A novel method for continuous monitoring of bilirubin production in unstressed rats

We describe a device for continuous infusion and monitoring of exhaled 14CO as a test of hepatic bilirubin production in rats. A Silastic catheter, implanted into a jugular vein under light ether anesthesia, was protected with a spring shield and a cannula swivel. The animals were kept in a modified Bollman cage. delta-[5-14C]aminolevulinic acid, a heme precursor yielding 14CO upon breakdown of heme to bilirubin, was infused at a constant rate. Exhaled 14CO was oxidized to 14CO2 and collected in ethanolamine. The efficiency of the system averaged 97.8%. In untreated animals 14CO production reached a plateau within 12 h; thereafter, it increased by 2.8% per day. The responsiveness of the system was tested by fasting the animals, which stimulated hepatic bilirubin production. Fasting increased 14CO production by 32.8 +/- 8% (mean +/- SD, P less than 0.005) after 72 h. This was associated with an increase in hepatic heme oxygenase activity (+48%, P less than 0.05) and a decrease in microsomal cytochrome P-450 content (-45%, P less than 0.05). Thus, our approach permits continuous monitoring of hepatic bilirubin production without subjecting the animals to the stress of handling, restraint, or anesthesia. The method can easily be applied to other breath tests involving formation of 14CO2.