Role of the premature action potential in contractile potentiation: a study of paired stimulation

Isometric contractions and transmembrane action potentials, recorded simultaneously from canine ventricular tissue, revealed a consistent relation between action potential configuration and the intensity of the associated contractile performance during paired stimulation and the return to the control state. The nature of this association consisted of an inverse relation between the magnitude of the contractile response and the duration of phase two of the associated action potential. This relation persists despite a characteristically non-linear augmentation of contractile force effected by paired stimulation. A second correlation was also observed between the phase 2 duration of each premature action potential and the magnitude of the subsequent contractile increment. It is speculated, then, that the premature action potential may be indirectly implicated in this type of contractile augmentation. Variation in the phase 2 prolongation of premature action potentials may reflect a variable calcium influx which, in turn, yields the observed diversity of contractile increments in subsequent post-extrasystolic beats.