ABSTRACT:Introduction: Hypogonadism is considered to be one of the major risk factors for osteoporosis in men. Here, we sequentially studied the effects of androgen deficiency on cortical bone in aged orchiectomy (ORX) rats. Materials and Methods: One hundred seventy 13-mo-old male Fischer-344 rats were either ORX or shamoperated. After in vivo fluorochrome labeling, groups of 8-15 SHAM and ORX rats each were killed at 2 wk and 1, 2, 3, 4, 6, and 9 mo after surgery. To examine the effects of testosterone replacement therapy, 9-mo-old ORX rats were supplemented with testosterone undecanoate at a weekly dose of 6 mg/kg for 4 mo. Cortical bone changes in the tibial shaft were monitored by pQCT analysis and by bone histomorphometry. Results: SHAM rats did not show age-related bone loss at the tibial diaphysis. pQCT analysis and bone histomorphometry showed cortical bone osteopenia in ORX rats, beginning from 2 mo after surgery until the end of the study. Androgen deficiency induced a sustained decrease in periosteal bone formation during the first 4 mo after ORX. However, although periosteal expansion of the tibial shaft tended to be slower in ORX rats compared with SHAM controls, the reduction in total cross-sectional area in ORX animals reached statistical significance only at 4 mo after surgery. The major mechanism for cortical bone loss in aged ORX rats was a progressive expansion of the marrow cavity, which was associated with an initial increase in endocortical eroded perimeter at 1 and 2 mo after surgery, followed by a sustained increase in endocortical bone formation until the end of the study. All these changes were prevented in aged ORX rats receiving testosterone supplementation in an insulin-like growth factor system-independent fashion. Conclusions: We conclude that androgen deficiency-induced cortical bone loss in aged, nongrowing rats is mainly caused by augmented endocortical bone remodeling.

The immunosuppressive drug cyclosporin A (CsA) is thought to be involved in the pathogenesis of posttransplantation osteoporosis. To evaluate further the skeletal effects of CsA, we treated aged male and female sham-operated and gonadectomized rats with low doses of CsA for 4 months. Here, we show that CsA is antiresorptive and bone-sparing in aged female rats but increases bone resorption and reduces bone mass in aged male rats. However, even in male rats, CsA treatment, at clinically relevant doses, increased bone resorption only transiently and did not result in pronounced long-term cancellous bone loss. The gender-specific skeletal effects of CsA were not modulated by sex hormones or gonadectomy. CsA did not influence sex steroid metabolism in male or female rats. However, endogenous estradiol in sham-operated female rats (and especially, exogenous administration of 17beta-estradiol in ovariectomized rats) markedly diminished blood levels of CsA, probably by increasing hepatic CsA metabolism. Although the mechanism for the gender-specific skeletal effects of CsA is still obscure, our findings may have important implications for clinical therapy with CsA.

We examined the long-term sensitivity of uterus and bone to low-dose 17␤-estradiol in a 4-month experiment in OVX rats and found that a dose of estradiol that fully protected against uterine atrophy did not protect against bone loss. Our results suggest higher estrogen sensitivity of the uterus compared with bone. Introduction: Estrogen is essential for the function of reproductive tissues and for the normal acquisition and maintenance of bone mass in females. This study was designed to examine the long-term sensitivity of the uterus and bone to low-dose estrogen. Materials and Methods:In preliminary experiments, we determined the lowest subcutaneous dose of 17␤-estradiol able to fully protect against uterine atrophy in ovariectomized (OVX) rats. This dose was found to be 1.5 g/kg, given five times per week. Subsequently, groups of sham-operated (SHAM) or OVX 6-month-old rats (n ϭ 8 each) were subcutaneously injected with vehicle or 1.5 g/kg 17␤-estradiol five times per week. All animals were killed 4 months after surgery. Serum osteocalcin and urinary deoxypyridinoline were measured as biochemical markers of bone turnover. Bones were analyzed by bone histomorphometry and pQCT. Results and Conclusions: Our study clearly showed that a dose of estradiol that restores physiological estradiol serum levels, fully maintains uterine weight in OVX rats at the SHAM control level, and suppresses serum follicle-stimulating hormone (FSH) by 67% relative to OVX vehicle controls does not provide significant protection against OVX-induced bone loss at different cancellous and cortical bone sites. We conclude that the long-term sensitivity of the uterus and the hypothalamus/pituitary axis to 17␤-estradiol is higher than that of bone in rats.