Abstract
The central purpose of this study was to evaluate the fiber type-specific satellite cell and myonuclear responses of endurance-trained cyclists to a block of intensified training, when supplementing with carbohydrate (CHO) vs. carbohydrate-protein (PRO). In a crossover design, endurance-trained cyclists (n = 8) performed two consecutive training periods, once supplementing with CHO (de facto “control” condition) and the other with PRO. Each training period consisted of 10 days of intensified cycle training (ICT–120% increase in average training duration) followed by 10 days of recovery (RVT–reduced volume training; 33% volume reduction vs. normal training). Skeletal muscle biopsies were obtained from the vastus lateralis before and after ICT and again following RVT. Immunofluorescent microscopy was used to quantify SCs (Pax7+), myonuclei (DAPI+), and myosin heavy chain I (MyHC I). Data are expressed as percent change ± 90% confidence limits. The 10-day block of ICTCHO increased MyHC I SC content (35 ± 28%) and myonuclear density (16 ± 6%), which remained elevated following RVTCHO (SC = 69 ± 50% vs. PRE; Nuclei = 17 ± 15% vs. PRE). MyHC II SC and myonuclei were not different following ICTCHO, but were higher following RVTCHO (SC = +33 ± 31% vs. PRE; Nuclei = 15 ± 14% vs. PRE), indicating a delayed response compared to MyHC I fibers. The MyHC I SC pool increased following ICTPRO (37 ± 37%), but without a concomitant increase in myonuclei. There were no changes in MyHC II SC or myonuclei following ICTPRO. Collectively, these trained endurance cyclists possessed a relatively large pool of SCs that facilitated rapid (MyHC I) and delayed (MyHC II) satellite cell proliferation and myonuclear accretion under carbohydrate conditions. The current findings strengthen the growing body of evidence demonstrating alterations in satellite cell number in the absence of hypertrophy. Satellite cell pool expansion is typically viewed as an advantageous response to exercise. However, when coupled with our previous report that PRO possibly enhanced whole muscle recovery and increased MyHC I and II fiber size, the limited satellite cell/myonuclear response observed with carbohydrate-protein seem to indicate that protein supplementation may have minimized the necessity for satellite cell involvement, thereby suggesting that protein may benefit skeletal muscle during periods of heavy training.
Highlights
The central dogma of muscle regeneration and stress adaptation involves a subset of multipotent skeletal muscle stem cells’ commitment to a cell lineage involving satellite cell proliferation, differentiation, and eventual fusion to target muscle fibers in the form of myonuclei of myotubes
While less is known about satellite cell behavior in response to endurance exercise, there is growing evidence that heavy endurance exercise can stimulate satellite cell proliferation following as little as one session (Darr and Schultz, 1987; Mackey et al, 2007) and that aerobic training programs can result in measurable increases in satellite cell number (Umnova and Seene, 1991; Charifi et al, 2003; Li et al, 2006; Verney et al, 2008; Shefer et al, 2010; Kurosaka et al, 2012; Fry et al, 2014; Hoedt et al, 2016; Murach et al, 2016)
Pooled SC density was Likely higher (22 ± 28%) following ICTCHO compared to PRECHO and remained Most Likely higher (44 ± 24%) than PRECHO following RVTCHO
Summary
The central dogma of muscle regeneration and stress adaptation involves a subset of multipotent skeletal muscle stem cells’ commitment to a cell lineage involving satellite cell proliferation, differentiation, and eventual fusion to target muscle fibers in the form of myonuclei of myotubes. While less is known about satellite cell behavior in response to endurance exercise, there is growing evidence that heavy endurance exercise can stimulate satellite cell proliferation following as little as one session (Darr and Schultz, 1987; Mackey et al, 2007) and that aerobic training programs can result in measurable increases in satellite cell number (Umnova and Seene, 1991; Charifi et al, 2003; Li et al, 2006; Verney et al, 2008; Shefer et al, 2010; Kurosaka et al, 2012; Fry et al, 2014; Hoedt et al, 2016; Murach et al, 2016). 6 weeks of cycle training, regardless of training intensity, led to an increase in satellite cell activity (Joanisse et al, 2015), altogether supporting the idea that satellite cells have a distinct role in non-hypertrophic remodeling
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