Abstract
1. The effects of abolition of the action potential on the tension development were investigated in the isolated frog ventricular muscle. A series of anodal pulses was applied at various time after the driving stimulus by using the separation chamber method.
2. Anodal pulses applied during an action potential markedly affected the tension development. Abolitions in the early phase of action potential produced strong inhibitory effects both on the rate of rise and time to peak of tension, while abolitions in the later phase caused less effects. According to the time point of abolition the coupling process between action potential and contraction followed at least four sequential phases.
3. The curve of maximum rate of tension development corresponding to each action potentials with different durations rose gradually and attained to its maximum 200 to 300 msec earlier than the peak of isometric tension. This curve, tentatively termed as the “activator-releasing” curve, was supposed to reflect the instantaneous amount of displaced activator calcium and determined to precede the development of active state.
4. The high calcium solution (5.6mM/l) augmented the degree of coupling in all sequential phases, while the low calcium solution (1.4mM/l) diminished it. 5. A distinct inflection was found between the phase 1 and phase 2 in the “activator-releasing” curve. This finding strongly suggests that there are at least two different sources for activator calcium in the frog ventricle.
