Cardiovascular responsiveness of the pig fetus to autonomic blockade.

We studied the development of responsiveness to pharmacological blockade of parasympathetic, alpha-adrenergic and beta-adrenergic control of the circulation of the pig fetus during the last 30 days of gestation (term is 114 days). Heart rate increased following administration of atropine, the increase in fetuses aged 85-90 days gestation being significantly smaller then at term. Infusion of phentolamine decreased fetal blood pressure, the fall in pressure seen in younger fetuses being less than that observed in fetuses near term. However, when expressed in relation to control pressures, the percentage change was the same at all ages. Propranolol slowed the heart rate of all fetuses with no significant differences between stages of gestation. We conclude that vagal control over the circulation continues to increase during the last month of gestation and that the substantial sympathetic activity seen at the earlier ages studied appeared to show no further development prior to term.