Mitochondrial membrane transitions in heart and other organs of a hibernator.

Critical temperatures (T) for transitions in both lipid structure and enzyme function of mitochondrial membranes from liver, kidney, brown fat, and heart tissues were determined for the hibernator Spermophilus lateralis at two weekly intervals from early summer to late autumn and during hibernation. For all tissues T fell into one of three groups: those below 4 degrees C (the minimal level of accurate determination), those centered about a mean of 11.9 +/- 1.4 degrees C, and those centered about a mean of 20.9 +/- 1.8 degrees C. The T for tissues from torpid animals and from heart, at all sampling periods, was below 4 degrees C. For liver, kidney, and brown fat the mean T was approximately 21 degrees C in early summer but was lowered later in the season in a two-step process, falling to below 4 degrees C before the animals were exposed to cold and entering torpor. It is concluded that for mitochondria the thermal response of the membrane lipids is altered such that the transition in structure and function is always below the minimum body temperature likely to be experienced by this animal. Heart tissue is exceptional in that the transition is at a temperature consistent with a body temperature of torpor even in summer-active animals.