Microalbuminuria in Rats Treated with D-Nitroarginine Methyl Ether.

Microalbuminuria is an early symptom and prognostic marker of the progression of renal pathology. The analysis of the role of anionic components of the renal glomeruli in the albumin retention and the development of a model of minimal changes in the glomerular filter leading to the appearance of microalbuminuria are relevant. The effect of organic cations D-arginine methyl esters (D-AME) and D-nitroarginine (D-NAME) on the excretion of albumin by the kidneys in rats was studied. D-AME had no effect on urinary albumin excretion in rats. D-NAME caused microalbuminuria, which persisted for more than a day and sharply increased after injection of vasopressin. The number of anionic sites labeled with polyethyleneimine decreased in the structures of the glomerular filter. D-NAME-induced microalbuminuria can later serve as a model for studying nephroprotective or damaging factors.

Role of Proglucagon Peptides in Osmoregulation.

Glucagon-like peptide-1 (GLP-1), a product of partial proteolysis of proglucagon, is involved not only in regulation of carbohydrates, but also in water-salt metabolism. The study examined the role of proglucagon derivatives GLP-1, GLP-2, and oxyntomodulin in rat osmoregulation. Of them, only blood plasma GLP-1 increased in response to water load (20 ml/kg). Administration of glucose (1.5 g/kg) elevated GLP-1 and oxyntomodulin but did not change the level of GLP-2. GLP-1 accelerated excretion of excess water during hyperhydration, whereas GLP-2 decreased this parameter. No physiological effects of oxyntomodulin in the kidneys were revealed. Probably, the blood levels of proglucagon derivatives are independently regulated for each peptide. In contrast to GLP-2 and oxyntomodulin, GLP-1 is involved in osmoregulation.

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.

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.

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.

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.

[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.

[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.

[Effects of neurohypophysial nonapeptides and their analogues on magnesium excretion by rat kidney].

Effects of neurohypophysial nonapeptides of vertebrates (vasopressin, vasotocin, and their synthetized analogues) on urinary magnesium excretion were studied in rats. Neurohypophysial hormones and their analogues at doses stimulating V2-receptors (0.0001-0.001 nmol/100 g BW) produced antidiuretic effect and reduced urinary magnesium excretion. At the higher doses activating V2- and V1a-receptors (0.025-0.1 nmol/100g BW), vasotocin and its analogues (deamino-vasotocin (dAVT), deamino-Thr4-vasotocin, deamino-hArg8-vasotocin, deaminomonocarbo-vasotocin) enhanced excretion of magnesium and sodium ions. Direct relation between increase in renal excretion of sodium and magnesium ions was found under these conditions. dAVT induced 10 times lesser increase in magnesium excretion after administration of a V1a-receptor antagonist. An antagonist of V2-receptors did not affect the dAVT-induced magniuresis. The obtained data suggest that V-receptors take part in regulation of magnesium transport in rat kidney.

[Ion-regulating renal function during oral and parenteral potassium loading].

Ion-regulating renal function and influence of vasopressin and its analogues on the rate and selectiveness of the urinary potassium excretion were investigated after short-term parenteral and oral potassium loading. In experiments with Wistar rats it was shown that increase in volume of orally administrated 1.25% KCl solution from 1 to 5 ml per 100 g body weight led to the proportional rise in potassium excretion. Hyperkalemia was already observed at 5 min after parenteral potassium loading, potassium excretion reached the maximum after administration of 2 ml of KCl solution per 100 g body weight. Kaliuresis induced by oral potassium load was higher and faster than after parenteral load and was followed by increase in diuresis, urinary sodium and magnesium excretion. Desmopressin and 1-deamino-Arg4-vasotocin prevented rise of diuresis, natriuresis and magniuresis under these conditions; 1-deamino-Arg4-vasotocin and vasopressin stimulated urinary potassium excretion during first 30 min after loading. After parenteral potassium load vasopressin analogues did not affect urinary potassium and sodium excretion. The data obtained suggest the participation of the gut in regulatory signal transduction to the kidney after potassium entering and prevention of significant changes in the internal environment.

[Diet and water-salt balance in rats].

We compared parameters of water-salt balance in Wistar female rats fed normal chows during more than 2 weeks. Potassium content was 1.4-fold higher in diet I than in diet II, and sodium end water content was 3.3- and 7.5-fold higher in diet II than in diet I. Blood osmolality and concentration of Na+, K+, Mg2+ were equal in rats fed different chow. In water-loaded rats (5 ml of water/100 bw per os) fed different chow, urine flow rate did not differ, but solute-free water excretion was higher by 40.2% in the rats fed diet II vs. diet I. The sort of diet did not affect the renal sodium excretion during oral administration of 5 ml 0.9% NaCl per 100 g bw to rats. After vasopressin injection solute-free water reabsorption was 1.5-fold higher in rats fed diet II. Natriuretic and hydruretic effect of exenatide, glucagon-like peptide 1 mimetic, was weaker in rats fed diet I. The data obtained indicate that organism can effectively maintain blood parameters. The modulation of hormone regulatory effects on water and sodium balance was found to depend on the state of organism under diet consumed continuously.