Metabolic clearance and plasma appearance rates of catecholamines in preterm and term fetal sheep.

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 X kg-1 min-1) is similar to preterm fetuses (205 +/- 22 ml X kg-1 min-1) as is MCR for E (193 +/- 28 versus 170 +/- 33 ml X kg-1 min-1, respectively). Estimates of MCR from cold hormone infusion were highly dependent on infusion rate with estimates 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.(ABSTRACT TRUNCATED AT 250 WORDS)