Arginine Vasopressin Response to an Osmotic Stimulus in the Fetal Sheep

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Abstract
Summary: Baseline plasma osmolality (pOsm) and plasma arginine vasopressin (pAVP) were measured in chronically catheterized fetal sheep. Mean baseline pAVP in fetuses from 101–120 days was 1.9 ± 0.46 μU/ml (mean ± SEM) with a pOsm of 293 ± 1.8 mOsm/kg. In fetuses of 121–141 days of gestation, mean pAVP was significantly lower, 0.77 ± 0.19 μU/ml (P < 0.05), with a similar pOsm (293 ± 1.9 mOsm/kg). The logarithmic baseline pAVP values were linearly correlated with pOsm for both groups. Hypertonic saline infusion resulted in a similar increase in the log pAVP corrected for the rise in pOsm in the 101–120-day fetuses and in 121–141-day fetuses. The slope of this response was similar to that of the steady state relationship. The data indicates that the fetal osmoreceptor system for control of arginine vasopressin secretion is fully functional in the last trimester of pregnancy. Speculation: The fetal hypothalamic posterior pituitary system appears to show a relatively heightened response to increases in plasma osmolality during the last trimester of gestation. This may reflect the heightened hypothalamic activity postulated to explain the high circulating growth hormone levels at this time. The higher baseline plasma vasopressin levels in the younger fetuses would support this view. Alternatively, the augmented responsiveness may represent an adaptive response to the immaturity of the fetal kidney with respect to free water clearance or an adaptive response to some extrarenal function of the hormone on salt and water homeostasis in utero.

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