The effect of propranolol, verapamil and dantrolene treatment on cardiac hypertrophy, enhanced myocardial contractility and tachycardia in the hyperthyroid rat

Experimentally induced hyperthyroidism is associated with cardiac hypertrophy, tachycardia and elevated myocardial contractility. To investigate the possibility of ameliorating the cardiac changes pharmacologically, hyperthyroid rats were treated with propranolol, verapamil or dantrolene. Cardiac hypertrophy was assessed from the heart mass: body mass ratio and cardiac function was measuredin vitro. Both verapamil and propranolol reversed the cardiac hypertrophy of the hyperthyroid animals from 0·92 ± 0·02 mg·g^-1 to 0·70±0·01 mg·g^-1 (P<0·001) and 0·72 ± 0·02 mg·g^-1 (P<0·001) respectively. Verapamil was effective in reducing the spontaneous heart rate from 331±8 beats·min^-1 to 273±7 beats·min^-1 (P<0·001) while propranolol reduced the dP/dt_max of the hyperthyroid hearts from 4089±87 mmHg·s^-1 to 3497±97 mmHg·s^-1 (P<0·001). Dantrolene had no effect on any parameter. We conclude from our results that cardiac hypertrophy of the hyperthyroid heart can be reversed by treatment with propranolol and verapamil probably via their inotropic and chronotropic properties.