Delayed Menarche Causes A Transient Reduction In Cancellous Bone Area In Female Rats

Abstract

Optimizing peak bone mass during late adolescence may help reduce the effects of osteoporosis. Multiple factors affect the structural development of the skeleton; including estrogen levels during growth which are an important factor in the pathogenesis of bone fragility. The delay of menarche decreases estrogen levels during adolescence and subsequent peak bone mass. Therefore, low estrogen levels during bone development decrease peak bone mass accrual in young adults and may lead to an increased risk of fracture later in life. PURPOSE The investigative hypothesis predicts that administration of a GnRH antagonist initiated prior to the onset of the first estrus cycle will reduce estrogen levels and impede the development of cancellous bone area in the proximal tibial metaphysis of female rats (6 weeks) and the peak bone area at maturity (6 months). METHODS Twenty-five day old female Sprague Dawley rats were randomly assigned into short-term control (C-ST) (n = 12), long-term control (C-LT) (n = 12), short-term GnRH antagonist (GnRH-a ST) (n = 12) and long-term GnRH antagonist groups (GnRH-a LT) (n = 12). Injections (0.2 ml) were given intraperitoneally for 18 days of either saline or GnRH antagonist (100 ug/day) and animals were monitored daily for vaginal opening. Both short-term and long-term groups received the GnRH antagonist for 18 days. However, the short-term groups were sacrificed after the last injection (day 42) and the long-term groups were sacrificed at 6 months of age. Uterine weights were measured. Serum Estradiol levels were assayed. Bone area (%) was measured at the proximal tibial metaphysis. Differences were detected using Students t-test. The Institutional Animal Care and Use Committee at Brooklyn College approved the study. RESULTS Day of vaginal opening was delayed 22% in the short term GnRH-a group and 30% in the long-term group. The delay in pubertal development was also confirmed by decreased uterine weights (74 %) in the GnRH-a ST group, uterine weights were normal in the GnRH-a LT group. GnRH-a ST estrogen levels were suppressed after the injection period (38.1 (13.6) pg/mL v 26.8 (16.6) pg/mL). Bone area was decreased 16.5% (p = 0.05) in the short-term groups but was not significantly different at 6 months of age. CONCLUSION GnRH antagonist given to animals prior to the first estrus cycle significantly decreased the bone area in the proximal tibial metaphysis at 6 weeks but not at 6 months of age. Delayed vaginal openings and decreased uterine weights in the GnRH-a ST group resulted in a reduction in the level of available estrogen. Supported by NIH R15 AG19654-01A1and PSC-CUNY 64293-00 33