Ageing-related changes in the in vivo function of rat liver macroautophagy and proteolysis.

Autophagy is a universal, highly regulated mechanism responsible for the degradation of long-lived proteins, cytomembranes and organelles during fasting and may be the cell repair mechanism that mediates the anti-ageing effects of calorie restriction (Bergamini and Gori, 1995). The function of autophagy was studied in vivo on male Sprague Dawley rats fed ad libitum or 40% food restricted. Autophagy was induced in overnight fasted rats by the injection of an anti-lipolytic agent and was investigated by electron microscopy. Changes in regulatory plasma nutrients and hormones were assessed and rate of proteolysis was calculated from the release of 14C(6)-valine from pre-labelled resident proteins. Results in rats fed ad libitum showed that autophagic-proteolytic response to antilypolitic agents was paramount in one month-old rats; was high but delayed in 2 month-old rats, decreased remarkably in 6 month-old rats and almost negligible at older age. Parallel ageing-related changes were observed in the effects of treatment lowering glucose and insulin plasma levels. Calorie restriction prevented all changes. In view of the known suppressive effects of insulin, it may be concluded that the age-changes of autophagy are secondary to the ageing-related alteration in glucose metabolism and hormone levels, whose appearance is delayed by calorie restriction. Data may support the hypothesis that ad libitum feeding accelerates the rate of ageing by raising insulin plasma levels and suppressing autophagy and membrane maintenance, and that calorie restriction may break this vicious circle.