PECULIARITIES OF VITAMIN-D AND OF THE CALCIUM AND PHOSPHATE HOMEOSTATIC SYSTEM IN HORSES

The aim of the present study was to investigate the importance of puta tive regulatory factors of the calcium (Ca) and inorganic phosphate (P -i) homeostatic system in the horse. The concentrations of Ca, P-i, vi tamin D metabolites, parathyroid hormone (PTH), the activity of the al kaline phosphatase (AP) and the concentration and binding properties o f vitamin D binding protein (DBP) were measured in the plasma. In addi tion, the ability of the renal cortex to hydroxylate calcidiol into 24 ,25(OH)(2)D-3 and 1,25(OH)(2)D-3 was evaluated in vitro. The plasma co ncentration of Ca (3.2 +/- 0.15 mmol.L-1, N = 100) showed no significa nt differences between different horse breeds and was not influenced b y Ca intake, exercise or by indoor maintenance. The concentration of p lasma P-i which ranged from 0.58 to 1.99 mmol.L-1 was negatively corre lated with age and positively correlated with the P content of the fee d. AP activities in plasma ranging from 131 to 852 U.L-1 were also neg atively correlated with age and tended to be higher in horses than in other domestic animals. Plasma concentrations of calcidiol and 24,25(O H)(2)D were much lower than in most other mammals and birds. The conce ntration and binding properties of DBP to calcidiol were not markedly different from those of other mammals. The mean plasma concentration o f calcitriol (55 +/- 24 pmol.L-1, N = 19) was much lower than in other mammals. The plasma concentration of PTH was 218 +/- 181 ng.L-1. In r enal cortex homogenates, only 25-hydroxycholecalciferol-24-hydroxylase activity could be detected (V-max: 0.42 +/- 0.11 pmol.min(-1) protein ; K-m: 373 +/- 263 nmol.L-1). In conclusion, this study promvided evid ence that in contrast to other species, vitamin D does not appear to p lay a key role in regulating Ca and P-i homeostasis in horses. (C) Inr a/Elsevier, Paris.