Persistence of Accelerated Rate in Isolated Hearts of Thyrotoxic Rabbits

Acute experiments with thyroid products, either on the intact animal or upon isolated tissues, have been uniformly unsuccessful in reproducing the phenomena of hyperthyroidism. The advent of crystalline thyroxine has demonstrated that most of the non-specific results have been due to proteins and other impurities in the material used. In hyperthyroidism induced by thyroid feeding and in spontaneous hyperthyroidism in the human, tachycardia is one of the most characteristic features. It has been ascribed variously to mechanical causes, to nervous influences, to the overwork incident to a heightened metabolism, to toxic damage of the heart and to an increased production of epinephrine. Upon removal of the cause of hyperthyroidism or discontinuance of thyroid feeding the heart rate falls gradually, over a number of days, to normal. During the course of the experiments reported below it was found that the hearts and auricles of thyroid-treated animals, when isolated, continued to beat at a much faster rate than similar preparations obtained from normal animals. This seemed to indicate the persistence of the specific thyroid effect on the isolated tissue and was the basis for a comparison of certain pharmacological reactions of the two types of hearts.

Healthy rabbits were used in all experiments. Some were fed thyroid gland (desiccated and powdered) in doses varying from 0.104 to 0.390 gm. daily over periods varying from 2 to 81 days. Others received Thyroxin (Roche) by intramuscular injection every second day in doses sufficient to average from 0.05 to 0.15 mg. daily over periods varying from 6 to 25 days. Weight, heart rates and temperatures were followed at frequent intervals. The usual tachycardia, rapid loss of weight, tachypnoea and sometimes increase of temperature and diarrhoea were observed.