Abstract
BackgroundWe sought to examine how 12 weeks of resistance exercise training (RET) affected skeletal muscle myofibrillar and sarcoplasmic protein levels along with markers of mitochondrial physiology in high versus low anabolic responders.MethodsUntrained college-aged males were classified as anabolic responders in the top 25th percentile (high-response cluster (HI); n = 13, dual x-ray absorptiometry total body muscle mass change (Δ) = +3.1 ± 0.3 kg, Δ vastus lateralis (VL) thickness = +0.59 ± 0.05 cm, Δ muscle fiber cross sectional area = +1,426 ± 253 μm2) and bottom 25th percentile (low-response cluster (LO); n = 12, +1.1 ± 0.2 kg, +0.24 ± 0.07 cm, +5 ± 209 μm2; p < 0.001 for all Δ scores compared to HI). VL muscle prior to (PRE) and following RET (POST) was assayed for myofibrillar and sarcoplasmic protein concentrations, myosin and actin protein content, and markers of mitochondrial volume. Proteins related to myofibril formation, as well as whole lysate PGC1-α protein levels were assessed.ResultsMain effects of cluster (HI > LO, p = 0.018, Cohen’s d = 0.737) and time (PRE > POST, p = 0.037, Cohen’s d = −0.589) were observed for citrate synthase activity, although no significant interaction existed (LO PRE = 1.35 ± 0.07 mM/min/mg protein, LO POST = 1.12 ± 0.06, HI PRE = 1.53 ± 0.11, HI POST = 1.39 ± 0.10). POST myofibrillar myozenin-1 protein levels were up-regulated in the LO cluster (LO PRE = 0.96 ± 0.13 relative expression units, LO POST = 1.25 ± 0.16, HI PRE = 1.00 ± 0.11, HI POST = 0.85 ± 0.12; within-group LO increase p = 0.025, Cohen’s d = 0.691). No interactions or main effects existed for other assayed markers.DiscussionOur data suggest myofibrillar or sarcoplasmic protein concentrations do not differ between HI versus LO anabolic responders prior to or following a 12-week RET program. Greater mitochondrial volume in HI responders may have facilitated greater anabolism, and myofibril myozenin-1 protein levels may represent a biomarker that differentiates anabolic responses to RET. However, mechanistic research validating these hypotheses is needed.
Highlights
Numerous studies have reported resistance exercise increases both whole/mixed fraction muscle protein synthesis (MPS) and myofibrillar protein synthesis (MyoPS) rates several days following a single exercise bout (Damas et al, 2016; Mitchell et al, 2012; Phillips et al, 1997, 1999; Wilkinson et al, 2008)
We observed resistance exercise training (RET) decreases whole-tissue citrate synthase activity regardless of cluster, levels were greater in HI versus LO responders throughout the intervention
The current data suggest myofibrillar or sarcoplasmic protein accretion differences do not exist between high versus low anabolic responders to prior to or following a 12-week RET. We interpret these findings to suggest myofibrillar or sarcoplasmic protein concentration alterations during a shorter-term RET program assume a minor role in hypertrophic magnitudes in previously untrained college-aged subjects
Summary
Numerous studies have reported resistance exercise increases both whole/mixed fraction muscle protein synthesis (MPS) and myofibrillar protein synthesis (MyoPS) rates several days following a single exercise bout (Damas et al, 2016; Mitchell et al, 2012; Phillips et al, 1997, 1999; Wilkinson et al, 2008). Several studies have reported weeks to months of resistance exercise training (RET) increases muscle fiber cross sectional area (fCSA) (Mitchell et al, 2013; Mobley et al, 2017; Petrella et al, 2008; Reidy et al, 2016; Staron et al, 1994). These parallel findings have led to a general consensus that RET-induced increases in fCSA likely coincide with increased myofibrillar protein content.
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