Possible involvement of Calcitonin I and II in calcium metabolism of the female reproductive physiology of goldfish (Caras-sius auratus)

Two types of Calcitonin (Calcitonin I (CtI), Calcitonin II (CtII)) were previously determined in goldfish. The present study examined the expression levels of CtI and CtII in the ultimobranchial glands (UBGs) using quantitative PCR with a TaqMan probe and their correlation with the reproductive physiology. The results showed that the mRNA expression of CtI and CtII in females had a significant relationship with their plasma Ca concentrations, while those in males did not correlate with their plasma Ca concentrations. Furthermore, there was a significant co-relationship between plasma CALCITONIN (CT) and Ca levels or gonad somatic index in female but not male goldfish. Additionally, the plasma CT and Ca levels in females were significantly higher than those in males. The calcium-sensing receptor mRNA expression in the UBGs of female goldfish was significantly higher than that in male goldfish, indicating that the UBGs in female goldfish respond sensitively to changing plasma Ca conditions during the reproductive period. On the other hand, it has been considered that the target of the secreted CT is osteoclasts in the fish scales, while it is bone in mammals. Next, we examined the CtI and CtII mRNA expressions in the scales of goldfish. In the scales of both sexes, we found that CtII mRNA was expressed, but CtI was not detected. CtII expression was immunohistochemically observed in the osteoblasts. Furthermore, CtII mRNA expression in scales increased significantly with E₂ treatment. Thus, in the present study, we demonstrate that CTI and CTII are involved in Ca metabolism, particularly in female goldfish reproductive physiology. CTII expressed in the ectopic organs such as scales has an ectopic function regarding Ca metabolism through paracrine or autocrine mechanisms during the female reproductive period.