Summation of Contractions in the Turtle Heart

Abstract

The phenomenon of summated contractions cannot be obtained in cardiac muscle under ordinary conditions. Mines, 1 however, found such an effect in the hearts of Torpedo ocellata which had been removed from the body for some time. He related this exception to the fact that the extraordinarily long refractory period of the heart had been shortened under the experimental conditions. Thus a second stimulus occurring sufficiently soon after a first would allow summation of one contraction upon another. Burridge 2 found that calcium salts greatly favored summation in frog ventricle muscle. Alcohol, ether, and alkali also had a somewhat similar effect.

In this laboratory while studying the vagus effects on the turtle heart it was discovered that in a limited number of cases extra-contractions could be summated upon normal contractions during vagus inhibition.

In these investigations hearts of Chrysemys elegans and Chrysemys troosti were used. It was found that summation of contractions was possible in either auricle with homolateral vagus stimulation, but the right auricle because of its greater size and contractility always demonstrated the phenomenon to an advantage. Accordingly, the right auricle was used in most of the experiments, its contractions being recorded on a smoked surface by means of a sensitive isometric lever. Since the negative chronotropic effect of the right vagus in this genus generally overshadows its negative inotropic action, it was found necessary to maintain rhythm of the auricles artificially during inhibition. This was accomplished by sending rhythmical break shocks into the non-recording auricle, thus maintaining contractions in all parts.

Fig. 1 is a characteristic result showing that an extra-systole during the period of vagus stimulation will produce a total height of contraction surpassing that of the regular driven beat. This is true only when this extra-contraction occurs within a short time after the normal, approximately 1.5 seconds in most cases at room temperature.