Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

James Staples

Abstract

During hibernation, daily torpor, and fasting, mammals reduce metabolic rate (MR) up to 99%, 95%, and 30%, respectively, compared to resting levels. Mitochondrial metabolic suppression likely contributes to this MR reduction, and the first objective of this study was to determine the relative contributions of active, regulated inhibition and passive thermal effects as body temperature (Tb) falls, to mitochondrial metabolic suppression, and to examine the mechanisms involved using top-down elasticity analysis and novel statistical approach. The second objective of this study was to determine how mitochondrial metabolic suppression affects mitochondrial reactive oxygen species (ROS) production, a topic which has been largely ignored previously. To accomplish these objectives, I measured in vitro respiration and ROS production rates of mitochondria from liver, skeletal muscle, and/or heart during hibernation in thirteen-lined ground squirrels (Ictidomys tridecemlineatus), spontaneous daily torpor and fasting in dwarf Siberian hamsters (Phodopus sungorus), and fasting-induced daily torpor and fasting in laboratory mice (Mus musculus) over a range of physiologically-relevant temperatures. In liver, state 3 respiration measured at 37°C was 70%, 35%, and 31% lower during hibernation, daily torpor, and fasting, respectively, resulting largely from substrate oxidation inhibition at complex I and/or II. In skeletal muscle, state 3 respiration measured at 37°C was reduced up to 32% during hibernation. By contrast, in heart, state 3 respiration measured at 37°C was 2-fold higher during daily torpor in hamsters. Therefore, active, regulated mitochondrial metabolic suppression in several tissues characterizes mammalian hypometabolism, accounting for up to 16% of the MR reduction observed. In all tissues, mitochondrial respiration declined with in vitro assay temperature, and differences among metabolic states were not observed at low temperatures (10-15°C), suggesting that passive thermal effects also play an important role, particularly during steady-state torpor when body temperature is low. In liver and heart (but not skeletal muscle), basal ROS production and/or free radical leak (FRL; proportion of electron flux leading to ROS production) was generally higher during hypometabolism when measured at 37°C, particularly at complex III. However, in all tissues, ROS production and FRL typically declined with temperature, suggesting that, while mitochondrial metabolic suppression may increase the potential for mitochondrial ROS production, perhaps leading to oxidative stress during fasting, low Tb during torpor may, in fact, alleviate the accumulation of oxidative damage.


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