Thyroid Hormones Regulate Mitochondrial Respiration as well as Antioxidant Defense in Teleosts Too!

Abstract

Our studies describe the effect of T₃ on the regulation of mitochondrial metabolism, lipid peroxidation and antioxidant enzyme homeostasis in a fresh water teleost, Anabas testudineus. These experiments suggest the possibility that certain of the clinical and biochemical manifestations of T₃ can be attributed to a direct effect of T₃ on mitochondria. Thyroid hormone enhances mitochondrial ATP production rate in the highly oxidative tissue such as liver. These in vitro and in vivo studies substantiate and confirm the earlier reported T₃-induced oxidative metabolism in lower vertebrates. These induced oxidative metabolisms create oxidative stress in tissues. This confirms that T₃ has an immediate response in in vivo and in vitro system on energy adjustment in the fish, A. testudineus. Tri-iodothyronine is also capable of combating the oxidative stress by activating the antioxidant system to remove the ROS. Tri-iodothyronine appears to have a dual role, as a stimulator of oxidative process and as a regulator of antioxidant enzyme activity. This establishes another example for the multi-functional role of T₃ in lower vertebrates also. However, the precise mechanism of action remains to be understood. The protein expression study revealed that T₃ administration in fish creates hypermetaolic state. This hypermetabolic state creates oxidative stress in fish. To maintain the homeostasis of the fish, physiology the main antioxidant CuZn SOD is directly altered. Due to augmented SOD utilization, expression of CuZn SOD is diminished in fish liver and brain. These findings conclude that thyroid hormone effectively maintains physiological status of fresh water fish. It is obvious that thyroid hormones have an over all effect on metabolism in responsive tissue and that their effect is a direct one. Theses studies establish multi-functional role of T₃ in lower vertebrates.