Calcitonin-like substance in the plasma of Cyclostomata and its putative role

The present study indicated that in the lamprey, Lampetra japonica, which undergoes a spawning migration from seawater to freshwater in its life cycle, a calcitonin (CT)-like substance was not detected in the plasma of seawater-adapted specimens (both sexes), but it was detected in the plasma of freshwater-adapted specimens (both sexes) by enzyme-linked immunosorbent assay using anti-salmon CT serum. Furthermore, this substance increased with the gonad somatic index (GSI) in both sexes. In lamprey, therefore, this CT-like substance may have some relationship with gonadal maturation. In the plasma of the hagfish, Eptatretus burgeri, on the other hand, the CT-like substance was always extremely high (47.2+/-4.2 ng/ml) independently of GSI in both sexes. However, a correlation was found between this substance and plasma calcium. In hagfish, the CT-like substance may participate in calcium homeostasis, as does calcium excretion via bile, because the hagfish has the highest bile calcium concentration determined to date among vertebrates, with a level corresponding to 12-fold that in plasma. This speculation is supported by reports that, in rat and cartilaginous fish, CT acts on calcium excretion via bile. In both fishes, moreover, the molecular weight of these substances was equal to that of salmon CT, which strongly suggests that these substances detected here are their CTs.