The rapid cooling contracture of toad cardiac muscles

The effect of rapid cooling on the mechanical activities of the cardiac muscles of the Japanese toad was examined. 1. The mechanical responses produced by rapid cooling (23 leads to 0 degrees C) in both auricular and ventricular muscle preparations were composed of an immediate effect (phasic contraction) and a delayed effect (tonic contraction). The immediate effect was blocked by TTX, but the delayed effect was not affected by TTX and Mn. 2. The tonic contraction of the delayed effect produced by rapid cooling was potentiated in Na-deficient Ringer solution in which an equivalent amount of NaC1 was substituted for Tris-C1. The relationship between the relative tension of the delayed effect produced by rapid cooling and the ratio of [Ca2+]/[Na+]2 showed an S-shaped curve. 3. Generation of tension development of the delayed effect produced by rapid cooling was obviously dependent upon the extracellular Ca concentrations. There waa an S-shaped relationship between the relative tension of the delayed effect and pCa in the extracellular medium. 4. In the test solution containing Ca less than pCa8.0, no tension development of the delayed effect was observed. However, the rapid cooling contracture was strongly elicited by application of caffeine (caffeine-RCC). 5. The caffeine-RCC was also observed in the presence of 100 mM KC1. 6. From the results of the present study, it was considered that, even though the Ca influx was abolished, the contraction of the cardiac muscle could be produced by another mechanism, probably due to increase of myoplasmic Ca released by raid cooling from the cell membrane itself and the sarcoplasmic reticulum.