In order to examine whether 8-isopentenylnaringenin (1), which has been proven to possess estrogen agonist activity in in vitro tests, also produces in vivo estrogenic properties, the effects of 1 on uterus and on bone metabolism were determined in ovariectomized rats. Rats were ovariectomized and treated with 1 at 30 mg/kg/day subcutaneously for two weeks or 17 beta-estradiol at 0.01 mg/kg/day subcutaneously for two weeks. Ovariectomy resulted in an increase in urinary excretion of bone resorption markers (hydroxyproline, pyridinoline and deoxypyridinoline) and a decrease in bone mineral density of the proximal tibia as well as reduced uterine weight. Treatment with 1 or 17 beta-estradiol completely suppressed these ovariectomy-induced bone and uterine changes in a qualitatively similar manner. These results demonstrate that 1 acts as an estrogen agonist in the uterus as well as in bone in vivo.

Recombinant human calcitonin (rh-CT) has been developed as an agent for patients with excessive bone resorption to replace calcitonins from animal species, which are associated with tolerance problems. In this study, inhibitory effects of rh-CT against bone resorption were examined in thyroparathyroidectomized (TPTX) rats, the animal model of accelerated bone resorption induced by administering a synthetic retinoid (arotinoid). The arotinoid-treated TPTX rats exhibited signs of stimulated bone resorption, such as hypercalcemia, reduced bone mineral density, and inferior bone strength. Significant improvements were seen in all of these changes after a daily treatment with rh-CT (30, 300 U/kg s.c.) for 1 week. A histomorphometrical analysis showed that the treatment with rh-CT markedly suppressed the reduction of trabecular bone volume and that of cortical thickness in the femur of arotinoid-treated TPTX rats. These results suggest that rh-CT may prevent osteopenia caused by accelerated bone resorption.