Endocrine Abstracts

Amphibia, through metamophosis, recapitulate the evolution of water homeostasis from aquatic life to terrestrial one. Whereas the tadpole has the status of a freshwater fish, the adult has developed a three osmoregulatory organ system, including kidney, bladder and skin, for facing terrestrial dehydration. Amphibia have differentiated epithelial hydroosmotic cells in each organ: principal cells in nephron collecting duct, granular cells in urinary bladder, principal cells in ventral skin. These cells, equipped with hormone receptors and effectors (aquaporins, ion channels, urea transporter) are largely controled by neurohypophysial hormones. Each vertebrate possesses two similar neurohypophysial nonapeptides. From the 13 peptides chemically characterized in the laboratory, we have traced two main evolutionary paralog lines: vasotocin (nonmammalian vertebrates) – vasopressin (mammals) involved in osmoregulation, and isotocin (bony fish) – mesotocin (nonmammalian tetrapods) – oxytocin (mammals) possibly implicated in reproduction.

Twelve amphibian species originating from Europa, North- and South-America, Africa and Asia have been investigated. Neurohypophysial secretory granules have been isolated from the neurointermediate pituitary by sucrose gradient centrifugation and their components, purified by HPLC, identified by aminoacid sequencing and/or coelution with synthetic peptides. Along with vasotocin ([Ile³]-vasopressin) and mesotocin (([Ile⁸]-oxytocin), vasotocinyl-Gly (hydrin2) has been identified in all species. This peptide results from a limited processing of the 141-residue provasotocin. A 4-enzyme cascade operating in secretory granules on vasotocinyl-Gly-Lys-Arg sequence leads usually to the alpha-amidated vasotocin but down-regulation of the last amidating enzyme gives, in amphibians only, vasotocinyl-Gly. Vasotocin and hydrin2 have different conformations and act on distinct receptors. Whereas vasotocin shows a water (re)absorption activity in kidney, bladder and skin, hydrin2 is devoid of antidiuretic activity and is more active than vasotocin on the skin. Hydrin2 is twice more abundant in species living in arid countries. Evolution has synchronized a new osmoregulatory organ (skin) with a new specific hormone, making two hormones from a single precursor.