A Preliminary Study on the Effects of Hyperosmotic Stress on Plasma Levels of Thyroid Hormones in Male Catfish, Clarias gariepinus

Male air-breathing catfish, Clarias gariepinus were exposed to hyperosmolar mannitol
solution (250 mM) to determine its effect on the plasma levels of thyroxine (T4) and
triiodothyronine (T3) during the winter, spring, summer and autumn seasons. During the
winter season, hyperosmolar mannitol solution significantly increased the plasma levels of T4
throughout the duration of exposure, whereas the plasma T3 levels significantly increased
only at 12-48 h. During the spring season, significant decreases in the plasma levels of T4
were observed only at 12 and 48 h, and significant increases in the plasma levels of T3 were
observed only at 6-24 h. During the summer season, the plasma levels of T4 significantly
increased at 6 h and decreased at 24-48 h, while the plasma levels of T3 significantly
increased only at 6 and 24 h. During the autumn season, the plasma levels of T4 significantly
increased at 6, 12 and 48 h and the plasma levels of T3 significantly decreased at 6, 24 and 48
h. These results indicate the season-dependent and differential effects of the hyperosmolar
mannitol solution on the plasma levels of thyroid hormones in Clarias gariepinus.