Effects of pharmacological doses of Vitamin D3 on mineral balance and profiles of plasma Vitamin D3 metabolites in horses.

Metabolism and functions of Vitamin D in horses differ from those in humans, pigs and rats. In horses, calcidiol and calcitriol concentrations in blood plasma are remarkably low (<10 nmol L(-1); 20-40 pmol L(-1), respectively). When a toxic amount of Vitamin D(3) is administered, the responsiveness of calcium and calcitriol concentrations in blood plasma is much reduced compared to the other domestic animal species but inorganic phosphate (Pi) response is much more marked, leading to an increase of the Ca x Pi product. Also, soft tissue calcifications have been observed to develop in horses during Vitamin D(3) intoxication. It was suggested that the elevation of the Ca x Pi product may play a causative role in this calcification process. To test this assumption, two horses were treated with 40,000 IU kg(-1) of Vitamin D(3) whilst dietary uptake of Ca and Pi was restricted to prevent or to diminish the increase of the Ca x Pi product. Distribution, number and severity of calcification centres were considerably less in these horses than in the control animals of a previous experiment which had received the same amount of Vitamin D(3) but where Ca and Pi intake was not restricted. It appears from these findings that in horses, the increase of the Ca x Pi product in blood plasma during a Vitamin D intoxication contributes to the soft tissue calcifications. It is further concluded that in the event of a Vitamin D intoxication, it is recommended to restrict the Ca and Pi uptake immediately. The authors believe that this may help to prevent or at least diminish soft tissue calcifications which are often fatal to the horse due to nephrocalcinosis.