Ammonia and urea in the maternal-fetal trophic relationship of the viviparous blenny (eelpout) Zoarces viviparus.

A high correlation was observed between the concentration of urea in the maternal plasma and the concentration of urea in the ovarian fluid during post-yolk sac growth of the embryos in the ovary of the viviparous blenny (eelpout) Zoarces viviparus. A high correlation was observed between maternal plasma and ovarian fluid ammonia as well. Ammonia but not urea was excreted to the external medium by the mother fish during pregnancy. Intraovarian loading with either urea or ammonia resulted in a steady decrease from initial high concentrations in the ovarian fluid and a concomitant increase in the maternal plasma levels of both nitrogenous compounds. Injected ammonia was eliminated much faster from the ovarian fluid than urea. No significant effect of any ammonia could be observed on urea excretion rates by embryos in vitro. Patterns of accumulation for urea and ammonia in the external medium were investigated by exposure of embryos in vitro to concentrations of urea and ammonia similar to those normally found in the ovarian fluid. No significant changes could be observed in the external urea or ammonia levels during the experiment. The results indicate that during post-yolk sac development of embryos in vivo, net catabolism of nitrogen-containing organics and formation of urea may be reduced by high ambient concentrations of urea in the ovarian fluid.