Modification of electrophysiological properties of rat heart with age.

The authors have determined the serum thyroid hormone levels [total (TT3) and free (FT3) triiodothyronine], the heart weight/body weight ratio and the heart rate of differently aged male rats. The variations of these parameters show a modification of thyroid state as a function of ageing. The authors have also recorded, at about 26 degrees C, resting and action potentials from single cells of papillary muscles isolated from the same groups of rats. The animals in the higher thyroid state exhibited a repolarization speed higher than the other animals. The thyroidectomy, performed on 50 day old rats, and T3 treatment of the thyroidectomized rats give rise to modifications of repolarization speed and then of action potential duration analogous to ones obtained in previous study for animals thyroidectomized at 30 days of age. These data demonstrate that the modifications of heart electrophysiological properties with age, are due fundamentally to thyroid state modifications. The results suggest also that the cardiac chronotropism modifications which the rat undergoes as a function of ageing are due to the changes of levels of thyroid hormone which might exert its effect by modifying the ion channel kinetics as well as the cardiac receptors.