The Treppe in Excitability in the Turtle Ventricle.

Abstract

In earlier reports, we have demonstrated a treppe in conduction through compressed cardiac muscle, i. e., after a period of quiescence, the conduction times for a series of impulses, more or less rapidly elicited, may show a progressive decrease. In attempting further to elucidate this phenomenon, it became necessary to determine whether, with repeated stimulation, the excitability would show a similar stepping up. Adrian, 1 in studying the supernormal recovery curve of the frog ventricle, determined the excitability at various intervals following a single response. There is nothing in his paper to show whether, as seemed highly probable, the excitability would have still further increased had there been 2 or more antecedent responses, i. e., whether a treppe of more than 1 step would have appeared.

We placed a freshly excised turtle ventricle in a moist chamber, and recorded the mechanical response while varying the strength of the break induction shock by changing a resistance in the primary. Six turtles were used in these experiments. Using the quiescent ventricle and starting with the least effective stimulus, we found that the effective strength could be progressively diminished when we stimulated repeatedly at intervals varying from 2 to 30 or more seconds. In 1 typical case, when the decrements were small and the interval 2 seconds, the resistance was changed from 25 to 46 ohms before failure of response ensued. On the other hand, with intervals of 30 seconds, the muscle responded only up to 31 ohms. If the decrements were great, e. g., over 0.2 ohm, failure of response appeared sooner because the treppe in excitability did not keep pace with the decrement in strength of stimulus. After a 3 minute rest, the least effective stimulus corresponded to 25 ohms.