Exercise-induced cell signalling responses of human skeletal responses of human skeletal muscle: the effects of reduced carbohydrate availability

Abstract

It is well documented that regular endurance exercise induces skeletal muscle mitochondrial biogenesis. However, the optimal training stimulus and nutritional intervention for which to maximize mitochondrial adaptations to endurance exercise is not well known. Developments in molecular techniques now permit the examination of the cell signalling responses to acute exercise therefore increasing our understanding of how manipulation of the training protocol and nutrient availability may enhance the training stimulus to a given bout of exercise. The primary aim of this thesis is to therefore characterise the skeletal muscle cell signalling responses thought to regulate mitochondrial biogenesis following an acute bout of high-intensity interval exercise and moderate- intensity continuous exercise. A secondary aim is to subsequently examine how manipulation of carbohydrate (CHa) availability may enhance the activation of key regulatory cell signalling pathways. The aim of the first study (Chapter 4) was to develop two exercise protocols of varied activity profile, which induced comparable total oxygen consumption and energy expenditure after being matched for average intensity, duration and distance ran. In a repeated measures and randomised design, eight active males performed an acute bout of high-intensity interval (HIT) running (6 x 3 min at 90 % V02max interspersed with 6 x 3 min at 50 % V02max also performed with a 7-min warm up and cool down at 70 % V02max) and an acute bout of moderate-intensity continuous (CaNT) running (50-m in continuous running at 70 % V02max). As a result of average intensity (70 % V02max) duration (50-min) and distance ran (9843 ± 176) being equal between protocols, total oxygen consumption (HIT; 162 ± 6, CaNT; 166 ± 10 L) and energy expenditure (HIT; 811 ± 30, CaNT; 832 ± 48 kcal) were matched between protocols (P > 0.05). Despite higher ratings of perceived exertion in HIT compared with CaNT (HIT; 14 ± 0.5, caNT; 13 ± 0.4 AU, P 0.05). Data therefore demonstrate comparable cell signalling responses between HIT and CaNT when matched for work done, average intensity, duration and distance ran. Furthermore, this is the first time exercise is shown to up-regulate p53 phosphorylation in human skeletal muscle therefore highlighting an additional pathway by which exercise may regulate mitochondrial biogenesis. Progressing from the role of the exercise stimulus in initiating mitochondrial biogenesis, the aim of the third study (Chapter 6) was to examine the effects of reduced CHO availability on modulating the exercise-induced activation of the cell signalling pathways as characterised in Chapter 5. Although HIT and CaNT protocols resulted in comparable signalling in Chapter 5, we chose HIT as our chosen exercise model given that it is perceived as more enjoyable than CaNT, has application for improving both human health and performance and also because of its relevance as a training modality for elite athletes in team and endurance sports. In a repeated measures and randomised design, muscle biopsies (vastus lateralis) were obtained from eight active males pre-, post and 3 h after performing an acute bout of high-intensity interval running with either high (HIGH) or low CHO availability (LOW). In LOW, subjects performed a bout of glycogen depleting exercise the night before and reported to the laboratory on the subsequent morning in a fasted state as well as restricting CHO before, during and after exercise. Subjects in HIGH CHO loaded for 24 h before reporting to the laboratory to perform HIT with CHO consumed before, during and after exercise. Resting muscle glycogen (HIGH, 467 ± 19; LOW, 103 ± 9 rnmol.kq" dw) and utilisation (HIGH, 142 ± 34; LOW, and 30 ± 12) was greater in HIGH compared with LOW (P < 0.05). Phosphorylation (P-) of ACCSer79 (HIGH, 1.4 ± 0.4; LOW, 2.9 ± 0.9), a marker for AMPK activity, and p53ser15 (HIGH, 0.9 ± 0.4; LOW, 2.6 ± 0.8) was higher in LOW immediately post- and 3 h post-exercise, respectively (P < 0.05). Before and 3 h post-exercise, mRNA content of PDK4, Tfam, COXIVand PGC-1a were greater in LOW compared with HIGH (P < 0.05) whereas CPT1 showed trend towards significance (P = 0.09). However, only PGC-1a expression was increased by exercise (P < 0.05) where 3-fold increases occurred independent of CHO availability. Data demonstrate that low CHO availability enhances p53 phosphorylation in a manner that may be related to upstream signalling through AMPK. Given the emergence of p53 as a potential molecular regulator of mitochondrial biogenesis, such nutritional modulation of contraction-induced p53 activation may have implications for both athletic and clinical populations. In summary, the work undertaken from the studies in this thesis provides novel information in relation to the regulation of exercise-induced cell signalling responses associated with mitochondrial biogenesis. Specifically, this is first report to examine cell-signalling responses to running exercise where comparable signalling between HIT and CaNT was observed when protocols are matched for average intensity and duration. Furthermore, these data provide the first report of an exercise-induced increase in p53 phosphorylation in which data demonstrate low CHO availability augments the exercise-induced increase in p53 signalling which may be related to upstream signalling through AMPK. Further studies would now benefit from addressing the nuclear and mitochondrial abundance of p53 in response to an acute exercise challenge as well as comprehensively examining how training status, exercise intensity and CHO availability affects p53 regulation and downstream target genes