THE EFFECTS OF COOLING, FREEZING, AND THAWING ON CARDIAC AND SKELETAL-MUSCLE OF THE TURTLE, CHRYSEMYS-PICTA

1. Seven adult painted turtles, after freeze anesthesia and pithing, were subjected to a prolonged cooling and freezing, followed by thawing and rewarming to room temperature. A 6 cm in diameter hole in the plastron allowed recording of ventricular temperature and tension plus the EKG. Stimulating electrodes and a thermocouple inserted into a hind leg were used to monitor the responsiveness of the leg musculature. 2. This preparation allowed simultaneous recording of ventricular contraction and EKG in relation to heart temperature and of leg muscle contraction relative to leg temperature. In spite of added insulation, the legs cooled and froze quicker and also thawed faster than did the body cavities. 3. Both muscle tension and contraction time were strongly temperature dependent during cooling. Although the leg muscles continued to function for several minutes after the onset of ice formation, they ceased to respond at an average exotherm temperature of -0.6-degrees-C. Recovery of responsiveness, noted in five of the seven turtles, was slow and only first noted at temperatures ranging from 1.9 to 17.4-degrees-C. 4. Cardiac tension and contraction times were likewise temperature dependent, and the contractions continued for several minutes after the onset of ice formation, eventually ceasing at an average exotherm temperature of -0.62-degrees-C. Some of the turtles showed a dramatic (i.e. ten-fold) slowing of the heart during the exotherm. Recovery of heart beat during the thaw was rapid and occurred at an average temperature of -0.4-degrees-C. The tensions during warming were higher than the corresponding tensions during cooling, perhaps due to a partial desiccation of the ventricle. 5. The EKG was the most persistent of the responses monitored, and, in at least four turtles, continued well past the point of cessation of cardiac contractility. The explanation for this uncoupling of electrical and mechanical activity remains uncertain.