The Effect of Parathyroid Hormone and Vitamin D on Serum Calcium in Rats

Conflicting evidence exists regarding the hypothesis that vitamin D is essential for the calcium‐mobilizing action of parathyroid hormone. A re‐investigation was undertaken by feeding weanling vitamin D‐deficient rats a vitamin D‐free diet containing 0.7 % calcium and 0.6 % phosphorus (diet D,) for 3 weeks. At day I, when the mean serum calcium level was 7.1 mg/100 ml, the diet was replaced by 10 % glucose in saline. Different groups received either A) 100 I.U. vitamin D on day 2; B) injections of 200 and 100 units parathyroid extract (PTE) on days 4 and 5, respectively; C) a combination of treatments A and B; or D) saline injections. The serum calcium levels, determined 6 hours after the last PTE injection, permitted calculation of mean individual increase expressed in mg/100 ml: A) + 1.04, B) + 0.86, C) + 2.58, D) – 0.57. Continuous feeding with diet D, or one containing less protein resulted also in a significant response to PTE. Parathyroidectomy, but not sham operation, resulted in a substantial fall in serum calcium in rats fed diet D., The conclusions are: 1) The calcium‐mobilizing action of parathyroid hormone in the rat is either independent of, or requires only minute amounts of, vitamin D. 2) At the one dose level investigated, no evidence for interaction between the effects of vitamin D and parathyroid extract was found when intestinal calcium absorption was minimal.