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Cardiovascular responses to autonomic blockade in intact fetal and newborn lambs

Sympathetic and parasympathetic activity have been studied in exteriorized fetal lambs but not in intact fetuses in utero. We placed vinyl catheters in a systemic artery and hindlimb vein in 20 fetuses from 92 days of gestation to term and also in 3 newborn lambs. The animals were allowed to recover from surgery for 2–3 days and then studied daily for several weeks. Maternal and fetal pH, PCO2 and PO2 were normal in all experiments. The blood pressure (BP) and heart rate (HR) responses to selective intravenous injection of propranolol (1 mg/kg), practolol (1–2 mg/kg), atropine (1.0 mg/kg), phentolamine (0.1 mg/kg), phenoxybenzamine (5 mg/kg) and tyramine (0.25 mg/kg) were recorded. The adequacy of autonomic blockade was confirmed by absence of response to acetylcholine (5–15 μg/kg), isoproterenol (0.1 μg/kg) or methoxamine (50 μg/kg). Change in HR from resting levels without significant BP change were observed as follows: propranolol 5–25% decrease, practolol 8–22% decrease and atropine 0–64% increase. Response to beta-adrenergic blockade did not vary significantly with gestational age. Phentolamine and phenoxybenzamine decreased systolic BP 2-14 and diastolic BP 2-12 mm Hg in 19/21 animals and tyramine increased systolic BP 7-60 and diastolic BP 10-46 mm Hg in 7/7 animals. No significant difference in sympathetic responses were observed in fetal and newborn lambs, indicating HR and BP to be under similar autonomic control at all ages studied.