1. Uraemic rats maintained on either a high or a low phosphate diet for 12 weeks were allocated to one of the following oral vitamin D treatment groups and received: (a) 1,25-dihydroxycholecalciferol [1,25-(OH)2D3], (b) 24,25-dihydroxycholecalciferol [24,25-(OH)2D3], (c) both 1,25-(OH)2D3 and 24,25-(OH)2D3, or (d) no vitamin D supplements.

2. Mean serum creatinine concentrations were elevated to a similar extent in all groups. Mean serum concentrations of calcium, phosphate and alkaline phosphatase were not significantly different from normal in any of the groups.

3. In the group receiving the high phosphate diet and no vitamin D supplements, calcified bone area measured by quantitative computerized histomorphometry was significantly lower than in (i) the group receiving the low phosphate diet and no vitamin D supplements (0.01 > P > 0.001), and in (ii) the groups receiving high phosphate diet and either 1,25-(OH)2D3 (0.01 > P > 0.001) or 24,25-(OH)2D3 (0.01 > P > 0.001).

4. We conclude that (i) uraemic rats maintained on a high phosphate diet for 12 weeks develop skeletal demineralization, (ii) this process does not occur in rats on a low phosphate diet, and (iii) a decrease in calcified bone area may be prevented by treatment with either 1,25-(OH)2D3 or 24,25-(OH)2D3.

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