[Specificity of osmoreceptors in the liver]

Effect of an increase in brain serotonin on the osmoregulatory response to a hypo- or hyperosmotic load in Wistar and vasopressin-deficient Brattleboro rats

Serotonin and its receptor agonists stimulate the release of arginine vasopressin (AVP) into peripheral blood under intraventricular injection. To test the hypothesis that brain serotonin can modulate the development of natural osmoregulatory responses, the effect of an increase in endogenous brain serotonin on the response to an intragastric hypo- or hyperosmotic loading was studied in Wistar and AVP-deficient Brattleboro rats. 5-Hydroxytryptophan (5-HTP), the rate-limiting serotonin biosynthesis precursor known to increase the brain level of serotonin, was injected intraperitoneally (5 mg/100 g body weight). The renal functional parameters (glomerular filtration rate [GFR], free water reabsorption, and urine flow rate) were monitored during the 4 h after intragastric infusion of water or a 2% NaCl solution (5% of body weight). Plasma AVP was measured by radioimmunoassay. In Wistar rats, intraperitoneal injection of 5-HTP at the same time as water loading prevented the development of the renal diuretic response: there was no increase in urine flow rate and GFR, and free water reabsorption remained at the high level. In AVP-deficient Brattleboro rats, unlike Wistar rats, 5-HTP treatment was without effect on the renal function parameters. In Wistar rats, injection of 5-HTP at the peak of water diuresis produced an abrogation of the diuretic response to water loading due to the increase in free water reabsorption. Plasma AVP increased from 1.2 +/- 0.4 to 4.2 +/- 1.6 pg/ml (n = 8 in each group, p < 0.01). Hyperosmotic treatment of Wistar rats with a 2% NaCl solution stimulated AVP secretion compared to baseline (from 3.2 +/- 0.1, n = 7 to 5.6 +/- 0.9, n = 7, p < 0.01), and the saluretic response developed on the background of high free water reabsorption. When injected concomitantly with NaCl solution, 5-HTP revealed no additive effect on plasma AVP and on free water reabsorption. We conclude that the 5-HTP-caused increase in brain serotonin contributed significantly to the dynamics of changes in the osmoregulatory response to the hypo-osmotic challenge due to stimulation of AVP secretion. 5-HTP had no additive effect on the osmoregulatory response to hyperosmotic loading. Peripherally injected 5-HTP had no effect on the renal function, being absent in AVP-deficient Brattleboro rats.

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.

Plasma arginine vasopressin response to oral, gastric, and intravenous water load in patients with cirrhosis

BACKGROUND/AIMS

Hepatic osmoreceptors are sensitive to changes in portal blood osmolality and cause variations in plasma antidiuretic hormone and water diuresis, which prevent major systemic osmotic changes. Insensitivity of hepatic osmoreceptors might contribute to maintaining increased plasma levels of antidiuretic hormone and negative free water clearance after oral hydration in some cirrhotic patients.

METHODS

We measured free water clearance and plasma arginine vasopressin levels in basal conditions and after oral, intravenous, and intragastric water overload in control subjects (group I) and patients with cirrhosis (some with a positive [group II] and some a negative [group III] free water clearance).

RESULTS

In groups I and II, no significant differences in plasma arginine vasopressin levels were found regardless of the route used for hydration. In group III, plasma levels were significantly higher between 0 and 75 minutes after intragastric hydration than after oral or intravenous hydration. No significant changes in plasma osmolality were detected between minute 0 and 20 after the end of oral or intragastric hydration, although plasma arginine vasopressin decreased significantly only 5 minutes after the oral hydration. Group III patients showed evidence of autonomic neuropathy. However, this dysfunction was unrelated to the insensitivity to gastric hydration.

CONCLUSIONS

Hepatic osmoreceptors seem to be disturbed in some cirrhotic patients, although this dysfunction may be compensated for by normally functioning oropharyngeal and hypothalamic osmoreceptors.

Validation of tests of completeness of vagotomy in rats

In recent years there has been renewed interest in the role of the gastrointestinal tract and the abdominal organs in regulating body energy balance. In an attempt to characterize vagal influence, vagotomy has been utilized as a routine manipulation. Recent anatomical findings, behavioral observations and physiological data have led to a re-examination of the validity of the tests usually performed to assess completeness of the operation. In this review, a comprehensive profile of the vagotomized rat is drawn, standard indices are described and discussed and finally, new tests are suggested. It is concluded that, for most of the studies, a set of well-chosen criteria is necessary to ascertain the success of the surgery.

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.

Anatomical evidence of direct projections from the nucleus of the solitary tract to the hypothalamus, amygdala, and other forebrain structures in the rat

Ascending projections from the caudal (general-visceroceptive) part of the nucleus of the solitary tract (NTS) were studied experimentally in the rat by the aid of the anterograde autoradiographic and the retrograde horseradish peroxidase (HRP) tracer techniques. Microelectrophoretic deposits of tritiated proline and leucine which involved the caudal part of the NTS, the dorsal motor nucleus of the vagus (dmX), and portions of the hypoglossal nucleus, nucleus intercalatus and/or nucleus gracilis were found to label ascending fibers that, besides going to numerous brain stem territories that included prominently the parabrachial area, could also be traced to serveral forebrain structures, namely, the bed nucleus of the stria terminalis (BST), the paraventricular (PA), dorsomedial (HDM) and arcuate (ARC) nuclei of the hypothalamus, the central nucleus of the amygdaloid complex (AC), the medial preoptic area (PM) and the periventricular nucleus of the thalamus (TPV). Smaller isotope injections almost completely confined to the NTS and dmX resulted in lighter labeling of a similar set of parabrachial and forebrain projections, whereas in another case, in which the deposit was almost exclusively limited to the nucleus gracilis, no label was seen in the aforementioned structures. In another series of experiments, aimed at further localizing the neurons of origin of the prosencephalic projections under consideration, small microelectrophoretic HRP injections confined almost totally to BST, PA, HDM, AC, PM or TPV, as well as both small and large injections involving ARC, resulted in labeled neurons situated in the dorsal medullary region, mainly in the medial portion of the NTS at the level of and caudal to the area postrema. Taken together, these observations indicate for the first time the existence of relatively direct conduction lines by which interoceptive information might be conveyed to limbic forebrain structures; some of the possible physiological correlates of these anatomical findings are discussed.

A comparison of sodium excretion in response to infusion of isotonic saline into the vena porta and vena cava of conscious rats

1. Indwelling non-occlusive catheters were placed in the vena porta and inferior vena cava of female rats several days before experimentation. Isotonic saline or isosmotic glucose (2% of body wt.) was infused into one vein followed one to several days later with an identical infusion into the other vein of each conscious animal. 2. Significantly higher urine flow and sodium excretion resulted from infusion of isotonic saline (0.5 ml/min) into the vena porta than into the vena cava. Modest prehydration or section of the hepatic branch of the right vagus did not affect the differential sodium response. Changes in endogenous creatinine clearance and potassium excretion were not significantly different for the two routes. Mean values for net peak sodium excretion and contemporaneous urine flow, urinary sodium concentration, and fractional sodium excretion were significantly higher for the portal than for the caval infusion while differences in glomerular filtration rate and filtered sodium load were insignificant. No significant difference in sodium excretion resulted from infusion of isosmotic glucose by the two routes. 3. Compared to the response promoted by the isotonic saline load infused at 0.5 ml/min, the differential response in sodium excretion was prolonged when the same load was infused at 0.375 ml/min. Sodium excretion was not significantly different for the two routes when the same isotonic saline load was infused at 0.75 ml/min. 4. These experiments provide evidence for participation of the liver in the control of sodium excretion and suggest release of a hepatic humoral factor which may be controlled by the duration of exposure of the hepatic circulation to an adequate load of isotonic saline.