1,25 dihydroxycholecalciferol treatment of uraemic rats after high magnesium feeding

The hypercalcaemic response to 1,25 dihydroxycholecalciferol (1,25(OH)2D3) treatment was studied in mild uraemic rats (5/6-nephrectomy) under conditions of varying magnesium intake of 0.05% (control) and 0.75% (experimental). After 3 weeks' exposure to the high magnesium diet a loss of the hypercalcaemic effect of 1,25(OH)2D3 when given as 40pmoles i.p. during 5 days was observed. By use of standard clearance experiments a similarity of the renal magnesium and calcium handling could be demonstrated. At high serum magnesium levels, the fraction of filtered magnesium excreted in the urine rose, concomitant with an increased fractional excretion of calcium (r=0.81). The bone magnesium content was correlated with serum levels and therefore increased after high dietary magnesium. Treatment with 1,25(OH)2D3 resulted in an increase of the bone calcium content (p less than 0.05) after low magnesium diet, but not after high magnesium feeding. It is concluded that high urinary calcium excretion after magnesium loading contributes to the impaired calcaemic response of 1,25(OH)2D3 in azotaemic rats, while calcium uptake of bone is impaired.