DECREASE IN RAT CARDIAC MYOSIN ATPASE WITH AORTIC CONSTRICTION - PREVENTION BY THYROXINE TREATMENT

Severe aortic constriction in rats produced cardiac hypertrophy and a chronic decrease in cardiac actomyosin ATPase activity during a six week postoperative period. Two weeks following aortic constriction, Ca2+ stimulated cardiac myosin ATPase activity was also depressed; the Km and Vmax were decreased by 86.2% (p < 0.0025) and 84.4% (p < 0.0025), respectively, when compared to sham operated controls. Administration of thyroxine (100 μg/kg/day for 14 days), which was initiated on the same day as aortic constriction, prevented, to a large extent, the decrease in cardiac myosin ATPase activity. The Km and Vmax of myosin from animals with aortic constriction showed substantially smaller decreases as a result of concomitant thyroxine administration (p < 0.0025 for the change from aortic constriction without thyroxine treatment). Thyroxine treatment in rats with aortic stenosis resulted in an additional increment of cardiomegaly when compared to animals with aortic constriction alone. The results of this study indicate that thyroxine, which normally has no effect on Ca2+ activated cardiac myosin ATPase in the rat, can prevent the decrease in myosin ATPase activity which results from severe aortic stenosis. © 1979.