After-Contractions and Restitution of Contractility in the Isolated Guinea-Pig Auricles

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Abstract
1. After-contractions (AC), i.e. oscillatory variations of the resting length of atrial myocardium which occur under strong positive inotropic influence following an arrest of rhythmic activity, have been studied on isolated electrically driven guinea-pig left auricles at isotonic conditions. Low temperature (15– 17.5°C) and a relatively high stimulating rate (about 1 p. sec.) were used as inotropic agents. 2. The amplitude and wave length of AC was found to depend: a) on the activity prior to the rest period, in which the AC occurred, namely, on the number and frequency of the preceding beats–see Fig. 2; b) on the Ca++ concentration in the medium. The higher the activity and/or Ca++ concentration, the greater the amplitude and the shorter the wave length of the AC. 3. Concommitant oscillatory variation in restitution of contractility has been described. The restitution is exactly in phase with AC, but has an opposite direction (cf. Fig. 3). The greater the AC at a given moment, the smaller the triggered contraction. This indicates some competition of both types of mechanical reaction, probably for a common driving factor. 4. The relation between the AC – and thus the initial muscle length – and triggered contraction is similar to that of Starling's Law.