Characterization of growth hormone nycthemeral plasma profiles in catheterized rainbow trout (Oncorhynchus mykiss).

This study was conducted in order to characterize the nycthemeral plasma profiles of growth hormone (GH) in 41 prepubertal (2+) rainbow trout (Oncorhynchus mykiss). The possible influences of day-night alternation and of the food factor (quantity ingested, timing of the feed) on these plasma profiles were also analysed. Blood samples were taken every hour during a 24-hour period through a catheter inserted in the dorsal aorta. An assay of cortisol levels showed that the fish were in satisfactory physiological condition. The plasma concentrations of GH fluctuated significantly during the course of a nycthemere. The basal level was very low (0.32 +/- 0.01 ng/ml), and was interrupted by peaks that occurred at a rate of 2.1 +/- 0.1 peaks/24 hr, an amplitude of 2.0 +/- 0.3 ng/ml, and a duration of 3.5 +/- 0.2 hr. As a result, mean GH levels over a 24-hr period were low (0.7 +/- 0.1 ng/ml). Peaks occurred irregularly and asynchronously in individual fish and displayed no rhythmicity. Our study shows that there is a sexual difference in GH profiles in rainbow trout. No significant difference was observed between daily and nightly levels of GH (0.6 +/- 0.07 vs. 0.8 +/- 0.1 ng/ml). No influence of the diet on the plasma profiles of GH was observed. Average levels of GH over a 24 hr period are not significantly correlated with the growth rate of the fish. This study shows that circulating levels of GH in rainbow trout are markedly lower than in other vertebrates.