Effects of Water Loading on Observed and Predicted Plasma Sodium, and Fluid and Urine Cation Excretion in Healthy Individuals

Classifying Hypotonic Hyponatremia by Projected Treatment Effects - A Quantitative 3-Dimensional Framework

Introduction

The diagnostic algorithms currently used for hypotonic hyponatremia focus primarily on impaired urinary dilution and often neglect the influence of free water intake and solute excretion. We hypothesized that, in each case of hypotonic hyponatremia different pathophysiological mechanisms play a role simultaneously.

Methods

Using clinical data of the previous observational Co-Med study, we defined each case of hypotonic hyponatremia concurrently in 3 dimensions as follows: (i) high net free water intake (HNFWI), (ii) impaired dilution of the urine (IDU), and (iii) low nonelectrolyte solute excretion (LNESE). For each dimension, a "standard delta sodium" (sdna) was calculated reflecting the expected difference to the serum sodium concentration, that would result from changing a dimension to a specific and equivalent target level.

Results

Results from 279 patients were used for this analysis. With target levels of free water intake and urine osmolality at the fifth percentile, and nonelectrolyte solute excretion at the 95th percentile, median (interquartile range) sdna values were 7.1 (4.8-10.2) for HNFWI, 11.8 (7.0-18.6) for IDU and 2.6 (1.6-4.2) mmol/l per 24 hours for LNESE. Sdna results in individual patients were highest with IDU in 68.5%, HNFWI in 30.8% and 0.7% with LNESE. At an sdna-level of at least 4mmol/l per 24 hours, the prevalence of HNFWI was 78.9%, IDU 87.1%, and LNESE 26.5%. 77.5% of patients had 2 or all 3 mechanisms present. Hyponatremia was mostly multifactorial in subgroups according to classic categories of hyponatremia and typical comorbidities as well.

Conclusion

Hypotonic hyponatremia can be quantitatively defined by 3 dimensions. Most cases should be considered multifactorial.

The role of glycosaminoglycans in blood pressure regulation

Essential hypertension (HT) is the global health problem and is a major risk factor for the development of cardiovascular and kidney disease. High salt intake has been associated with HT and impaired kidney sodium excretion is considered to be a major mechanism for the development of HT. Although kidney has a very important role in regulation of BP, this traditional view of BP regulation was challenged by recent findings suggesting that nonosmotic tissue sodium deposition is very important for BP regulation. This new paradigm indicates that sodium can be stored and deposited nonosmotically in the interstitium without water retention and without increased BP. One of the major determinants of this deposition is glycosaminoglycans (GAGs). By binding to GAGs found in the endothelial surface layer (ESL) which contains glycocalyx, sodium is osmotically inactivated and not induce concurrent water retention. Thus, GAGs has important function for homeostatic BP and sodium regulation. In the current review, we summarized the role of GAGs in ESL and BP regulation.

The kidney, volume homeostasis and osmoregulation in space: current perspective and knowledge gaps

Although we have sent humans into space for more than 50 years crucial questions regarding kidney physiology, volume regulation and osmoregulation remain unanswered. The complex interactions between the renin-angiotensin-aldosterone system, the sympathetic nervous system, osmoregulatory responses, glomerular function, tubular function, and environmental factors such as sodium and water intake, motion sickness and ambient temperature make it difficult to establish the exact effect of microgravity and the subsequent fluid shifts and muscle mass loss on these parameters. Unfortunately, not all responses to actual microgravity can be reproduced with head-down tilt bed rest studies, which complicates research on Earth. Better understanding of the effects of microgravity on kidney function, volume regulation and osmoregulation are needed with the advent of long-term deep space missions and planetary surface explorations during which orthostatic intolerance complaints or kidney stone formation can be life-threatening for astronauts. Galactic cosmic radiation may be a new threat to kidney function. In this review, we summarise and highlight the current understandings of the effects of microgravity on kidney function, volume regulation and osmoregulation and discuss knowledge gaps that future studies should address.

Predictive correction of serum sodium concentration with formulas derived from the Edelman equation in patients with severe hyponatremia

Severe hyponatremia can cause life-threatening cerebral edema. Treatment comprises rapid elevation of serum sodium concentration; however, overcorrection can result in osmotic demyelination. This study investigated potential factors, including predictive correction based on the Edelman equation, associated with appropriate correction in 221 patients with a serum sodium concentration ≤ 120 mEq/L who were admitted to a hospital in Nagoya, Japan. Appropriate correction was defined as an elevation in serum sodium concentration in the range of 4-10 mEq/L in the first 24 h and within 18 mEq/L in the first 48 h after the start of the correction. Appropriate corrections were made in 132 (59.7%) of the 221 patients. Multivariate analysis revealed that predictive correction with an infusate and fluid loss formula derived from the Edelman equation was associated with appropriate correction of serum sodium concentration (adjusted odds ratio, 7.84; 95% confidence interval, 2.97-20.64). Relative without its use, the predictive equation results in a lower proportion of undercorrection (14.3% vs. 48.0%, respectively) and overcorrection (1.0% vs. 12.2%, respectively). These results suggest that predictive correction of serum sodium concentrations using the formula derived from the Edelman equation can play an essential role in the appropriate management of patients with severe hyponatremia.

Reconsidering the Edelman equation: impact of plasma sodium concentration, edema and body weight

BACKGROUND

Guidelines recommend treatment of dysnatremias to be guided by formulas based on the Edelman equation. This equation describes the relation between plasma sodium concentration and exchangeable cations. However, this formula does not take into account clinical parameters that have recently been associated with local tissue sodium accumulation, which occurs without concurrent water retention. We investigated to what extent such clinical factors affect the Edelman equation and dysnatremia treatment.

METHODS

We performed a post-hoc analysis with original data of the Edelman study. Linear regression was used to examine the effect of age, sex, weight, edema, total body water (TBW) and heart and kidney failure on the Edelman equation. With attenuated correction, we corrected for measurement errors of both variables. Using piecewise regression, we analyzed whether the Edelman association differs for different plasma sodium concentrations.

RESULTS

Data was available for 82 patients; 57 males and 25 females with a mean (SD) age of 57 (15) years. The slope of the Edelman equation was significantly affected by weight (p=0.01) and edema (p=0.03). Also, below and above plasma sodium levels of 133 mmol/L the slope of the Edelman equation was significantly different (1.25 x0025vs 0.58x0025, p<0.01).

CONCLUSION

Edelman's equation's coefficients are significantly affected by weight, edema and plasma sodium, possibly reflecting differences in tissue sodium accumulation capacity. The performance of Edelman-based formulas in clinical settings may be improved by taking these clinical characteristics into account.

Modelling sodium requirements of athletes across a variety of exercise scenarios - identifying when to test and target, or season to taste

AbstractEvidence suggests the focus for sodium replacement during exercise should be maintenance of plasma sodium concentration ([Na⁺]_plasma) for any given total body water (TBW) volume. The sodium intake to achieve stable [Na⁺]_plasma given known fluid and electrolyte intakes and losses can be mathematically estimated. Therefore the aim of this investigation was to model sodium requirements of athletes during exercise, observing the influence of sweat rate, exercise duration, body mass, baseline [Na⁺]_plasma and sweat potassium [K⁺]_sweat, and relevance to competition (soccer, elite marathon, and 160 km ultramarathon running). Models were constructed across a range of sweat sodium concentrations ([Na⁺]_sweat) (20-80 mmol·L^-1), sweat rates (0.5-2.5 L·h^-1) and fluid replacement (10-90% of losses). In the competition-specific scenarios, fluid replacement was calculated to achieve 2% TBW losses. Sodium requirements were driven by fluid replacement (% of losses) and [Na⁺]_sweat, with minimal or no influence of other variables. Replacing sodium was unnecessary in all realistic scenarios modelled for a soccer match and elite marathon. In contrast, the 160 km ultramarathon required ≥47% sodium replacement when [Na⁺]_sweat was ≥40 mmol·L^-1 and >80% of fluid losses were replaced. In conclusion, sodium requirements to maintain stable [Na⁺]_plasma during exercise depend on both the proportion of fluid losses replaced, and [Na⁺]_sweat. Only when prolonged exercise is coupled with aggressive fluid replacement (>80%) and whole body [Na⁺]_sweat ≥40 mmol·L^-1 does sweat composition testing and significant, targeted sodium replacement appear necessary.

Edelman Revisited: Concepts, Achievements, and Challenges

The key message from the 1958 Edelman study states that combinations of external gains or losses of sodium, potassium and water leading to an increase of the fraction (total body sodium plus total body potassium) over total body water will raise the serum sodium concentration ([Na]_S), while external gains or losses leading to a decrease in this fraction will lower [Na]_S. A variety of studies have supported this concept and current quantitative methods for correcting dysnatremias, including formulas calculating the volume of saline needed for a change in [Na]_S are based on it. Not accounting for external losses of sodium, potassium and water during treatment and faulty values for body water inserted in the formulas predicting the change in [Na]_S affect the accuracy of these formulas. Newly described factors potentially affecting the change in [Na]_S during treatment of dysnatremias include the following: (a) exchanges during development or correction of dysnatremias between osmotically inactive sodium stored in tissues and osmotically active sodium in solution in body fluids; (b) chemical binding of part of body water to macromolecules which would decrease the amount of body water available for osmotic exchanges; and (c) genetic influences on the determination of sodium concentration in body fluids. The effects of these newer developments on the methods of treatment of dysnatremias are not well-established and will need extensive studying. Currently, monitoring of serum sodium concentration remains a critical step during treatment of dysnatremias.

Comparative Effects of Metamizole (Dipyrone) and Naproxen on Renal Function and Prostacyclin Synthesis in Salt-Depleted Healthy Subjects - A Randomized Controlled Parallel Group Study

Aim: The objective was to investigate the effect of metamizole on renal function in healthy, salt-depleted volunteers. In addition, the pharmacokinetics of the four major metamizole metabolites were assessed and correlated with the pharmacodynamic effect using urinary excretion of the prostacyclin metabolite 6-keto-prostaglandin F1α. Methods: Fifteen healthy male volunteers were studied in an open-label randomized controlled parallel group study. Eight subjects received oral metamizole 1,000 mg three times daily and seven subjects naproxen 500 mg twice daily for 7 days. All subjects were on a low sodium diet (50 mmol sodium/day) starting 1 week prior to dosing until the end of the study. Glomerular filtration rate was measured using inulin clearance. Urinary excretion of sodium, potassium, creatinine, 6-keto-prostaglandin F1α, and pharmacokinetic parameters of naproxen and metamizole metabolites were assessed after the first and after repeated dosing. Results: In moderately sodium-depleted healthy subjects, single or multiple dose metamizole or naproxen did not significantly affect inulin and creatinine clearance or sodium excretion. Both drugs reduced renal 6-keto-prostaglandin F1α excretion after single and repeated dosing. The effect started 2 h after intake, persisted for the entire dosing period and correlated with the concentration-profile of naproxen and the active metamizole metabolite 4-methylaminoantipyrine (4-MAA). PKPD modelling indicated less potent COX-inhibition by 4-MAA (EC₅₀ 0.69 ± 0.27 µM) compared with naproxen (EC₅₀ 0.034 ± 0.033 µM). Conclusions: Short term treatment with metamizole or naproxen has no significant effect on renal function in moderately sodium depleted healthy subjects. At clinically relevant doses, 4-MAA and naproxen both inhibit COX-mediated renal prostacyclin synthesis.

Can Changes in the Plasma Sodium Concentration Be Predicted Based on the Mass Balance of Sodium, Potassium, and Water in the Face of Osmotically Inactive Sodium Storage?

CONTEXT

Alterations in the plasma sodium concentration ([Na+]p) is predicted based on changes in the mass balance of Na+, K+, and H2O. However, it is well appreciated that Na+ retention results in both osmotically active and osmotically inactive Na+ storage and that only osmotically active Na+ contributes to the modulation of the [Na+]p. Subject of Review: Recent clinical studies suggested that prediction of changes in the [Na+]p based on the mass balance of Na+, K+, and H2O is inaccurate since the osmotically inactive Na+ storage pool is dynamically regulated. In contrast, animal studies demonstrated that changes in the [Na+]p can be predicted if the total body Na+, K+, and H2O were to be accurately accounted for. Second Opinion: Our analysis demonstrated that alterations in the [Na+]p are predictable at the total body level if all sources of input and output of Na+, K+, and H2O can be accurately accounted for despite the paradoxical finding that there are changes in the osmotically inactive Na+ storage pool at the tissue level. However, future prospective clinical studies are needed to corroborate the findings in the animal studies. We proposed that the fundamental question as to whether changes in the [Na+]p can be predicted in the face of osmotically inactive sodium storage is best addressed by serial measurements of total body exchangeable Na+ and K+ and total body water by isotope dilution at different time intervals.

Distinct osmoregulatory responses to sodium loading in patients with altered glycosaminoglycan structure: a randomized cross-over trial

Background

By binding to negatively charged polysaccharides called glycosaminoglycans, sodium can be stored in the body—particularly in the skin—without concurrent water retention. Concordantly, individuals with changed glycosaminoglycan structure (e.g. type 1 diabetes (DM1) and hereditary multiple exostosis (HME) patients) may have altered sodium and water homeostasis.

Methods

We investigated responses to acute (30-min infusion) and chronic (1-week diet) sodium loading in 8 DM1 patients and 7 HME patients in comparison to 12 healthy controls. Blood samples, urine samples, and skin biopsies were taken to investigate glycosaminoglycan sulfation patterns and both systemic and cellular osmoregulatory responses.

Results

Hypertonic sodium infusion increased plasma sodium in all groups, but more in DM1 patients than in controls. High sodium diet increased expression of nuclear factor of activated t-cells 5 (NFAT5)—a transcription factor responsive to changes in osmolarity—and moderately sulfated heparan sulfate in skin of healthy controls. In HME patients, skin dermatan sulfate, rather than heparan sulfate, increased in response to high sodium diet, while in DM1 patients, no changes were observed.

Conclusion

DM1 and HME patients show distinct osmoregulatory responses to sodium loading when comparing to controls with indications for reduced sodium storage capacity in DM1 patients, suggesting that intact glycosaminoglycan biosynthesis is important in sodium and water homeostasis.

Trial registration These trials were registered with the Netherlands trial register with registration numbers: NTR4095 (https://www.trialregister.nl/trial/3933 at 2013-07-29) and NTR4788 (https://www.trialregister.nl/trial/4645 at 2014-09-12).

Evaluation of intradialytic sodium shifts during sodium controlled hemodialysis

Plasma sodium shifts during hemodialysis treatments can be minimized by application of a sodium control algorithm. The present randomized cross-over trial was designed to apply this option on a large patient cohort and to observe the time course of plasma sodium over the treatment. In one study phase, patients received post-dilution online hemodiafiltration treatments with sodium control over the entire treatment. In the other study phase, patients received isolated ultrafiltration during the first 90 min followed by post-dilution online hemodiafiltration with sodium control for the remainder of the session, with the purpose to follow a possible initial equilibration process without the influence of a diffusive solute transfer. Each phase included six treatments and was delivered in randomized order. Eighty-one patients were enrolled, 77 patients could be analyzed as intention-to-treat population. The difference of the mean plasma sodium concentration between start and end of the treatment was −0.60 mmol/L (confidence interval −0.88 to −0.32) and −0.15 mmol/L (confidence interval −0.43 to 0.13), for sodium control and isolated ultrafiltration during the first 90 min followed by post-dilution online hemodiafiltration with sodium control, respectively. The functionality of the sodium control option could be confirmed and further reproduced in a bigger population of dialysis patients, providing the basis to investigate the clinical benefit of individually adjusting dialysate sodium in further clinical studies.

Clinical impact of tissue sodium storage

In recent times, the traditional nephrocentric, two-compartment model of body sodium has been challenged by long-term sodium balance studies and experimental work on the dermal interstitium and endothelial surface layer. In the new paradigm, sodium can be stored without commensurate water retention in the interstitium and endothelial surface layer, forming a dynamic third compartment for sodium. This has important implications for sodium homeostasis, osmoregulation and the hemodynamic response to salt intake. Sodium storage in the skin and endothelial surface layer may function as a buffer during periods of dietary depletion and excess, representing an extra-renal mechanism regulating body sodium and water. Interstitial sodium storage may also serve as a biomarker for sodium sensitivity and cardiovascular risk, as well as a target for hypertension treatment. Furthermore, sodium storage may explain the limitations of traditional techniques used to quantify sodium intake and determine infusion strategies for dysnatraemias. This review is aimed at outlining these new insights into sodium homeostasis, exploring their implications for clinical practice and potential areas for further research for paediatric and adult populations.

Body Fluid-Independent Effects of Dietary Salt Consumption in Chronic Kidney Disease

The average dietary salt (i.e., sodium chloride) intake in Western society is about 10 g per day. This greatly exceeds the lifestyle recommendations by the WHO to limit dietary salt intake to 5 g. There is robust evidence that excess salt intake is associated with deleterious effects including hypertension, kidney damage and adverse cardiovascular health. In patients with chronic kidney disease, moderate reduction of dietary salt intake has important renoprotective effects and positively influences the efficacy of common pharmacological treatment regimens. During the past several years, it has become clear that besides influencing body fluid volume high salt also induces tissue remodelling and activates immune cell homeostasis. The exact pathophysiological pathway in which these salt-induced fluid-independent effects contribute to CKD is not fully elucidated, nonetheless it is clear that inflammation and the development of fibrosis play a major role in the pathogenic mechanisms of renal diseases. This review focuses on body fluid-independent effects of salt contributing to CKD pathogenesis and cardiovascular health. Additionally, the question whether better understanding of these pathophysiological pathways, related to high salt consumption, might identify new potential treatment options will be discussed.

Abnormal sodium and water homeostasis in mice with defective heparan sulfate polymerization

Glycosaminoglycans in the skin interstitium and endothelial surface layer have been shown to be involved in local sodium accumulation without commensurate water retention. Dysfunction of heparan sulfate glycosaminoglycans may therefore disrupt sodium and water homeostasis. In this study, we investigated the effects of combined heterozygous loss of heparan sulfate polymerization genes (exostosin glycosyltransferase 1 and 2; Ext1^+/-Ext2^+/-) on sodium and water homeostasis. Sodium storage capacity was decreased in Ext1^+/-Ext2^+/- mice as reflected by a 77% reduction in endothelial surface layer thickness and a lower skin sodium-to-glycosaminoglycan ratio. Also, these mice were characterized by a higher heart rate, increased fluid intake, increased plasma osmolality and a decreased skin water and sodium content, suggesting volume depletion. Upon chronic high sodium intake, the initial volume depletion was restored but no blood pressure increase was observed. Acute hypertonic saline infusion resulted in a distinct blood pressure response: we observed a significant 15% decrease in control mice whereas blood pressure did not change in Ext1^+/-Ext2^+/- mice. This differential blood pressure response may be explained by the reduced capacity for sodium storage and/or the impaired vasodilation response, as measured by wire myography, which was observed in Ext1^+/-Ext2^+/- mice. Together, these data demonstrate that defective heparan sulfate glycosaminoglycan synthesis leads to abnormal sodium and water homeostasis and an abnormal response to sodium loading, most likely caused by inadequate capacity for local sodium storage.