Influence of estrogen and the endocrine disruptor on the calcitonin functions in teleosts

In this review, we discuss the function of calcitonin (CT) during teleost reproduction and the influence of estrogen and bisphenol-A (BPA) on CT functions.CT acts as a hypocalcemic hormone in mammals.However, the hypocalcemic effect of CT remains unclear in teleosts.The effect of CT on the plasma calcium level varies among fish species and experimental conditions.On the other hand, CT has significant functions in ter ms of teleost reproduction.Plasma CT levels in some teleosts are higher in females than in males.Additionally, a correlation between plasma CT concentration and the gonadal somatic index is observed in female teleosts.In the reproductive period, estrogen (17ß-estradiol), a sex steroid, induces the synthesis of vitellogenin in the liver and promotes calcium resorption from the scales by activating osteoclasts.Vitellogenin possesses a calciumbinding property and is incorporated into oocytes through the vitellogenin receptor.The estrogen receptor is present in the ultimobranchial gland (UBG), which is the source of CT in non-mammals.The estrogen receptor is expressed in the UBG, and specific estrogen binding is detected in the UBG.In addition, estrogen induces CT secretion from the UBG.Furthermore, using the scale in vitro assay system, we found that osteoclastic activity accelerated by the estrogen was suppressed by CT.These findings imply that CT protects the scales from excess degradation of calcium at the time of teleost vitellogenesis.Additionally, we introduce the function of an endocrine disruptor such as BPA in calcium metabolism.The actions of BPA in calcium homeostasis are different from those of 17ß-estradiol.BPA suppressed both osteoclastic and osteoblastic activities and decreased both the plasma CT and calcium levels.Collectively, these issues lead to the conclusion that CT has significant function(s) during specific stages, such as reproduction, and that BPA acts differently from estrogen and disrupts the CT functions.