Fetal sheep were thyroidectomized at 80 days' gestation and reoperated at 118-122 days for insertion of
vascular catheters. The effects of hypoxaemia and intravenous
tyramine infusion on plasma
catecholamine concentrations, blood pressure and heart rate were then determined in experiments
at 125-135 days' gestation. Age matched intact fetuses were also studied.
Thyroidectomy was associated with increased concentrations of
noradrenaline,
adrenaline and
dopamine in some thoracic and abdominal organs, increased
noradrenaline concentrations in the cerebellum, and decreased
adrenaline concentrations in the hypothalamus, cervical spinal cord, and superior cervical and inferior mesenteric ganglia. Arterial pressure was significantly lower in the thyroidectomized fetuses (34.0 +/- 0.15 mmHg) than in intact fetuses (44.7 +/- 0.2 mmHg; p less than 0.001). In contrast, plasma
noradrenaline concentrations were significantly higher in the thyroidectomized fetuses (2.04 +/- 0.25 ng/ml) compared to the intact fetuses (0.99 +/- 0.08 ng/ml; P less than 0.001). In the intact fetuses there was a significant increase in plasma
noradrenaline concentration and blood pressure during hypoxaemia, and
bradycardia at the onset of hypoxaemia. In contrast, in the thyroidectomized fetuses hypoxaemia did not cause significant change in plasma
catecholamine concentrations, blood pressure or heart rate. Infusion of
tyramine produced a 1.9-fold increase of plasma
noradrenaline in thyroidectomized fetuses compared to a 9.2-fold increase in the intact fetuses (P less than 0.05).
Tyramine infusion caused a similar proportional increase of blood pressure in both thyroidectomized and intact fetuses. Heart rate decreased during the
tyramine-induced
hypertension in the intact fetus, but increased in the thyroidectomized fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)