Thyroxine-induced alterations in protein and nucleic acid contents of fat body of female silkworms during different developmental stages

Abstract

The fat body protein content of silkworms increased with 0.5 μg of thyroxine (T4) per gram of body weight and more so with 1 μg/g on days 4 and 8 after injection. Thyroxine with 1 μg/g dose, however, caused an earlier increase in protein content on day 2. The higher doses of T4 (2 and 5 μg/g) decreased the protein content. All T4 treated groups (except with an ineffective dose of 0.25 μg/g) showed reduced protein level in the adult stage in comparison to the control. Thyroxine with a dose of 0.25, 0.5 and 1 μg/g increased the fat body RNA content over the control level which was observed on days 2, 4 and 8. But on day 13, the RNA content in these groups returned to the control level. Higher doses of T4 (2 and 5 μg/g) decreased the RNA content during the developmental period. DNA content was not altered by lower doses of T4 (0.25,0.5 and 1 μg/g), while the higher doses (2 and 5 μg/g) surprisingly caused a reduction in DNA content during the larval, pupal and adult stages. The increase in protein and RNA contents with lower doses of T4 and decrease with higher doses, indicates a biphasic nature of thyroid hormone action in silkworm fat body.