Changes in mRNA levels of alkaline phosphatase and tartrate-resistant acid phosphatase in femur of ovariectomized rats: effects of estrogen and unloading.

Our previous studies demonstrated that estrogen (E2) prevents the development of disuse atrophy of the femur in tail-suspended rats. To elucidate the mechanisms of this E2 action, we investigated the effects of E2 on the expression of alkaline phosphatase (ALP, a marker for bone formation) and tartrate-resistant acid phosphatase (TRAP, a marker for bone resorption) in the femur of ovariectomized and tail-suspended rats. One group of ovariectomized rats received estradiol dipropionate (OVX-E2), and the other the vehicle alone (OVX). Each group was subjected to tail-suspension. After 1, 3, 5 or 7 days of suspension, ALP and TRAP mRNA levels were determined by Northern blot analysis. The ALP mRNA level was not altered by suspension in the OVX group, but it gradually increased in the OVX-E2 group, the highest level being observed at day 5 of suspension. In contrast, TRAP mRNA significantly increased at days 5 and 7 in the OVX group, while it is decreased significantly from day 3 to 7 in the OVX-E2 group. These results indicate that E2 prevents disuse atrophy of the femur in an ovariectomized and tail-suspended rat model by stimulating bone formation and by inhibiting bone resorption.