Absorption of Vitamin B12 from the Large Intestine of Rats

Publisher Website
Google Scholar
Abstract
Labeled vitamin B12 (17 to 180 of mμg 57Co- or 60Co-labeled vitamin B12 corresponding to 15,000 to 400,000 count/min) was injected into the colon ascendens or cecum of 104 rats (3 groups). The animals were killed at different time intervals (from 20 minutes to 29 days) and the tissue radioactivity was determined in a well-type scintillation counter. The total uptake, calculated for each animal, was expressed as percentage of the injected dose; large intestine (injection site) radioactivity was excluded. Thirty rats fed a stock diet containing non-labeled vitamin B12 (subdivided in 2 series) showed an average total uptake of 2.9% (range zero to 17.3) and of 2.0% of the injected dose (range 0.9 to 4.3), respectively. Animals fed vitamin B12-deficient diets showed a higher uptake of the labeled vitamin. In one series of 24 rats a total uptake of 8.3% of the injected dose (range 2.2 to 17.7) was obtained. Thirteen rats of a similar experiment showed an uptake of 5.0% of the injected dose (1.3 to 13.4) and 12 rats of 2 additional series showed an uptake of 6.8% (range 4.8 to 9.6) and 11.4% of the injected dose (range 4.0 to 17.3), respectively. The mechanism of the uptake of the radiovitamin was studied in the third group of 25 animals (series 7). In 23 of these rats the passage at the ileocecal valve was occluded by ligation prior to injection. The average uptake was similar to that obtained in a parallel series. No evidence for leakage of the injected vitamin, either intraperitoneally or into the small intestine (by anti-peristaltic movement), could be found. It appears therefore that the labeled vitamin was absorbed from the large intestine, perhaps by non-specific passive diffusion. Similarly, vitamin B12 synthesized in the large intestine by microbial flora could gain access to the tissues by such a mechanism. The possible role of intestinal vitamin B12 of microbial origin in rats, and perhaps in man, and factors affecting the utilization of this vitamin are discussed.