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Adaptation to Low Dietary Calcium in Magnesium-Deficient Rats^1,2,

Department of Biochemistry, University of Ottawa, Ottawa, Ontario, Canada K1H 8M5

To determine if adaptation to low calcium (Ca) diets is impaired by magnesium (Mg) deficiency, weanling rats were pair-fed control (700 mg Mg/kg diet) or low Mg (70 mg Mg/kg diet) diets containing 5000 mg Ca/g diet for 7 or 10 d. Half of the animals from each group were then killed; the remainder continued on their previous Mg intakes and were subjected to low (500 mg Ca/kg diet) dietary Ca for 7 or 15 d. After 10 d of Mg deficiency, rats fed adequate Ca were hypomagnesemic and hypercalcemic, relative to controls, and had elevated circulating parathyroid hormone (PTH) but normal 1,25-dihydroxycholecalciferol (1,25-DHCC). After 7 d of low dietary Ca, there were no significant differences in plasma Ca, 1,25-DHCC or PTH between control and Mg-deficient rats despite a 50% reduction in plasma Mg in the latter group. After 15 d of low Ca stress, control rats remained normocalcemic, but Mg-deficient rats developed relative hypocalcemia despite similar, high circulating levels of PTH. Plasma 1,25-DHCC in the hypocalcemic, Mg-deficient rats was significantly lower than that of control rats also exposed to low dietary Ca, suggesting an impairment in 1,25-DHCC production during chronic Mg depletion. Significantly elevated renal Ca content was present in the Mg-deficient rats, which was unrelated to dietary Ca and preceded the development of hypocalcemia. The results indicate that both the duration of Mg deficiency and the Ca content of the experimental diet are important determinants of an animal's ability to maintain Ca homeostasis during Mg depletion.

KEY WORDS: • magnesium deficiency • hypocalcemia • vitamin D • 1,25-dihydroxycholecalciferol • parathyroid hormone

¹ Supported by the National Sciences and Engineering Research Council of Canada.

² Presented in part at the Annual Meeting of the Federation of American Societies for Experimental Biology, Washington, DC, April, 1987 [Welsh, J. & Weaver, V. (1987) Adaptation to low dietary calcium in magnesium deficiency. Fed. Proc. 46: 1410 (abs.)].

Manuscript received 10 September 1987. Revision accepted 16 February 1988.