The repolarization phase of the cardiac action potential: A comparative study of rate-induced changes in its waveform

Abstract

The rapidly equilibrating changes in the shape of the repolarization phase of the action potential of cardiac muscle (as reflected in changes in its area or voltage-time integral), which occur when the rate of stimulation (contraction) is suddenly changed, have been examined for the frog, guinea pig, chicken and rabbit heart and have been analyzed and compared. In frog and chicken heart these changes are of one kind, being describable mathematically in terms of a hypothetical variable, S, which decreases on excitation and which is regenerated with time between contractions in a simple way. In mammalian hearts, on the other hand, such changes, when they occur, mask more complex changes of another kind, a kind originally described for the rabbit heart (so-called N-type changes, 5), and which would seem to be a characteristic feature of action potential area-frequency relationships of mammals in general. In the guinea pig heart these N-type changes were demonstrable, although small in comparison to accompanying S-type changes. Similarly, in the rabbit heart, when cooled, the N-type changes, so characteristic of this heart, became largely obscured by the appearance of S-type changes that do not normally occur in this heart. The possible relationship between these differences in function to the abundance of well-differentiated couplings on the one hand and the relative sparsity of poorly-differentiated couplings on the other hand in these animals is discussed.