The Autophagic Response to Acute Exercise in Young Men is Exercise Intensity Dependent and Influenced by Exposure to Heat

Autophagy is a vital cellular mechanism that maintains normal cellular function during acute stressors, including exercise and heat stress, by degrading and recycling damaged or dysfunctional cellular constituents, thereby enabling normal cellular processes to survive the stress insult. Despite implications of exercise being a potent stimulus of autophagy, the relationship between exercise intensity and activation of autophagy remains unclear in humans. Further, while cellular stress associated with exercise is exacerbated when performed in the heat, it is unknown if this is associated with a corresponding change in autophagy. Therefore, we evaluated the hypothesis that autophagy would be exercise‐intensity dependent and that the autophagic response would be enhanced during exercise in the heat. To evaluate this hypothesis, on four separate days, 10 young men (mean [SD]; 22 [2] years) performed 30‐minutes of low‐, moderate‐, and high‐intensity semi‐recumbent cycling (equivalent to 40, 55, and 70% of maximal oxygen consumption) in a non‐heat stress environment (25°C). To assess the superimposing effects of an environmental heat stress, high‐intensity exercise was also performed in the heat (40°C). Mean body temperature (MBT; 0.64*rectal temperature + 0.36*skin temperature) was measured throughout, while autophagy‐related proteins (microtubule associated protein 1 light chain 3 (LC3)‐II and sequesterome‐1/p62 (p62)) were assessed via Western blot before, immediately after, and following 3h and 6h post‐exercise recovery. All proteins were normalized to β‐actin and reported as fold change relative to the respective baseline. Data were compared via a two‐way repeated measures ANOVA with Tukey’s test (α=0.05). No change in MBT occurred during low‐intensity exercise, although a step‐wise increase was observed at end‐exercise during moderate‐ (36.01 [0.36]°C; p<0.01), and high‐intensity exercise (36.32 [0.35]°C; p<0.01), with further increases in the heat (37.40 [0.50]°C; p<0.01) as assessed in the high‐intensity exercise only. While autophagy proteins did not change during low‐intensity exercise, LC3‐II was elevated at the end of moderate‐intensity exercise (1.31 [0.38]; p=0.02), which returned to baseline within 3h. During high‐intensity exercise, LC3‐II increased at end‐exercise (1.59 [0.16]; p<0.01), which remained elevated above baseline at 3h (1.46 [0.28]; p<0.01) and 6h (1.31 [0.22]; p=0.04). Similarly, while LC3‐II was elevated at end‐exercise in the heat above the non‐heat stress condition (2.38 [0.98]; p=0.01), the recovery response was similar between conditions. Changes in p62 were only observed during high‐intensity exercise with (0.65 [0.27]; p=0.03) and without (0.69 [0.10]; p=0.04) heat, which returned to basal levels within 3‐h in both conditions, with no differences between conditions. Taken together, we show that autophagy is exercise‐intensity dependent (given LC3‐II accumulation and p62 degradation are indicative of elevated autophagy) and this response is amplified by the added burden of heat.

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