Increased spillover and reduced clearance both contribute to rise in plasma catecholamines after birth in lambs

We studied the relationship between changes in norepinephrine (NE) and epinephrine (Epi) levels, total body spillover, and total body clearance at birth. Near-term fetal lambs were chronically instrumented under general anesthesia with arterial, venous, and left atrial catheters. One week later, NE and Epi kinetics (isotope dilution methodology) and systemic blood flows (radioactive microspheres) were measured in fetuses and then in the same animals 1 and 4 h after cesarean section delivery. Comparing fetal and 1-h lambs 1) systemic output fell by 33% (P < 0.005); 2) systemic plasma NE increased by 144% (P < 0.005), and plasma Epi increased sevenfold (P < 0.005); 3) total body NE spillover rose by 63% (P < 0.01) and Epi spillover by fivefold (P < 0.005); and 4) total body NE clearance decreased by 34% (P < 0.005) and Epi clearance by 37% (P < 0.005). Systemic blood flow and kinetic data were similar in 1- and 4-h lambs and were therefore pooled to define the interrelationship among perinatal changes in NE and Epi plasma levels, spillover, and clearance. Between fetal and newborn lambs, plasma NE rose by 1,375 +/- 207 pg/ml, of which 604 +/- 119 pg/ml (approximately 44%) resulted from increased NE total body spillover and 771 +/- 160 pg/ml from reduced NE total body clearance. In the same interval, plasma Epi rose by 292 +/- 30 pg/ml, of which 123 +/- 18 pg/ml (approximately 42%) was due to Epi total body spillover and 169 +/- 19 pg/ml to reduced Epi total body clearance. These findings indicate that 1) sympathoadrenal activity increases with birth, and 2) increased total body spillover and reduced total body clearance contribute a similar portion of the plasma NE and Epi surge at birth.