THE MODE OF ACTION OF VITAMIN D

1. The purpose of this review article is to re-evaluate and integrate many of the observations related to the physiological effects of vitamin D, using as a working hypothesis the concept that the vitamin may be acting analogously to a steroid hormone in terms of its ability to interact with genetic information and ultimately elicit a physiological response. Prior to this time the problem of the mechanism of action of vitamin D has primarily been approached from the point of view that the vitamin was acting as a cofactor for some specific enzymic reaction. 2. The physiological activities of vitamin D are integrated with those of parathyroid hormone to provide a homeostatic control for the regulation of primarily calcium and secondarily phosphate metabolism. It is proposed that the role of vitamin D in this homeostatic control mechanism is older and more fundamental than that of parathyroid hormone. The interaction of vitamin D on skeletal calcium metabolism may have evolved before the effects of the vitamin on intestinal calcium absorption. 3. There are several physiological defects of calcium metabolism—rickets, osteo-malacia, vitamin D-resistant rickets and idiopathic hypercalcaemia—all of which may be a consequence of an aberration in one or another of the interlocking steps of the vitamin D-dependent and calcium-dependent homeostatic control mechanism. 4. The most thoroughly established action of vitamin D in vivo is to promote or facilitate the intestinal absorption of calcium. Although the exact biochemical details of this process are not available, this may involve vitamin D-mediated synthesis of the appropriate enzyme systems or the alteration of membrane structure necessary for calcium absorption. It is not yet unequivocally established whether calcium absorption is an energy-dependent active transport process or is a passive carrier-mediated or simple diffusion process. 5. The exact action of vitamin D on bone metabolism is not as well established, but the primary effect of the vitamin is likely to mediate bone resorption. The vitamin D-dependent activities of the cell in both the intestine and bone are to absorb calcium and transfer it to the blood. 6. No direct effects of vitamin D on intestinal absorption of phosphate have been found. Furthermore the validity of a vitamin D-mediated renal reabsorption of phosphate is questioned, for the major effects of vitamin D are cation oriented. If the renal effects of vitamin D are true, it is postulated that the mechanism of action of the vitamin here on the anion, phosphate, is fundamentally different from its cation oriented mechanism. 7. There is a lag in the action of vitamin D on the vitamin mediated: (a) transport of calcium both in vivo in rats and chicks, and in vitro with everted intestinal slices; (b) the apparent increased permeability of intestinal mucosa; (c) increased levels of citric acid in serum or bone; (d) the increased incorporation of radioactive inorganic phosphorus into intestinal mucosa phospholipids. As shown by the use of radioactive vitamin D, this lag is not due to a lack of the vitamin in the target organs. 8. Whereas large, unphysiological doses of radioactive vitamin D localize in all tissues and all subcellular fractions, small physiological doses of radioactive vitamin D localize predominantly in the nucleus of the intestinal mucosa. The amount of vitamin D localized in the nucleus would appear to be too low for the vitamin to function as a cofactor, and is more indicative of an interaction on or with deoxy-ribonucleic acid. 9. Actinomycin D, an inhibitor of DNA-directed RNA synthesis, inhibits the action of vitamin D in mediating intestinal calcium absorption and bone resorption. Vitamin D also stimulates messenger-RNA synthesis in intestinal mucosa within 1/2 hr. of vitamin treatment. Vitamin D may play a crucial role, along with parathyroid hormone and calcium, in a DNA, gene-dependent, homeostatic control mechanism for cal, ium metabolism. In this system the vitamin D molecule has certain very specific structural requirements which are probably a reflection of the specificity of its receptor molecule, rather than structural requirements for a cofactor-enzyme relationship.