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 measured in 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 less than 0.001) and 0.72 +/- 0.02 mg.g-1 (P less than 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 less than 0.001) while propranolol reduced the dP/dtmax of the hyperthyroid hearts from 4089 +/- 87 mmHg.s-1 to 3497 +/- 97 mmHg.s-1 (P less than 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.