VITAMIN-E ADMINISTRATION ATTENUATES THE TRI-IODOTHYRONINE-INDUCED MODIFICATION OF HEART ELECTRICAL-ACTIVITY IN THE RAT

This work was designed to determine whether the thyroid-hormone-induce d modifications of heart electrical activity are, at least in part, du e to increased free radical production. For this study, 60-day-old eut hyroid, hyperthyroid and hyperthyroid vitamin-E-treated rats were used . Hyperthyroidism, elicited by a 10 day treatment with tri-iodothyroni ne, induced an increase in lipid peroxidation without changing the lev el of antioxidants. Intraperitoneal vitamin administration to hyperthy roid rats led to a reduction in lipid peroxidation and a non-significa nt increase in antioxidant level. The hyperthyroid state was also asso ciated with an increase in heart rate measured in vivo and a decrease in the duration of the ventricular action potential recorded in vitro. Administration of vitamin E attenuated the thyroid-hormone-induced ch anges in heart rate and action potential duration, which were, however , significantly different from those of the control euthyroid rats. Th ese results suggest that vitamin E protects hyperthyroid heart against lipid peroxidation by mechanisms that may be independent of the chang es in antioxidant systems. Moreover, the reduction in the tri-iodothyr onine effects on heart electrophysiological properties indicates that such effects are mediated, at least in part, through a membrane modifi cation, probably related to increased lipid peroxidation, involving a free radical mechanism.