Seasonal and temperature-related changes in mitochondrial membranes associated with torpor in the mammalian hibernator Spermophilus richardsonii

Abstract Seasonal variations in the thermal response of liver mitochondrial membranes from Richardson's ground squirrels ( Spermophilus richardsonii ) were determined by measuring succinate-cytochrome c reductase activity and spin label motion over a temperature range of 2 °C to 35 °C. For seven summer animals from the field the Arrhenius-type plots for enzyme activity and spin label motion were biphasic indicating a transition in structure and function at 22 + 2.3°C and 23 ± 1.9°C , respectively; typical of homeothermic mammals. For 12 winter animals maintained at 19°C, the transition in structure and function was lowered to 12 ± 1.1°C and 13 ± 1.4°C , respectively. The transition for 5 of 11 winter animals which were kept at 4°C and maintained normal activity and body temperature was similar to animals maintained at 19°C, while for the other six the transition was further lowered to less than 4°C. The transition for seven winter animals which were in deep hibernation was less than 4°C. The results for liver mitochondria show that lowering of the transition in membrane structure and function occurs as a two-stage process of about 10 deg. C for each stage and that the lowering is a requisite for hibernation rather than a response to the low-body temperatures experienced during hibernation.