Dissociation of natriuresis and diuresis by oxytocin molecular forms in rats

In the rat, oxytocin (OT) produces dose-dependent diuretic and natriuretic responses. Post-translational enzymatic conversion of the OT biosynthetic precursor forms both mature and C-terminally extended peptides. The plasma concentrations of these C-terminally extended peptides (OT-G; OT-GK and OT-GKR) are elevated in newborns and pregnant rats. Intravenous injection of OT-GKR to rats inhibits diuresis, whereas injection of amidated OT stimulates diuresis. Since OT and OT-GKR show different effects on the urine flow, we investigated whether OT-GKR modulates renal action by inhibition of the arginine-vasopressin (AVP) receptor V 2 (V 2 R), the receptor involved in renal water reabsorption. Experiments were carried out in the 8-week-old Wistar rats receiving intravenous (iv) injections of vehicle, OT, OT-GKR or OT+OT-GKR combination. OT (10 μmol/kg) increased urine outflow by 40% ( P <0.01) and sodium excretion by 47% ( P <0.01). Treatment with OT-GKR (10 μmol/kg) decreased diuresis by 50% ( P <0.001), decreased sodium excretion by 50% ( P <0.05) and lowered potassium by 42% ( P <0.05). OT antagonist (OTA) reduced diuresis and natriuresis exerted by OT, whereas the anti-diuretic effect of OT-GKR was unaffected by OTA. The treatment with V 2 R antagonist (V 2 A) in the presence and absence of OT induced diuresis, sodium and potassium outflow. V 2 A in the presence of OT-GKR only partially increased diuresis and natriuresis. Autoradiography and molecular docking analysis showed potent binding of OT-GKR to V 2 R. Finally, the release of cAMP from CHO cells overexpressing V 2 receptor was induced by low concentration of AVP (EC50:4.2e-011), at higher concentrations of OT (EC50:3.2e-010) and by the highest concentrations of OT-GKR (EC50:1.1e-006). OT-GKR potentiated cAMP release when combined with AVP, but blocked cAMP release when combined with OT. These results suggest that OT-GKR by competing for the OT renal receptor (OTR) and binding to V 2 R in the kidney, induces anti-diuretic, anti-natriuretic, and anti-kaliuretic effects.