Fish Physiology and Biochemistry vol:19 issue:2 pages:135-143
Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T-3 and T-4 concentrations when compared to the nd libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T-4 values but not of plasma T-3 Plasma T-3 remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T-3 observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T-3 in fish. Although the deiodinases important for the gross regulation of plasma T-3 during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T-3 availability at the cellular or plasma levels, respectively.