Introduction
Skeletal muscle stem cells, commonly referred to as satellite cells (SC), play a key role in the skeletal muscle’s adaptive response. Following various stimuli, such as damage-inducing exercise, SC are activated, proliferate and can fuse to existing myofibres to support repair and remodelling. We have previously demonstrated that greater skeletal muscle capillarization resulted in enhanced SC responses to damaging exercise1, however the effect of aerobic training to alter the SC response to damaging exercise has yet to be explored.
Aims/objectives
The aim of the study was to determine the effect of aerobic exercise training on acute SC responses to a bout of damaging exercise. We hypothesized that aerobic exercise would augment the acute SC response to damaging exercise.
Methods
Fourteen (n=8 males; n=6 females; 21 ± 2years) healthy, recreationally active individuals were recruited. Appropriate ethical approval was granted (HiREB #3885) and procedures conformed to the Declaration of Helsinki. Participants completed 6 weeks of unilateral aerobic training on a cycle ergometer (3x45min sessions/wk; at 50% unilateral work peak) followed by an acute bout of 300 maximal eccentric contractions (180 degrees/second). Muscle biopsies were taken from the m.vastus lateralis of the aerobically trained (EX) and control (CTL) legs before (Pre), 24h and 48h hours post eccentric contractions. Immunofluorescence microscopy was used to determine SC content (Pax7+ cells), activation status (Pax7+/MyoD+ cells) and capillary to fibre perimeter exchange ratio (CFPE), a measure of capillarization. qPCR was used to determine mRNA expression of Pax7 and MyoD.
Results
Following unilateral aerobic training type I fibre CFPE (capillaries/1000 μm) was greater in EX (7.0 ± 1.2) compared to CTL (5.1 ± 1.1) (p<.05). Type I SC content (Pax7+ cells/100 fibres), was greater in EX (Pre 6.21 ± 2.86, 24h 11.31 ± 6.21, 48h 9.77 ± 3.70) compared to CTL (Pre 4.97 ± 2.23, 24h 7.11 ± 2.20, 48h 8.25 ± 6.25) across all time points (p<0.05). Type II SC content (Pax7+ cells/100 fibres) increased from Pre (3.84 ± 1.50) to 48h (10.85 ± 5.42) in the EX leg and at 24h (8.27 ± 3.71) and 48h (8.73 ± 3.64) compared to Pre (4.03 ± 1.67) in the CTL leg (p0.05).
Damaging exercise increased Pax7 mRNA expression (p<.05) to a greater extent in EX (Pre 1.20±0.52, 24h 1.01±0.41, 48h 1.62±0.36) compared to CTL (Pre 1.00±0.21, 24h 1.04±0.34, 48h 1.18±0.26; p<0.05) across all time points. MyoD mRNA expression also increased following damaging exercise (p0.05).
Discussion
Our results suggest that aerobic training augments the SC response to a bout of damaging exercise and potentially due to an increase in capillarization. Maximising the SC response can accelerate the muscle repair process which can have far reaching implications from athletic populations to compromised individuals e.g. older persons.