The intracellular distribution of [1-3H]cholecalciferol in the intestine of vitamin D-deficient and -supplemented rats.

1. [1-(3)H]Cholecalciferol was administered orally at two dosages to vitamin D-deficient and -supplemented rats, and the intracellular distribution of the vitamin in the intestinal mucosa studied. 2. The concentration of cholecalciferol was highest in a fraction consisting of brush borders and nuclei. The microsomal fraction contained a higher concentration of the vitamin than the mitochondrial fraction in deficient rats, irrespective of the dose, whereas in the vitamin D-supplemented rats the concentration was the same in the two fractions. 3. Appreciable metabolism of the cholecalciferol occurred only in the supplemented rats and the metabolites were found predominantly in the mitochondrial fraction. 4. The cholecalciferol is more tightly bound to the microsomal fraction than to the mitochondrial fraction. 5. Experiments conducted in vitro have shown that all the intracellular particles combine with the vitamin either when dissolved in ethanol or bound to albumin. However, such an uptake does not account for the high concentration of radioactivity found in vivo in the fraction containing nuclei and brush border, nor for the tightly bound vitamin in the microsomal fraction.