Ventricular electrophysiological properties: is interspecies variability related to thyroid state?

Whereas action potential (AP) duration, area, and repolarization vary considerably, ventricular AP characteristics such as resting membrane potential, AP amplitude, and maximal upstroke velocity of phase 0 depolarization appear similar in a variety of mammalian species. As it has been shown that modifications of the thyroid state are associated with alterations in ventricular electrophysiological properties, we hypothesized that some variations in transmembrane potentials among mammalian species result in part from differences in the individual thyroid states. To test this hypothesis, we used standard microelectrode techniques to study ventricular APs in the dog, mole-rat, guinea pig, rat, mouse, and shrew, which encompass a wide range of thyroid states. In these species O2 consumption ranges from 0.38 (in the shrew) to 4.05 ml O2 X g-1 X h-1 (in the dog). We found that resting membrane potential and AP amplitude, duration, and area were inversely correlated with O2 consumption. The correlation coefficients between these parameters with O2 consumption were -0.92, -0.78, -0.91, and -0.92, respectively. We further tested the hypothesis in guinea pigs in which we modified the thyroid state in opposite directions by thyroxine administration and by propylthiouracil treatment. In the hypothyroid myocardium, AP duration (cycle length = 2,000 ms) markedly increased (P less than 0.001), whereas in the hyperthyroid myocardium, resting membrane potential and AP amplitude (P less than 0.01) and duration (P less than 0.001) decreased. We conclude that interspecies variations in ventricular electrophysiological properties can be correlated with the thyroid state, which may be an important determinant of these properties.