Direct measurement of biliary bilirubin excretion in ponies during fasting.

Biliary excretion of bilirubin, including the conjugate composition, was studied during feeding and during a 2.5-day fast of three pony mares with chronic external biliary (T-tube) fistulas. Fasting bilirubin excretion (1.96 +/- 0.74 microgram/min/kg of body weight), after establishing a new steady state, was not different from excretion during feeding (1.99 +/- 0.45 microgram/min/kg). Hyperbilirubinemia of fasting resulted from a reduced removal of plasma bilirubin rather than from an increased input of bilirubin into the plasma. Relative plasma excretion of the individual conjugate fractions of biliary bilirubin was not changed by fasting.