Muscle mitochondrial density after exhaustive exercise in dogs: prolonged restricted activity and retraining

The effect of exhaustive treadmill exercise on mitochondrial density (MD) and ultrastructural changes in quadriceps femoris muscle was studied in 7 normal, healthy, male mongrel dogs (2-4 years, 15-20 kg) before and after restricted activity (RA) (2 months for 5 dogs, 5 months for 2 dogs), and following a subsequent 2-month exercise retraining period (2-months group). Mean (+/- S.E.) time to exhaustion in the 2-month group decreased from 177 +/- 11 min before to 90 +/- 16 min (delta = -49%, p < 0.05) after RA; retraining increased tolerance to 219 +/- 36 min (delta = +24%, p < 0.05) above the pre-RA and 143% (p < 0.05) above the post-RA time. Post-RA exhaustion time in the 5-months group was 25 and 45 min (35 +/- 10 min). Muscle samples taken after RA showed abnormalities indicative of degeneration, which were reversed by retraining. Resting MD decreased (p < 0.05) from a control level of 27.8% to 14.7% (2 months) and 16.3% (5 months), and was restored to 27.1% (NS) after retraining. Exhaustive exercise caused an increase in MD under control conditions and after RA, but not following retraining. Disruption of mitochondria after exercise was evident after 5-months confinement. Factors causing mitochondrial changes and eventually their disruption during exercise after restricted activity are not related as much to the state of fatigue as to the pre-exercise quality of the muscle modified by disease or training.