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, w ere subjected to a prolonged cooling and freezing, followed by thawing and rewarming to room temperature. A 6 cm in diameter hole in the pla stron allowed recording of ventricular temperature and tension plus th e 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 contra ction and EKG in relation to heart temperature and of leg muscle contr action 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 t emperature dependent during cooling. Although the leg muscles continue d to function for several minutes after the onset of ice formation, th ey ceased to respond at an average exotherm temperature of -0.6-degree s-C. Recovery of responsiveness, noted in five of the seven turtles, w as slow and only first noted at temperatures ranging from 1.9 to 17.4- degrees-C. 4. Cardiac tension and contraction times were likewise temp erature dependent, and the contractions continued for several minutes after the onset of ice formation, eventually ceasing at an average exo therm temperature of -0.62-degrees-C. Some of the turtles showed a dra matic (i.e. ten-fold) slowing of the heart during the exotherm. Recove ry 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 part ial desiccation of the ventricle. 5. The EKG was the most persistent o f the responses monitored, and, in at least four turtles, continued we ll past the point of cessation of cardiac contractility. The explanati on for this uncoupling of electrical and mechanical activity remains u ncertain.