In vivo rat assay: bone remodeling and steroid effects on juvenile bone by pQCT quantification in 7 days. Am J Physiol Endocrinol Metab 284: Dawley weanling rats were scanned for bone mineral density (BMD) values after 7 days of treatment to determine whether resorption/growth at the proximal tibia can be quantified by peripheral quantitative computed tomography scanning techniques. Because the weanling rat is in a rapid growth stage, all groups showed significant increases in change from

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... ne values of BMD. Bisphosphonate treatment produced significant dose-related changes in BMD with average increases of 195 and 241% (10 and 20 g/kg) vs. 86% in control rats. We further characterized this model to determine effects of steroids on growing bone. Graded doses of glucocorticoid (3.5, 7.0, 10.5, 14.0, 28.0, and 42.0 mg ⅐ kg Ϫ1 ⅐ wk Ϫ1 ) caused no significant differences in trabecular BMD in 7 days between control and treated rats. Significant decreases in growth (weights) and increases in cortical bone area were observed, indicating that this model may be useful in comparing effects of nonsteroid, anti-inflammatory alternatives on juvenile bone. Although the relevance of this model to adult disease remains to be elucidated, it also provides a tool for mechanistic evaluation of therapeutic modalities or efficacy assessment for dose selection for longerterm models. osteoporosis; bone mineral density; glucocorticoid; computed tomography; animal model ANIMAL MODELS that simulate osteoporosis, such as ovariectomized rodents (postmenopausal) and glucocorticoid-induced and senescence-related osteopenia, are typically conducted in aged animals over long periods of time. In the case of senescence-induced osteoporosis, decrements in bone density may not be observed for 3 to 6 mo. The cost of boarding and large amounts of drugs that are needed to sustain these chronic models are thus often prohibitive. Moreover, this protracted study time is rate limiting to evaluation and development of novel therapeutic agents. Normal bone growth involves both bone resorption and bone formation in a well-controlled balance. It is proposed that administration of an anti-resorptive agent to the 21-day-old growing rat may disrupt the normal equilibrium at this time of fast growth, leading to a significantly higher bone mass in treated rats compared with age-matched untreated rats. In early studies, Schenk and colleagues [Muhlbauer et al. (16) and Schenk et al. (21) ] utilized this 21-day-old model to screen bisphosphonate candidates. These studies showed that 7-day treatment with bisphosphonates yielded bones that demonstrated large increases in metaphysial density as measured by histology. The purpose of our study was to determine whether increases or decreases in apparent bone density can be quantified by peripheral quantitative computed tomography (pQCT) at an early time period. The 21-day-old rat represents a juvenile model of bone growth. The juvenile bone is particularly sensitive to corticosteroids. Corticosteroids are prescribed for severe asthma, juvenile rheumatoid arthritis, or dermatologic disease and are known to reduce bone turnover, stunt bone growth, and decrease bone mineral density (BMD) (5, 6, 9, 15, 23 ). An average daily dose of 5 mg⅐kg Ϫ1 ⅐day Ϫ1 will cause loss of BMD in the prepubertal child, and monitoring of bone density is recommended even for adults taking 7.5 mg⅐kg Ϫ1 ⅐day Ϫ1 for Ն6 mo (17). In their retrospective review of 212 patients, Hougardy et al. (10) reported that the median daily dose was 10 mg prednisolone equivalent and the median duration of oral corticosteroid treatment was 50 wk, which is well above the dosage capable of eliciting bone loss. Moreover, at an average dose of 0.67 mg⅐m Ϫ2 ⅐day Ϫ1 of inhaled steroids, there is a reduction in the acquisition of bone mineral in prepubertal children that compromises their peak bone mass and predisposes them to osteoporosis and a higher fracture risk as an adult (2). Therefore, we also wanted to determine whether we could quantify by pQCT with this model negative effects of corticosteroids on juvenile bone. Although pQCT scanning cannot replace the information obtained from histostaining, immunostaining, or histomorphometric analysis, it allows for the rapid and noninvasive evaluation of potential therapeutic anti-resorptive compounds in a shorter amount of time.