Nephrol Dial Transplant (2016) 31: 1078–1081 doi: 10.1093/ndt/gfv343 Advance Access publication 25 September 2015
Balancing wobbles in the body sodium Jens Titze1,2, Natalia Rakova1,4, Christoph Kopp1, Anke Dahlmann1, Jonathan Jantsch3 and Friedrich C. Luft2,4 1
Interdisciplinary Center for Clinical Research and Department for Nephrology and Hypertension, Nikolaus-Fiebiger Center for Molecular
Medicine, Nashville, TN, USA, 3Institute of Clinical Microbiology and Hygiene, Universitätsklinikum Regensburg and Universität Regensburg, Regensburg, Germany and 4Experimental and Clinical Research Center, an institutional cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany
Correspondence and offprint requests to: Friedrich C. Luft; E-mail: [email protected]
FULL REVIEW
A B S T R AC T
Astronomers rely on ‘wobbles’, consistent recurrent deviations from the predicted, to discover the astounding. Through wobbles, planets in our solar system, and now planets in other solar systems, have been found. Our observations were made through wobbles, namely numbers that did not add up in the available equations. Current teaching states that when sodium intake is increased from low to high levels, total body sodium (TBNa) and water increase until daily sodium excretion again equals intake. When sodium intake is reduced, sodium excretion briefly exceeds intake until the excess TBNa and water are eliminated, at which point sodium excretion again equals intake. Body weight changes parallel sodium intake. Our doubts were raised when we were unable to verify these conclusions on the basis of a long-term spaceflight simulation in which subjects collected all urine made and in which we knew precisely how much salt (NaCl) they had eaten. We learned that sodium excretion failed to precisely correspond to intake, but instead meandered even if intake was known. Aldosterone excretion was related. More disturbingly, TBNa (sodium in minus sodium out) corresponded to nothing, including body weight, even though TBNa went up and down in the subjects independent of body weight and blood pressure [1]. A short check disclosed other studies in which sodium accumulation and body weight did not coincide [2]. Studies examining extremes of sodium intake also did not really add up, but the difference was generally brushed off as variability and faulty subject compliance [3]. But then again, perhaps the data are not the problem, possibly the hypothesis is all wet. Our group has conducted studies (2000–present) exploring the discrepancies. Through ashing and atomic absorption spectrometry of cations, we found that sodium resides largely in the skin, presumably bound to negatively charged proteoglycans, as well as elsewhere as reviewed recently [4]. We found in the course of these studies that immune cells regulate third-space sodium
Sodium balance is achieved within a matter of days and everything that enters should come out; sodium stores are of questionable relevance and sodium accumulation is accompanied by weight gain. Careful balance studies oftentimes conflicted with this view, and long-term studies suggested that total body sodium (TBNa) fluctuates independent of intake or body weight. We recently performed the opposite experiment in that we fixed sodium intake for weeks at three levels of sodium intake and collected all urine made. We found weekly (circaseptan) patterns in sodium excretion that were inversely related to aldosterone and directly related to cortisol. TBNa was not dependent on sodium intake, but instead exhibited far longer (greater than or equal to monthly) infradian rhythms independent of extracellular water, body weight or blood pressure. To discern the mechanisms further, we delved into sodium magnetic resonance imaging (NaMRI) to identify sodium storage clinically. We found that sodium stores are greater in men than in women, increase with age and are higher in hypertensive than normotensive persons. We have suggestive evidence that these sodium stores can be mobilized, also in dialysis patients. The observations are in accordance with our findings that immune cells regulate a hypertonic interface in the skin interstitium that could serve as a protective barrier. Returning to our balance studies, we found that due to biological variability in 24-h sodium excretion, collecting urine for a day could not separate 12, 9 or 6 g/day sodium intakes with the precision of tossing a coin. Every other daily urine sampling correctly classified a 3-g difference in salt intake less than half the time, making the gold standard 24-h urine collection of little value in predicting salt intake. We suggest that wobbles in expected outcomes can lead to novel clinical insights even with respect to banal salt questions. Keywords: hemodialysis, hypertension, magnetic resonance imaging, salt, sodium © The Author 2015. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved.
1078
Downloaded from http://ndt.oxfordjournals.org/ at Serials Dept / Cornell University Medical Library on September 29, 2016
Medicine, Friedrich-Alexander-University, Erlangen-Nürnberg, Germany, 2Division of Clinical Pharmacology, Vanderbilt University School of
1079
FULL REVIEW
Balancing wobbles in the body sodium
[9]. We have observed that anaerobic exercise increases the sodium content of skeletal muscle, whereas aerobic exercise does not [14]. Skin sodium was not affected by exercise. We also found that sodium accumulates at the site of bacterial skin infection in humans and in mice. This finding prompted us to raise the hypothesis that perhaps sodium accumulation and the resultant local increase in osmolality could serve as an innate protection against invaders. We used the protozoan parasite Leishmania major as a model of skin-prone macrophage infection to test the hypothesis that skin sodium storage facilitates antimicrobial host defense [15]. Activation of macrophages in the presence of high salt concentrations modified epigenetic markers and enhanced p38 mitogen-activated protein kinase-dependent TonEBP/NFAT5 activation. The highsalt response resulted in elevated type 2 nitric oxide synthase (Nos2)–dependent NO production and improved L. major control. We then studied mice conditionally deficient in NFAT5 in myeloid cells. We observed that increasing sodium content in the skin by a high-salt diet boosted activation of macrophages in an NFAT5-dependent manner and promoted cutaneous antimicrobial defense. We suggested that the hypertonic microenvironment could serve as a barrier to infection. This possibility would supply a teleological explanation of why sodium is stored in skin. The idea has not escaped us that the kidney and the skin harbor similarities in terms of osmotic gradient generation. Both contain looping vascular structures forming hairpins. Such hairpins raise the possibility that a countercurrent mechanism exists. Preliminary evidence suggests that the skin is a functional, kidney-like countercurrent system. Dermal vascular countercurrents could multiply an electrolyte concentration gradient, which is presumably initiated in the keratinocyte layer. Lymph capillaries could serve as a tubular/urethra-like drainage system of the hypertonic subepidermal fluid layer. On the basis of our own data and information accrued from the literature, we have modeled the skin in terms of Gibbs’ energy, electrical potential and osmotic pressure calculations [16]. We suggest that the skin interstitium concentrates electrolytes and thereby may provide a physiological barrier, which induces a continuous solvent drag for water, very much like the renal medullary interstitium. The pathophysiological function of this ‘Henle loop’ is not well characterized, although presumably a hypertonic electrolyte fluid barrier under the skin could coincide with hypertension and may represent a cardiovascular risk. Recently we have been confronted with our long-term balance data to answer the question of 24-h urine sodium collections and their utility in estimating salt intake. Our long-term balance Mars simulation flight gave us the opportunity to test separating 6, 9 and 12 g/day [17]. We relied not only on our 24-h urine specimens, but also reviewed the 27 279 individual servings our subjects ingested to accurately determine their sodium intake on each and every day. We found that 83% of all servings were consumed completely (as we had requested), 16.5% were incompletely eaten (confounding variable) and 0.5% were incompletely consumed (error). Urinary recovery of dietary salt was 92% of recorded intake, indicating long-term steady-state sodium balance. We defined a ±25 mmol deviation from the average difference between recorded sodium intake
Downloaded from http://ndt.oxfordjournals.org/ at Serials Dept / Cornell University Medical Library on September 29, 2016
(and presumably somehow chloride) storage via the tonicity element binding protein transcription factor (TonEBP/NFAT5) [5]. The findings indicated that local areas exist, particularly in skin at proteoglycan interfaces, where osmolality is higher than that in the plasma space. We confirmed these claims with independent separate techniques [6]. Other investigators subsequently expanded these hypotheses, implying a far-reaching immunological effect of our findings [7]. The observations imply a third space of sodium storage. We were quite smug about these discoveries. However, a more precise search of the literature disclosed that we were in error. Ivanova and colleagues reported almost 40 years ago that sodium is stored in skin [8]. They concluded that the increase in exchangeable sodium in the skin under the condition of a high-sodium regimen is, evidently, due to an increase in sulfated glycosaminoglycans. But the findings go back even further. Wahlgren noted >100 years ago that skin is a site of chloride storage and accounted for 30% of chloride in the body [9]. He could not measure sodium accurately because the flame photometer was not invented; that technology came 50 years later. Can we see this phenomenon in patients? To that end, we established parallel studies to accomplish just that. We developed magnetic resonance imaging of sodium (Na-MRI) for animals and humans. As a matter of fact, MRI allows us to measure sodium and water content simultaneously. The sodium images do not have the same striking resolution quality as proton images. After all, there are a lot more hydrogen than sodium atoms in the body. However, they allow accurate determinations even as required by controlled ashing and absorption spectrometry [10]. We have expanded these studies to cohorts of normal men and women, persons with essential hypertension [11] and hemodialysis patients [12]. We detected higher sodium stores in men compared with women. Hypertensive persons of both sexes had higher sodium stores than normotensive persons. Sodium stores increased with age in normotensive and hypertensive men and women. We elected to investigate hemodialysis patients because they should have a very obvious problem with sodium and sodium storage [12]. Furthermore, they rely on dialysis treatment for almost all their sodium removal. We again found that age was associated with higher tissue sodium content. This increase was paralleled by an age-dependent decrease of circulating vascular endothelial growth factor C (VEGF-C) levels. After hemodialysis treatments, patients with low VEGF-C levels had significantly higher skin sodium content compared with patients with high VEGF-C levels. The circulating VEGF-C antagonist soluble Flt4 largely behaved conversely. Our dialysis study showed that sodium stores could be mobilized with dialysis treatment and suggested possible regulators. We are aware that clinically important changes in the serum sodium concentration (hypernatremia/hyponatremia) are also reflected in changes in tissue sodium stores [13]. Such findings could perhaps explain why equated predictions of serum sodium change with therapy oftentimes are misleading. Our Na-MRI technique allows differentiating sodium stores in the skin from those in muscle. Muscle also acts as a sizeable storage site. Interestingly, the second-highest chloride depot found by Wahlgren (18%) was located in skeletal muscle
FULL REVIEW
intake was higher than the recorded because of subject preference. When daily recorded sodium intake and the average difference between sodium intake and UNaV due to extrarenal sodium loss is taken into account (Bland–Altman plots), only every other UNaV collection accurately measures sodium intake within the 3-g prediction interval. Repetitive collections reduce the number of UNaV misclassifications of
Balancing wobbles in the body sodium Jens Titze1,2, Natalia Rakova1,4, Christoph Kopp1, Anke Dahlmann1, Jonathan Jantsch3 and Friedrich C. Luft2,4 1
Interdisciplinary Center for Clinical Research and Department for Nephrology and Hypertension, Nikolaus-Fiebiger Center for Molecular
Medicine, Nashville, TN, USA, 3Institute of Clinical Microbiology and Hygiene, Universitätsklinikum Regensburg and Universität Regensburg, Regensburg, Germany and 4Experimental and Clinical Research Center, an institutional cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany
Correspondence and offprint requests to: Friedrich C. Luft; E-mail: [email protected]
FULL REVIEW
A B S T R AC T
Astronomers rely on ‘wobbles’, consistent recurrent deviations from the predicted, to discover the astounding. Through wobbles, planets in our solar system, and now planets in other solar systems, have been found. Our observations were made through wobbles, namely numbers that did not add up in the available equations. Current teaching states that when sodium intake is increased from low to high levels, total body sodium (TBNa) and water increase until daily sodium excretion again equals intake. When sodium intake is reduced, sodium excretion briefly exceeds intake until the excess TBNa and water are eliminated, at which point sodium excretion again equals intake. Body weight changes parallel sodium intake. Our doubts were raised when we were unable to verify these conclusions on the basis of a long-term spaceflight simulation in which subjects collected all urine made and in which we knew precisely how much salt (NaCl) they had eaten. We learned that sodium excretion failed to precisely correspond to intake, but instead meandered even if intake was known. Aldosterone excretion was related. More disturbingly, TBNa (sodium in minus sodium out) corresponded to nothing, including body weight, even though TBNa went up and down in the subjects independent of body weight and blood pressure [1]. A short check disclosed other studies in which sodium accumulation and body weight did not coincide [2]. Studies examining extremes of sodium intake also did not really add up, but the difference was generally brushed off as variability and faulty subject compliance [3]. But then again, perhaps the data are not the problem, possibly the hypothesis is all wet. Our group has conducted studies (2000–present) exploring the discrepancies. Through ashing and atomic absorption spectrometry of cations, we found that sodium resides largely in the skin, presumably bound to negatively charged proteoglycans, as well as elsewhere as reviewed recently [4]. We found in the course of these studies that immune cells regulate third-space sodium
Sodium balance is achieved within a matter of days and everything that enters should come out; sodium stores are of questionable relevance and sodium accumulation is accompanied by weight gain. Careful balance studies oftentimes conflicted with this view, and long-term studies suggested that total body sodium (TBNa) fluctuates independent of intake or body weight. We recently performed the opposite experiment in that we fixed sodium intake for weeks at three levels of sodium intake and collected all urine made. We found weekly (circaseptan) patterns in sodium excretion that were inversely related to aldosterone and directly related to cortisol. TBNa was not dependent on sodium intake, but instead exhibited far longer (greater than or equal to monthly) infradian rhythms independent of extracellular water, body weight or blood pressure. To discern the mechanisms further, we delved into sodium magnetic resonance imaging (NaMRI) to identify sodium storage clinically. We found that sodium stores are greater in men than in women, increase with age and are higher in hypertensive than normotensive persons. We have suggestive evidence that these sodium stores can be mobilized, also in dialysis patients. The observations are in accordance with our findings that immune cells regulate a hypertonic interface in the skin interstitium that could serve as a protective barrier. Returning to our balance studies, we found that due to biological variability in 24-h sodium excretion, collecting urine for a day could not separate 12, 9 or 6 g/day sodium intakes with the precision of tossing a coin. Every other daily urine sampling correctly classified a 3-g difference in salt intake less than half the time, making the gold standard 24-h urine collection of little value in predicting salt intake. We suggest that wobbles in expected outcomes can lead to novel clinical insights even with respect to banal salt questions. Keywords: hemodialysis, hypertension, magnetic resonance imaging, salt, sodium © The Author 2015. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved.
1078
Downloaded from http://ndt.oxfordjournals.org/ at Serials Dept / Cornell University Medical Library on September 29, 2016
Medicine, Friedrich-Alexander-University, Erlangen-Nürnberg, Germany, 2Division of Clinical Pharmacology, Vanderbilt University School of
1079
FULL REVIEW
Balancing wobbles in the body sodium
[9]. We have observed that anaerobic exercise increases the sodium content of skeletal muscle, whereas aerobic exercise does not [14]. Skin sodium was not affected by exercise. We also found that sodium accumulates at the site of bacterial skin infection in humans and in mice. This finding prompted us to raise the hypothesis that perhaps sodium accumulation and the resultant local increase in osmolality could serve as an innate protection against invaders. We used the protozoan parasite Leishmania major as a model of skin-prone macrophage infection to test the hypothesis that skin sodium storage facilitates antimicrobial host defense [15]. Activation of macrophages in the presence of high salt concentrations modified epigenetic markers and enhanced p38 mitogen-activated protein kinase-dependent TonEBP/NFAT5 activation. The highsalt response resulted in elevated type 2 nitric oxide synthase (Nos2)–dependent NO production and improved L. major control. We then studied mice conditionally deficient in NFAT5 in myeloid cells. We observed that increasing sodium content in the skin by a high-salt diet boosted activation of macrophages in an NFAT5-dependent manner and promoted cutaneous antimicrobial defense. We suggested that the hypertonic microenvironment could serve as a barrier to infection. This possibility would supply a teleological explanation of why sodium is stored in skin. The idea has not escaped us that the kidney and the skin harbor similarities in terms of osmotic gradient generation. Both contain looping vascular structures forming hairpins. Such hairpins raise the possibility that a countercurrent mechanism exists. Preliminary evidence suggests that the skin is a functional, kidney-like countercurrent system. Dermal vascular countercurrents could multiply an electrolyte concentration gradient, which is presumably initiated in the keratinocyte layer. Lymph capillaries could serve as a tubular/urethra-like drainage system of the hypertonic subepidermal fluid layer. On the basis of our own data and information accrued from the literature, we have modeled the skin in terms of Gibbs’ energy, electrical potential and osmotic pressure calculations [16]. We suggest that the skin interstitium concentrates electrolytes and thereby may provide a physiological barrier, which induces a continuous solvent drag for water, very much like the renal medullary interstitium. The pathophysiological function of this ‘Henle loop’ is not well characterized, although presumably a hypertonic electrolyte fluid barrier under the skin could coincide with hypertension and may represent a cardiovascular risk. Recently we have been confronted with our long-term balance data to answer the question of 24-h urine sodium collections and their utility in estimating salt intake. Our long-term balance Mars simulation flight gave us the opportunity to test separating 6, 9 and 12 g/day [17]. We relied not only on our 24-h urine specimens, but also reviewed the 27 279 individual servings our subjects ingested to accurately determine their sodium intake on each and every day. We found that 83% of all servings were consumed completely (as we had requested), 16.5% were incompletely eaten (confounding variable) and 0.5% were incompletely consumed (error). Urinary recovery of dietary salt was 92% of recorded intake, indicating long-term steady-state sodium balance. We defined a ±25 mmol deviation from the average difference between recorded sodium intake
Downloaded from http://ndt.oxfordjournals.org/ at Serials Dept / Cornell University Medical Library on September 29, 2016
(and presumably somehow chloride) storage via the tonicity element binding protein transcription factor (TonEBP/NFAT5) [5]. The findings indicated that local areas exist, particularly in skin at proteoglycan interfaces, where osmolality is higher than that in the plasma space. We confirmed these claims with independent separate techniques [6]. Other investigators subsequently expanded these hypotheses, implying a far-reaching immunological effect of our findings [7]. The observations imply a third space of sodium storage. We were quite smug about these discoveries. However, a more precise search of the literature disclosed that we were in error. Ivanova and colleagues reported almost 40 years ago that sodium is stored in skin [8]. They concluded that the increase in exchangeable sodium in the skin under the condition of a high-sodium regimen is, evidently, due to an increase in sulfated glycosaminoglycans. But the findings go back even further. Wahlgren noted >100 years ago that skin is a site of chloride storage and accounted for 30% of chloride in the body [9]. He could not measure sodium accurately because the flame photometer was not invented; that technology came 50 years later. Can we see this phenomenon in patients? To that end, we established parallel studies to accomplish just that. We developed magnetic resonance imaging of sodium (Na-MRI) for animals and humans. As a matter of fact, MRI allows us to measure sodium and water content simultaneously. The sodium images do not have the same striking resolution quality as proton images. After all, there are a lot more hydrogen than sodium atoms in the body. However, they allow accurate determinations even as required by controlled ashing and absorption spectrometry [10]. We have expanded these studies to cohorts of normal men and women, persons with essential hypertension [11] and hemodialysis patients [12]. We detected higher sodium stores in men compared with women. Hypertensive persons of both sexes had higher sodium stores than normotensive persons. Sodium stores increased with age in normotensive and hypertensive men and women. We elected to investigate hemodialysis patients because they should have a very obvious problem with sodium and sodium storage [12]. Furthermore, they rely on dialysis treatment for almost all their sodium removal. We again found that age was associated with higher tissue sodium content. This increase was paralleled by an age-dependent decrease of circulating vascular endothelial growth factor C (VEGF-C) levels. After hemodialysis treatments, patients with low VEGF-C levels had significantly higher skin sodium content compared with patients with high VEGF-C levels. The circulating VEGF-C antagonist soluble Flt4 largely behaved conversely. Our dialysis study showed that sodium stores could be mobilized with dialysis treatment and suggested possible regulators. We are aware that clinically important changes in the serum sodium concentration (hypernatremia/hyponatremia) are also reflected in changes in tissue sodium stores [13]. Such findings could perhaps explain why equated predictions of serum sodium change with therapy oftentimes are misleading. Our Na-MRI technique allows differentiating sodium stores in the skin from those in muscle. Muscle also acts as a sizeable storage site. Interestingly, the second-highest chloride depot found by Wahlgren (18%) was located in skeletal muscle
FULL REVIEW
intake was higher than the recorded because of subject preference. When daily recorded sodium intake and the average difference between sodium intake and UNaV due to extrarenal sodium loss is taken into account (Bland–Altman plots), only every other UNaV collection accurately measures sodium intake within the 3-g prediction interval. Repetitive collections reduce the number of UNaV misclassifications of
