Alterations in rat cardiac myosin isozymes induced by whole-body irradiation are prevented by 3,5,3′- l-triiodothyronine

Abstract

Changes in cardiac myosin isozymes and serum thyroid hormone levels were investigated in rats following 10 Gy whole-body gamma irradiation. The percent β-myosin heavy chain increased from 21.3 ± 1.8 to 28.1 ± 6.8 (NS) at 3-day postirradiation, 37.7 ± 1.9 ( P<.001) at 6-day postirradiation, and 43.8 ± 3.3 ( P<.001) at 9-day postirradiation. Along with the change in myosin isozymes was a significant 53% decrease ( P<.001) in the serum thyroxine (T 4) level by day 3 postirradiation, remaining depressed through day 9 postirradiation. The serum 3,5,3′-triiodothyronine (T 3) level, however, was normal until day 9, when significant depression was also observed. In contrast, the thyroid-stimulating hormone (TSH) level was significantly increased by fourfold at day 3, returning to near normal values by day 9 postirradiation. Daily injections of physiological doses of T 3 ( 0.3 μ g 100 g body weight ) prevented the change in the myosin isozymes following whole-body irradiation. Daily pharmacological injections of T 3 ( 3.0 μ g 100 g body weight ) to the irradiated rats produced a further decrease in the percent β-myosin heavy chain (below control values) indicating tissue hyperthyroidism. Thus, this study suggests that the change in myosin isozymes following whole-body irradiation is caused by an alteration in thyroid hormone activity.