Abstract
This study tested the hypothesis that aging exacerbates apoptotic signaling in rat fast plantaris muscle during muscle unloading. Plantaris muscle mass was 22% lower in aged animals and the apoptotic index was 600% higher, when compared to those in young adult animals. Following 14 days of hind-limb unloading, absolute plantaris muscle mass was 20% lower in young adult animals with a corresponding 200% higher elevation of the apoptotic index. Unloading had no affect on muscle weight or apoptotic index of aged plantaris muscles. The changes in pro-apoptotic messenger RNA (mRNA) for apoptotic protease activating factor-1 (Apaf-1), Bax, and inhibitor of differentiation protein-2 (Id2) were exacerbated with aging. Bax and Bcl-2 protein levels were also altered differently in aged muscle, compared to young. Significant positive correlations were observed between the changes in Id2 and Bax mRNA, and Id2 and caspase-9 mRNA. These data suggest that a pro-apoptotic environment may contribute to aging-associated atrophy in fast skeletal muscle, but apoptotic signaling differs by age.