Peculiarities of vitamin D and of the calcium and phosphate homeostatic system in horses.

The research investigates the regulatory factors of calcium and inorganic phosphate homeostatic system in horses. The study shows that unlike in other species, vitamin D does not seem to play a significant role in regulating these systems in horses.

Calcium and Inorganic Phosphate Concentration

• The research measured calcium, inorganic phosphate (P(i)), vitamin D metabolites, parathyroid hormone (PTH), and alkaline phosphatase (AP) activity in horse plasma. The calcium concentration did not show significant differences across different horse breeds or intake.
• The concentration of inorganic phosphate varied from 0.58 to 1.99 mmol.L-1 and found correlation with age and the phosphate content of the feed – it decreased with age and increased with higher phosphate in the diet.

Activity of Alkaline Phosphatase

• The activities of AP in horse plasma were also negatively correlated with age – it decreases as a horse ages. The range of AP activities tended to be higher in horses than in other domestic animals.

Vitamin D Metabolites and Binding Protein

• Compared to other species, the concentration level of vitamin D metabolites such as calcidiol and 24,25(OH)2D were much lower in horses.
• The concentration and the binding properties of the Vitamin D Binding Protein (DBP) which binds to calcidiol were comparable to those found in other mammals. However, the plasma concentration of calcitriol was much lower than what is found in other mammals.

Parathyroid Hormone and Vitamin D Activity in Renal Cortex

• In the kidneys, only the 25-hydroxycholecalciferol-24-hydroxylase activity was detected. The mean plasma concentration of Parathyroid Hormone (PTH), which plays a key role in calcium homeostasis by regulating its levels in the blood, was 218 +/- 181 ng.L-1.
• The research also evaluated the ability of the renal cortex in horses to convert calcidiol into 24,25(OH)2D3 and 1,25(OH)2D3, forms of vitamin D.

Significance of Vitamin D in Horses

• Based on the tests and observations, the study concluded that vitamin D does not appear to play a key role in regulating the calcium and inorganic phosphate homeostasis in horses. This is in contrast to other species where vitamin D is crucial for calcium and phosphate regulation.