Urea content and urea production via the ornithine-urea cycle pathway during the ontogenic development of two teleost fishes

Abstract

1. 1. In the oviparous S. gairdnerii and in the ovoviviparous P. reticulata. the urea level is decreasing at the end of the embryonic development toward the low values found in adult fishes. 2. 2. In P. reticulata, the urea content increases sharply at the end of vitellogenesis and remains high in early embryos. 3. 3. These modifications in urea level can be related, at least partly, to the possibilities of urea formation through the urea cycle pathway. 4. 4. The differences in urea formation capabilities of the two species can be best explained when considering differences in the NH 2-group metabolism related to reproduction characteristics. 5. 5. Application of hyperosmotic media leads to an increase in urea content and formation during the embryonic development of both species.