SUMMARY

This study was undertaken to compare the rates of succinate-supported hepatic mitochondrial respiration between 12 months (adult) and 29 months (old) male Fischer 344 rats. Experiments were also performed to determine the activity of adenine nucleotide translocase and the effect of its inhibition on mitochondrial respi-ration. Succinate-supported state 3 mitochondrial respiration was found to decline 20 % between 12 and 29 months of age in rat liver, along with a similar 25 % decrease in the respiratory control ratio with age. Adenine nucleotide translocase activity is shown to decrease 39 % from adult to old rat liver mitochondria. This decrease does not, however, account for the decline in state 3 respiration, since translocase activity is approximately 50 % greater than state 3 respiration in both adult and old rats. Therefore, adenine nucleotide translocase is not rate-limiting for state 3 mitochon-drial respiration. Neither the rate of succinate permeation into the mitochondrion or the rate of electron transport is rate-limiting for state 3 respiration, indicated by the greatly in-creased oxygen consumption with addition of the uncoupler carbonyl cyanide m-chlorophenyl hydrazone (m-CCCP). These processes, therefore, are not responsible for the observed ecline in state 3 respiration. The implications and possible cause of the age-related ecrease in the maximal rate of ATP-synthesis are discussed. Key words. " Liver mitochondria; Respiration; Adenine nucleotide translocase; Agin