Tissue respiration changes in chronic exercise

Summary

The influence of physical activity on tissue metabolism is still largely unknown, in spite of the fact that several studies have been reported in this area. The purpose of this study was to determine more precisely whether a daily endurance type of exercise has any influence on the resting aerobic metabolism of the key tissues involved in exercise.

The oxygen consumption of tissue slices of heart, skeletal muscle and liver of trained and nontrained rats was determined using the Warburg technique. The tissue respiration was measured using a gas phase of 95% O₂, 5% CO₂ and with added substrates of either glucose or pyruvate. 22 rats were progressively trained for 5–8 weeks in a motorized running wheel apparatus, reaching a rate of 1 mile per hour for an hour each day at the end of the 4th week. They were then held at this running rate each day until sacrificed. 20 rats were used as controls; these remained in their cages with no exercise available.

The results show that an endurance type of training, as used here, has little effect on the resting aerobic metabolism of heart or skeletal muscle tissue, regardless of the substrate employed. The oxygen consumption of liver slices, however, was significantlylower in the chronic exercised rats. The trained rats had$Q_{{\text{O}}_{\text{2}} }$ values of 9.9 and 12.6 μl per milligram dry weight per hour with glucose and pyruvate, respectively, whereas the nontrained liver slices had higher$Q_{{\text{O}}_{\text{2}} }$ values of 12.3 and 13.9 with glucose and pyruvate substrates. Thus, training appears to shift the liver resting metabolism to a lower level. Also, training appears to cause an increase in density of trained tissues, as reflected in lower wet weight/dry weight ratios found in the trained tissues.