Natriuretic effect of oxytocin on saline-expanded neonatal dogs

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 sodium (adult dogs undergoing similar volume expansion excrete about 65-8% of the filtered sodium). After oxytocin was infused in these saline-expanded puppies at 8 mU x kg-1 x h-1, fractional sodium 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 sodium excretion averaged 31%. In contrast, distal nephron blockade in saline-expanded animals resulted in fractional sodium excretion of 55%. When oxytocin was infused in either saline-expanded or nonexpanded puppies undergoing distal nephron blockade, there was no increase in sodium excretion. These results support the following conclusions. In the newborn dog 1) the attenuated natriuretic response to saline expansion is related to increased reabsorption of sodium in the distal nephron; 2) an adult type of natriuretic response to saline expansion can be induced in puppies by infusion of oxytocin; and 3) oxytocin inhibits sodium reabsorption in the distal nephron.