Recovery of Excitability in the Turtle Ventricle.

Abstract

Ashman and Hafkesbring 1 report a progressive increase in the excitability of turtle ventricle with repeated stimulation, i. e., a treppe of excitability. One of us in a recent study 2 on the recovery of contractility in turtle ventricle, has indicated that the treppe in contractility depends upon the alteration of the rate of recovery after contraction. And further, that this rate of recovery varies with 2 factors: (1) the amplitude of the preceding contraction, and (2) the interval between the 2 preceding contractions. In the course of the experiments leading to these conclusions certain observations were made on excitability which were not published at that time.

In these experiments ventricular strips were caused to contract rhythmically by the application of electrical stimuli. It was found that when an extra contraction was produced following one of the regular rhythmical responses (by the introduction of an extra stimulus), the absolute refractory period following this contraction is shorter than that following the regular rhythmical contractions. As an example: Ventricular strip responding rhythmically every 4.4 seconds. Very strong stimuli ineffective during interval of 2.1 seconds following the rhythmic responses. Response obtained at 2.2 seconds after one of the regular contractions. Following this response another was elicited with the same strength of stimulus after only 2.0 seconds.

The foregoing experiment indicates that the recovery of excitability is more rapid after 2 contractions which follow closely upon each other than when these contractions are separated by a longer interval of time. Thus it appears that the rate of recovery of excitability depends upon the interval between preceding contractions. These findings parallel those for the mechanical responses, 2 and are quite in accord with those of Ashman and Hafkesbring.

Further experiments are in progress which it is hoped may assist in the explanation of the treppe in excitability and the summation of stimuli.