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

Abstract

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.

Keywords: kidney; oxytocin; reabsorption of solute-free water; saluresis; vasopressin.

Fig. 1.

Correlation between the excretion of osmotically active substances and renal clearance of lithium in rats after oxytocin injection. The abscissa axis shows lithium clearance (С_Li), and the ordinate axis shows the excretion of osmotically active substances (U_OsmV) after the injection of 0.25 nM oxytocin (n = 15).

Fig. 2.

Correlation between the excretion of osmotically active substances and oxytocin by the rat kidney. The abscissa axis shows the excretion of osmotically active substances (U_OsmV) after the injection of 0.25 nM oxytocin (n = 15), and the ordinate axis shows the excretion of oxytocin (U_OTV).

Fig. 3.

Correlation between the sodium excretion and renal clearance of lithium in rats after oxytocin injection. The abscissa axis shows the sodium excretion (U_NaV) after injection of 0.25 nM oxytocin (n = 15), and the ordinate shows lithium clearance (C_Li).

Fig. 4.

Correlation between the excretion of lithium and reabsorption of solute-free water in the rat kidney after the oxytocin injection. The abscissa axis shows the lithium excretion (U_LiV), and the ordinate axis shows the reabsorption of solute-free water () after the injection of 0.25 nM oxytocin (n = 15).