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ABSTRACT
The purpose of these studies was to determine whether the responsiveness of the kidney to parathyroid hormone (PTH) and calcitonin changed with age. Experiments were performed in young (3 months old), adult (12–14 months old) and old (22–24 months old) male Fischer 344 rats fed normal diets and thyroparathyroidectomized. Parathyroid hormone was administered i.p. at 24, 12 and 2 h before death and calcitonin was given i.p. at 12 and 2 h before death. Parathyroid hormone significantly increased the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) by renal slices from young but not adult or old animals. A similar age-related decline in the capacity of PTH to raise serum 1,25-dihydroxyvitamin D (1,25-(OH)2D) levels was also seen. Parathyroid hormone significantly decreased tubular reabsorption of phosphorus, increased concentrations of urinary cyclic AMP (cAMP) and increased serum concentrations of calcium in all age groups. In contrast, calcitonin significantly increased 1,25-(OH)2D3 production by renal slices from both young and adult animals. Calcitonin decreased serum concentrations of calcium in young but not in adult rats. These results suggest that there are maturational changes in the PTH- and cAMP-dependent pathways in the kidney but not in the calcitonin- and cAMP-independent pathways. The changes in the PTH- and cAMP-dependent pathways affect the stimulation of 1,25-(OH)2D production but not the inhibition of phosphate transport.
J. Endocr. (1987) 114, 173–178
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Search for other papers by H. J. Armbrecht in
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Search for other papers by L. R. Forte in
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ABSTRACT
Growth hormone stimulates intestinal calcium absorption. This action has been linked to vitamin D metabolism. We have investigated the effects of hypophysectomy and GH treatment on renal metabolism of 25-hydroxycholecalciferol (25-OH-D3). Renal hydroxylation of 25-OH-D3 was measured in vitro using the renal slice technique. Experiments were performed in young F344 rats fed a vitamin D-replete, low calcium diet for 4 weeks. In hypophysectomized rats, renal conversion of 25-OH-D3 to 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) was markedly reduced compared with sham-operated rats. Renal conversion of 25-OH-D3 to 24,25-(OH)2D3 was markedly increased in hypophysectomized rats compared with sham-operated rats. Treatment of hypophysectomized rats with rat GH (rGH) for 10 days resulted in a significant increase in renal conversion of 25-OH-D3 to 1,25-(OH)2D3 and a significant decrease in conversion to 24,25-(OH)2D3. Rat GH treatment caused no significant changes in serum levels of immunoreactive parathyroid hormone. Serum calcium concentrations were similar in all groups, and serum phosphorus was low in hypophysectomized rats. Treatment of hypophysectomized rats with ovine GH for 6 days caused changes which were much less pronounced than those induced by rGH. Renal conversion of 25-OH-D3 to 1,25-(OH)2D3 and 24,25-(OH)2D3 correlated well with growth rate (weight gain). These results suggest that GH, either directly or indirectly, modulates renal metabolism of 25-OH-D3.
J. Endocr. (1984) 101, 333–338
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Search for other papers by T L Hodam in
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Abstract
The biologically active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D), acts on intestinal, renal, and bone cells to regulate skeletal and mineral metabolism. 1,25(OH)2D also induces 24-hydroxylase activity in these target cells. The 24-hydroxylase hydroxylates 1,25(OH)2D to 1,24,25-trihydroxyvitamin D and 25(OH)D to 24,25-dihydroxyvitamin D. The production of 1,24,25-trihydroxyvitamin D is thought to be the first step in the inactivation of 1,25(OH)2D by its target tissues. Previous studies have characterized the induction of the 24-hydroxylase by 1,25(OH)2D in clonal cell lines from intestine and bone. The purpose of these studies was to characterize the induction of the 24-hydroxylase by 1,25(OH)2D in the kidney, using the clonal rat renal cell line NRK-52E. 1,25(OH)2D (10−7 m) increased the mRNA levels for the cytochrome P450 component of the 24-hydroxylase (P450cc24) by sevenfold after 36 h in NRK-52E cells. 1,25(OH)2D increased P450cc24 mRNA levels in a dose-dependent manner with an EC50 of 10−8 m. In parallel experiments, 1,25(OH)2D significantly increased 24-hydroxylase enzyme activity after 48–72 h. The increase in P450cc24 mRNA induced by 1,25(OH)2D required on-going transcription and translation and was inhibited by H-7, a protein kinase C inhibitor. Tetradecanoyl phorbol acetate markedly increased the magnitude of the tissue responsiveness to 1,25(OH)2D by a protein kinase C-dependent pathway. These studies demonstrate that 1,25(OH)2D increases P450cc24 mRNA levels in NRK-52E cells by a mechanism requiring new protein synthesis and involving protein kinase C. This is in contrast to the action of 1,25(OH)2D in intestinal cells, which does not require new protein synthesis, and in osteoblastic cells, which does not involve protein kinase C.
Journal of Endocrinology (1997) 153, 199–205
