Studies on the mechanism of diet-induced nephrocalcinosis: calcium and phosphorus metabolism in the female rat.

The disposition of calcium and phosphorus in female Sprague-Dawley rats fed either a low- or a high-calcium semi-synthetic diet for up to 9 wk from weaning has been investigated. The rats fed the low-calcium diet (calcium to phosphorus ratio approximately 1:1) developed cortico-medullary nephrocalcinosis within 6 wk, whereas those fed the high-calcium diet (calcium to phosphorus ratio greater than 1.5:1) did not develop any lesion during this time. Intake and excretion of calcium was greater at all times in the animals fed the high-calcium diet than in those on the low-calcium diet. However, the true absorption and retention of calcium was not significantly different between the two diet groups. Phosphorus intake and total excretion were similar for both groups of animals, although urinary excretion accounted for less than 1% of the total in the animals fed the high-calcium diet but more than 50% in those fed the low-calcium diet. Bioavailability of phosphorus decreased with age in both groups. Recovery of injected 45Ca in urine and faeces increased during the experimental period from approximately 3 to 11% of the dose in rats on the low-calcium diet and from approximately 9 to 13% in the high-calcium group. Recovery of both injected and orally administered 32P was substantially greater in the low-calcium group than in the high-calcium group at wk 3 and 5, but at wk 9 was only greater after intramuscular administration. At the earlier times the urine was the major route of 32P excretion. It appears that no gross disturbances of calcium or phosphorus metabolism occurred in female rats maintained on a diet that resulted in the rapid development of kidney calcification.