ELECTROPHYSIOLOGICAL PROPERTIES OF THE HYPERTHYROID RAT-HEART

We have studied the effects of in vivo administration of different T-3 doses to thyroidectomized rats on electrophysiological properties, measured in vitro, of papillary muscle fibers. The treatment with increasing T-3 doses was associated with a significant reduction of the action potential duration up to a dose as large as 25 mu g/100 g body weight every second day. The treatment with larger doses of T, tended to restore the values of the action potential duration present in animals treated with physiological doses (5 mu g/100 g body weight every second day). Action potential duration is frequency dependent. As the stimulation rate was increased from 1 to 5 Hz, this duration increased in all groups. However the difference between the rat groups remained significant. The cardiac frequency measured in unanaesthetized rats increased as the T, doses. Furthermore the intrinsic frequency showed a similar increase, indicating a direct effect of T-3 On the pacemaker cells in all thyroid states. The mechanism of this action of the thyroid hormone is not, however clear.