Comparison of renal responses to 5% saline infusions into vena portae and vena cava in conscious dogs

Gastrointestinal osmoreceptors and renal sodium excretion in humans

The hypothesis that natriuresis can be induced by stimulation of gastrointestinal osmoreceptors was tested in eight supine subjects on constant sodium intake (150 mmol NaCl/day). A sodium load equivalent to the amount contained in 10% of measured extracellular volume was administered by a nasogastric tube as isotonic or hypertonic saline (850 mM). In additional experiments, salt loading was replaced by oral water loading (3.5% of total body water). Plasma sodium concentration increased after hypertonic saline (+3.1 +/- 0.7 mM), decreased after water loading (-3.8 +/- 0.8 mM), and remained unchanged after isotonic saline. Oncotic pressure decreased by 9.4 +/- 1.2, 3.7 +/- 1.2, and 10.7 +/- 1.3%, respectively. Isotonic saline induced an increase in renal sodium excretion (104 +/- 15 to 406 +/- 39 micromol/min) that was larger than seen with hypertonic saline (85 +/- 15 to 325 +/- 39 micromol/min) and water loading (88 +/- 11 to 304 +/- 28 micromol/min). Plasma ANG II decreased to 22 +/- 6, 35 +/- 6, and 47 +/- 5% of baseline after isotonic saline, hypertonic saline, and water loading, respectively. Plasma atrial natriuretic peptide (ANP) concentrations and urinary excretion rates of endothelin-1 were unchanged. In conclusion, stimulation of osmoreceptors by intragastric infusion of hypertonic saline is not an important natriuretic stimulus in sodium-replete subjects. The natriuresis after intragastric salt loading was independent of ANP but can be explained by inhibition of the renin-angiotensin system.

The intestinal tract and the pathophysiology of arterial hypertension: an experimental study on Dahl rats

Salt depleted rabbits and humans excrete an oral sodium load more quickly via the kidneys than an intravenous one. This has been ascribed to the presence of a sodium sensor in the gastrointestinal tract which in some way can influence renal function. The purpose of this study was to investigate this response in the Dahl rats. Renal and faecal sodium excretion was followed in the two strains of rats (normotensive, saltresistant (SR/Jr) and hypertensive, saltsensitive (SS/Jr) rats). After 4 days on a low salt diet they were given NaCl (1.5 mmol k(-1) body wt) either by gavage or intravenously. SR/Jr rats showed an increased renal sodium excretion both after oral and intravenous sodium repletion. The excretion was 2-3 times greater after th oral than after the intravenous administration. The SS/Jr rats augmented their renal sodium excretion only after the oral load, although the sodium excretion was significantly less than in SR/Jr rats. In fact, during the first 8 h after giving sodium orally the renal excretion of sodium was on an average eight times larger in the SR/Jr than in the SS/Jr rats. Renal excretion of sodium was similar in the two strains after intravenous administration. We conclude that the hypertensive SS/Jr rats have great difficulties in excreting an oral sodium load, a phenomenon that may be of importance in the pathophysiology of arterial hypertension in this strain of rats.

Effect of chronic hypertonic saline ingestion on vasopressin gene expression in the rat

The present study was undertaken to examine vasopressin gene expression in response to a normal versus hypertonic sodium chloride (506 mOsm/kg H2O) intake for 7 days in Sprague-Dawley rats. The animals in both groups demonstrated precision in maintaining constancy of body fluid composition in spite of large differences in sodium and water intakes. Compared with the rats on a normal diet, chronic ingestion of hypertonic sodium chloride resulted in significant increases in total fluid intake (210 +/- 8 mL v 471 +/- 48 mL, P < 0.001) and total urine output (86 +/- 5 mL v 347 +/- 48 mL, P < 0.001), while glomerular filtration rate, hematocrit, serum urea nitrogen, creatinine, serum sodium, and plasma osmolality were unchanged. Without detectable changes in plasma osmolality or intravascular volume, vasopressin release from the pituitary, as measured by plasma and pituitary vasopressin concentrations (1.5 +/- 0.1 pg/mL v 5.9 +/- 1.5 pg/mL, P < 0.01 and 2.0 +/- 0.5 micrograms/pituitary v 0.86 +/- 0.1 micrograms/pituitary, P < 0.01, respectively), was increased in the animals ingesting hypertonic sodium chloride. In addition, vasopressin gene expression as measured by hypothalamic vasopressin mRNA concentrations was significantly increased 1.85-fold (P < 0.001) in the animals ingesting hypertonic sodium chloride. In summary, Sprague-Dawley rats ingesting hypertonic sodium chloride (506 mOsm/kg H2O) were able to maintain sodium and water homeostasis over a 7-day period. Yet, in these animals plasma vasopressin increased, pituitary vasopressin stores decreased, and hypothalamic vasopressin gene expression was stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatorenal reflex plays an important role in natriuresis after high-NaCl food intake in conscious dogs

Responses of renal nerve activity and urinary Na+ and Cl- excretion were examined in chronically instrumented conscious dogs through feedings of boiled rice with or without NaCl. The boiled rice (20 g/kg body wt) without NaCl did not influence plasma Na+ and Cl- concentrations, renal nerve activity, or urinary Na+ excretion but decreased urinary Cl- excretion. On the other hand, boiled rice containing NaCl (0.4 g/kg body wt) increased plasma Na+ (+3.8 +/- 0.7 meq/l) and Cl- (+3.0 +/- 1.5 meq/l) concentrations, then decreased renal nerve activity by 61 +/- 4%, and increased urinary Na+ and Cl- excretions. In dogs with hepatic denervation, a decrease in renal nerve activity, which was observed in intact dogs in response to the high-NaCl food intake, was completely abolished along with significant attenuation of postprandial natriuresis. That is, only 9 +/- 5% of the loaded Na+ and 7 +/- 3% of the loaded Cl- were excreted during 4 consecutive hours in hepatic-denervated dogs, whereas 36 +/- 5% of the loaded Na+ and 36 +/- 4% of the loaded Cl- were excreted in intact dogs. In dogs with renal denervation, postprandial natriuresis was also attenuated. These results indicate that the high-NaCl food intake elicits a decrease in renal nerve activity, the decrease is predominantly mediated by the hepatic nerves, and the decrease in renal nerve activity plays an important role in augmentation of urinary Na+ and Cl- excretion. Thus, the hepatorenal reflex may play an important role in controlling extracellular fluid homeostasis during food intake.

Ventral pontine catecholaminergic pathway mediates the vasopressin response to splanchnic osmostimulation in conscious rats

This study examines the role of catecholamines, cell bodies and fibers of passage within the subcoeruleus area (subLC) in the arginine vasopressin (AVP) response to splanchnic osmotic stimulation and hemorrhage. Bilateral chemical lesions were induced into the subLC, approximately 1 mm ventral to the locus coeruleus (LC), using 6-hydroxydopamine (6-OHDA) and ibotenic acid to selectively destroy catecholaminergic components and cell bodies, respectively. Vehicle and 5,7-dihydroxytryptamine (5,7-DHT) injections into the subLC area, 6-OHDA injections into the LC, as well as systemic desipramine pretreatment, were performed as controls for the possible non-specific effects of the lesions. Seven and 8 days later, plasma AVP level, plasma osmolality, mean arterial pressure and heart rate were measured following either gastric infusion of hypertonic (598 mOsm/kg; 2 ml/4 min) or isotonic (290 mOsm/kg) saline or a mild hemorrhage (2.5 ml/300 g) in conscious rats with indwelling tail artery catheters and naso-gastric tubes. 6-OHDA injections into subLC reduced the AVP response to the osmotic stimulation by 62.3% (P less than 0.01), as compared to vehicle-injected controls. These same rats demonstrated a normal AVP response to hemorrhage implying a specificity of the disrupted pathway. All controls confirmed that the effects of the 6-OHDA were due to specific action on noradrenergic components within the subLC area. Ibotenic acid lesions in the subLC did not significantly decrease the AVP response, demonstrating that mainly fibers and not cell bodies in this region are part of the pathway. 6-OHDA injections just anterior to the LC, where the dorsal noradrenergic bundle (DNAB) forms, reduced the AVP secretion due to hemorrhage by 77.0% (P less than 0.05), but had minor effects on the response to osmotic stimulation. These results indicate that catecholaminergic fibers travelling primarily within the subLC, in the ventral noradrenergic bundle (VNAB), carry splanchnic osmotic input to the hypothalamus, whereas the DNAB may mediate the AVP response to hemorrhage.

Splanchnic control of vasopressin secretion in conscious rats

Saline solutions (NaCl, 2 ml, pH 7.4, 10-598 mosmol/kgH2O) were infused over 4 min in conscious rats, via tail artery catheter or intragastric tube. Intragastric infusions of hyper- and hypotonic solutions caused, within 14.4 +/- 2.2 min, a maximal increase and decrease, respectively, of plasma vasopressin (AVP) relative to time controls (r = 0.97; P less than 0.00001) without affecting systemic plasma osmolality (r = -0.09; P less than 0.92). Mean changes of plasma AVP between 11 and 21 min were also correlated with the osmolality of gastric infusion (r = 0.72; P less than 0.000001), whereas systemic osmolality was unchanged (r = 0.14; P less than 0.42). Systemic infusions caused within 9.0 +/- 2.0 min a maximal change in both plasma AVP (r = 0.82; P less than 0.00001) and systemic osmolality (r = 0.97; P less than 0.00001). However, mean changes of plasma AVP between 11 and 21 min weakly correlated with the osmolality of systemic infusions (r = 0.27; P less than 0.20), although correlations between mean changes of systemic osmolality and the osmolality of systemic infusions were significant (r = 0.72; P less than 0.00001). Lack of correlations with mean arterial pressure and heart rate suggest that hemodynamic changes did not mediate the AVP responses. Pretreatment with atropine methyl bromate (2 mg/kg) abolished the AVP response to gastric but not systemic infusions of hypertonic saline. These results indicate that a splanchnic cholinergic receptor mechanism modulates AVP secretion during a moderate gastric intake of salt or water.

Sodium homeostasis in dogs with chronic renal insufficiency

Volume homeostasis in the fasting rate and 24-hr sodium balance are maintained in chronic renal insufficiency as a result of adaptations in the residual nephrons. This study evaluates the limitations to these adaptations and the dynamics of sodium excretion (UNaV) after an acute challenge with 100 mEq of sodium chloride in normal dogs (GFR 50 ml/min) and in dogs with one remnant kidney and moderate chronic renal insufficiency (GFR 15 ml/min). When food was administered with the sodium challenge, no or minimal changes in serum protein and hematocrit values occurred, and the natriuretic responses were small and equivalent in normal and remnant dogs. On the other hand, when the sodium challenge was given without food, the natriuretic response was large in normal dogs and markedly blunted in remnant. Within 5 hr of the sodium challenge, the various groups of normal dogs excreted 40 to 63% of the sodium load, but the remnant animals eliminated only 12 to 22% (P less than 0.001). The blunted natriuresis in remnant dogs was associated with a prolonged hemodilution of circulating proteins, indicating a longer lasting expansion of the intravascular volume. The blunted response was independent of sodium diet, of the administration route (p.o. vs. i.v.) or strength (isotonic vs. hypertonic) of the sodium load, and appears to result from an inability of the remnant kidney to rapidly excrete a sodium load. Thus, administration of sodium to dogs with chronic renal insufficiency leads to prolonged sodium retention, prolonged extracellular fluid (ECF) volume expansion, and requires a prolonged excretory cycle to restore 24-hr balance.

Osmosensitivity of the hepatic portal vein area and vasopressin release in rats

1. The role of intraperitoneal osmoreceptors in hypothalamo-neurohypophyseal control was studied in urethane- or nembutal-anaesthetized rats. Plasma samples were taken for radioimmunoassay of arginine vasopressin, and the electrical activity of single supraoptic endocrine neurones and of the hypothalamo-neurohypophyseal tract were monitored during superfusion of the hepatic portal vein with hypo-, iso- and hypertonic solutions. 2. Plasma arginine vasopressin increased within 1 min following superfusion with 0.3-0.9 osmolal NaCl solutions in a dose-related manner from basal levels of 30 pg/ml, to 170 pg/ml. Prior superfusion with xylocaine or intravenous infusions of 800 micrograms atropine-methyl bromate abolished this response, although vasopressin was still released to nicotine in atropine-blocked rats. 3. Portal vein superfusions had no significant effects on arterial blood pressure, plasma osmolality and plasma Na concentrations. 4. Forty supraoptic neurones were antidromically activated from the neural lobe/stalk region. Superfusions of the portal vein with NaCl solutions (0.33-1.20 osmole/kg, 37 degrees C, 5-120 sec) stimulated seven out of eight phasically firing and eight out of twenty-four continuously firing neurones. One phasically active, ten continuously firing and four silent cells were not affected, and six continuously firing neurones were inhibited by the superfusions. 5. The amplitude decreases of antidromic compound action potentials in the hypothalamo-neurohypophyseal tract, reflecting an increase of the orthodromic nerve impulse traffic, ranged from 17 to 22% for superfusions with 1.2 osmolal NaCl or LiCl solutions, from 8 to 11% for 1.2 osmolal Na isethionate or choline Cl and from 3 to 9% for 1.2 osmolal glucose; there was no effect when 1.2 osmolal urea and isotonic or hypotonic NaCl solutions were applied. 6. Responses of the amplitude of compound action potentials to superfusions with 1.2 osmolal NaCl solutions or with 0.1 mumole ACh, but not to electrical stimulation of the portal vein or its superfusion with 1.2 osmolal KCl, were abolished by prior application of 0.3 mumole atropine sulphate. Prior superfusions with xylocaine abolished the responses to all stimuli above. 7. These results suggest that within the hepatic portal vein area there are osmosensitive receptor cells and/or nerve terminals which activate the hypothalamoneurohypophyseal system through a peripheral cholinergic mechanism.

Participation of the liver receptors in the regulation of ion composition osmolality and extracellular fluid volume

Characteristics of 106 fibres dissected from liver afferent nerves were examined on 52 cats anaesthetized with intravenous nembutal administration. The afferent discharge of these fibres has been recorded during intraportal infusions of the different test solutions (75, 300, 450 mM sodium chloride; 8, 10.7 mM potassium chloride in normal saline; 300, 600 mM glucose and mannitol in normal saline and water). The results of the experiments showed that hepatic afferent fibres differ from each other in the following: spontaneous discharge rate, response to different stimuli, threshold, and the adaptation rate. Various receptor units responded specifically to a change either in the intraportal volume, or the osmolality, or both to NaCl and KCl concentration in the portal circulation. These data suggest an existence in the liver of highly specific receptor's populations: mechano(volume)-, osmo-, Na+-sensitive and K+-sensitive one. These receptors can serve as an informative link in the regulation of the water-salt homeostasis.

Sodium-chloride sensitive receptors located in hepatic portal vein of the rat

The superfusion of the hepatic portal vein of anaesthetized rats with hypertonic solutions of NaCl elicits a significant increase of neural activity measured in the hypothalamo-neurohypophysial tract. Superfusions of the liver or hepatic veins with hypertonic saline solutions had no effect, whereas superfusions of the portal vein also increased the firing rates of supraoptic endocrine neurones and the intramammary pressure. Results suggest that sodium chloride sensitive receptors (perhaps osmoreceptors) are located within the wall of the hepatic portal vein and mediate neurophypophysial hormone release.