Colonic absorption of bacterially synthesized vitamin K2 in the rat

Colonic absorption of bacterially synthesized vitamin K2 ([3H]menaquinone-9) was studied with everted rat colonic sacs in vitro. The mean +/- SE rate of absorption of the vitamin by the colon was 20 +/- 1.45 pmol/min per 100 mg tissue at 300 nM mucosal concentration of the vitamin. The rate of absorption did not change (P greater than 0.10) with the addition of 2,4-dinitrophenol, Na azide, or KCN to the mucosal incubation medium. No evidence for transmural transport of the vitamin was detectable. When the concentration of the vitamin was increased in a stepwise fashion up to 900 nM, the absorption rate remained linear with respect to the mucosal fluid concentration (r = 0.98). Autoradiography indicated that the vitamin accumulated in the mucosal and submucosal layers of the large bowel. Absorption of the vitamin by the large bowel takes place via a passive, nonsaturable process that shows no evidence of energy dependence or carrier mediation. It was concluded that vitamin K2 (bacterial origin) is absorbable by the rat colon in amounts sufficient to meet the daily requirement of the animal and may explain the lack of bleeding problems in the face of episodic lack of dietary vitamin K.