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1. Using a calcium buffer system, the effect of severe calcium lack on the shape of K-contracture of the guinea-pig's taenia coli was studied. Under conditions of calcium lack, the initial phasic response was preferentially affected and it disappeared completely at concentrations below 10^-7 M, while the ensuing tonic response persisted, though considerably diminished in size, even at the concentration of 10^-8 M.

2. In calcium-free media, various multivalent cations, which according to Frank (1962) can support the K-contracture of a skeletal twitch muscle fibre in calcium-free solution, augmented the remaining tonic response, but did not restore the phasic response, when it was eliminated in calcium free environment.

3. When K-contractures were induced in normal calcium media, these cations produced, in contrast, an abolition of the phasic response together with a partial depression of the tonic phase. They also inhibited a part of the fully developed contracture. The last effect was no longer obtainable in calcium-free media.

4. It is concluded that the phasic response and a part of the tonic response of taenia coli depend upon the extracellular calcium for their initiation (and also for maintenance of tension in the case of the latter) and that the rest of the tonic response draws on a store of `bound' calcium for its evolution.

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