Individual diurnal plasma profiles of thyroid hormones in rainbow trout (Oncorhynchus mykiss) in relation to cortisol, growth hormone, and growth rate

In order to characterize the individual diurnal plasma profiles of triiodothyronine (T-3) and thyroxine (T-4) in rainbow trout (Oncorhynchus mykiss), blood samples from 41 fish were taken every hour during a 24-hr period, through a catheter inserted into the dorsal aorta. The possible influences of day-night alternation, sex, and diet (feed intake, time of meals) on thyroid hormone (TH) profiles were analyzed. The existence of relations between diurnal plasma profiles of T-3, T-4, T-3/T-4 ratio, and those of the growth hormone (GH), cortisol (previously described in Gomez et al., J. Exp. Zool. 274, 171-180, 1996), and the growth rate was monitored. Average daily T-3 and T-4 concentrations were, respectively, 2.6 +/- 0.2 and 5.5 +/- 0.3 ng/ml (n = 41). Our study showed little or no variation in plasma T-3 concentrations during one 24-hr period, while those of T-4 fluctuated markedly. T-4 peaks occurred from a baseline of 4.0 +/- 0.2 ng/ml at a frequency of 2.5 +/- 0.2 peaks/24 hr, with an amplitude of 3.0 +/- 0.4 ng/ml, and a duration of 4.3 +/- 0.4 hr. There was a significant difference between the average circulating T-3 level during the day and that at night (2.4 +/- 0.2 vs 2.7 +/- 0.2 ng/ml). No influence of sex or food factors ias observed on daily TH concentrations. TH peaks occurred irregularly and asynchronously without apparent influence of day-night alternation, sex, and diet. The growth rate was significantly correlated with the daily T-3 concentration (r = 0.77), but not with T-4. No significant relationships were found between daily concentrations of T-3, T-4, GH, and cortisol. The absence of a relationship between TH and GH concentrations suggests that, in salmonids, GH may have no observable short-term action on the conversion of T-4 to T-3. (C) 1997 Academic Press.