The effect of metabolic depression on proton leak rate in mitochondria from hibernating frogs

Futile cycling of protons across the mitochondrial inner membrane accounts for 20% or more of the total standard metabolic rate of a rat. Approximatel y 15% of this total is due to proton leakage inside the skeletal muscle alo ne. This study examined whether the rate of proton leak is downregulated as a part of a coordinated response to energy conservation during metabolic d epression in cold-submerged frogs. We compared the proton leak rate of skel etal muscle mitochondria isolated from frogs at different stages of hiberna tion (control, 1 month and 4 months of submergence ill normoxia and hypoxia ). The kinetics of mitochondrial proton leak rate was unaltered throughout normoxic and hypoxic submergence. The state 4 respiration rates did not dif fer between control animals and frogs hibernating in normoxia. In contrast, the state 4 respiration rates obtained from frogs submerged in hypoxic wat er for 4 months were half those of control animals, This 50% reduction in r espiration rate in hypoxic hibernation was due to a reduction in electron t ransport chain activity and consequent decrease in mitochondrial membrane p otential. We conclude that proton leak rate is reduced during metabolic dep ression as a secondary result of a decrease in electron transport chain act ivity, but that the proton conductance is unchanged. In addition, we show t hat the rate of proton leakage and the activity of the electron transport c hain are lower in frogs than in rats, strengthening the observation that mi tochondria from ectotherms have a lower proton conductance than mitochondri a from endotherms.