The influence of 24,25(OH)2D3 on the calcemic effect of 1,25(OH)2D in rats with chronic renal failure is parathyroid hormone dependent.

Previous studies from our laboratory have shown that 24,25(OH)2D3 attenuates the calcemic effect of 1,25(OH)2D3 in rats with reduced renal mass. This study was undertaken to clarify the role of parathyroid hormone in this response. Adult rats (n = 27) with reduced renal mass after parathyroidectomy with an initial plasma calcium of 3.7 +/- 0.1 mEq/liter were divided into four groups: (i) control rats and rats treated with (ii) 24,25(OH)2D3, (iii) 1,25(OH)2D3, and (iv) both 1,25 and 24,25(OH)2D3. After 4 days significant hypercalcemia was seen in PTX animals receiving 1,25(OH)2D3 alone or in combination with 24,25(OH)2D3. Plasma calcium in the combined therapy rats (7.42 +/- 0.22 mEq/liter) was significantly higher than in those treated with 1,25(OH)2D3 alone (6.68 +/- 0.22 mEq/liter, P less than 0.05). After 8 days, plasma calcium was higher in the rats treated with 1,25(OH)2D3 but was of same magnitude in those treated with 1,25(OH)2D3 alone or in combination with 24,25(OH)2D3. In contrast, in a subset of rats (n = 35) with reduced renal mass but intact parathyroid glands similarly treated with the vitamin D metabolites, a blunted calcemic response was seen after the combination of 1,25(OH)2D3 with 25,25(OH)2D3 administration alone. These results show that in rats with reduced renal mass, 24,25(OH)2D3 attenuates the calcemic effect of 1,25(OH)2D3 only in the presence of intact parathyroid glands. The different calcemic responses to 1,25 or combined 1,25 and 24,25(OH)2D3 in intact or parathyroidectomized rats with chronic renal insufficiency may result from different interaction between the vitamin D metabolites and the parathyroid hormone, presumably at the level of bone.