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, me asured in vitro, of papillary muscle fibers. The treatment with increa sing T-3 doses was associated with a significant reduction of the acti on potential duration up to a dose as large as 25 mu g/100 g body weig ht every second day. The treatment with larger doses of T, tended to r estore the values of the action potential duration present in animals treated with physiological doses (5 mu g/100 g body weight every secon d day). Action potential duration is frequency dependent. As the stimu lation rate was increased from 1 to 5 Hz, this duration increased in a ll groups. However the difference between the rat groups remained sign ificant. The cardiac frequency measured in unanaesthetized rats increa sed as the T, doses. Furthermore the intrinsic frequency showed a simi lar increase, indicating a direct effect of T-3 On the pacemaker cells in all thyroid states. The mechanism of this action of the thyroid ho rmone is not, however clear.