Aging causes a progressive decline in skeletal muscle mass and function, alongside shifts in fibre composition that reduce physical capacity. Glucagon-like peptide-1 (GLP-1) receptor agonists, like semaglutide, are widely used to reduce adiposity; however, their effects on aging skeletal muscle remain poorly defined. This study investigated low-dose semaglutide’s effects on contractile function and micro-architecture in locomotor (soleus) and respiratory (diaphragm) muscles of young and aged mice.
Young (3-month) and aged (24-month) C57BL/6J mice were assigned to placebo or semaglutide groups (young placebo (YP, n=6), young semaglutide (YS, n=6), old placebo (OP, n=6), old semaglutide (OS, n=7)). Semaglutide was administered for two weeks (60 µg/kg/day, 3 days, week 1; 30 µg/kg/day, 3 days, week 2). In vitro contractile function of the soleus and diaphragm was assessed to determine force–frequency relationships and fatigue resistance (40 Hz stimulation every 2 seconds for 5 mins). A subset of samples underwent immunofluorescent staining for fibre type composition (type I, IIa, IIx/b), cross-sectional area (FCSA), capillarisation indices, and roundness using semi-automatic image analysis.
Body weight remained stable throughout the intervention due to the low doses administered (p=0.3846). Absolute muscle wet mass did not differ between groups; however, hindlimb muscle mass normalised to body weight was reduced in older mice (p=0.0118). Semaglutide reduced subcutaneous (p=0.0010) and visceral (p=0.0071) fat and liver mass (p=0.0021).
Semaglutide exerted age-dependent effects on function. In the soleus, twitch force was reduced in semaglutide-treated aged mice only (p=0.0116). A trend occurred for maximal specific force (N/cm²), which was lower in older mice receiving semaglutide (p=0.0888). In the diaphragm, semaglutide showed a trend toward reduced twitch force in both age groups (p=0.0696) alongside an age × treatment interaction for maximal force (p=0.0286), indicating impaired force production in aged semaglutide-treated mice. Conversely, fatigue resistance improved following semaglutide treatment in soleus muscle, showing enhanced force maintenance during repeated stimulation (40 Hz every 2 s for 5 min) in both young and aged mice (p=0.0461). Diaphragm fatigue properties were unaffected by treatment or age.
Histological analysis revealed prominent structural remodelling underlying these functional shifts. The treatment and age interaction caused a reduction in type 1 FCSA (p=0.0018), with its SD altered by age (p=0.0073). Type 2a FCSA demonstrated a reduction when considering the interaction of treatment and age (p=0.0019) and age effect (p=0.0405), with its SD showing interaction (p=0.0483), treatment (p=0.0337), and age (p=0.0220) effects. Fibre proportions differed according to age (Type 1 increase: p<0.0001; Type 2a decrease: p=0.0024; Type 2b/x decrease: p=0.0005). Capillary-to-fibre ratio increased with age (p=0.0194) and age × treatment interaction (p=0.0108), while a trend toward reduction occurred with treatment (p=0.0795).
While low-dose semaglutide benefits mouse body composition without weight loss, this is accompanied by unfavourable changes in the older muscle. We observed a reduction in FCSA, while muscle mass remained unchanged, suggesting an altered muscle composition. These structural changes may contribute to the impaired force-generating capacity observed in aged skeletal muscle. These data highlight the need to consider muscle-specific and age-dependent responses when evaluating the physiological impacts of GLP-1 receptor agonist therapies.