Biliary excretion of 14C-labeled vitamin B-6 in rats

The biliary excretion of vitamin B-6 was studied in the intact rat and isolated perfused rat liver. In the whole animal, 21% of the radioactivity administered intravenously as [14C]pyridoxine was excreted in the urine over 4 hours, whereas only 2.1% was recovered in the bile. In the perfused liver, 3% of the radioactivity added to the medium was detected in the bile after 4 hours of perfusion. These data suggest that biliary excretion and enterohepatic circulation of vitamin B-6 probably play only a minor role in the overall economy of this vitamin. The concentration of the radiolabeled B-6 compounds in the bile of perfused liver was much higher than that in the perfusate and the pattern of the distribution of radioactivity among difference B-6 compounds was also different. These results suggest that [14C]pyridoxine and its metabolites are released separately by the hepatocytes into the bile and the perfusate and that paracellular transport of vitamin B-6 is not the predominant pathway for the biliary excretion of this vitamin.