Intracellular distribution of tritiated bilirubin during hepatic uptake and excretion

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Abstract
Physiologic amounts of bilirubin-H3 were injected into normal rats and the distribution of isotope was determined in serum, subcellular fractions of the liver, and bile at intervals during the phases of uptake and excretion. Recovery in the three spaces after 2, 5, 15, and 30 min was 75–85%, indicating little bilirubin entered the extrahepatic tissues. At 5 min, 50% of the dose (excluding trapped serum bilirubin-H3) was in the liver, a bilirubin concentration greater than in serum; biliary excretion of bilirubin was less than 3%. By 30–60 min, most of the label was excreted in the bile as intact, diazotizable and crystallizable bilirubin. Bilirubin-H3 appeared principally in the hepatic cell sap at a rate and concentration too high for an albumin-linked transfer. No differences in the distribution of isotope in subcellular fractions were detectable during the phases of uptake or excretion or when the quantity injected was increased 20–40 times. Thus, intracellular distribution was primarily a constant function of intracellular bilirubin concentration. When compared to in vitro experiments, significant amounts of bilirubin accumulated within the microsomes and the acid hydrolase-rich fraction (lysosomes) but not in the mitochondrial or nuclear fractions.