Ventricular electrophysiological properties in normal and congenitally hypothyroid neonatal rats

The serum thyroid hormone levels [total (TT3) and free (FT3) triiodothyronine] and the heart rates were determined in neonatal rats of different ages (1-5-10 days). Thyroid hormone levels increase gradually in the first 10 days of age. The heart rate, tested at a body temperature of 37 degrees C, also increases during the same period. As the increase in heart rate in this phase of rat life is not due to the catecholamines, it is suggested that such an increase might depend on the increased thyroid hormone activity. On the other hand in congenitally hypothyroid rats the levels of both hormones and heart rates are lower than in normal animals of the same age. The electrophysiological properties of ventricular muscle fibres include a longer action potential, irrespective of stimulation frequency, in younger, naturally hypothyroid animals. The duration of action potential is greater in the congenitally hypothyroid animals, at all ages. These data demonstrate that, as in young and adult rats, the age-related modifications in heart rate, found in neonatal rats, might be due to thyroid dependent modifications of cardiac electrophysiology.