Calcitonin Functions Both as a Hypocalcemic Hormone and Stimulator of Steroid Production and Oocyte Maturation in Ovarian Follicles of Common Carp, Cyprinus carpio

Calcitonin (CT) is a calcium regulating hormone produced mainly by the parafollicular C cells of the thyroid gland in mammals. In lower vertebrates, these cells are concentrated primarily in a specialized gland, the ultimobranchial gland. The common structure of CT is 32-amino acid residues with a seven-residue cyclic loop formed by disulfide bond between cysteines at position 1 and 7 and prolinamide at carboxyl terminal. Calcitonin in mammals and birds is acknowledged to be the principal hypocalcemic agent, but the situation in fish is less clear. Several laboratories although showed a hypocalcemic action of CT in fish, conflicting results are often reported. Moreover, fish have other hypocalcemic factor like Staniocalcin, a product of Corpuscles of Stannius, secretion of which is positively regulated by extracellular calcium levels. Control of calcium homeostasis in fish may likely to be different from that in terrestrial vertebrates because of the aquatic environment. In addition to the involvement of CT in calcium homeostasis in mammals, an endocrine role of endogenous CT at brain, pituitary and gonad has also been suggested. Similarly, a role for CT in calcium ion regulation in fish may be subordinate to other functions which include acting as neurotransmitter or inducer of ovarian steroid hormone synthesis. A major question thus ariseswhat is the exact role of CT in fresh water teleost? Does it really has any role in calcium homeostasis or involved in many other functions including reproduction? The evolutionary history and functions of CT in aquatic vertebrate therefore require further investigation and as part of ongoing studies in to endocrine/paracrine factors involved in calcium homeostasis and the role of CT, if any, on reproduction in fish, we tried to answer these questions.