimming is involved in the enhanced ERAD of the cotransporter. Moreover, depriving ManIA of its catalytic domain fully abolished its effect on NKCC2. In summary, our data demonstrate the presence of a ManIA-mediated ERAD pathway in renal cells promoting retention and degradation of misfolded NKCC2 proteins. They suggest a model whereby Golgi ManIA contributes to ERAD of NKCC2, by promoting the retention, recycling, and ERAD of misfolded proteins that initially escape protein quality control surveillance within the ER.

Predictive effect of salt intake on patient and kidney survival in non-dialysis CKD: competing risk analysis in older versus younger patients under nephrology care

BACKGROUND

The optimal level of salt intake remains ill-defined in non-dialysis chronic kidney disease (CKD) patients under regular nephrology care. This unanswered question becomes critical in older patients who are exposed to higher risk of worsening of cardiorenal disease due to volemic changes.

METHODS

In this pooled analysis of four prospective studies in CKD, we compared the risk of all-cause mortality and end-stage kidney disease (ESKD) between patients ≤65 and >65 years of age stratified by salt intake level (<6, 6-8 and >8 g/day) estimated from two measurements of 24-h urinary sodium.

RESULTS

The cohort included 1785 patients. The estimated glomerular filtration rate was 37 ± 21 mL/min/1.73 m2 overall, 41 ± 25 in younger patients and 34 ± 16 in older patients (P < 0.001). The median 24-h urinary sodium excretion was 143 mEq [interquartile range (IQR) 109-182] in all, 147 (112-185) in younger patients and 140 (106-179) in older patients (P = 0.012). Salt intake was ≤6, 6-8 and >8 g sodium chloride/day in 21.9, 26.2 and 52.0% of older patients and 18.6, 25.2 and 56.2% in younger patients, respectively (P = 0.145). During a median follow-up of 4.07 years we registered 383 ESKD and 260 all-cause deaths. In the whole cohort, the risks of ESKD and all-cause death did not differ by salt intake level. In older patients, ESKD risk [multi-adjusted hazard ratio (HR) and 95% confidence interval (CI)] was significantly lower at salt intakes of 6-8 g/day [HR 0.577 (95% CI 0.361-0.924)] and >8 g/day [HR 0.564 (95% CI 0.382-0.833)] versus the reference group (<6 g/day). Mortality risk was higher in older versus younger patients, with no difference across salt intake categories. No effect of salt intake on ESKD and mortality was observed in younger patients.

CONCLUSIONS

CKD patients under nephrology care show a moderate salt intake (8.4 g/day) that is lower in older versus younger patients. In this context, older patients are not exposed to higher mortality across different levels of salt intake, while salt intake <6 g/day poses a greater risk of ESKD.

Lysine-Specific Demethylase-1 Deficiency Increases Agonist Signaling Via the Mineralocorticoid Receptor

LSD1 (lysine-specific demethylase-1) is an epigenetic regulator of gene transcription. LSD1 risk allele in humans and LSD1 deficiency (LSD1^+/-) in mice confer increasing salt-sensitivity of blood pressure with age, which evolves into salt-sensitive hypertension in older individuals. However, the mechanism underlying the relationship between LSD1 and salt-sensitivity of blood pressure remains elusive. Here, we show that LSD1 genotype (in humans) and LSD1 deficiency (in mice) lead to similar associations with increased blood pressure and urine potassium levels but with decreased aldosterone levels during a liberal salt diet. Thus, we hypothesized that LSD1 deficiency leads to an MR (mineralocorticoid receptor)-dependent hypertensive state. Yet, further studies in LSD1^+/- mice treated with the MR antagonist eplerenone demonstrate that hypertension, kaliuria, and albuminuria are substantially improved, suggesting that the ligand-independent activation of the MR is the underlying cause of this LSD1 deficiency-mediated phenotype. Indeed, while MR and epithelial sodium channel expression levels were increased in LSD1^+/- mouse kidney tissues, aldosterone secretion from LSD1^+/- glomerulosa cells was significantly lower. Collectively, these data establish that LSD1 deficiency leads to an inappropriate activation and increased levels of the MR during a liberal salt regimen and suggest that inhibiting the MR pathway is a useful strategy for treatment of hypertension in human LSD1 risk allele carriers.

Sodium-hydrogen exchanger regulatory factor-1 (NHERF1) confers salt sensitivity in both male and female models of hypertension in aging

Hypertension is a risk factor for premature death and roughly 50% of hypertensive patients are salt-sensitive. The incidence of salt-sensitive hypertension increases with age. However, the mechanisms of salt-sensitive hypertension are not well understood. We had demonstrated decreased renal sodium‑hydrogen exchanger regulatory factor 1 (NHERF1) expression in old salt-resistant F344 rats. Based on those studies we hypothesized that NHERF1 expression is required for the development of some forms of salt-sensitive hypertension. To address this hypothesis, we measured blood pressure in NHERF1 expressing salt-sensitive 4-mo and 24-mo-old male and female Fischer Brown Norway (FBN) rats male and female 18-mo-old NHERF1 knock-out (NHERF1-/-) mice and wild-type (WT) littermates on C57BL/6J background after feeding high salt (8% NaCl) diet for 7 days. Our data demonstrate that 8% salt diet increased blood pressure in both male and female 24-mo-old FBN rats but not in 4-mo-old FBN rats and in 18-mo-old male and female WT mice but not in NHERF1-/- mice. Renal dopamine 1 receptor (D1R) expression was decreased in 24-mo-old rats, compared with 4-mo-old FBN rats. However, sodium chloride cotransporter (NCC) expression increased in 24-mo-old FBN rats. In FBN rats, age had no effect on NaK ATPase α1 and NKCC2 expression. By contrast, high salt diet increased the renal expressions of NKCC2, and NCC in 24-mo-old FBN rats. High salt diet also increased NKCC2 and NCC expression in WT mice but not NHERF1-/- mice. Our data suggest that renal NHERF1 expression confers salt sensitivity with aging, associated with increased expression of sodium transporters.

Extracellular matrix roles in cardiorenal fibrosis: Potential therapeutic targets for CVD and CKD in the elderly

Whereas hypertension, diabetes, and dyslipidemia are age-related risk factors for cardiovascular disease (CVD) and chronic kidney disease (CKD), aging alone is an independent risk factor. With advancing age, the heart and kidney gradually but significantly undergo inflammation and subsequent fibrosis, which eventually results in an irreversible decline in organ physiology. Through cardiorenal network interactions, cardiac dysfunction leads to and responds to renal injury, and both facilitate aging effects. Thus, a comprehensive strategy is needed to evaluate the cardiorenal aging network. Common hallmarks shared across systems include extracellular matrix (ECM) accumulation, along with upregulation of matrix metalloproteinases (MMPs) including MMP-9. The wide range of MMP-9 substrates, including ECM components and inflammatory cytokines, implicates MMP-9 in a variety of pathological and age-related processes. In particular, there is strong evidence that inflammatory cell-derived MMP-9 exacerbates cardiorenal aging. This review explores the potential therapeutic targets against CVD and CKD in the elderly, focusing on ECM and MMP roles.

Mechanisms of altered renal sodium handling in age-related hypertension

The prevalence of hypertension rises with age to approximately two out of three adults over the age of 60 in the United States. Although the mechanisms underlying age-related hypertension are incompletely understood, sodium homeostasis is critical to the long-term regulation of blood pressure and there is strong evidence that aging is associated with alterations in renal sodium handling. This minireview focuses on recent advancements in our understanding of the vascular, neurohumoral, and renal mechanisms that influence sodium homeostasis and promote age-related hypertension.

Renal sodium transport in renin-deficient Dahl salt-sensitive rats

Objective:

The Dahl salt-sensitive rat is a well-established model of salt-sensitive hypertension. The goal of this study was to assess the expression and activity of renal sodium channels and transporters in the renin-deficient salt-sensitive rat.

Methods:

Renin knockout (Ren^−/−) rats created on the salt-sensitive rat background were used to investigate the role of renin in the regulation of ion transport in salt-sensitive hypertension. Western blotting and patch-clamp analyses were utilized to assess the expression level and activity of Na⁺ transporters.

Results:

It has been described previously that Ren^−/− rats exhibit severe kidney underdevelopment, polyuria, and lower body weight and blood pressure compared to their wild-type littermates. Here we found that renin deficiency led to decreased expression of sodium-hydrogen antiporter (NHE3), the Na⁺/H⁺ exchanger involved in Na⁺ absorption in the proximal tubules, but did not affect the expression of Na-K-Cl cotransporter (NKCC2), the main transporter in the loop of Henle. In the distal nephron, the expression of sodium chloride cotransporter (NCC) was lower in Ren^−/− rats. Single-channel patch clamp analysis detected decreased ENaC activity in Ren^−/− rats which was mediated via changes in the channel open probability.

Conclusion:

These data illustrate that renin deficiency leads to significant dysregulation of ion transporters.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Pediatric Acute Respiratory Distress Syndrome: Fluid Management in the PICU

The administration of an appropriate volume of intravenous fluids, while avoiding fluid overload, is a major challenge in the pediatric intensive care unit. Despite our efforts, fluid overload is a very common clinical observation in critically ill children, in particular in those with pediatric acute respiratory distress syndrome (PARDS). Patients with ARDS have widespread damage of the alveolar-capillary barrier, potentially making them vulnerable to fluid overload with the development of pulmonary edema leading to prolonged course of disease. Indeed, studies in adults with ARDS have shown that an increased cumulative fluid balance is associated with adverse outcome. However, age-related differences in the development and consequences of fluid overload in ARDS may exist due to disparities in immunologic response and body water distribution. This systematic review summarizes the current literature on fluid imbalance and management in PARDS, with special emphasis on potential differences with adult patients. It discusses the adverse effects associated with fluid overload and the corresponding possible pathophysiological mechanisms of its development. Our intent is to provide an incentive to develop age-specific fluid management protocols to improve PARDS outcomes.

Changes in urinary excretion of water and sodium transporters during amiloride and bendroflumethiazide treatment

AIM: To quantify changes in urinary excretion of aquaporin2 water channels (u-AQP2), the sodium-potassium-chloride co-transporter (u-NKCC2) and the epithelial sodium channels (u-ENaC) during treatment with bendroflumethiazide (BFTZ), amiloride and placebo.

METHODS: In a randomized, double-blinded, placebo-controlled, 3-way crossover study we examined 23 healthy subjects on a standardized diet and fluid intake. The subjects were treated with amiloride 5 mg, BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day. On the examination day, glomerular filtration rate was measured by the constant infusion clearance technique with ⁵¹Cr-EDTA as reference substance. To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENaC, u-NKCC2, the gamma fraction of ENaC (u-ENaCγ), and u-AQP2 were measured at baseline and after infusion with 3% hypertonic saline. U-NKCC2, u-ENaCγ, u-AQP2 and plasma concentrations of vasopressin (p-AVP), renin (PRC), angiotensin II (p-ANG II) and aldosterone (p-Aldo) were measured, by radioimmunoassay. Central blood pressure was estimated by applanation tonometry and body fluid volumes were estimated by bio-impedance spectroscopy. General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences. Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group.

RESULTS: At baseline there were no differences in u-NKCC2, u-ENaCγ and u-AQP2. PRC, p-Ang II and p-Aldo were increased during active treatments (P < 0.001). After hypertonic saline, u-NKCC2 increased during amiloride (6% ± 34%; P = 0.081) and increased significantly during placebo (17% ± 24%; P = 0.010). U-AQP2 increased significantly during amiloride (31% ± 22%; P < 0.001) and placebo (34% ± 27%; P < 0.001), while u-NKCC2 and u-AQP2 did not change significantly during BFTZ (-7% ± 28%; P = 0.257 and 5% ± 16%; P = 0.261). U- ENaCγ increased in all three groups (P < 0.050). PRC, AngII and p-Aldo decreased to the same extent, while AVP increased, but to a smaller degree during BFTZ (P = 0.048). cDBP decreased significantly during BFTZ (P < 0.001), but not during amiloride or placebo. There were no significant differences in body fluid volumes.

CONCLUSION: After hypertonic saline, u-NKCC2 and u-AQP2 increased during amiloride, but not during BFTZ. Lower p-AVP during BFTZ potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium.

Sodium-pump gene-expression, protein abundance and enzyme activity in isolated nephron segments of the aging rat kidney

Aging is associated with alteration in renal tubular functions, including sodium handling and concentrating ability. Na-K-ATPase plays a key role in driving tubular transport, and we hypothesized that decreased concentrating ability of the aging kidney is due in part to downregulation of Na-K-ATPase. In this study, we evaluated Na and K balance, aldosterone levels, and Na-K-ATPase gene expression, protein abundance, and activity in aging rat kidney. Na-K-ATPase activity (assayed microfluorometrically), mRNA (RT-PCR), and protein abundance (immunoblotting) were quantitated in the following isolated nephron segments: PCT, PST, MTAL, DCT, and CCD from 2, 8, 15, and 24 month-old-rats. In the course of aging, creatinine clearance decreased from 0.48 ± 0.02 mL/min/100 g BW to 0.28 ± 0.06 (P < 0.001) and aldosterone decreased from 23.6 ± 0.8 ng/dL to 13.2 ± 0.6 (P < 0.001). Serum Na⁺ and K⁺ increased by 4.0% and 22.5%, respectively. Na-K-ATPase activity, mRNA, and protein abundance of the α₁ subunit displayed similar trends in all assayed segments; increasing in PCT and PST; decreasing in MTAL and DCT; increasing in CCD: in PCT they increased by 40%, 75%, and 250%, respectively; while in PST they increased by 80%, 50%, and 100%, respectively (P < 0.001). In MTAL they declined by 36%, 24%, and 34%, respectively, and in DCT by 38%, 59%, and 60%, respectively (P < 0.001). They were higher in CCD by 110%, 115%, and 246%, respectively (P < 0.001). Rats maintained Na/K balance; however with a steady state elevated serum K⁺. These results reveal quantitative changes in axial distribution of Na-K-ATPase at the level of gene expression, protein abundance, and activity in the nephrons of aging animals and may explain, in part, the pathophysiology of the senescent kidney.

Aquaporins, vasopressin, and aging: current perspectives

Functioning of the hypothalamic-neurohypophyseal-vasopressin axis is altered in aging, and the pathway may represent a plausible target to slow the process of aging. Arginine vasopressin, a nine-amino acid peptide that is secreted from the posterior pituitary in response to high plasma osmolality and hypotension, is central in this pathway. Vasopressin has important roles in circulatory and water homoeostasis mediated by vasopressin receptor subtypes V1a (vascular), V1b (pituitary), and V2 (vascular, renal). A dysfunction in this pathway as a result of aging can result in multiple abnormalities in several physiological systems. In addition, vasopressin plasma concentration is significantly higher in males than in females and vasopressin-mediated effects on renal and vascular targets are more pronounced in males than in females. These findings may be caused by sex differences in vasopressin secretion and action, making men more susceptible than females to diseases like hypertension, cardiovascular and chronic kidney diseases, and urolithiasis. Recently the availability of new, potent, orally active vasopressin receptor antagonists, the vaptans, has strongly increased the interest on vasopressin and its receptors as a new target for prevention of age-related diseases associated with its receptor-altered signaling. This review summarizes the recent literature in the field of vasopressin signaling in age-dependent abnormalities in kidney, cardiovascular function, and bone function.

Age-related declines in thirst and salt appetite responses in male Fischer 344×Brown Norway rats

The F344×BN strain is the first generational cross between Fischer 344 (F344) and Brown Norway (BN) rats. The F344×BN strain is widely used in aging studies as it is regarded as a model of "healthy" aging (Sprott, 1991). In the present work, male F344×BN rats aged 4mo (young, n=6) and 20mo (old, n=9) received a series of experimental challenges to body fluid homeostasis to determine their thirst and salt appetite responses. Corresponding urinary responses were measured in some of the studies. Following sodium depletion, old rats ingested less saline solution (0.3M NaCl) than young rats on a body weight basis, but both ages drank enough saline solution to completely repair the accrued sodium deficits. Following intracellular dehydration, old rats drank less water than young rats, again on a body weight basis, and were less able than young rats to drink amounts of water proportionate to the osmotic challenge. Compared with young rats, old rats drank less of both water and saline solution after combined food and fluid restriction, and also were refractory to the stimulatory effects of low doses of captopril on water drinking and sodium ingestion. Age differences in urinary water and sodium excretion could not account for the age differences in accumulated water and sodium balances. These results extend observations of diminished behavioral responses of aging animals to the F344×BN rat strain and support the idea that impairments in behavior contribute more to the waning ability of aging animals to respond to body fluid challenges than do declines in kidney function. In addition, the results suggest that behavioral defense of sodium homeostasis is less diminished with age in the F344×BN strain compared to other strains so far studied.

Dietary salt intake modulates differential splicing of the Na-K-2Cl cotransporter NKCC2

Both sodium reabsorption in the thick ascending limb of the loop of Henle (TAL) and macula densa salt sensing crucially depend on the function of the Na/K/2Cl cotransporter NKCC2. The NKCC2 gene gives rise to at least three different full-length NKCC2 isoforms derived from differential splicing. In the present study, we addressed the influence of dietary salt intake on the differential splicing of NKCC2. Mice were subjected to diets with low-salt, standard salt, and high-salt content for 7 days, and NKCC2 isoform mRNA abundance was determined. With decreasing salt intake, we found a reduced abundance of the low-affinity isoform NKCC2A and an increase in the high-affinity isoform NKCC2B in the renal cortex and the outer stripe of the outer medulla. This shift from NKCC2A to NKCC2B during a low-salt diet could be mimicked by furosemide in vivo and in cultured kidney slices. Furthermore, the changes in NKCC2 isoform abundance during a salt-restricted diet were partly mediated by the actions of angiotensin II on AT1 receptors, as determined using chronic angiotensin II infusion. In contrast to changes in oral salt intake, water restriction (48 h) and water loading (8% sucrose solution) increased and suppressed the expression of all NKCC2 isoforms, without changing the distribution pattern of the single isoforms. In summary, the differential splicing of NKCC2 pre-mRNA is modulated by dietary salt intake, which may be mediated by changes in intracellular ion composition. Differential splicing of NKCC2 appears to contribute to the adaptive capacity of the kidney to cope with changes in reabsorptive needs.

Urine concentrating and diluting ability during aging

Urine concentrating ability is reduced during normal aging in people and rats. The abundance of many of the key transport proteins that contribute to urine concentrating ability is reduced in the kidney medulla of aged rats. The reductions in water, sodium, and urea transport protein abundances, and their reduced response to water restriction, contribute to the reduced ability of aged rats to concentrate their urine and conserve water. If similar mechanisms occur in human kidneys, it would provide a molecular explanation for the reduced urine concentrating ability in aging and may provide opportunities for novel therapeutic approaches to improve urine concentrating ability and/or nocturnal polyuria.

Aging kidney: modern perspectives for an ‘old’ problem

The scientific community and health systems will have to address the increasing incidence of reduced kidney function because of progressive global aging. The aim of this review is to describe the morphological and functional alterations that characterize the aging kidney and to suggest not only pharmaceutical pathways but also lifestyle changes that could be beneficially targeted.

Dysfunction of the thick loop of Henle and senescence: from molecular biology to clinical geriatrics

The sodium-potassium-2 chloride bumetanide-sensitive transporter (NKCC2), a protein coded by gene SLC12A1, allows salt reabsorption in the thick ascending loop of Henle (TALH). The functional and clinical exploration of the TALH can be carried out using the Chaimowitz's test, which is based on the exploration of the tubular response to an acute overload of a hypotonic sodium chloride solution. Since this segment is normally responsible for the generation of free water clearance, its function can be assessed via the calculation of such clearance from the parameters obtained during this test. By applying the Chaimowitz's test, the presence of incompetence for sodium reabsorption in TALH in healthy old people was documented. Additionally, it was documented that in water-restricted old rats, a situation that normally induces an increase in the number of NKCC2 in young rats is absent in old ones. In the clinical setting, the increased urinary sodium loss usually found in healthy old people predisposes them to dehydration, hypotension and or hyponatremia when they are on low-sodium diet or under treatment with diuretics. These are commonly found in elderly people with geriatric syndromes such as delirium, gait disorders and incontinence.

CONCLUSION

The NKCC2 transporter decrease in the thick ascending loop of Henle secondary to the ageing could explain the reduced sodium reabsorption of this segment in the healthy elderly and its potential clinical consequences of dehydration and serum sodium abnormalities.

Hypertensive effects of central angiotensin II infusion and restraint stress are reduced with age

OBJECTIVE

We investigated the effect of age on cardiovascular responses mediated by central angiotensin II (AngII) after intracerebroventricular infusion of AngII, and during restraint stress.

METHODS

Blood pressure (BP) and heart rate (HR) of young (5-month-old) and old (27-month-old) male Fischer-344 x Brown-Norway rats were measured using radiotelemetry. AngII was infused intracerebroventricularly using osmotic minipumps (10 ng/0.5 microl/h for 11 days). BP and HR responses to stress were evaluated by placing animals in restrainers for 20 min before and after intracerebroventricular infusion of the AngII-type-1 receptor inhibitor losartan (15 microg/microl per h for 3 days).

RESULTS

Resting BP was significantly elevated and HR was significantly lower in old rats compared with young. AngII-induced BP increase was markedly reduced in old rats, but HR responses were similar. Diurnal variation of both BP and HR was lower in old animals, and AngII reduced the amplitude of BP variation in young rats, but not in old. Restraint stress-induced BP and HR elevations were reduced with age. BP responses were diminished by central losartan infusion in both young and old, but this effect was more significant in young rats. In addition, expression of CuZn-superoxide dismutase and catalase declined significantly with age in the hypothalamus, whereas baseline oxidative stress increased. In contrast, AngII-induced increase in hypothalamic oxidative stress decreased with age.

CONCLUSION

This study demonstrates that the role of central AngII diminishes with age in the regulation of BP both during baseline conditions and during stress, whereas the involvement of AngII in the regulation of HR remains unaffected.

Electrolytes in the aging

The elderly population in the United States continues to grow and is expected to double by 2050. With aging, there are degenerative changes in many organs and the kidney is no exception. After 40 years of age, there is an increase in cortical glomerulosclerosis and a decline in both glomerular filtration rate and renal plasma flow. These changes may be associated with an inability to excrete a concentrated or a dilute urine, ammonium, sodium, or potassium. Hypernatremia and hyponatremia are the most common electrolyte abnormalities found in the elderly and both are associated with a high mortality. Under normal conditions, the elderly are able to maintain water and electrolyte balance, but this may be jeopardized by an illness, a decline in cognitive ability, and with certain medications. Therefore, it is important to be aware of the potential electrolyte abnormalities in the elderly that can arise under these various conditions to prevent adverse outcomes.

Article Commentary: The Alternatively Spliced Form “b” of the Epithelial Sodium Channel α subunit (α ENaC): Any Prior Evidence of its Existence?

The epithelial sodium channel (ENaC) is critical in maintaining sodium balance across aldosterone-responsive epithelia. ENaC is a combined channel formed of three subunits (αβγ) with α ENaC subunit being the most critical for channel functionality. In a previous report, we have demonstrated the existence and mRNA expression levels of four alternatively spliced forms of the α ENaC subunit denoted by -a, -b, -c and -d in kidney cortex of Dahl S and R rats. Of the four alternatively spliced forms presently identified, α ENaC-b is considered the most interesting for the following reasons: Aside from being a salt-sensitive transcript, α ENaC-b mRNA expression is ∼32 fold higher than α ENaC wildtype in kidney cortex of Dahl rats. Additionally, the splice site used to generate α ENaC-b is conserved across species. Finally, α ENaC-b mRNA expression is significantly higher in salt-resistant Dahl R rats versus salt-sensitive Dahl S rats. As such, this commentary aims to highlight some of the previously published research articles that described the existence of an additional protein band on α ENaC western blots that could account for α ENaC-b in other rat species.

Urinary concentration and dilution in the aging kidney

Aged people and rats have a reduced ability to maximally concentrate their urine. Many of the key transport proteins that contribute to urine concentrating ability are reduced in the medulla of aged rats. The reductions in the abundances of water, sodium, and urea transport proteins, and their reduced response to water restriction, contributes to the reduced ability of aged rats to concentrate their urine and conserve water. If similar mechanisms occur in human kidneys, it would provide a molecular explanation for the reduced concentrating ability in aging and may provide opportunities for novel therapeutic approaches to improve urine concentrating ability.

Aortic aging in the Fischer 344 / NNiaHSd x Brown Norway / BiNia Rat

Aging is now recognized as one of major risk factors for cardiovascular disease (CVD). It is well documented that elderly populations show increased incidence of CVD symptomology but whether these changes are directly related to aging is not well understood since the possibility exists that other age-associated pathologies in different organ systems could impact on cardiovascular function. Hence, the development of an aging model with reduced systemic illness could invigorate efforts to understand the direct role of aging in CVD progression. The Fischer 344 / NNIaHSD x Brown Norway / BiNia rat (F344BN) has been proposed as a potential model for aging that exhibits reduced systemic pathology and increased longevity compared to other models. Here we examine the current literature regarding the F344BN, focusing on age-associated changes in aortic structure and function.

Molecular mechanisms and regulation of furosemide-sensitive Na-K-Cl cotransporters

PURPOSE OF REVIEW

Relevant advances towards understanding how furosemide-sensitive Na-K-Cl cotransporters (NKCC) are regulated by alternative splicing, phosphorylation and membrane expression have been made, which are critical to comprehending the role of NKCCs in blood pressure homeostasis.

RECENT FINDINGS

A major breakthrough has been the description of a macromolecular complex responsible for the regulatory phosphorylation of NKCCs, involving members of two families of novel serine-threonine kinases: WNK kinases and Ste-20-related kinases SPAK and OSR1. A new regulatory pathway has been defined, with WNK lying upstream of SPAK-OSR1 and the latter kinases directly phosphorylating NKCC. New evidence has arisen supporting regulation of NKCC membrane expression, possibly through the same mechanisms regulating phosphorylation. Alternative splicing of kidney-specific NKCC2 also appears to be a regulated process. Renal roles for NKCC1 have been described, including an unexpected role in controlling renin secretion.

SUMMARY

We now begin to understand the biochemical pathways mediating NKCC regulatory phosphorylation, which are governed by kinases that, like NKCCs, have been linked to the genesis of hypertension. Complementary long-term regulation of NKCC membrane expression, alternative splicing or gene transcription, however, should not be overlooked. Deciphering the relationships between these processes will enhance our understanding of the pathogenesis of hypertension.