Effect of oral contraceptive treatment on bone mass acquisition in skeletally immature young female rats

The objective of the present study was to investigate the effect of oral contraceptive (OC) treatment on bone mass accrual in skeletally immature young female rats. Animals in the baseline group were killed at the beginning of the experiment and were subjected to bone density assessment by peripheral quantitative computerized tomography (pQCT). The control group was fed a base diet free of phytoestrogens, while animals in the contraceptive group received the same base diet mixed with 2.67 μg desogestrel/100 g body weight and 0.0533 μg ethinyl estradiol/100 g body weight. The duration of the treatment period was 16 weeks. Densitometric measurements by dual energy x-ray absorptiometry and serum bone markers assessment were carried out at baseline, at 8 weeks and at 16 weeks, while pQCT densitometry took place after sacrifice. All bone mineral density and bone mineral content indices measured by dual energy x-ray absorptiometry increased significantly throughout the study period in both the OC and control group. Concerning pQCT measurements, animals in both the OC and the control group had significantly higher cortical density compared with baseline (midtibia: p=.0003 and .0003, respectively). Total area and periosteal circumference were significantly higher in OC group, both in proximal (p=.003 and .003, respectively) and midtibia (p=.048 and .042, respectively) compared with baseline. Osteoprotegerin serum levels increased in both groups, and at the end of the experiment, circulating osteoprotegerin was significantly higher in the OC group compared with controls (p=.032). At the end of the experiment, carboxyl-terminal telopeptides of collagen type I levels were significantly lower in the OC-treated animals compared with controls (p=.046). Our results suggest that OC administration to skeletally immature female rats allows normal bone accrual and may even improve bone geometry. This effect may be mediated through enhanced inhibition of bone resorption.