CNS-induced natriuresis during dopamine receptor blockade. Further support for the existence of, at least, two separate natriuretic hormonal systems

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.

CNS-induced natriuresis, neurohypophyseal peptides and renal dopamine and noradrenaline excretion in prehypertensive salt-sensitive Dahl rats

To identify defects in the salt-sensitive Dahl rat (Dahl-S), the natriuretic, catecholaminergic and pressor responses to 60-min elevation of the cerebroventricular sodium concentration (CNS-induced natriuresis) were compared between prehypertensive salt-sensitive Dahl-S and salt-resistant Dahl rats (Dahl-R). The plasma concentrations of the rat natriuretic hormone oxytocin, which has implications for the development of hypertension, and vasopressin (AVP) were also measured. Basal sodium and catecholamine excretion and mean arterial blood pressure (MAP) were similar in both strains. Sodium excretion during CNS stimulation increased more than 15-fold in Dahl-R but only 10-fold in Dahl-S. Dopamine excretion increased only transiently and similarly in both strains. Noradrenaline excretion and response to CNS stimulation were similar, suggesting a comparable sympathetic nervous activity between the strains. MAP increased comparably in Dahl-R and Dahl-S. Plasma AVP concentration was similar in both strains while plasma oxytocin concentration after CNS stimulation was more than 2-fold higher in Dahl-S than in Dahl-R. In conclusion, the prehypertensive Dahl-S has an attenuated natriuretic response to elevations of the cerebroventricular fluid sodium concentration and a higher plasma level of the natriuretic hormone oxytocin. Dopamine is not a mediator of CNS-induced natriuresis in neither strain. The attenuated natriuretic response may partly explain the salt-sensitivity in Dahl-S, and the higher plasma oxytocin value may either represent an effort to compensate for the deficient natriuretic response or reflect a primary defect in this system. Due to the known involvement of oxytocin in central MAP regulation in some hypertensive animal models, the findings warrant further investigation.

Cerebral salt wasting syndrome

There is significant evidence to show that many patients with hyponatremia and intracranial disease who were previously diagnosed with SIADH actually have CSW. The critical difference between SIADH and CSW is that CSW involves renal salt loss leading to hyponatremia and volume loss, whereas SIADH is a euvolemic or hypervolemic condition. Attention to volume status in patients with hyponatremia is essential. The primary treatment for CSW is water and salt replacement. The mechanisms underlying CSW are not understood but may involve ANP or other natriuretic factors and direct neural influence on renal function. Future investigation is needed to better define the incidence of CSW in patients with intracranial disease, identify other disorders that can lead to CSW, and elucidate the mechanisms underlying this syndrome.

Renal dopamine and noradrenaline excretion during CNS-induced natriuresis in spontaneously hypertensive rats: influence of dietary sodium

Abnormalities in dopamine (DA) and noradrenaline (NA) activities and sodium handling may be involved in the pathogenesis of hypertension. The present study was designed to investigate whether any differences exist between normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) in urinary excretion of DA, NA and sodium after 15 weeks on a low, medium or high sodium diet and during a subsequent elevation of the cerebroventricular fluid sodium concentration (CNS-induced natriuresis). Seven features were noted: (1) Basal sodium and DA excretion after the diet regimen was correlated to the dietary sodium content in both strains, except that sodium and DA excretion in SHR showed no further increase after the high sodium diet over and above that after medium sodium diet. (2) For any given sodium diet, SHR excreted more DA and NA as compared with WKY. (3) Blood pressure in SHR, as opposed to that in WKY, was higher after medium and high sodium diet than after low sodium diet. (4) During CNS-induced natriuresis NA excretion decreased or remained unchanged in WKY, but increased in SHR. (5) The DA/NA excretion ratio during CNS-induced natriuresis increased in WKY while decreased in SHR, which would not favour a natriuretic/vasodilatory response in the latter. (6) The ability of SHR to respond with CNS-induced natriuresis was attenuated after high sodium diet. (7) The magnitude of CNS-induced natriuresis was in both strains correlated to the sodium diet; the higher the dietary sodium content, the greater the natriuretic response. In conclusion, the study shows some clear differences in the catecholamine and sodium handling between WKY and SHR which may be involved in the pathogenesis of hypertension in SHR. Furthermore, increased sodium in the diet sensitizes the brain and kidney to increase the ability to respond with natriuresis for a given sodium stimulus.

Cerebral salt wasting syndrome: a review

Hyponatremia is frequently seen in neurosurgical patients and is often attributed to inappropriate secretion of antidiuretic hormone. A number of studies in recent years have shown that hyponatremia in many patients with intracranial disease may actually be caused by cerebral salt wasting, in which a renal loss of sodium leads to hyponatremia and a decrease in extracellular fluid volume. The appropriate treatment of cerebral salt wasting fluid and salt replacement, is opposite from the usual treatment of hyponatremia caused by inappropriate secretion of antidiuretic hormone. This review summarizes the evidence in favor of cerebral salt wasting in patients with intracranial disease, examines the possible mechanisms responsible for this phenomenon, and discusses methods for diagnosis and treatment.

Natriuresis obtained by intracerebroventricular infusion of hypertonic NaCl in rats with papillary necrosis

Raising the sodium concentration in the third cerebral ventricle increases renal sodium, potassium and water excretion. The identification and characterization of the factor(s) mediating the centrally evoked natriuresis would be greatly facilitated if the exact intrarenal effector site were known. We have assessed the importance of inner medullary structures for the effects of CNS stimulation by examining its ability to alter renal excretion in rats with papillary necrosis, induced 2 d earlier with 2-bromoethylamine hydrobromide (BEA), 250 mg kg-1 body wt i.v. Male Lewis x DA rats were divided into a BEA-treated group (n = 6) and a control group receiving vehicle alone (n = 6). In contrast to the white papillae normally seen, the papillae of BEA-treated animals were bright red and showed a clear line of demarcation at their base. The rats were anaesthetized i.p. with Inactin (120 mg kg-1 body wt). Artificial cerebrospinal fluid (CSF) was infused (520 nL min-1) via a cannula into the left lateral ventricle. After 45 min CSF containing 1 M NaCl was used. Stimulation of the control rats with hypertonic CSF increased urine flow rate five-fold (5.4 +/- 0.8 to 27.1 +/- 6.1 microL min-1), Na excretion 23-fold (0.4 +/- 0.1 to 7.6 +/- 1.8 mumol min-1) and K excretion fourfold (0.6 +/- 0.18 to 3.8 +/- 0.5 mumol min-1). When the concentration mechanisms were damaged with BEA, the basal excretion rates of water and Na increased. The natriuretic response to ICV stimulation was severely impaired in these rats, but the kaliuretic effect was sustained.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dopamine receptor blockade on natriuresis is dependent on the degree of hypervolemia

A number of different physiological factors and systems have been suggested to be responsible for the natriuretic effect following acute isotonic volume expansion (VE). The variation in suggestions may depend on the prevailing status of the systems governing fluid and electrolyte balance before VE, on the expansion medium and on the rate and degree of VE. A study was performed to determine whether the previously documented attenuating effect of dopamine receptor blockade on natriuresis induced by VE is dependent on the degree of hypervolemia. Anesthetized rats were pretreated with the dopamine receptor blockers haloperidol (1 mg.kg-1 body weight, i.p.), SCH 23390 (30 micrograms.hr-1.kg-1 i.v.) or vehicle and then subjected to VE at 2, 5 or 10% of body weight per hour. VE at 2, 5 and 10% increased sodium excretion in vehicle-pretreated animals 6-, 29- and 130-fold, respectively. In the haloperidol-pretreated animals the natriuretic response (accumulated sodium excretion) to VE was attenuated by 67% (P less than 0.05), 46% (P less than 0.05) and 22% (NS) at the three degrees of expansion, respectively. The corresponding attenuation in SCH 23390-treated animals were 60% (P less than 0.05), 56% (P less than 0.05) and 19% (NS), respectively. The gradual decrease in attenuation indicates that at varying degrees of hypervolemia, different physiological systems contribute differently to the renal natriuretic response. The dopamine system seems to be relatively more important in promoting natriuresis at the lower (physiological) range of hypervolemia whereas in the high range other factors have a greater impact.

Atrial natriuretic factor, urinary catechol compounds and electrolyte excretion in rats during normal hydration and isotonic volume expansion. Influence of dopamine receptor blockade

To investigate the influence of acute isotonic volume expansion (VE) on the plasma concentration of atrial natriuretic factor (ANF), the excretion of catechol compounds and electrolytes and the whole kidney glomerular filtration rate (GFR), these variables were measured before and during 60 min of VE (2% of body weight per hour). Atrial natriuretic factor was measured at the end of the experiment. In a control group (n = 7) without volume expansion, plasma ANF was 58 +/- 4 pg ml-1. The excretion of sodium, dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), noradrenaline (NA) and GFR did not change during the control study. In VE animals (n = 7) plasma ANF was 82 +/- 7 pg ml-1, significantly higher than in the control group. Sodium excretion increased more than 17-fold. The excretion of the DA increased by 38% and that of DOPAC by 30%. Noradrenaline excretion remained unchanged while GFR increased by 20%. In haloperidol-pretreated animals subjected to VE (n = 7), plasma ANF was 81 +/- 8 pg ml-1 during VE, significantly higher than in the control animals. Although the sodium excretion increased more than ninefold in this group during VE, this increase was only 55% of that in the VE group not given haloperidol. The DA and DOPAC excretion was increased by haloperidol, indicating a feedback effect of receptor blockade. DOPAC excretion was not increased further by VE, but the excretion of DA increased by 15% and GFR increased by 19%.(ABSTRACT TRUNCATED AT 250 WORDS)