Satellite cell proliferation in response to a chronic laboratory-controlled uphill vs. downhill interval training intervention
Thesis (MSc (Physiological Sciences))--University of Stellenbosch, 2006.
Despite a growing interest into the mechanisms of the repeated bout effect, little is known about the consequences of chronic eccentrically biased training and the possible advantageous such training may offer to athletes as well as patients with muscle-debilitating disease. This study investigated the role of satellite cells in the muscle adaptation in response to either downhill or uphill high intensity training (HIT). Welltrained endurance runners were divided into two training groups matched for training volume and 10 km running times (n = 6, uphill training, UP; n = 6, downhill training, DH) and subjects in both groups completed 10 HIT sessions over a period of 4 weeks. Running performance was tested before and after the training intervention by a 10 km road race and peak treadmill speed (PTS) in horizontal and inclined (+5%) laboratory incremental tests to exhaustion. Skeletal muscle biopsies were sampled at baseline, after 2 HIT sessions, and after 4 weeks of HIT. Muscle was analysed immunohistochemically for satellite cell frequency as identified by CD56 and M-cadherin (Mcad) expression. Myogenin protein contents of muscle homogenates were determined by western blotting. Myosin heavy chain (MyHC) isoform proportions and mean fibre crosssectional area was measured. During the HIT intervention, UP exercised at a higher percentage of their HRmax than DH (mean ± SD, 97 ± 1 vs. 92 ± 3 %HRmax, p < 0.005), but at a similar rate of perceived exertion (RPE). DH completed more intervals per session and covered greater distance per session than their UP counterparts. Both training groups increased their training intensity but decreased their training volume during the 4 weeks of HIT. The combined group of 12 athletes improved their PTSgradient (mean ± SD, 16.7 ± 0.8 vs. 17.3 ± 1.0 km/h, p < 0.05). No significant differences between groups were found for PTS, VO2max or 10 km performance. Satellite cell frequency in this cohort of trained runners (48.9 ± 10.3 km/week) at baseline was similar to healthy young males (CD56+ cells/fibre, 0.19 ± 0.10). Satellite cell frequency increased significantly in DH after 4 weeks (Mcad, 123%; CD56, 138%) and non-significantly in UP (Mcad, 45%, CD56, 39%). No significant differences were found after two training sessions or at any time between groups. Mcad and CD56 expression correlated well (r = 0.95, p < 0.0001). Muscle myogenin content increased for both groups (UP: 56%; DH: 60%) after 4 weeks. No notable changes were seen after two training sessions. However, myogenin levels 2 days after session 1 correlated well (r= 0.99, p<0.005) with muscle pain experienced on the same day, as measured by the visual analogue scale. No changes were seen in the MyHC proportions or the fibre cross-sectional area after the training intervention. It was concluded that the training intervention was too short to induce changes in MyHC distribution or fibre area. Is seems likely that satellite cell proliferation was initiated as an early response to DOMS, but the response was maintained for 4 weeks. However, due to the lack of change in fibre morphology and myonuclear number, the role of satellite cell proliferation in fibre type transformation or muscle hypertrophy could not be established. Similarly, various possible roles for increased myogenin protein are offered, but since the origin of myogenin expression (satellite cells vs. myonuclei) was not determined, no definite conclusion regarding the precise function can be made. In conclusion, this study is the first to definitively indicate satellite cell proliferation in well-trained endurance runners in response to a change in training, including specifically downhill HIT. This response was early and sustained. This study asks several questions about the role of satellite cells during muscle adaptation to repetitive downhill training, and lays a foundation for further research into this unexplored field.