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Calcium and Vitamin D Metabolism in the Dairy Cowl

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Calcium and Vitamin D Metabolism in the Dairy Cowl SYMPOSIUM: CALCIUM METABOLISM AND UTILIZATION Calcium and Vitamin D Metabolism in the Dairy Cowl R. L. HORST,* J. P. GOFF, and 1. A. REINHARDT USDA, Agricultural Research Service National Animal Disease Center Metabolic Diseases and immunology Research Unit Ames, IA 50010-0070 ABSTRACT D receptor, VDRE = vitamin D response ele- Most daq cows experience some de- ments, S-(OH)D3 = 25-hydroxycholecal- gree of hypocalcemia during the peripar- ciferol, 1,25-(OH)zD = 1,25-dihydroxyvitamin turient period. There is, however, a sub- D, 1,25-(0H)& = 1,25-dihydroxycholeca1- group of dairy cows that experience a ciferol, 1,24,25-(OH)$12 = 1,24,25-trihydroxy- breakdown in their ability to maintain vitamin Dz, 24,26-(OH)2D2 = 24,26-dihy- plasma calcium and, consequently, suffer droxyvitamin D2, 1,24,26-(OH)$12 = from severe hypocalcemia. This condi- 1,24,26-trihydroxyvitamin D,. tion is also known as milk fever and usually occurs in cows in their third or INTRODUCTlON greater lactation. The precise metabolic lesions responsible for the onset of milk Calcium is required for the normal function- fever have not yet been defined. Re- ing of a wide variety of tissues and physiologic search has shown that milk fever is not processes. Calcium is needed for bone forma- the result of inadequate production of tion, muscle contraction, nerve transmission, calcitropic hormones (parathyroid hor- blood clotting, and as a second messenger mone and 1.25-dihydroxyvitamin D), but regulating the actions of many hormones (30). rather is more likely a result of inade- In general, vertebrates maintain Ca with quate receptor numbers or receptor dys- remarkable precision. An exception is the aged function in the target cell of these hor- parturient dauy cow, which develops the meta- mones. This report reviews vitamin D bolic disease, milk fever. The hypocalcemia of and calcium metabolism, giving empha- milk fever results from a breakdown in the sis to 1,25-dihydroxyvitamin D receptor homeostatic mechanisms needed to replenish regulation and function as related to the Ca lost hmthe extracellular Ca pool at the periparturient dairy cow. The report also initiation of lactation (30). Diet and bone are focuses on providing insights into nutri- the primary sources of Ca in mammals and tional (anionic diets) and endocrine birds (11). The enhancement of intestinal Ca strategies that have proved useful in milk absorptive and bone Ca resorptive processes fever management. are under the influence of the Ca-regulating (Key words: calcium, vitamin D, hormones, parathyroid hormone (PTH),which parathyroid hormone, milk fever) is secreted by the parathyroid glands, and 1,25-dihydroxyvitamin D [1,25-(OH)zD], Abbreviation key: CaBP = calcium-binding which is produced in the kidney (11). Because protein, DCAB = dietary cation-anion balance, many endocrine disorders result from primary NAF = nuclear activation factor, PTH = hormone deficiencies or excess, failure to se- parathyroid hormone, RAF = receptor aux- cret PTH or 1,25-(OHhD was once hypothe- iliary factor, RAR = retinoic acid receptor, sized as the primary defect in cows with milk RXR = retinoid X receptors, VDR = vitamin fever. However, these hypotheses were dis- proved when researchers found that PTH and 1,25-(OH)2D were higher in blood of cows suffering from milk fever (33, 59). The cellular Received June 16, 1993. lesions involved in milk fever, therefore, still Accepted October 15, 1993. remain to be identified. This article reviews the 'No endorsements an herein implied. 2To whom reprint requests should be addressed (tele- known homeostatic controls involved in the phone 515/239-8312; FAX 515l239-8458). regulation and maintenance of Ca, and empha- 1994 J Dairy Sci 77:1936-1951 1936 SYMPOSIUM: CALCIUM METABOLISM AND UTILIZATION 1937 sis is given to the biological basis and control therefore, would have evolved with vitamin D2 of milk fever in dairy cows. as their major (if not only) source of vitamin D. Vitamin D3, however, would have served as the major vitamin source in most diurnal Vitamin Dp and Vitamin D3 Metabollrm D species. Both vitamin D2 and vitamin D3 are used Figure 1 summarizes the major metabolic for supplementation of animal and human diets pathways for the metabolism of vitamin D3. in the United States. Vitamin D3 is the form of Activation of vitamin D3 is initiated by C25 vitamin D that is synthesized by vertebrates hydroxylation in the liver to form 25- (29), and vitamin D2 is the major naturally hydroxycholecalciferol [25-(OH)D3], which is occurring form of the vitamin in plants. Vita- the major circulating form of vitamin D and, in min D3 also occurs naturally in plants and may the normal cow, is present in the plasma at constitute as much as 1% of the total vitamin concentrations of 20 to 50 ng/ml. Concentra- D in alfalfa (38). Whether or not vitamin D3 tions of <5 ng/ml would be indicative of vita- occurs naturally in other plant species is min D deficiency, and concentrations of 200 to presently unknown. Nocturnal herbivores, 300 ng/d would indicate vitamin D toxicosis. Figure 1. Pathways for the metabolism of vitamin D3. The bold mows indicate the major pathways for activation and catabolism. PTH = Parathyroid hormone. Journal of Dairy Science Vol. 77, No. 7, 1994 1938 HORST ETAL The 25-(OH)D3 is then la-hydroxylated in the and favors stimulation of the la-hydroxylase. kidney to form 1,25-dihydroxycholecalciferol The intestine, however, does not contain recep- [1,25-(OH)zD3], the active form of vitamin tors for PTH. The C24 oxidative enzymes in D3. The control of the la-hydroxylation proc- the intestine are, therefore, regulated mainly by ess is influenced by many factors (11). How- 1,25-(OH)2D, ever, the protein hormone PTH is most active As with vitamin D3, the major physiologic and most important in regulating the lu- pathway for activation of vitamin D2 is in- hydroxylase enzyme (1 1). The concentration of itiated by hydroxylation at C25 to form PTH in plasma is regulated mainly by plasma 25-(OH)&. However, the differences in side- Ca. As Ca concentration in the plasma declines chain chemistry between vitamin D2 and vita- e10 mg/dl, the parathyroid glands are stimu- min D3 offer opportunities for divergence in lated to produce PTH. In turn, PTH stimulates side-chain oxidation of the two sterols. For the activation of Z-(OH)D3 by upregulating example, the 24-position in vitamin D2 is a la-hydroxylase enzyme in the kidney in the tertiary carbon, as is the 25-position in vitamin kidney to form 1,25-(0H)2D.3 If plasma Ca is D2 and vitamin D3. In addition, the 24-position >lo mg/dl, PTH secretion is depressed, and for vitamin D2 is an allelic carbon, making it a 1,25-(OH)zD synthesis is depressed. In the far more reactive site than the corresponding adult nonpregnant, nonlactating cow, 1,25- position in vitamin D3. On the basis of these (OmD circulates in a range of 5 to 20 pg/ml. chemical differences, 24-hydroxylation of vita- In late pregnancy, circulating 1,25-(OH)zD min D2 may be a quantitatively significant may rise to a range of 20 to 50 pg/ml. During pathway for further metabolism of vitamin D2. parturition and initiation of lactation, In support of this argument is the recent find- ing of Horst et al. who provided the first 1,25-(OH)zD rises to values ranging from (M), lo0 quantitative report evaluating the 24- to >300 pg/ml during severe cases of hydroxylation of vitamin D2 to form 24- hypocalcemia (33). (0H)Q as a relatively minor but intact path- Both 25-(OH)D3 and 1,25-(OH)zD3 are sub- way for vitamin D2 activation. The 24-(0H)D2 ject to catabolic enzymes, which are present can then be la-hydroxylated to form mainly in the intestine and kidney. Catabolism 1,24-(OH)2D2, which, like 1,25-(OH)2D3, is ac- is initiated by C24 oxidation (50, 82). The C24- tive in target cells. Metabolism of 25-(0H)D2 oxidized metabolites then act as substrates for and 1,25-(OHhD2 deviates somewhat from the further oxidation at C23 (61, 62, 74), leading to classic vitamin D3 pathways. For example, the cleavage between C23 and C24 (13, 73). Minor 24-hydroxy derivatives of 25-(OH)2D2 and pathways of catabolism include c26 oxidation 1,24,25-trihydroxyvitamin D2 [1,24,25-(OH)3 and formation of 26,234actones (39, 83). Un- Dz], can be further hydroxylated at c28 or c26 der the influence of vitamin D toxicity or if (73). However, to date, neither 24-keto nor C23 plasma 1,25-(O&D concentrations are oxidized vitamin D2 metabolites have been elevated by giving exogenous 1,25-(OH)zD, identified. The C22 alkene and C24 (S)-methyl the C24 oxidative pathway of catabolism is up- groups in vitamin D2 apparently preclude these regulated in both the intestine and kidney. classic side-chain oxidation reactions. Also, However, if endogenous synthesis of 125- the compound 24-(0H)& [and probably (OmD is stimulated by a low Ca diet, the 1,24-(0H)24, as well] is a poor substrate for catabolic enzymes in the kidney are depressed, C25 hydroxylation. Rather, these metabolites but these same enzymes are stimulated in the proceed through the c26 hydroxylation path- intestine (22). The difference in these tissue way (49) to form 24,26-dihydroxyvitamin D2 responses to exogenous and endogenous [24,26-(0H)2D2] and 1,24,26-trihydroxy- 1,25-(OH)2D is that the kidney possesses vitamin D2 [1,24,26-(OH)&] (Figure 2). receptors for PTH.
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