HYPOPHOSPHATAEMIA AND VITAMIN D METABOLISM IN SHEEP

¹ Department of Animal Physiology and Nutrition, University of Leeds, Leeds LS2 9JT

The effect of dietary induced hypophosphataemia on vitamin D metabolism was studied in four conscious sheep. No change in the plasma concentrations of 25-hydroxyvitamin D₃ and 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) were found during hypophosphataemia when compared with normophosphataemia. In addition both the metabolic clearance rate and the production rate of plasma 1,25(OH)₂D₃ were unaltered during hypophosphataemia. This suggests that changes in circulating levels of 1,25(OH)₂D₃ were not necessary for the increased efficiency of intestinal phosphorus absorption seen in phosphorus-depleted hypophosphataemic sheep, and may explain why there is no increase in efficiency of intestinal calcium absorption in phosphorus depleted sheep. Increases in phosphataemia induced acutely by intravenous phosphate infusion for 3 h also had no effect on plasma 1,25(OH)₂D₃ concentration. It is concluded that in the sheep, unlike non-ruminant animals, chronic hypophosphataemia does not increase plasma 1,25(OH)₂D₃ concentrations and plays no physiological role in regulating its production or metabolism.

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