Increased prostaglandin e₂ has a positive correlation with plasma calcium during goldfish reproduction

We recently demonstrated that prostaglandin E₂ PG¿ increases osteoclastic activity and induces bone resorption in both in vitro and in vivo experiments using goldfish. In the fish reproductive period, the plasma calcium (Ca) level in female teleosts increases remarkably to make vitellogenin, which is a major component of egg protein and a Ca-binding protein. In this period, however, there is no reported relationship between PGE₂ and Ca metabolism in fish. To clarify the Ca metabolism in fish reproduction, we examined plasma PGE₂ and Ca levels and measured tartrate-resistant acid phosphatase (TRAP) activities as an indicator of osteoclastic activity in goldfish. Plasma PGE₂ levels in the reproductive stage significantly increased as compared with those in non-reproductive stages. Also, both plasma Ca and TRAP increased in the reproductive stage. Significant positive correlations were recognized between plasma Ca and the gonad somatic index (r=0.81, p<0.001), plasma Ca and plasma PGE₂ levels (r=0.635, p<0.05), and plasma Ca and plasma TRAP activities (r=0.584, p<0.05) from the analysis using samples of both reproductive and nonreproductive stages. Taking these data into consideration, we suggested that PGE, acts on osteoclasts and increases plasma Ca as a result of osteoclastic bone resorption, and we concluded that PGE, is an important hormone in Ca metabolism during fish reproduction.