Effects of 1,25-dihydroxyvitamin D₃ on calcium and phosphorus homeostasis in sheep fed diets either adequate or restricted in calcium content☆

Abstract 

It was the aim of the present study to collect basic data on calcium (Ca) and phosphorus (P) homoeostasis in sheep. Two series of experiments were carried out to investigate the effects of 1,25-dihydroxyvitammin D₃ (calcitriol) in supraphysiological dosage in combination with varying alimentary Ca supply. In the first series, blood samples were collected over 72h to determine the concentrations of total Ca (Ca), ionized Ca (Ca²⁺), inorganic phosphate (P_i), and the bone resorption marker CrossLaps (CL). In the second series, measurements were carried out over 12h. In addition, urine samples were collected to calculate the fractional excretions (FE) of Ca and P_i. Changes in plasma macromineral concentrations (P<0.01) as well as in CL (P<0.001) and endogenous calcitriol (P<0.05) were observed in the alimentary Ca-restricted animals, indicating that the reduction of daily Ca intake challenged the animals’ macromineral homeostatic mechanisms. However, the Ca-restricted diet had an effect on neither FE of Ca nor on FE of P_i. The treatment resulted in peak serum calcitriol concentrations between 1,900 and 2,500pg/mL, and supraphysiological concentrations were maintained for the next 48h. Irrespective of dietary Ca, calcitriol had hypercalcemic and hyperphosphatemic effects. An increase in CL was revealed only in the Ca-restricted, calcitriol-treated sheep (P<0.01), reflecting a remarkable enhancement of Ca mobilization from the bone by calcitriol exclusively in this group. From these data, it can be concluded that the sheep can be a suitable animal model for studying catabolic effects of Ca deficiency and calcitriol on bone metabolism.

Keywords: Sheep, Bone, Renal Excretion, Calcium, Phosphorus, 1,25-dihydroxyvitamin D₃