Animals spend as much as 20% of their daily energy budget in leaking protons across the mitochondrial inner membrane, a process known as proton leak. Proton leak uncouples cellular respiration from ATP production,resulting in metabolic inefficiency. However, when animals are faced with adverse environmental conditions, maintenance of metabolic efficiency is paramount to their survival. In order to increase survival time in the face of limited energy resources, some animals have developed the extraordinary capacity of drastically reducing their daily energy budget to preserve vital physiological functions, a state called metabolic depression. During metabolic depression, proton leak must be reduced otherwise it would dominate metabolism, threatening energy homeostasis. Recent advances have revealed that proton leak is reduced during metabolic depression in ectotherms, through a decrease in substrate oxidation capacity; the generating force for proton leak. The study by Barger and collaborators brings this question to a new level, by examining if proton leak is reduced during hibernation in mammals.
Arctic ground squirrels were used as the animal model for hibernation. Proton leak was compared between active and hibernating squirrels by simultaneously determining respiration, which is used as surrogate for proton leak, and the membrane potential of the animal's mitochondria. The mitochondria were isolated from liver and skeletal muscle, two tissues that contribute significantly to standard metabolic rate. In addition, the authors measured mitochondrial state 3 respiration, which represents the maximal capacity of mitochondria to oxidize substrates and generate ATP.
Liver mitochondria from hibernating squirrels displayed reduced state 3 respiration and proton leak rate compared with those from active animals. Basically, this implies that the capacity for mitochondrial ATP production is reduced to parallel the lowered cellular ATP demand and that metabolic efficiency is conserved. The reduction in proton leak was caused by a decrease in membrane potential due to a reduced substrate oxidation capacity and not to a lowered permeability of the membrane to protons. Since proton leak rate increases exponentially with membrane potential, a slight drop in membrane potential can lead to a significant reduction in proton leak rate. In other words, the strategy adopted by liver cells to preserve metabolic efficiency during hibernation is to limit proton leak by reducing mitochondrial membrane potential.
Contrary to liver mitochondria, skeletal muscle mitochondria from hibernating squirrels did not display lowered state 3 respiration nor proton leak compared with those from active animals. Indeed, the bioenergetic properties of skeletal muscle mitochondria appeared very similar between control and hibernating animals. Together, the results with liver and skeletal muscle mitochondria illustrate tissue-specific regulation of proton leak during hibernation.
With the present study, a general principle starts to emerge: during metabolic depression, proton leak is reduced to preserve metabolic efficiency by a decrease in substrate oxidation capacity and not by a lowered permeability of the mitochondrial membrane to protons. The fact that proton leak is not abolished during metabolic depression highlights that this process must play a crucial role in cell survival. Many hypotheses have been put forward but, as of yet, there is no definitive answer. The search continues.