Vitamin D has long been recognized as an essential factor in the absorption of calcium from the gut. As mentioned in Chapter 12, deficiency of this vitamin in infants and children causes rickets, which is characterized by abnormal endochondral calcification, resulting in bones that are hypocalcified and soft. Rickets can be induced experimentally in rats fed on diets lacking vitamin D or low in phosphorus.

Though the mechanism by which vitamin D exerts its effects on calcium metabolism has only comparatively recently been elucidated, it has long been realized that the vitamin is intimately concerned with movement of calcium across the intestinal wall. This was shown to be an active process by experiments in vitro using everted gut sacs from healthy animals. Calcium ions were found to be moved from the mucosal to the serosal surface against a concentration gradient, provided that a metabolizable carbohydrate and oxygen were both present. The mechanism is fairly specific for calcium although strontium can be transported competitively but at a much slower rate. This discrimination against strontium is fortunate since it minimizes uptake of the radioactive fission product strontium-90, from the ‘fall-out’ of nuclear weapons, and so lessens damage to cells by radiation from this isotope.

Transport of calcium across the gut wall completely stops in animals that have been depleted of vitamin D but begins again, after a definite time-lag, when vitamin D is restored to their diet. However, if vitamin D is added in vitro to the everted gut sac from an animal starved of the vitamin, it has no effect in stimulating transport of calcium against a concentration gradient. This apparent paradox was resolved by a series of investigations, the success of which depended upon, firstly, the ability to produce labelled cholecalciferol with a sufficiently high specific activity for it to be studied at physiological dose levels and, secondly, the development of chromatographic techniques for the separation and subsequent identification of nanogram (10⁻⁹ g) levels of steroids and related compounds. By means of these techniques it has been shown that cholecalciferol is first converted in the liver to 25-hydroxycholecalciferol (25-HCC) (Figure 30.1), which is more biologically active than cholecalciferol itself. This substance is then hydroxylated further, in the kidney, to produce 1,25-dihydroxycholecalciferol (1,25-DHCC), the most potent antirachitic substance known. Furthermore, 1,25-DHCC, unlike cholecalciferol itself, can stimulate calcium transport in isolated gut sacs in vitro. The 1,25-DHCC finds its way to the nucleus of the surface intestinal cells, where it unmasks a specific gene which, by transcribing the appropriate mRNA, codes for a calcium-binding protein CaBP. This protein is located in the intestinal brush border and effects transport of calcium across the wall of the intestine. The need for two successive hydroxylations of cholecalciferol in the liver and kidney and the subsequent migration of the 1,25-DHCC to the target cells, followed successively by unmasking, transcription and protein synthesis, accounts for the time-lag observed before cholecalciferol produces its effect in deficient animals. Administration of actinomycin D, an inhibitor of protein synthesis, has been shown to block the physiological response to vitamin D by preventing the synthesis of the calcium-binding protein.

Dihydroxycholecalciferol is able to act on a number of tissues with columnar epithelial cells, including intestinal mucosa, kidney tubules, the shell gland of birds and probably also various types of bone cell where it may assist the synthesis of osteocalcin (page 161). Its mode of action is very similar to that of steroid hormones (Figure 30.1). In this respect its precursor, vitamin D₃, may be considered to function as a prohormone rather than a vitamin. The ability of 1,25-DHCC and other metabolites of vitamin D₃ to act on bone and kidney cells, as well as those of the intestine, means that vitamin D plays a key role in calcium and phosphorus metabolism (Figure 30.2).

The kidney cells are adaptable in that they can hydroxylate 25-HCC in alternative positions according to the need of the body for calcium. When the plasma calcium concentration tends to be low, the highly active 1,25-DHCC is formed but in normal and hypercalcaemic conditions the isomer 21,25-DHCC is produced instead. The latter is less active in promoting absorption of calcium but acts on the kidney to increase calcium excretion.

A practical outcome of these discoveries concerns the treatment of patients suffering from vitamin D-resistant rickets, who respond only to huge doses of the vitamin. This condition can result from chronic renal failure or from an inherited metabolic defect affecting the enzyme responsible for hydroxylating 25-HCC at the 1-position. Consequently the kidney cells are unable to produce 1,25-DHCC. Minute doses of this active metabolite are able to alleviate the failure of calcium absorption, bone disorders and muscular weakness. Unfortunately 1,25-DHCC is at present in short supply; however, synthetic 1-HCC is almost as effective as 1,25-DHCC to which it is probably converted in the body.