RESUMEN
Total bilirubin production and relative rates of early labeling of bilirubin (ELB) were determined in rats during postnatal development. Total production was estimated by measuring endogenous rate of excretion of carbon monoxide (VeCO), while ELB was determined by measuring the incorporation of glycine-2-14C and delta-aminolevulinic acid-5-14C(delta-ALA-5-14C) into expired 14CO over a 30-h period after isotope injection. VeCO was considerably higher in 1- and 4-day-old rat pups than in adults, but fell rapidly toward adult values by 9 days of age. 14CO excretion from both isotopic precursors of bilirubin was significantly greater in suckling animals than in postweanling and young adult animals when expressed as a percent of the administered radioactivity. The activity of hepatic heme oxygenase showed a similar pattern of postnatal change to 14CO excretion from both glycine-2-14C and delta-ALA-5-14C. However, phenobarbital administration to young weanling animals significantly increased 14CO excretion from glycine-2-14C, but did not result in a change in the activity of hepatic heme oxygenase. The activity of hepatic heme oxygenase does not always reflect in vivo ELB.