1. The rate of radioactive vitamin B12 excretion into plasma and bile from the isolated perfused rat liver and into bile in vivo has been measured after the intramuscular injection of radioactive cyanocobalamin at various time-intervals before study.

2. With an interval of labelling of 10 or more days, the release of radioactive vitamin B12 over 4 h by the isolated perfused liver was linear and con-stituted approximately 35% and 1% of the hepatic radioactivity into plasma and bile respectively.

3. In contrast, after a shorter period of prelabelling (less than 7 days), there was a biphasic release of radioactive vitamin Blz: an initial rapid rate followed by a slower rate after about 1 h of perfusion. The total radioactive vitamin Blz was considerably increased (e.g. 25% and 5% of hepatic radioactivity into plasma and bile respectively during a 4 h perfusion of a liver labelled 18 h previously).

4. Confirmation of coexisting stable and labile pools of intrahepatic vitamin B12 was provided by: (a) the patterns of hepatic release ifter double labelling with 57Co-labelled and 58Co-labelled cyanocobalamin at different times before liver per-fusion; (b) the rates of biliary excretion of 57Co-labelled and 58Co-labelled vitamin B12 in vivo after different periods of prelabelling.

5. The rate and pattern of release were not altered by changes in the quantity of precursor cyanocobala-min, by phenobarbitone treatment or by the addition of cycloheximide to the perfusion.

6. The injection into rats of subcellular preparations of rat liver labelled in viuo with radioactive vitamin B12 demonstrated that hepatic heterogeneity did not depend on physical compartmentation.

7. Despite the rapid release rate from the liver recently administered radioactive cyanocobalamin, the hepatic radioactivity increased progressively with time after labelling in vivo, in contrast to the other tissues, where it decreased. In the presence of rapid bidirectional fluxes the ability of the liver to store vitamin B can be largely explained by the reduction in the rate of hepatic release that continues for about 10 days after parenteral administration of cyano-cobalamin.

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