Further studies on the interrelationship between parathyroid hormone and vitamin D.

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Abstract
Recent opinion regarding the parathyroid hormone (PTH)-vitamin D relationship has varied from that of a complete dependence of the hormone action upon the presence of the vitamin to that of a complete independence of action. This problem was studied directly by comparing the osseous and renal responses of vitamin D fed and D deficient, non-rachitic sham operated, parathyroidectomized or thyroparathyroidectomized rats to the long term intravenous perfusion of PTH. Previous studies had suggested that in the absence of vitamin D, PTH exerted its usual effect on the renal excretion of PO4 but was unable to mobilize Ca from bone. The results of the present work fully confirmed this notion and suggests the following: the presence of chronic secondary hyperparathyroidism decreases the phosphaturic response to exogenous PTH; excess thyrocalcitonin is not responsible for the failure of PTH to cause Ca mobilization in vitamin D deficient rats, excess PTH causes depletion of intracellular PO4 stores, and the clear demonstration of the described dependency relationships depends upon the adequacy of vitamin D depletion and the absence of endogenous parathyroid hormone in experimental animals.