Effect of age and dietary calcium on renal 25(OH)D metabolism, serum 1,25(OH)2D, and PTH.

The purpose of this study was to determine how serum 1,25(OH)2D, renal production of [3H]1,25(OH)2D and [3H]24,25(OH)2D from [3H]25(OH)D, and serum IPTH change with age and dietary Ca restriction. Male Fischer 344 rats aged 3, 13, and 25 mo were placed on either a high-Ca (1.2%) or low-Ca (0.02%) vitamin D-replete diet. After 4 wk, serum was collected, and renal conversion of [3H]25(OH)D3 to [3H]1,25(OH)2D3 and [3H]24,25(OH)2D3 was measured in vitro using isolated renal cortical slices. Serum 1,25(OH)2D and renal [3H]1,25(OH)2D3 production were markedly reduced in 13- and 25-mo-old rats compared with 3-mo-old rats fed the low-Ca diet. In 3-mo-old rats, feeding the low-Ca diet increased serum 1,25(OH)2D by 18-fold and renal [3H]1,25(OH)2D3 production by threefold compared with feeding the high-Ca diet. In 25-mo-old rats, dietary Ca had no effect on these parameters. Renal [3H]24,25(OH)2D3 production was increased in the 13- and 25-mo-old rats compared with the 3-mo-old rats. Serum IPTH increased with age regardless of diet and was significantly increased by the low-Ca diet in 3-mo but not in 13- or 25-mo-old rats. The changes in serum 1,25(OH)2D and renal [3H]1,25(OH)2D3 production observed in this study may account for the previously observed age-related decline in intestinal Ca absorption in this animal model.