Bone pathology and parathyroid gland activity in hypocalcemic magnesium-deficient chicks

Growing chicks fed magnesium-deficient (150 ppm Mg) diets for 14 or 21 days developed significant hypomagnesemia and hypocalcemia compared to control chicks fed 1,000 ppm Mg. The hypocalcemia was accompanied by significant parathyroid gland hyperactivity, suggesting that magnesium deficiency did not impair parathyroid gland function. Despite parathyroid gland hyperactivity, however, bone resorption was decreased in the magnesium-deficient chicks, although bone formation was not affected by magnesium depletion. The decrease in bone resorption in the magnesium-deficient chicks was correlated with significant bone magnesium depletion and resulted in increased bone calcium content. These findings suggested that the development of hypocalcemia in magnesium-deficient chicks was related to decreased bone resorption, due to impaired osteocytic function rather than parathyroid gland insufficiency. The results also offer an explanation for the decrease in skeletal responsiveness to PTH which has been reported during magnesium depletion.