Cerebral mechanisms influencing renal sodium excretion in dehydrated sheep

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.

Increased osmolality of conscious water-deprived rats supports arterial pressure and sympathetic activity via a brain action

To test the hypothesis that high osmolality acts in the brain to chronically support mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA), the osmolality of blood perfusing the brain was reduced in conscious water-deprived and water-replete rats by infusion of hypotonic fluid via bilateral nonoccluding intracarotid catheters. In water-deprived rats, the intracarotid hypotonic infusion, estimated to lower osmolality by approximately 2%, decreased MAP by 9+/-1 mmHg and LSNA to 86+/-7% of control; heart increased by 25+/-8 beats per minute (bpm) (all P<0.05). MAP, LSNA, and heart rate did not change when the hypotonic fluid was infused intravenously. The intracarotid hypotonic fluid infusion was also ineffective in water-replete rats. Prior treatment with a V1 vasopressin antagonist did not alter the subsequent hypotensive and tachycardic effects of intracarotid hypotonic fluid infusion in water-deprived rats. In summary, acute decreases in osmolality of the carotid blood of water-deprived, but not water-replete, rats decreases MAP and LSNA and increases heart rate. These data support the hypothesis that the elevated osmolality induced by water deprivation acts via a region perfused by the carotid arteries, presumably the brain, to tonically increase MAP and LSNA and suppress heart rate.

Effects of central losartan on plasma renin and centrally mediated natriuresis

Because intracerebroventricular (ICV) infusion of hypertonic saline or angiotensin II (Ang II) both induce water drinking, vasopressin secretion, natriuresis and increased arterial pressure, the possibility that common neural pathways mediate responses to ICV Ang II and hypertonic saline has been investigated. This was done by testing the effect of ICV infusion of the Ang II antagonist losartan on the natriuretic and pressor responses to ICV hypertonic NaCl in sheep. The effect of ICV losartan on plasma renin concentration (PRC) was also investigated. Infusion of losartan (1 mg/hr) into a lateral ventricle prevented both natriuretic and pressor responses to infusion of 0.6 mol/liter NaCl into a lateral ventricle at 1 ml/hr. In another experiment, ICV losartan at 1 mg/hr caused a pronounced increase in the PRC of Na-depleted sheep, while ICV Ang II at 3 micrograms/hr decreased PRC. The results suggest that: (i) a central angiotensinergic pathway may mediate osmoregulatory responses to centrally administered hypertonic saline, and (ii) a central angiotensinergic pathway may have a tonic inhibitory influence on renin secretion in Na-depleted animals.

Opening the blood-brain barrier to zinc

Sheep with guide tubes implanted over the brain lateral ventricles, in order to facilitate episodic sampling of cerebrospinal fluid (CSF), were used to determine the effects of increasing cranial blood osmolality or electroconvulsive shock (ECS) on the permeability of the blood-brain barrier (BBB) to zinc. Zinc acetate solution (1 mg Zn/ml) was infused intravenously (i.v.) at 1.0 ml/min for 30 min and then continuously at 0.125 ml/min. This infusion increased plasma total zinc concentration (pZn) approximately 10-fold without altering CSF zinc concentration (CSFZn). After 1.5-3.5 h, 4 M NaCl was infused at 5-10 ml/min for 10 min into one carotid artery with the other carotid artery occluded, or the animals were anaesthetized and given an ECS (140 V, 2 s). Paired samples of blood and CSF were collected before and after these treatments. Results were: (i) CSFZn was approximately one tenth of pZn; (ii) zinc administered i.v. was almost completely excluded from the CSF; (iii) increased cranial blood osmolality or ECS increased CSFZn in all experiments, but the time course and extent of the rise were variable. CSFZn reached the concentrations of zinc in normal sheep plasma in some experiments; (iv) CSFZn subsequently fell towards the low values of zinc in normal CSF; (v) the animals suffered no evident ill-effects from either procedure. The procedures may, therefore, be used for reversible opening of the BBB to particles such as zinc in conscious or anaesthetized sheep with no troublesome sequelae.

Cerebral regulation of renal sodium excretion in sheep infused intravenously with hypertonic NaCl

1. The natriuretic response to intravenous infusion of 2 M-NaCl was investigated in six conscious sheep. This hypertonic NaCl load resulted in relatively small, physiological (2-3 mmol l-1) increases in plasma Na+ concentration and was followed by a natriuresis with a maximum mean urinary sodium excretion 5 times higher than pre-infusion values. 2. Intravenous infusion of isotonic NaCl, delivering the same Na+ load as hypertonic NaCl infusion, did not induce natriuresis. This suggested, therefore, that with the hypertonic sodium load administered in the present study, the rise in plasma Na+ and/or tonicity rather than increase in blood volume is important in evoking the natriuretic response. 3. Intracerebroventricular infusion of low-Na+ artificial cerebrospinal fluid (CSF) reduced CSF Na+ concentration, decreased plasma vasopressin (AVP) levels and caused a copious water diuresis. This was associated with excessive loss of water and large increases in plasma Na+ concentration and osmolality. 4. The natriuresis induced by intravenous hypertonic NaCl load could be blocked by lowering CSF Na+ concentration in situations where water diuresis was either prevented or reduced by intravenous infusion of AVP or by delayed intracerebroventricular infusion of low-Na+ CSF, respectively. 5. The results of the present study provide further evidence that renal sodium excretion can be controlled by the central nervous system.

Renal denervation does not prevent dehydration-induced natriuresis in sheep

Normal sheep or sheep in which the renal nerves had been extirpated were deprived of water for 2 days in order to determine whether changes in renal nerve activity contribute to natriuresis during water deprivation. Both groups of sheep showed a considerable natriuresis throughout the period of water deprivation and increases in plasma osmolality and plasma Na concentration. Renal denervation, as indicated by the absence of catecholamine fluorescence in kidney sections, was extensive. Previous experiments have suggested cerebral involvement in the induction of dehydration-induced natriuresis. The present results indicate that the efferent pathway mediating this cerebral influence on renal sodium excretion does not involve the renal nerves, suggesting a hormonal mechanism as the likely pathway.

Humoral control of water and electrolyte excretion during water restriction

The goals of the present study were twofold: first, to assess the renal excretory and hormonal responses to chronic water restriction in dogs whose sodium retaining mechanisms had been stimulated through dietary sodium (Na+) deprivation; second, to determine the mediator(s) of the natriuresis which was observed with water restriction in these sodium deprived dogs. Three groups of dogs maintained on a low Na+ diet (5 mEq/day) for two weeks underwent a three day period of water restriction. In normal, intact dogs Group 1 (N = 5), water restriction resulted in a significant increase in Na+ excretion with a net cumulative loss of 26.3 +/- 2.6 mEq over three days. The natriuresis was associated with a significant increase in plasma vasopressin (PAVP) (1.7 to 10.2 pg/mliter) and a significant fall in plasma aldosterone (PALDO) from the levels observed with Na+ restriction alone (24.9 to 12.4 ng/dliter). The natriuresis could not be explained by decreases in food intake as determined by control studies in four dogs. Group 2 (N = 6) dogs had a decrease in PALDO with water restriction that was prevented by means of continuous i.v. aldosterone infusion (6.0 micrograms/kg/day). Dogs in this group failed to demonstrate a natriuresis during three days of water restriction, despite the fact that PAVP rose from 3.3 +/- 0.8 to a peak level of 14.95 +/- 1.9 pg/mliter. Group 3 (N = 6) dogs underwent selective neurohypophysectomy, thus preventing the rise in PAVP during three days of water restriction. In this group, PALDO also remained unchanged from the Na+ deprived level during water restriction, and no natriuresis was observed. We conclude: 1) that the natriuresis which occurs with water restriction is a potent physiological response that occurs even in the Na+ restricted state; and 2) this natriuresis can be explained by a fall in PALDO and not the rise in PAVP.

The influence of intracerebroventricular infusions on osmotically induced urine excretion in the pigeon (Columba livia)

This study deals with the influence of intracerebroventricular (ICV) infusions on urine volume and electrolyte excretion in response to a peripherally administered osmotic load in conscious behaving pigeons. The ICV infusions were intended to influence the cerebrospinal fluid (CSF) sodium (Na+) concentration that would have been increased by the hypertonic NaCl or sucrose solutions infused intravenously (IV). Urine and electrolyte excretion following IV infusion of 0.5 M NaCl were enhanced by simultaneous ICV infusion of 0.3 M NaCl and essentially unchanged by ICV infusions of 0.3 M or 0.9 M sucrose, or water. Infusions (ICV) of water, isotonic and hypertonic NaCl or sucrose did not significantly influence urine and electrolyte excretions following IV infusion of 1.0 M NaCl or sucrose (except K+ in the case of ICV/IV sucrose). Isotonic (0.3 M) or hypertonic (0.9 M) sucrose infused ICV enhanced urine and electrolyte excretion following IV infusion of 1.5 M sucrose. Similar amounts of sodium were excreted following IV infusion of 0.5 M NaCl, 1.0 M sucrose or 1.5 M sucrose plus the ICV infusions. The results suggest that the concentration of Na+ in the CSF is one of the factors that play a role in urine and electrolyte excretion following IV administration of osmotic stimuli in the pigeon.