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Sodium Excretion and Renin Secretion after Continuous Versus Pulsatile Infusion of Oxytocin in Rats¹

Department of Physiology, Biomedical Center, Uppsala University (M.S., E.J.), Uppsala S-75123, Sweden; the Department of Physiology, University of Odense (O.S.), Odense DK-5000, Denmark; and the Department of Neuroscience, University of Pittsburgh (W.H., E.M.S., A.F.S.), Pittsburgh, Pennsylvania 15260 U.S.A.

Address all correspondence and requests for reprints to: Dr. Mats Sjöquist, Department of Physiology, Box 572, Biomedical Center, S-75123 Uppsala, Sweden. E-mail: mats.sjoquist{at}physiology.uu.se

Neurohypophyseal oxytocin (OT), secreted continuously under conditions of hyperosmolality, is a potent natriuretic hormone in rats. In contrast, OT secretion during lactation is pulsatile and is not accompanied by increased urinary Na⁺ excretion. The present experiments compared the effects of continuous and pulsatile infusion of OT on natriuresis in rats. In male rats anesthetized with Inactin, continuous infusion of OT (125 ng/kg·h) increased plasma OT to about 70 pg/ml; renal Na⁺ excretion increased 10-fold, and urine volume and K⁺ excretion also were elevated. However, when OT was administered iv in the same amount but in pulses given once every 5 or 10 min, to simulate 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 receptors are present in the macula densa, increased 2-fold when OT was infused either continuously or in pulses. These results indicate that the effects of OT administration on urinary Na⁺ excretion in rats varies depending on whether the infusion is pulsatile or continuous, whereas the effects of OT on renin secretion show no such difference.

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