The distribution and storage of vitamin D and its metabolites in human tissues

1. The distribution of vitamin D and its metabolites in human tissues has been studied by the combined use of radioactive cholecalciferol and biological assays of antiricketic activity in tissue extracts.

2. Injected radioactive cholecalciferol was cleared rapidly from the blood; unchanged vitamin D and various metabolites were detected subsequently in all tissues examined. The highest concentration of biological activity and radioactivity was in fat; this and, to a lesser extent, other tissues were shown to retain activity over a prolonged period of time.

3. Adipose tissue and voluntary muscle are the principal sites of storage of vitamin D in man. Pre-existing tissue pools of vitamin D can, in some circumstances, invalidate the use of radioactively labelled cholecalciferol to trace the pattern of body distribution.

4. Metabolically produced 25-hydroxycholecalciferol is also taken up from the blood into many tissues, probably by protein-binding.

5. Vitamin D is excreted in bile principally as more polar metabolites. Smaller amounts of cholecalciferol and 25-hydroxycholecalciferol are also excreted, and antiricketic activity has been demonstrated in the bile of individuals treated with vitamin D.

6. An excess of cholecalciferol (or of 25-hydroxycholecalciferol) in the blood appears to be eliminated by the physicochemical processes of partition into tissue lipid and binding to tissue proteins. It is inferred tentatively that an increased concentration of vitamin D or of 25-hydroxycholecalciferol in the liver also causes an increased biliary excretion of these substances, and an increased hepatic formation and elimination of more polar metabolites of the vitamin.

Hormonal control of the renal conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol

Isolated renal tubules from vitamin D-deficient chicks catalyse the in vitro conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol. This conversion is stimulated by 5 x 10(-10) M bovine parathyroid hormone, or by 10(-6) M cyclic AMP. It is inhibited by 10(-9) M porcine calcitonin. It is concluded that these hormonal controls of the synthesis of the renal hormone 1,25-dihydroxycholecalciferol are of particular physiological significance in coordinating the activities of the various organs involved in extracellular calcium homeostasis.

Control of 25-hydroxycholecalciferol metabolism by parathyroid glands

Thyroparathyroidectomy of rats on a diet low in calcium reduces production of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol to negligible levels within 40 hr, and increases production of another metabolite, called Va. Parathyroid extract, at a dose of 20 units per day, prevents these changes. When 40 units per day of parathyroid extract is given 48 hr after thyroparathyroidectomy, 1,25-dihydroxycholecalciferol production is restored almost to control levels within 36 hr. The change brought about by parathyroid extract cannot be attributed to resulting changes in serum calcium or phosphorus concentration. It appears that the parathyroid hormone serves as a tropin for production of 1,25-dihydroxycholecalciferol, the hormonal form of vitamin D responsible for calcium mobilization from intestinal contents and bone.

1,25-dihydroxycholecalciferol: metabolite of vitamin D3 active on bone in anephric rats

Nephrectomy prevents completely the bone calcium mobilization response to 25-hydroxycholecalciferol. In contrast it does not prevent this response to 1,25-dihydroxycholecalciferol. Because it is known that the kidney is the site of 1,25-dihydroxycholecalciferol formation, these results provide evidence that 1,25-dihydroxycholecalciferol or a further metabolite thereof and not 25-hydroxycholecalciferol is the metabolically active form of vitamin D(3) responsible for bone calcium mobilization.

Phenobarbital-induced alterations in vitamin D metabolism

The metabolic fate of intravenously injected vitamin D(3)-1,2-(3)H (D(3)-(3)H) was studied in two normal individuals on chronic phenobarbital therapy. Silicic acid column chromatography of lipid-soluble plasma extracts obtained serially for 96 hr after D(3)-(3)H injection demonstrated a decreased plasma D(3)-(3)H half-life and increased conversion to more polar metabolites. The polar metabolites formed included several with chromatographic mobility similar to known biologically inactive vitamin D metabolites and one with chromatographic mobility identical to 25-hydroxycholecalciferol. Disappearance of this latter material was also accelerated. A child with rickets and a normal volunteer studied before and after a 2 wk course of phenobarbital therapy demonstrated similar alterations in D(3)-(3)H metabolism. When liver microsomes from 3-wk-old Sprague-Dawley rats treated with phenobarbital were incubated with D(3)-(3)H, polar metabolites were produced with chromatographic mobility similar to the plasma D(3)-(3)H metabolites from phenobarbital-treated humans. Similar incubations employing 25-hydroxy-cholecalciferol-26-27-(3)H as the substrate also demonstrated an increased conversion to polar metabolites. The data suggest that the reported increased incidence of osteomalacia observed in patients on chronic anticonvulsant therapy may be the result of an accelerated conversion of vitamin D and its active metabolite, 25-hydroxycholecalciferol, to polar metabolites by druginduced liver microsomal enzymes.

Regulation by calcium of in vivo synthesis of 1,25-dihydroxycholecalciferol and 21,25-dihydroxycholecalciferol

Tritiated 1,25-dihydroxycholecalciferol accumulates in several tissues, to an extent that varies with dietary calcium content, 12 hr after the administration of 325 pmoles of tritiated 25-hydroxycholecalciferol to rats. As the dietary and serum calcium concentrations increase, the amount of 1,25-dihydroxycholecalciferol is diminished and the concentration of 21,25-dihydroxycholecalciferol increases. This correlation is especially evident in rats given vitamin D(3). In vitamin D-deficient rats, the repression of 1,25-dihydroxycholecalciferol formation occurs with a diet containing 3% calcium and 20% lactose. The results suggest that the production of 1,25-dihydroxycholecalciferol, believed to be the metabolically active form of vitamin D in the intestine, is responsible for the adaptation of calcium absorption to low dietary concentrations of calcium.

Mechanism of action of 1,25-dihydroxycholecalciferol on intestinal calcium transport

The prior administration of actinomycin D prevents the metabolism of [(3)H]25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, a metabolite of vitamin D(3) that is effective in the stimulation of intestinal calcium transport. In this paper, the question of whether the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is sensitive to actinomycin D was examined. While the response of intestinal transport to physiological amounts of 25-hydroxycholecalciferol is blocked by actinomycin D, the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is insensitive to the antibiotic. These results suggest that 1,25-dihydroxycholecalciferol, or a further metabolite thereof, is the metabolically active form of vitamin D in the intestine, that it functions by a process not involving transcription of DNA, and that the step sensitive to actinomycin D in the action of vitamin D on the intestine does not occur in the intestine, but is the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidney.