Animal models to study cognitive impairment of chronic kidney disease

Mild cognitive impairment (MCI) is common in people with chronic kidney disease (CKD) and its prevalence increases with progressive loss of kidney function. MCI is characterized by a decline in cognitive performance greater than expected for an individual age and education level but with minimal impairment of instrumental activities of daily living. Deterioration can affect one or several cognitive domains (attention, memory, executive functions, language, and perceptual motor or social cognition). Given the increasing prevalence of kidney disease, more and more people with CKD will also develop MCI causing an enormous disease burden for these individuals, their relatives and society. However, the underlying pathomechanisms are poorly understood and current therapies mostly aim at supporting patients in their daily life. This illustrates the urgent need to elucidate the pathogenesis, and potential therapeutic targets and test novel therapies in appropriate preclinical models. Here, we will outline the necessary criteria for experimental modelling of cognitive disorders in CKD. We discuss the use of mice, rats and zebrafish as model systems and present valuable techniques through which kidney function and cognitive impairment can be assessed in this setting. Our objective is to enable researchers to overcome hurdles and accelerate preclinical research aimed at improving therapy of people with CKD and MCI.

Fecal-oral transmission of SARS-CoV-2: A systematic review of evidence from epidemiological and experimental studies

BACKGROUND

SARS-CoV-2 ribonucleic acid (RNA) has been detected in feces, but RNA is not infectious. This systematic review aims to answer if fecal SARS-CoV-2 is experimentally infectious and if evidence of human fecal-oral SARS-CoV-2 transmission exists.

METHODS

On September 19, 2022, we searched PubMed, Embase, Web of Science, medRxiv, and bioRxiv. Biomedical studies inoculating SARS-CoV-2 from feces, rectal, or anal swabs in cells, tissue, organoids, or animals were included. Epidemiological studies of groups differing in exposure to fecal SARS-CoV-2 were included. Risk of bias was assessed using standardized tools. Results were summarized by vote counting, tabulation, and a harvest plot. PROSPERO registration no. CRD42020221719.

RESULTS

A total of 4,874 studies were screened; 26 studies were included; and 13 out of 23 biomedical studies (56.5%) succeeded in infection. Two (66.7%) epidemiological studies found limited evidence suggesting fecal-oral transmission. All studies had concerns about the risk of bias.

CONCLUSIONS

It is possible to experimentally infect cell cultures, organoids, and animals with fecal SARS-CoV-2. No strong epidemiologic evidence was found to support human fecal-oral transmission. We advise future research to study fecal infectivity at different time points during infection, apply appropriate controls, use in vivo models, and study fecal exposure as a risk factor of transmission in human populations.

The SGLT2 inhibitor dapagliflozin improves kidney function in glycogen storage disease XI

Glycogen storage disease XI, also known as Fanconi-Bickel syndrome (FBS), is a rare autosomal recessive disorder caused by mutations in the SLC2A2 gene that encodes the glucose-facilitated transporter type 2 (GLUT2). Patients develop a life-threatening renal proximal tubule dysfunction for which no treatment is available apart from electrolyte replacement. To investigate the renal pathogenesis of FBS, SLC2A2 expression was ablated in mouse kidney and HK-2 proximal tubule cells. GLUT2Pax8Cre+ mice developed time-dependent glycogen accumulation in proximal tubule cells and recapitulated the renal Fanconi phenotype seen in patients. In vitro suppression of GLUT2 impaired lysosomal autophagy as shown by transcriptomic and biochemical analysis. However, this effect was reversed by exposure to a low glucose concentration, suggesting that GLUT2 facilitates the homeostasis of key cellular pathways in proximal tubule cells by preventing glucose toxicity. To investigate whether targeting proximal tubule glucose influx can limit glycogen accumulation and correct symptoms in vivo, we treated mice with the selective SGLT2 inhibitor dapagliflozin. Dapagliflozin reduced glycogen accumulation and improved metabolic acidosis and phosphaturia in the animals by normalizing the expression of Napi2a and NHE3 transporters. In addition, in a patient with FBS, dapagliflozin was safe, improved serum potassium and phosphate concentrations, and reduced glycogen content in urinary shed cells. Overall, this study provides proof of concept for dapagliflozin as a potentially suitable therapy for FBS.

Deletion of aquaporin-4 improves capillary blood flow distribution in brain edema

Brain edema is a feared complication to disorders and insults affecting the brain. It can be fatal if the increase in intracranial pressure is sufficiently large to cause brain herniation. Moreover, accruing evidence suggests that even slight elevations of intracranial pressure have adverse effects, for instance on brain perfusion. The water channel aquaporin-4 (AQP4), densely expressed in perivascular astrocytic endfeet, plays a key role in brain edema formation. Using two-photon microscopy, we have studied AQP4-mediated swelling of astrocytes affects capillary blood flow and intracranial pressure (ICP) in unanesthetized mice using a mild brain edema model. We found improved regulation of capillary blood flow in mice devoid of AQP4, independently of the severity of ICP increase. Furthermore, we found brisk AQP4-dependent astrocytic Ca2+ signals in perivascular endfeet during edema that may play a role in the perturbed capillary blood flow dynamics. The study suggests that astrocytic endfoot swelling and pathological signaling disrupts microvascular flow regulation during brain edema formation.

Loss of the Secretin Receptor Impairs Renal Bicarbonate Excretion and Aggravates Metabolic Alkalosis in Mice during Acute Base-Loading

SIGNIFICANCE STATEMENT

During acute base excess, the renal collecting duct β -intercalated cells ( β -ICs) become activated to increase urine base excretion. This process is dependent on pendrin and cystic fibrosis transmembrane regulator (CFTR) expressed in the apical membrane of β -ICs. The signal that leads to activation of this process was unknown. Plasma secretin levels increase during acute alkalosis, and the secretin receptor (SCTR) is functionally expressed in β -ICs. We find that mice with global knockout for the SCTR lose their ability to acutely increase renal base excretion. This forces the mice to lower their ventilation to cope with this challenge. Our findings suggest that secretin is a systemic bicarbonate-regulating hormone, likely being released from the small intestine during alkalosis.

BACKGROUND

The secretin receptor (SCTR) is functionally expressed in the basolateral membrane of the β -intercalated cells of the kidney cortical collecting duct and stimulates urine alkalization by activating the β -intercalated cells. Interestingly, the plasma secretin level increases during acute metabolic alkalosis, but its role in systemic acid-base homeostasis was unclear. We hypothesized that the SCTR system is essential for renal base excretion during acute metabolic alkalosis.

METHODS

We conducted bladder catheterization experiments, metabolic cage studies, blood gas analysis, barometric respirometry, perfusion of isolated cortical collecting ducts, immunoblotting, and immunohistochemistry in SCTR wild-type and knockout (KO) mice. We also perfused isolated rat small intestines to study secretin release.

RESULTS

In wild-type mice, secretin acutely increased urine pH and pendrin function in isolated perfused cortical collecting ducts. These effects were absent in KO mice, which also did not sufficiently increase renal base excretion during acute base loading. In line with these findings, KO mice developed prolonged metabolic alkalosis when exposed to acute oral or intraperitoneal base loading. Furthermore, KO mice exhibited transient but marked hypoventilation after acute base loading. In rats, increased blood alkalinity of the perfused upper small intestine increased venous secretin release.

CONCLUSIONS

Our results suggest that loss of SCTR impairs the appropriate increase of renal base excretion during acute base loading and that SCTR is necessary for acute correction of metabolic alkalosis. In addition, our findings suggest that blood alkalinity increases secretin release from the small intestine and that secretin action is critical for bicarbonate homeostasis.

Sulfate and acid-base balance

It has been acknowledged for years that compounds containing sulfur (S) are an important source of endogenous acid production. In the metabolism, S is oxidized to sulfate, and therefore the mEq sulfate excreted in the urine is counted as acid retained in the body. In this study we show that pH in fluids with constant [Na] and [HEPES] declines as sulfate ions are added, and we show that titratable acidity increases exactly with the equivalents of sulfate. Therefore, sulfate excretion in urine is also acid excretion per se. This is in accordance with the down-regulation of proximal sulfate reabsorption under acidosis and the observation that children with distal renal tubular acidosis may be sulfate depleted. These results are well explained using charge-balance modeling, which is based only on the three fundamental principles of electroneutrality, conservation of mass, and rules of dissociation as devised from physical chemistry. In contrast, the findings are in contrast to expectations from conventional narratives. These are unable to understand the decreasing pH as sulfate is added since no conventional acid is present. The results may undermine the traditional notion of endogenous acid production since in the case of sulfur balance, S oxidation and its excretion as sulfate exactly balance each other. Possible clinical correlates with these findings are discussed.

Serial intravital 2-photon microscopy and analysis of the kidney using upright microscopes

Serial intravital 2-photon microscopy of the kidney and other abdominal organs is a powerful technique to assess tissue function and structure simultaneously and over time. Thus, serial intravital microscopy can capture dynamic tissue changes during health and disease and holds great potential to characterize (patho-) physiological processes with subcellular resolution. However, successful image acquisition and analysis require significant expertise and impose multiple potential challenges. Abdominal organs are rhythmically displaced by breathing movements which hamper high-resolution imaging. Traditionally, kidney intravital imaging is performed on inverted microscopes where breathing movements are partly compensated by the weight of the animal pressing down. Here, we present a custom and easy-to-implement setup for intravital imaging of the kidney and other abdominal organs on upright microscopes. Furthermore, we provide image processing protocols and a new plugin for the free image analysis software FIJI to process multichannel fluorescence microscopy data. The proposed image processing pipelines cover multiple image denoising algorithms, sample drift correction using 2D registration, and alignment of serial imaging data collected over several weeks using landmark-based 3D registration. The provided tools aim to lower the barrier of entry to intravital microscopy of the kidney and are readily applicable by biomedical practitioners.

Acid content and buffer-capacity: a charge-balance perspective

Rational treatment and thorough diagnostic classification of acid-base disorders requires quantitative understanding of the mechanisms that generate and dissipate loads of acid and base. A natural precondition for this tallying is the ability to quantify the acid content in any specified fluid. Physical chemistry defines the pH-dependent charge on any buffer species, and also on strong ions on which, by definition, the charge is pH-invariant. Based, then, on the requirement of electroneutrality and conservation of mass, it was shown in 1914 that pH can be calculated and understood on the basis of the chemical composition of any fluid. Herein we first show that this specification for [H⁺] of the charge-balance model directly delivers the pH-dependent buffer-capacity as defined in the literature. Next, we show how the notion of acid transport as proposed in experimental physiology can be understood as a change in strong ion difference, ΔSID. Finally, based on Brønsted-Lowry theory we demonstrate that by defining the acid content as titratable acidity, this is equal to SID_ref - SID, where SID_ref is SID at pH 7.4. Thereby, any chemical situation is represented as a curve in a novel diagram with titratable acidity = SID_ref - SID as a function of pH. For any specification of buffer chemistry, therefore, the change in acid content in the fluid is path invariant. Since constituents of SID and titratable acidity are additive, we thereby, based on first principles, have defined a new framework for modeling acid balance across a cell, a whole organ, or the whole-body.

MO675: A New in Vivo Multi-Photon Microscopy Based Approach to Study the Peritoneal Membrane Changes Induced by Peritoneal Dialysis

BACKGROUND AND AIMS

Peritoneal dialysis (PD) is a renal replacement therapy that allows the elimination of metabolic waste products and excess body fluid through the peritoneal membrane. The exposure to PD solution contributes to membrane aging and fibrosis resulting in ultrafiltration and clearance failure.

The glucose, used as an osmotic factor, in PD dialysate triggers several processes involved in the pathogenesis of peritoneal fibrosis, angiogenesis and epithelial to mesenchymal transition.

Studies on natural extracts, such as Oleuropein (Ole), a powerful antioxidant with remarkable antifibrotic and protective effects on the peritoneal membrane, are currently being validated.

This study aims to develop a method based on multi-photon microscopy to study the physiology of the peritoneal membrane during dialysis exchange and to validate in vivo the effects of the Ole in animal models of fibrosis undergoing dialysis treatment.

METHOD

Multi-photon microscopy allows in vivo evaluation of the microcirculation that supply the peritoneum and also to study the framework of mesothelial cells and their underlying layer of collagen fibers that contributes to the sub-mesothelial space.

We have implemented the surgical procedure in order to optimize the stability of a flap of parietal peritoneum to directly observe at the scope. With this approach the peritoneal membrane is evaluable at baseline condition and during exposure to dialysate solutions.

RESULTS

Our method allows building a three-dimensional render of the peritoneal membrane, with the evaluation of all the single layers without the use of specific markers. In this way we could assess specifically the phenomena induced by the fibrotic process: the thickening of the sub-mesothelial interstitium and the greater density of the vascular network.

Furthermore, in vivo measurements of flow in the vessels of microcirculation (arterioles, capillaries, post-capillary venules) determine that exposure to hypotonic solutions increases significantly the flow in large-diameter vessels due to better permeability and hemodilution.

Finally in order to evaluate the possibility to detect morphological and functional changes in pathogenic model we have used the well-established model of peritoneal fibrosis derived by 15 days’ long exposure of the peritoneal membrane with 3.86% glucose dialysate. Whth this approach we could detect the significant increase in the parameters of cellularity, vascularization, fibrosis and thickening of collagen fibers.

In order to test the sensibility of our approach to the evaluation of the peritoneal membrane senescence parameters, we tested the effect of Ole in preventing the damage induced by high glucose–containing dialysate. Ole reduced both the thickness and the organization of the collagen fibers and the vascular network, including the number of branch points.

CONCLUSION

The developed method has potential for a dynamic and reliable in vivo approach. Studies are ongoing to validate the effects of drugs and dialysates with different osmotic and electrolytic compositions on the dialysis capacities of the peritoneal membrane and on the blood flow in peritoneal capillaries.

This method offers great potential for testing new pharmacological approaches aimed at preserving the structural and functional integrity of the peritoneum and for the validation of substances, such as natural extracts with beneficial effects against the damage induced in the long term by conventional dialysis solutions.

Single nephron glomerular filtration rate measured by linescan multiphoton microscopy compared to conventional micropuncture

Renal micropuncture, which requires the direct access to the renal tubules, has for long time been the technique of choice to measure the single nephron glomerular filtration rate (SNGFR) in animal models. This approach is challenging by virtue of complex animal preparation and numerous technically difficult steps. The introduction of intravital multiphoton microscopy (MPM) offers another approach to the measure of the SNGFR by mean of the high laser-tissue penetration and the optical sectioning capacity. Previous MPM studies measuring SNGFR in vivo relied on fast full-frame acquisition during the filtration process obtainable with high performance resonant scanners. In this study, we describe an innovative linescan–based MPM method. The new method can discriminate SNGFR variations both in conditions of low and high glomerular filtration, and shows results comparable to conventional micropuncture both for rats and mice. Moreover, this novel approach has improved spatial and time resolution and is faster than previous methods, thus enabling the investigation of SNGFR from more tubules and improving options for data-analysis.

SARS-CoV-2 prevalence and transmission in swimming activities: Results from a retrospective cohort study

There is an urgent need for research on the epidemiology of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) causing coronavirus disease 2019 (COVID‐19), as the transmissibility differs between settings and populations. Here we report on a questionnaire‐based retrospective cohort study of the prevalence and transmission of SARS‐CoV‐2 among participants in swimming activities in Denmark in the last 5 months of 2020 during the COVID‐19 pandemic.

Eight of 162 swimming activities with a SARS‐CoV‐2 positive participant led to transmission to 23 other participants. Overall, the percentage of episodes leading to transmission was 4.9% (competitive swimming 8.9%; recreational swimming 1.3%). Overall, the incidence rate of transmission was 19.5 participants per 100 000 pool activity hours (corresponding values: 43.5 and 4.7 for competitive and recreational swimming, respectively).

Compliance with precautionary restrictions was highest regarding hand hygiene (98.1%) and lowest in distancing personal sports bags (69.9%). As a result of low statistical power, the study showed no significant effect of restrictions.

Insight into the risk of transmission of SARS‐CoV‐2 during indoor swimming is needed to estimate the efficiency of restrictive measures on this and other sports and leisure activities. Only when we know how the virus spreads through various settings, optimal strategies to handle the COVID‐19 pandemic can be developed.

MO429A NOVEL METHOD LINESCAN-DRIVEN FOR SNGFR MEASUREMENTS AS ALTERNATIVE TO HIGH FULL FRAME MULTIPHOTON MICROSCOPY ACQUISITION

Background and Aims

Renal micropuncture, which requires the direct access to the renal tubules, has been for long time the technique of choice to measure the single nephron glomerular filtration rate (SNGFR) in animal models, but this approach is challenging by virtue of complex animal preparation and numerous careful steps. The introduction of intravital multiphoton microscopy (MPM) permitted to improve the study of renal functions exploiting the high laser penetration and the optical sectioning capacity. Previous MPM studies measuring in vivo the SNGFR relied on fast full frame acquisition during the filtration process obtainable with microscope resonant scanners, which represent optional expensive equipment able to reach very high acquisition speed. In this work we propose an innovative linescan-based MPM method to calculate SNGFR in rodents doable without using the fast acquisition rate offered by resonant scanners.

Method

An in vivo MPM approach was used to measure the SNGFR in control Munich Wistar Frömter rats (MWF) and to test the feasibility of the innovative linescan approach. In order to validate this method in conditions known for reduced and increased SNGFR, it was applied to ischemia reperfusion injury (IRI) and low-dose dopamine treated conditions, respectively.

Results

The glomeruli connected to S1 proximal tubules extending at least 100 μm from the exit of the Bowman’s space were chosen for the measurement. A linescan path starting from the urinary pole and crossing many times the tubular lumen orthogonally to the cellular wall was hand drawn. The linescan was acquired soon after a i.v. bolus of low-molecular weight fluorescent marker was injected. The tubular length, the mean diameter and the transit time of the fluorescent marker within two lines of interest (called cross1 and cross2) were measured to obtain the SNGFR. SNGFR measured in control rats was comparable with previous reported data both at MPM and micropuncture. Significantly higher values compared to control were obtained in 3 μg/kg/min dopamine-treated rats. In IRI-treated rats the SNGFR was reduced about 35% compared to the controls.

Conclusion

The results achieved with our linescan method were quite similar to those obtained with conventional micropuncture, suggesting that the two methods overlap for the normal, dopamine and IRI treatment. Our results show that linescan approach is a promising and cheap alternative to the fast full frame acquisition for the investigation of SNGFR in health and disease, offering results comparable to conventional micropuncture with unprecedent temporal resolution.

Localization and regulation of claudin-14 in experimental models of hypercalcemia

Variations in the claudin-14 (CLDN14) gene have been linked to increased risk of hypercalciuria and kidney stone formation. However, the exact cellular localization of CLDN14 and its regulation remain to be fully delineated. To this end, we generated a novel antibody that allowed the detection of CLDN14 in paraffin-embedded renal sections. This showed CLDN14 to be detectable in the kidney only after induction of hypercalcemia in rodent models. Protein expression in the kidney is localized exclusively to the thick ascending limbs (TALs), mainly restricted to the cortical and upper medullary portion of the kidney. However, not all cells in the TALs expressed the tight junction protein. In fact, CLDN14 was primarily expressed in cells also expressing CLDN16 but devoid of CLDN10. CLDN14 appeared in very superficial apical cell domains and near cell junctions in a belt-like formation along the apical cell periphery. In transgenic mice, Cldn14 promotor-driven LacZ activity did not show complete colocalization with CLDN14 protein nor was it increased by hypercalcemia, suggesting that LacZ activity cannot be used as a marker for CLDN14 localization and regulation in this model. In conclusion, CLDN14 showed a restricted localization pattern in the apical domain of select cells of the TAL.

Quantification of Capillary Perfusion in an Animal Model of Acute Intracranial Hypertension

Intracranial hypertension (IH) is a common feature of many pathologies, including brain edema. In the brain, the extended network of capillaries ensures blood flow to meet local metabolic demands. Capillary circulation may be severely affected by IH, but no studies have quantified the effect of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) on capillary perfusion during the development of brain edema. We used optical coherence tomography angiography to quantify relative changes of fractional perfused volume (FPV) in cortical capillaries and simultaneously monitored ICP and blood pressure (BP) in anesthetized male C57Bl/6NTac mice during development of brain edema induced by water intoxication (WI) within 30 min. WI induced severe IH and brain herniation. ICP and CPP reached 90.2 mm Hg and 38.4 mm Hg, respectively. FPV was significantly affected already at normal ICP (ICP <15 mm Hg, slope ≈ -1.46, p < 0.001) and, at the onset of IH (ICP = 20-22 mm Hg), FPV was 17.9 ± 13.3% lower than baseline. A decreasing trend was observed until the ICP peak (Δ%FPV = -43.6 ± 19.2%). In the ICP range of 7-42 mm Hg, relative changes in FPV were significantly correlated with ICP, BP, and CPP (p < 0.001), with ICP and CPP being the best predictors. In conclusion, elevated ICP induces a gradual collapse of the cerebral microvasculature, which is initiated before the clinical threshold of IH. In summary, the estimate of capillary perfusion might be essential in patients with IH to assess the state of the brain microcirculation and to improve the availability of oxygen and nutrients to the brain.

Potassium depletion induces cellular conversion in the outer medullary collecting duct altering Notch signaling pathway

Potassium depletion affects AQP2 expression and the cellular composition of the kidney collecting duct. This, in turn, contributes to the development of a secondary form of nephrogenic diabetes insipidus and hypokalemic nephropathy. Here we show that after 14 days of potassium depletion, the cellular fraction of A-type intercalated cells increases while the fraction of principal cells decreases along the outer medullary collecting duct in rats. The intercalated cells acquired a novel distribution pattern forming rows of cells attached to each other. These morphological changes occur progressively and reverse after 7 days of recovery on normal rat chow diet. The cellular remodeling mainly occurred in the inner stripe of outer medulla similar to the previously seen effect of lithium on the collecting duct cellular profile. The cellular remodeling is associated with the appearance of cells double labelled with both specific markers of principal and type-A intercalated cells. The appearance of this cell type was associated with the downregulation of the Notch signaling via the Hes1 pathways. These results show that the epithelium of the collecting duct has a high degree of plasticity and that Notch signaling likely plays a key role during hypokalemia.

A new experimental mouse model of water intoxication with sustained increased intracranial pressure and mild hyponatremia without side effects of antidiuretics

The most used experimental mouse model of hyponatremia and elevated intracranial pressure (ICP) is intraperitoneal injection of water in combination with antidiuretics. This model of water intoxication (WI) results in extreme pathological changes and death within 1 h. To improve preclinical studies of the pathophysiology of elevated ICP, we characterized diuresis, cardiovascular parameters, blood ionogram and effects of antidiuretics in this model. We subsequently developed a new mouse model with mild hyponatremia and sustained increased ICP. To investigate the classical protocol (severe WI), C57BL/6mice were anesthetized and received an intraperitoneal injection of 20% body weight of MilliQ water with or without 0.4 µg·kg^-1 desmopressin acetate (dDAVP). Corresponding Sham groups were also studied. In the new WI protocol (mild WI), 10% body weight of a solution containing 6.5 mM NaHCO₃, 1.125 mM KCl and 29.75 mM NaCl was intraperitoneally injected. By severe WI, ICP and mean arterial pressure increased until brain stem herniation occurred (23 ± 3 min after injection). The cardiovascular effects were accelerated by dDAVP. Severe WI induced a halt to urine production irrespective of the use of dDAVP. Following the new mild WI protocol, ICP also increased but was sustained at a pathologically high level without inducing herniation. Mean arterial pressure and urine production were not affected during mild WI. In conclusion, the new mild WI protocol is a superior experimental model to study the pathophysiological effects of elevated ICP induced by water intoxication.

H+-ATPase B1 subunit localizes to thick ascending limb and distal convoluted tubule of rodent and human kidney

The vacuolar-type H⁺-ATPase B1 subunit is heavily expressed in the intercalated cells of the collecting system, where it contributes to H⁺ transport, but has also been described in other segments of the renal tubule. This study aimed to determine the localization of the B1 subunit of the vacuolar-type H⁺-ATPase in the early distal nephron, encompassing thick ascending limbs (TAL) and distal convoluted tubules (DCT), in human kidney and determine whether the localization differs between rodents and humans. Antibodies directed against the H⁺-ATPase B1 subunit were used to determine its localization in paraffin-embedded formalin-fixed mouse, rat, and human kidneys by light microscopy and in sections of Lowicryl-embedded rat kidneys by electron microscopy. Abundant H⁺-ATPase B1 subunit immunoreactivity was observed in the human kidney. As expected, intercalated cells showed the strongest signal, but significant signal was also observed in apical membrane domains of the distal nephron, including TAL, macula densa, and DCT. In mouse and rat, H⁺-ATPase B1 subunit expression could also be detected in apical membrane domains of these segments. In rat, electron microscopy revealed that the H⁺-ATPase B1 subunit was located in the apical membrane. Furthermore, the H⁺-ATPase B1 subunit colocalized with other H⁺-ATPase subunits in the TAL and DCT. In conclusion, the B1 subunit is expressed in the early distal nephron. The physiological importance of H⁺-ATPase expression in these segments remains to be delineated in detail. The phenotype of disease-causing mutations in the B1 subunit may also relate to its presence in the TAL and DCT.

Hemoglobin polymerization via disulfide bond formation in the hypoxia-tolerant turtle Trachemys scripta: implications for antioxidant defense and O2 transport

The ability of many reptilian hemoglobins (Hbs) to form high-molecular weight polymers, albeit known for decades, has not been investigated in detail. Given that turtle Hbs often contain a high number of cysteine (Cys), potentially contributing to the red blood cell defense against reactive oxygen species, we have examined whether polymerization of Hb could occur via intermolecular disulfide bonds in red blood cells of freshwater turtle Trachemys scripta, a species that is highly tolerant of hypoxia and oxidative stress. We find that one of the two Hb isoforms of the hemolysate HbA is prone to polymerization in vitro into linear flexible chains of different size that are visible by electron microscopy but not the HbD isoform. Polymerization of purified HbA is favored by hydrogen peroxide, a main cellular reactive oxygen species and a thiol oxidant, and inhibited by thiol reduction and alkylation, indicating that HbA polymerization is due to disulfide bonds. By using mass spectrometry, we identify Cys5 of the α^A-subunit of HbA as specifically responsible for forming disulfide bonds between adjacent HbA tetramers. Polymerization of HbA does not affect oxygen affinity, cooperativity, and sensitivity to the allosteric cofactor ATP, indicating that HbA is still fully functional. Polymers also form in T. scripta blood after exposure to anoxia but not normoxia, indicating that they are of physiological relevance. Taken together, these results show that HbA polymers may form during oxidative stress and that Cys5α^A of HbA is a key element of the antioxidant capacity of turtle red blood cells.

Measurement of total CO2 in microliter samples of urine and other biological fluids using infrared detection of CO2

The purpose of this study is to describe a low-cost and simply made instrument capable of measuring the total CO₂ content of microliter volumes of biological fluids utilizing a commercially available CO₂ sensor based on a NDIR detector. The described instrument is based on transformation of dissolved HCO₃^- to CO₂ by acidification and subsequent measurement of the produced CO₂. The instrument has a linear response in the range 0.025-10 μmol HCO₃^-, which enables measurements in fresh urine and plasma samples down to 5 μl. The values from plasma were compared to measurements made on 65 μl whole blood in an automatic blood gas analyzer and found not to differ significantly. Compared to currently commercially available instruments applying the same principles to measure total CO₂, this study provides a simple and robust alternative which even can be used on smaller sample volumes.

An innate antiviral pathway acting before interferons at epithelial surfaces

Mucosal surfaces are exposed to environmental substances and represent a major portal of entry for microorganisms. The innate immune system is responsible for early defense against infections and it is believed that the interferons (IFNs) constitute the first line of defense against viruses. Here we identify an innate antiviral pathway that works at epithelial surfaces before the IFNs. The pathway is activated independently of known innate sensors of viral infections through a mechanism dependent on viral O-linked glycans, which induce CXCR3 chemokines and stimulate antiviral activity in a manner dependent on neutrophils. This study therefore identifies a previously unknown layer of antiviral defense that exerts its action on epithelial surfaces before the classical IFN response is operative.

A single simple procedure for dewaxing, hydration and heat-induced epitope retrieval (HIER) for immunohistochemistry in formalin fixed paraffin-embedded tissue

Heat-induced epitope retrieval (HIER) is widely used for immunohistochemistry on formalin fixed paraffin-embedded tissue and includes temperatures well above the melting point of paraffin. We therefore tested whether traditional xylene-based removal of paraffin is required on sections from paraffin-embedded tissue, when HIER is performed by vigorous boiling in 10 mM Tris/0.5 mM EGTA-buffer (pH=9). Immunohistochemical results using HIER with or without prior dewaxing in xylene were evaluated using 7 primary antibodies targeting proteins located in the cytosol, intracellular vesicles and plasma membrane. No effect of omitting prior dewaxing was observed on staining pattern. Semiquantitative analysis did not show HIER to influence the intensity of labelling consistently. Consequently, quantification of immune labelling intensity using fluorescent secondary antibodies was performed at 5 dilutions of primary antibody with and without prior dewaxing in xylene. No effect of omitting prior dewaxing on signal intensity was detectable indicating similar immunoreactivity in dewaxed and non-dewaxed sections. The intensity of staining the nucleus with the DNA-stain ToPro3 was similarly unaffected by omission of dewaxing in xylene. In conclusion, the HIER procedure described and tested can be used as a single procedure enabling dewaxing, hydration and epitope retrieval for immunohistochemistry in formalin fixed paraffin-embedded tissue.

Ultrastructural and immunohistochemical localization of plasma membrane Ca2+-ATPase 4 in Ca2+-transporting epithelia

Plasma membrane Ca(2+)-ATPases (PMCAs) participate in epithelial Ca(2+) transport and intracellular Ca(2+) signaling. The Pmca4 isoform is enriched in distal nephron isolates and decreased in mice lacking the epithelial transient receptor potential vanilloid 5 Ca(2+) channel. We therefore hypothesized that Pmca4 plays a significant role in transcellular Ca(2+) flux and investigated the localization and regulation of Pmca4 in Ca(2+)-transporting epithelia. Using antibodies directed specifically against Pmca4, we found it expressed only in the smooth muscle layer of mouse and human intestines, whereas pan-specific Pmca antibodies detected Pmca1 in lateral membranes of enterocytes. In the kidney, Pmca4 showed broad localization to the distal nephron. In the mouse, expression was most abundant in segments coexpressing the epithelial ransient receptor potential vanilloid 5 Ca(2+) channel. Significant, albeit lower, expression was also evident in the region encompassing the cortical thick ascending limbs, macula densa, and early distal tubules as well as smooth muscle layers surrounding renal vessels. In the human kidney, a similar pattern of distribution was observed, with the highest PMCA4 expression in Na(+)-Cl(-) cotransporter-positive tubules. Electron microscopy demonstrated Pmca4 localization in distal nephron cells at both the basolateral membrane and intracellular perinuclear compartments but not submembranous vesicles, suggesting rapid trafficking to the plasma membrane is unlikely to occur in vivo. Pmca4 expression was not altered by perturbations in Ca(2+) balance, pointing to a housekeeping function of the pump in Ca(2+)-transporting epithelia. In conclusion, Pmca4 shows a divergent expression pattern in Ca(2+)-transporting epithelia, inferring diverse roles for this isoform not limited to transepithelial Ca(2+) transport.

Furosemide-induced urinary acidification is caused by pronounced H+ secretion in the thick ascending limb

The loop diuretic furosemide inhibits NaCl reabsorption in the thick ascending limb (TAL). In addition, furosemide acidifies the urine, which is traditionally explained by increased Na+ loading to the distal tubule causing an activation of H+ secretion via H+-ATPase in α-intercalated cells. The inability to acidify urine in response to furosemide serves to diagnose distal renal tubular acidosis (dysfunction of α-intercalated cells). Since the TAL is important for acid/base regulation, we speculated that it is involved in furosemide-induced urinary acidification. Luminal furosemide (100 μM) caused major, stable, and reversible intracellular alkalization (7.27 ± 0.06 to 7.6 ± 0.04) in isolated perfused murine medullary TAL and pronounced H+ secretion. This H+ secretion was fully inhibited with luminal amiloride (1 mM) and the Na+/H+ exchanger (NHE)3-specific antagonist #4167 (1 μM). Moreover, furosemide triggered a substantial drop of intracellular Na+ concentration in the medullary TAL. These results suggest that the furosemide-induced H+ secretion is a consequence of a drop in intracellular Na+ concentration, increasing the driving force for NHE3. Intriguingly, in whole animal experiments, furosemide-induced urinary acidification and net acid excretion were markedly reduced by specific NHE3 inhibition. Furthermore, the furosemide-induced urinary acidification was partially preserved during epithelial Na+ channel inhibition with benzamil. These results provide new insights in the mechanism of furosemide-induced urinary acidification and emphasize the role of the TAL in renal acid/base handling.

Tubulovascular cross-talk by vascular endothelial growth factor a maintains peritubular microvasculature in kidney

Vascular endothelial growth factor A (VEGFA) production by podocytes is critical for glomerular endothelial health. VEGFA is also expressed in tubular epithelial cells in kidney; however, its physiologic role in the tubule has not been established. Using targeted transgenic mouse models, we found that Vegfa is expressed by specific epithelial cells along the nephron, whereas expression of its receptor (Kdr/Vegfr2) is largely restricted to adjacent peritubular capillaries. Embryonic deletion of tubular Vegfa did not affect systemic Vegfa levels, whereas renal Vegfa abundance was markedly decreased. Excision of Vegfa from renal tubules resulted in the formation of a smaller kidney, with a striking reduction in the density of peritubular capillaries. Consequently, elimination of tubular Vegfa caused pronounced polycythemia because of increased renal erythropoietin (Epo) production. Reducing hematocrit to normal levels in tubular Vegfa-deficient mice resulted in a markedly augmented renal Epo production, comparable with that observed in anemic wild-type mice. Here, we show that tubulovascular cross-talk by Vegfa is essential for maintenance of peritubular capillary networks in kidney. Disruption of this communication leads to increased renal Epo production and resulting polycythemia, presumably to counterbalance microvascular losses.

Fluctuations in surface pH of maturing rat incisor enamel are a result of cycles of H(+)-secretion by ameloblasts and variations in enamel buffer characteristics

It is disputed if ameloblasts in the maturation zone of the enamel organ mainly buffer protons released by hydroxyapatite (HA) crystal growth or if they periodically secrete protons to create alternating acidic and alkaline conditions. The latter hypothesis predicts alternating pH regimes in maturing enamel, which would be affected by pharmacological interference with ameloblast H(+)-secretion. This study tests these predictions. Colorimetric pH-indicators and ratiometric fluorometry were used to measure surface pH in maturation zone enamel of rat incisors. Alternating acidic (down to pH6.24±0.06) and alkaline zones (up to pH7.34±0.08) were found along the tooth coinciding with ameloblast morphological cycles. Underlying the cyclic pattern, a gradual decrease in pH towards the incisal edge was seen. Vinblastine or FR167356 (H(+)-ATPase-inhibitor) disturbed ameloblast acid-secretion, especially in the early parts of acidic zones. Enamel surface pH reflects the titration state of surface PO4(3-)-ions. At the pH-values observed, PO4(3-) would be protonated (pKa>12) and HA dissolved. However, by molecular dynamics simulations we estimate the pKa of HPO4(2-) at an ideal HA surface to be 4.3. The acidic pH measured at the enamel surface may thus only dissolve non-perfect domains of HA crystals in which PO4(3-) is less electrostatically shielded. During repeated alkaline/acidic cycles, near-perfect HA-domains may therefore gradually replace less perfect HA-domains resulting in near-perfect HA-crystals. In conclusion, cyclic changes in ameloblast H(+)-secretion and the degree of enamel maturation determine enamel surface pH. This is in accordance with a hypothesis implicating H(+)-ATPase mediated acid-secretion by ameloblasts.

Interleukin-21 receptor signalling is important for innate immune protection against HSV-2 infections

Interleukin (IL) -21 is produced by Natural Killer T (NKT) cells and CD4(+) T cells and is produced in response to virus infections, where IL-21 has been shown to be essential in adaptive immune responses. Cells from the innate immune system such as Natural Killer (NK) cells and macrophages are also important in immune protection against virus. These cells express the IL-21 receptor (IL-21R) and respond to IL-21 with increased cytotoxicity and cytokine production. Currently, however it is not known whether IL-21 plays a significant role in innate immune responses to virus infections. The purpose of this study was to investigate the role of IL-21 and IL-21R in the innate immune response to a virus infection. We used C57BL/6 wild type (WT) and IL-21R knock out (KO) mice in a murine vaginal Herpes Simplex Virus type 2 (HSV-2) infection model to show that IL-21 - IL-21R signalling is indeed important in innate immune responses against HSV-2. We found that the IL-21R was expressed in the vaginal epithelium in uninfected (u.i) WT mice, and expression increased early after HSV-2 infection. IL-21R KO mice exhibited increased vaginal viral titers on day 2 and 3 post infection (p.i.) and subsequently developed significantly higher disease scores and a lower survival rate compared to WT mice. In addition, WT mice infected with HSV-2 receiving intra-vaginal pre-treatment with murine recombinant IL-21 (mIL-21) had decreased vaginal viral titers on day 2 p.i., significantly lower disease scores, and a higher survival rate compared to infected untreated WT controls. Collectively our data demonstrate the novel finding that the IL-21R plays a critical role in regulating innate immune responses against HSV-2 infection.

Abstract 24: Tubular Vegfa is Required for Renal Microvasculature and Oxygen Sensing

Adequate renal oxygenation is pivotal for maintaining essential functions of the kidney. Vascular endothelial growth factor A (VEGFA) plays a key role in vascular formation and maintenance, processes that are critical for sufficient tissue perfusion and oxygenation. Here we show that Vegfa is expressed in select tubular epithelial cells within the nephron, while expression of its receptor ( Kdr/Vegfr2 ) is restricted largely to capillary beds. Early genetic ablation of tubular Vegfa allows the formation of a grossly normal kidney. However, microvascular density is markedly reduced, leading to increased renal hypoxia. As a consequence the mice develop pronounced polycythemia (81 ± 1.62 vs. 58 ± 0.87 % Hematocrit), due to an increase in serum erythropoietin concentration (426.5 ± 49.3 vs. 249.3 ± 13.6 pg/ml). The elevation in circulating erythropoietin is elicited by an augmented renal production of the hormone, and occurs independent of changes in hepatic expression. In kidney, expression of classical hypoxia-inducible factor-1 responsive transcripts remains unaltered; suggesting that erythropoiesis partly compensates the capillary rarefaction. An increase in diastolic blood pressure (DBP, 112 ± 3.7 vs. 104 ± 1.6 mmHg) is present in mice lacking tubular Vegfa maintained on a normal diet, while systolic blood pressure (SBP, 139 ± 2.8 vs. 135 ± 2.8 mmHg) does not change. The difference in diastolic pressure is ablated when dietary NaCl content changed (DBP: 107 ± 3.1 vs. 103 ± 2.7 mmHg; SBP: 132 ± 4.1 vs. 131 ± 2.9 mmHg; for low NaCl), or when angiotensin II is infused (DBP: 137 ± 6.7 vs. 134 ± 3.8 mmHg; SBP: 161 ± 7.6 vs. 162 ± 9.3 mmHg). Moreover, renal electrolyte excretion remains unperturbed by reductions in tubular Vegfa. In conclusion, tubular Vegfa is critical for adequate development of renal microvasuculature, a prerequisite for proper oxygenation of the kidney. Compensatory polycythemia likely prevents severe perturbations of renal function in this model. In addition, increased diastolic blood pressure may occur as a consequence of capillary rarefaction within the kidney.

Opposing effects of NaCl restriction and carbohydrate loading on urine volume in diabetic rats

AIM

To test the effects of dietary NaCl and carbohydrate content on urine volume in diabetic rats.

METHODS

Streptozotocin-induced diabetic rats were subjected to NaCl restriction using either a NaCl-deficient carbohydrate-rich synthetic diet (Altromin C1036) supplemented to contain 0.16% NaCl (C1036 + lowNaCl) or a modified normal cereal-based diet (Altromin 1320) containing 0.086% NaCl (lowNaCl-1320). Normal diet contained 0.2683% NaCl.

RESULTS

Using the C1036 + lowNaCl diet, earlier reported paradoxical increases in water intake and urine volume of diabetic rats were reproduced. However, water intake and urine volume also increased in diabetic rats offered the synthetic C1036 diet supplemented with NaCl to normal levels. Using the lowNaCl-1320 diet, water intake and urine volume were markedly reduced. Highly significant correlations between urine volume and both osmotic output and urinary glucose excretion were found in diabetic rats on normal diet, but these correlations were absent in diabetic rats on synthetic diet, which showed higher urine volumes than expected from the correlations. In contrast, urine volume was significantly correlated with carbohydrate intake in diabetic rats, irrespective of the diet.

CONCLUSIONS

(i) The synthetic diet dramatically increases the urine volume in STZ-DM rats irrespectively of NaCl content. (ii) Rats with STZ-DM on a normal diet show reduced water intake and urine volume in response to dietary NaCl restriction. (iii) A shift to high carbohydrate diet induces polyuria in STZ-DM rats. (iv) Urine volume in all STZ-DM rats only shows correlation with dietary carbohydrate intake. (v) Glucose-driven osmotic diuresis is unlikely to explain the carbohydrate-induced polyuria.

Ion transporters in secretory and cyclically modulating ameloblasts: a new hypothesis for cellular control of preeruptive enamel maturation

Mature enamel consists of densely packed and highly organized large hydroxyapatite crystals. The molecular machinery responsible for the formation of fully matured enamel is poorly described but appears to involve oscillative pH changes at the enamel surface. We conducted an immunohistochemical investigation of selected transporters and related proteins in the multilayered rat incisor enamel organ. Connexin 43 (Cx-43) is found in papillary cells and ameloblasts, whereas Na+-K+-ATPase is heavily expressed during maturation in the papillary cell layer only. Given the distribution of Cx-43 channels and Na+-K+-ATPase, we suggest that ameloblasts and the papillary cell layer act as a functional syncytium. During enamel maturation ameloblasts undergo repetitive cycles of modulation between ruffle-ended (RA) and smooth-ended (SA) ameloblast morphologies. Carbonic anhydrase II and vacuolar H+-ATPase are expressed simultaneously at the beginning of the maturation stage in RA cells. The proton pumps are present in the ruffled border of RA and appear to be internalized during the SA stage. Both papillary cells and ameloblasts express plasma membrane acid/base transporters (AE2, NBC, and NHE1). AE2 and NHE1 change position relative to the enamel surface as localization of the tight junctions changes during ameloblast modulation cycles. We suggest that the concerted action of the papillary cell layer and the modulating ameloblasts regulates the enamel microenvironment, resulting in oscillating pH fluctuations. The pH fluctuations at the enamel surface may be required to keep intercrystalline spaces open in the surface layers of the enamel, enabling degraded enamel matrix proteins to be removed while hydroxyapatite crystals grow as a result of influx of calcium and phosphate ions.

Acute and chronic effects of growth hormone on renal regulation of electrolyte and water homeostasis

For decades, growth hormone (GH) has been known to influence electrolyte and water handling in humans and animals. However, the molecular mechanisms underlying the GH-induced anti-natriuretic and anti-diuretic effects have remained elusive. This review will examine the existing literature on renal electrolyte and water handling following acute and chronic GH-exposure. Renal responses to GH differ in acute and chronic models. Acute application of GH results in a reduced urinary electrolyte and water excretion, whereas the chronic effects of GH are more diverse, as this state likely represents a complex mixture of primary and secondary actions of GH as well as compensatory mechanisms. During chronic GH-exposure an initial sodium retaining state often occurs, followed by a normalization of the urinary sodium excretion, although extracellular volume expansion still persists. We recently described a possible mechanism by which GH acutely increases renal electrolyte and water reabsorption, by modulation of the kidney specific Na(+), K(+), 2Cl(-) co-transporter (NKCC2). The primary aim of this review is to investigate how GH-induced regulation of NKCC2 may be involved in the complex renal changes previously described during acute and chronic GH. We propose, that the GH-induced increase in NKCC2 activity may explain the initial water and sodium retention seen in a number of studies. Moreover, renal changes seen during prolonged GH-exposure may now be seen on the background of the acute stimulation of NKCC2. Additionally, GH also promotes renal acidification, thus influencing renal acid/base handling. The GH-induced renal acidification is partly compatible with changes in NKCC2 activity. Finally, we review the available data on changes in hormonal systems affecting tubular transport during acute and chronic GH-exposure.

Renal compensation to chronic hypoxic hypercapnia: downregulation of pendrin and adaptation of the proximal tubule

The molecular basis for the renal compensation to respiratory acidosis and specifically the role of pendrin in this condition are unclear. Therefore, we studied the adaptation of the proximal tubule and the collecting duct to respiratory acidosis. Male Wistar-Hannover rats were exposed to either hypercapnia and hypoxia [8% CO(2) and 13% O(2) (hypercapnic, n = 6) or normal air (controls, n = 6)] in an environmental chamber for 10 days and were killed under the same atmosphere. In hypercapnic rats, arterial pH was lower than controls (7.31 +/- 0.01 vs. 7.39 +/- 0.01, P = 0.03), blood HCO(3)(-) concentration was increased (42 +/- 0.9 vs. 32 +/- 0.24 mM, P < 0.001), arterial Pco(2) was increased (10.76 +/- 0.4 vs. 7.20 +/- 0.4 kPa, P < 0.001), and plasma chloride concentration was decreased (92.2 +/- 0.7 vs. 97.2 +/- 0.5 mM, P < 0.001). Plasma aldosterone levels were unchanged. In the proximal tubule, immunoblotting showed an increased expression of sodium/bicarbonate exchanger protein (188 +/- 22 vs. 100 +/- 11%, P = 0.005), confirmed by immunohistochemistry. Total Na/H exchanger protein expression in the cortex was unchanged by immunoblotting (119 +/- 10 vs. 100 +/- 11%, P = 0.27) and immunohistochemistry. In the cortex, the abundance of pendrin was decreased (51 +/- 9 vs. 100 +/- 7%, P = 0.003) by immunoblotting. Immunohistochemistry revealed that this decrease was clear in both cortical collecting ducts (CCDs) and connecting tubules (CNTs). This demonstrates that pendrin expression can be regulated in acidotic animals with no changes in aldosterone levels and no external chloride load. This reduction of pendrin expression may help in redirecting the CNT and CCD toward chloride excretion and bicarbonate reabsorption, contributing to the increased plasma bicarbonate and decreased plasma chloride of chronic respiratory acidosis.

A Role of myosin Vb and Rab11-FIP2 in the aquaporin-2 shuttle

Arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Its binding to Gs-coupled vasopressin V2 receptors increases cyclic AMP (cAMP) and subsequently elicits the redistribution of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane (AQP2 shuttle), thereby facilitating water reabsorption from primary urine. The AQP2 shuttle is a paradigm for cAMP-dependent exocytic processes. Using sections of rat kidney, the AQP2-expressing cell line CD8, and primary principal cells, we studied the role of the motor protein myosin Vb, its vesicular receptor Rab11, and the myosin Vb- and Rab11-binding protein Rab11-FIP2 in the AQP2 shuttle. Myosin Vb colocalized with AQP2 intracellularly in resting and at the plasma membrane in AVP-treated cells. Rab11 was found on AQP2-bearing vesicles. A dominant-negative myosin Vb tail construct and Rab11-FIP2 lacking the C2 domain (Rab11-FIP2-DeltaC2), which disrupt recycling, caused condensation of AQP2 in a Rab11-positive compartment and abolished the AQP2 shuttle. This effect was dependent on binding of myosin Vb tail and Rab11-FIP2-DeltaC2 to Rab11. In summary, we identified myosin Vb as a motor protein involved in AQP2 recycling and show that myosin Vb- and Rab11-FIP2-dependent recycling of AQP2 is an integral part of the AQP2 shuttle.

Angiotensin II mediates downregulation of aquaporin water channels and key renal sodium transporters in response to urinary tract obstruction

The renin-angiotensin system is well known to be involved in the pathophysiological changes in renal function after obstruction of the ureter. Previously, we demonstrated that bilateral ureteral obstruction (BUO) is associated with dramatic changes in the expression of both renal sodium transporters and aquaporin water channels (AQPs). We now examined the effects of the AT1-receptor antagonist candesartan on the dysregulation of AQPs and key renal sodium transporters in rats subjected to 24-h BUO and followed 2 days after release of BUO (BUO-2R). Consistent with previous observations, BUO-2R resulted in a significantly decreased expression of AQP1, -2, and -3 compared with control rats. Concomitantly, the rats developed polyuria and reduced urine osmolality. Moreover, expression of the type 2 Na-phosphate cotransporter (NaPi-2) and type 1 bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) was markedly reduced, consistent with postobstructive natriuresis. Candesartan treatment from the onset of obstruction attenuated the reduction in GFR (3.1 ± 0.4 vs. 1.7 ± 0.3 ml·min−1·kg−1) and partially prevented the reduction in the expression of AQP2 (66 ± 21 vs. 13 ± 2%, n = 7; P < 0.05), NaPi-2 (84 ± 6 vs. 57 ± 10%, n = 7; P < 0.05), and NKCC2 (89 ± 12 vs. 46% ± 11, n = 7; P < 0.05). Consistent with this, candesartan treatment attenuated the increase in urine output (58 ± 4 vs. 97 ± 5 μl·min−1·kg−1, n = 7; P < 0.01) and the reduction in sodium reabsorption (433 ± 62 vs. 233 ± 45 μmol·min−1·kg−1, n = 7; P < 0.05) normally found in rats subjected to BUO. Moreover, candesartan treatment attenuated induction of cyclooxygenase 2 (COX-2) expression in the inner medulla, suggesting that COX-2 induction in response to obstruction is regulated by ANG II. In conclusion, candesartan prevents dysregulation of AQP2, sodium transporters, and development of polyuria seen in BUO. This strongly supports the view that candesartan protects kidney function in response to urinary tract obstruction.

Acute growth hormone administration induces antidiuretic and antinatriuretic effects and increases phosphorylation of NKCC2

Growth hormone (GH) has antidiuretic and antinatriuretic effects in rats and humans, but the molecular mechanisms responsible for these effects are unknown. The aim of this study was to investigate the mechanisms behind the acute renal effects of GH in rats. Female rats received rat (r)GH (2.8 mg/kg sc) or saline and were placed in metabolic cages for 5 h. Urinary excretion of electrolytes and urinary volume were reduced after rGH injection, while urine osmolality was increased. Creatinine and lithium clearance remained unchanged, suggesting that rGH increases reabsorption in segments distal to the proximal tubule. Total plasma insulin-like growth factor I (IGF-I) levels did not change, while cortical IGF-I mRNA abundance was increased. The relative abundance of total and Ser(256)-phosphorylated aquaporin 2 was found to be unchanged by immunoblotting, whereas a significant increase of Thr(96) and Thr(101)-phosphorylated NKCC2 (renal Na(+), K(+), 2Cl(-) cotransporter) was found in the inner stripe of outer medulla thick ascending limbs (mTAL). Additionally, an increased NKCC2 expression was observed in the cortical region. Immunohistochemistry confirmed these findings. The density of NKCC2 molecules in the apical membrane of mTAL cells appeared to be unchanged after rGH injection evaluated by immunoelectron microscopy. Basolateral addition of rGH or IGF-I to microperfused rat mTAL segments did not change transepithelial voltage. In conclusion, GH appears to exert its acute antinatriuretic and antidiuretic effects through indirect activation of NKCC2 in the mTAL.

Clinical review: Renal tubular acidosis--a physicochemical approach

The Canadian physiologist PA Stewart advanced the theory that the proton concentration, and hence pH, in any compartment is dependent on the charges of fully ionized and partly ionized species, and on the prevailing CO₂ tension, all of which he dubbed independent variables. Because the kidneys regulate the concentrations of the most important fully ionized species ([K⁺], [Na⁺], and [Cl^-]) but neither CO₂ nor weak acids, the implication is that it should be possible to ascertain the renal contribution to acid–base homeostasis based on the excretion of these ions. One further corollary of Stewart's theory is that, because pH is solely dependent on the named independent variables, transport of protons to and from a compartment by itself will not influence pH. This is apparently in great contrast to models of proton pumps and bicarbonate transporters currently being examined in great molecular detail. Failure of these pumps and cotransporters is at the root of disorders called renal tubular acidoses. The unquestionable relation between malfunction of proton transporters and renal tubular acidosis represents a problem for Stewart theory. This review shows that the dilemma for Stewart theory is only apparent because transport of acid–base equivalents is accompanied by electrolytes. We suggest that Stewart theory may lead to new questions that must be investigated experimentally. Also, recent evidence from physiology that pH may not regulate acid–base transport is in accordance with the concepts presented by Stewart.

Regulation of the Cl-/HCO3- exchanger AE2 in rat thick ascending limb of Henle's loop in response to changes in acid-base and sodium balance

The Cl(-)/HCO(3)(-) exchanger AE2 is believed to be involved in transcellular bicarbonate reabsorption that occurs in the thick ascending limb of Henle's loop (TAL). The purpose of this study was to test whether chronic changes in acid-base status and sodium intake regulate AE2 polypeptide abundance in the TAL of the rat. Rats were subjected to 6 d of loading with NaCl, NH(4)Cl, NaHCO(3), KCl, or KHCO(3). AE2 protein abundance was estimated by semiquantitative immunoblotting in renal membrane fractions isolated from the cortex and the outer medulla of treated and control rats. In the renal cortex, AE2 abundance was markedly increased in response to oral loading with NH(4)Cl or with NaCl. In contrast, AE2 abundance was unchanged in response to loading with KCl or with NaHCO(3) and was decreased by loading with KHCO(3). The response of AE2 in the outer medulla differed from that in the cortex in that HCO(3)(-) loading increased AE2 abundance when administered with Na(+) but had no effect when administered with K(+). Immunohistochemistry revealed that NaCl loading increased AE2 abundance in the basolateral membrane of both the cortical and the medullary TAL. In contrast, NH(4)Cl loading increased AE2 abundance only in the cortical TAL but not in the medullary TAL. These results suggest that regulation of the basolateral Cl(-)/HCO(3)(-) exchanger AE2 plays an important role in the adaptation of bicarbonate absorption in the TAL during chronic acid-base disturbances and high sodium intake. The present study also emphasizes the contribution of cortical TAL adaptation in the renal regulation of acid-base status.

Characterization of basolateral chloride/bicarbonate exchange in macula densa cells

Functional and immunohistological studies were performed to identify basolateral chloride/bicarbonate exchange in macula densa cells. Using the isolated, perfused thick ascending limb with attached glomerulus preparation dissected from rabbit kidney, macula densa intracellular pH (pH(i)) was measured with fluorescence microscopy and BCECF. For these experiments, basolateral chloride was reduced, resulting in reversible macula densa cell alkalinization. Anion exchange activity was assessed by measuring the maximal net base efflux on readdition of bath chloride. Anion exchange activity required the presence of bicarbonate, was independent of changes in membrane potential, did not require the presence of sodium, and was inhibited by high concentrations of DIDS. Inhibition of macula densa anion exchange activity by basolateral DIDS increased luminal NaCl concentration-induced elevations in pH(i). Immunohistochemical studies using antibodies against AE2 demonstrated expression of AE2 along the basolateral membrane of macula densa cells of rabbit kidney. These results suggest that macula densa cells functionally and immunologically express a chloride/bicarbonate exchanger at the basolateral membrane. This transporter likely participates in the regulation of pH(i) and might be involved in macula densa signaling.

Sodium coupled bicarbonate transporters in the kidney, an update

Recently five genes have been cloned, which code for sodium dependent bicarbonate transport proteins. These genes belong to the SLC4A gene family. This short review summarizes our knowledge of these gene products with respect to their renal distribution and function. The best characterized members are the SLC4A4 and SLC4A7. SLC4A4 codes for an electrogenic Na(+), HCO(3) (-)-cotransporter (NBCe1), which is present in the basolateral membranes of proximal tubules and is responsible for the bicarbonate efflux here, and thus about 80% of the renal bicarbonate reabsorption. SLC4A7 codes for an electroneutral NBC (called NBC3 and NBCn1), which is present basolaterally in the thick ascending limb and the distal part of the collecting ducts and in intercalated cells (either apically or basolaterally) in the connecting and collecting tubules. In the thick ascending limb NBCn1 may be important for NH(4) (+) reabsorption. SLCA5 codes for an electrogenic NBC (called NBC4 and NBCe2), which based on RT-PCR is located to the kidney but the exact localization awaits a good antibody. This is also the case for the SLC4A8 and SLC4A10 gene products, which are sodium dependent Cl(-), HCO(3) (-) exchangers. The recent development in this field substantially increases our understanding of the complex renal regulation of acid base status.

Non-muscle myosin II and myosin light chain kinase are downstream targets for vasopressin signaling in the renal collecting duct

We have previously demonstrated that vasopressin increases the water permeability of the inner medullary collecting duct (IMCD) by inducing trafficking of aquaporin-2 to the apical plasma membrane and that this response is dependent on intracellular calcium mobilization and calmodulin activation. Here, we address the hypothesis that this water permeability response is mediated in part through activation of the calcium/calmodulin-dependent myosin light chain kinase (MLCK) and regulation of non-muscle myosin II. Immunoblotting and immunocytochemistry demonstrated the presence of MLCK, the myosin regulatory light chain (MLC), and the IIA and IIB isoforms of the non-muscle myosin heavy chain in rat IMCD cells. Two-dimensional electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry identified two isoforms of MLC, both of which also exist in phosphorylated and non-phosphorylated forms. 32P incubation of the inner medulla followed by autoradiography of two-dimensional gels demonstrated increased 32P labeling of both isoforms in response to the V2 receptor agonist [deamino-Cys1,D-Arg8]vasopressin (DDAVP). Time course studies of MLC phosphorylation in IMCD suspensions (using immunoblotting with anti-phospho-MLC antibodies) showed that the increase in phosphorylation could be detected as early as 30 s after exposure to vasopressin. The MLCK inhibitor ML-7 blocked the DDAVP-induced MLC phosphorylation and substantially reduced [Arg8]vasopressin (AVP)-stimulated water permeability. AVP-induced MLC phosphorylation was associated with a rearrangement of actin filaments (Alexa Fluor 568-phalloidin) in primary cultures of IMCD cells. These results demonstrate that MLC phosphorylation by MLCK represents a downstream effect of AVP-activated calcium/calmodulin signaling in IMCD cells and point to a role for non-muscle myosin II in regulation of water permeability by vasopressin.

AE2 isoforms in rat kidney: immunohistochemical localization and regulation in response to chronic NH4Cl loading

Three splice variants of anion exchanger (AE)2 (AE2a, b, and c) have been described in the rat, but their relative distribution in rat kidney is not known. The purpose of this study was to describe the segmental and cellular distribution of the AE2 isoforms in the rat kidney and to evaluate whether the expression levels of these AE2 isoforms are regulated independently in response to chronic NH(4)Cl loading. Two polyclonal antibodies were generated, respectively, recognizing a NH(2)-terminal peptide unique to AE2a and an amino acid sequence common to AE2a and AE2b. Antibody specificities were tested using cells transfected separately with the AE2a, AE2b, and AE2c isoforms. Immunohistochemistry on sections of paraffin-embedded rat kidneys showed a distribution of AE2a/AE2b labeling in the kidney similar to the distribution of AE2 in the rat kidney reported previously. AE2 is highly expressed in the medullary thick ascending limb, cortical thick ascending limb (cTAL), and macula densa. The pattern of AE2a-specific labeling differed from the pattern of AE2a/AE2b labeling in that relatively more of the total immunolabel was observed in the terminal inner medullary collecting duct. NH(4)Cl loading (0.033 mmol NH(4)Cl/g body wt for 7 days) did not change the labeling of AE2 isoforms in the medulla, whereas the labeling in the cortex was intensified and included more distal parts of the cTAL. Immunoblotting confirmed upregulation of AE2a/b expression in the cortex. These results indicate that AE2a and AE2b are differentially expressed and regulated in the rat kidney. The regulation following NH(4)Cl loading of AE2b in the cTAL suggests a role for AE2 in transepithelial bicarbonate reabsorption in this segment.

NBCn1 is a basolateral Na+-HCO3- cotransporter in rat kidney inner medullary collecting ducts

Primary cultures of rat inner medullary collecting duct (IMCD) cells Na(+) dependently import HCO(3)(-) across the basolateral membrane through an undefined transport protein. We used RT-PCR, immunoblotting, and immunohistochemistry to identify candidate proteins for this basolateral Na(+)-HCO(3)(-) cotransport. The mRNA encoding the electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1 was detected as the only Na(+)-HCO(3)(-) cotransporter in the rat inner medulla (IM) among the five characterized Na(+)-dependent HCO(3)(-) transporters. The mRNA of a yet uncharacterized transporter-like protein, BTR1, was also present in the IM, but its expression in microdissected tubules seemed restricted to the thin limbs of Henle's loop. Immunoblotting confirmed the presence of NBCn1 as an approximately 180-kDa protein of the rat IM. Anti-NBCn1 immunolabeling was confined to the basolateral plasma membrane domain of IMCD cells in the papillary two-thirds of the IM. Consistent with the presence of NBCn1, IMCD cells possessed stilbene-insensitive, Na(+)- and HCO(3)(-)-dependent pH recovery after acidification, as assessed by fluorescence microscopy using a pH-sensitive intracellular dye. In furosemide-induced alkalotic rats, NBCn1 protein abundance was decreased in both the IM and inner stripe of outer medulla (ISOM) as determined by immunoblotting and immunohistochemistry. In contrast, NBCn1 abundance in the IM and ISOM was unchanged in NaHCO(3)-loaded animals, and the NBCn1 abundance increased only in the ISOM after NH(4)Cl loading. In conclusion, NBCn1 is a basolateral Na(+)-HCO(3)(-) cotransporter of IMCD cells and is differentially regulated in IMCD and medullary thick ascending limb.