Aging sustains the hypertrophy-associated elevation of apoptotic suppressor X-linked inhibitor of apoptosis protein (XIAP) in skeletal muscle during unloading.

This study tested the hypotheses that apoptotic suppressors: (a) increase during muscle overload, (b) decrease in response to unloading following hypertrophy, and (c) respond to unloading in an aging-dependent fashion. Following 14 days of stretch-induced overloading, the X-linked inhibitor of apoptosis protein (XIAP) was elevated by 140% and 116% in patagialis (PAT) muscles of young and old quail, respectively, when compared to the contralateral control side. XIAP messenger RNA (mRNA) or protein was not different in experimental and control muscles of young birds after 7 or 14 days of unloading. In old birds, PAT XIAP mRNA and protein were 47% and 67% greater in experimental than in control muscles, respectively, after 7 days of unloading. Furthermore, XIAP mRNA had returned to control level by 14 days of unloading, but XIAP protein content was 57% greater than control muscles after 14 days of unloading. Higher levels of XIAP during unloading in old than in young muscles may be an attempt to counterbalance apoptosis-induced muscle atrophy.