Sodium excretion and renin secretion after continuous versus pulsatile infusion of oxytocin in rats

Neurohypophyseal oxytocin (OT), secreted continuously under conditions of h yperosmolality, is a potent natriuretic hormone in rats. In contrast, OT se cretion during lactation is pulsatile and is not accompanied by increased u rinary Na+ excretion. The present experiments compared the effects of conti nuous and pulsatile infusion of OT on natriuresis in rats. In male rats ane sthetized with Inactin, continuous infusion of OT (125 ng/kg.h) increased p lasma OT to about 70 pg/ml; renal Na+ excretion increased 10-fold, and urin e volume and K+ excretion also were elevated. However, when OT was administ ered iv in the same amount but in pulses given once every 5 or 10 min, to s imulate the pattern of OT secretion during lactation, rats did not excrete significantly more urine, Na+, or K+ than did vehicle-treated animals. The plasma renin concentration, measured in these experiments because OT recept ors are present in the macula densa, increased 2-fold when OT was infused e ither continuously or in pulses. These results indicate that the effects of OT administration on urinary Na+ excretion in rats varies depending on whe ther the infusion is pulsatile or continuous, whereas the effects of OT on renin secretion show no such difference.