Ammonia and urea in plasma of juvenile turbot ( Scophthalmus maximus L.) in response to external ammonia

Abstract

Turbot juveniles ( Scophthalmus maximus L.) fed to satiation at 15.9°C increased their ammonia (NH 3+NH 4 +) excretion within the first hour after feeding. The ammonia excretion reached a plateau after about 3–4 h. The concentration of ammonia (NH 3+NH 4 +) in plasma showed a linear increase in response to increasing levels of ammonia in the ambient water. However, the relation between plasma ammonia and the isolated fraction of non-ionized ammonia in the water revealed a stronger correlation. Although the statistical difference from a linear model was subtle and not significant, indications towards a non-linear correlation between plasma ammonia and non-ionized ammonia in the water were found. These results reflect the passive excretion of non-ionized ammonia and active regulation of ionized ammonia. The concentration of urea in the plasma of unexposed turbot juveniles was at a similar level as the concentration of ammonia in plasma, with concentrations of approximately 0.25 mM. The concentration of urea in plasma, calculated in percent of the sum of urea and ammonia, showed an exponential decrease from a maximum of 53.6% to a minimum of 0.2% in response to increasing levels (0–0.68 mg NH 3-N l −1; 0–4.86×10 −2 mM NH 3-N) of external non-ionized ammonia.