Metabolic Clearance and Plasma Appearance Rates of Catecholamines in Preterm and Term Fetal Sheep

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Abstract
Plasma catecholamines increase markedly during labor and delivery. Moreover we have noted greater increases at birth in preterm than term lambs. It was unclear whether these differences were due to differences in secretion or clearance. We therefore designed experiments to compare the metabolic clearance rates (MCR) and plasma appearance rates of norepinephrine (NE) and epinephrine (E) in chronically catheterized term (143 ± 1 days) and preterm (131 ± 1 days) fetal sheep. Two different techniques, radioisotope tracer techniques and infusion of cold hormones for estimation of clearance rates, were compared systematically in the same group of animals. The results demonstrate that MCR of NE in term fetuses (178 ± 28 ml·kg−1 min−1) is similar to preterm fetuses (205 ± 22 ml·kg−1 min−1) as is MCR for E (193 ± 28 versus 170 ± 33 ml·kg−1 min−1, respectively). Estimates of MCR from cold hormone infusion were highly dependent on infusion rate with estimtes as much as 150% above that determined by isotope tracer infusions. Plasma appearance rates for both NE and E in term and preterm animals were similar. There were no detectable physiologic alterations during the tracer isotope infusions whereas infusions of cold hormone were associated with marked alterations in heart rate and blood pressure. These results suggest: 1) there are no differences in MCR or plasma appearance rates of NE or E between 130 days gestational age and term fetal sheep; 2) cold hormone infusions overestimate MCR and plasma appearance rates of both NE and E when compared to radioisotope tracer infusions; 3) differences in catecholamine concentrations at birth between term and preterm animals appear to be due to differences in secretion rather than clearance.