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Skeletal and Plasma Calcium Changes in Chicks during Recovery from Vitamin D Deficiency with Normal and Low Calcium Intakes¹

Veterans Administration Hospital, Lexington, Kentucky 40507, and Department of Physiology and Biophysics, University of Kentucky, Lexington, Kentucky 40506

Tracer techniques were used to study calcium status in chicks fed a vitamin D₃-deficient diet for 14 days. Similar studies were continued between 14 and 21 days with and without vitamin D₃ repletion in chicks fed either a 1.0 or 0.08% calcium diet. Hypocalcemia which was evident at 14 days was corrected by vitamin D dosage within 48 hours in chicks fed the 1% calcium diet. During the initial 24-hour period, when only partial correction of the hypocalcemic condition was observed, vitamin D appeared to function mainly by influencing skeletal mobilization to increase blood calcium levels. Thereafter, the improved uptake of calcium from the gut was predominant, suggesting that the skeletal tissue responded sooner to vitamin D₃ than did the gut. The relative bone ⁴⁵Ca specific activities were highly correlated between the compact and cancellous bone fractions of the tibia in the control chicks and the four experimental groups. Vitamin D₃ appeared to enhance the movement of ⁴⁵Ca from each area, but the relative effect was greater in the cancellous than in compact bone. This resulted in a gradual increase in the compact/cancellous ratio of ⁴⁵Ca specific activity. This effect appeared to be independent of parathyroid hormone and thyrocalcitonin influence since it occurred in chicks exhibiting both hypocalcemia and normal calcemia. It also occurred in growing bone and in bone tissue where growth had virtually been arrested. These results indicate that vitamin D or a metabolite of this compound may play a direct role in bone remodeling, particularly the resorptive phase.

Manuscript received 19 January 1970.