Mitochondrial ATP production rate in 55 to 73-year-old men: effect of endurance training.

The effect of 6-week endurance training on mitochondrial ATP production rate was investigated in 14 elderly men. Mean age, body weight and height were 63 +/- 6 yr, 75.6 +/- 9.2 kg and 174 +/- 4 cm, respectively. Subjects trained on a Monark cycle ergometer at 79 +/- 8% of their maximal heart rate for 1 h day-1, 4 days week-1. Muscle samples were obtained at rest, before and after endurance training, by a needle biopsy technique and used for determination of mitochondrial ATP production rate in isolated mitochondria and enzyme assays. Endurance training resulted in a significant increase in maximal oxygen uptake (L min-1) (P < 0.01). Citrate synthase activity, a mitochondrial marker enzyme, and hexokinase activity increased significantly (both P < 0.01) in response to training while 3-hydroxyacyl-CoA dehydrogenase and carnitine palmitoyltransferase I activities remained statistically unchanged. A higher mitochondrial ATP production rate was observed after endurance training with the substrate combinations pyruvate+palmitoyl-L-carnitine+L-glutamate+malate (P < 0.01), L-glutamate (P < 0.001), pyruvate+malate (P < 0.05) and palmitoyl-L-carnitine+malate (P < 0.01). The largest increase was obtained with L-glutamate (170%). Significant correlations were observed between the percent increase in citrate synthase activity and those of mitochondrial ATP production rates. It was concluded that the increased mitochondrial ATP production rate of aged human skeletal muscle with training seems mainly to occur through an increased mitochondrial content, and in a way similar to those observed in young men.