Changes in cardiac function following administration of thyroid hormones in thyroidectomised rats: assessment using the isolated working rat heart preparation.

We investigated the changes in cardiac function following alterations in thyroid state of rats, using the isolated working heart preparation. This technique allows heart rate, contractility, and pump function to be studied under controlled conditions. Surgical thyroidectomy resulted in significant impairment of cardiac function. Single doses of 3,5,3'-triiodo-L-thyronine (T3) or L-thyroxine (T4) were given to thyroidectomised rats. Cardiac function was restored to normal within 48 h of administration of 0.5 mg/kg T3 or 2.5 mg/kg T4 without significant changes in heart weight. The dose-response relationships between T3 and T4 and cardiac function showed that significant changes occurred after single doses of T3, 0.02 mg/kg, or T4, 0.5 mg/kg. An increase in contractility was seen within 12 h of administration of T3, before other physiological effects of T3 are reported to occur. This indicates that the change in contractility is the result of a direct effect of thyroid hormones on the heart and is not secondary to changed loading conditions of the heart. Changes in heart rate occurred between 18 and 48 h. The maximum heart rate achieved did not exceed that in hearts from euthyroid rats, although contractility could be increased above euthyroid levels.