VENTRICULAR ELECTROPHYSIOLOGICAL PROPERTIES IN NORMAL AND CONGENITALLY HYPOTHYROID NEONATAL RATS

The serum thyroid hormone levels [total (TT3) and free (FT3) triiodoth yronine] and the heart rates were determined in neonatal rats of diffe rent 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 increa se in heart rate in this phase of rat Life is not due to the catechola mines, it is suggested that such an increase might depend on the incre ased thyroid hormone activity. On the other hand in congenitally hypot hyroid rats the levels of both hormones and heart rates are lower than in normal animals of the same age. The electrophysiological propertie s of ventricular muscle fibres include a longer action potential, irre spective of stimulation frequency, in younger, naturally hypothyroid a nimals. The duration of action potential is greater in the congenitall y hypothyroid animals, at all ages. These data demonstrate that, as in young and adult rats, the age-related modifications in heart rate, fo und in neonatal rats, might be due to thyroid dependent modifications of cardiac electrophysiology.