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.

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.

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.

[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.

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.

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.

[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.

[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.

[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.

Exenatide enhances kaliuresis under conditions of hyperkalemia

Experiments on Wistar rats showed that exenatide (0.015-0.5 nmol per 100 g body weight) somewhat increased renal excretion of potassium from 7±1 to 16±1 μmol/h/100 g body weight (p<0.05) in animals with normal serum concentration of glucose (4.6±0.4 mM) and potassium (4.3±0.1 mM). Exenatide dramatically enhanced excretion of potassium under conditions of hyperkalemia (11.4±0.4 mM) produced by intraperitoneal injection of 1.25% KCl solution (5 ml per 100 g body weight). During the first postinjection hour, potassium excretion increased 2-fold and attained 97±11 μmol/h/100 g body weight in comparison with potassium load alone (47±9 μmol/h/100 g body weight, p<0.05). The data attest to a possible role of peptide regulators in normalization of potassium balance via renal mechanisms.

[Incretin role in osmotic homeostasis in rats]

Solute-free water clearance increased by 186% in water loaded rats treated with glucagon-like peptide 1 compared to untreated group. The effect of glucagon-like peptide 1 was reproduced in experiments with its synthetic analogue exenatide. Serum osmolality in water loaded rats treated with exenatide was closer to normal level on 30, 45, 60 min of experiment compared to the untreated group and reached the control value in 60 min. Administration of glucagon-like peptide 1 receptor antagonist to water loaded rats delayed (5 ml per 100 g bw) or prevented (2 ml per 100 g bw) increase in solute-free water clearance. The obtained data suggest the involvement of glucagon-like peptide 1 in osmotic homeostasis and indicate the unique physiologic role of incretins in regulation of water balance.

[Synthesis and investigation of effects of conopressin S analogues on ion and water excretion by rat kidney]

The investigation deals with effect of analogues of conopressin S--a peptide of the vasopressin family with amino acid replacement in the 2nd and 4th positions (characteristic of invertebrate peptides)--on transport of water and ions in the rat kidney. At administration to female Wistar rats, 1-deamino-conopressin S produced a weak action on water transport and had no effect on urinary K+ and Na+ excretion. Its analogue, 1-deamino-Tyr2-conopressin S, caused antidiuretic and kaliuretic action without affecting the Na+ excretion. Estimation of significance of the variant of optic isomer of arginine in the 4th and 8th position of the molecular for the antidiuretic and kaliuretic action of the peptide showed that 1-deamino-Tyr2,D-Arg4-conopressin S and 1-deamino-Tyr2,D-Arg4,8-conopressin S did not affect the urinary K+ excretion and renal water reabsorption, whereas action of 1-deamino-Tyr2,D-Arg8-conopressin S did not differ from action of 1-deamino-Tyr2-conopressin S. Thus, it has been established that the selective kaliuretic action of analogues of conopressin S on rat kidney depends on the presence of tyrosine in the 2nd and of L-arginine, but not of D-arginine, in the 4th position of the molecule.

[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.

[Natriuretic effect of exenatide: high efficacy and site of action]

Exenatide injection to female Wistar rats induced an increase in diuresis, urinary sodium and mild potassium excretion. The maximum natriuretic effect of exenatide was 18500 times more pronounced compared to that of furosemide in equimolar dose. The natriuretic and diuretic effects after combined administration of maximum doses of exenatide and furosemide were additive, which was indicative of different mechanisms of their action on the nephron. Exenatide probably inhibits sodium reabsorption in renal proximal segment. The data obtained might be essential for assessment of the clinical use of exenatide to excrete extracellular fluid excess in cases of edemation and to maintain potassium balance.

[Physiological analysis of various types of osmotic diuresis]

Efficacy of drugs reduced proximal reabsorption was compared in experiments with female Wistar rats. Urine flow rate for the 1st h of experiment was enhanced after polyethylene glycol-400 (PEG) and 6% Na2SO4 infusion by over 30-fold, exenatide--40-fold, glycerol--11-fold as compared with the control. The maximal values of Na+ excretion were observed during Na2SO4 and exenatide administration (280 +/- 31 micromol/h vs. 3.2 +/- 0.6 Imol/h/100 g bw). The highest K+ excretion was revealed in experiments with glycerol administration (41 +/- 5 micromol/h vs. 7 +/- 2 micromol/h/100 g bw), Mg2+ --after exenatide injection (5.3 +/- 1.3 micromol/h vs. 0.16 +/- 0.03 micromol/ h/100 g bw). Diuretic effects were additive after combined administration of maximal doses of exenatide and PEG which suggests a different mechanism of action of solutes filtrated (PEG) to the proximal nephron segment and generated due to Na+/HW-exchange inhibition (exenatide). Osmotic diuretics differ by potency, mechanism of diuretic action and selectivity of ion excretion).

[Influence of new neurohypophyseal hormone analogs on sodium and water reabsorption in rat kidney]

New analogs of some neurohypophyseal hormones (oxypressin, hydrin, glumitocin, vasotocin) have been synthesized. Experiments with injection of these peptides to rats showed that substitution of C-terminal glycinamide on beta-ethanolamine (glycinol) or ethylamine in 1-deamino-arginine vasotocin resulted in loss of natriuretic but not antidiuretic activity. Analogs of oxypressin and hydrin exhibited neither natriuretic activity nor ability to affect water reabsorption. Glumitocin analog induced renal sodium ion excretion and did not influence potassium ion excretion.