Fig. 7. Model summarizing mitochondrial bioenergetics in response to cold exposure and fasting. Effects on proton motive force (PMF, measured as membrane potential) and respiration (R, measured as state 4 respiration) in liver and skeletal muscle mitochondria. The boxes show the four conditions analyzed and include the state 4 membrane potentials (mV). Arrows indicate the changes in membrane potential and/or respiration in response to cold and/or fasting: black arrows, an increase; white arrows, a decrease; broken arrows, no change. In the liver, cold or fasting lead to an increase in membrane potential, while additional fasting in the cold causes an decrease in state 4 membrane potential and respiration rate. In skeletal muscle of cold-acclimated toads, fasting leads to a decrease in state 4 membrane potential.