The renal actions of oxytocin were studied in the conscious unrestrained rat infused with 0.077 M saline at a rate of 150 µl/min. During the control period volume and sodium excretion reached stable equilibria, the rates being equal to those infused. Administration of oxytocin at 200 pmol/min produced plasma oxytocin levels of 26.0±2.1 pmol/1 and caused a significant diuresis and natriuresis. Renal responses could also be seen with a lower dose of 30 pmol/min which produced plasma levels of 5.1 ±0.5 pmol/1 while a dose of 15 pmol/min which produced no significant increase in the plasma oxytocin had no renal effect. It appears that oxytocin has a natriuretic action in concentrations within the physiological range.
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Previous studies from our laboratory have reported a marked reduction in glomerular filtration rate (GFR) and sodium reabsorption in renal proximal tubule during intravenous infusion of P1,P4-diadenosine tetraphosphate (Ap4A) at dose of 1.0 µmol/kg + 10 nmol/kg/min (i.v., injection followed by infusion) in anaesthetized Wistar rats. In the present study, the changes of GFR and urine sodium excretion were investigated in response to systemic infusion of Ap4A at different doses. Ap4A at dose of 0.1 µmol/kg + 1.0 nmol/kg/min did not change GFR and sodium urinary excretion whereas 2-fold higher dose produced significant (3.4-fold) increase in sodium excretion without changes in GFR. Significant but transient reduction in GFR by ~21% was observed during infusion of Ap4A at dose of 0.5 µmol/kg + 5.0 nmol/kg/min. Higher doses of Ap4A (1.0 µmol/kg + 10 nmol/kg/min and 2.0 µmol/kg + 20 nmol/kg/min) produced sustained reduction in GFR and marked natriuresis. Our results suggest that tubular sodium transport systems are more sensitive to Ap4A than systems involved in GFR regulation.