Cleaved Caspase-3 Response to Acute Resistance Exercise in Young and Old Men and Women : Relationship to Muscle Glycogen Content and 5'-AMP-Activated Protein Kinase (AMPK) Activity

Abstract

In addition to suppressing protein synthesis and activating protein degradation in skeletal muscle and other cell types, 5'-AMP-activated protein kinase (AMPK) is known to stimulate nuclear apoptosis in non-muscle cells through cleaved (activated) caspase-3, one of the final steps in the apoptosis cascade. Although it is unknown whether AMPK stimulates caspase-3 cleavage or nuclear apoptosis in skeletal muscle cells in vivo, AMPK activity and nuclear apoptosis are elevated at rest in aged rat skeletal muscle. AMPK phosphorylation and activity are also higher in old vs. young rats and humans in response to overload or resistance exercise. Furthermore, older individuals display lower muscle glycogen content, a condition known to accentuate AMPK activity at rest and during aerobic exercise. We hypothesized that skeletal muscle cleaved caspase-3 content would be higher after acute resistance exercise in older versus younger individuals. Seven young (21.7 ± 2.1 yrs) and 11 old (67.0 ± 8.6 yrs) subjects performed an acute bout of leg extension resistance exercise. Muscle biopsies were obtained pre-exercise (PRE), immediately post-exercise (0P), 1-hour post-exercise (1P), and 2-hours post-exercise (2P). Glycogen content was measured in muscle samples, as were the phosphorylations (via western blot) of AMPK and acetyl-CoA carboxylase (ACC; a marker of AMPK activity). Procaspase-3 and cleaved (activated) caspase-3 contents were also assessed via western blot. AMPK phosphorylation was significantly (p < 0.05) increased in old, but not young, subjects immediately post-exercise. In both age groups, AMPK activity (assessed by ACC phosphorylation) was elevated vs. PRE at the 0P and 1P time points, and cleaved caspase-3 content was elevated vs. PRE at the 0P, 1P, and 2P time points. However, there was no effect of exercise on procaspase-3 content in either age group, and no differences between age groups in AMPK activity, procaspase-3 content, or cleaved caspase-3 content at any time point. There were significant, or close to significant, relationships between glycogen content and AMPK activity at time points PRE, 0P, and 1P regardless of age. However, no significant correlations between AMPK activity and cleaved caspase-3 content were observed at any time point. In summary, these data indicate that cleaved caspase-3 (e.g., caspase-3 activity) increases in response to acute resistance exercise in both young and old subjects. However, exercise-induced AMPK activation may not be the mechanism by which this occurs.