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.

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.

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.

Role of Vasopressin in the Regulation of Renal Sodium Excretion: Interaction with Glucagon-Like Peptide-1.

The present study aimed to investigate the potential physiological role of vasopressin and the incretin hormone of the gastrointestinal tract (glucagon-like peptide-1; GLP-1) in the regulation of the water-salt balance in a hyperosmolar state as a result of sodium loadings. In rats, the administration of hypertonic NaCl solution resulted in a significant increase in natriuresis, which correlated with the vasopressin excretion rate. Natriuresis following an i.p. NaCl load (23.2 ± 1.4 µmol/min/kg) was enhanced by inhibition of V2 receptors (51.6 ± 3.7 µmol/min/kg, P < 0.05) and was reduced by a V1a antagonist injection (6.3 ± 1.1 µmol/min/kg, P < 0.05). Compared to i.p. salt administration, oral NaCl loading induced a significant increase in the plasma GLP-1 level within 5 min and resulted in more prominent natriuresis and a smaller increase in blood sodium concentration. It was hypothesised that the basis for the fast elimination of excess sodium following an oral NaCl load could be the involvement of GLP-1 in osmoregulation combined with vasopressin. It was demonstrated that GLP-1 mimetic exenatide (1.5 nmol/kg) produced a significant decrease in proximal reabsorption and an increase in fractional sodium excretion (from 0.15 ± 0.04% to 9 ± 1%). It was also shown that vasopressin at doses of 1-10 µg/kg and the selective V1a agonist (1 µg/kg) induced an increase in sodium fractional excretion to 10 ± 2% and 8 ± 2%, respectively. Combined administration of exenatide and V1a agonist revealed their cumulative natriuretic effect, and sodium fractional excretion increased by up to 18 ± 2%. These data suggest that GLP-1 combined with vasopressin could be involved in the regulation of sodium balance in the hyperosmolar state as a result of NaCl loading. Vasopressin regulates the reabsorption of a significant portion of filtered sodium in the distal segment of the nephron and modulates the natriuretic effect of GLP-1.

[STUDY OF THE EFFICACY OF REGULATION OF THE BLOOD SERUM PHYSICOCHEMICAL PARAMETERS IN CHILDREN WITH DYSFUNCTION OF PHYSIOLOGICAL SYSTEMS].

The physicochemical parameters of blood serum (osmolality, concentration of several ions, total protein, glucose) were studied in 200 children of different age with various forms of pathology. The variability of each parameter was calculated. A high level of stability of the parameters studies was revealed in healthy children and in children with dysfunction of various systems (disease of the respiratory system, gastrointestinal tract, renal and urinary tract, nervous and endocrine systems). However, estimation of their coefficients of variation showed significant individual deviations of these parameters from the average value of the examined patients. This fact reflects the extent of efficacy of activity of different organs and regulatory systems under pathological processes. Combination of clinical and ontogenetic methods of evolutionary physiology in this study opens new possibilities for understanding the nature of regulation of water-salt balance in humans and points out to the expedience of using these approaches in the practical medicine.

[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.

[Effect of byetta on renal osmoregulatory function in patients with diabetes mellitus].

The renal osmoregulatory function was studied in patients with type 2 diabetes mellitus (DM). The renal response to water loading (0.7% b.w.) and simultaneous exenatide (byetta) injection (5 microg) exhibited variation and was dependent on the degree of hyperglycemia. Effective solute-free water excretion was observed in patients with well-controlled DM (HbAlc 6.0 +/- 0.1%), in which CH20 changed from -0.67 +/- 0.2 mL/min to 0.72 +/- 0.2 mL/min. This reaction was absent in patients with poorly controlled DM (HbAlc 8.8 +/- 0.6%) and the process of solute-free water reabsorption prevailed: -CH20 = -1.06 +/- 0.1 mL/min in control period vs. -0.99 +/- 0.1 mL/min after treatment. Thus, byetta increases the efficiency of osmoregulation and accelerates the excretion of excess water in patients with compensated carbohydrate metabolism.

[Ratio of cations in hemolymph, blood serum of aquatic and terrestrial organisms].

A wide range of differences of cations concentrations (Na, K, Ca, Mg) in hemolymph, blood serum of multicellular animals and humans was revealed. It was established that constancy of K/Na ratio 0.034 +/- 0.001 is strictly maintained in these organisms. Except for some freshwater and marine organisms, Mg/Na ratio is sustained on the same level in the wide range of concentrations of these ions in serum of vertebrates and hemolymph of invertebrates. Ca/Na ratio is higher in hemolymph of freshwater mollusks. The question about possible reasons of physiological differences of Ca/Na and Mg/Na rations compared to constancy of K/Na ratio is discussed.

[Exenatide stimulated solute-free water excretion by human kidney].

Exenatide effect was studied in 55 human including 38 patients with type 2 diabetes mellitus without signs of diabetic nephropathy and primary renal diseases. Preliminary study with water load in volume from 0.5 to 1% b.w. showed that water load equal to 0.7% b.w. caused significant increase in urine excretion. The rise of diuresis after 10 microg Byetta (exenatide) injection and simultaneous water load of 0.7% of b.w. depended on increase in solute-free water clearance. Thus, exenatide (mimetic of GLP-1) action let us to consider its possible role as component of osmoregulation system in human. This peptide provides high efficiency of renal function in osmotic homeostasis.

[K+/Na+ in the animal extracellular fluid at weathering of granitoids and problem of the origin of life].

Leaching of granitoids of the paleoproterozoic age was performed from several seconds to 360 days in water solutions (pH < 6.0) in the oxygen or argon medium. Under these conditions the entrance of K+ into the solution has been shown to occur at the higher rate than that of Na+. The obtained data are considered as evidence in favor of a possibility of the existence at the earlier stages of the Earth geologic history of water reservoirs with predominance of K+ over Na+. The K+/Na+ ratio exceeding 1 has been shown in prokaryotes, in cells and tissues of the free living and parasitic species of invertebrate and vertebrate animals. At the same time in the extracellular fluid of the fresh water, marine, and terrestrial animals, in which the Na+ concentration varies from 13 to 482 mmol/l, the K+/Na+ ratio is preserved at the level of 0.034 +/- 0.001. These results are discussed in connection with the problem of origin of protocells and of concentration ratios of monovalent cations in water phases of multicellular organisms.

[The effect of exenatide analogs on hydruretic function of rat kidney under water load].

Injection (0.05 nmol/100 g b.w.) of exenatide or its amino-acid-substituted synthetic analogs I (substitution at positions 14 and 39) and II (substitution at positions 14, 35, and 39) led to an increase in diuresis and excretion of sodium, magnesium and potassium, but only exenatide caused the excretion of solute free water. In experiments with 1% water load, only exenatide analog I stimulated the solute free water excretion. These features of exenatide and its analogs show the possibility of searching for substances with various power of action upon ion and water excretion by the kidney, which may have a clinical significance.

[Comparison of reabsorption of osmotically free water in the bud and energy of interaction of vasotocin analogs with a V2-receptor].

The calculated values of the binding energy of nonapeptides with receptors in docking with their influence on reabsorption of osmotically free water in a rat bud in vivo were compared. Vasotocin and some its analogs were intramuscularly introduced to non-narcotized rat females of the Wistar line in doses from 0.1 pmol to 0.5 nmol/kg of body weight against the background of peroral water load (50 ml/kg of body weight). A significant correlation between the calculated interaction energy of peptides with V2-receptors and an increase of reabsorption of osmotically free water in the rat bud stimulated by injection of nonapeptides was found. The results evidence that alterations in rat bud in vivo caused by analogs of vasotocin and their interactions with V2-receptors can be accurately simulated.

Effect of a blocker of nitric oxide production on albumin excretion by rat kidney.

Experiments on Wistar rats showed that single intraperitoneal injection nonselective NO-synthase inhibitor L-NAME in a dose of 50 mg/kg was followed by transient proteinuria and albuminuria. This effect was not reproduced by injection of ODQ, an inhibitor of intracellular effects of NO, and arginine, but D-NAME, an optical isomer of L-NAME not blocking NO-synthase, produced similar, though less pronounced effect. The degree of proteinuria and albuminuria increased in combined treatment with nitroarginine methyl esters and 1-deamino-arginine vasotocin or arginine vasopressin. Proteinuria during treatment with arginine derivatives attests to not only their effect on the charge of the filtration membrane, but also the participation of NO-dependent processes in the regulation of ultrafiltration in renal glomeruli.

[Influence of the angiotensin converting enzyme inhibition on the functional proteinuria in rats].

Influence of enalaprilat, angiotensin converting enzyme inhibitor, on a functional proteinuria associated with increase in diuresis unduced by furosemide, 1-deamino-arginine vasotocin (1d-AVT) injection or water loading was investigated in experiments with Wistar rats. Intraperitoneal injection of 0.1 mg/100 g b.w. of enalaprilat resulted in reduction of glomerular filtration rate, solute-free water reabsorption and solutes excretion, particularly potassium excretion, after 1d-AVT administration and decrease in diuresis and solute-free water excretion after oral water loading. Enalaprilat injection did not influence on the level of proteinuria induced by the various types ofdiuresis and albuminuria during water diuresis and 1d-AVT-dependent saluresis. The data obtained have shown that decrease in angiotensin II production in the renal structures does not affect protein excretion rate during examined forms of proteinuria and suggest existence of a multicomponent system of the pressure stabilization in the glomerular apparatus.

[Determination of ion content and localization of vasopressin and aquaporin receptors in the gerbil kidney after the flight aboard biosatellite "Foton M3"].

After 12 days on space fight, body mass of gerbils decreased by 32.9%; however, the effective systems of regulation did not let changes in the concentration of Na, K, Mg ions in fresh samples of the renal cortex. These data are viewed as a reflection of the regulation systems strategy to preserve the optimal conditions for functioning of the kidney, a vital body organ. The cations were reduced significantly in the renal medulla when calculated for dry mass and compared with the synchronous control. This could have changed the osmoregulating function of the kidney during space fight and, due to small cations accumulation, caused the drop of the cortex/medulla osmotic gradient. Locations of V1a-, V1b- and V2-receptors and aquaporin 1 and 2 in renal tubule cells of gerbils before and fight are described.