High Intensity Exercise Cause Sarcoplasmic Reticulum Ca2+ Leak Due To Severe Ryanodine Receptor Modification.

Abstract

PURPOSE: High-intensity interval training (HIIT) is a form of exercise which is shown to be highly beneficial in VO2 max gain over short amounts of time. For instance, a single session of repeated 30s ‘all-out’ cycling has been shown to induce mitochondrial biogenesis. However, little is known as to how this relatively short exercise can have such a large effect on muscle adaptation. METHODS and RESULTS: Human subjects were severely fatigued by performing six repeated Wingate cycling exercises (i.e. 6x30s ‘all-out’ cycling). Maximal voluntary (MVC) and low frequency (10Hz) force were significantly decreased 5 min post exercise (MVC 61±1% and 10Hz 34±1% of pre-exercise (mean±SEM); P < 0.001). This decrease was not due to impaired activation, i.e. EMG and M-wave signals were not changed. Skinned fibers from muscle biopsies showed no decrease in maximum force production after exercise. Western blot showed a severe and specific proteolysis of the sarcoplasmic reticulum (SR) Ca2+ release channels (the ryanodine receptor 1; RyR1) 24 h after the exercise, resulting in specific fragments sized ∼375, 81 and 64 kDa. In mouse flexor digitorum brevis (FDB) stimulated with a ‘simulated’ Wingate protocol (6x30s of 100Hz, 250ms tetani every 500ms), Western blot showed RyR1 malondialdehyde adducts 5 min after stimulations, indicating buildup of reactive oxygen species during exercise. This was followed by proteolysis of RyR1, similar to that seen in the human biopsies. Accordingly, tetanic [Ca2+]i was decreased in FDB fibers following the repeated stimulations. Moreover, baseline [Ca2+]i was still increased 2 h post exercise (72±1 vs. 56±1 nM pre; P < 0.001), indicating SR Ca2+ leak. Intriguingly; the RyR1 proteolysis was prevented when FDB muscles were stimulated in the presence of antioxidant N-acetylcysteine (NAC). Furthermore, elite endurance athletes (VO2max 67 ± 2 ml/min/kg) showed no sign of RyR1 breakdown following the same repeated cycling exercises. These athletes had a higher level of the natural antioxidant super oxide dismutase. CONCLUSIONS: HIIT has been shown to be an effective way to stimulate mitochondrial biogenesis and increase endurance. Our results provide a possible mechanism: HIIT induced RyR1 modifications with increased SR Ca2+ leak. Elevated [Ca2+]i can stimulate mitochondrial biogenesis.