Effects of 3,5,3′-triiodo-L-thyronine (T3) and 6-n-propyl-2-thiouracil (PTU) on growth of GH-transgenic coho salmon, Oncorhynchus kitsutch

GH-transgenic coho salmon ( Oncorhynchus kitsutch) juveniles were fed diets containing 3,5,3"-triiodo-L-thyronine (T-3; 30 ng/g fish) or 6-n-propyl-2-thiouracil (PTU; 20 ug/g fish), to assess the effect of these drugs on the physiology, growth and survival in comparison with untreated transgenic and non-transgenic salmon. After 84 days, food intake, feed efficiency, survival, growth, hepato-somatic index (HSI), viscera-somatic index (VSI), plasma L-thyroxine (T-4), T-3 and growth hormone (GH) levels, and cranial morphological abnormalities were determined. Growth of transgenic salmon was significantly faster than the nontransgenic salmon, and was increased by exogenous T-3 and reduced by PTU. Food intake of transgenic salmon was higher than that of the nontransgenic group, but was reduced by exogenous PTU administration. Food conversion efficiency of transgenic salmon was lower than that of nontransgenic salmon, and also was increased by T-3, but reduced by PTU in the transgenic fish. The survival rate in all transgenic groups was significantly higher than that of nontransgenic, and transgenic T3 and PTU treatment groups showed higher survivals than the transgenic-control group. The HSI and VSI of the transgenic fish were higher than the nontransgenic fish; and both parameters in the transgenic salmon were increased by PTU, but reduced by T-3. The plasma T-4 level in transgenic salmon was approximately 1.5-fold higher relative to the nontransgenic fish, whereas no difference was observed among the transgenic groups. Plasma T-3 levels in transgenic salmon were also approximately 2-fold higher relative to the nontransgenic fish. However, the plasma T-3 level in transgenic animals was increased by exogenous T-3 administration, but was reduced by exogenous PTU to that observed in nontransgenic salmon. The plasma GH level of transgenic fish was higher than that of the nontransgenic salmon, and the level was increased by the exogenous T-3, whereas exogenous PTU did not reduce significantly GH levels in transgenic salmon. Transgenic fish also displayed cranium, jaw and opercular abnormalities typical of the effects of this gene construct in coho salmon, indicating that some imbalance in growth processes has been induced. However, these abnormalities ( especially cranial disruptions) were diminished by administration of exogenous PTU. In conclusion, exogenous T-3 and PTU treatments can induce hyperthyroidism and hypothyroidism, respectively, and have inverse effects on growth and skeletal abnormalities of transgenic salmon constitutively expressing GH.