Differential effects of experimental and cold-induced hyperthyroidism on factors inducing rat liver oxidative damage

Thyroid hormone-induced increase in metabolic rates is often associated with increased oxidative stress. The aim of the present study was to investigate the contribution of iodothyronines to liver oxidative stress in the functional hyperthyroidism elicited by cold, using as models cold-exposed and 3,5,3'-triiodothyronine (T3)- or thyroxine (T4)-treated rats. The hyperthyroid state was always associated with increases in both oxidative capacity and oxidative damage of the tissue. The most extensive damage to lipids and proteins was found in T3-treated and cold-exposed rats, respectively. Increase in oxygen reactive species released by mitochondria and microsomes was found to contribute to tissue oxidative damage, whereas the determination of single antioxidants did not provide information about the possible contribution of a reduced effectiveness of the antioxidant defence system. Indeed, liver oxidative damage in hyperthyroid rats was scarcely related to levels of the liposoluble antioxidants and activities of antioxidant enzymes. Conversely, other biochemical changes, such as the degree of fatty acid unsaturation and hemoprotein content, appeared to predispose hepatic tissue to oxidative damage associated with oxidative challenge elicited by hyperthyroid state. As a whole, our results confirm the idea that T3 plays a key role in metabolic changes and oxidative damage found in cold liver. However, only data concerning changes in glutathione peroxidase activity and mitochondrial protein content favour the idea that dissimilarities in effects of cold exposure and T3 treatment could depend on differences in serum levels of T4.

Effect of prolonged exercise on oxidative damage and susceptibility to oxidants of rat tissues in severe hyperthyroidism

We investigated effects of prolonged aerobic exercise and severe hyperthyroidism on indices of oxidative damage, susceptibility to oxidants, and respiratory capacity of homogenates from rat liver, heart and skeletal muscle. Both treatments induced increases in hydroperoxide and protein-bound carbonyl levels. Moreover, the highest increases were found when hyperthyroid animals were subjected to exercise. These changes, which were associated to reduced exercise endurance capacity, were in part due to higher susceptibility to oxidants of hyperthyroid tissues. Levels of oxidative damage indices were scarcely related to changes in antioxidant enzyme activities and lipid-soluble antioxidant concentrations. However, the finding that, following exercise the scavenger levels generally decreased in liver homogenates and increased in heart and muscles ones, suggested a net shuttle of antioxidants from liver to other tissues under need. Aerobic capacity, evaluated by cytochrome oxidase activity, was not modified by exercise, which, conversely, affected the rates of oxygen consumption of hyperthyroid preparations. These results seem to confirm the higher susceptibility of hyperthyroid tissues to oxidative challenge, because the mechanisms underlying the opposite changes in respiration rates during State 4 and State 3 likely involve oxidative modifications of components of mitochondrial respiratory chain, different from cytochrome aa3.

Functional and biochemical characteristics of mitochondrial fractions from rat liver in cold-induced oxidative stress

We determined characteristics of rat liver mitochondrial fractions, resolved at 1000 (M1), 3000 (M3), and 10,000 g (M10) after 2 and 10 days cold exposure. In all groups, the M1 fraction exhibited the highest oxidative capacity, oxidative damage, H2O2 production rate, and susceptibility to stress conditions, and the lowest antioxidant levels. Cold exposure increased cytochrome oxidase activity in all fractions and succinate-supported O2 consumption in the M1 and M10 fractions during state 3 and state 4 respiration, respectively. With succinate, the H2O2 release rate increased in all fractions during state 4 and state 3 respiration, whereas with pyruvate/malate, it increased only during state 4 respiration. Increases in tissue mitochondrial proteins caused a faster H2O2 flow from the mitochondrial to cytosolic compartment, which was limited by the reduction in the M1 fraction. Despite increased liposoluble antioxidant levels, cold also caused enhanced oxidative damage and susceptibility to oxidative challenge and Ca2+-induced swelling in all fractions. These changes leading to elimination of H2O2-overproducing mitochondria avoided excessive tissue damage. We propose that triiodothyronine, whose levels increase in the cold environment, brings about the biochemical changes producing oxidative damage and those limiting its extent.