Differences in the thermal behaviour of myocardial membranes relative to hibernation

1. 1. Phase transitions and changes in the molecular ordering of membrane lipids were detected by differential scanning calorimetry (DSC) and spin labelling using myocardial membranes from summer-active and winter-hibernating ground squirrels (Spermophilus richardsonii), dog (Canis familiaris) and trout (Salmo gairdnei). 2. 2. Two phase transitions, one at 26°C and the other at 15°C, were detected by DSC in the membranes from the dog, whereas only one transition was found in the membranes from the ground squirrel. This was at 26°C in membranes from summer-active animals but was at 16°C in membranes from winter-hibernating squirrels. No transition was detected in membranes from cold water, adapted trout. 3. 3. Spin labelling showed changes in lipid ordering at about the same temperature as the thermal transitions detected by DSC except in the membranes from the dog where only the transition at 26°C was observed. 4. 4. The membrane lipids from the winter-hibernating squirrel contained a higher proportion of unsaturated fatty acids than those of the summer-active squirrel. 5. 5. The transition at 15°C in membranes from dog correlates with the reported temperature for the loss of cardiac function during hypothermia and suggests a causal relationship between the thermal response of membrane lipid structure and cardiac function. 6. 6. The absence of a transition at 15°C in the membranes of the summer-active ground squirrel is consistent with the ability of this animal to maintain rhythmic heart beat at tissue temperatures near 0°C.