Quantitative and qualitative adaptation of skeletal muscle mitochondria to increased physical activity

Endurance capacity rely on high muscle oxidative capacity but should also involve a tighter coupling between energy production and utilization within the myocyte. The present study examined the responses of muscle oxidative capacity and the regulation of oxidative phosphorylation by phosphate acceptors in locomotor muscles of voluntary running rats (n = 8), using saponin permeabilized fibers of the deep and superficial parts of plantaris muscle (dPLA and sPLA, respectively). Non-ADP stimulated respiration of skinned fibers increased by 33% (P < 0.05) and 100% (P < 0.001) in sPLA and dPLA, respectively. The maximal ADP-stimulated respiration was 57% (P < 0.001) and 32% (P < 0.01) higher in active rats than in sedentary rats (n = 8), in sPLA and dPLA, respectively. This finding was consistent with a 72% increase in the CS activity in plantaris muscle of exercising rats (P < 0.01). Voluntary running induced a 334% increase in the apparent Km for ADP in sPLA (P < 0.001), and a 61% increase in dPLA (P < 0.05), showing a lower affinity for cytosolic ADP of mitochondria present in both, predominantly glycolytic, and oxidative fibers. There was an increase in the creatine kinase efficacy in both sPLA and dPLA (131%, 75%, P < 0.001, respectively), consistent with an increase in the activity of the mitochondrial isoform of creatine kinase (106%, P < 0.01). It is concluded that, in addition to the well-known increased oxidative capacity, voluntary running is associated with changes in the regulation of oxidative phosphorylation by phosphate acceptors, in both glycolytic and oxidative fibers, in the direction of increased coupling between energy production and energy utilization.