Distribution of bilirubin-H³ in the fetal and maternal rat

¹ Gastrointestinal Laboratory and Metabolic Unit, Department of Medicine, and the Departments of Biochemistry and Anatomy, University of California Medical Center, San Francisco, California

Bilirubin-H³ in maternal or fetal serum was injected in physiologic amounts into the jugular vein of 19- or 21-day-old fetal rats, and distribution of activity in fetal and maternal tissues was measured. Two hours after injection, 93–98% of the activity was recovered in whole fetal homogenates. Activity was not detectable in either maternal serum or liver, indicating less than 2.5% of the total dose was in either tissue. Approximately 1–4% of the total dose appeared in maternal bile. In other experiments, activity in fetal tissues, after correction for trapped serum bilirubin-H³, was: carcass 27%, serum 21%, liver 10%, and intestine 2.5%. Significant activity was detectable in lungs, brain, kidney, and stomach, and traces were noted in heart, placenta, umbilical vessels, and spleen. Based on rate of bilirubin secretion and synthesis in adult rats, fetal synthesis would be 2.5–10 times placental transfer. Results suggest that placental transfer of bilirubin or bilirubin metabolites may not completely explain the low levels of bilirubin found in fetal circulation but indicate that a slow rate of bilirubin production or increased bilirubin storage in fetal tissue are contributing factors.

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