Vitamin D, Dietary Calcium and Parathyroid Hormone Interactions in Chicks1

The effect of dietary vitamin D levels on the response to iv injected parathyroid hormone (PTH) was studies in chicks fed one of three diets: D-deficient, Control-D (1.4IU cholecalciferol/g diet), or High-D (70 IU cholecalciferol/g diet) during the first 4 weeks post-hatching. Compared to chicks on Control-D diet, chicks on the D-deficient diet had significantly decreased plasma Ca levels at 2 and 4 weeks and increased plasma P levels at 17 and 21 days. The plasma Ca response to a low dose of PTH (15 USP U/100 g body wt) 1 hr postinjection was normal at 1 week, reduced at 2 weeks and absent at 4 weeks in D-deficient chicks. However, a 4-16-fold higher dose of PTH did elicit a significant, though subnormal, response in this group at 3 and 4 weeks. Chicks fed the D-deficient diet with 2.8% Ca, compared to 1.4% Ca, showed a near normal plasma Ca level and bone ash content and only a small increase in plasma P at 17 and 21 days. However, the plasma Ca response to 15 U PTH/100 g body wt in this group was significantly increased only at 17 days and not at 21 days. In contrast, the hyperphosphatemic response to PTH was not markedly diminished in the D-deficient group, and it was restored to Control-D levels in the D-deficienyt High Ca group. These data suggest that different mechanisms may be involved in the Ca and P responses.