Low urine flow reduces the capacity to excrete a sodium load in humans

Chronic activation of vasopressin-2 receptors induces hypertension in Liddle mice by promoting Na+ and water retention

The renin-angiotensin-aldosterone and arginine vasopressin-V2 receptor-aquaporin-2 (AQP2) systems converge on the epithelial Na+ channel (ENaC) to regulate blood pressure and plasma tonicity. Although it is established that V2 receptors initiate renal water reabsorption through AQP2, whether V2 receptors can also induce renal Na+ retention through ENaC and raise blood pressure remains an open question. We hypothesized that a specific increase in V2 receptor-mediated ENaC activity can lead to high blood pressure. Our approach was to test effects of chronic activation of V2 receptors in Liddle mice, a genetic mouse model of high ENaC activity, and compare differences in ENaC activity, urine Na+ excretion, and blood pressure with control mice. We found that ENaC activity was elevated in Liddle mice and could not be stimulated further by administration of desmopressin (dDAVP), a V2 receptor-specific agonist. In contrast, Liddle mice showed higher levels of expression of AQP2 and aquaporin-3, but they could still respond to dDAVP infusion by increasing phospho-AQP2 expression. With dDAVP infusion, Liddle mice excreted smaller urine volume and less urine Na+ and developed higher blood pressure compared with control mice; this hypertension was attenuated with administration of the ENaC inhibitor benzamil. We conclude that V2 receptors contribute to hypertension in the Liddle mouse model by promoting primary Na+ and concomitant water retention.NEW & NOTEWORTHY Liddle syndrome is a classic model for hypertension from high epithelial Na+ channel (ENaC) activity. In the Liddle mouse model, vasopressin-2 receptors stimulate both ENaC and aquaporin-2, which increases Na+ and water retention to such an extent that hypertension ensues. Liddle mice will preserve plasma tonicity at the expense of a higher blood pressure; these data highlight the inherent limitation in which the kidney must use ENaC as a pathway to regulate both plasma tonicity and blood pressure.

The Influence of Salt Sensitivity Phenotype on Sodium Excretion and Diuresis: A Chrononutrition Pilot Study

BACKGROUND

Chrononutrition studies on interaction of diet/nutrients on endogenous circadian clocks and meal timing on metabolic homeostasis may be of importance in the management of nocturnal polyuria (NP), owing to loss of circadian rhythm in nighttime urination. Dietary salt restriction is an increasingly popular lifestyle recommendation for NP patients.

AIM

This study aims to evaluate the effect of an acute salt load on diuresis and to study the phenomenon of salt sensitivity. Methodology. Young, healthy men (n = 21, fasted and sober) ingested 500 ml of water on the control day and 8 g and 12 g of salt with water (500 ml) on two other days. Blood and urine samples were collected at 0 hrs, 2 hrs, and 4 hrs and voided volumes were recorded. Diuresis, serum and urine osmolality, sodium, potassium, urea, and creatinine were measured. Salt sensitivity was determined based on the rate of sodium excretion.

RESULTS

Compared to 8 g, ingestion of 12 g of salt significantly increased diuresis after 4 hrs. Pure water load induced fast diuresis, whereas salt and water load initially reduced diuresis and promoted late increase in diuresis. The total voided volume was significantly lower in the salt sensitive individuals.

CONCLUSION

Taken together, salt sensitivity profile and type and time of fluid intake are important considerations to build effective personalized lifestyle recommendations for NP patients, which needs further investigation.

Fluid-induced harm in the hospital: look beyond volume and start considering sodium. From physiology towards recommendations for daily practice in hospitalized adults

PURPOSE

Iatrogenic fluid overload is a potential side effect of intravenous fluid therapy in the hospital. Little attention has been paid to sodium administration as a separate cause of harm. With this narrative review, we aim to substantiate the hypothesis that a considerable amount of fluid-induced harm is caused not only by fluid volume, but also by the sodium that is administered to hospitalized patients.

METHODS

We show how a regular dietary sodium intake is easily surpassed by the substantial amounts of sodium that are administered during typical hospital stays. The most significant sodium burdens are caused by isotonic maintenance fluid therapy and by fluid creep, defined as the large volume unintentionally administered to patients in the form of dissolved medication. In a section on physiology, we elaborate on the limited renal handling of an acute sodium load. We demonstrate how the subsequent retention of water is an energy-demanding, catabolic process and how free water is needed to excrete large burdens of sodium. We quantify the effect size of sodium-induced fluid retention and discuss its potential clinical impact. Finally, we propose preventive measures, discuss the benefits and risks of low-sodium maintenance fluid therapy, and explore options for reducing the amount of sodium caused by fluid creep.

CONCLUSION

The sodium burdens caused by isotonic maintenance fluids and fluid creep are responsible for an additional and avoidable derailment of fluid balance, with presumed clinical consequences. Moreover, the handling of sodium overload is characterized by increased catabolism. Easy and effective measures for reducing sodium load and fluid retention include choosing a hypotonic rather than isotonic maintenance fluid strategy (or avoiding these fluids when enough free water is provided through other sources) and dissolving as many medications as possible in glucose 5%.

Biobehavioral variation in human water needs: How adaptations, early life environments, and the life course affect body water homeostasis

Body water homeostasis is critical for optimal physiological and cognitive function for humans. The majority of research has illustrated the negative biological consequences of failing to meet water needs. The human body has several mechanisms for detecting, regulating, and correcting body water deficits and excesses. However, variation exists in total water intake and how people meet those water needs as well as thirst thresholds and how well people tolerate water restriction. An evolutionary and developmental framework provides an underexplored perspective into human water needs by examining how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape these critical factors in body water homeostasis. This article first reviews biological and behavioral adaptations to water scarcity among animals and humans. It then examines human variation in water intake in a mostly water secure environment through the analysis of National Health and Nutrition Examination Survey dietary data and the link between water intake patterns and hydration biomarkers. Next, it reviews existing evidence of how maternal water restriction in utero and during lactation shape vasopressin release, thirst thresholds, drinking patterns, and body water homeostasis for the infant. Early life water restriction appears to have implications for hydration status, body size, and cardiovascular health. Finally, it examines how life course changes in health states and behaviors, including obesity, sleep, and parasitic infection, affect body water homeostasis. This article poses new questions about the plasticity and shaping of human water needs, thirst, and hydration behaviors.

Estimated Daily Urine Volume and Solute Excretion from Spot Urine Samples to Guide the Therapy of Hyponatremia in SIADH

Background: In hyponatremia, due to the inappropriate secretion of antidiuretic hormone (SIADH), a high versus low solute intake will affect the urine volume (UV) and, hence, the SNa level. The clinical implication of the fractional solute excretion is presented. Methods: In 35 normal controls and 24 patients with SIADH and urine osmolality higher than serum osmolality, we compared exact solute intake obtained from 24 h urine collection, with the estimated value obtained on a urine morning spot sample by the formula: eGFR (L/min) × Sosm × 1440 × FE.Osm (%) = mmol/24 h. The exact UV was compared with the estimated value given by the formula: eGFR × 1440 × S.Creat/U.Creat (for eGFR the MDRD was used). In 65 patients with chronic SIADH, from which a morning spot urine sample was available, we determined the estimated fluid and solute intake. Results: A good correlation was observed between the measured solute output or urine volume and the estimated values obtained from the controls (r = 0.86) as well as in SIADH (r = 0.91). Conclusion: Patients with low solute intake (FE.Osm <1.4%) and low diuresis (V/eCcr <0.8%) should increase their intake by taking oral urea, for example. Patients with high solute intake (FE.Osm >2.5%) and high diuresis (V/eCcr >1.5%) could usually be treated by mild water restriction (<1.5–21/24 h).

Chronic recurrent dehydration associated with periodic water intake exacerbates hypertension and promotes renal damage in male spontaneously hypertensive rats

Epidemiological evidence links recurrent dehydration associated with periodic water intake with chronic kidney disease (CKD). However, minimal attention has been paid to the long-term impact of periodic water intake on the progression of CKD and underlying mechanisms involved. Therefore we investigated the chronic effects of recurrent dehydration associated with periodic water restriction on arterial pressure and kidney function and morphology in male spontaneously hypertensive rats (SHR). Arterial pressure increased and glomerular filtration rate decreased in water-restricted SHR. This was observed in association with cyclic changes in urine osmolarity, indicative of recurrent dehydration. Additionally, water-restricted SHR demonstrated greater renal fibrosis and an imbalance in favour of pro-inflammatory cytokine-producing renal T cells compared to their control counterparts. Furthermore, urinary NGAL levels were greater in water-restricted than control SHR. Taken together, our results provide significant evidence that recurrent dehydration associated with chronic periodic drinking hastens the progression of CKD and hypertension, and suggest a potential role for repetitive bouts of acute renal injury driving renal inflammatory processes in this setting. Further studies are required to elucidate the specific pathways that drive the progression of recurrent dehydration-induced kidney disease.

A review of the nonpressor and nonantidiuretic actions of the hormone vasopressin

The pressor and antidiuretic actions of arginine vasopressin (AVP) have been well documented. This review focuses on the less widely appreciated actions of AVP which also have important physiologic functions and when better understood may provide important insights into common disease states. These actions include effects on pain perception and bone structure as well as important relationships to the varied components of metabolic syndrome. These include effects on blood glucose, lipid levels, and blood pressure. AVP may also play a role in the progression of chronic kidney disease and effect physiologic changes relating to aging, abnormal social behavior, and cognitive function. Important cellular responses including cell proliferation, inflammation, and control of infection and their relationship to AVP are described. Finally, the effects of AVP on hemostasis and the hypothalamic-pituitary-adrenal axis are noted. The goal of this summary of the various actions of AVP is to direct attention to the potential benefits of research in these underemphasized areas of importance.

Hyperosmolarity drives hypertension and CKD--water and salt revisited

An epidemic of chronic kidney disease (CKD) in Mesoamerica is providing new insights into the mechanisms by which salt and water might drive hypertension and CKD. Increasingly, evidence suggests that recurrent dehydration and salt loss might be a mechanism that causes CKD, and experimental studies suggest a key role for increased plasma osmolarity in activating both intrarenal (polyol-fructokinase) and extrarenal (vasopressin) pathways that drive renal injury. Thus, we propose that water and salt might influence blood pressure and kidney disease through the timing and combination of their intake, which affect plasma osmolarity as well as intrarenal and extrarenal mechanisms of renal injury. The type of fluid intake might also be important, as fluids containing fructose can trigger activation of these pathways. Future studies should investigate the effects of salt, sugar and fluid intake on plasma osmolarity as a potential pathogenetic mechanism in renal injury and high blood pressure.

Vasopressin: a novel target for the prevention and retardation of kidney disease?

After several decades during which little attention was paid to vasopressin and/or urine concentration in clinical practice, interest in vasopressin has renewed with the availability of new, potent, orally active vasopressin-receptor antagonists--the vaptans--and with the results of epidemiological studies evaluating copeptin (a surrogate marker of vasopressin) in large population-based cohorts. Several experimental studies in rats and mice had previously shown that vasopressin, acting via vasopressin V2 antidiuretic receptors, contributes to the progression of chronic kidney disease; in particular, to autosomal dominant polycystic kidney disease. New epidemiological studies now suggest a role for vasopressin in the pathogenesis of diabetes mellitus and metabolic disorders via activation of hepatic V1a and/or pancreatic islet V1b receptors. The first part of this Review describes the adverse effects of vasopressin, as revealed by clinical and experimental studies in kidney diseases, hypertension, diabetes and the metabolic syndrome. The second part provides insights into vasopressin physiology and pathophysiology that may be relevant to the understanding of these adverse effects and that are linked to the excretion of concentrated nitrogen wastes and associated hyperfiltration. Collectively, the studies reviewed here suggest that more attention should be given to the vasopressin-thirst-urine concentration axis in clinical investigations and in patient care. Whether selective blockade of the different vasopressin receptors may provide therapeutic benefits beyond their present indication in hyponatraemia requires new clinical trials.

Low abundance of sweat duct Cl- channel CFTR in both healthy and cystic fibrosis athletes with exceptionally salty sweat during exercise

To understand potential mechanisms explaining interindividual variability observed in human sweat sodium concentration ([Na(+)]), we investigated the relationship among [Na(+)] of thermoregulatory sweat, plasma membrane expression of Na(+) and Cl(-) transport proteins in biopsied human eccrine sweat ducts, and basal levels of vasopressin (AVP) and aldosterone. Lower ductal luminal membrane expression of the Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR) was observed in immunofluorescent staining of sweat glands from healthy young adults identified as exceptionally "salty sweaters" (SS) (n = 6, P < 0.05) and from patients with cystic fibrosis (CF) (n = 6, P < 0.005) compared with ducts from healthy young adults with "typical" sweat [Na(+)] (control, n = 6). Genetic testing of healthy subjects did not reveal any heterozygotes ("carriers") for any of the 39 most common disease-causing CFTR mutations in the United States. SS had higher baseline plasma [AVP] compared with control (P = 0.029). Immunostaining to investigate a potential relationship between higher plasma [AVP] (and sweat [Na(+)]) and ductal membrane aquaporin-5 revealed for all groups a relatively sparse and location-dependent ductal expression of the water channel with localization primarily to the secretory coil. Availability of CFTR for NaCl transport across the ductal membrane appears related to the significant physiological variability observed in sweat salt concentration in apparently healthy humans. At present, a heritable link between healthy salty sweaters and the most prevalent disease-causing CFTR mutations cannot be established.

Vasopressin V2 receptors, ENaC, and sodium reabsorption: a risk factor for hypertension?

Excessive sodium reabsorption by the kidney has long been known to participate in the pathogenesis of some forms of hypertension. In the kidney, the final control of NaCl reabsorption takes place in the distal nephron through the amiloride-sensitive epithelial sodium channel (ENaC). Liddle's syndrome, an inherited form of hypertension due to gain-of-function mutations in the genes coding for ENaC subunits, has demonstrated the key role of this channel in the sodium balance. Although aldosterone is classically thought to be the main hormone regulating ENaC activity, several studies in animal models and in humans highlight the important effect of vasopressin on ENaC regulation and sodium transport. This review summarizes the effect of vasopressin V2 receptor stimulation on ENaC activity and sodium excretion in vivo. Moreover, we report the experimental and clinical data demonstrating the role of renal ENaC in water conservation at the expense of a reduced ability to excrete sodium. Acute administration of the selective V2 receptor agonist dDAVP not only increases urine osmolality and reduces urine flow rate but also reduces sodium excretion in rats and humans. Chronic V2 receptor stimulation increases blood pressure in rats, and a significant correlation was found between blood pressure and urine concentration in healthy humans. This led us to discuss how excessive vasopressin-dependent ENaC stimulation could be a risk factor for sodium retention and resulting increase in blood pressure.

Activation of the epithelial Na+ channel in the collecting duct by vasopressin contributes to water reabsorption

We used patch-clamp electrophysiology on isolated, split-open murine collecting ducts (CD) to test the hypothesis that regulation of epithelial sodium channel (ENaC) activity is a physiologically important effect of vasopressin. Surprisingly, this has not been tested directly before. We ask whether vasopressin affects ENaC activity distinguishing between acute and chronic effects, as well as, parsing the cellular signaling pathway and molecular mechanism of regulation. In addition, we quantified possible synergistic regulation of ENaC by vasopressin and aldosterone associating this with a requirement for distal nephron Na+ reabsorption during water conservation vs. maintenance of Na+ balance. We find that vasopressin significantly increases ENaC activity within 2-3 min by increasing open probability (P(o)). This activation was dependent on adenylyl cyclase (AC) and PKA. Water restriction (18-24 h) and pretreatment of isolated CD with vasopressin (approximately 30 min) resulted in a similar increase in P(o). In addition, this also increased the number (N) of active ENaC in the apical membrane. Similar to P(o), increases in N were sensitive to inhibitors of AC. Stressing animals with water and salt restriction separately and jointly revealed an important effect of vasopressin: conservation of water and Na+ each independently increased ENaC activity and jointly had a synergistic effect on channel activity. These results demonstrate a quantitatively important action of vasopressin on ENaC suggesting that distal nephron Na+ reabsorption mediated by this channel contributes to maintenance of water reabsorption. In addition, our results support that the combined actions of vasopressin and aldosterone are required to achieve maximally activated ENaC.

Activation of the epithelial Na+ channel in the collecting duct by vasopressin contributes to water reabsorption

We used patch-clamp electrophysiology on isolated, split-open murine collecting ducts (CD) to test the hypothesis that regulation of epithelial sodium channel (ENaC) activity is a physiologically important effect of vasopressin. Surprisingly, this has not been tested directly before. We ask whether vasopressin affects ENaC activity distinguishing between acute and chronic effects, as well as, parsing the cellular signaling pathway and molecular mechanism of regulation. In addition, we quantified possible synergistic regulation of ENaC by vasopressin and aldosterone associating this with a requirement for distal nephron Na+ reabsorption during water conservation vs. maintenance of Na+ balance. We find that vasopressin significantly increases ENaC activity within 2-3 min by increasing open probability (P(o)). This activation was dependent on adenylyl cyclase (AC) and PKA. Water restriction (18-24 h) and pretreatment of isolated CD with vasopressin (approximately 30 min) resulted in a similar increase in P(o). In addition, this also increased the number (N) of active ENaC in the apical membrane. Similar to P(o), increases in N were sensitive to inhibitors of AC. Stressing animals with water and salt restriction separately and jointly revealed an important effect of vasopressin: conservation of water and Na+ each independently increased ENaC activity and jointly had a synergistic effect on channel activity. These results demonstrate a quantitatively important action of vasopressin on ENaC suggesting that distal nephron Na+ reabsorption mediated by this channel contributes to maintenance of water reabsorption. In addition, our results support that the combined actions of vasopressin and aldosterone are required to achieve maximally activated ENaC.

Sodium excretion in response to vasopressin and selective vasopressin receptor antagonists

The mechanisms by which arginine vasopressin (AVP) exerts its antidiuretic and pressor effects, via activation of V2 and V1a receptors, respectively, are relatively well understood, but the possible associated effects on sodium handling are a matter of controversy. In this study, normal conscious Wistar rats were acutely administered various doses of AVP, dDAVP (V2 agonist), furosemide, or the following selective non-peptide receptor antagonists SR121463A (V2 antagonist) or SR49059 (V1a antagonist). Urine flow and sodium excretion rates in the next 6 h were compared with basal values obtained on the previous day, after vehicle treatment, using each rat as its own control. The rate of sodium excretion decreased with V2 agonism and increased with V2 antagonism in a dose-dependent manner. However,for comparable increases in urine flow rate, the V2 antagonist induced a natriuresis 7-fold smaller than did furosemide. Vasopressin reduced sodium excretion at 1 mug/kg but increased it at doses >5 umg/kg,an effect that was abolished by the V1a antagonist. Combined V2 and V1a effects of endogenous vasopressin can be predicted to vary largely according to the respective levels of vasopressin in plasma,renal medulla (acting on interstitial cells), and urine (acting on V1a luminal receptors). In the usual range of regulation, antidiuretic effects of vasopressin may be associated with variable sodium retention. Although V2 antagonists are predominantly aquaretic, their possible effects on sodium excretion should not be neglected. In view of their proposed use in several human disorders, the respective influence of selective (V2) or mixed (V1a/V2) receptor antagonists on sodium handling in humans needs reevaluation.

[Difference in urine concentration according to gender and ethnicity: possible involvement in the different susceptibility to various renal and cardiovascular diseases]

Men and African-Americans are known to be at greater risk of urolithiasis and cardiovascular and renal diseases than women and Caucasians. Previous studies suggest that the antidiuretic effects of vasopressin and/or a greater urine concentration are associated with the rate of progression of these diseases. The present review addresses possible sex and ethnic-related differences in urine volume and osmolality which could participate in this male and black higher predominance. We reanalyzed 24h-urine data collected previously by different investigators for other purposes. In studies concerning healthy subjects (six studies) or patients with chronic kidney disease or Diabetes mellitus (three studies), men excreted a larger osmolar load than women, with a 15 to 30% higher urinary osmolality (or another index of urine concentration based on the urine/plasma creatinine concentration ratio) and a similar 24h urine volume than in women. In two American studies, African-Americans showed a significantly higher urinary concentration than Caucasians and a lower 24h-urine volume. Sex and ethnic differences in thirst threshold, vasopressin level, or other regulatory mediators may contribute to the higher urinary concentration of men and of African Americans. These differences could play a role in the greater susceptibility of these subjects to these pathologies. New prospective studies should take into account the antidiuretic effects of vasopressin as a potential risk factor in the initiation and progression of cardiovascular and renal diseases.

Sex difference in urine concentration across differing ages, sodium intake, and level of kidney disease

Men are known to be at greater risk of urolithiasis and cardiovascular and renal diseases than women. Previous studies suggest that greater urine concentration is associated with acceleration of progression of chronic kidney disease (CKD), increased urinary albumin excretion, and delayed renal sodium excretion. The present review addresses possible sex-related differences in urine volume and osmolality (U(osm)) that could participate in this male risk predominance. Because of the scarcity of information, we reanalyzed 24-h urine data collected previously by different investigators for other purposes. In nine studies concerning healthy subjects (6 studies) or patients with CKD or diabetes mellitus, U(osm) (or another index of urine concentration based on the urine/plasma creatinine concentration ratio) was 21-39% higher (i.e., about a 150 mosm/kgH2O difference) in men than in women. Urine volume was not statistically different. Thus, the larger osmolar load of men (related to their higher food intake) is excreted in a more concentrated urine with no difference in urine volume. This sex difference was not influenced by the level of sodium excretion and was still present in CKD patients. Sex differences in thirst threshold, AVP level, and other regulatory mediators may all contribute to the higher male U(osm). Because of the previously demonstrated adverse effects of vasopressin and/or high urine concentrating activity, the greater tendency of men to concentrate urine could participate in their greater susceptibility to urolithiasis and hypertension and to the faster progression towards end-stage renal failure.

Mild dehydration, vasopressin and the kidney: animal and human studies

Water balance depends essentially on fluid intake and urine excretion. Mild dehydration and the consequent hypertonicity of the extracellular fluid induce an increase in vasopressin secretion, thus stimulating urine concentrating processes and the feeling of thirst. The osmotic threshold for the release of vasopressin is lower than that for thirst and also shows appreciable individual variation. Sustained high levels of vasopressin and low hydration induce morphological and functional changes in the kidney. However, they could also be risk factors in several renal disorders, such as chronic renal failure, diabetic nephropathy and salt-sensitive hypertension.

"Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"?

Despite the seemingly ubiquitous admonition to "drink at least eight 8-oz glasses of water a day" (with an accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking. This review sought to find the origin of this advice (called "8 x 8" for short) and to examine the scientific evidence, if any, that might support it. The search included not only electronic modes but also a cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids. No scientific studies were found in support of 8 x 8. Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence, precisely the population and conditions that the "at least" in 8 x 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates. Since it is difficult or impossible to prove a negative-in this instance, the absence of scientific literature supporting the 8 x 8 recommendation-the author invites communications from readers who are aware of pertinent publications.

Abnormal regulation of ENaC: syndromes of salt retention and salt wasting by the collecting duct

Although the aldosterone-responsive segments of the nephron together reabsorb <10% of the filtered Na+, certain single-gene defects that affect the epithelial Na+ channel (ENaC) in the luminal membrane of the collecting duct (CD) or its regulation by aldosterone cause severe hypertension, whereas others cause salt wasting and hypotension. These rare defects illustrate the key role of the distal nephron in maintaining normal extracellular volume and blood pressure. Genetic defects that increase the Cl- conductance of the junctional complexes may also lead to salt retention and hypertension. Less dramatic alterations in regulatory actions of other hormones such as vasopressin (VP), either alone or with other genetic variations, diet, or environmental factors, may also produce Na+ retention or loss. Although VP acts primarily to regulate water balance, it is also an antinatriuretic hormone. Elevated basal plasma VP levels, and/or augmented VP release with increased Na+ intake, have been linked to essential hypertension in humans and in animal models of congestive heart failure and cirrhosis. Norepinephrine, dopamine, and prostaglandin E2 can inhibit the antinatriuretic effects of VP, and changes in the actions of these autocrine and paracrine regulators may also be involved in abnormal regulation of Na+ reabsorption.

Natriuresis induced by mild hypernatremia in humans

The hypothesis that increases in plasma sodium induce natriuresis independently of changes in body fluid volume was tested in six slightly dehydrated seated subjects on controlled sodium intake (150 mmol/day). NaCl (3.85 mmol/kg) was infused intravenously over 90 min as isotonic (Iso) or as hypertonic saline (Hyper, 855 mmol/l). After Hyper, plasma sodium increased by 3% (142.0 +/- 0.6 to 146.2 +/- 0.5 mmol/l). During Iso a small decrease occurred (142.3 +/- 0.6 to 140.3 +/- 0.7 mmol/l). Iso increased estimates of plasma volume significantly more than Hyper. However, renal sodium excretion increased significantly more with Hyper (291 +/- 25 vs. 199 +/- 24 micromol/min). This excess was not mediated by arterial pressure, which actually decreased slightly. Creatinine clearance did not change measurably. Plasma renin activity, ANG II, and aldosterone decreased very similarly in Iso and Hyper. Plasma atrial natriuretic peptide remained unchanged, whereas plasma vasopressin increased with Hyper (1.4 +/- 0.4 to 3.1 +/- 0.5 pg/ml) and decreased (1.3 +/- 0.4 to 0.6 +/- 0.1 pg/ml) after Iso. In conclusion, the natriuretic response to Hyper was 50% larger than to Iso, indicating that renal sodium excretion may be determined partly by plasma sodium concentration. The mechanism is uncertain but appears independent of changes in blood pressure, glomerular filtration rate, the renin system, and atrial natriuretic peptide.

Regulation by sodium intake of type 1 angiotensin II receptor mRNAs in the kidney of Sabra rats

OBJECTIVE

To study the relationship between the sensitivity to sodium content of the diet in terms of development of hypertension and the regulation of the expression of type 1 angiotensin II receptor subtypes by such a diet.

METHODS

The expression of angiotensin II receptor subtype (AT1A and AT1B) mRNAs was studied by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the four zones of the kidneys of Sabra rats, sensitive or resistant to DOCA salt-induced hypertension (SBH/y and SBN/y, respectively). Rats were fed a high (8%) or normal (0.4%) NaCl diet. As vasopressin is known to be elevated in SBH/y rats and to be involved in DOCA-salt hypertension, we studied an additional group of SBH/y rats, fed a high sodium diet, enriched in water.

RESULTS

With the absence of DOCA, SBH/y rats did not develop hypertension. The high sodium diet induced a greater fall in the plasma renin activity in the SBH/y (-95%) than in the SBN/y (-63%). In the cortex (C) and inner stripe (IS), the high sodium diet decreased AT1A and AT1B mRNAs in SBH/y and SBN/y, with a higher magnitude for SBH/y, than for SBN/y (C, -28 versus -20%; IS, -42 versus -20%). The addition of water to the high sodium diet lessened the effect of sodium in the C and IS, although the plasma renin activity (PRA) was not altered.

CONCLUSION

A high sodium diet significantly decreases both AT1A and AT1B gene expression in two specific zones of the rat kidney containing the target cells of angiotensin II (C and IS). This down-regulation is organ-specific since it was observed in the kidney and adrenals, but not in the liver. Finally, SBH/y and SBN/y rats differ in the basal level of AT1 mRNA expression in the IS, and in the ability to modulate AT1 mRNA level under sodium intake.