Effect of Changes in Frequency of Stimulation Upon Rabbit Ventricular Action Potential

It has been demonstrated that in rabbit ventricular muscle, the area of the action potential is at a maximum at a stimulus frequency ranging from 1 to 3 sec. -1 . At frequencies on either side of this range the action potential decreases in area. The changes in action-potential shape at high rates of stimulation are caused by an earlier "take off" of the fast phase of repolarization, and at low rates of stimulation the changes in shape are predominantly caused by a drop in the height of the plateau. It has also been shown that the ratio of the area of the extra action potential to the area of the normal action potential depends upon the stimulus frequency and the interval between the normal and extra action potentials. Upon any chauge in the stimulus frequency, the area of the ventricular action potential overshoots its final value before stabilizing. The shape of the action potential is dependent upon the previous history of the muscle. By altering the frequency of stimulation, it has also been shown that the changes in action-potential shape at low frequencies of stimulation, in contrast to those at high frequencies, are very rapid and take place immediately when the basic stimulus frequency is altered.