Plasma vasopressin levels during hypoxaemia and the cardiovascular effects of exogenous vasopressin in foetal and adult sheep.

1. Fotetal plasma vasopressin levels were measured by bio-assay in chronically catheterized sheep from 110 to 145 days gestation. 2. In foetuses in good condition resting circulating vasopressin concentrations were generally undetectable (less than 5 micromicron./ml.). In 15% of the samples low concentrations (5-10 micromicron./ml.) were observed. 3. Hypoxaemia in the foetus was caused by allowing the ewe to breathe 9% O2-3% CO2 in N2 for 1 hr. Plasma vasopressin levels rose in the foetus to 119 +/- 32 micromicron./ml., whereas the hormone levels in the ewe were not routinely increased. In the foetus, the rise in plasma vasopressin levels was significantly related to the fall in pH and Pa, O2 during the hypoxia. 4. In foetuses in which the cervical vagosympathetic trunks were cut, the rise in plasma vasopressin levels (to 48 +/- 25 micromicron./ml.) during hypoxaemia was less than in intact foetuses. The increase was related only to the fall in arterial pH and the regression coefficient was less than in intact foetuses. 5. During hypoxaemia arterial pressure rose and heart rate fell in the normal foetuses. The rise in arterial pressure was greatest when the plasma vasopressin concentration was highest. 6. Spontaneous episodes of hypoxaemia (Pa, O2 less than 15 mmHg) and/or acidaemia (pH less than 7.30) occurred in four intact foetuses and four foetuses in which the cervical vagosympathetic trunks were cut; all of the latter lambs died in utero. Plasma vasopressin levels were elevated and the concentrations were inversely related to arterial pH. 7. Intravenous infusions of vasopressin to foetuses increased plasma vasopressin levels to 6-202 micromicron./ml.; the rate of clearance of the hormone was three times that in adult ewes. There was a large increase in arterial pressure and bradycardia. The hypertensive effects of vasopressin were relatively much greater in the foetus than in adult ewes.