Thyroxine influences on contractile proteins from atrial and ventricular myocardium.

When thyroxine is administered to rats or rabbits, both heart ventricles eventually undergo hypertrophy. Both atria and ventricles show a relative weight increase and this mainly involves sarcoplasmic proteins. Ventricular myosin from thyrotoxic rabbits has higher Ca2+-ATPase activity when compared with euthyroid rabbits, the hyperthropied atria however, show no change in myosin ATPase after thyroxine administration.