The effect of tin protoporphyrin on the bilirubin production rate in newborn rats.

Using a flow-through system, the pulmonary excretion rate of carbon monoxide (VECO) was determined by gas chromatography and used as an index of bilirubin production in newborn rats treated with tin protoporphyrin. Hepatic and splenic heme oxygenase activities were determined spectrophotometrically. No significant differences in the VECO were found between experimental and control animals despite significant decreases in hepatic heme oxygenase activity (P less than .0005) and splenic heme oxygenase activity (P less than .025). These results suggest that 1) there is no simple relation between heme oxygenase activity and bilirubin production; 2) heme oxygenase is present in excess amounts in neonatal rats; and 3) the lowering of serum bilirubin levels caused by tin protoporphyrin cannot be attributed to decreased bilirubin production and may be owing instead to increased uptake, conjugation, or excretion of bilirubin, or decreased enterohepatic circulation of bilirubin.