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 putative regulatory factors of the calcium (Ca) and inorganic phosphate (P(i)) homeostatic system in the horse. The concentrations of Ca, P(i), vitamin D metabolites, parathyroid hormone (PTH), the activity of the alkaline phosphatase (AP) and the concentration and binding properties of vitamin D binding protein (DBP) were measured in the plasma. In addition, the ability of the renal cortex to hydroxylate calcidiol into 24,25(OH)2D3 and 1,25(OH)2D3 was evaluated in vitro. The plasma concentration of Ca (3.2 +/- 0.15 mmol.L-1, N = 100) showed no significant differences between different horse breeds and was not influenced by Ca intake, exercise or by indoor maintenance. The concentration of plasma P(i) which ranged from 0.58 to 1.99 mmol.L-1 was negatively correlated with age and positively correlated with the P content of the feed. AP activities in plasma ranging from 131 to 852 U.L-1 were also negatively correlated with age tended to be higher in horses than in other domestic animals. Plasma concentrations of calcidiol and 24,25(OH)2D were much lower than in most other mammals and birds. The concentration and binding properties of DBP to calcidiol were not markedly different from those of other mammals. The mean plasma concentration of 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 renal cortex homogenates, only 25-hydroxycholecalciferol-24-hydroxylase activity could be detected (Vmax: 0.42 +/- 0.11 pmol.min-1.mg-1 protein; Km: 373 +/- 263 nmol.L-1). In conclusion, this study provided evidence that in contrast to other species, vitamin D does not appear to play a key role in regulating Ca and P(i) homeostasis in horses.