It is well known that a short rest period during the regular stimulation of isolated cardiac muscle induces a transient increase of subsequent twitch tension, although the mechanism of postextrasystolic potentiation (PESP) is still unclear. This paper is concerned with the contribution of an extrasystole to the PESP using isolated papillary muscle of guinea pigs. We have found that the length of the coupling interval of the extrastimulation prior to the rest interval affected the degree of PESP

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... in a coupling interval-dependent way at the same rest period, and the PESP of the same coupling ratio increased and decreased as the rest period was prolonged. Theophylline increased the strength of steady-state twitch tensions, whereas the dependency of PESP on a coupling ratio of extrastimulation was removed and the PESP decreased with prolonged rest periods. The evidence suggests that the coupling ratio of extra-stimulation might vary the relative contributions of trans-sarcolemmal Ca influx and Ca release from the sarcoplasmic reticulum.