Characterization of a Biochemical Mouse Model of Primary Aldosteronism for Thermal Therapies

Introduction

Aldosterone-producing adenoma (APA) is the most common cause of endocrine-related hypertension but surgery is not always feasible. Current medical interventions are associated with significant side effects and poor patient compliance. New APA animal models that replicate basic characteristics of APA and give physical and biochemical feedback are needed to test new non-surgical treatment methods, such as image-guided thermal ablation.

Methods

A model of APA was developed in nude mice using HAC15 cells, a human adrenal carcinoma cell line. Tumor growth, aldosterone production, and sensitivity to angiotensin II were characterized in the model. The utility of the model was validated via treatment with microwave ablation and characterization of the resulting physical and biochemical changes in the tumor.

Results

The APA model showed rapid and relatively homogeneous growth. The tumors produced aldosterone and steroid precursors in response to angiotensin II challenge, and plasma aldosterone levels were significantly higher in tumor bearing mice two hours after challenge verses non-tumor bearing mice. The model was useful for testing microwave ablation therapy, reducing aldosterone production by 80% in treated mice.

Conclusion

The HAC15 model is a useful tumor model to study and develop localized treatment methods for APA.

Aldosterone and aldosterone synthase inhibitors in cardiorenal disease

Modulation of the renin-angiotensin-aldosterone system is a foundation of therapy for cardiovascular and kidney diseases. Excess aldosterone plays an important role in cardiovascular disease, contributing to inflammation, fibrosis, and dysfunction in the heart, kidneys, and vasculature through both genomic and mineralocorticoid receptor (MR)-mediated as well as nongenomic mechanisms. MR antagonists have been a key therapy for attenuating the pathologic effects of aldosterone but are associated with some side effects and may not always adequately attenuate the nongenomic effects of aldosterone. Aldosterone is primarily synthesized by the CYP11B2 aldosterone synthase enzyme, which is very similar in structure to other enzymes involved in steroid biosynthesis including CYP11B1, a key enzyme involved in glucocorticoid production. Lack of specificity for CYP11B2, off-target effects on the hypothalamic-pituitary-adrenal axis, and counterproductive increased levels of bioactive steroid intermediates such as 11-deoxycorticosterone have posed challenges in the development of early aldosterone synthase inhibitors such as osilodrostat. In early-phase clinical trials, newer aldosterone synthase inhibitors demonstrated promise in lowering blood pressure in patients with treatment-resistant and uncontrolled hypertension. It is therefore plausible that these agents offer protection in other disease states including heart failure or chronic kidney disease. Further clinical evaluation will be needed to clarify the role of aldosterone synthase inhibitors, a promising class of agents that represent a potentially major therapeutic advance.

Extra-adrenal aldosterone: a mini review focusing on the physiology and pathophysiology of intrarenal aldosterone

PURPOSE

Accumulating evidence has demonstrated the existence of extra-adrenal aldosterone in various tissues, including the brain, heart, vascular, adipocyte, and kidney, mainly based on the detection of the CYP11B2 (aldosterone synthase, cytochrome P450, family 11, subfamily B, polypeptide 2) expression using semi-quantitative methods including reverse transcription-polymerase chain reaction and antibody-based western blotting, as well as local tissue aldosterone levels by antibody-based immunosorbent assays. This mini-review highlights the current evidence and challenges in extra-adrenal aldosterone, focusing on intrarenal aldosterone.

METHODS

A narrative review.

RESULTS

Locally synthesized aldosterone may play a vital role in various physio-pathological processes, especially cardiovascular events. The site of local aldosterone synthesis in the kidney may include the mesangial cells, podocytes, proximal tubules, and collecting ducts. The synthesis of renal aldosterone may be regulated by (pro)renin receptor/(pro)renin, angiotensin II/Angiotensin II type 1 receptor, wnt/β-catenin, cyclooxygenase-2/prostaglandin E2, and klotho. Enhanced renal aldosterone release promotes Na+ reabsorption and K+ excretion in the distal nephron and may contribute to the progress of diabetic nephropathy and salt-related hypertension.

CONCLUSIONS

Inhibition of intrarenal aldosterone signaling by aldosterone synthase inhibitors or mineralocorticoid receptor antagonists may be a hopeful pharmacological technique for the therapy of diabetic nephropathy and saltrelated hypertension. Yet, current reports are often conflicting or ambiguous, leading many to question whether extra-adrenal aldosterone exists, or whether it is of any physiological and pathophysiological significance.

Renal calcification in children with renal tubular acidosis: What a paediatrician ‎should ‎know‎

Renal tubular acidosis (RTA) can lead to renal calcification in children, which can cause various complications and impair renal function. This review provides pediatricians with a comprehensive understanding of the relationship between RTA and renal calcification, highlighting essential aspects for clinical management. The article analyzed relevant studies to explore the prevalence, risk factors, underlying mechanisms, and clinical implications of renal calcification in children with RTA. Results show that distal RTA (type 1) is particularly associated with nephrocalcinosis, which presents a higher risk of renal calcification. However, there are limitations to the existing literature, including a small number of studies, heterogeneity in methodologies, and potential publication bias. Longitudinal data and control groups are also lacking, which limits our understanding of long-term outcomes and optimal management strategies for children with RTA and renal calcification. Pediatricians play a crucial role in the early diagnosis and management of RTA to mitigate the risk of renal calcification and associated complications. In addition, alkaline therapy remains a cornerstone in the treatment of RTA, aimed at correcting the acid-base imbalance and reducing the formation of kidney stones. Therefore, early diagnosis and appropriate therapeutic interventions are paramount in preventing and managing renal calcification to preserve renal function and improve long-term outcomes for affected children. Further research with larger sample sizes and rigorous methodologies is needed to optimize the clinical approach to renal calcification in the context of RTA in the pediatric population.

Corin Deficiency Diminishes Intestinal Sodium Excretion in Mice

Sodium excretion, a critical process in sodium homeostasis, occurs in many tissues, including the kidney and intestine. Unlike in the kidney, the hormonal regulation of intestinal sodium excretion remains unclear. Atrial natriuretic peptide (ANP) is a crucial hormone in renal natriuresis. Corin is a protease critical for ANP activation. Corin and ANP are expressed mainly in the heart. In this study, we investigated corin, ANP, and natriuretic peptide receptor A (Npra) expression in mouse intestines. Corin and ANP expression was co-localized in enteroendocrine cells, whereas Npra expression was on the luminal epithelial cells. In Corin knockout (KO) mice, fecal Na⁺ and Cl^- excretion decreased compared with that in wild-type (WT) mice. Such a decrease was not found in conditional Corin KO mice lacking cardiac corin selectively. In kidney conditional Corin KO mice lacking renal corin, fecal Na⁺ and Cl^- excretion increased, compared to that in WT mice. When WT, Corin KO, and the kidney conditional KO mice were treated with aldosterone, the differences in fecal Na⁺ and Cl^- levels disappeared. These results suggest that intestinal corin may promote fecal sodium excretion in a paracrine mechanism independent of the cardiac corin function. The increased fecal sodium excretion in the kidney conditional Corin KO mice likely reflected an intestinal compensatory response to renal corin deficiency. Our results also suggest that intestinal corin activity may antagonize aldosterone action in the promotion of fecal sodium excretion. These findings help us understand the hormonal mechanism controlling sodium excretion the intestinal tract.

EAST/SeSAME Syndrome and Beyond: The Spectrum of Kir4.1- and Kir5.1-Associated Channelopathies

In 2009, two groups independently linked human mutations in the inwardly rectifying K+ channel Kir4.1 (gene name KCNJ10) to a syndrome affecting the central nervous system (CNS), hearing, and renal tubular salt reabsorption. The autosomal recessive syndrome has been named EAST (epilepsy, ataxia, sensorineural deafness, and renal tubulopathy) or SeSAME syndrome (seizures, sensorineural deafness, ataxia, intellectual disability, and electrolyte imbalance), accordingly. Renal dysfunction in EAST/SeSAME patients results in loss of Na+, K+, and Mg2+ with urine, activation of the renin-angiotensin-aldosterone system, and hypokalemic metabolic alkalosis. Kir4.1 is highly expressed in affected organs: the CNS, inner ear, and kidney. In the kidney, it mostly forms heteromeric channels with Kir5.1 (KCNJ16). Biallelic loss-of-function mutations of Kir5.1 can also have disease significance, but the clinical symptoms differ substantially from those of EAST/SeSAME syndrome: although sensorineural hearing loss and hypokalemia are replicated, there is no alkalosis, but rather acidosis of variable severity; in contrast to EAST/SeSAME syndrome, the CNS is unaffected. This review provides a framework for understanding some of these differences and will guide the reader through the growing literature on Kir4.1 and Kir5.1, discussing the complex disease mechanisms and the variable expression of disease symptoms from a molecular and systems physiology perspective. Knowledge of the pathophysiology of these diseases and their multifaceted clinical spectrum is an important prerequisite for making the correct diagnosis and forms the basis for personalized therapies.

Prior fluid and electrolyte imbalance is associated with COVID-19 mortality

Background

The COVID-19 pandemic represents a major public health threat. Risk of death from the infection is associated with age and pre-existing comorbidities such as diabetes, dementia, cancer, and impairment of immunological, hepatic or renal function. It remains incompletely understood why some patients survive the disease, while others do not. As such, we sought to identify novel prognostic factors for COVID-19 mortality.

Methods

We performed an unbiased, observational retrospective analysis of real world data. Our multivariable and univariable analyses make use of U.S. electronic health records from 122,250 COVID-19 patients in the early stages of the pandemic.

Results

Here we show that a priori diagnoses of fluid, pH and electrolyte imbalance during the year preceding the infection are associated with an increased risk of death independently of age and prior renal comorbidities.

Conclusions

We propose that future interventional studies should investigate whether the risk of death can be alleviated by diligent and personalized management of the fluid and electrolyte balance of at-risk individuals during and before COVID-19.

A Pilot Study of the Safety and Efficacy of Alkali Therapy on Vascular Function in Kidney Transplant Recipients

Introduction

Metabolic acidosis is associated with cardiovascular events, graft function, and mortality in kidney transplant recipients (KTRs). We examined the effect of alkali therapy on vascular endothelial function in KTRs.

Methods

We performed an 18-week, randomized, double-blind, placebo-controlled crossover pilot study examining the effect of sodium bicarbonate therapy versus placebo on vascular function in 20 adult KTRs at least 1 year from transplant with an estimated glomerular filtration rate (eGFR) ≥45 ml/min per 1.73 m² and a serum bicarbonate level of 20 to 26 mEq/L. Each treatment period was 8 weeks in duration with a 2-week washout period between treatments. The primary outcome was change in brachial artery flow-mediated dilation (FMD) between sodium bicarbonate treatment and placebo.

Results

Twenty patients completed the study and were included in the primary analysis. The mean (SD) baseline eGFR of participants was 75 (22) ml/min per 1.73 m², respectively. Serum bicarbonate levels did not increase significantly with treatment (0.3 [1.5] mEq/L, P = 0.37). Sodium bicarbonate therapy was not associated with worsening blood pressure, weight gain, or hypokalemia. There was no significant increase in FMD after 8 weeks of sodium bicarbonate therapy compared to placebo (mean change in FMD 2.2%, 95% CI -0.1 to 4.6, P = 0.06). There were no significant changes in high-sensitivity C-reactive protein, interleukin-6, eGFR, or urinary albumin-to-creatinine ratio during treatment. Urinary ammonium excretion decreased by 9 mmol/d (P=0.003), with sodium bicarbonate.

Conclusions

Sodium bicarbonate therapy is safe and feasible in KTRs, and our results strengthen the need for a larger randomized controlled trial.

Acid base disorders in patients with COVID-19

Purpose

Acid–base derangement has been poorly described in patients with coronavirus disease 2019 (COVID-19). Considering the high prevalence of pneumonia and kidneys injury in COVID-19, frequent acid–base alterations are expected in patients admitted with SARS-Cov-2 infection. The study aimed to assess the prevalence of acid–base disorders in symptomatic patients with a diagnosis of COVID-19.

Methods

The retrospective study enrolled COVID-19 patients hospitalized at the University Hospital of Modena from 4 March to 20 June 2020. Baseline arterial blood gas (ABG) analysis was collected in 211 patients. In subjects with multiple ABG analysis, we selected only the first measurement. A pH of less than 7.37 was categorized as acidemia and a pH of more than 7.43 was categorized as alkalemia.

Results

ABG analyses revealed a low arterial partial pressure of oxygen (PO₂, 70.2 ± 25.1 mmHg), oxygen saturation (SO₂, 92%) and a mild reduction of PO₂/FiO₂ ratio (231 ± 129). Acid–base alterations were found in 79.7% of the patient. Metabolic alkalosis (33.6%) was the main alteration followed by respiratory alkalosis (30.3%), combined alkalosis (9.4%), respiratory acidosis (3.3%), metabolic acidosis (2.8%) and other compensated acid–base disturbances (3.6%). All six patients with metabolic acidosis died at the end of the follow-up.

Conclusion

Variations of pH occurred in the majority (79.7%) of patients admitted with COVID-19. The patients experienced all the type of acid–base disorders, notably metabolic and respiratory alkalosis were the most common alterations in this group of patients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11255-021-02855-1.

A Case Report of Recurrent Hypokalemia During Pregnancies Associated With Nonaldosterone-Mediated Renal Potassium Loss

Rationale

Geller et al reported a rare mutation in the mineralocorticoid receptor (MR) resulting in constitutive MR activity. Progesterone, normally an MR antagonist, acts as a potent agonist with this mutation. Progesterone levels can increase 100-fold during pregnancy and thus lead to increased MR activity in this setting, resulting in hypertension (HTN) and hypokalemia during pregnancy and resolution of hypokalemia after delivery.

Presenting concerns

Our patient was a 33-year-old African American female with a history of pregnancy-induced HTN associated with hypokalemia during her last pregnancy. She presented with muscle weakness from profound hypokalemia complicated by nephrogenic diabetes insipidus (DI) and rhabdomyolysis.

Diagnosis

Her admission potassium was 1.9 mmol/L (3.5-5.1 mmol/L) with a 24-hour urine potassium of 35 mmol per day and an unmeasurable serum aldosterone level. Her potassium normalized 1 day after delivery off potassium supplementation and amiloride, which were last given 1 day prior to her delivery. Recurrent hypokalemia from nonaldosterone-mediated renal potassium wasting during pregnancy (with normal potassium in a nongestational state) is consistent with the cases of gain-of-function mutation in MR that Geller et al report. A definite diagnosis requires genetic analysis.

Interventions

Her hypokalemia was refractory to potassium replacement but quickly responded to an inhibitor of the epithelial sodium channel (ENaC), amiloride.

Outcomes

Her potassium normalized on amiloride 10 mg per day and KCL 40 mEq daily during the remainder of her pregnancy, and her nephrogenic DI resolved after this correction of hypokalemia. After her delivery, her potassium remained normal off the potassium supplements and amiloride.

Novel findings

Pregnancy-induced hypokalemia from an activating MR mutation has rarely been reported. Pregnancy-induced HTN is often the first differential diagnosis in a patient who develops worsening in her HTN during pregnancy. We should also consider the possibility of a gain-of-function mutation in MR in these patients who also have associated hypokalemia.

Primary aldosteronism is associated with risk of urinary bladder stones in a nationwide cohort study

We analyzed database from the Taiwan National Health Insurance to investigate whether primary aldosteronism (PA) increases the risk of bladder stones. This retrospective nationwide population-based cohort study during the period of 1998–2011 compared patients with and without PA extracted by propensity score matching. Cox proportional hazard models and competing death risk model were used to estimate the hazard ratios (HRs), sub-hazard ratios (SHRs) and corresponding 95% confidence intervals (CIs). There were 3442 patients with PA and 3442 patients without PA. The incidence rate of bladder stones was 5.36 and 3.76 per 1000 person-years for both groups, respectively. In adjusted Cox hazard proportional regression models, the HR of bladder stones was 1.68 (95% CI 1.20–2.34) for patients with PA compared to individuals without PA. Considering the competing risk of death, the SHR of bladder stones still indicates a higher risk for PA than a comparison cohort (SHR, 1.79; 95% CI 1.30–2.44). PA, age, sex, and fracture number were the variables significantly contributing to the formation of bladder stones. In conclusion, PA is significantly associated with risk of bladder stones.

Re-visiting pH-adjusted potassium to avoid hypokalemic crisis during management of diabetic ketoacidosis: A conceptual framework

BACKGROUND AND AIMS

Diabetic ketoacidosis (DKA) treatment guidelines recommend to initiate potassium-replacement when serum potassium (S_K) drops within normal range, and to withhold insulin if S_K is below normal. Despite strict recommendations, hypokalemia is frequently observed in DKA.

METHODS

Scientific literature was thoroughly searched to find 1) DKA treatment guidelines, 2) studies reporting hypokalemia in DKA, 3) and literature elaborating mechanisms involved in hypokalemia.

RESULTS

Acidosis affects S_K and its regulators including insulin, catecholamines and aldosterone. Current conceptual framework is an argument to gauge the degree of hypokalemia before it strikes DKA patients utilizing S_K level after adjusting it with pH. Suggested approach will reduce hypokalemia risk and its associated complications. The nomogram calculates pH-adjusted potassium and expected potassium loss. It also ranks hypokalemia associated risk, and proposes the potassium-replacement rate over given time period. The differences between current DKA treatment guidelines and proposed strategy are also discussed. Moreover, reasons and risk of hyperkalemia due to early initiation of potassium replacement and remedial actions are debated.

CONCLUSION

In light of proposed strategy, utilizing the nomogram ensures reduced incidence of hypokalemia in DKA resulting in improved clinical and patient outcomes. Pharmacoeconomic benefits can also be expected when avoiding hypokalemia ensures early discharge.

The therapeutic importance of acid-base balance

Baking soda and vinegar have been used as home remedies for generations and today we are only a mouse-click away from claims that baking soda, lemon juice, and apple cider vinegar are miracles cures for everything from cancer to COVID-19. Despite these specious claims, the therapeutic value of controlling acid-base balance is indisputable and is the basis of Food and Drug Administration-approved treatments for constipation, epilepsy, metabolic acidosis, and peptic ulcers. In this narrative review, we present evidence in support of the current and potential therapeutic value of countering local and systemic acid-base imbalances, several of which do in fact involve the administration of baking soda (sodium bicarbonate). Furthermore, we discuss the side effects of pharmaceuticals on acid-base balance as well as the influence of acid-base status on the pharmacokinetic properties of drugs. Our review considers all major organ systems as well as information relevant to several clinical specialties such as anesthesiology, infectious disease, oncology, dentistry, and surgery.

"Summer hypokalemia" as an initial presentation of cystic fibrosis in a morbidly obese African American adult: case report

BACKGROUND

Most patients with cystic fibrosis (CF) present with respiratory or digestive symptoms. About 3% of patients have electrolyte disturbances at the time of diagnosis, but most of the described cases presenting with this manifestation have been in children. Only 3 adult patients are identified in the literature who first presented with hypokalemia. We describe a morbidly obese African American adult who presented with severe hypokalemia and metabolic alkalosis, which eventually led to the diagnosis of CF after multiple hospitalizations over 4 consecutive summers. Besides being the first African American adult with this presentation, he had the highest BMI, lowest serum potassium, highest pH, and highest bicarbonate level.

CASE PRESENTATION

In the summer of 2015, a 26 year-old African American man presented to the hospital for generalized weakness. His BMI was 54 kg/M², and he had been on a special diet for a few months with a weight loss of 50 pounds. He sweated profusely while working as a chef. Laboratory tests showed severe hypokalemia and metabolic alkalosis. Further work-up pointed toward extrarenal losses of potassium. He was treated with intravenous normal saline and potassium chloride. After discharge, his potassium level remained normal through the winter while the potassium was tapered off. However, over the following three summers, he repeatedly presented to hospitals for the same problems. Cystic fibrosis was suspected and confirmed by an abnormal pilocarpine sweat test. Gene test revealed two mutations of cystic fibrosis transmembrane conductance regulator (CFTR). Thereafter, his potassium level remained normal with potassium replacement during summertime. Unexpectedly, however, his BMI rose to 83 kg/M² after he stopped the special diet for weight reduction. The reason for the delayed diagnosis is discussed.

CONCLUSION

We present an exceedingly rare case of CF in a morbidly obese African American adult male whose only manifestation of CF was hypokalemia and metabolic alkalosis. Clinicians should keep an open mind to the diagnosis of CF in ethnically diverse populations, even if it seems unlikely at first glance. For "summer hypokalemia", consider cystic fibrosis.

Salt-sensitive hypertension in chronic kidney disease: distal tubular mechanisms

Chronic kidney disease (CKD) causes salt-sensitive hypertension that is often resistant to treatment and contributes to the progression of kidney injury and cardiovascular disease. A better understanding of the mechanisms contributing to salt-sensitive hypertension in CKD is essential to improve these outcomes. This review critically explores these mechanisms by focusing on how CKD affects distal nephron Na+ reabsorption. CKD causes glomerulotubular imbalance with reduced proximal Na+ reabsorption and increased distal Na+ delivery and reabsorption. Aldosterone secretion further contributes to distal Na+ reabsorption in CKD and is not only mediated by renin and K+ but also by metabolic acidosis, endothelin-1, and vasopressin. CKD also activates the intrarenal renin-angiotensin system, generating intratubular angiotensin II to promote distal Na+ reabsorption. High dietary Na+ intake in CKD contributes to Na+ retention by aldosterone-independent activation of the mineralocorticoid receptor mediated through Rac1. High dietary Na+ also produces an inflammatory response mediated by T helper 17 cells and cytokines increasing distal Na+ transport. CKD is often accompanied by proteinuria, which contains plasmin capable of activating the epithelial Na+ channel. Thus, CKD causes both local and systemic changes that together promote distal nephron Na+ reabsorption and salt-sensitive hypertension. Future studies should address remaining knowledge gaps, including the relative contribution of each mechanism, the influence of sex, differences between stages and etiologies of CKD, and the clinical relevance of experimentally identified mechanisms. Several pathways offer opportunities for intervention, including with dietary Na+ reduction, distal diuretics, renin-angiotensin system inhibitors, mineralocorticoid receptor antagonists, and K+ or H+ binders.

Effect of Galectin 3 on Aldosterone-Associated Risk of Cardiovascular Mortality in Patients Undergoing Coronary Angiography

Recent experimental studies have suggested that galectin-3 has an interaction with aldosterone, and modifies its adverse effects. We therefore aimed to elucidate whether the relationship between plasma aldosterone concentrations (PACs) and long-term fatal cardiovascular (CV) events would depend on plasma galectin-3 levels. A total of 2,457 patients (median age: 63.5 [interquartile range (IQR) = 56.3 to 70.6] years, 30.1% women) from the LUdwigshafen RIsk and Cardiovascular Health study, with a median follow-up of 9.9 (IQR = 8.5 to 10.7) years, were included. We tested the interaction between aldosterone and galectin-3 for CV-mortality using a multivariate Cox proportional hazard model, reporting hazard ratios (HRs) with 95% confidence intervals (95%CIs). Adjustments for multiple CV risk factors as well as medication use were included. Mean PAC was 79.0 (IQR = 48.0 to 124.0) pg/ml and there were 558 (16.8%) CV deaths. There was a significant interaction between PAC and galectin-3 (p = 0.021). When stratifying patients by the median galectin-3, there was a significant association between aldosterone and CV-mortality for those above (HR per 1 standard deviation = 1.14; 95%CI [1.01 to 1.30], p = 0.023), but not below the cut-off value (HR per 1 standard deviation = 1.00; 95%CI [0.87 to 1.15], p = 0.185). In conclusion, the current study demonstrates for the first time a modifying effect of galectin-3 on the association between aldosterone and CV-mortality risk in humans. These findings indicate that galectin-3 is an intermediate between aldosterone and adverse outcomes.

Molecular Pathophysiology of Acid-Base Disorders

Acid-base balance is critical for normal life. Acute and chronic disturbances impact cellular energy metabolism, endocrine signaling, ion channel activity, neuronal activity, and cardiovascular functions such as cardiac contractility and vascular blood flow. Maintenance and adaptation of acid-base homeostasis are mostly controlled by respiration and kidney. The kidney contributes to acid-base balance by reabsorbing filtered bicarbonate, regenerating bicarbonate through ammoniagenesis and generation of protons, and by excreting acid. This review focuses on acid-base disorders caused by renal processes, both inherited and acquired. Distinct rare inherited monogenic diseases affecting acid-base handling in the proximal tubule and collecting duct have been identified. In the proximal tubule, mutations of solute carrier 4A4 (SLC4A4) (electrogenic Na⁺/HCO₃^--cotransporter Na⁺/bicarbonate cotransporter e1 [NBCe1]) and other genes such as CLCN5 (Cl^-/H⁺-antiporter), SLC2A2 (GLUT2 glucose transporter), or EHHADH (enoyl-CoA, hydratase/3-hydroxyacyl CoA dehydrogenase) causing more generalized proximal tubule dysfunction can cause proximal renal tubular acidosis resulting from bicarbonate wasting and reduced ammoniagenesis. Mutations in adenosine triphosphate ATP6V1 (B1 H⁺-ATPase subunit), ATPV0A4 (a4 H⁺-ATPase subunit), SLC4A1 (anion exchanger 1), and FOXI1 (forkhead transcription factor) cause distal renal tubular acidosis type I. Carbonic anhydrase II mutations affect several nephron segments and give rise to a mixed proximal and distal phenotype. Finally, mutations in genes affecting aldosterone synthesis, signaling, or downstream targets can lead to hyperkalemic variants of renal tubular acidosis (type IV). More common forms of renal acidosis are found in patients with advanced stages of chronic kidney disease and are owing, at least in part, to a reduced capacity for ammoniagenesis.

Pathogenesis of hypertension in a mouse model for human CLCN2 related hyperaldosteronism

Human primary aldosteronism (PA) can be caused by mutations in several ion channel genes but mouse models replicating this condition are lacking. We now show that almost all known PA-associated CLCN2 mutations markedly increase ClC-2 chloride currents and generate knock-in mice expressing a constitutively open ClC-2 Cl⁻ channel as mouse model for PA. The Clcn2^op allele strongly increases the chloride conductance of zona glomerulosa cells, provoking a strong depolarization and increasing cytoplasmic Ca²⁺ concentration. Clcn2^op mice display typical features of human PA, including high serum aldosterone in the presence of low renin activity, marked hypertension and hypokalemia. These symptoms are more pronounced in homozygous Clcn2^op/op than in heterozygous Clcn2^+/op mice. This difference is attributed to the unexpected finding that only ~50 % of Clcn2^+/op zona glomerulosa cells are depolarized. By reproducing essential features of human PA, Clcn2^op mice are a valuable model to study the pathological mechanisms underlying this disease.

Mutations in the chloride channel ClC-2 have been found in primary aldosteronism (PA). Here, Göppner et al. generate transgenic mice expressing a mutant form of ClC-2 that displays increased chloride currents like patient mutations, and find it recapitulates the key pathological features of PA.

ANP-stimulated Na+ secretion in the collecting duct prevents Na+ retention in the renal adaptation to acid load

We have recently reported that type A intercalated cells of the collecting duct secrete Na⁺ by a mechanism coupling the basolateral type 1 Na⁺-K⁺-2Cl^- cotransporter with apical type 2 H⁺-K⁺-ATPase (HKA2) functioning under its Na⁺/K⁺ exchange mode. The first aim of the present study was to evaluate whether this secretory pathway is a target of atrial natriuretic peptide (ANP). Despite hyperaldosteronemia, metabolic acidosis is not associated with Na⁺ retention. The second aim of the present study was to evaluate whether ANP-induced stimulation of Na⁺ secretion by type A intercalated cells might account for mineralocorticoid escape during metabolic acidosis. In Xenopus oocytes expressing HKA2, cGMP, the second messenger of ANP, increased the membrane expression, activity, and Na⁺-transporting rate of HKA2. Feeding mice with a NH₄Cl-enriched diet increased urinary excretion of aldosterone and induced a transient Na⁺ retention that reversed within 3 days. At that time, expression of ANP mRNA in the collecting duct and urinary excretion of cGMP were increased. Reversion of Na⁺ retention was prevented by treatment with an inhibitor of ANP receptors and was absent in HKA2-null mice. In conclusion, paracrine stimulation of HKA2 by ANP is responsible for the escape of the Na⁺-retaining effect of aldosterone during metabolic acidosis.

Novel discoveries in acid-base regulation and osmoregulation: A review of selected hormonal actions in zebrafish and medaka

Maintenance of internal ionic and acid-base homeostasis is critical for survival in all biological systems. Similar to mammals, aquatic fishes have developed sophisticated homeostatic mechanisms to mitigate metabolic or environmental disruptions in ionic and acid-base status of systemic body fluids via hormone-controlled transport of ions or acid equivalents. The present review summarizes newly discovered actions of several hormones in zebrafish (Danio rerio) and medaka (Oryzias latipes) that have greatly contributed to our overall understanding of ionic/acid-base regulation. For example, isotocin and cortisol were reported to enhance transport of various ions by stimulating the proliferation and/or differentiation of ionocyte progenitors. Meanwhile, stanniocalcin-1, a well-documented hypocalcemic hormone, was found to suppress ionocyte differentiation and thus downregulate secretion of H⁺ and uptake of Na⁺ and Cl^-. Estrogen-related receptor and calcitonin gene-related peptide also regulate the differentiation of certain types of ionocytes to either stimulate or suppress H⁺ secretion and Cl^- uptake. On the other hand, endothelin and insulin-like growth factor 1 activate the respective secretion of H⁺ and Na⁺/Cl through fast actions. These new findings enhance our understanding of how hormones regulate fish ionic and acid-base regulation while further providing new insights into vertebrate evolution, mammalian endocrinology and human disease-related therapeutics.

Unravelling purinergic regulation in the epididymis: activation of V-ATPase-dependent acidification by luminal ATP and adenosine

KEY POINTS

In the epididymis, elaborate communication networks between epithelial cells are important with respect to establishing an optimal acidic luminal environment for the maturation and storage of spermatozoa, which is essential for male fertility. Proton secretion by epididymal clear cells is achieved via the proton pumping V-ATPase located in their apical membrane. In the present study, we dissect the molecular mechanisms by which clear cells respond to luminal ATP and adenosine to modulate their acidifying activity via the adenosine receptor ADORA2B and the pH-sensitive ATP receptor P2X4. We demonstrate that the hydrolysis of ATP to produce adenosine by ectonucleotidases plays a key role in V-ATPase-dependent proton secretion, and is part of a feedback loop that ensures acidification of the luminal compartment These results help us better understand how professional proton-secreting cells respond to extracellular cues to modulate their functions, and how they communicate with neighbouring cells.

ABSTRACT

Cell-cell cross-talk is crucial for the dynamic function of epithelia, although how epithelial cells detect and respond to variations in extracellular stimuli to modulate their environment remains incompletely understood. In the present study, we used the epididymis as a model system to investigate epithelial cell regulation by luminal factors. In the epididymis, elaborate communication networks between the different epithelial cell types are important for establishing an optimal acidic luminal environment for the maturation and storage of spermatozoa. In particular, clear cells (CCs) secrete protons into the lumen via the proton pumping V-ATPase located in their apical membrane, a process that is activated by luminal alkalinization. However, how CCs detect luminal pH variations to modulate their function remains uncharacterized. Purinergic regulation of epithelial transport is modulated by extracellular pH in other tissues. In the present study, functional analysis of the mouse cauda epididymis perfused in vivo showed that luminal ATP and adenosine modulate the acidifying activity of CCs via the purinergic ADORA2B and P2X4 receptors, and that luminal adenosine content is itself regulated by luminal pH. Altogether, our observations illustrate mechanisms by which CCs are activated by pH sensitive P2X4 receptor and ectonucleotidases, providing a feedback mechanism for the maintenance of luminal pH. These novel mechanisms by which professional proton-secreting cells respond to extracellular cues to modulate their functions, as well as how they communicate with neighbouring cells, might be translatable to other acidifying epithelia.

The role of the clinical laboratory in diagnosing acid-base disorders

Acid-base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body's acid-base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid-base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid-base disorder is generally responsible for a patient's signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid-base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid-base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid-base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid-base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid-base disorders.

Effect of Treatment of Metabolic Acidosis on Vascular Endothelial Function in Patients with CKD: A Pilot Randomized Cross-Over Study

BACKGROUND AND OBJECTIVES

We examined the effect of alkali replacement for metabolic acidosis on vascular endothelial function in patients with CKD.

METHODS

We performed a pilot, prospective, open-label 14-week crossover study examining the effect of oral sodium bicarbonate treatment on vascular function in 20 patients with an eGFR of 15-44 ml/min per 1.73 m2 with low serum bicarbonate levels (16-21 mEq/L). Each period was 6 weeks in duration with a 2-week washout period in between. Patients were treated to goal serum bicarbonate of ≥23 mEq/L. The primary end point was change in brachial artery flow-mediated dilation (FMD) between treatment and control conditions. Secondary end points included changes in markers of inflammation, bone turnover, mineral metabolism, and calcification.

RESULTS

Eighteen patients completed the study and were included in the primary efficacy analysis. The mean (SD) age and eGFR were 59 (12) years and 26 (8) ml/min per 1.73 m2, respectively. Serum bicarbonate increased significantly with sodium bicarbonate treatment (+2.7±2.9 mEq/L, P≤0.001), whereas there was no change in bicarbonate levels in the control group. FMD significantly improved after sodium bicarbonate therapy (mean±SD, FMD baseline: 4.1%±4.1%; 6 weeks: 5.2%±2.9%; P=0.04) There was no significant change in FMD in the control group (mean±SD, FMD baseline: 4.6%±3.1%; 6 weeks: 4.1%±3.4%; P=0.20). Compared with control, sodium bicarbonate treatment resulted in a significant increase in FMD (mean, 1.8%; 95% confidence interval, 0.3 to 3.3; P=0.02). There was no significant change in bone markers or serum calcification propensity with treatment. Serum phosphorus and intact fibroblast growth factor 23 increased significantly during treatment.

CONCLUSIONS

Treatment of metabolic acidosis with sodium bicarbonate significantly improved vascular endothelial function in patients with stages 3b and 4 CKD.

Adenylyl cyclase 6 in acid-base balance - adding complexity

Systemic acid-base balance is tightly controlled within a narrow range of pH. Disturbances in systemic acid-base homeostasis are associated with diverse detrimental effects. The kidney is a key regulator of acid-base balance, capable of excreting HCO₃^- or H⁺, and chronic kidney disease invariably leads to acidosis. However, the regulatory pathways underlying the fine-tuned acid-base sensing and regulatory mechanisms are still incompletely understood. In the article published recently in Clinical Science (vol 132 (16) 1779-1796), Poulson and colleagues investigated the role of adenylyl cyclase 6 (AC6) in acid-base homeostasis. They uncovered a complex role of AC6, specifically affecting acid-base balance during HCO₃^- load, which causes pronounced alkalosis in AC6-deficient mice. However, the phenotype of AC6-deficient mice appears much more complex, involving systemic effects associated with increased energy expenditure. These observations remind us that there is much to be learned about the intricate signaling pathways involved in renal control of acid-base balance and the complex ramifications of acid-base regulation.

Aldosterone deficiency in mice burdens respiration and accentuates diet-induced hyperinsulinemia and obesity

Aldosterone synthase inhibitors (ASIs) should alleviate obesity-related cardiovascular and renal problems resulting partly from aldosterone excess, but their clinical use may have limitations. To improve knowledge for the use of ASIs, we investigated physiology in aldosterone synthase-knockout (ASKO) mice. On regular chow diet (CD), ASKO mice ate more and weighed less than WT mice, largely because they hyperventilated to eliminate acid as CO2. Replacing CD with high-fat diet (HFD) lessened the respiratory burden in ASKO mice, as did 12- to 15-hour fasting. The latter eliminated the genotype differences in respiratory workload and energy expenditure (EE). Thus, aldosterone deficiency burdened the organism more when the animals ate carbohydrate-rich chow than when they ate a HFD. Chronic HFD exposure further promoted hyperinsulinemia in ASKO mice that contributed to visceral fat accumulation accompanied by reduced lipolysis, thermogenic reprogramming, and the absence of weight-gain-related EE increases. Intracerebroventricular aldosterone supplementation in ASKO mice attenuated the HFD-induced hyperinsulinemia, but did not affect EE, suggesting that the presence of aldosterone increased the body's energetic efficiency, thus counteracting the EE-increasing effect of low insulin. ASIs may therefore cause acid-overload-induced respiratory burden and promote obesity. Their use in patients with preexisting renal and cardiopulmonary diseases might be contraindicated.

Evaluation of a self-regulated in vitro hypoxic system by using chemical reactions

In this study, we established an in vitro hypoxic system driven by a self-regulated chemical reaction that proved effective for cell culture. The hypoxic device was modified from a 1.5 L polypropylene preservation box normally employed for food storage. Pyrogallic acid, sodium hydroxide, and sodium carbonate were dissolved in water and injected into the box. Sodium dihydrogen phosphate solution was injected into the box after 15 min. We measured the concentrations of oxygen and carbon dioxide in the box to determine viability of the hypoxic system. It maintained low levels of oxygen less than 0.2% and stabilizing levels of carbon dioxide at 5% for at least 96 h. Therefore, this device sustained a stable hypoxic environment that may be applicable for cell culture and in vitro studies of hypoxia.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

Extracellular Adenosine Stimulates Vacuolar ATPase-Dependent Proton Secretion in Medullary Intercalated Cells

Acidosis is an important complication of AKI and CKD. Renal intercalated cells (ICs) express the proton pumping vacuolar H⁺-ATPase (V-ATPase) and are extensively involved in acid-base homeostasis. H⁺ secretion in type A intercalated cells (A-ICs) is regulated by apical vesicle recycling and stimulated by cAMP. In other cell types, cAMP is increased by extracellular agonists, including adenosine, through purinergic receptors. Adenosine is a Food and Drug Administration-approved drug, but very little is known about the effect of adenosine on IC function. Therefore, we investigated the role of adenosine in the regulation of V-ATPase in ICs. Intravenous treatment of mice with adenosine or agonists of ADORA2A and ADORA2B purinergic P1 receptors induced V-ATPase apical membrane accumulation in medullary A-ICs but not in cortical A-ICs or other IC subtypes. Both receptors are located in A-IC apical membranes, and adenosine injection increased urine adenosine concentration and decreased urine pH. Cell fractionation showed that adenosine or an ADORA2A or ADORA2B agonist induced V-ATPase translocation from vesicles to the plasma membrane and increased protein kinase A (PKA)-dependent protein phosphorylation in purified medullary ICs that were isolated from mice. Either ADORA2A or ADORA2B antagonists or the PKA inhibitor mPKI blocked these effects. Finally, a fluorescence pH assay showed that adenosine activates V-ATPase in isolated medullary ICs. Our study shows that medullary A-ICs respond to luminal adenosine through ADORA2A and ADORA2B receptors in a cAMP/PKA pathway-dependent mechanism to induce V-ATPase-dependent H⁺ secretion.

Electrolyte and Acid-Base Disturbances in End-Stage Liver Disease: A Physiopathological Approach

Electrolyte and acid-base disturbances are frequent in patients with end-stage liver disease; the underlying physiopathological mechanisms are often complex and represent a diagnostic and therapeutic challenge to the physician. Usually, these disorders do not develop in compensated cirrhotic patients, but with the onset of the classic complications of cirrhosis such as ascites, renal failure, spontaneous bacterial peritonitis and variceal bleeding, multiple electrolyte, and acid-base disturbances emerge. Hyponatremia parallels ascites formation and is a well-known trigger of hepatic encephalopathy; its management in this particular population poses a risky challenge due to the high susceptibility of cirrhotic patients to osmotic demyelination. Hypokalemia is common in the setting of cirrhosis: multiple potassium wasting mechanisms both inherent to the disease and resulting from its management make these patients particularly susceptible to potassium depletion even in the setting of normokalemia. Acid-base disturbances range from classical respiratory alkalosis to high anion gap metabolic acidosis, almost comprising the full acid-base spectrum. Because most electrolyte and acid-base disturbances are managed in terms of their underlying trigger factors, a systematic physiopathological approach to their diagnosis and treatment is required.

Current views on the diagnosis and management of hypokalaemia in children

Hypokalaemia is a common electrolyte disorder in children, caused by decreased potassium intake, increased gastrointestinal and urinary losses or transcellular shift. Patients with severe hypokalaemia may suffer from symptoms such as life-threatening cardiac arrhythmias. The aim of our study was to review the aetiology of hypokalaemia, suggest a diagnostic algorithm and discuss the management of patients with various aetiologies of hypokalaemia.

CONCLUSION

Understanding the pathophysiology of hypokalaemic states, along with a detailed medical history, physical examination and specific laboratory tests are required for proper diagnosis and appropriate treatment.

Aldosterone changes after consumption of a sodium-bicarbonated mineral water in humans. A four-way randomized controlled trial

Abnormally high aldosterone levels are associated to hypertension and cardiovascular disease. A sodium-rich mineral water was previously shown to reduce several markers of cardiovascular risk and did not increase blood pressure in healthy adults. We aimed to study the effects of consuming the same mineral water compared to a control water on aldosterone levels, and if the effects vary due to the presence of meal in healthy adults. The design was a four-way randomized controlled crossover 120-min-postprandial trial. Twenty-one healthy men and women participated in the study. Exclusion criteria are diabetes, hypertension, and being a usual consumer of carbonic mineral water. Two different mineral waters, high-sodium and bicarbonate mineral water (BW, sodium, 1 g/L; bicarbonate, 2 g/L) and low-mineral content control water (CW), were consumed with or without a standard meal (500 mL per meal). Statistical analysis was performed by repeated measures ANOVA. The results are as follows: serum sodium did not vary, and serum potassium decreased throughout the assay (p = 0.01) without water influence. Consumption of BW significantly decreased aldosterone levels at 30 (p = 0.046), 60 (p = 0.009), and 120 (p = 0.025) min when consumed alone, and at 120 min (p = 0.019) when consumed with meal, compared to CW. Moreover, the effect of BW on aldosterone levels was significant in women but not in men. In conclusion, consumption of a sodium-bicarbonated mineral water, in presence or absence of meal, induces aldosterone inhibition in healthy women, which is suggested to be a physiological response that protects them against hypertension. This trial is registered at clinicaltrial.gov as NCT01334840.

Context-dependent mechanisms modulating aldosterone signaling in the kidney

The aldosterone-mineralocorticoid receptor (MR) system serves as the major regulator of fluid homeostasis, and is an important drug target for the treatment of hypertension, heart failure, and chronic kidney disease. While the ligand aldosterone plays a central role in facilitating MR activity, recent studies have revealed that MR signaling is modulated through distinct mechanisms at the levels of the receptor and the downstream targets. Notably, phosphorylation of the ligand-binding domain in MR regulates the ability of the receptor to bind to ligand in renal intercalated cells, providing an additional layer of regulation that allows the cell-selective control of MR signaling. These mechanisms are involved in the context-dependent effects of aldosterone in the distal nephron. In this article, the recent progress in the understanding of mechanisms regulating the action of aldosterone is discussed, focusing on the connecting tubules and collecting duct in the kidney.

On the pleotropic actions of mineralocorticoids

Classical effects of mineralocorticoids include stimulation of Na(+) reabsorption and K(+) secretion in the kidney and other epithelia including colon and several glands. Moreover, mineralocorticoids enhance the excretion of Mg(2+) and renal tubular H(+) secretion. The renal salt retention following mineralocorticoid excess leads to extracellular volume expansion and hypertension. The increase of blood pressure following mineralocorticoid excess is, however, not only the result of volume expansion but may result from stiff endothelial cell syndrome impairing the release of vasodilating nitric oxide. Beyond that, mineralocorticoids are involved in the regulation of a wide variety of further functions, including cardiac fibrosis, platelet activation, neuronal function and survival, inflammation as well as vascular and tissue fibrosis and calcification. Those functions are briefly discussed in this short introduction to the special issue. Beyond that, further contributions of this special issue amplify on mineralocorticoid-induced sodium appetite and renal salt retention, the role of mineralocorticoids in the regulation of acid-base balance, the involvement of aldosterone and its receptors in major depression, the mineralocorticoid stimulation of inflammation and tissue fibrosis and the effect of aldosterone on osteoinductive signaling and vascular calcification. Clearly, still much is to be learned about the various ramifications of mineralocorticoid-sensitive physiology and pathophysiology.