CONTROL OF VASOPRESSIN (AVP) SECRETION IN THE LAMB

The newborn is capable of secreting AVP, but there are no data regarding relative responsiveness to the several known physiological stimuli. We examined AVP, plasma sodium (Ma) and osmolality (Osm) in response to phlebotomy, water loading, hypertonic saline and mild dehydration in 7-11 lambs, 2-49 (mean = 19) days of age. AVP was measured by radioimmunoassay.Repeated phlebotomies (total 20 ml/kg) raised AVP from 4.8 ± 2.1 (mean ± SEM) to 74 ± 19 μU/ml (p < .01), while Na and Osm remained unchanged. When 100 ml/kg 2.5% dextrose/water was infused over 60 minutes, AVP fell from 3.,4 ± 1.2 to 0.7 ± 0.6 μU/ml by 60 minutes (p < .05). Na and Osm fell from 140 to 125 mEq/L and 283 to 262 mOsm/kg respectively (p < .05) Hypertonic (23%) sodium chloride infusion (10 mEq/kg) increased Na from 142 to 159 mEq/L and Osm from 271 to 318 mOsm/kg over a 30 min period (p < .05). In response to this stimulus, AVP increased from 2.9 ± 0.7 to 22.2 ± 9 μU/ml (p < .05). After 18 hours of dehydration, AVP rose from 0 6 ± 0.1 to 34 ± 1 8 μU/ml, Na from 134 to 140 mEq/L (p < 05) and Osm from 293 to 306 mOsm/kg (p < .05).Thus the newborn lamb is capable of responding to both volume and osmolar stimuli. The quantitative stimulus-response ratios (SRR = Mog AVP/ΔOsm) were similar for water loaded and saline stimulated newborns and similar to responses in the adult. The dehydration SRR was not accountable by osmolar change alone but also reflected volume change.