Measurements of serum thyroxine and the proportions of rhodopsin and porphyropsin in rainbow trout

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Abstract
Immersion of yearling rainbow trout (Salmo gairdneri) in 0.1 or 1.0 ppm quantities of thyroxine (T4) significantly increased both T4 in the serum and the proportion of porphyropsin visual pigment in the retina. In fish not treated with T4, levels of porphyropsin were higher at 6 °C than at 14 °C. However, under different conditions of light or temperature there was no consistent correlation between serum T4 and percentage of porphyropsin. Furthermore, there was no tendency for individuals higher in porphyropsin to have higher titre of T4, or vice versa. The results generally do not support the hypothesis that serum T4 titre and porphyropsin are physiologically interdependent.