Two natriuretic substances in extracts of urine from normal man when salt-depleted and salt-loaded

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

Natriuretic hormones, endogenous ouabain, and related sodium transport inhibitors

The work of deWardener and colleagues stimulated longstanding interest in natriuretic hormones (NHs). In addition to the atrial peptides (APs), the circulation contains unidentified physiologically relevant NHs. One NH is controlled by the central nervous system (CNS) and likely secreted by the pituitary. Its circulating activity is modulated by salt intake and the prevailing sodium concentration of the blood and intracerebroventricular fluid, and contributes to postprandial and dehydration natriuresis. The other NH, mobilized by atrial stretch, promotes natriuresis by increasing the production of intrarenal dopamine and/or nitric oxide (NO). Both NHs have short (<35 min) circulating half lives, depress renotubular sodium transport, and neither requires the renal nerves. The search for NHs led to endogenous cardiotonic steroids (CTS) including ouabain-, digoxin-, and bufadienolide-like materials. These CTS, given acutely in high nanomole to micromole amounts into the general or renal circulations, inhibit sodium pumps and are natriuretic. Among these CTS, only bufalin is cleared sufficiently rapidly to qualify for an NH-like role. Ouabain-like CTS are cleared slowly, and when given chronically in low daily nanomole amounts, promote sodium retention, augment arterial myogenic tone, reduce renal blood flow and glomerular filtration, suppress NO in the renal vasa recta, and increase sympathetic nerve activity and blood pressure. Moreover, lowering total body sodium raises circulating endogenous ouabain. Thus, ouabain-like CTS have physiological actions that, like aldosterone, support renal sodium retention and blood pressure. In conclusion, the mammalian circulation contains two non-AP NHs. Identification of the CNS NH should be a priority.

Renal effects of a novel endogenous natriuretic agent xanthurenic acid 8-o-β-d-glucoside in rats

Xanthurenic acid 8-o-β-d-glucoside is an endogenous derivative of tryptophan metabolism, isolated from urine of patients with chronic renal disease. This compound was suggested previously to act as a natriuretic hormone based on its ability to block short circuit currents in a frog skin assay and to induce a sustained natriuresis when injected into rats (C. D. Cain et al., Proc. Natl. Acad. Sci. USA 2007: 17873-17878). The present communication describes the effects of the compound on renal clearance and hemodynamic parameters in male Sprague-Dawley rats maintained on a normal salt (0.4-0.5%) diet. Intravenous administration of synthetic xanthurenic acid 8-o-β-d-glucoside in two consecutive incremental doses (6.3 and 31.5 nmol) resulted in a significant increase (P < 0.05), in urine flow (43.91 ± 6.31 μL/min vs. 10.54 ± 2.21 μL/min), absolute rate of sodium excretion (3.99 ± 0.95 μEq/min vs. 1.15 ± μEq/min), and percentage sodium excretion (1.63 ± 0.46% vs. 0.37 ± 0.12%, peak response vs. baseline, respectively). The natriuretic/diuretic effect was associated also with a significant increase in potassium excretion. These effects were not related to changes in renal hemodynamics or in arterial blood pressure. Pretreatment with the sodium channel blocker, amiloride, completely abolished the natriuretic and kaluretic actions of the compound. Administration of the xanthurenic acid derivative caused a dose-related increase in urinary nitrite/nitrate excretion. Moreover, under chronic nitric oxide blockade by l-NG-Nitro-Arginine-Methyl-Esther (l-NAME) sodium excretion was similar in rats treated or untreated with the compound. Our data demonstrate that xanthurenic acid 8-o-β-d-glucoside has significant diuretic/natriuretic and kaluretic properties. An intact amiloride-sensitive sodium channel is required for the renal effects of the compound. The data further suggest that the natriuretic effect is mediated in part by a nitric oxide-dependent mechanism.

Identification of xanthurenic acid 8-O-beta-D-glucoside and xanthurenic acid 8-O-sulfate as human natriuretic hormones

Hormonal regulation of salt excretion and water balance by the kidneys is well documented. Before 1961, it was widely believed that the glomerular filtration rate and the steroid hormone aldosterone controlled sodium balance in the body. In 1961, deWardener et al. [de Wardener HE, Mills IH, Clapham WF, Hayter CJ (1961) Clin Sci 21:249-258] showed that when these two variables were controlled, the kidney was still able to increase sodium excretion in response to a salt load. Several lines of evidence argued for a small-molecule signal as a definitive modulator of sodium excretion by the kidney. However, the chemical nature of the suspected natriuretic agent remained unknown. Here we report the identification and natriuretic activity of two closely related small molecules isolated from human urine, xanthurenic acid 8-O-beta-d-glucoside and xanthurenic acid 8-O-sulfate. The two compounds were partially purified by activity-guided fractionation and subsequently identified by using NMR spectroscopic analyses of enriched active fractions. Both compounds caused substantial and sustained (1- to 2-h) natriuresis in rats and no or minimal concomitant potassium excretion. We believe these compounds constitute a class of kidney hormones that also could influence sodium transport in nonkidney tissues given that these tryptophan metabolites presumably represent evolutionarily old structures.

Role of renal Na+,K(+)-ATPase in the regulation of sodium excretion under normal conditions and in acute congestive heart failure

BACKGROUND

Cardiac glycosides have traditionally been used as inotropic agents in the treatment of congestive heart failure (CHF). The renal actions of cardiac glycosides independent of inotropic effects are not characterized. The presence of endogenous digitalis-like factors (EDLFs) with characteristic Na+,K(+)-ATPase activity has been postulated in volume-expanded states such as CHF, and recent studies have demonstrated that at least one EDLF shares structural homology with ouabain. This study was undertaken to evaluate the renal actions of ouabain in normal dogs and those with CHF.

METHODS AND RESULTS

After surgical preparation, normal dogs (n = 6) and dogs in pacing-induced CHF (n = 6) received intrarenal ouabain in sequential doses of 0.167 micrograms/kg/min, 0.334 micrograms/kg/min, and 0.668 micrograms/kg/min. Hemodynamics and renal function were evaluated during the infusion. There was no change in heart rate or mean arterial pressure during the infusion compared with baseline in both groups. Sodium excretion and urine volume significantly increased in both groups. Plasma renin activity, activated by the onset of pacing in the CHF group, was inhibited by the administration of intrarenal ouabain in this group only.

CONCLUSIONS

These studies demonstrate that ouabain has diuretic and natriuretic actions independent of cardiac hemodynamics that are preserved in CHF. Furthermore, intrarenal ouabain suppresses activation of renin in CHF.

Digoxin-like immunoreactivity in cord blood plasma extracts is not only due to endogenous corticosteroids

Methods for quantitative extraction and enrichment of digoxin-like immunoreactive substances (DLIS) from human cord blood plasma (CBP) using organic solvents combined with a solid state absorbent are presented. Sephadex LH-20 column chromatography is more suitable than sephadex G-25 for DLIS separation and purification from CBP extract. The elution pattern of sephadex LH-20 chromatography of deproteinized and desalted CBP shows two distinct DLIS peaks. The first peak coelutes with the steroids aldosterone, cortisol, progesterone, 17-OH-progesterone, testosterone and estradiol, while the second DLIS peak coelutes with dehydroepiandrosterone sulfate (DHEA-S). The Na+,K(+)-ATPase inhibitory activity peak partially overlaps with the first DLIS peak. Coeluted steroids could account only partially for digoxin-like immunoreactivity in the first DLIS peak; however, they did not contribute to the Na+,K(+)-ATPase inhibitory activity, whereas the second DLIS peak could be ascribed to endogenous DHEA-S.

Chemical and clinical studies of endogenous digitalis-like factor in hypertension

Endogenous digitalis-like factor (endalin) was investigated by measuring the ability of rat and human plasma and urine to inhibit [3H]ouabain-specific binding, digoxin-antidigoxin antibodies interaction, and renal Na+, K+-ATPase activity. Endalin was detected in plasma (and urine) of one third of 112 patients with sustained and moderate hypertension (Na+ intake = 110 mmol/l). Endalin tended to be increased in the more pronounced hypertensives. No correlation with any other clinical and biological parameter could be detected. An activity to inhibit Na+, K+-ATPase was also detected in the rat after acute and chronic Na+ loading, in reduced renal mass-type hypertension and in SHRs as compared to WKY rats. Comparison of the plasma and urine inhibitory effects in the different tests revealed some chemical heterogeneity. However, a compound possessing the biochemical and pharmacological characteristics of digitaline was extracted from human urine. Chromatographic and spectral analysis of about 1,000 liters revealed a compound with apparent chemical homogeneity, molecular weight around 500, devoid of peptidic bound and of aliphatic structure.

What's new in cellular cation exchange and humoral factors in primary hypertension?

In essential hypertension abnormalities of cellular Na+ and Ca++ handling have been described. The latter disturbance probably offers an explanation for the increased vasoconstriction in essential hypertension. The etiology of those disturbances in cellular Na+ and Ca++ metabolism may be related to genetic disturbances of membrane transport, or to a humoral factor influencing cellular cation metabolism.

Na-K ATPase activity of erythrocyte membrane correlated with urinary Na/K ratio in normotensive adolescents with a family history of hypertension

In female adolescents with a normal blood pressure range, the correlation between urinary sodium excretion and Na-K ATPase activity of erythrocyte membrane was investigated. 34 subjects with a family history of hypertension showed a significantly positive correlation between body mass index and mean blood pressure (r = 0.64, p less than 0.01), and a significantly lower urinary Na/Cr and Na/K ratio compared with 37 subjects without the same family history (p less than 0.05 and p less than 0.01, respectively). The positive correlation between urinary Na/K ratio and Na-K ATPase activity of erythrocyte membrane was demonstrated in only subjects with a family history of hypertension (r = 0.41, p less than 0.05). The authors discuss these significant correlations in terms of the abnormal membrane cation fluxes associated with essential hypertension.

Diuretic and natriuretic actions of extract prepared from rat erythrocytes

Freeze-dried preparation of rat erythrocytes was extracted into distilled water and after heat treatment the extract was chromatographed on Sephadex G-25 column. The effluent fractions were pooled, freeze-dried, dissolved in Ringer's solution and bioassayed for diuretic and natriuretic responses in anaesthetized non-diuretic rats. The active fractions potently enhanced urine output (about 9-fold control) and urinary sodium excretion (about 7-fold control). The responses started to develop after a lag period of approximately 10-20 min, peaked between 60-80 min post-injection and then diminished. The augmentation of sodium excretion persisted longer than the increased diuresis. The biological activity of the extract was retained after dialysis in a tube with nominal molecular weight cut-off below 1,000.

Observations on a low molecular weight natriuretic and Na-K-ATPase inhibitory material in urine

Natriuretic and Na-K-ATPase inhibitory material prepared from urine by gel filtration on G25 Sephadex was previously found to be of low molecular weight, polar and non-peptide. Although activity appeared to depend on an amino group, tests and radioenzymatic assays for catecholamines suggested that these were not implicated in the natriuretic activity. Further purification of the material included solvent extraction, cation exchange and high performance liquid chromatography. At each stage, fractions were assayed for natriuretic activity, stimulation of G6PD and inhibition of Na-K-ATPase in cytochemical assays, and for digoxin-like activities i.e. inhibition of dog kidney Na-K-ATPase (Sigma), displacement of 3H ouabain bound to cell membranes and cross reaction with antidigoxin antibody. The crude material possessed all activities, but with successive purifications the activities separated from each other and were thus due to different substances. Analyses for catecholamines with HPLC and electrochemical detection revealed that the natriuretic activity was due to dopamine.

Observations on the "cascade" of Na-K-ATPase inhibitory and digoxin-like immunoreactive material in human urine: possible relevance to essential hypertension

Previous investigations have demonstrated an increased amount of a sodium pump inhibitor (N.H.) in plasma from humans with essential hypertension and from animals with various forms of experimental hypertension. The present study has employed Sephadex column and C18 reverse phase separation of urines from patients with essential hypertension and normal controls to distinguish "high", "intermediate" and "low" molecular weight forms of N.H., measured through properties of Na-K-ATPase inhibition and digoxin-like immunoreactivity. The major difference between hypertensive and normotensive urines was a highly significant increase in the "intermediate" molecular weight form of N.H., as measured by Na-K-ATPase inhibition. In contrast, digoxin-like immunoreactivity was significantly decreased in urine from hypertensive patients. The results are compatible with an hypothesis that the defect in some forms of essential hypertension may be partial inhibition of enzymatic conversion of intermediate to final form of N.H., with the increased sodium pump inhibition primarily related to the precursor.

Natriuretic and smooth muscle responses to human urinary natriuretic hormone in rats: relation to blood pressure and kallikrein excretion in patients

Human urine contains a small molecular weight natriuretic substance and similar material isolated from the kidney inhibits Na/K ATPase. Such action on smooth muscle in blood vessels would cause contraction. Human urinary natriuretic material caused contraction of the smooth muscle in the rat anococcygeus muscle and this activity correlated with its natriuretic activity. Known vasoactive substances could not explain the activity of the natriuretic factor when tested on the anococcygeus muscle. The best correlation with blood pressure of the patient was with the log of the ratio of natriuretic activity divided by the kallikrein excretion. A normotensive woman with severe renal failure had very high kallikrein excretion as well as increased natriuretic activity and her data fitted the same correlation as the hypertensives' data.

Intrarenal pressure and sodium excretion in hypertension of chronic glomerulonephritis in humans

The relationship between the fractional excretion of filtered sodium (FENa) and the peritubular capillary physical factors (PCPF) in the hypertension (HT) of chronic glomerulonephritis (GN) was examined in hydropenia (C) and during sustained isotonic saline volume expansion (E; 3% net increase of body weight) in 32 GN patients (16 with HT), and compared with our previous findings in 20 normal individuals (NORM) and 19 patients with essential hypertension (EH). Fourteen GN patients (seven with HT) had a 75% reduction of glomerular filtration rate (GFR), the others (nine with HT) had normal or near normal GFR. The PCPF were estimated from the intrarenal venous (wedged) pressure (IRVP) and the calculated efferent arteriolar protein concentration (EAPC). In C, IRVP correlated to GFR (r = 0.682, p less than 0.001) and (FENa) (r = -0.357, p less than 0.05), but IRVP and EAPC were similar in HT and normotension at comparable levels of GFR. The increase of FENa during E (delta FENa) was exaggerated in all HT groups even at reduced levels of GRF, and could not be related to changes in renal hemodynamics or PCPF. delta FENa correlated with mean arterial pressure in C both in GN (r1 = 0.702, p less than 0.01) and in the combined NORM/EH group (r2 = 0.478, p less than 0.01), with r1 greater than r2 (p less than 0.005). The findings indicate that the pathogenesis of hypertension of chronic glomerulonephritis is independent of changes in the PCPF, and are compatible with the idea that humoral factors are the main mediators of the altered sodium excretion during saline volume expansion in the HT of both chronic GN and EH.

Atrial natriuretic factor. Characterization and partial purification

One of the main differences between atrial and ventricular cardiocytes is the presence in former of specific granules with morphological characteristics very similar to secretory granules found in peptide-secreting endocrine cells. It has been suggested that these granules are the storage place for the atrial natriuretic factor. In the rat, water deprivation produces an increase in atrial granularity but a significant decrease in acid-extractable diuretic and natriuretic activity, suggesting that the number of atrial specific granules does not necessarily represent natriuretic activity. The atrial natriuretic factor activity is destroyed by incubation with several proteases and does not inhibit the sodium-potassium ATPase, suggesting that the active substance is a small peptide that is probably different from the so-called natriuretic hormone. After a series of chromatographic steps in Sep-Pak cartridges, Bio-Gel P-10, CM Bio-Gel, and Mono S columns, the specific activity of the atrial natriuretic factor was increased from 193, corresponding to atrial homogenates, to 242,000, which corresponds to the last chromatographic step representing a 1250-fold purification. This material showed a potent natriuretic activity, as 10 picomoles increased natriuresis by 100%.

The permeability of collecting ducts to 22Na+ and 36Cl- in rat isolated papillae

1. The diffusional permeability of collecting ducts to 22Na+ and 36Cl- was measured in rat papillae in vitro. 2. The permeability of the collecting duct to 36Cl- was 0.72 (s.e.m. = 0.01; n = 356) microns/sec which was significantly higher than the value of 0.51 (s.e.m. = 0.01; n = 356) microns/sec measured for 22Na+. 3. Collecting ducts in papillae taken from rats on a high sodium intake had a 22Na+ permeability of 0.63 (s.e.m. = 0.04; n = 53) microns/sec which was significantly higher than the value on a normal salt intake (0.50, s.e.m. = 0.04; n = 46 microns/sec). 4. When papillae from normal rats were studied in plasma taken from salt loaded rats, the 22Na+ permeability of 0.59 (s.e.m. = 0.04; n = 18) microns/sec was significantly higher than when incubated in plasma from normal rats (0.44, s.e.m. = 0.05; n = 12) microns/sec. 5. An extract of urine with natriuretic activity had no effect on 22Na+ permeability when tested in this system. 6. Adrenalectomy, PGE2, indomethacin and antidiuretic hormone had no significant effect on 22Na+ and 36Cl- permeability. 7. A substance exists in plasma from salt loaded animals that increases the permeability of collecting ducts to sodium. This effect could explain the component of the natriuresis that follows saline infusion which is independent of changes in glomerular filtration rate, aldosterone, or proximal tubule reabsorption.

Influence of uranyl nitrate upon tubular reabsorption and glomerular filtration in blood perfused isolated dog kidneys

The early changes in tubular reabsorption, glomerular filtration, blood flow and sodium excretion brought about by uranyl nitrate were investigated in isolated, blood-perfused dog kidneys during water diuresis. No significant changes in urine volume were observed; the decrease in fluid reabsorption was counterbalanced quantitatively by a reduction in glomerular filtration rate; only a small diminution of renal blood flow was found. The balance between reabsorption and filtration was observed as well when angiotensin action or prostaglandin synthesis were inhibited. The intrarenal venous pressure rose, suggesting that an increase in proximal intratubular hydrostatic pressure caused the decrease in filtration. Tubular back-leak of fluid, or back-diffusion, induced by the toxin, were excluded. The presence of natriuretic compounds in the urine was confirmed.

Further observations on a low-molecular-weight natriuretic substance in the urine of normal man

Large quantities of the low-molecular-weight natriuretic material (F4), which appears after the salts when fractionated on G-25 Sephadex column, were obtained from the urine of normal man on a normal diet. The natriuretic substance in F4 was (1) untrafiltrable through a membrane with a claimed molecular-weight cut-off of 500 daltons (Amicon UMO5); (2) soluble in more polar organic solvents; (3) totally soluble in 95% acetone when specific activity was doubled; (4) relatively resistant to heating at 100 degrees C for 1 hour at a pH of 10, and to heating at 110 degrees C in 6 N hydrochloric acid for up to 90 hours under anaerobic conditions, and treatment with nitrous acid; it was less resistant to these procedures when extracted into 95% acetone; (5) not destroyed by trypsin, chymotrypsin, pronase, pepsin, leucine aminopeptidase, and subtilysin, nor was it destroyed by pepsin, leucine aminopeptidase, subtilysin, carboxypeptidase A and B, and aminopeptidase M, or by monoamine oxidase, aryl sulphatase, and beta-glucuronidase when extracted into 95% acetone. The natriuretic substance in the 95% acetone-soluble F4 was totally destroyed by incubation with prolidase. The least amount of 95% acetone-soluble F4 required to produce a significant natriuresis in the bioassay rat was that derived from a 7-min sample of urine. The maximal response was obtained from a 30-min sample of urine. Continuous i.v. infusion of the 95% acetone-soluble F4 for 40 min produced a sustained natriuresis, whereas a greater amount injected as a bolus produced an effect which was not sustained beyond 20 min.

Role of the basal sodium intake in the rats on their response to a natriuretic factor

A previously described natriuretic factor found in urine from man and dogs receiving a high salt diet has been postulated to be of hormonal nature. In order to test this hypothesis, natriuretic factor obtained from a pool of human urine of a standard activity has been injected into rats fed 0.55 (group A), 3.55 (group B), 6.55 (group C) mEq sodium a day as it has been shown that the endogenous activity of this factor depends on the sodium content of the diet. This material induces an increment of the UNaV and FENa in the rats averaging respectively 429.6 +/- 150.1% and 317.5 +/- 71.14% for the rats from group A, 103 +/- 29% and 99.7 +/- 26.25% for the rats from group B, 56.3 +/- 34.4% and 63.2 +/- 35.8% for the rats from group C reflecting the degree of statistical significance observed between the absolute values obtained during the control and experimental periods for each group. In addition, a dose-response curve has been established. Altogether the results suggest the hormonal nature of the natriuretic factor and design the kidney as the target organ. These data allow to define the best conditions under which the rats have to be in order to provide a valuable bioassay for this natriuretic material.

The sodium-potassium pump in volume expanded hypertension

Decreased arterial Na+-K+ pump and cardiac Na+, K-+ATPase activities have now been demonstrated in several types of experimental volume expanded hypertension. The changes are not secondary to elevated pressure since they also occur in veins and right ventricle where the pressure is not elevated. Decreased arterial Na+-K+ pump activity can be reproduced by acute volume expansion of the normal rat and plasma extracts from this rat suppress pump activity when applied to arteries from another rat. Suppression of Na+-K+ pump activity in arteries, veins and heart, with ouabain for example, leads to increased contractile activity. Thus the volume expansion, reduced pump activity, and hypertension appear to be causally related through an ouabain-like humoral agent. Certain other evidence suggests that the pump defect extends to the sympathetic nerve endings, thereby reducing the efficiency of neural compensatory mechanisms.

Sodium metabolism

This brief review attempts to summarize important basic concepts of sodium metabolism including sodium ion distribution, sodium balance and the renal regulation of sodium excretion. Finally, an attempt has been made to relate these basic concepts to the mechanisms and management of common clinical situations of abnormal salt balance.