Age-related changes in the thyroid hormone effects on malondialdehyde-modified proteins in the rat heart.

To determine the age-related changes in thyroid hormone (TH) effects on malondialdehyde (MDA)-modified proteins in cardiac tissue, rats at 4, 12, and 25 months of age were studied. Hyperthyroidism was induced with daily injection of L-triiodothyronine (15 microg/100 g) intraperitoneally for 10 days. Hypothyroidism was induced with 0. 025% methimazole in the drinking water for 4 weeks. MDA proteins were measured with immunoblots using a specific anti-MDA antiserum. MDA was measured as thiobarbituric acid reactive substance. Hypothyroidism in 4-month-old rats was associated with significant reduction in MDA proteins compared to euthyroid rats (13.4 +/- 5.9% vs. 99.8 +/- 10.4% of controls P < 0.001). Hyperthyroidism did not result in a significant change of MDA proteins. In aged rats, neither hypothyroidism nor hyperthyroidism was associated with significant changes in cardiac MDA proteins. The changes in MDA proteins did not correlate with cardiac MDA concentrations. In young rats, the MDA concentrations (nmol/mg) were significantly reduced in hypothyroidism (2.71 +/- 0.21) and were increased in hyperthyroidism (8.19 +/- 0.78) compared to euthyroid values (5.06 +/- 0.71) P < 0. 01. In aged rats, cardiac MDA content was significantly increased during both hyperthyroidism and hypothyroidism. We conclude that alterations in MDA protein content is yet another potential biochemical effect of TH in cardiac tissue. This particular effect is significantly blunted with age.