Postextrasystolic contractile decay always contains exponential and alternans components in canine heart

In isolated, blood-perfused canine hearts, postextrasystolic potentiation (PESP) decays monotonically after a noncompensatory pause following a spontaneous extrasystole (ES). The monotonic PESP decay yields myocardial internal Ca²⁺ recirculation fraction (RF). We have found that after a compensatory pause (CP), PESP decays in alternans, consisting of an exponential and a sinusoidal decay component. We have proposed that this exponential component also yields RF. In the present study, we examined the reliability of this alternative method by widely changing the ES coupling interval (ESI), CP, and heart rate in the canine excised, cross-circulated left ventricle. We found that all PESP decays consisted of the sum of an exponential and a sinusoidal decay component of variable magnitudes whether a CP existed or not. Their decay constants as well as the calculated RF were independent of the ESI and CP. This confirmed the utility of our alternative RF determination method regardless of the ESI, CP, and heart rate. Direct experimental evidence of Ca²⁺ dynamics supportive of this alternative method, however, remains to be obtained.