Mode of Electrical Activity of Cardiac Muscle Fibers Under Hypothermia

Effects of hypothermia on the intracellular potentials were studied on the surviving strips of the ventricular muscle from the mouse, and mode of activities of the fibers was analysed.2. Lowered muscle temperature caused no appreciable change in the magnitude of both resting potential (RP) and action potential (AP) down to 19°C, but below this temperature, sudden decrease of the potentials were observed. The decrease was more conspicuous in AP than RP, and AP was no longer elicitable at 13°C or below.3. The ventricular muscle fiber often became spontaneously active at 37°C to 25°C. Double spike was also frequently encountered particularly within the temperatures of 32°C to 22°C, more often in the right ventricle (65%) than in the left one (35%).4. Both rising phase and repolarization phase of AP were prolonged with the fall of temperature. At temperatures of 19°C and below, AP appeared peculiarly deformed and in about half the number of cases of penetrated cells, inflection came into sight in the rising phase and on some occasions, the spike was fallen off with the slow potential left alone.5. On the basis of above findings, the possibility is discussed that the intracellular AP of the cardiac muscle is made up of two components, i.e. spike component and slow potential component, and that the latter is a myo-myo-junction potential related to the excitation transmission among the muscle fibers.