Restoration of Parathyroid Responsiveness in Vitamin D—Deficient Rats by Parenteral Calcium or Dietary Lactose*

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Abstract
The effects of treatment with vitamin D, calcium, or lactose on the responsiveness of vitamin D—deficient rats to parathyroid hormone were compared. In the absence of vitamin D, parenteral calcium or dietary lactose administration resulted in increases in serum calcium concentration although not to the normal values obtained in animals given vitamin D. Dietary lactose also partially restored the low bone calcium content of vitamin D—deficient rats. Untreated vitamin D—deficient rats showed no significant changes in serum calcium concentration after parathyroidectomy or parathyroid extract administration. Vitamin D, lactose, and calcium all restored responsiveness to parathyroid hormone; serum calcium concentration decreased after parathyroidectomy and showed a dose-related increase in response to parathyroid extract. Hence, the unresponsiveness to parathyroid hormone in vitamin D deficiency may be due to a lack of calcium at a local site of action, presumably bone, rather than to the absence of vitamin D as a specific cofactor.