ALTERATIONS IN RAT CARDIAC MYOSIN ISOZYMES INDUCED BY WHOLE-BODY IRRADIATION ARE PREVENTED BY 3,5,3'-L-TRIIODOTHYRONINE

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 (T4) level by day 3 postirradiation, remaining depressed through day 9 postirradiation. The serum 3,5,3′-triiodothyronine (T3) 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 T3 ( 0.3 μg 100 g body weight) prevented the change in the myosin isozymes following whole-body irradiation. Daily pharmacological injections of T3 ( 3.0 μg 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. © 1990.