Calcium and phosphorus metabolism in chronically vitamin D-deficient laboratory rabbits

Calcium and phosphorus metabolism were investigated in chronically vitamin D-deficient adult albino rabbits that were consuming a diet containing 1% calcium and 0.5% phosphorus. Mineral balances, net intestinal and renal handling of calcium and phosphorus, and serum concentrations of calcium, phosphorus, parathyroid hormone, 25-hydroxycholecalciferol, and calcitriol were measured during metabolic balance studies. Chronic vitamin D deficiency (with undetectable serum concentrations of 25-hydroxycholecalciferol or calcitriol) was associated with mild hypocalcemia, moderate hypophosphatemia, and generally elevated serum parathyroid hormone concentrations. Net intestinal absorption of calcium or phosphorus was equal in chronically vitamin D-deficient adult rabbits and nutritionally matched, vitamin D-supplemented controls. In contrast, urinary excretion rates of both minerals were reduced significantly in the vitamin D-deficient group, indicating the action of homeostatic mechanisms within the kidneys. As a result of the decreased urinary excretions of calcium and phosphorus, the net external balance for each mineral during the metabolic balance studies was significantly more positive in the chronically vitamin D-deficient rabbits than in the vitamin D-supplemented controls. These observations demonstrate the importance of the renal conservation of Ca and P to mineral homeostasis in the chronically vitamin D-deficient adult rabbit.