Effects of inorganic mmercury on osteoclasts and osteoblasts of the goldfish scales in vitro

Abstract

The fish scales are the major source of internal calcium requirement due to having a higher internal calcium reservoir than the body skeleton during the periods of drastic calcium demand, such as sexual maturation. Therefore, we developed original in vitro assay system using goldfish scales that contain osteoclasts and osteoblasts, and examined the direct effect of inorganic mercury (HgCl2) on osteoclasts and osteoblasts. In this assay system, we measured the activities of tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) as respective indicators of each activity in osteoclasts and osteoblasts. TRAP activity in the scales significantly decreased by the treatment of HgCl2 (10-5 to 10-3M) during 6 hrs of incubation. In addition, mRNA expressions of osteoclastic markers: TRAP and cathepsin K significantly decreased compared with control. In our knowledge, this is the first report of a direct effect of inorganic mercury on osteoclasts. On the other hand, ALP activity decreased after exposures of HgCl2 at a concentration of 10-6, 10-5 or 10-4M for 36 and 64 hrs, although its activity did not change after 6 and 18 hrs. The mRNA expression of metallothionein (MT) which is a metal-binding-protein that protects the organism from heavy metal, significantly increased by HgCl2 (10-4M) although insulin-like growth factor-I (osteoblastic marker) was less than those of control scales by treatment with HgCl2 (10-4M). These results suggests that osteoblasts may synthesize MT and protect from mercury until 18 hrs incubation. Thus, the scale in vitro assay system would be a useful means for analysis of heavy metal on bone metabolism.