Absorption, excretion and tissue distribution of oral cyano-cobalamin in rabbits

The appearance of radioactivity in plasma, following an oral dose of Co60 cyanocobalamin, reached maximum values after 7 hours. The plasma activity during the succeeding 26-day experimental period decreased in a logarithmic fashion which was resolved into three components with biological half-lives of 29 hours, 7.1 days and 15.8 days. The 7.1-day half-life component is significantly longer than the 4-day half-life found in rabbits following intramuscular or intravenous administration of Co60 cyanocobalamin. This indicates a different, more stable, protein-cyanocobalamin complex formed during absorption. The excretion of radioactivity in urine and feces following oral dosage of Co60 cyanocobalamin demonstrates the limited absorption of cyanocobalamin from the gut of rabbits. The urinary excretion of the radioactive dose increased from 2.2% to 20.7% in animals previously injected with nonradioactive cyanocobalamin and liver tissue retained more radioactivity following nonradioactive cyanocobalamin supplementation. It is concluded that the cyanocobalamin absorption mechanism of rabbits is similar to that of human subjects and other laboratory animals.

Insulin and tissue distribution of pentose in nephrectomized cats

A comparison has been made of the ‘pentose space’ and ‘chloride space’ of various tissues in the nephrectomized cat injected intravenously with various pentoses, and of the effect of insulin injection on the pentose space. The four pentoses tested, d- and l-arabinose and D- and L-xylose, were found to pass readily into the intracellular compartment of liver. With this exception, and the apparent failure to penetrate the blood-brain barrier, d-arabinose and l-xylose, which have steric configurations about carbon atoms 1, 2 and 3 opposite to that of D-glucose, showed purely extracellular distribution which was not affected by insulin. In the absence of exogenous insulin, extracellular distribution of L-arabinose and D-xylose was found in lung, intestine, skin, spleen and muscle; under these conditions brain and heart showed partial penetration of these sugars into the intracellular compartment. The injection of insulin resulted in partial intracellular penetration into muscle, increased penetration intracellularly in brain and heart, and was without apparent effect on the other tissues analyzed. The findings are discussed in terms of the postulate of Levine that insulin acts to facilitate the transfer across the cell membrane of glucose and of those sugars with the same steric configuration as D-gluclose about carbon atoms 1, 2 and 3. It is concluded that if the mechanism of transport of glucose across the cell membrane is the same as that of transport of these pentoses, then the entry of glucose into the cell interior precedes any phosphorylation reaction.