Noradrenergic control of blood vessels in the premature lamb fetus

Experiments were conducted with fetal lambs of 113-120 days gestation (0.8 term) to assess the integrity of adrenergic neurotransmission at the level of the blood vessels. Regional blood flow and the distribution of blood from the inferior vena cava were compared when the arterial pressure was increased either by an infusion of exogenous noradrenaline or when tyramine was used to evoke a local release of the neurotransmitter. Most vascular beds perfused via the descending aorta were constricted to a similar extent by both drugs although the renal circulation did not respond to tyramine. Noradrenaline increased the distribution of blood from the inferior vena cava to the fetal lungs and both drugs increased placental blood flow. These data indicate that mechanisms for effective adrenergic control of vascular are developed in many organs of the premature lamb fetus by 0.8 of term.