Abstract
1. Effects of massive AC stimulation on the activity of bullfrog heart were investigated and comparisons in the effects were made among the whole heart and the isolated strips of various portions. Dependency of the responses on frequency of the AC stimulation was also studied.
2. General effects of mass stimulation were observed to be divided into immediate and after-effects. The latter was further subdivided into after-inhibition and after-acceleration of augmentation.
3. The effects on the whole heart were considerably different depending on the position of electrode and on the voltage of stimulation. High voltages to the sinus venosus produced dominantly a negative chronotropic effect while those to the ventricle caused a negative inotropic effect. The atrial stimulation produced an intermediate response. Low voltage of stimulation elicited mainly the after-augmentation in every portion.
4. Isolated strips of the sinus, venosus, atrium and ventricle hehaved quite similar in the responses to those of the sinus, atrial and ventricular stimulation of the whole heart respectively. Only exception was a slenderness of after-augmentation in the isolated sinus.
5. In the sinus venosus in situ, indirect stimulation of the vagosympathetic nerve produced almost the same effects to those of direct mass stimulationc applied on the sinus. The sustained contraction, however, was never produced by indirect nerve stimulation.
6. Increase in frequency of AC stimulation produced a decrease of the immediate- and after-effects. Low frequencies elicited a marked sustained contraction and an after-inhibition followed by a long lasting augmentation while high frequencies produced a fast but temporal augmentation. Thus, the frequency dependent changes appeared similar to the voltage dependent changes in such a way that high frequencies corresponded to low voltages and vice versa.
7. In all cases of the isolated strips and whole heart examined, the inhibitory after-effect was strengthened by ACh and completely blocked by atropine. The secondary after-effect of augmentation was depressed by adrenergic blockers such as inderal and reserpine.
8. These results suggest that the mixed electro-release responses can occur in a wide range of frequency of stimulation and also in different portions of the heart, though the responses vary depending upon areal difference of the electro-release and of function of the tissues.
