title: 24-Hydroxylase: potential key regulator in hypervitaminosis D3 in growing dogs.
abstract: A group of growing dogs supplemented with cholecalciferol (vitamin D(3); HVitD) was studied vs. a control group (CVitD; 54,000 vs. 470 IU vitamin D(3)/kg diet, respectively) from 3 to 21 wk of age. There were no differences in plasma levels of P(i) and growth-regulating hormones between groups and no signs of vitamin D(3) intoxication in HVitD. For the duration of the study in HVitD vs. CVitD, plasma 25-hydroxycholecalciferol levels increased 30- to 75-fold; plasma 24,25-dihydroxycholecalciferol levels increased 12- to 16-fold and were accompanied by increased renal 24-hydroxylase gene expression, indicating increased renal 24-hydroxylase activity. Although the synthesis of 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)] was increased in HVitD vs. CVitD (demonstrated by [(3)H]1,25(OH)(2)D(3) and increased renal 1alpha-hydroxylase gene expression), plasma 1,25(OH)(2)D(3) levels decreased by 40% as a result of the even more increased metabolic clearance of 1,25(OH)(2)D(3) (demonstrated by [(3)H]1,25(OH)(2)D(3) and increased gene expression of intestinal and renal 24-hydroxylase). A shift of the Ca set point for parathyroid hormone to the left indicated increased sensitivity of the chief cells. Effective counterbalance was provided by hypoparathyroidism, hypercalcitoninism, and the key regulator 24-hydroxylase, preventing the development of vitamin D(3) toxicosis.
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, 3508 TD Utrecht, The Netherlands. M.A.Tryfonidou@vet.uu.nl
authors.forename: M A
authors.initials: MA
authors.lastname: Tryfonidou
authors.forename: M A
authors.initials: MA
authors.lastname: Oosterlaken-Dijksterhuis
authors.forename: J A
authors.initials: JA
authors.lastname: Mol
authors.forename: T S G A M
authors.initials: TS
authors.lastname: van den Ingh
authors.forename: W E
authors.initials: WE
authors.lastname: van den Brom
authors.forename: H A W
authors.initials: HA
authors.lastname: Hazewinkel
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000235:genetics; D000375:Aging / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn / Q000254:growth & development / Q000378:metabolism; D002118:Calcium / Q000097:blood / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D002762:Cholecalciferol / Q000008:administration & dosage / Q000506:poisoning*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D004492:Edetic Acid / Q000494:pharmacology; D015870:Gene Expression / Q000187:drug effects; D013250:Steroid Hydroxylases / Q000235:genetics / Q000378:metabolism*; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D002614:Chelating Agents; D002762:Cholecalciferol; D003577:Cytochrome P-450 Enzyme System; D004492:Edetic Acid; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002118:Calcium