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 tri iodothyronine (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 pos sible influences of day-night alternation, sex, and diet (feed intake, time of meals) on thyroid hormone (TH) profiles were analyzed. The ex istence 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 we re, respectively, 2.6 +/- 0.2 and 5.5 +/- 0.3 ng/ml (n = 41). Our stud y showed little or no variation in plasma T-3 concentrations during on e 24-hr period, while those of T-4 fluctuated markedly. T-4 peaks occu rred 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 aver age 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 obser ved on daily TH concentrations. TH peaks occurred irregularly and asyn chronously without apparent influence of day-night alternation, sex, a nd diet. The growth rate was significantly correlated with the daily T -3 concentration (r = 0.77), but not with T-4. No significant relation ships were found between daily concentrations of T-3, T-4, GH, and cor tisol. The absence of a relationship between TH and GH concentrations suggests that, in salmonids, GH may have no observable short-term acti on on the conversion of T-4 to T-3. (C) 1997 Academic Press.