The Mechanism of the Solute-Free Water Reabsorption Increase in the Rat Kidney by Oxytocin Saluresis.

We found an experimental solution to the paradox when the reabsorption of solute-free water increases with a simultaneous increase in diuresis and saluresis in the rat kidney under the oxytocin action. Injection of oxytocin to rats (0.25 nmol/100 g of body weight) increases diuresis from 0.16 ± 0.03 to 0.26 ± 0.02 mL/h, the excretion of solutes from 134 ± 13.7 to 300 ± 16.3 µOsm/h, and the reabsorption of solute-free water, which correlates with the renal excretion of oxytocin (p < 0.001). The mechanism of the effect is that oxytocin decreases the reabsorption of ultrafiltrate in the proximal tubule (the clearance of lithium increases) and increases the fluid flow through the distal segment of the nephron. In vivarium rats, urine osmolality (1010 ± 137 mOsm/kg H2O) and the concentration of vasopressin are high, this causes an increase in the reabsorption of solute-free water. Thus, oxytocin increases saluresis, which, against the background of a high level of endogenous vasopressin, increases the water reabsorption in the collecting ducts.

Subtypes of Neurohypophyseal Nonapeptide Receptors and Their Functions in Rat Kidneys.

The nonapeptides of neurohypophysis, vasotocin and mesotocin, detected in most vertebrates, are replaced by vasopressin and oxytocin in mammals. Using bioinformatics methods, we determined the spectrum of receptor subtypes for these hormones in mammals and their physiological effects in the kidneys of rats. A search for sequences similar to the vertebrate vasotocin receptor by proteomes and transcriptomas of nine mammalian species and the rat genome revealed three subtypes of vasopressin receptors (V1a, V1b, and V2) and one type of oxytocin receptors. In the kidneys of non-anesthetized rats, which received a water load of 2 ml per 100 g of body weight, three effects of vasopressin were revealed: 1) increased reabsorption of water and sodium, 2) increased excretion of potassium ions, and 3) increased excretion of sodium ions. It has been suggested that each of the effects on the kidney is associated with selective stimulation of the vasopressin receptor subtypes V2, V1b, and V1a depending on the concentration of nonapeptide. In experiments on non-anaesthetized rats with a water load, the injection of oxytocin reduces the reabsorption of solute-free water in the kidneys and increases the excretion of sodium ions. The possible physiological mechanisms behind the realization of both effects with the participation of a single type of oxytocin receptors are being analyzed. Thus, the spectrum of activated receptor subtypes varies depending on the current concentration of neurohypophyseal hormones, as a result of which the predominant effect on renal function changes, which ensures precise regulation of water-salt homeostasis.

The Cascade System of Osmotic Homeostasis Regulation.

The human and animal osmoregulation system is aimed at stabilizing serum osmolality in order to maintain cell volume. It has been shown that the introduction of 5 mL water per 100 g of body weight into the stomach of rats decreases serum osmolality and the concentration of Na and Ca, but not K and Mg. The cascade system of osmotic homeostasis increases secretion of glucagon-like peptide-1 (GLP-1) and oxytocin, and decreases secretion of vasopressin, which reduces the osmotic permeability of collecting duct. After water loading and the injection of 0.015 nM exenatide (GLP-1 mimetic), the time of excretion of 50% of water was halved from 112 ± 4 to 57 ± 5 min (p < 0.01), and after the injection of 0.015 nM oxytocin, it decreased to 83 ± 6 min (p < 0.01). The physiological mechanism of renal effect of the aforementioned hormones which accelerate the recovery of osmotic homeostasis has been found.

Mechanism of Natriuretic Effect of Oxytocin.

In rats, intramuscular injection of oxytocin (0.25 nmol/100 g body weight) increased sodium excretion from 19±5 to 120±11 µmol/min. A significant correlation (p<0.001) was revealed between renal excretion of oxytocin and sodium ions. Under the action of oxytocin, natriuresis was characterized by diminished reabsorption of fluid in the proximal tubule of the nephron attested by elevated lithium clearance rate and from stimulation of V1a receptors in the cells of thick ascending loop of Henle. Pmp-Tyr(Me)-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, a V1a receptors antagonist, prevented the natriuretic effect of oxytocin.

Vasopressin receptor subtypes and renal sodium transport.

In mammals, three subtypes of V-receptors have been identified in the kidney. The effects of vasopressin, a hormone synthesized in the hypothalamus, are triggered by three distinct receptor isoforms: V2, V1a, and V1b. Stimulation of V2-receptors regulates urine osmotic concentration by increasing sodium reabsorption in the thick ascending limb of the loop of Henle and enhancing osmotic permeability of the epithelium cells in the collecting duct. Stimulation of V1a-receptors inhibits renal sodium reabsorption and induces natriuresis, comparable to the effect of the diuretic furosemide, in the thick ascending limb of the loop of Henle. Stimulation of V1b-receptors induces potassium secretion in the final parts of the distal segments and initial parts of the collecting ducts. In this review, we discuss the role of vasopressin and its interaction with V-receptor subtypes in natriuresis and for stabilizing the physicochemical parameters of the internal environment and water-salt homeostasis in humans. A better understanding of these systems and their regulation is necessary to facilitate identification of additional system components and mechanisms, clarify their contribution during various normal and pathological functional states, and suggest novel strategies for the development of therapeutic interventions.

Correlation between Urinary Excretion of Arginine-Vasopressin and Renal Reabsorption of Sodium and Water.

We developed an approach for quantitative assay of injected vasopressin in urine samples by ELISA under conditions of physiological suppression of hormone secretion from the neurohypophysis into the blood. In experiments on unanesthetized rats, water load (5 ml/100 g body weight) almost completely blocked secretion of arginine-vasopressin. Injection of arginine-vasopressin in a dose of 0.1 nmol/100 g body weight after water load enhanced reabsorption of solute-free water and renal excretion of Na+, K+, and Mg2+ by 13.3, 5.5, and 5.0 times, respectively; urinary excretion of Ca2+ remained unchanged. It was found that urinary excretion of arginine-vasopressin directly correlated with reabsorption of solute-free water and renal sodium excretion.

Stimulus for Glucagon-Like Peptide 1 Secretion in Rats.

Blood concentration of glucagon-like peptide-1 (GLP-1) increased 5 min after per os administration of water, sodium chloride solution, or glucose solution. Changes in blood osmolality or blood glucose level did not stimulate GLP-1 release. A method of short-term increase in the gastric capacity in rats using an inflating balloon attached to the Foley catheter was developed in order to test the hypothesis that excitation of the upper gastrointestinal tract receptors is a primary signal for the GLP-1 secretion during oral intake of the substances. Mechanical gastric distension in rats caused elevation of the blood GLP-1 concentration which was comparable to the effects of oral administration of water, sodium chloride, and glucose solutions.

A New Functional Role of Oxytocin: Participation in Osmoregulation.

After a water load in rats hyperhydration occurs and the secretion of oxytocin increases by the neurohypophysis. This increases the water diuresis, the water is quickly excreted by the kidney, and the recovery of osmotic homeostasis is accelerated.

Involvement of Glucagon-Like Peptide-1 in the Regulation of Selective Excretion of Sodium or Chloride Ions by the Kidneys.

An increase of total glucagon-like peptide-1 (GLP-1) concentration in the plasma in rats was revealed 5 min after oral, but not intraperitoneal administration of NaCl or Trizma HCl solutions. The increase in GLP-1 level was similar to that after oral glucose administration. After intraperitoneal administration of 2.5% NaCl, GLP-1 mimetic exenatide accelerated natriuresis and urinary chloride excretion. Under conditions of normonatriemia and hyperchloremia induced by injection of 6.7% Trizma HCl, exenatide stimulated chloride excretion and reabsorption of sodium ions in the kidneys. These findings suggest that GLP-1 participates in selective regulation of the balance of sodium and chloride ions.

The mechanism of glucagon-like peptide-1 participation in the osmotic homeostasis.

We have found the physiological mechanism of intensification of the excessive fluid removal from the body under the action of glucagon-like peptide-1 and its analog exenatide. Under the water load in rats, exenatide significantly increased the clearance of lithium, reduced fluid reabsorption in the proximal tubule of the nephron and intensified reabsorption of sodium ions in the distal parts, which contributed to the formation of sodium-free water and faster recovery of osmotic homeostasis. Blocking this pathway with a selective antagonist of glucagon-like peptide-1 receptors slowed down the elimination of excessive water from the body.

Effects of Selective Agonists of V1a, V2, and V1b Receptors on Sodium Transport in Rat Kidney.

The role of subtypes of vasopressin receptors in modulation of renal sodium reabsorption was studied in in vivo experiments on Wistar rats. Selective V1a receptor agonist reduced sodium reabsorption in the kidneys and expression of these receptors increased by practically 100 times. This effect was similar to the effect of furosemide. Selective V2 receptor agonist enhanced sodium reabsorption in the kidney and simultaneously increased reabsorption of solute-free water. Stimulation of V1b receptors did not affect sodium transport. Our findings attest to the key role of V1a receptors in the regulation of renal excretion of sodium ions.

[EFFECTS OF ORALLY AND PARENTERALLY ADMINISTERED ANALOGS OF PEPTIDE HORMONES ON RENAL WATER AND ION TRANSPORT].

In experiments with female Wistar rats it was shown that analogs of neurohypophysial hormones administered to gastrointestinal tract preserved their specific physiological activity - they increased solute-free water reabsorption and urinary sodium and potassium excretion. Doses of deamino-vasotocin exerted antidiuretic and natriuretic effects following its oral administration were 50 and 200 times higher compared to maximal effective ones after intramuscular injection. Inhibition of gastrointestinal proteases by aprotinine enhanced effects of nonapeptides; the amount of peptide absorbed from the intestine under these conditions was approximately 0.5 % of orally introduced substance. In contrast to analogs of neurohypophysial nonapeptides, glucagon-like peptide-1 mimetic (exenatide) did not exert its physiological effects after oral administration, whereas it increased urinary sodium and potassium excretion following intramuscular injection.

[COEVOLUTION OF PHYSIOLOGICAL SYSTEMS].

Coevolution - the interaction of different species of animals that are closely related biologically but do not exchange genetic information. The article discusses the problem of coevolution of physiological systems within an organism as the interrelated development of structure, function and systems of their re- gulation during the formation of living systems. We consider the coevolution of osmoregulation and type of metabolism of nitrogen compounds, systemic and individual strategy of coevolution of cell volume re- gulation in poikiloosmotic and homoioosmotic animals, coevolution of effectory organs and endocrine factors in the development of water-salt homeostasis, interrelationship of nonapeptides of neurohypophysis and glucagon-like peptide 1 in regulation of renal function for stabilization of physicochemical para- meters of fluids of the internal environment.

[DIFFERENCES OF EXENATIDE EFFECTS ON GLYCEMIA AND RENAL WATER AND ION EXCRETION IN FROGS AND RATS].

The aim of the present study was to compare effects of glucagon-like peptide-1 (GLP-1) mimetic exenatide on glucose and water-salt homeostasis in animals with different level of renal tubular proximal reabsorption - rats (Rattus norvegicus) and frogs (Rana temporaria). Following glucose tolerance test, in rats exenatide promoted rapid recovery of normoglycemia, whereas in frogs delayed this process. In water-loaded rats exenatide essentially augmented solute-free water clearance and enhanced natriuresis in furosemide-treated rats.-In frogs exenatide did not alter urinary flow rate, urinary sodium excretion and solute-free water clearance under water diuresis and furosemide treatment. It is suggested that the invol- vement of GLP- 1 in regulation of water-salt homeostasis in mammals should be preceded by key evoluti- onary transformation - increase in glomerular filtration rate and proximal tubular reabsorption.

Redistribution of Proximal and Distal Reabsorption of Water and Ions in Rat Kidney After Treatment with Glucagon-Like Peptide-1 Mimetic.

Injection of a glucagon-like peptide-1 mimetic accelerated recovery of the initial status of water-salt balance in rats after water or saline load (2.5% NaCl). This effect is mediated by a decrease in proximal fluid reabsorption and change in ion and water transport in the distal part of renal tubules. We developed a new approach to calculation of additional fluid inflow from the proximal tubule to the distal tubule and distal sodium reabsorption under the influence of this mimetic in rats. The expected values corresponded to experimental results, which confirmed our hypothesis on the physiological mechanism for the involvement of the kidneys in the homeostatic effect of glucagon-like peptide-1 under variations in water-salt balance.

[PHYSIOLOGICAL APPROACH TO RESTORING OSMOTIC HOMEOSTASIS IN RATS WITH HYPERNATREMIA].

The aim of the study was a search of physiological approach to restoring osmotic homeostasis in rats with hypernatremia. Intraperitoneal administration of 1.8 ml/100 g BW 2.5% NaCl solution to Wistar rats induced hyperosmia (306 +/- 1 mOsm/kg H2O) and hypernatremia (150.3 +/- 0.3 mM in 60 min of experiment), increase in urinary sodium excretion (from 8 +/- 1 to 230 +/- 10 micromol/100 g BW for 2 h). Under these conditions enhancement of natriuresis up to 465 +/- 29 micromol/100 g BW and 667 +/- 24 micromol/100 g BW for 2 h was observed after injections of vasopressin analogue, deamino-vasotocin (dAVT, 0.05 microg/100 g BW), or loop diuretic, furosemide (1 mg/100 g BW), respectively. dAVT-induced natriuresis was accompanied by increase in solute-free water reabsorption; serum osmolality (301 +/- 1 mOsm/kg H2O) and sodium concentration (145.8 +/- 0.5 mM) were close to normal values by 60 min of experiment. Furosemide caused relatively greater excretion of water, than sodium; hypernatremia (150.2 +/- 0.4 mM) and hyperosmia (311 +/- 1 mOsm/kg H2O) persisted during 60 min of experiment. Thus, in rats with hypernatremia dAVT due to decrease in renal sodium reabsorption and increase in solute-free water reabsorption promotes recovery of serum osmolality and sodium concentration.

[INFLUENCE OF OLEAMIDE OF WATER AND ION TRANSPORT IN THE OSMOREGULATORY ORGANS].

Application of oleamide (final concentration of 10 µM) at the skin basal surface of the frog, Rana temporaria L., augmented the short-circuit current (SCC) from 59.8 ± 2.5 to 78.2 ± 1.4 µA/cm2. Oleamide added to the serous membrane of the frog urinary bladder at a final dose of 1 µM induced more than 30-fold increase of osmotic permeability. The addition of arginine-vasotocin on the background of oleamide action further increased SCC across the isolated frog skin and osmotic permeability of the frog urinary bladder. Intraperitoneal injection of oleamide at a dose of 0.1 mM/100 g BW to water-loaded non-anesthetized Wistar rats decreased diuresis by 22%, enhanced solute-free water reabsorption and urinary sodium excretion by 31% and 55% respectively, but did not affect the renal potassium excretion. The results obtained provide evidence of similarity of oleamide and neurohypophyseal hormones effects on water and ion transport in epithelial cells of osmoregulatory organs in vertebrates.