Cerebrospinal fluid and plasma vasopressin in the fetal lamb: Basal concentration and the effect of hypoxia

The concentrations of vasopressin in the plasma and cerebrospinal fluid (CSF) of the chronically catheterized fetal lamb were measured under basal and hypoxic conditions. Under basal conditions, samples were obtained from 13 fetal lambs of 117-146 days gestation. The mean ± SEM vasopressin level in CSF was 19.5 ± 1.5 pg/ml; the mean plasma vasopressin level of 1.9 ± 0.2 pg/ml was significantly less (P < 0.001). No consistent change in concentrations of vasopressin in CSF was observed with gestational maturation in 3 animals sampled sequentially or in individual samples obtained over the last 32 days of gestation. The mean vasopressin concentration in the CSF of the pregnant ewe was 5.1 ± 0.4 pg/ml. The gradients for osmolality, sodium, and potassium between fetal plasma and CSF were: osmolality, 298.4 ± 1.6 to 304.3 ± 1.4 mosmol/kg; sodium, 140.9 ± 0.5-142.5 ± 0.5 meq/liter; and potassium, 4.3 ± 0.1 to 3.3 ± 0.1 meq/liter. Fetal hypoxia was induced by exposure of the ewe to 10% O₂ in N₂ for 30 min. The concentration of vasopressin increased from 1.7 ± 0.3 to 277 ± 144 pg/ml (P < 0.001) in fetal plasma and from 21.4 ± 3.8 to 47.1 ± 9.9 pg/ml (P < 0.04) in fetal CSF. When the ewe was exposed to room air under comparable experimental conditions, no similar changes in plasma or CSF vasopressin levels were observed in the fetus. Infusion of vasopressin into the fetal jugular vein at 1.0 mU/min for 30 min increased plasma concentrations from 2.3 ± 0.5 to 83 ± 17 pg/ ml, while the CSF vasopressin values were 31.9 ± 5.9 (basally) and 30.7 ± 4.8 pg/ml (after infusion). Mean plasma and CSF osmolality, sodium, and potassium were not changed by any of these experimental interventions. We conclude that 1) under basal conditions, high concentrations of vasopressin are present in the CSF of the fetal lamb, the blood-CSF barrier appears to be impermeable to vasopressin, and concentrations of the hormone in fetal plasma are less than those in CSF; and 2) hypoxia is a potent stimulus of vasopressin release in both fetal plasma and CSF. The route of vasopressin released into the fetal CSF may be distinct from that released into plasma.