[Water homeostasis in amphibia: vasotocin and hydrins].

Water homeostasis is ensured in vertebrates through neuroendocrine reflexes involving hormones and specialized exchange organs depending whether the habitat is freshwater, sea or land. Amphibians have a development recapitulating two adaptive programs, first a freshwater fish program as a tadpole, then a land vertebrate program as an adult. The second, however, is imperfect because of the great evaporative water loss through the skin when the animal is in the open air. This loss must be compensated by water reabsorption through the nephron, the urinary bladder and mainly by water uptake through the skin. Water reabsorption by the nephron is not as efficient as in higher vertebrates but water uptake through the skin is crucial because adult amphibians, like tadpoles, do not drink. Adaptation occurred at the level of three organs, nephron, urinary bladder and skin whose permeability is under control of specific hormones. Aside from vasotocin, active on these three organs, differential maturation of provasotocin led to processing-arrested intermediates, namely vasotocinyl-Gly in virtually all anuran amphibians and vasotocinyl-Gly-Lys-Arg in Xenopus laevis. These intermediates result from a down regulation of the alpha-amidating enzyme or carboxypeptidase E, respectively. They have been termed hydrin 2 and hydrin 1 because they are endowed with hydroosmotic properties equal or superior to those of vasotocin on the skin and the bladder. However, in contrast to vasotocin, they are devoid of antidiuretic activity. Adaptive evolution has created, aside from the osmoregulatory vasotocin-nephron system that was preserved in strictly terrestrial nonmammalian tetrapods, additional functions such as the hydrin-skin and the hydrin-bladder rehydrations with specific messengers and organs. The adjuvant systems disappeared in true land vertebrates because the vasotocin-nephron system became more efficient.