Evidence for a hypothalamic control of renal sodium excretion

Effect of intracerebroventricular epinephrine microinjection on blood pressure and urinary sodium handling in gestational protein-restricted male adult rat offspring

In this study, we hypothesized that blunting of the natriuresis response to intracerebroventricularly (ICV) microinjected adrenergic agonists is involved in the development of hypertension in maternal low-protein intake (LP) offspring. A stainless steel cannula was stereotaxically implanted into the right lateral ventricle (LV), then we evaluated the ICV administration of adrenergic agonists at increasing concentrations, and of α1 and α2-adrenoceptor antagonists on blood pressure and urinary sodium handling in LP offspring relative to an age-matched normal-protein intake (NP) group. We confirmed that epinephrine (Epi) microinjected into the LV of conscious NP rats leads to enhanced natriuresis followed by a reduction in arterial pressure. This response is associated with increased proximal and post-proximal sodium excretion accompanied by an unchanged glomerular filtration rate. The current study showed, in both NP and LP offspring, that the natriuretic effect of Epi injection into the LV was abolished by prior local microinjection of an α1-adrenoceptor antagonist (prazosin). Conversely, LV α2-adrenoceptor antagonist (yohimbine) administration potentiated the action of Epi. The LV yohimbine pretreatment normalized urinary sodium excretion and reduced the blood pressure in LP compared with age-matched NP offspring. These are, as far as we are aware, the first results showing the role of central adrenergic receptors’ interaction on hypertension pathogenesis in maternal LP fetal-programming offspring. This study also provides good evidence for the existence of central nervous system adrenergic mechanisms consisting of α1 and α2-adrenoceptors, which work reciprocally on the control of renal sodium excretion and blood pressure. Although the precise mechanism of the different natriuretic response of NP and LP rats is still uncertain, these results lead us to speculate that inappropriate neural adrenergic pathways might have significant effects on tubule sodium transport, resulting in the inability of the kidneys to control hydrosaline balance and, consequently, an increase in blood pressure.

Summary: We evaluated the effect of intracerebroventricular microinjections of adrenergic agonists at increasing concentrations, and μ1 and μ2-adrenergic receptors antagonists on blood pressure and urinary sodium in hypertensive low- and normal-protein rat offdpring.

Altered urinary sodium excretion response after central cholinergic and adrenergic stimulation of adult spontaneously hypertensive rats

In this study, we hypothesized that blunting of the natriuresis response to intracerebroventricularly (i.c.v.) microinjected cholinergic and adrenergic agonists is involved in the development of hypertension in spontaneously hypertensive rats (SHR). We evaluated the effect of i.c.v. injection of cholinergic and noradrenergic agonists, at increasing concentrations, and of muscarinic cholinergic and α1 and α2-adrenoceptor antagonists on blood pressure and urinary sodium handling in SHR, compared with age-matched Wistar Kyoto rats (WR). We confirmed that CCh and NE microinjected into the lateral ventricle (LV) of conscious rats leads to enhanced natriuresis. This response was associated with increased proximal and post-proximal sodium excretion accompanied by an unchanged rate of glomerular filtration. We showed that cholinergic-induced natriuresis in WR and SHR was attenuated by previous i.c.v. administration of atropine and was significantly lower in the hypertensive strain than in WR. In both groups the natriuretic effect of injection of noradrenaline into the LV was abolished by previous local injection of an α1-adrenoceptor antagonist (prazosin). Conversely, LV α2-adrenoceptor antagonist (yohimbine) administration potentiated the action of noradrenaline. The LV yohimbine pretreatment normalized urinary sodium excretion in SHR compared with age-matched WR. In conclusion, these are, as far as we are aware, the first results showing the importance of interaction of central cholinergic and/or noradrenergic receptors in the pathogenesis of spontaneous hypertension. These experiments also provide good evidence of the existence of a central adrenergic mechanism consisting of α1 and α2-adrenoceptors which works antagonistically on regulation of renal sodium excretion.

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.

Altered renal sodium handling in spontaneously hypertensive rats (SHR) after hypertonic saline intracerebroventricular injection: role of renal nerves

The mechanism by which blood pressure rises in the SHR strain remains to be elucidated. Also, there is a surprising lack of experimental data on the natriuretic mechanisms induced by intracerebroventricular (ICV) injection of hyperosmotic saline (HoS) in SHR. In normotensive animals ICV injection of HoS causes coordinated responses including natriuresis and inhibition of renal sympathetic nerve activity. In the present study, we hypothesized that presumable blunting of the sympathoinhibitory response to centrally injected HoS may contribute to a lack of suppression of efferent renal nerve outflow in SHR. To test this hypothesis, the present study evaluates the influence of renal denervation after central HoS injection at increasing concentration on urinary sodium handling in SHR compared with age-matched normotensive WKy rats. The study confirmed previous data showing pronounced natriuretic response to centrally HoS stimuli but also demonstrated that the creatinine clearance (C(Cr)) and fractional sodium excretion responses diminished as graded NaCl concentrations were increased in WKy rats but not in SHR. In SHR, increased FE(Na) obtained by central administration of 0.90 M NaCl was produced by increases in proximal (FEP(Na)) and post-proximal fractional urinary sodium rejection without changes in C(Cr), indicating a direct tubular effect. Renal denervation caused significant antinatriuresis by decreased C(Cr) and increased FEP(Na) reabsorption in WKy but not in SHR. This study suggests that natriuresis observed only after higher centrally HoS stimuli with a rightward shift of dose-response curve provides evidence of a down-regulation of target organ responsiveness of periventricular areas of genetic hypertensive rats.

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.

Intracerebroventricular infusion of hypertonic NaCl increases urinary CGMP in healthy and cirrhotic rats

Implication of serum atrial natriuretic peptide (ANP) and endothelin-1 (ET1) in the central nervous system (CNS)-induced natriuresis and hypertension respectively, was investigated in healthy and cirrhotic rats. Both healthy and nonascitic CCl(4)-induced cirrhotic rats under pentobarbital anesthesia received either normotonic (140 mmol/L) or hypertonic (320 mmol/L) NaCl artificial cerebrospinal fluid into the CNS lateral ventricle at a rate of 8.3 microl/min for 120 min. A sham operated group, but not centrally infused, served as matched control. Hypertonic NaCl solution significantly increased mean arterial pressure (MAP) similarly in both healthy (n = 5) ((MAP: 16 mm Hg, 13%) and cirrhotic rats (n = 6) ((MAP: 20 mm Hg, 15%) (ANOVA, p <.001) although the latter showed a slower increment. Under hypertonic NaCl infusion, natriuresis was also significantly increased in a similar manner in both healthy (U (Na) V: baseline: 0.38 +/- 0.22 micromol/min x 100 g; experiment: 2.36 +/- 0.90 micromol/min x 100 g; mean +/- SD) and cirrhotic rats (0.69 +/- 0.48 vs. 3.16 +/- 0.87; p <.001). By contrast, central hypertonic NaCl solutions did not show a significant modification of serum ANP in neither healthy (62 +/- 18 fmol/ml vs. 51 +/- 17 fmol/ml) nor cirrhotic rats (126 +/- 61 vs. 115 +/- 30). Likewise, ET-1 was not significantly modified under central hypertonic NaCl infusion in neither healthy (352 +/- 46 pg/ml vs. 344 +/- 39 pg/ml) nor cirrhotic rats (287 +/- 58 vs. 277 +/- 61). Despite no modification in serum ANP, there was a significant increment in urinary excretion of cGMP under central hypertonic NaCl infusions in bo th healthy (6.8 +/- 4.1 pmol/min x 100 g vs. 13.0 +/- 6.5 pmol/min x 100 g; p <.05) and cirrhotic rats (8.6 +/- 1.7 vs. 11.1 +/- 1.3; p <.05). Our data indicate the preservation of the mechanisms of central natriuresis in a model of non-ascitic CCl(4 )-induced cirrhosis in rats. An increment in urinary cGMP could potentially be implicated in the natriuretic response obtained by intracerebroventricular hypertonic NaCl stimulus in both healthy and cirrhotic rats. The lack of modification of serum ANP and ET-1 does not appear to support a systemic implication of these peptides in the natriuretic and hypertensive responses respectively induced by this manoeuvre.

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)

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.

Natriuresis in conscious dogs caused by increased carotid [Na+] during angiotensin II and aldosterone blockade

The renal response to a selective increase in the Na+ concentration of the blood perfusing the central nervous system was investigated in conscious dogs treated with the converting enzyme inhibitor enalaprilat and the aldosterone antagonist canrenoate. In split-infusion experiments the plasma [Na+] of carotid blood was increased (approx. 6 mM) by bilateral infusion of hypertonic NaC1. Concomitantly distilled water was infused into the v. cava making the sum of the infusions isotonic. In control experiments isotonic saline was infused at identical rates into all three catheters. Na+ excretion increased markedly in both series, 103 +/- 14 to 678 +/- 84 mumol min-1 during split-infusion and 90 +2- 14 to 496 +/- 74 mumol min-1 during the isotonic volume expansion. Peak rate of excretion, peak fractional sodium excretion, and cumulative sodium excretion were all significantly higher (P < 0.05) during split-infusion than during control experiments. Plasma vasopressin increased only during split-infusion (0.68 +/- 0.11 to 2.4 +/- 0.8 pg ml-1) while the increases in plasma atrial natriuretic peptide were similar in the two series. Urinary excretion of urodilatin (ANP95-126) increased significantly more during split-infusion (46 +/- 11 to 152 +/- 28 fmol min-1) than during the isotonic volume expansion (45 +/- 14 to 84 +/- 16 fmol min-1) (P < 0.05). It is concluded that the natriuretic mechanisms activated by a selective increase in the Na+ concentration of carotid blood and associated with increased excretion of urodilatin cannot be eliminated by blockade of the renin-angiotensin-aldosterone system.

C-fos expression in arcuate nucleus following intracerebroventricular hypertonic saline injections

Experiments were done in conscious rats to investigate the effect of i.c.v. infusions of hypertonic NaCl solutions on the induction of the protein Fos in the arcuate nucleus (Arc). Neurons containing Fos-like immunoreactivity were observed throughout the rostrocaudal extent of Arc after i.c.v. infusions of hypertonic saline solutions (337-744 mM). However, most of the labelled neurons were confined to the middle third of the nucleus, in the region of the dorsomedial and ventromedial subnuclei. Few, if any Fos-labelled neurons were observed in Arc of animals that received i.c.v. infusions of isotonic (142 mM) or mild hypertonic (173 mM) saline solutions or a hyperosmotic (660 mOsm/kg) saline solution of mannitol. No Fos-labelled neurons were found in the subfornical organ, although a few were observed scattered throughout the organum vasculosum laminae terminalis (OVLT) in all the animals studied. The density nor the distribution pattern of Fos-labelled neurons in OVLT was altered in animals receiving i.c.v. infusions of hypertonic saline or hyperosmotic solutions. These data demonstrate that Arc neurons are activated during a hypertonic saline challenge and suggest that Arc may function as a sodium-sensitive structure that is involved in body-fluid and circulatory homeostasis.

Natriuresis after a water load in humans under different sodium body content

The present study was aimed at observing the diuretic and natriuretic responses after a water load (2% body weight) in four groups of young consenting volunteers submitted previously, during three days, to a hypersodic (500 mEq Na/day), hyposodic (35 mEq Na/day), and normosodic (200 mEq Na/day) diet, or treated with furosemide (Lasix, 40 mg/day). During the treatment urine was collected each day. On the fourth day, in the morning, the bladder was emptied, the water load was ingested, and the urine collected during 10 periods of 20 min each. The urinary, sodium, and chloride flows were determined. The four groups displayed diuretic curves following a similar pattern. In contrast, the natriuretic curves of the four groups were completely different; totally flat with low values for the furosemide group and a large initial natriuretic curve for the hypersodic group with a gradual decrease but maintaining high values. The results indicate that the way the organism compensates for the excess of water by means of urinary water loss is independent of the body sodium content, whereas the way in which sodium loss is accomplished is determined by its body content and is independent of the way in which the water is lost.

Increase in the blood pressure and decrease in the norepinephrine release in the ventrolateral medulla during intraventricular administration of hypertonic NaCl

Norepinephrine (NE) release in the ventrolateral medulla (VLM) was serially measured in anesthetized male Wistar rats during the rise in the blood pressure (BP) produced by acute intraventricular (ICV) administration of hypertonic (1.5 M) NaCl. Catecholamine release was determined by a brain microdialysis method using high performance liquid chromatography and electrochemical detector. The release of NE in the VLM was significantly decreased after ICV 1.5 M NaCl. In another set of rats, the pressor response to acute ICV 1.5 M NaCl was attenuated by selective administration of NE to the VLM using the microdialysis method. Chronic and continuous ICV infusion of 1.5 M NaCl to conscious rats caused an increase in BP on day 10 which was associated with a decrease in NE release in the VLM; concomitant ICV infusion of NE or of a synthetic NE precursor, L-threo-3,4-dihydroxyphenylserine (L-DOPS) prevented the rise in BP as well as the reduction in NE release. These results suggest that a decrease in the NE release of the VLM may contribute to the change in BP induced by ICV infusion of hypertonic saline.

Lack of evidence for a cerebral sodium modulating mechanism in the monkey

Experiments were carried out in seven conscious macaque monkeys undergoing a water diuresis to determine the effects of raising carotid blood sodium concentration on renal sodium excretion and free water clearance. On separate days each animal received an intracarotid infusion of hypertonic sodium chloride (90 Eq NaCl/kg.body wt./min) for 5 to 10 min, the same hypertonic infusion intravenously, and an intracarotid infusion of isotonic NaCl. None of the infusions produced a change in sodium excretion. However, the intracarotid hypertonic infusion produced a sustained decrease in free water excretion while the other infusion did not. Creatinine clearance was not affected by any of the infusions. The results of these experiments support the view that while the brain of the primate contains an osmotic sensing mechanism it does not contain a mechanism which modulates sodium excretion.

Ammonium chloride induced acidosis and aldosterone secretion in the goat

Responses to 30-min intraduodenal infusion of NH4Cl (total amount 75 mmol) were studied in conscious goats. The infusion caused an immediate, transient rise in plasma aldosterone concentration (PA) from a mean of 78 to 221 pmo l-1. As expected, the NH4Cl administration also induced metabolic acidosis, initially subjected to partial respiratory compensation. The acidosis did not become fully developed until 1 h after cessation of the infusion, when PA had almost returned to its initial level. Renal compensation of the acidosis was shown by acidification of the urine and reduced Na excretion being most pronounced 1-2 h post-infusion. During the infusion blood haemoglobin concentration and the haematocrit increased by 25 and 13%, respectively, without simultaneous increase in plasma protein concentration and with persisting ear vasodilatation, indicating a mobilization of stored erythrocytes in the absence of a general increase in sympathetic tone. The results suggest that the reduction of blood pH is not the cause of the increase in PA occurring in association with NH4Cl-induced metabolic acidosis, but leave open the possibility that this increase may be due to some centrally mediated or direct adrenal influence of NH+4. As regards the apparent NH4Cl-induced mobilization of stored erythrocytes, it is suggested that such a response may play a role in the defence against acidosis by increasing the buffering capacity of the circulating blood.

Redistribution of glomerular filtration and renal plasma flow in CNS-induced natriuresis

Infusion of hypertonic sodium chloride solution into the third cerebral ventricle results in a marked increase in renal sodium output, indicating an important regulator of extracellular volume homeostasis. The intrarenal events governing the enhanced excretion have not been thoroughly studied previously. In 12 anaesthetized male rats a stainless steel cannula was introduced stereotaxically into the right lateral cerebral ventricle. Urine volume and excretion rates, Na, K, and osmotically active particles were measured during control infusion of artificial cerebrospinal fluid and during stimulation of central mechanisms with I M NaCl (520 nl min-1). At the end of the stimulation period, regional renal plasma flow (86RbCl) and glomerular filtration rate (51Cr-EDTA) were measured with single injection techniques. A second group of 12 non-stimulated rats served as controls. During ICV stimulation, the urine flow rate increased from 1.8 +/- 0.19 to 6.4 +/- 1.01 microliter min-1 (P less than 0.001). The urinary concentrations of Na and K increased, leading to a rise in the excretion rates of these ions from 0.12 +/- 0.025 to 0.96 +/- 0.352 mumol min-1 (P less than 0.001) and 0.40 +/- 0.083 to 1.70 +/- 0.196 (P less than 0.001), respectively. The osmolar excretion rate was 2.9 +/- 0.35 mu Osm min-1 before stimulation and 9.6 +/- 1.09 higher (P less than 0.001) during stimulation. Simultaneously the inner medullary plasma flow rose two-fold from 0.7 +/- 0.06 to 1.4 +/- 0.12 microliter min-1 tissue (P less than 0.008).(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of the renin-angiotensin system in captopril-induced sodium appetite in the rat

The angiotensin converting enzyme inhibitor, captopril, given to rats in their drinking water (about 40 mg/day) for 6 days caused an increase in intake of hypertonic NaCl solution which began 1-2 days after the captopril was started and reached a plateau after 4-5 days. Twice-daily subcutaneous injections of captopril (15 mg per injection) elicited a sodium appetite similar in pattern to that seen with oral administration. The rats remained in sodium and fluid balance during oral captopril treatment and the haematocrit did not alter. Captopril infused directly into the ventricles (12 micrograms/h), or captopril reaching the brain from the periphery across a leaky blood-brain barrier, suppressed the sodium appetite which normally follows oral captopril. Continuous intravenous infusion of captopril at rates high enough to block angiotensin converting enzyme in the brain (25, 50 or 500 mg/day) did not cause sodium appetite. As soon as the rate was reduced to a low value (5 mg/day), NaCl intake increased. In conclusion, moderate levels of circulating captopril which do not cross the blood-brain barrier in sufficient amounts to block cerebral angiotensin converting enzyme, result in an increase in circulating angiotensin I which stimulates sodium appetite when it is converted to angiotension II in the brain.

Chronic cerebroventricular infusion of hypertonic sodium chloride in rats reduces hypothalamic sympatho-inhibition and elevates blood pressure

To determine whether or not salt loading restricted to the brain would elevate blood pressure, chronically implanted osmotic minipumps were used to infuse hypertonic sodium chloride solutions made in artificial cerebrospinal fluid into a jugular vein intravenously or the third cerebral ventricle, intracerebroventricularly, for 11 days in awake rats. During intracerebroventricular infusion of hypertonic sodium chloride, tail-cuff systolic pressures began to rise on day 4 and were significantly elevated by day 9. In contrast, infusions of either artificial cerebrospinal fluid alone, intracerebroventricularly, or hypertonic sodium chloride, intravenously, were ineffective. Confirming the blood pressure elevation thereby detected, systolic and mean pressures recorded directly from indwelling aortic catheters after the same rats had been anesthetized with urethane on day 11, were also significantly higher following intracerebroventricular infusion of hypertonic sodium chloride than after infusion of artificial cerebrospinal fluid alone. Magnitude of depressor and sympatho-inhibitory responses elicited by graded electrical stimulation of the anterior hypothalamus invariably increased with the current strength used for stimulation. At all current strengths used for hypothalamic stimulation, depressor responses as well as attendant decreases in sympathetic neural firing, were smaller in rats that had been infused intracerebroventricularly with hypertonic sodium chloride than in any of the controls. Inhibition seemed specific for the anterior hypothalamus because pressor responses to stimulation of the ventromedial hypothalamus were the same whether or not the intracerebroventricular infusion contained hypertonic sodium chloride. An explanation based on diminished cardiovascular sensitivity also appeared unlikely, since depressor responses to intravenously injected histamine were almost equal in both groups. Our results are compatible with the interpretation that hypertonic sodium chloride infused chronically into the 3rd ventricle acts by reducing anterior hypothalamic inhibition of sympathetic vasomotor tone, and that this, in turn, then elevates blood pressure.

Renin-induced sodium appetite: effects on sodium balance and mediation by angiotensin in the rat

1. Injection of pig renin or purified renin from the mouse submaxillary gland into the preoptic region or third ventricle of the rat caused thirst within a minute or so of injection followed shortly afterwards by increased sodium appetite. Renin from two widely different sources produced identical responses.2. The stimulating effect of renin on intake of water and hypertonic (2.7%) NaCl was continuous and persisted for at least a week after the largest (265 ng) dose of purified renin.3. The stimulating effect was also very large. A single preoptic injection of less than 0.75 pmol (26.5 ng) purified mouse renin caused mean intakes of 250.4 +/- 26.2 ml water and 44.8 +/- 12.5 ml 2.7% NaCl by five naive rats in 24 h. After the largest dose (265 ng) intakes of water and 2.7% NaCl reached about 80% and 20% body weight respectively.4. Weekly injections of renin resulted in progressively larger intakes of NaCl and water in response to the injections.5. Even after repeated injections, carbachol did not stimulate sodium appetite. The stimulating effect on water intake was quickly over and showed no progressive increase with repeated injections. Overnight intake of water was generally depressed after carbachol.6. Preoptic injection of renin caused some increase in sodium excretion but this was small compared with the stimulating effect on sodium appetite.7. Detailed temporal analysis of fluid and sodium balance shows that the increased intakes of water and 2.7% NaCl were not secondary to renin-induced urinary losses. Increased intakes of water and 2.7% NaCl caused by renin resulted in the rats going into and remaining in positive fluid and sodium balance throughout the 24 h experiment.8. Renin-induced sodium appetite and thirst were inhibited by the converting enzyme inhibitors teprotide or captopril, or by the angiotensin antagonist saralasin. Inhibition was longer lasting after captopril. Carbachol-induced thirst was unaffected.9. In conclusion, renin injected into the preoptic region or third ventricle is a potent stimulus to sodium appetite as well as thirst. The effect is mediated by local generation of angiotensin II and it is not secondary to increased urinary loss.

Modulation of renal tubular function by renal interstitium

Rat micropuncture experiments have been performed to investigate the influence of renal interstitial pressure conditions on proximal tubular fluid absorption and on the tubulo-glomerular feedback mechanism. The feedback mechanism works by sensing the distal tubular fluid flow rate at the macula densa segment and adjusting the tonus of the arterioles with resulting changes in blood flow and glomerular filtration rate. It was found that in situations with high hydrostatic and low oncotic pressure within the interstitium like in saline volume expansion and post-nephrectomy situations, proximal tubular fluid absorption and the sensitivity of the tubulo-glomerular feedback control are reduced giving rise to increased urine production without activating the feedback mechanism. In situations with low hydrostatic and high oncotic interstitial pressures like in dehydration, hypotension and hypovolemia proximal fluid absorption and feedback sensitivity is increased, feedback sensitivity even so much that the mechanism is activated to reduce GFR even though the load to the distal portion of the nephron is reduced below normal. In this way renal interstitial pressure-volume conditions can modulate renal function in response to extracellular fluid needs and blood pressure level.

Heterogeneity in regulation of glomerular function

The aim was to study differences in filtration driving forces and glomerular filtration rates between superficial and deep nephrons when urine flow rate was altered at the macula densa region. In young rats stop-flow pressures and single nephron glomerular filtration rates (SNGFR) were measured in the superficial proximal tubules and in the loops of Henle in the papilla. SNGFR was also measured with a modified Hanssen technique. The stop-flow pressures of superficial nephrons amounted to 30.9 +/- 0.8 mmHg (mean +/- SE) and those of juxtamedullary nephrons to 52.2 +/- 1.6 mmHg. In the stop-flow condition the net driving filtration forces were calculated to be about 19 mmHg and 50 mmHg for the superficial and deep glomeruli, respectively. In free flow conditions both net driving forces were calculated to be 19 mmHg. The micropuncture technique gave a SNGFR value for superficial nephrons of 29.6 +/- 2.9 and for deep nephrons of 84.1 +/- 8.5 nl x min-1 . g-1 kidney weight (KW). With a modified Hanssen technique the corresponding values were 25.8 +/- 3.3 and 27.7 +/- 2.9 nl . min-1 . g-1 KW. The tubuloglomerular feedback mechanism is considered to have a powerful regulatory influence on the glomerular filtration rate of deep nephrons.

Renal renin output during continuous intracarotid infusions of iso- and hypertonic sodium chloride solutions in the rat

On male Sprague Dawley rats isotonic NaCl during control and 1 M NaCl were infused either into the carotid artery or i.v. Glomerular filtration rate, sodium output and plasma renin activity were determined. Five of 19 animals reacted with no increase in sodium output for 1 M NaCl and are treated as a different group from the animals reacting with an increased sodium output. For the animals reacting with an increased sodium output a decrease in plasma renin activity was found together with an increase in glomerular filtration rate. In both groups the plasma sodium was constant. The animals not reacting with an increased sodium output had a higher initial plasma renin activity, which did not change during 1 M NaCl infusion. The responses were equal for both intracarotid and i.v. infusions but with a somewhat longer delay before the response occurred with i.v. infusions. These results might be explained by a central nervous effect and by a direct renal effect. Also during 1 M NaCl infusion a possible extracellular volume expansion, derived from water withdrawal from the cells into the extracellular space might occur.

Influence of prostaglandin E1 on cerebral mechanisms involved in the control of fluid balance

The effects of infusions of PGE1 (30 ng/kg min-1) into the lateral cerebral ventricle were studied in the conscious, hydrated goat. The infusions caused release of antidiuretic hormone and increased renal sodium excretion. When PGE1 was infused together with hypertonic NaCl these effects became markedly enhanced and the infusion also induced drinking and a rise in the arterial blood pressure. Much weaker effects were obtained by the infusion of the hypertonic NaCl alone. This sodium-PGE1 interaction is discussed in relation to previously observed, central sodium-angiotensin II interaction. A more pronounced drinking effect was obtained in response to the intraventricular infusion of PGE1 + angiotensin II, than to the infusion of either substance separately. The PGE1 administered into the lateral cerebral ventricle did not induce any febrile response.

Studies of cerebral osmoreceptors in anesthetized dogs: the effect of intravenous and intracarotid infusion of hyper-osmolar sodium chloride solutions during sustained water diuresis

The function of the suggested hypothalamic osmoreceptors was investigated in dogs during light chloralose anesthesia. The dogs were subjected to an i.v. load of 40 ml/kg b.w.t. of a hypo-osmolar solution of sodium chloride and glucose. This degree of hydration was kept constantly by a specially constructed servo system based on the weight of the animal. During water diuresis the renal free water clearance remained essentially constant for several hours (mean about 0.2 ml/kg b.w.t. min). Renal sodium excretion was low (mean 0.82 mumol/kg b.w.t. min) and decreased continuously throughout the experimental period. I.v. infusion of hyperosmolar sodium chloride solution (1.33 mmol/kg b.w.t. in 30 min) was followed by prolonged parallel increases in free water clearance and sodium excretion, without any detectable change in the excretion of osmoles and potassium. The renal response to bilateral infusion of hyper-osmolar NaCl (1.33 mmol/kg b.w.t. in 30 min) into the common carotid arteries was identical to the response to i.v. infusion. The estimated increase in the osmolality of the carotid blood was 2.2%. In seven out of eight experiments intracarotid infusion of NaCl (1.33 mmol/kg b.w.t. in 8 min) did not elicit any reduction in free water clearance. On the contrary, an increase was found similar to that obtained after i.v. infusion. The estimated increase in the osmolality of the carotid blood was 8.4%. The present results question the validity of the currently held view that hypothalamic osmoreceptors play an important role in the control of the osmolality of plasma.

Natriuresis, kaliuresis and diuresis in the rat following microinjections of carbachol into the septal area

The effects of intraseptal injection of carbachol on natriuresis, kaliuresis and diuresis has been studied in conscious, unrestrained water-loaded male rats. Urinary sodium and potassium excretion increased following injections into the septal area. The intensity of the natriuresis and kaliuresis was dose-related. An antidiuretic effect was also observed. The Na+/K/ ratio increased with increasing doses of carbachol, indicating that the rise in urinary sodium exceeded that of potassium. Systematic mapping of the septal area yielded about the same results for all sites, excepting a zone located in the anterior-dorsal part of the medial nucleus which appeared more sensitive. The natriuretic effect of intraseptal carbachol in adrenalectomized rats demonstrated the secondary role played by the adrenals. Contrariwise the decrease of the natriuretic effect observed either in hypophysectomized rats or in rats bearing a median eminence lesion receiving intraseptal carbachol showed the important participation of these structures in urinary Na+ excretion. Adrenalectomy or median eminence lesions did not modify the kaliuretic response while hypophysectomy produced a transitory diminution. This fact favours the hypothesis of different mechanisms involved in Na+ and K+ excretion following intraseptal carbachol. These results leave open the question as to mechanism of action but suggest a possible role of the pituitary in mediating the responses. Also, the possibility of a role played by hemodynamic shifts is suggested.

A central osmosensitive receptor for renal sodium excretion

1. The effect on renal Na and water excretion of increasing the NaCl concentration of blood supplying the brain was investigated in conscious water-loaded sheep. Intracarotid infusion ot 4 M-NACl at 0-8 ml./min for 60 min was compared with equivalent intrajugular infusion. 2. A more rapid increase in renal Na excretion and urine osmolality occurred with the intracarotid infusions than with intrajugular infusions. 3. Intracarotid infusions of 2 M sucrose or fructose at 1-6 ml./min for greater increase in renal Na excretion, urine osmolality and a decrease in urine flow rate. 4. The results suggest that there are receptors in the brain sensitive to changes in extracellular tonicity which influence renal Na excretion. It is possible that changes in ADH secretion alone mediate the early natriuresis seen with intracarotid hypertonic infusions although an alternative concurrent mechanism cannot be ruled out.

Plasma saluretic activity: its nature and relation to oxytocin analogs

Plasma natriuretic activity was evoked in cows and dogs by infusion of saline with or without dextran. Deproteinized samples were fractionated on both Sephadex and Bio-Gel columns; the activity was separated, the approximate molecular weight being in the region of 1000. Incubation with chymotrypsin destroyed the activity, suggesting that it might be a polypeptide. A similar activity in blood resulted from intracarotid injection of either oxytocin or either of two synthetic analogs. Possibly the latter are saluretic by virtue of a releasing action on some intracranial structure for another natriuretic peptide.