Renal parathyroid hormone-dependent adenylate cyclase activity after repletion of vitamin D-deficient rats with vitamin D-2

Rats fed a diet deficient in both vitamin D and Ca²⁺ exhibited a greater depression of the renal parathyroid hormone (PTH)-dependent adenylate cyclase than was observed in rats fed diets deficient in either vitamin D or calcium. Total serum Ca²⁺ was decreased from a control level of 11.2 mg/dl to 8.5 mg/dl in rats fed the diet deficient in calcium alone, and to 5.4 mg/dl in rats fed the diet deficient in vitamin D. Serum calcium was decreased further to 4.3 mg/dl in rats fed the diet deficient in both vitamin D and Ca²⁺. Serum immunoreactive PTH was significantly elevated over control levels when rats were fed the test diets; however, there were no significant differences between the elevated levels in the three experimental groups. Repletion of rats deficient in vitamin D only with a single oral dose of 3200 I.U. vitamin D-2 resulted in restoration of serum calcium to normal levels, a return of serum PTH to the control state, and an associated increase in PTH-dependent adenylate cyclase activity to the control level by 72 h. Repletion of rats deficient in both vitamin D and Ca²⁺ with the same dose of vitamin D-2 raised serum Ca²⁺ to 7.2 mg/dl by 72 h, but did not cause a reduction in circulating PTH, nor did it result in any significant improvement in the responsiveness of the membrane adenylate cyclase to PTH. These results suggest that elevated PTH is a factor in the down regulation of the PTH-dependent adenylate cyclase, but do not rule out a role for calcium as a regulatory factor.