The participation of bone in total body sodium metabolism in the rat

Hyponatremia and aging-related diseases: key player or innocent bystander? A systematic review

BACKGROUND

Hyponatremia is frequent in older age; whether it is a key player, a surrogate marker, or an innocent bystander in age-related diseases is still unclear.

OBJECTIVE

To understand the role of hyponatremia in falls, osteoporosis, fractures, and cognitive impairment in old patients.

METHOD

Eligibility criteria for study inclusions were: written in English, peer-reviewed observational and intervention studies, clinical trial, prospective and retrospective controlled cohort studies, and case-controlled studies without limitations regarding the date of publication.

INFORMATION SOURCES

Protocol available on the International Prospective Register of Systematic Reviews (PROSPERO, CRD42021218389). MEDLINE, Embase, and PsycINFO were searched. Final search done on August 8, 2021. Risk-of-bias assessment: Risk-of-Bias Assessment tool for Non-randomized Studies (RoBANS) and the Bradford Hill's criteria for causality.

RESULTS

Includes studies: One-hundred thirty-five articles retained for the revision. Synthesis of results - Falls: Eleven studies were included. Strong association between hyponatremia and falls in all the studies was found. Osteoporosis and fractures: nineteen articles were included. The association between hyponatremia and osteoporosis is unclear. Cognitive impairment: Five articles were included. No association between hyponatremia and cognitive impairment was found.

DISCUSSION

Interpretation: Falls, osteoporosis, and fractures are multifactorial. Hyponatremia is not temporally related with the outcomes; we suggest that hyponatremia may be regarded as a marker of unhealthy aging and a confounder instead of a causal factor or an innocent bystander for falls and fractures. Concerning cognitive impairment, there are no evidence supporting a real role of hyponatremia to be regarded as an innocent bystander in neurodegeneration.

A biokinetic model for systemic sodium

This paper describes an updated biokinetic model for systemic sodium (Na), developed for use in a series of reports by the International Commission on Radiological Protection (ICRP) on occupational intake of radionuclides. In contrast to the ICRP's previous model for intake of radio-sodium by workers, the updated model depicts realistic directions of movement of Na in the body including recycling of activity between blood and tissues. The updated model structure facilitates extension of the baseline transfer coefficients for adults to different age groups and to special exposure scenarios such as transfer of radio-sodium from the mother to the foetus or the nursing infant. Dose coefficients for²²Na and²⁴Na based on the updated model generally do not differ greatly from those based on the ICRP's previous Na model when both models are connected to the ICRP's latest dosimetry system. The main exception is that the updated model yields roughly twofold higher dose coefficients for endosteal bone surface than does the previous model due to the dosimetrically cautious assumption in the updated model that exchangeable Na in bone resides on bone surface.

Magnetic resonance imaging determination of tissue sodium in patients with chronic kidney disease

Excess sodium is a major modifiable contributor to hypertension and cardiovascular risk. Knowledge of sodium storage and metabolism has derived mainly from indirect measurements of dietary sodium intake and urinary sodium excretion, however both attempt to measure body sodium and fluid in a two-compartment model of intracellular and extracellular spaces. Our understanding of total body sodium has recently included a storage pool in tissues. In the last two decades, sodium-23 magnetic resonance imaging (²³ Na MRI) has allowed dynamic quantification of tissue sodium in vivo. Tissue sodium is independently associated with cardiovascular dysfunction and inflammation. This review explores (i) The revolution of our understanding of sodium physiology, (ii) The development and potential clinical adoption of ²³ Na MRI to provide improved measurement of total body sodium in CKD and (iii) How we can better understand mechanistic and clinical implications of tissue sodium in hypertension, cardiovascular disease and immune dysregulation, especially in the CKD population.

Chronic hyponatremia in the elderly: association with geriatric syndromes (review of literature)

Hyponatremia is more common in the elderly. A number of studies suggest that even mild chronic hyponatremia is a serious danger, increasing the risks of developing geriatric syndromes, falls, low bone density, fractures, cognitive impairment, and death of all causes. Although the mechanism for the development of such complications is currently not completely clear. Questions remain about the need and methods for correcting this condition, although there is some evidence that the correction of hyponatremia improves cognitive functions, postural balance, minimizing the risk of falls and fractures. These issues are addressed in this review of the literature.

Tissue sodium concentrations in chronic kidney disease and dialysis patients by lower leg sodium-23 magnetic resonance imaging

BACKGROUND

Sodium-23 magnetic resonance imaging (23Na MRI) allows direct measurement of tissue sodium concentrations. Current knowledge of skin, muscle and bone sodium concentrations in chronic kidney disease (CKD) and renal replacement therapy patients is limited. In this study we measured the tissue sodium concentrations in CKD, hemodialysis (HD) and peritoneal dialysis (PD) patients with 23Na MRI of the lower leg and explored their correlations with established clinical biomarkers.

METHODS

Ten healthy controls, 12 CKD Stages 3-5, 13 HD and 10 PD patients underwent proton and 23Na MRI of the leg. The skin, soleus and tibia were segmented manually and tissue sodium concentrations were measured. Plasma and serum samples were collected from each subject and analyzed for routine clinical biomarkers. Tissue sodium concentrations were compared between groups and correlations with blood-based biomarkers were explored.

RESULTS

Tissue sodium concentrations in the skin, soleus and tibia were higher in HD and PD patients compared with controls. Serum albumin showed a strong, negative correlation with soleus sodium concentrations in HD patients (r = -0.81, P < 0.01). Estimated glomerular filtration rate showed a negative correlation with tissue sodium concentrations (soleus: r = -0.58, P < 0.01; tibia: r = -0.53, P = 0.01) in merged control-CKD patients. Hemoglobin was negatively correlated with tissue sodium concentrations in CKD (soleus: r = -0.65, P = 0.02; tibia: r = -0.73, P < 0.01) and HD (skin: r = -0.60, P = 0.04; tibia: r = -0.76, P < 0.01).

CONCLUSION

Tissue sodium concentrations, measured by 23Na MRI, increase in HD and PD patients and may be associated with adverse metabolic effects in CKD and dialysis.

Chronic hyponatremia and association with osteoporosis among a large racially/ethnically diverse population

Chronic hyponatremia may contribute to decreased bone density. We studied 341,003 men and women who underwent DXA testing and observed that individuals with chronic hyponatremia (sodium < 135 mEq/L) had an 11% greater likelihood of having osteoporosis. There was a dose-dependent effect with lower sodium and stronger association with osteoporosis.

INTRODUCTION

Chronic hyponatremia has been associated with both neurologic deficits and increased risk of gait abnormalities leading to falls and resultant bone fractures. Whether chronic hyponatremia contributes to decreased bone density is uncertain. We evaluated whether chronic, mild hyponatremia based on serial sodium measurements was associated with increased risk of osteoporosis within a large, ethnically diverse population.

METHODS

This is a retrospective cohort study between January 1, 1998 and December 31, 2014 within Kaiser Permanente Southern California, an integrated healthcare delivery system. Men and women were aged ≥ 55 years with ≥ 2 serum sodium measurements prior to dual-energy X-ray absorptiometry (DXA) testing. Time-weighted (TW) mean sodium values were calculated by using the proportion of time (weight) elapsed between sodium measurements and defined as < 135 mEq/L. Osteoporosis defined as any T-score value ≤ - 2.5 of lumbar spine, femoral neck, or hip.

RESULTS

Among 341,003 individuals with 3,330,903 sodium measurements, 11,539 (3.4%) had chronic hyponatremia and 151,505 (44.4%) had osteoporosis. Chronic hyponatremic individuals had an osteoporosis RR (95% CI) of 1.11 (1.09, 1.13) compared to those with normonatremia. A TW mean sodium increase of 3 mEq/L was associated with a lower risk of osteoporosis [adjusted RR (95% CI) 0.95 (0.93, 0.96)]. A similar association was observed when the arithmetic mean sodium value was used for comparison.

CONCLUSIONS

We observed a modest increase in risk for osteoporosis in people with chronic hyponatremia. There was also a graded association between higher TW mean sodium values and lower risk of osteoporosis. Our findings underscore the premise that chronic hyponatremia may lead to adverse physiological effects and responses which deserves better understanding.

Using Electrolyte Free Water Balance to Rationalize and Treat Dysnatremias

Dysnatremias or abnormalities in plasma [Na+] are often termed disorders of water balance, an unclear physiologic concept often confused with changes in total fluid balance. However, most clinicians clearly recognize that hypertonic or hypotonic gains or losses alter plasma [Na+], while isotonic changes do not modify plasma [Na+]. This concept can be conceptualized as the electrolyte free water balance (EFWB), which defines the non-isotonic components of inputs and outputs to determine their effect on plasma [Na+]. EFWB is mathematically proportional to the rate of change in plasma [Na+] (dPNa/dt) and, therefore, is actively regulated to zero so that plasma [Na+] remains stable at its homeostatic set point. Dysnatremias are, therefore, disorders of EFWB and the relationship between EFWB and dPNa/dt provides a rationale for therapeutic strategies incorporating mass and volume balance. Herein, we leverage dPNa/dt as a desired rate of correction of plasma [Na+] to define a stepwise approach for the treatment of dysnatremias.

Structure and thermal stability of sodium and carbonate-co-substituted strontium hydroxyfluorapatites

Physicochemical characterization of sodium and carbonate co-substituted strontium hydroxyfluorapatite resulting in pure apatite with ionic vacancies as a biomaterial.

3 Tesla 23Na Magnetic Resonance Imaging During Acute Kidney Injury

RATIONALE AND OBJECTIVES

Sodium and proton magnetic resonance imaging (²³Na/¹H-MRI) have shown that muscle and skin can store Na⁺ without water. In chronic renal failure and in heart failure, Na⁺ mobilization occurs, but is variable depending on age, dialysis vintage, and other features. Na⁺ storage depots have not been studied in patients with acute kidney injury (AKI).

MATERIALS AND METHODS

We studied 7 patients with AKI (mean age: 51.7 years; range: 25-84) and 14 age-matched and gender-matched healthy controls. All underwent ²³Na/¹H-MRI at the calf. Patients were studied before and after acute hemodialysis therapy within 5-6 days. The ²³Na-MRI produced grayscale images containing Na⁺ phantoms, which served to quantify Na⁺ contents. A fat-suppressed inversion recovery sequence was used to quantify H₂O content.

RESULTS

Plasma Na⁺ levels did not change. Mean Na⁺ contents in muscle and skin did not significantly change following four to five cycles of hemodialysis treatment (before therapy: 32.7 ± 6.9 and 44.2 ± 13.5 mmol/L, respectively; after dialysis: 31.7 ± 10.2 and 42.8 ± 11.8 mmol/L, respectively; P > .05). Water content measurements did not differ significantly before and after hemodialysis in muscle and skin (P > .05). Na⁺ contents in calf muscle and skin of patients before hemodialysis were significantly higher than in healthy subjects (16.6 ± 2.1 and 17.9 ± 3.2) and remained significantly elevated after hemodialysis.

CONCLUSIONS

Na⁺ in muscle and skin accumulates in patients with AKI and, in contrast to patients receiving chronic hemodialysis and those with acute heart failure, is not mobilized with hemodialysis within 5-6 days.

Hyponatremia and fractures: should hyponatremia be further studied as a potential biochemical risk factor to be included in FRAX algorithms?

The Fracture Risk Assessment Tool (FRAX®) was developed by the WHO Collaborating Centre for metabolic bone diseases to evaluate fracture risk of patients. It is based on patient models that integrate the risk associated with clinical variables and bone mineral density (BMD) at the femoral neck. The clinical risk factors included in FRAX were chosen to include only well-established and independent variables related to skeletal fracture risk. The FRAX tool has acquired worldwide acceptance despite having several limitations. FRAX models have not included biochemical derangements in estimation of fracture risk due to the lack of validation in large prospective studies. Recently, there has been an increasing number of studies showing a relationship between hyponatremia and the occurrence of fractures. Hyponatremia is the most frequent electrolyte abnormality measured in the clinic, and serum sodium concentration is a very reproducible, affordable, and readily obtainable measurement. Thus, we think that hyponatremia should be further studied as a biochemical risk factor for skeletal fractures prediction, particularly those at the hip which carries the greatest morbidity and mortality. To achieve this will require the collection of large patient cohorts from diverse geographical locations that include a measure of serum sodium in addition to the other FRAX variables in large numbers, in both sexes, over a wide age range and with wide geographical representation. It would also require the inclusion of data on duration and severity of hyponatremia. Information will be required both on the risk of fracture associated with the occurrence and length of exposure to hyponatremia and to the relationship with the other risk variables included in FRAX and also the independent effect on the occurrence of death which is increased by hyponatremia.

Hyponatremia and bone disease

Hip fractures represent a serious health risk in the elderly, causing substantial morbidity and mortality. There is now a considerable volume of literature suggesting that chronic hyponatremia increases the adjusted odds ratio (OR) for both falls and fractures in the elderly. Hyponatremia appears to contribute to falls and fractures by two mechanisms. First, it produces mild cognitive impairment, resulting in unsteady gait and falls; this is probably due to the loss of glutamate (a neurotransmitter involved in gait function) as an osmolyte during brain adaptation to chronic hyponatremia. Second, hyponatremia directly contributes to osteoporosis and increased bone fragility by inducing increased bone resorption to mobilize sodium stores in bone. Low extracellular sodium directly stimulates osteoclastogenesis and bone resorptive activity through decreased cellular uptake of ascorbic acid and the induction of oxidative stress; these effects occur in a sodium level-dependent manner. Hyponatremic patients have elevated circulating arginine-vasopressin (AVP) levels, and AVP acting on two receptors expressed in osteoblasts and osteoclasts, Avpr1α and Avpr2, can increase bone resorption and decrease osteoblastogenesis. Should we be screening for low serum sodium in patients with osteoporosis or assessing bone mineral density (BMD) in patients with hyponatremia? The answers to these questions have not been established. Definitive answers will require randomized controlled studies that allocate elderly individuals with mild hyponatremia to receive either active treatment or no treatment for hyponatremia, to determine whether correction of hyponatremia prevents gait disturbances and changes in BMD, thereby reducing the risk of fractures. Until such studies are conducted, physicians caring for elderly patients must be aware of the association between hyponatremia and bone disorders. As serum sodium is a readily available, simple, and affordable biochemical measurement, clinicians should look for hyponatremia in elderly patients, especially in those receiving medications that can cause hyponatremia. Furthermore, elderly patients with an unsteady gait and/or confusion should be evaluated for the presence of mild hyponatremia, and if present, treatment should be initiated. Finally, elderly patients presenting with an orthopedic injury should have serum sodium checked and hyponatremia corrected, if present.

Water Intake and NaCl Appetite in Sodium Depletion

Rats were sodium depleted by intraperitoneal dialysis (IPD) to determine: (a) whether there is a thirst mechanism not dependent on increases in effective osmotic pressure and (b) whether a sodium chloride appetite could be induced by this rapid means of sodium depletion. IPD resulted in increased intakes of water and 3% NaCl solution when these were tested independently (single-stimulus method) A second IPD performed after 2 days of recovery produced even greater increases in intake. When water and 3% NaCl were available simultaneously after both dialyses, the increased salt intake was shown to be the predominant effect. The increased water intake occurred in spite of cellular overhydration and decreased serum Na level.

Low extracellular sodium promotes adipogenic commitment of human mesenchymal stromal cells: a novel mechanism for chronic hyponatremia-induced bone loss

Hyponatremia represents an independent risk factor for osteoporosis and fractures, affecting both bone density and quality. A direct stimulation of bone resorption in the presence of reduced extracellular sodium concentrations ([Na(+)]) has been shown, but the effects of low [Na(+)] on osteoblasts have not been elucidated. We investigated the effects of a chronic reduction of extracellular [Na(+)], independently of osmotic stress, on human mesenchymal stromal cells (hMSC) from bone marrow, the common progenitor for osteoblasts and adipocytes. hMSC adhesion and viability were significantly inhibited by reduced [Na(+)], but their surface antigen profile and immuno-modulatory properties were not altered. In low [Na(+)], hMSC were able to commit toward both the osteogenic and the adipogenic phenotypes, as demonstrated by differentiation markers analysis. However, the dose-dependent increase in the number of adipocytes as a function of reduced [Na(+)] suggested a preferential commitment toward the adipogenic phenotype at the expense of osteogenesis. The amplified inhibitory effect on the expression of osteoblastic markers exerted by adipocytes-derived conditioned media in low [Na(+)] further supported this observation. The analysis of cytoskeleton showed that low [Na(+)] were associated with disruption of tubulin organization in hMSC-derived osteoblasts, thus suggesting a negative effect on bone quality. Finally, hMSC-derived osteoblasts increased their expression of factors stimulating osteoclast recruitment and activity. These findings confirm that hyponatremia should be carefully taken into account because of its negative effects on bone, in addition to the known neurological effects, and indicate for the first time that impaired osteogenesis may be involved.

Mild prolonged chronic hyponatremia and risk of hip fracture in the elderly

BACKGROUND

Hip fractures are among the most serious bone fractures in the elderly, producing significant morbidity and mortality. Several observational studies have found that mild hyponatremia can adversely affect bone, with fractures occurring as a potential complication. We examined if there is an independent association between prolonged chronic hyponatremia (>90 days duration) and risk of hip fracture in the elderly.

METHODS

We performed a retrospective cohort study in adults >60 years of age from a prepaid health maintenance organization who had two or more measurements of plasma sodium between 2005 and 2012. The incidence of hip fractures was assessed in a very restrictive population: subjects with prolonged chronic hyponatremia, defined as plasma sodium values <135 mmol/L, lasting >90 days. Multivariable Cox regression was performed to determine the hazard ratio (HR) for hip fracture risk associated with prolonged chronic hyponatremia after adjustment for the propensity to have hyponatremia, fracture risk factors and relevant baseline characteristics.

RESULTS

Among 31 527 eligible patients, only 228 (0.9%) had prolonged chronic hyponatremia. Mean plasma sodium was 132 ± 5 mmol/L in hyponatremic patients and 139 ± 3 mmol/L in normonatremic patients (P < 0.001). The absolute risk for hip fracture was 7/282 in patients with prolonged chronic hyponatremia and 411/313 299 in normonatremic patients. Hyponatremic patients had a substantially elevated rate of hip fracture [adjusted HR 4.52 (95% CI 2.14-9.6)], which was even higher in those with moderate hyponatremia (<130 mmol/L) [adjusted HR 7.61 (95% CI 2.8-20.5)].

CONCLUSION

Mild prolonged chronic hyponatremia is independently associated with hip fracture risk in the elderly population, although the absolute risk is low. However, proof that correcting hyponatremia will result in a reduction of hip fractures is lacking.

Magnetic resonance-determined sodium removal from tissue stores in hemodialysis patients

We have previously reported that sodium is stored in skin and muscle. The amounts stored in hemodialysis (HD) patients are unknown. We determined whether (23)Na magnetic resonance imaging (sodium-MRI) allows assessment of tissue sodium and its removal in 24 HD patients and 27 age-matched healthy controls. We also studied 20 HD patients before and shortly after HD with a batch dialysis system with direct measurement of sodium in dialysate and ultrafiltrate. Age was associated with higher tissue sodium content in controls. This increase was paralleled by an age-dependent decrease of circulating levels of vascular endothelial growth factor-C (VEGF-C). Older (>60 years) HD patients showed increased sodium and water in skin and muscle and lower VEGF-C levels compared with age-matched controls. After HD, patients with low VEGF-C levels had significantly higher skin sodium content compared with patients with high VEGF-C levels (low VEGF-C: 2.3 ng/ml and skin sodium: 24.3 mmol/l; high VEGF-C: 4.1 ng/ml and skin sodium: 18.2 mmol/l). Thus, sodium-MRI quantitatively detects sodium stored in skin and muscle in humans and allows studying sodium storage reduction in ESRD patients. Age and VEGF-C-related local tissue-specific clearance mechanisms may determine the efficacy of tissue sodium removal with HD. Prospective trials on the relationship between tissue sodium content and hard end points could provide new insights into sodium homeostasis, and clarify whether increased sodium storage is a cardiovascular risk factor.

Regulation of bone remodeling by vasopressin explains the bone loss in hyponatremia

Although hyponatremia is known to be associated with osteoporosis and a high fracture risk, the mechanism through which bone loss ensues has remained unclear. As hyponatremic patients have elevated circulating arginine-vasopressin (AVP) levels, we examined whether AVP can affect the skeleton directly as yet another component of the pituitary-bone axis. Here, we report that the two Avp receptors, Avpr1α and Avpr2, coupled to Erk activation, are expressed in osteoblasts and osteoclasts. AVP injected into wild-type mice enhanced and reduced, respectively, the formation of bone-resorbing osteoclasts and bone-forming osteoblasts. Conversely, the exposure of osteoblast precursors to Avpr1α or Avpr2 antagonists, namely SR49059 or ADAM, increased osteoblastogenesis, as did the genetic deletion of Avpr1α. In contrast, osteoclast formation and bone resorption were both reduced in Avpr1α(-/-) cultures. This process increased bone formation and reduced resorption resulted in a profound enhancement of bone mass in Avpr1α(-/-) mice and in wild-type mice injected with SR49059. Collectively, the data not only establish a primary role for Avp signaling in bone mass regulation, but also call for further studies on the skeletal actions of Avpr inhibitors used commonly in hyponatremic patients.

Fracture risk after thiazide-associated hyponatraemia

BACKGROUND/AIM

Although thiazide-type diuretics can promote a positive calcium balance, thiazide can be associated with hyponatraemia, which is recently linked with heightened fracture risk. We examine the chance of developing fracture in patients with and without hyponatraemia after taking thiazide diuretics.

METHODS

In this single-centre retrospective study, we followed up a previously published cohort of patients with (n= 223) and without (n= 216) thiazide-induced hyponatraemia.

RESULTS

A total of 61 osteoporotic fractures was recorded during a mean follow-up period of 82 months. Using univariate regression analysis, the hazard ratio of thiazide-induced hyponatraemia was 1.78 (95% confidence interval (CI), 1.05-3.03; P= 0.033). Cox proportional hazards regression analysis, however, showed that age, body mass index and diabetes mellitus were the only independent predictors of osteoporotic fractures. No association of a history of thiazide-induced hyponatraemia and risk of fracture was evident in the final model.

CONCLUSION

Since a history of thiazide-induced hyponatraemia is associated with osteoporotic fracture in univariate but not multivariate analyses, an alternative explanation is that confounding factors of older age and low body mass index accounted for the apparently increased risk of osteoporotic fracture in patients with thiazide-induced hyponatraemia.

Is chronic hyponatremia a novel risk factor for hip fracture in the elderly?

Hip fractures represent a serious health risk in the elderly, with significant associated morbidity and mortality. There is now an emerging literature that suggests that chronic hyponatremia increases the adjusted odds ratio (OR) for both falls and fractures in the elderly. Hyponatremia appears to contribute to falls and fractures by two mechanisms: (i) it produces mild cognitive impairment resulting in unsteady gait and falls and (ii) it directly contributes to osteoporosis and increased bone fragility by inducing increased bone resorption to mobilize sodium. There is debate over the effect of hyponatremia on the production of osteoporosis, as one study found decreased bone mineral density (BMD) and another did not. Should we be screening for low serum sodium in patients with osteoporosis or assessing BMD in patients with hyponatremia? The final answer is yet to come from prospective studies that allocate elderly individuals with mild hyponatremia to receive active treatment or not for hyponatremia and see if this intervention prevents gait disturbances and changes in BMD reducing fracture risk. In the meantime, physicians caring for elderly patients must be aware of the association between hyponatremia and bone problems. As serum sodium is a readily available, simple and affordable biochemical measurement, clinicians should look for hyponatremia in elderly patients who take medications that can cause hyponatremia. Also, elderly patients with unsteady gait and/or confusion should be checked for the presence of mild hyponatremia and if present it should not be ignored. Finally, elderly patients presenting with an orthopedic injury should have serum sodium checked and corrected if hyponatremia is present.

Syndrome of inappropriate ADH secretion and severe osteoporosis

OBJECTIVE

Recent studies indicate an association between hyponatremia and osteoporosis. We report a clinical case that supports this statement.

CASE REPORT

A 36-yr-old man was diagnosed with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) at the age of 22. In the years that followed, sodium levels in the blood remained low from 111-130 mmol/liter (137-144). At the age of 34, he was diagnosed with osteoporosis after a magnetic resonance imaging scan showed moderate compression fractures at Th9-11 and L2. A dual energy x-ray absorptiometry scan showed a Z-score of -3.9 at the lumbar spine (L3-L4) and a Z-score of -1.3 in the total hip. He had no other known risk factors for osteoporosis. Urinary excretion of calcium and sodium were elevated. Plasma vasopressin level was inappropriately elevated. One year after treatment with 5-mg zoledronic acid, there has been no significant change in the bone mineral density in the lumbar spine and a small increase in bone mineral density in the total hip. The symptoms from SIADH have not changed.

CONCLUSION

The case illustrates that severe idiopathic SIADH and chronic hyponatremia can have severe side effects on bone metabolism and can lead to secondary osteoporosis. We support the recommendation that patients with chronic SIADH should be screened for osteoporosis and suggest that early bone protective treatment should be considered to reduce or delay osteoporosis onset.

Hyponatraemia: more than just a marker of disease severity?

Hyponatraemia--the most common serum electrolyte disorder--has also emerged as an important marker of the severity and prognosis of important diseases such as heart failure and cirrhosis. Acute hyponatraemia can cause severe encephalopathy, but the rapid correction of chronic hyponatraemia can also profoundly impair brain function and even cause death. With the expanding elderly population and the increased prevalence of hyponatraemia in this segment of society, prospective studies are needed to examine whether correcting hyponatraemia in the elderly will diminish cognitive impairment, improve balance and reduce the incidence of falls and fractures. Given that polypharmacy is also common in the elderly population, the various medications that may stimulate arginine vasopressin release and/or enhance the hormone's action to increase water absorption must also be taken into consideration. Whether hyponatraemia in a patient with cancer is merely a marker of poor prognosis or whether its presence may alter the patient's quality of life remains to be examined. In any case, hyponatraemia can no longer be considered as just a biochemical bystander in the ill patient. A systematic diagnostic approach is necessary to determine the specific aetiology of a patient's hyponatraemia. Therapy must then be dictated not only by recognized reversible causes such as advanced hypothyroidism, adrenal insufficiency, diuretics or other medicines, but also by whether the hyponatraemia occurred acutely or chronically. Information is emerging that the vast majority of cases of hyponatraemia are caused by the nonosmotic release of arginine vasopressin. Now that vasopressin V2-receptor blockers are available, a new era of clinical investigation is necessary to examine whether hyponatraemia is just a marker of severe disease or whether correction of hyponatraemia could improve a patient's quality of life. Such an approach must involve prospective randomized studies in different groups of patients with hyponatraemia, including those with advanced heart failure, those with cirrhosis, patients with cancer, and the elderly.

Internal sodium balance in DOCA-salt rats: a body composition study

The idea that Na(+) retention inevitably leads to water retention is compelling; however, were Na(+) accumulation in part osmotically inactive, regulatory alternatives would be available. We speculated that in DOCA-salt rats Na(+) accumulation is excessive relative to water. Forty female Sprague-Dawley rats were divided into four subgroups. Groups 1 and 2 (controls) received tap water or 1% saline (salt) for 5 wk. Groups 3 and 4 received subcutaneous DOCA pellets and tap water or salt. Na(+), K(+), and water were measured in skin, bone, muscle, and total body by desiccation and consecutive dry ashing. DOCA-salt led to total body Na(+) excess (0.255 +/- 0.022 vs. 0.170 +/- 0.010 mmol/g dry wt; P < 0.001), whereas water retention was only moderate (0.685 +/- 0.119 vs. 0.648 +/- 0.130 ml/g wet wt; P < 0.001). Muscle Na(+) retention (0.220 +/- 0.029 vs. 0.145 +/- 0.021 mmol/g dry wt; P < 0.01) in DOCA-salt was compensated by muscle K(+) loss, indicating osmotically neutral Na(+)/K(+) exchange. Skin Na(+) retention (0.267 +/- 0.049 vs. 0.152 +/- 0.014 mmol/g dry wt; P < 0.001) in DOCA-salt rats was not balanced by K(+) loss, indicating osmotically inactive skin Na(+) storage. We conclude that DOCA-salt leads to tissue Na(+) excess relative to water. The relative Na(+) excess is achieved by two distinct mechanisms, namely, osmotically inactive Na(+) storage and osmotically neutral Na(+) retention balanced by K(+) loss. This "internal Na(+) escape" allows the maintenance of volume homeostasis despite increased total body Na(+).

Reduced osmotically inactive Na storage capacity and hypertension in the Dahl model

Recent evidence suggested that Na can be stored in an osmotically inactive form. We investigated whether osmotically inactive Na storage is reduced in a rat model of salt-sensitive (SS) hypertension. SS and salt-resistant (SR) Dahl-Rapp rats as well as Sprague-Dawley (SD) rats were fed a high (8%)- or low (0.1%)-NaCl diet for 4 wk (n = 10/group). Mean arterial pressure (MAP) was measured at the end of the experiment. Wet and dry weights, water content, total body Na (TBS), and bone Na content were measured by dessication and dry ashing. MAP was higher in both Dahl strains than in SD rats. In SS rats, 8% NaCl led to Na accumulation, water retention, and hypertension due to impaired renal Na excretion. There was no dietary-induced Na retention in SR and SD rats. TBS was variable; nevertheless, TBS was significantly correlated with body water and MAP in all strains. However, the extent of Na-associated volume and MAP increases was strain specific. Osmotically inactive Na in SD rats was threefold higher than in SS and SR rats. Both SS and SR Dahl rat strains displayed reduced osmotically inactive Na storage capacity compared with SD controls. A predisposition to fluid accumulation and high blood pressure results from this alteration. Additional factors, including impaired renal Na excretion, probably contribute to hypertension in SS rats. Our results draw attention to the role of osmotically inactive Na storage.

The effect of normal development and of severe undernutrition on some minor components of cortical bone

1. The amounts of calcium, magnesium, sodium and citric acid in the bones of undernourished pigs 1 year old were compared with the amounts in the bones of smaller newborn animals, normal animals of the same weight aged 4 weeks and of the same age weighing 170kg. 2. The differences that were found between 4 weeks and 1 year of age in the normal animals were expected as effects of aging. However, between birth and 4 weeks of age the changes in composition were in the opposite direction to those between 4 weeks and 1 year. 3. Undernutrition produced a bone that resembled chemically that of an animal 1 year old.