Mitochondrial proton conductance, standard metabolic rate and metabolic depression

Proton cycling across the mitochondrial inner membrane makes up a significa nt proportion (10-30%) of Standard Metabolic Rate (SMR) in rats, if proton cycling is equally important in other animals, those that metabolically dep ress to 25% or less of SMR have a problem: either their entire energy budge t will be wasted by proton cycling, or they have to suppress the leak, of p rotons across the mitochondrial membrane. Muscle mitochondria from metaboli cally depressed, hypoxic overwintering frogs (Rana temporaria) do have decr eased proton leak rate. This is achieved not by decreasing the proton condu ctance of the membrane, but by lowering the protonmotive force (the driving force for the leak). Protonmotive force is lowered aerobically by restrict ing electron supply, and in anoxia by restricting mitochondrial ATPase acti vity. There is also a temperature component to the physiological depression of overwintering: frogs. The proton conductance of frog muscle mitochondri a decreases steeply with temperature. Frog hepatocytes also respond strongl y to temperature, and decrease their proton cycling in parallel to other re actions, so preserving metabolic efficiency at different temperatures. Hepa topancreas cells from the land snail (Helix aspersa) provide a good new mod el system to study biochemical mechanisms of depression without the complic ations of temperature change. Cells from aestivating animals show a persist ent metabolic depression to 30% of controls, partly through intrinsic effec ts and partly through the extrinsic effects of pH and pO(2). In depressed c ells, proton cycling decreases at least as much as cellular respiration rat e. These results using frogs;md snails show that mitochondrial proton cycli ng is strongly suppressed in metabolic depression, so that metabolic effici ency is maintained or even enhanced.