Age and exercise-related changes in myocardial mitochondria in mice

A quantitative histochemical and an ultrastructural study was performed on the left ventricular myocardium of C57BL/6J female mice to evaluate the effects of aging and running exercise on mitochondrial structure and SDH activity. A comparison was made between 6-month-old ("young") and 27-month-old ("old") sedentary mice with age-matched mice subjected to running schedules (10 or 30 min/d) for 6 weeks. In longer-term studies, 27-month-old mice were given similar daily running schedules over a 10 month period ("long term runners"). No significant differences were found in SDH activity during the normal course of aging in mice (6 to 27 months of age), however, the response to running differed markedly in young versus old mice. "Young" trained mice showed significant increase in SDH activity compared with age-matched sedentary mice, whereas "old" trained mice showed significantly reduced SDH activity. Electron microscopy showed ultrastructural changes in mitochondria associated with aging including the development of large aggregations of mitochondria in subsarcolemmal and paranuclear sites. Running schedules, especially in aged runners, caused an increase in interfibrillar mitochondria hypertrophy, loss of matrix and cristae, incorporation of lipid inclusions and the formation of giant mitochondria. These abnormal mitochondrial changes are interpreted as being degenerative and possibly contributing to the reduced SDH levels found in cardiac myocytes of aged runners. Our results indicate that whereas regular exercise in young animals enhanced SDH activity, in aged mice it may be detrimental rather than beneficial.