Influence of plasma lipid changes in response to 17β‐oestradiol stimulation on plasma growth hormone, somatostatin, and thyroid hormone levels in immature rainbow trout

Plasma total lipids were significantly higher in 17β‐oestradiol(E 2 )‐treated immature rainbow trout Oncorhynchus mykiss at week 4 after implantation, due to increases in polar and neutral lipids. The lipid classes responding were phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, sterols and sterol esters, in a proportion that approximately reflected the increase in plasma vitellogenin (VtG) levels as measured by a non‐competitive enzyme‐linked immunosorbent assay (ELISA). Plasma non‐esterified fatty acids and triacylglycerol were not affected by E 2 treatment. Plasma growth hormone GH levels were increased, and plasma somatostatin‐14 (SRIF) levels decreased in E 2 ‐treated fish, responses which could be secondary to elevated plasma lipid (VtG) content, although a direct E 2 action on somatotroph function is possible. Plasma T 4 concentrations were not affected by E 2 treatment, but plasma T 3 concentrations were significantly lower than in controls 1 week after implantation when plasma E 2 concentrations were the highest; this is in support of the hypothesis that E 2 has a suppressive action on T 3 production.