We recently developed a new in vitro assay system for the evaluation of bone metabolism using goldfish scale. This system can simultaneously detect the activities of both scale osteoblasts (formation cells) and osteoclasts (resorption cells) with alkaline phosphatase and tartrate-resistant acid phosphatase as markers. Using this scale in vitro assay system, in the present study, we analyzed the effect of ultrasound (US) stimulation on osteoblastic and osteoclastic activities. The osteoblastic activity significantly increased by pulsed low-density US (1MHz, 60mW/cm^2 I_<SATA>, 50% duty factor at 0.5Hz, 180 pulses) in 18 hrs of incubation at 15℃ after US treatment but not in shorter incubation periods, while the osteoclastic activity did not change in the same incubation period. To examine the mechanism of US in osteoblasts, the estrogen receptor (ER) and insulin-like growth factor-I (IGF-I) mRNA expressions in the cultured scales were analyzed by RT-PCR. ER mRNA expression was found to be higher in the US-treated scales than in the control scales in 18 hrs of incubation at 15℃ after treatment, although ER mRNA expression did not change in 3 hrs of incubation. On the other hand, IGF-I mRNA expression increased in 3 hrs of incubation at 15℃ after US treatment. Therefore, IGF-I mRNA expression was more rapid to respond to US than ER mRNA expression, and IGF-I may have an important function in the activation of osteoblasts by US treatment. We found that both osteoblastic and osteoclastic activities increased in the remaining ontogenic scales of the left side at 3 days after the removal of all scales in the right side. Using these scales, we examined the effects of US stimulation on osteoblasts and osteoclasts under the same conditions described above. In 18 hrs of incubation at 15℃ after US treatment, the osteoblastic activity increased by US treatment in the ontogenic scale as well as the normal scale. Furthermore, we indicated that the osteoclastic activity in the ontogenic scale decreased by US treatment. The conditions of osteoblasts and osteoclasts in the ontogenic scale are similar to those in the human osteoporosis, indicating that US stimulation may contribute to the cure of osteoporosis.
