The plasma sodium, osmolality, and arginine vasopressin (AVP) responses to phlebotomy, hypertonic saline, water loading and fluid restriction were studied in 2–49 day old lambs. Phlebotomy of 10 and 20% of the lamb's estimated blood volume produced 37-and 44-fold increments in plasma AVP, without a concomitant change in plasma sodium or osmolality. The infusion of 10 mEq/kg sodium chloride produced a 12% rise in plasma sodium concentration accompanied by a 7-fold rise in plasma AVP. Water loading with 100 ml/kg hypotonic fluid produced a significant fall in plasma sodium concentration (10.7%) and a decrease in plasma AVP. Eighteen hr of water deprivation evoked a 7-fold increase in AVP.Summary
These results indicate that the newborn lamb is capable of responding appropriately to known stimuli for AVP secretion. The stimulus response ratio (SRR):Summary
SRR=Log [AVP]1 - Log [AVP]2/δ osmolalitySummary
of newborn lambs was nearly identical after hypertonic saline and water loading and was also quite similar to that of the adult ewe after a saline challenge. The SRR of water deprived lambs was greater than that after the other stimuli, presumably reflecting combined volume and osmolar stimuli. We conclude that the neurohypophysis and the volume receptor systems of the newborn lamb are capable of appropriate, mature AVP responsiveness during the first days of extrauterine life.Speculation
The present studies indicate that AVP secretion in the newborn sheep is responsive to both volume and osmolar stimuli. Quantitative responses are equivalent to those of mature ewes. If the newborn human is both osmo- and volume sensitive, as seems likely, AVP secretion may be important in fluid and electrolyte homeostasis in the newborn period. Thus, lack of AVP secretion does not explain the limited ability to concentrate urine demonstrated by the newborn infant.