A note on the use of plasma urea level to validate the arginine requirement assessed by growth data in sea bass (D. labrax)

Responsiveness of urea synthesis to dietary arginine is believed to be predictable in the European Sea Bass (D. labrax) because of the high level of arginase activity in this species (Corti et al., 1985). In rainbow trout fry, Cho et al., (1992) have used post-prandial serum urea level to confirm the arginine requirement assessed by growth parameters. A similar approach was applied in the present study to fingerling sea bass by measuring plasma urea levels. The experiment started at the end of a seven week trial carried out to assess the arginine requirement of fingerling sea bass (Tibaldi et al., unp. res.). Fish (12–18 g) were kept in 65 1 flow-through tanks (temperature 25°C and salinity 15 ‰) and fed seven isonitrogenous (47% N × 6.25) isolipidic (12% EE) diets containing graded levels of Arginine (L-ARG). A basal diet (diet 1) was formulated to be limiting in arginine (1% by weight). It contained maize gluten (300 g/kg), fish meal (100 g/kg) and mixtures of essential and dispensable AA to simulate the AA composition of sea bass muscle protein. Diets from 2 to 7 were obtained by adding 3, 6, 9, 12, 15 and 18 g/kg of crystalline L-ARG to the basal diet. Diet 4 was used in a preliminary study in order to determine the time pattern of post-prandial changes in plasma urea levels. Fish were withdrawn for blood sampling 1,3,5,7,9,12 and 24 hr after the morning meal. Peak urea level was found 5 hr post-prandial so that all the blood samples were taken 5 hr after the single meal. Blood was collected by severing the caudal peduncle and plasma was stored under liquid N and analysed for urea content within 24 hr. Three fish were sampled for each time period (preliminary study) and per diet. The arginine requirement of fingerling sea bass estimated by the dose-growth relationship was found to be 4% of the dietary protein. The 5 hr post-prandial plasma urea level varying with the arginine content of the diet and increased from 0.40 ± 0.04 mM (diet 1, ARG content 2.2% of the protein) to 0.61 ± 0.08 mM (diet 2, ARG 2.9%) without any further change up to diet 4 (ARG 4.2%, plasma urea 0.63 ± 0.04 mM). A sharp increase was noted thereafter (± 180%) by elevating the dietary arginine up to 5.4 % of the protein (diet 6, plasma urea 1.11 ± 0.03 mM). The critical dietary arginine value of 4.2% based on plasma urea level was similar to the dietary requirement estimated by growth rate (4%).