On the Role of Calcium Ion in the Contractility of the Frog's Stomach Muscle

1. The isotonis sontrastion (shortening) of the muscle-strip from the pyloric portion of frog's stomach was investigated in the physiological solution, in which the calcium or the potassium cantent was either decreased or increased. 2. When the calcium ion was deprived from the bathing solution, the contractile response to acetylcholine could not be observed after one hour or more. Generally, the preparation relaxed gradually to a full extent in the calcium-free solution. If the bath was changed to that containing the calcium, it shortened again and the responsibility to acetylholine reappeared. 3. The contractile response to acetylcholine or to rlrctric stimulus could be observed with the depolarized muscle in a potassium-rich solution. The shortening of contracture induced by high-potassium was maintained in Ca-containing solution but was not, in Ca-free one. 4. In the contractile response to acetylcholine, two kinds of shortening, a fast component and a slow component, could be distinguished in different proportions. The slow component disappeared much earlier than the fast one in a Ca-free solution. There was no finding to differentiate the former and the maintained shortening or contracture. 5. Concerning the role of calcium ion in the contractile mechanism, it was discussed as a working hypothesis that the entrance of calcium ion from the extracellular fluid or the protoplasmic membrane into the maintained state of shortening would have a relationship with the concentration of calsium existing inside the muscle fibre.