A new study has revealed that differences in muscle responses to exercise in older versus younger men also reflect differences on the hormonal, molecular, and gene-expression level.
These differences include genetic up-regulation of a key enzyme related to muscle breakdown in older men.
The results help in understanding the 'molecular control points' for aging-related muscle atrophy-and may lead to new approaches to mitigating the adverse effects of muscle wasting in older adults, said senior author Chad M. Kerskick of the University of Oklahoma, Norman.
The researchers compared responses to resistance exercise (weightlifting) in a group of older and younger men, with an average age between 21 and 68 years.
Before and after the men performed a series of exercises (squat, leg press, and leg extension exercises), blood samples were obtained to measure key hormones involved in muscle responses to exercise.
Samples of muscle tissue (biopsies) were also obtained to measure gene expression of two specific enzymes -atrogin-1 and MuRF-1-involved with the process of skeletal muscle breakdown.
Recent studies have suggested that these enzymes reflect age-related differences in muscle metabolism and muscle response to exercise.
The researchers found that younger men had higher levels of the hormones cortisol and insulin-like growth factor-1 (IGF-1), before and after exercise.
In addition, 24 hours after exercise, the level of IGF-1 had increased in older men. Recent studies have suggested that IGF-1 may reduce degradation of skeletal muscle protein.
Before exercise, the older men had higher expression of the MuRF- gene, which has been linked to age-related muscle atrophy (wasting).
There was no age-related difference in expression of atrogin-1.
The changes in MuRF-1 and atrogin-1 after exercise did not differ significantly between the younger and older men. However, men with higher expression of MuRF-1 had lower levels of IGF-1.
Muscle mass and strength decrease with aging, due to reductions in the levels of anabolic hormones and growth factors. Muscle growth response to exercise (hypertrophy) is also decreased with aging.
The new results help to clarify some of the differences between younger and older muscle. The increased expression of MuRF-1 in older men could be a precursor to muscle atrophy, or may possibly represent an adaptive change to help maintain muscle mass.
The study is published in the current issue of The Journal of Strength and Conditioning Research.