[Calcitonin and stanniocalcin. Particular aspects of the endocrine regulation of phospho-calcium metabolism in mammals and fish].

The hypocalcemic and hypophosphatemic peptide calcitonin (CT) is secreted by mammalain thyroid parafollicular cells and fish ultimobranchial body. Over a dozen species of CTs have been cloned and/or sequenced. They can be separated into three classes based on structural and biological similarities: teleost/avian, artiodactyl, and human/rat. In mammals, CT exerts its anti-hypercalcemic and hypophosphatemic effects by inhibiting osteoclastic bone resorption and renal tubular phosphate reabsorption, respectively. CT receptors (CTRs) are members of a subfamily of seven-transmembrane domain, G protein-coupled receptors that include those for several other peptide hormones. Basic amino acid substitutions within the CT molecule enhance potency, probably by conferring a helical structure to the peptide. This might explain the enhanced potency of fish CTs for mammalian CTRs. The presence and secretion of salmon CT-like immunoreactive material have been described in both the murine and human central nervous systems, which possess CTRs. These findings are consistent with a role for this peptide acting as a neurotransmitter in mammals. Stanniocalcin (STC) is another hypocalcemic hormone originally identified in fish. In fish STC exerts its anti-hypercalcemic effect by regulating calcium and phosphate transports by the gills, intestine and kidney. Although fish ultimobranchial cells are much less responsive to the secretagogic effects of Ca2+ than mammalian parafollicular cells, the secretion of both CT and STC are positively regulated by extracellular calcium. STC has also been recently identified in humans and rats. It is released by some renal tubular cells and might play a role in the regulation of phosphate metabolism. Nevertheless, the true physiologic roles for CT in fish and STC in mammals, respectively, remain unknown.