Mechanism and site of small intestinal uptake of vitamin D3 in pharmacological concentrations

The site and mechanism of initial uptake of 1,2-3H vitamin D3 pharmacological concentrations was investigated using everted rat small bowel sacs incubated in a micellar medium. The mean +/- SE uptake rates of the vitamin at 300 muM incubation solution concentration by proximal, medial, and distal small bowel segments were 6.7 +/- 0.26, 7.8 +/- 0.54, and 3.3 +/- 0.20 nmole/min/100 mg tissue, respectively. Incubation with the addition of 10(-3) M 2,4-dinitrophenol, or 10(-3) M KCN, or under nitrogen atmosphere did not change (P greater than 0.05) the above rates of absorption. Incremental increases in the concentration of vitamin D in the incubation medium up to 1200 muM resulted in a linear increase in the uptake rate indicating lack of saturation kinetics. In all the above experiments, greater rate of uptake of the vitamin occurred in the proximal and medial small bowel than the distal small bowel (P less than 0.01). The above experiments indicate that vitamin D3 in this range of concentrations is taken up by enterocytes by a nonsaturable passive diffusion mechanism showing no evidence for carrier mediation. The rate of intestinal uptake is highest in the proximal and medial segments of the small bowel.