Modification of mitochondrial respiration by aging and dietary restriction.

Effects of aging and of dietary restriction on mitochondrial recovery and respiratory capacities have been assessed in mice. Old mice (23-26 months) did not differ from adult mice (9-12 months) in amounts of protein recovered in mitochondrial fractions of liver, brain and spleen, but did show a decline in specific activity of cytochrome c oxidase (cyt. c ox.) in liver and spleen. Age effects on in vitro respiration by mitochondria occurred in liver and spleen. In liver, only one substrate (beta-hydroxybutyrate) of four tested was respired at a different rate by old than by young mitochondria. Depression of state 3 respiration and 2,4-dinitrophenol (DNP)-uncoupled rates was observed for this substrate; however, this effect depended on expressing respiration on the basis of mitochondrial protein and was less overt if data were expressed per unit of cyt. c ox. activity. Old spleen mitochondria exhibited a grosser defect, showing a 40% decrease in the respiratory control index (RCI) for (succinate + rotenone)- supported respiration (the only substrate tested) due to a possible increase in state 4 rates. Effects of dietary restriction were assessed in liver and brain of 3-7-month-old mice underfed since weaning. Dietary restriction reduced recovery of total liver mitochondrial protein and liver cyt. c ox. specific activity. Liver mitochondria from restricted mice generally showed increased state 3 rates with no differences from controls in state 4 rates for respiration supported by glutamate or pyruvate + malate, resulting in an increased RCI for these substrates. DNP-uncoupled rates were also raised by dietary restriction. Unlike effects observed in old versus young mice, these differences obtained whether the data were expressed on the basis of mitochondrial protein or on cyt. c ox. activity. Electron microscopy of liver mitochondrial preparations revealed more non-mitochondrial contaminants in old mice and larger mitochondria in dietarily restricted mice. These findings are compatible with reports of age-dependent losses of liver mitochondria and suggest that dietary restriction may retard this loss.