Effect of adrenal steroids on bone resorption in rats

Rats labeled with strontium-85 (85Sr) were rejected with adrenocortical steroids for 2 wk. The urinary-to-tibial (U/T) 85Sr ratio was used as an index of bone resorption. The glucocorticoids caused an inhibition of skeletal resorption, as judged by the 50% reduction in the U/T ratio, and decreased excretion of hydroxyproline. Thyroidal calcitonin levels were slightly elevated in glucocorticoid-treated animals, suggestive of a possible retardation of calcitonin release. The U/T ratios of thyroparathyroidectomized (TPTX) rats injected with corticosteroids were 50% of control values. The results indicate that glucocorticoids inhibit bone resorption independent of the action of calcitonin. Cortisol treatment increased the tibial density as measured by a radiographic technique. However, bone density was decreased and the U/T ratio increased in steroid-treated rats fed a low-calcium diet. In TPTX cortisol-treated rats, parathyroid extract (PTE) increased the U/T ratio and serum calcium but not to the degree observed in TPTX PTE-injected control animals. These experiments indicate that in rats glucocorticoids inhibit the rate of bone resorption but this effect can be overcome in part by PTE.