Natriuretic effect of oxytocin on saline-expanded neonatal dogs

Abstract

The natriuretic effect of oxytocin was studied in 40 anesthetized newborn dogs 2-33 days of age. After saline expansion, puppies excreted only 2.86% of the filtered Na; adult dogs undergoing similar volume expansion excreted about 6-8% of the filtered Na. After oxytocin was infused in these saline-expanded puppies at 8 mU .cntdot. kg-1 per h, fractional Na excretion increased to 7.51%. Oxytocin had no natriuretic effect in nonexpanded puppies. In nonexpanded animals, when distal nephron function was blocked by a combination of ethacrynic acid, chlorothiazide and amiloride, fractional Na excretion averaged 31%. Distal nephron blockade in saline-expanded animals resulted in fractional Na excretion of 55%. When oxytocin was infused in either saline-expanded or nonexpanded puppies undergoing distal nephron blockade, there was no increase in Na excretion. In the newborn dog the attenuated natriuretic response to saline expansion apparently is related to increase reabsorption of Na in the distal nephron; an adult type of natriuretic response to saline expansion can be induced in puppies by oxytocin infusion. Oxytocin evidently inhibits Na reabsorption in the distal nephron.