Circulatory effects of chemical sympathectomy in fetal, neonatal, and adult sheep.

Circulatory effects of chemical sympathectomy with 6-hydroxydopamine (6-HD), including changes in vascular sensitivity to neurotransmitters, were investigated in chronically instrumented unanesthetized fetal, neonatal, and adult sheep. In all animals acute response to 6-HD consisted of hypertension and tachycardia, which lasted 2-3 h. Chronic phase of sympathetic ablation then followed during which arterial pressure and heart rate of sympathectomized fetus and neonate returned to and remained at levels observed in intact animals, whereas adult sheep were slightly hypotensive throughout this period. In all animals supersensitivity of peripheral circulation to norepinephrine occurred, but only the fetus exhibited supersensitivity to acetylcholine and isoproterenol. Our conclusions are as follows: 1) acute phase of chemical sympathectomy in fetal, neonatal, and adult sheep simulates strong adrenergic stimulation; 2) absence of significant alteration in arterial pressure of fetus and neonate during the chronic phase of sympathectomy suggests a minor role for the adrenergic system in the maintenance of resting tone of peripheral circulation; and 3) on the basis of present and previous data, we postulate that the supersensitivity of fetal circulation to acetylcholine and isoproterenol may be secondary to changes occurring in pulmonary vascular bed.