THE RELATIONSHIP BETWEEN VITAMIN D AND PARATHYROID HORMONE*

Abstract

The effect of parathyroidectomy and parathyroid hormone administration upon the concentrations of plasma calcium and phosphate have been measured in vitamin D-deficient rats and the results compared to those observed in D-fed animals. Also, the influence of dietary calcium and phosphate content upon these responses has been investigated. Parathyroidectomy in the D-fed animal results in a rapid fall in plasma calcium and a biphasic plasma phosphate response with an initial fall followed by a striking rise. In contrast, parathyroidectomy in the D-deficient rat is followed by no significant change in plasma calcium and an immediate rise in plasma phosphate that can be prevented by the administration of parathyroid hormone. Dietary phosphate restriction abolished the effect of parathyroidectomy in both groups of animals. Administration of a dose of 500 U of hormone to the intact D-deficient rat had little influence upon plasma calcium; a dose of 2,000 U did increase plasma calcium. A dose of 25 U was sufficient to increase the plasma calcium of the D-fed animal.