Vitamin D and calcium metabolism in horses in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Institute of Veterinary, Animal and Biomedical Sciences (IVABS), Massey University, Manawatū, New Zealand
The physiology of vitamin D in horses has not been studied in great depth. Few studies on vitamin D metabolites (25OHD2, 25OHD3, and 1,25(OH)2D) and their relationship to other serum analytes exist. In addition, some studies suggest that equine vitamin D physiology may be different from other species. This thesis aimed to investigate aspects of vitamin D metabolism in horses.
The effect of blanketing on vitamin D synthesis and its relationship with other analytes involved in calcium homeostasis, including vitamin D metabolites (25-hydroxyvitamin D2 (25OHD2), and 25-hydroxyvitamin D3 (25OHD3), 1,25-dihydroxyvitamin D (1,25(OH)2D)), ionised calcium (iCa), total calcium (tCa), phosphorus (P), total magnesium (tMg) and parathyroid hormone (PTH) were studied in horses. Regardless of blanketing, 25OHD3 was undetectable in equine serum and 25OHD2 was the main form of 25OHD in circulation. A strong seasonal variation in serum 25OHD2, 1,25(OH)2D, iCa, tCa, P, tMg and PTH concentrations was detected, although no differences were seen between horses that were blanketed and those that were not. The circadian rhythms of serum vitamin D metabolites, iCa, tCa, P, tMg, and PTH concentrations in horses was studied over 48 h on the summer and winter solstices. A significant difference was seen between the serum concentrations of studied analytes between solstices, with no rhythm detected in winter. An in vivo study suggested that equine skin may be unable to convert 7-dehydrocholesterol (7-DHC) to vitamin D3 after exposure to ultraviolet B (UVB) light. Quantitative PCR was performed on equine kidney to study the expression of vitamin D responsive and calcium transporting genes, which were then compared to genes in sheep and dogs.
The results suggested that TRPV6, calD9k /calD28k, and PMCA were the main calcium transporting pathways in the kidney of these species, and there was a high correlation between VDR and other studied genes. It was concluded that 25OHD2 is the main metabolic precursor for 1,25(OH)2D and should be considered the best available index of vitamin D status in unsupplemented horses, and that horses most likely rely on diet as their primary source of vitamin D.