TEMPERATURE EFFECTS ON TRANSMEMBRANE POTENTIALS OF RAT VENTRICLE

To elucidate the effects of hypothermia on myocardium, the sensitive parameters of resting and action transmembrane potentials-measured by intracellular glass electrodes-were recorded as the temperature was lowered by 5°C decrements. Slivers of rat ventricle in oxygenated Tyrode's solution at 37°C maintained a normal resting potential (RP) of -80 ± 4 mV and a normal action potential (AP) for at least 6 hrs. On cooling action potential waveforms lengthened-especially phase 2 which corresponds to the ST segment-though phases 0 and 3 were prolonged below 15°C. Below 10°C the AP could no longer be stimulated. Resting potentials fell to -40 mV at 4°C. Recovery was slow, irregular, and incomplete. Myocardium stimulated at 30 beats per min in Tyrode's became inexcitable at 8°C and had higher RP values (-60 mV at 4°C), but incomplete recovery. Low molecular weight dextran (LMD) was detrimental if added at normothermia, but beneficial at lower temperatures. Addition of cool LMD lowered the arrest temperature at 9°C, and the RP values at 4°C averaged -70 mV and were quite stable. Recovery during warmup was prompt. LMD was removed at 25°C, and with continued warming to 37°C the RP and AP values and waveforms were comparable to the controls. These studies demonstrate significant deleterious changes in myocardial cellular function resulting from deep hypothermia even without freezing, which are ameliorated by LMD. Like many cryoprotective agents, LMD is found to have significant though reversible deleterious effects at normothermia. © 1969.