Measurement of bone resorption by strontium excretion in prelabelled rats

The body handles strontium (Sr) in a similar way to calcium (Ca) in that Sr is absorbed by the gut, concentrated in bone and excreted in urine and feces. In this study, rats were labelled with Sr during growth and later subjected to various treatments affecting bone resorption and Sr excretion was measured during and after treatment. Six weeks old Wistar rats were repeatedly s.c. injected with SrCl2. After a period of 2 weeks after the last Sr injection the rats were subjected to various treatments. Sr clearance was then measured weekly for 2 weeks. In the first experiment, the Sr labelled rats were sham-operated (sham) or ovariectomized (ovx) and urine collected afterwards. Sham rats were either treated with 4 daily s.c. clodronat injections at the beginning of the urine sampling, fed a low Ca diet (0.08% Ca) during the second sampling week or injected with saline. Urinary Sr excretion was decreased in the clodronate group during the first sampling week and increased in the Ca depleted group during feeding the low Ca diet. Sr excretion by ovx rats was similar to the sham control. In the second experiment, the effect of high-dose treatment with 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) or clodronate on bone resorption induced by Ca depletion was assessed by Sr output in urine and feces. Sr labelled rats were fed a low Ca diet and daily injected with 24,25(OH)2D3 or clodronate for 14 consecutive days. Clodronate significantly decreased Sr output during both sampling weeks. Treatment with 24,25(OH)2D3 resulted in an increased Sr output indicating an increase in bone resorption.