Pfliigers Arch (1984) 402:321-324 Pfl gers Archiv EuropeanJournal of Physiology 9 Springer-Verlag1984 Postextrasystolic potentiation in isolated rat myocardium: dependence on resting muscle length C. Poggesi, R. Bottinelli, M. Vitale, and S. Testa Istituto di Fisiologia Umana, Universit~ di Pavia, Via Forlanini 6, 1-27100, Pavia, Italy Abstract. The degree and decay of postextrasystolic poten- tiation were studied in isolated rat papillary muscle at L .... the length at which developed force was maximum, and at shorter lengths. Potentiation curves were constructed relating the degree of potentiation with the time interval between any given extrasystole and the last steady-state beat. At all stimulation rates tested (ranging from 3 to 60/rain), potentiation curves were found to be dependent on muscle length: at any interval the degree of potentiation increased progressively with decreasing initial muscle length. At all lengths and stimulation frequencies tested, the decay of potentiation was a function of the number of beats following the extrasystole. This relationship could be fitted by a single exponential equation. The exponential constant of the decay was significantly smaller at Lmax than at shorter lengths. These results suggest that muscle length affects more than one step of the E-C coupling processes. Key words: Rat myocardium - Postextrasystolic potentia- tion - E-C coupling - Length-tension relationship Introduction In mammalian myocardium, an early extrasystole after a steady-state contraction deterraines an enhancement of the strength of the subsequent contraction, a phenomenon known as postextrasystolic potentiation (Woodworth 1902). The degree of potentiation varies among species and is re- lated to the interval between the premature stimulus and the last steady-state stimulus (Kruta and Braveny 1960). The increased contractile force induced by the extrasystole takes some contractions to decay back to control value in a manner that is mainly beat dependent (Wood et al. 1969). The transient potentiation and its decay reflect some features of the activation processes in the myocardium. Several experimental approaches have shown that con- tractile activation in the heart depends on resting muscle length (Allen et al. 1974; Fabiato and Fabiato 1975; Lakatta and Spurgeon 1981; Allen and Kurihara 1982). The present study was carried out to determine whether the degree of postextrasystolic potentiation and the kinetics of its decay are affected by initial muscle length in isolated rat myocar- dium. Offprints requests to: C. Poggesi at the above address Methods Papillary muscles from the left ventricle of adult male Wistar rats were used. Preparations with a mean diameter larger than 1 mm or showing spontaneous activity were discarded. After dissection under a Wild stereo microscope at 10 x magnification, the muscle was mounted horizontally in a muscle chamber between two glass hooks, one of which was connected to a force transducer (Cambridge rood. 404). The position of the other hook, and the resting length of the muscle, could be adjusted by means of a micrometer device. The chamber was continuously perfused with Krebs- bicarbonate solution bubbled with 5% CO2 in 02 (pH = 7.45). The temperature was maintained thermostati- cally at 26 ~C. Supramaximal (50% above the threshold value for excitation) stimulus pulses of 3 ms duration delivered by a stimulator developed in our laboratory were applied to the muscle through platinum plate electrodes placed on either side of the preparation. The papillary muscle was stimulated to contract isometrically at constant ("control") intervals. Postextrasystolic potentiation (Fig. 1) was induced by apply- ing to the muscle a "test" stimulus at a shorter interval (test interval) after a control stimulus: the test contraction (7) was depressed, as expected from the mechanical restitution phenomenon (Kruta and Braveny 1960). The stimulus next to the test one was delivered at the control interval and resulted in a potentiated contraction (P). This stimulation pattern allowed us to avoid the influence of restitution on the potentiated response (Kruta and Braveny 1960). After the potentiated twitch, the control interval of stimulation was maintained long enough to study the decay of potentia- tion and to ensure its complete removal before the effect of a new test interval was tested. The test interval was varied from 0.15 to 1.5 s. The potentiation induced by test contractions at varying intervals was studied at Lmax (the muscle length at which peak developed tension was maximum) and at shorter muscle lengths with different control frequencies of stimula- tion (3/min, 12/rain, 30/min). In most experiments only one length shorter than Lmax(ca. 0.9 Lm,x) was tested; in 4 experi- ments more points on the ascending limb of the length- tension relation were explored (ca. 0.95, 0.90, 0.85 Lm,O. The sequence at which each length was tested varied within experiments and between experiments. After every change in length, the muscle was allowed to reach a stable contractile performance before potentiation was examined. The signals from the force transducer and the stimulator were displayed and photographed on a storage oscilloscope