Failure of propranolol to prevent chronic hyperthyroid induced cardiac hypertrophy and multifocal cellular necrosis in the rat

The effect of excess thyroid and thyroid plus propranolol on right and left ventricular structure was examined. Perfusion fixed hearts from untreated adults and those treated for 10 weeks with thyroid, propranolol, and thyroid plus propranolol were embedded in plastic and sectioned for light and electron microscopy. Hyperthyroidism stimulated a significant increase in heart rate, oxygen consumption, and systolic blood pressure. Propranolol was effective in normalizing the heart rate of hyperthyroid rats, but did not affect oxygen consumption or blood pressure. Both left and right ventricular weight increased similarly in thyroid and thyroid-propranolol treated animals, but right ventricular mass increased more than left ventricular mass in both groups. The majority (21 of 24) of right ventricles from thyroid and thyroid-propranolol treated animals had multifocal areas of histologically detected fibrosis. Left ventricular fibrosis was absent or minimal in both hyperthyroid groups. Neither left nor right ventricular fibrosis was observed in control or propranolol only treated animals. We conclude that beta adrenergic stimulation is unnecessary for thyroid induced cardiomegaly, and excess thyroid hormones induce right ventricular fibrosis which is not prevented by propranolol.