Effects of hypoxia on vasopressin concentrations in cerebrospinal fluid and plasma of sheep.

The relationship between concentrations of vasopressin in plasma and cerebrospinal fluid (CSF) was examined under basal and hypoxic conditions in unanesthetized and unrestrained adult female sheep. Under basal conditions mean (+/- SE) concentration of vasopressin (4.1 +/- 0.3 pg/ml) in CSF was significantly (p less than 0.001) higher than the concentration in arterial plasma (2.4 +/- 0.2 pg/ml). There was a significant positive correlation between CSF and plasma vasopressin concentrations (r = 0.66, p less than 0.01). The introduction of severe hypoxia by exposure of the sheep to an inspired gas mixture of 5% O2 in N2, a known stimulus to systemic release of vasopressin, was associated with an increase in the concentration of vasopressin in plasma to 339 +/- 65 pg/ml and CSF to 19 +/- 3.9 pg/ml. This increase of vasopressin in CSF was delayed in time with respect to the increases in the plasma and of a lesser magnitude. With moderate hypoxia (10% O2 in N2 exposure) no increase in CSF or plasma vasopressin concentration was observed, suggesting a threshold response. Intravenous infusion of vasopressin to achieve plasma values comparable to those seen with hypoxia was not accompanied by a significant increase in CSF vasopressin concentration. Thus, severe hypoxia is a potent stimulus for release of vasopressin into both CSF and plasma. Furthermore, data suggest that vasopressin may be released into CSF via a separate route from that released into plasma.