BONE-RESORPTION AND SERUM LEVELS OF VITAMIN-D METABOLITES IN THE HYPOPHOSPHATEMIC RAT

The supplementation of a low phosphate diet with vitamin D has been sh own to result in an increase in bone resorption in the hypophosphataem ic rat. The aim of the present study was to determine if administratio n of vitamin D to rats fed a vitamin D- and phosphate-depleted diet wo uld result in an increase in the circulatory levels of the active vita min D metabolite 1,25(OH)(2)D-3 and an associated increase in bone res orption. Three groups of weanling Sprague-Dawley rats were used. The f irst group consisted of control animals on a normal laboratory stock d iet and the second and third groups were experimental animals receivin g a vitamin D- and phosphate-deficient diet with the third group recei ving vitamin D supplementation. All animals were housed in the dark. A fter 30 days on the diet the experimental animals received 0.1 mmol Na H2PO4 by intraperitoneal injection. Blood was sampled at zero, 3, 6, 1 8 and 48 h post-injection and analysed for the vitamin D metabolites 2 5(OH)D-3 and 1,25(OH)(2)D-3, calcium and inorganic phosphate (Pi). The serum analyses revealed that the level of 25(OH)D-3 in the hypophosph ataemic animals was significantly lower than that of the control anima ls. However the 1,25(OH)(3)D-3 level was initially significantly highe r, then dropped to the control level at 18 h post-intraperitoneal inje ction of phosphate. Further, the serum levels of 25(OH)D-3 and 1,25(OH )(2)D-3, calcium and Pi in the hypophosphataemic animals supplemented with vitamin D were significantly higher than those of the vitamin D-d eficient animals. Also the vitamin D-supplemented animals exhibited si gnificantly greater levels of bone resorption, These results therefore , are consistent with a role of 1,25(OH)(2)D-3 in bone resorption in h ypophosphataemic rats.