The supplementation of a low
phosphate diet with
vitamin D has been shown to result in an increase in
bone resorption in the hypophosphataemic rat. The aim of the present study was to determine if administration of
vitamin D to rats fed a
vitamin D- and
phosphate-depleted diet would result in an increase in the circulatory levels of the active
vitamin D metabolite
1,25(OH)2D3 and an associated increase in
bone resorption. Three groups of weanling Sprague-Dawley rats were used. The first group consisted of control animals on a normal laboratory stock diet and the second and third groups were experimental animals receiving a
vitamin D- and
phosphate-deficient diet with the third group receiving
vitamin D supplementation. All animals were housed in the dark. After 30 days on the diet the experimental animals received 0.1 mmol NaH2PO4 by
intraperitoneal injection. Blood was sampled at zero, 3, 6, 18 and 48 h post-injection and analysed for the
vitamin D metabolites 25(
OH)D3 and
1,25(OH)2D3,
calcium and
inorganic phosphate (Pi). The serum analyses revealed that the level of 25(
OH)D3 in the hypophosphataemic animals was significantly lower than that of the control animals. However, the 1,25(
OH)3D3 level was initially significantly higher, then dropped to the control level at 18 h post-
intraperitoneal injection of
phosphate. Further, the serum levels of 25(
OH)D3 and
1,25(OH)2D3,
calcium and Pi in the hypophosphataemic animals supplemented with
vitamin D were significantly higher than those of the
vitamin D-deficient animals. Also the
vitamin D-supplemented animals exhibited significantly greater levels of
bone resorption. These results therefore, are consistent with a role of
1,25(OH)2D3 in
bone resorption in hypophosphataemic rats.