Mechanism of Action in the Intestine

It is well known that vitamin D stimulates intestinal calcium absorption. Throu the work of several groups has come the understanding that in response to vitamin D, calcium is transported against an electrochemical potential gradient in the small intestine The most rapid rate of calcium transport is in the duodenum followed by jejunum and ileum Harrison and Harrison have also shown clearly that vitamin D improves intestinal calcium transport even in the colon Physiologically, because of the time during which calcium is subject to absorption, it seems evident that the distal portions of the small intestine are primarily responsible for the bulk of intestinal calcium absorption. 

The movement of calcium against an electrical and concentration gradient requires metabolic energy The site of vitamin D activation of the calcium transport process is not entirely settled, although there is some agreement that vitamin D in some way alters the characteristics of the brush border membrane to permit entry of calcium but the evidence on this point is equivocal The 

The response of intestinal calcium transport to vitamin D is sensitive to the pretreatment by actinomycin However, it is unclear whether actinomycin D blocks the intestinal response to 1,25-(OH)2D3 There is some evidence which suggests that the lifetime of the 25-OH-D3-l-hydroxylase and its messenger is sufficiently short that the apparent block in vitamin D action on intestinal calcium transport may be due to the decay of the messenger and of the enzyme for the 1-hydroxylation reaction in the kidney Once the 1-hydroxylase is bypassed by the administration of 1,25- OH)2D3, actinomycin D does not block the transport process in the rat at least This finding has been confirmed and extended Tsai et al. have provided conflicting evidence in the chick that actinomycin D does block the intestinal response to 1,25-(OH)2D3 However, they found it necessary to administer the actinomycin D every 2 hr before a block could be effected. Whether this is a bona fide block or a toxic reaction to the antibiotic remains undetermined. In any case the use of RNA and protein synthesis inhibitors in vivo to deduce the mechanism of action of 1,25-(OH)2D3 results in unclear and difficult to interpret results, especially if RNA and protein synthesis is incompletely blocked  

A fluorescent antibody study of the calcium binding protein location has suggested that it is formed in the goblet cells and is secreted from the goblet cells along the surface of the columnar epithelial tissues A wider dispersion of this protein has been suggested from other immunofluorescent antibody studies but the question of specificity of antibody must be taken into account. The exact cellular and subcellular localization of calcium binding protein is as of this date unresolved. 

Work with radioactive vitamin D metabolites has shown that the nuclear fraction contains as much as 80% of the administered radioactive 1,25-(OH)2D3 However, it is not at all certain whether all of this radioactivity is contained within the pure nuclei. Unfortunately methods of preparing pure nuclei from other tissues have not been successfully applied to the intestine. Even the method specifically developed for this purpose results in a low yield of intestinal nuclei of the order of 20 to 40% Recently the synthesis of high specific activity tritiated 25-OH-D3 has been completed converted to 1,25-(OH)2D3 and used to study subcellular localization by frozen section autoradiography and physiological doses Specific nuclear location of l,25-(OH)2[ H]D3 could be demonstrated in intestinal villus and crypt cells but not in muscle, liver, submucosa and most of the kidney (Fig. 8). This localization preceded the initiation of intestinal calcium absorption by 1,25-(0H)2D3 Thus at least a portion of the mechanism of action of 1,25-(OH)2D3 must involve nuclear function. 

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