Apoptosis in Young and Old Denervated Rat Skeletal Muscle

Abstract

Introduction: The purpose of this study was to investigate the apoptotic response to different degrees of denervation in two age groups.

Methods: A total of thirty young (3 mos.

n=15) and older (22 mos.

n=15) Sprague-Dawley rats were randomized into three groups: control (C), partial denervation (PD), and complete denervation (CD). The right sciatic nerve was injured in the PD and CD groups. Four weeks after the injury, the muscle wet weight and myosin heavy chain (MHC) isoform composition (via SDS-PAGE) were determined in gastrocnemius (GCM) and soleus (SOL) muscles. Histological appearance and fiber cross-sectional area (FCSA) were studied in GCM. Apoptotic responses in GCM were determined by studying changes in myonuclei and expression of Bcl-2 and BAX. Statistical analysis was done using nonparametric methods and generalized linear model analysis.

Results: No definite interaction between age group and degree of denervation was seen. In general, older animals were heavier (p<0.001) but showed non-significant tendency of lower muscle weight to body weight ratio (MBR, p>0.05). However, within control rats, we found significant difference of MBR between age groups (p=0.036). PD and CD resulted in significant reductions in muscle weight (GCM and SOL) and FCSA (GCM) in young and older rats. Increases in connective tissue were seen after denervation. Significant changes were seen in the expression of types I (increase, p=0.008) and IIb (decrease, p=0.008) MHC isoforms in young GCM after denervation but no significant changes were seen in older animals. In SOL, CD resulted in significant changes compared to C in type I (decrease, p=0.016) and IIa (increase, p=0.016) MHC isoforms in young rats. In general, there was no difference of apoptotic response between age groups (P>0.05). But, within control group, older rats had significantly more broken myonuclei (p=0.033), higher expression of BAX (p=0.036) and Bcl-2 (p=0.028). A larger number of apoptotic nuclei was seen in PD and CD compared to C in young and older animals (p=0.008). PD and CD resulted in significantly higher expression of BAX and Bcl-2 in both young and older rats (p<0.05) but the BAX/Bcl-2 ratio did not change (p>0.05).

Conclusions: Older age was associated with an increased level of apoptosis but older muscle was not more vulnerable to the effect of denervation on cell death.