Effect of the thyroid state on ventricular action potential characteristics in the mammalian heart

Certain ventricular action potential characteristics such as action potential duration and amplitude vary considerably among mammalian species. As previous studies showed that changes in the thyroid state are associated with alterations in action potential duration, we tested the hypothesis that these variations might be related to differences in the thyroid state of the species. Ventricular action potential characteristics from several species were studied, encompassing a wide range of metabolic rates. The species were dog, guinea pig, mole rat, rat, mouse, and shrew. Resting membrane potential and action potential amplitude, and area were found to be inversely correlated with heart rate, body mass, and metabolic rate, expressed as oxygen consumption of the species. Modification of the thyroid state in guinea pigs and rats by the administration of thyroxine (T4) or pro-pylthiouracil, yielded alterations in ventricular action potential characteristics. Hyperthyroidism was found to be associated with a reduction in action potential duration and a faster rate of repolarization, whereas hypothyroidism was associated with a marked increase in action potential duration.