The effect of in vivo mechanical loading on estrogen receptor α expression in rat ulnar osteocytes

The presence of estrogen receptor alpha (ERalpha) in osteocytes was identified immunocytochemically in transverse sections from 560 to 860 mum distal to the midshaft of normal neonatal and adult male and female rat ulnas (n = 3 of each) and from adult male rat ulnas that had been exposed to 10 days of in vivo daily 10-minute periods of cyclic loading producing peak strains of either -3000 (n = 3) or -4000 microstrain (n = 5). Each animal ambulated normally between loading periods, and its contralateral ulna was used as a control. In animals in which limbs were subject to normal locomotor loading alone, 14 +/- 1.2% SEM of all osteocytes in each bone section were ERa positive. There was no influence of either gender (p = 0.725) or age (p = 0.577) and no interaction between them (p = 0.658). In bones in which normal locomotion was supplemented by short periods of artificial loading, fewer osteocytes expressed ERalpha (7.5 +/- 0.91% SEM) than in contralateral control limbs, which received locomotor loading alone (14 +/- 1.68% SEM; p = 0.01; median difference, 6.43; 95% CI, 2.60, 10.25). The distribution of osteocytes expressing ERa was uniform across all sections and thus did not reflect local peak strain magnitude. This suggests that osteocytes respond to strain as a population, rather than as individual strain-responsive cells. These data are consistent with the hypothesis that ERa is involved in bone cells' responses to mechanical strain. High strains appear to decrease ERa expression. In osteoporotic bone, the high strains assumed to accompany postmenopausal bone loss may reduce ERa levels and therefore impair the capacity for appropriate adaptive remodeling.