[Physiological significance of calcitonin in fish]

The physiological role of calcitonin (CT) in fish has been partially elucidated. In teleosts, CT did not affect the calcium (Ca) and phosphate contents in bone and scales. In an experiment using Ca(45), CT did not show any change in the Ca flux of the scales. To examine the effect of CT on osteoclasts in teleosts, therefore, we developed a new culture system using goldfish scales. This system can detect osteoclastic and osteoblastic activities very sensitively using tartrate-resistant acid phosphatase and alkaline phosphatase as respective markers. We are the first to demonstrate that CT suppresses osteoclastic activities in teleosts as well as mammals. Using the sandwich type of an ELISA system with two polyclonal antibodies, we demonstrated that a considerable increase (more than twice) of plasma Ca triggered the secretion of CT from ultimobranchial glands in the feeding period but a slight increase of plasma Ca did not promote CT secretion. From the analyses of the plasma CT level and the expression of a CT receptor, we found that in teleosts, CT has the most important role in the reproductive period compared to other periods. In cartilaginous fish and cyclostomes as well as teleosts, CT has some roles in the reproductive period. These results suggest that CT in fish has a more important role in the reproductive physiology than in the Ca metabolism.