Effect of haemorrhage on plasma lysine vasopressin and the cardiovascular responses to vasopressin in the pig fetus.

The effect of haemorrhage on plasma lysine vasopressin concentrations and the cardiovascular effects of intravenous injections of synthetic lysine vasopressin were studied in twenty-four chronically catheterized pig fetuses aged between 81 and 114 d gestation (term = 114 +/- 1 d). Removal of 15-20% of the estimated blood volume reduced mean arterial blood pressure and elevated plasma vasopressin concentrations above the mean basal value of 2.2 +/- 0.3 muu./ml. The vasopressin concentration in plasma of fetuses younger than 104 d was insignificantly affected at 30 and 60 min after the bleed, whereas the corresponding values in older fetuses were raised by 12.7 +/- 7.2 muu./ml (d.f. = 7) and 16.0 +/- 6.0 muu./ml (d.f. = 7) respectively. The pituitary concentration of vasopressin was also greater towards the end of gestation. Plasma osmolality was unaffected by fetal blood loss. Maternal concentrations of vasopressin did not change from the basal values of 1.0 +/- 0.1 muu./ml (d.f. = 16). Injection of vasopressin raised fetal blood pressure and decreased heart rate. These results demonstrate that lysine vasopressin is present in the circulation of the pig fetus at 81 d (0.72 gestation), and that near term fetuses respond to haemorrhage more rapidly, and to a greater degree than younger fetuses.